GLP-1 and the heart: clinical and preclinical data

See also DPP-4 and the Cardiovascular System

Three cardiovascular outcome studies with GLP-1R agonists have reported and published results, the ELIXA trial with lixisenatide, the liraglutide (LEADER) study. and SUSTAIN-6 for semaglutide

Topline results for exenatide once weekly in the EXSCEL study were reported (cardiovascular safety, non-significant reduction in MACE events) in a Press Releasein May 2017, and will be presented at EASD in September 2017.

An overview of basic and clinical cardiovascular GLP-1 science is provided at The Cardiovascular Biology of Glucagon-like Peptide-1 Cell Metab. 2016 Jul 12;24(1):15-30

In the ELIXA trial, Lixisenatide in Patients with Type 2 Diabetes and Acute Coronary Syndrome N Engl J Med. 2015 Dec 3;373(23):2247-57 the cardiovascular safety of once daily lixisenatide was evaluated in subjects with type 2 diabetes with a recent MI or hospitalization for ACS within the past 180 days. 6068 patients who underwent randomization were followed for a median of 25 months-major trial results showed CV safety of lixisenatide compared to usual care in the control group. The primary endpoint was 3 point MACE: cardiovascular death, myocardial infarction, stroke, or hospitalization for unstable angina. There was no difference between groups for any of the key endpoints, nor for hospitalization for heart failure. Hence, lixisenatide is a safe GLP-1R gonist to use in patients with T2D with pre-existing cardiovascular disease.

Results for the LEADER trial were reported in a Press Release on March 4 2016 followed by a full publication in the NEJM June 13 2016 Liraglutide and Cardiovascular Outcomes in Type 2 Diabetes  Liraglutide demonstrated superiority, with a statistically significant reduction in cardiovascular risk (MACE) in subjects with T2D and existing CV disease, or older subjects at high risk for CVD. The primary endpoint of the LEADER study was defined as the composite outcome of the first occurrence of cardiovascular death, non-fatal myocardial infarction or non-fatal stroke. The superior reduction of major adverse cardiovascular events demonstrated by Liraglutide (Victoza®) was derived from all three components of the endpoint but principally reflect a reduction in CV death. A significant reduction in total MI was also seen with liraglutide therapy, as was a reduction in microvascular disease (proteinuria, principally new onset macroalbuminuria).

LEADER trial design and baseline data LEADER 2 Calcitonin LEADER 3 Lipase & Amylase LEADER 4 Blood Pressure LEADER 5 Cardiometabolic risk

The second GLP-1R agonist to report positive data from a cardiovascular outcome study, designed primarily to look at safety, was once weekly Semaglutide. In the Trial to Evaluate Cardiovascular and Other Long-term Outcomes with Semaglutide in Subjects with Type 2 Diabetes (SUSTAIN-6), 3297 patients with type 2 diabetes who were on a standard-care regimen were randomized to receive once-weekly semaglutide (0.5 mg or 1.0 mg) or placebo for 104 week. The primary outcome was classic 3 point MACE.  At baseline, 3297 subjects were randomized, 2735 of the patients (83.0%) had established cardiovascular disease, chronic kidney disease, or both. The primary outcome occurred in 108 of 1648 patients (6.6%) in the semaglutide group and in 146 of 1649 patients (8.9%) in the placebo group. The difference in primary outcomes was primarily driven by a reduction in non-fatal stroke in subjects treated with semaglutide. Fewer serious events and less severe hypoglycemia was reported in the semaglutide group, despite a significantly lower end of study A1c with semaglutide. See

Semaglutide and Cardiovascular Outcomes in Patients with Type 2 Diabetes NEJM 2016DOI: 10.1056/NEJMoa1607141

The baseline characteristics of subjects enrolled in the EXSCEL trial have been described. Baseline characteristics of patients enrolled in the Exenatide Study of Cardiovascular Event Lowering (EXSCEL) Am Heart J. 2017 May;187:1-9. Baseline A1c 8%, 16% with prior history of heart failure, 73% with one prior CV event ( 70% coronary artery disease, 24% peripheral arterial disease, 22% cerebrovascular disease).

The cardiovascular safety of Dulaglutide is being examined in the REWIND (Researching cardiovascular Events with a Weekly INcretin in Diabetes) trial, with 9,901 subjects, 46% women, mean duration of diabetes 10 yrs. A much lower baseline A1c of 7.3%, and only 31% had prior cardiovasclar disease at study entry. Design and Baseline Characteristics of Participants in the Researching cardiovascular Events with a Weekly INcretin in Diabetes (REWIND) Trial of Dulaglutide's Cardiovascular Effects Diabetes Obes Metab. 2017 Jun 1. doi: 10.1111/dom.13028

The cardiovascular safety of once weekly Albiglutide is being examined in HARMONY Outcomes, in subjects with type 2 diabetes and pre-existing cardiovascular disease. Effect of Albiglutide, When Added to Standard Blood Glucose Lowering Therapies, on Major Cardiovascular Events in Subjects With Type 2 Diabetes Mellitus

The cardiovascular safety of ITCA-650, an investigational continuously released exenatide has been reported in a Press Release. Results of the FREEDOM-CVO trial studies the CV safety over 3 years in ~4,000 subjects. 160 MACE events were reported, and the CV safety profile of ITCA-650 was analyzed across the FREEDOM program to support a NDA filing.

GLP-1, and ischemic heart disease-Clinical Data

The actions of GLP-1R agonists in the context of ischemic heart disease in human subjects have been examined in several short term studies. Lonborg and colleagues examined the effects of acute exenatide administration in a randomized placebo-controlled double blind study in human subjects with ST elevation myocardial infarction (STEMI) treated with percutaneous vessel revascularization (PCI). Exenatide infusion was commenced 15 minutes prior to PCI and continued for 6 hrs. Plasma exenatide levels ranged from 0.03-0.3 nMol/L, with mean levels of 177 nM. Exenatide therapy increased the salvage index and reduced infarct size relative to area at risk (by 23%), assessed by cardiac MRI 90 days after infarction. There was no significant effect on peak troponin levels, or on LV function or clinical events assessed out to 30 days. Exenatide reduces reperfusion injury in patients with ST-segment elevation myocardial infarction Eur Heart J. 2011 Sep 14. [Epub ahead of print]. Subsequent analysis of the data set revealed that the apparent cardioprotection associated with exenatide infusion was observed in both normoglycemic non-diabetic and hyperglycemic diabetic subjects, suggestion that the mechanisms linking exenatide to reduced infarct size are not strictly glucose-dependent. Impact of acute hyperglycemia on myocardial infarct size, area at risk and salvage in patients with ST elevation myocardial infarction and the association with exenatide treatment - results from a randomized study Diabetes. 2014 Feb 28

Woo and colleagues examined the effects of exenatide or placebo administration in a randomized controlled clinical trial in 58 patients with STEMI undergoing PCI using drug-eluting stents. After randomization, patients with impaired left ventricular (LV) systolic function (n=18), patients with thrombolysis in myocardial infarction flow grade 1, 2 or 3 on angiography (n=40) were also excluded, leaving n=58. About 26% of the patients had diabetes. 18 patients ended up receiving exenatide, and 40 received placebo. Patients assigned to the exenatide group were treated with 10 μg subcutaneous and intravenous bolus 10 μg injection of exenatide 5 min before the onset of reperfusion and 10 μg subcutaneous injection of exenatide was continued twice daily on the following 2 days Exenatide reduced infarct size and infarct mass assessed at 1 month by MRI. Exenatide also transiently increased systolic and diastolic BP over the first 48h and was associated with substantial rates of nausea and vomiting. Cardioprotective effects of exenatide in patients with ST-segment-elevation myocardial infarction undergoing primary percutaneous coronary intervention: results of exenatide myocardial protection in revascularization study Arterioscler Thromb Vasc Biol. 2013 Sep;33(9):2252-60.

A pilot study employing a similar design examined the feasibility, safety, and putative efficacy of administering a higher dose of exenatide, a 5 ug intravenous loading dose followed by a continuous infusion of 20ug exenatide/24h period for 72 h. Study subjects were non-diabetic, predominantly male patients presenting with a first STEMI and single vessel disease, or without a known dominant coronary lesion, or with a TIMI score of 3 (normal flow) were excluded. 22 subjects received exenatide, 21 placebo. There was no significant difference in hypo or hyperglycemia, however nausea and the use of anti-emetics was significantly greater in subjects treated with exenatide. No differences in major cardiac endpoints (LV function, infarct size:area at risk) were observed. The authors considered this a pilot and feasibility study. Effect of additional treatment with EXenatide in patients with an Acute Myocardial Infarction: The EXAMI study Int J Cardiol. 2012 Oct 17. doi:pii: S0167-5273(12)01325-3.

Chen and colleagues studied 92 patients randomized to receive liraglutide (45) or placebo (49) for 7 days after PCI for STEMI. The treatment started 30 minutes before PCI with a single injection of 1.8 mg of liraglutide in the ambulance. Subsequently, patients received 06, 1.2 and 1.8 mg liraglutide daily, for 2, 2 and 3 days, respectively. The primary endpoint was LV ejection fraction 3 months after PCI, which was 4.1% greater (p<.001) in subjects treated with liraglutide after 3 months. No major differences in other clinical endpoints was detected during a total F/U period of 6 months. Infarct size was not assessed in this study. Effects of liraglutide on left ventricular function in patients with ST-segment elevation myocardial infarction undergoing primary percutaneous coronary intervention Am Heart J. 2015 Nov;170(5):845-54   A related study from the same group using a slightly different treatment protocol, detected a very similar 4.7% increase in EF at 3 months after 1 week of liraglutide in patients with non-STEMI MI. Effects of liraglutide on left ventricular function in patients with non-ST-segment elevation myocardial infarction Endocrine. 2015 Nov 16

Nikolaidis and colleagues examined the consequences of 72 h infusion of native recombinant GLP-1 infusion in human subjects with acute myocardial infarction following angioplasty in a non-randomized study of 10 patients with LV dysfunction (LVEF less than 40%). Patients, both diabetic and non-diabetic, receiving GLP-1 ( 1.5 pmol/kg per minute ) exhibited improvements in LV function, reduced hospital stay, and some of the improvements persisted out to 30 days post hospitalization Effects of glucagon-like peptide-1 in patients with acute myocardial infarction and left ventricular dysfunction after successful reperfusion Circulation. 2004 Mar 2;109(8):962-5 

A 6 hr infusion of exenatide in 20 male subjects with T2DM and class II or IV CHF demonstrated that exenatide increased heart rate and hence cardiac index, with concomitant favorable improvements in hemodynamics. Glucose and insulin were simultaneously improved to prevent hypoglycemia. Two patients with atrial fibrillation experienced increases in HR that required digoxin administration. Plasma exenatide levels after 6 hrs were 132 pmol/L. Effects of intravenous exenatide in type 2 diabetic patients with congestive heart failure: a double-blind, randomised controlled clinical trial of efficacy and safety DOI: 10.1007/s00125-011-2440-x

Read and colleagues carried out dobutamine stress echocardiography with or without (studies done 1 week apart in the same subjects in the absence of b-blocker therapy) acute GLP-1 administration (14 mg/kg/min) commenced 30 minutes prior to the induction of ischemia in 14 human subjects (4 with diet controlled T2DM) with ischemic heart disease and normal ventricular function awaiting revascularization. Plasma GLP-1 levels were increased from 10 to 108.8 pM. Plasma insulin levels increased from 67.56 + 30.7 to 126.9 + 66.7). The GLP-1 infusion was discontinued 30 minutes after cessation of the dobutamine infusion. GLP-1 had no effect on LV function at rest in the absence of dobutamine. however in the presence of ischemia, GLP-1 improved LV function (10% increase in injection fraction) and improved the function of iscehmic ventricular segments. GLP-1 infusion also mitigated the extent of'ventricular stunning' observed pursuant to ischemia Cardioprotection against ischaemia induced by dobutamine stress using glucagon-like peptide-1 in patients with coronary artery disease. Heart. 2011 May 10. [Epub ahead of print]

A related study examined the response to acute GLP-1 administration (1.2 pmol/kg/min for 30 minutes) administered immediately after sequential balloon occlusion (2 separate 1 minute occlusions were employed prior to PCI and stenting) at the site of stenosis in 20 non-diabetic subjects with normal LV function and single vessel (LAD) disease at the time of elective PCI. Mean plasma levels of GLP-1 were about 100 pg/ml (14-fold higher than baseline) at the end of the infusion and insulin levels were higher after GLP-1. Systolic dysfunction was evident after the second balloon occlusion in the control group, however the GLP-1-infused subjects exhibited relative preservation of LV function and little evidence for LV stunning. A Pilot Study to Assess Whether Glucagon-Like Peptide-1 Protects the Heart From Ischemic Dysfunction and Attenuates Stunning After Coronary Balloon Occlusion in Humans Circ Cardiovasc Interv. 2011 May 17. [Epub ahead of print].

Subsequently, McCormick and colleagues administered an intravenous of GLP-1 (1.2 pmol/kg/min) or placebo to 19 subjects (10/9) with single vessel CAD just prior to and continued for several hrs after 1 minute of low pressure balloon occlusion of the LAD coronary artery. Plasma GLP-1 levels were elevated ~40-fold by the infusion to a mean level of 123 pg/ml. Subjects infused with GLP-1 exhibited a smaller deterioration in LV function during ischemia and improved ventricular function during the 30 minute recovery period, without changes in myocardial glucose extraction.  Pre-treatment with glucagon-like Peptide-1 protects against ischemic left ventricular dysfunction and stunning without a detected difference in myocardial substrate utilization. JACC Cardiovasc Interv. 2015 Feb;8(2):292-301

Database and Meta-analyses: Best and colleagues interrogated the IMS LifeLink Healthplan Claims Database for cardiovascular events in patients with type 2 diabetes filling prescriptions for new glucose-lowering medications after June 1 2005. Patients with recent CVS events (stroke, MI, revascularization) during the preceding 9 months were excluded from the analysis. Patient event information was collected from the date of the new prescription to time of disenrollment, first CV event, or March 31 2009. Patients were assigned to the 'Exenatide' cohort as long as they filled a new Rx for Exenatide, irrespective of the other anti-diabetic agents they were taking. A total of 383,525 patients were analyzed, comprising 21,754 patients starting Exenatide and 361,771 patients initiating other anti-diabetic agents. Analysis of the different populations suggested that patients treated with Exenatide may have had more obesity, a longer duration of diabetes and more diabetes-associated comorbidities (dyslipidemia and hypertension). Exenatide-treated patients were significantly less likely to record a CVD event (Hazard Ratio of0.81; 95% CI 0.68-0.95). Similar trends were observed in the ITT analysis. Exenatide-treated patients also had significantly lower rates of hospitalization for both CVD and non-CVD diagnoses See Risk of Cardiovascular Disease Events in Patients with Type 2 Diabetes Prescribed the GLP-1 Receptor Agonist Exenatide Twice Daily or Other Glucose-Lowering Therapies: A Retrospective Analysis of the LifeLinkTM Database Diabetes Care. 2010 Oct 7.

Atherosclerosis, Blood vessels and Blood Pressure

Clinical data

Although native GLP-1 improves flow mediated vasodilation in human subjects, whether degradation-resistant GLP-1R agonists produce similar effects is less clear. Kelly and colleagues compared the effects of exenatide vs. metformin therapy in obese non-diabetic subjects with IGT, IFG, or mild elevation in HbA1c. Study subjects were treated for 3 months after assessment of baseline endothelial function. Microvascular function was assessed by quantification of digital reactive hyperemia using finger probes, prior to and after OGTT. Three months of exenatide or metformin therapy had similar effects on parameters of microvascular endothelial function , inflammation, and oxidative stress, with the majority of biomarkers or endpoints showing no significant change from baseline Effects of Exenatide vs. Metformin on Endothelial Function in Obese Patients with Pre-Diabetes: A Randomized Trial Cardiovasc Diabetol. 2012 Jun 8;11(1):64

A reduction in blood pressure (usually 2-3 mm systolic BP) is commonly seen in clinical trials of GLP-1R agonists, such as exenatide or liraglutide. For example, analysis of pooled data from 6 clinical trials and 2171 subjects demonstrated a modest but significant reduction in systolic blood pressure in exenatide-treated subjects compared to BP in subjects treated with placebo or insulin. Some of the reduction in BP was likely associated with weight loss in the exenatide-treated group, however weight loss alone was not likely to account for the BP reductions seen in these patients. Effects of Exenatide on Systolic Blood Pressure in Subjects With Type 2 Diabetes Am J Hypertens. 2009 Dec 17. [Epub ahead of print]. Similar blood presure reductions have been observed in pooled analysis of the LEAD 1-5 trials with liraglutide therapy The safety and efficacy of liraglutide with or without oral antidiabetic drug therapy in type 2 diabetes: an overview of the LEAD 1-5 studies. Diabetes Obes Metab. 2009 Dec;11 Suppl 3:26-34.

Lovshin and colleague examined the effects of liraglutide on atrial natriuretic protein (ANP), blood pressure (BP), heart rate (HR) and urine sodium excretion in the BOLT (Blood Pressure Outcomes with LiraglutideTherapy) study in 18 human diabetic subjects with hypertension using a randomized placebo-controlled cross over design. Median BP at entry was 145/85. Each study subject received daily injections of placebo or liraglutide for 21 days, separated by a washout period. Liraglutide had no effect on mean plasma levels of ANP (although 4 patients showed robust ANP responses), and reduction of BP was not statistically significant in this small exploratory study. Heart rate was significantly but transiently increased by liraglutide which returned to normal by ~ 4hrs after the last injection. Although ANP levels were not increased liraglutide significantly increased night time and 24 hr urine sodium excretion without changes in the albumin:creatinine ratio. Liraglutide significantly reduced HbA1c, fasting glucose and body weight but no changes in serum creatinine, or plasma angiotensin II concentrations were detected after 21 days. Hence ANP does not likely contribute meaningfully to the natriuretic actions of GLP-1R agonists in diabetic hypertensive subjects. See Liraglutide Promotes Natriuresis but Does Not Increase Circulating Levels of Atrial Natriuretic Peptide in Hypertensive Subjects With Type 2 Diabetes Diabetes Care published ahead of print November 20, 2014, doi:10.2337/dc14-1958

The most extensive study (755 patients randomized, ~250 per treatment group) of BP reduction to date was reported by Ferdinand who studied 2 different doses (0.75 and 1.5 mg) of once weekly dulaglutide vs. placebo in diabetic subjects, with and without mild hypertension (from 90/60 to 140/90 seated, on 3 or fewer medications for hypertension). Ambulatory BP monitoring (ABPM) was done at baseline, 4, 16 and 26 weeks. Dulaglutide 1.5mg significantly reduced mean 24h BP at 16 and 26 weeks, with reductions observed in both daytime and nighttime systolic BP. Dulaglutide 1.5 mg also significantly increased heart rate (HR) by ~3-4 bpm. Dulaglutide 0.75 mg also significantly increased nocturnal HR. Mechanistically, no significant changes were observed within or between the dulaglutide and placebo groups for serum aldosterone, plasma renin activity, plasma metanephrines and normetanephrines, or N-terminal pro–brain natriuretic peptide, whereas HbA1c was reduced by ~1.1%.

Effects of the once-weekly glucagon-like peptide-1 receptor agonist dulaglutide on ambulatory blood pressure and heart rate in patients with type 2 diabetes mellitus Hypertension. 2014 Oct;64(4):731-7

In contrast, not all GLP-1R agonists lower blood pressure in human subjects. Gustavson and colleagues evaluated the effects of PF-04603629 , a high molecular weight GLP-1R agonist (exendin-4-nonglycosylated human transferrin fusion protein), in human subjects with T2DM. Heart rate increased by a mean 23 bpm and unexpectedly, diastolic blood pressure increased by 10.5 mm Hg. The increase in BP was noted form 16-72h after a single dose. No significant concentrations of the drug were found in CSF, making CNS exposure very unlikely. Effects of a Long-Acting GLP-1 Mimetic (PF-04603629) on Pulse Rate and Diastolic Blood Pressure in Patients with Type 2 Diabetes Mellitus  Diabetes Obes Metab. 2011 Aug 3. doi: 10.1111/j.1463-1326.2011.01479.

 

Blood pressure-Preclinical data

GLP-1 and GLP-1R agonists have acute effects on the vascular system to increase BP in rodents, but sustained GLP-1R activation lowers blood pressure in rodents and humans . Multiple mechanisms linking GLP-1R activation to reduction of blood pressure have been proposed, including modulation of sympathetic and parasympathetic nervous system activity regulating vascular tone, direct vasodilator actions of GLP-1 or GLP-1R agonists on blood vessels, direct actions of GLP-1 on the kidney to enhance sodium excretion, and direct modulation of RAS activity. Many of these studies have included demonstration of GLP-1R positive cells in heart, kidney, and blood vessels, using immunocytochemical and Western blot analyses. However recent studies have raised questions about the sensitivity and specificity of most commercially available GLP-1R antisera, see GLP-1 Receptor Activation Indirectly Reduces Hepatic Lipid Accumulation But Does Not Attenuate Development of Atherosclerosis in Diabetic Male ApoE−/− Mice Endocrinology 2013 Jan;154(1):127-39 and The glucagon-like peptide-1 receptor--or not? Endocrinology. 2013 Jan;154(1):4-8

Kim and colleagues now demonstrate that GLP-1R agonists, including liraglutide, exenatide, and GLP-1, lower blood pressure in hypertensive mice via stimulation of atrial natriuretic factor(ANF) secretion. The Glp1r was localized exclusively to cardiac atria, but not ventricles, in analyses using PCR primers that spanned the entire Glp1r coding region. Liraglutide did not directly produce vasorelaxation of preconstricted aortic segments, however cardiac perfusate from liraglutide-treated mice increased Vasp phosphorylation, cGMP formation, and produced vasorelaxation. Both acute and chronic administration of liraglutide increased plasma levels of ANF and reduced blood pressure in two different models of hypertension, AngII-infused mice and mice with transverse aortic constriction. In contrast, liraglutide-stimulated cardiac perfusate from Nppa (ANF)-/- mice had no effect on vasorelaxation. and liraglutide failed to lower blood pressure in hypertensive Nppa-/- mice. Liraglutide, exenatide and GLP-1 exhibited a sustained ability to increase plasma ANF levels and reduce blood pressure in AngII-infused hypertensive mice over a 3 week period, although the magnitude of the effect waned over time. Liraglutide, exenatide and GLP-1 robustly increased ANF secretion from the perfuse heart and atrial cardiomyocyte cultures in vitro, actions insensitive to the PKA inhibitor H-89, but mimicked by the Epac activator 8CPT-2Me-cAMP, also known as ESCA-AM. GLP-1R agonists failed to lower blood pressure, relax blood vessels, or increased urine sodium excretion in hypertensive Nppa-/- mice; liraglutide increased translocation of Epac2 to the plasma membrane and enhanced ANF secretion in WT atrial cardiomyocytes, whereas liraglutide failed to increase ANF secretion, enhance ANF plasma levels or reduce BP in Rapgef4(Epac2)-/- mice. Adenoviral transduction of Rapgef4-/- cardiomyocytes with Epac2 restored the ability of liraglutide to stimulate ANF secretion. Plasma ANF levels increased following refeeding in WT mice but not in Glp1r-/- mice. These findings define a GLP-1-heart-blood vessel/kidney axis for the control of experimental hypertension and raise questions in regard to how GLP-1R agonists exert cardioprotective actions in the absence of ventricular Glp1r expression. GLP-1 receptor activation and Epac2 link atrial natriuretic peptide secretion to control of blood pressure Nature Medicine 2013 doi:10.1038/nm.3128

Even moderate doses of GLP-1R agonists infused at levels not sufficient to lower blood glucose result in activation of central sympathetic neurons and adrenal medullary chromaffin cells  that produce catecholamines. Centrally and peripherally administered GLP-1R agonists including native GLP-1 and the lizard peptide exendin-4 dose-dependently increased blood pressure and heart rate in rats. GLP-1R activation induced c-fos expression in the adrenal medulla and neurons in autonomic control sites in the rat brain, including medullary catecholamine neurons providing input to sympathetic preganglionic neurons. Furthermore, GLP-1R agonists rapidly activated tyrosine hydroxylase transcription in AP neurons which express the GLP-1R, as shown in Glucagon-Like Peptide-1-Responsive Catecholamine Neurons in the Area Postrema Link Peripheral Glucagon-Like Peptide-1 with Central Autonomic Control Sites. J Neurosci. 2003 Apr 1;23(7):2939-2946.  

These findings suggest that the central GLP-1 system represents a regulator of sympathetic outflow leading to downstream activation of cardiovascular responses in the rodent, and are consistent with previous reports demonstrating that GLP-1R systems function as a component of neural networks transducing the CNS response to aversive stimuli. See Glucagon-like peptide-1 receptor stimulation increases blood pressure and heart rate and activates autonomic regulatory neurons J. Clin. Invest. 2002;110 43-52

The importance of cholinergic and nicotinic acid receptors for transduction of the central cardiovascular response to GLP-1 was determined in normal rats. The nicotinic receptor antagonist mecamylamine and the muscarinic receptor antagonist atropine prevented the stimulatory effect of GLP-1 on blood pressure whereas only mecamylamine blocked the GLP-1-dependent increase in heart rate. Intraarterial application of a V(1) receptor antagonist blocked the GLP-1 effects on blood pressure. See Effects of intracerebroventricularly injected glucagon-like peptide-1 on cardiovascular parameters; role of central cholinergic system and vasopressin. Regul Pept. 2004 Apr 15;118(1-2):33-8

In contrast to data suggesting that acute administration of GLP-1 may increase heart rate and blood pressure in rodents, chronic 14 day treatment of salt-sensitive rats on a high salt diet with recombinant GLP-1 reduced the development of hypertension, proteinuria and improved endothelial function with decreased  renal and cardiac damage. The authors postulated that the protective effects of GLP-1 were attributable to increased urine flow and sodium excretion notable for the first 3 days following elevation in sodium intake. See Antihypertensive effect of glucagon-like peptide 1 in Dahl salt-sensitive rats. J Hypertens. 2003 Jun;21(6):1125-1135. Similarly, continuous administration of an exenatide-related GLP-1R agonist AC3174 for 4 weeks to salt-sensitive rats improved survival, lowered blood pressure, and attenuated renal damage. More striking improvements in cardiovascular and renal parameters were observed in combination with captopril. See The exenatide analogue AC3174 attenuates hypertension, insulin resistance, and renal dysfunction in Dahl salt-sensitive rats Cardiovasc Diabetol. 2010 Aug 3;9(1):32. [Epub ahead of print].  These data are also consistent with findings reported in in salt-sensitive obese db/db mice and angiotensin II (angII)-infused C57BLK6/J mice, where exendin-4 treatment attenuated the development of hypertension and enhanced sodium excretion Exendin-4 has an anti-hypertensive effect in salt-sensitive mice model Biochem Biophys Res Commun. 2009 Feb 27;380(1):44-9

Crajoinas and colleagues examined mechanisms through which native GLP-1 promotes sodium and fluid excretion. Continuous infusion of native GLP-1 in rats increased urinary cyclic AMP excretion, urine flow, GFR, renal plasma flow, lithium clearance, fractional potassium excretion and sodium excretion. Glp1r mRNA transcripts were detected by RT-PCR in glomerulus and proximal convoluted tubule. The Abcam polyclonal GLP-1R antibodu was used to detect immunoreactive GLP-1R protein predominantly to protein fractions from brush border microvilli. GLP-1 decreased bicarbonate flux in perfused proximal tubules in a PKA-dependent manner in association with increased phosphorylation of NHE3 (sodium hydrogen exchanger 3). Exendin-4, and to a lesser extent, the DPP-4 inhibitor P32/98 also increased urinary cAMP and bicorbonate excretion; exendin-4 potently enhanced urine flow, sodium and potassium excretion, GFR, RPF and lithium clearance in rats. Both exendin-4 and P32/98 increased NHE3 phosphorylation in vivo. Mechanisms mediating the diuretic and natriuretic actions of the incretin-hormone glucagon-like peptide-1 Am J Physiol Renal Physiol. 2011 May 18. Similar studies carried out by the same group demonstrated that sitagliptin therapy (oral gavage twice daily for 8 days) reduced mean arterial BP in young (5 weeks old) SHR rats, together with increased sodium excretion, urine flow, and decreased NHE3 activity and expression in the renal proximal tubule. Sitagliptin had no effect on blood pressure or parameters of renal function in older (14 weeks) SHR rats Dipeptidyl peptidase IV inhibition attenuates blood pressure rising in young spontaneously hypertensive rats J Hypertens. 2011 Mar;29(3):520-8

Atherosclerosis and vascular biology

Nagashima and colleagues studied the effects of native GLP-1 and GIP, and the N-terminal metabolites, GLP-1 1(9-36)amide and GIP(3-42) on the development of atherosclerosis in 17 week old ApoE-/- mice. Peptides were continuously administered by osmotic minipump for 4 weeks. GLP-1 administration reduced the extent of aortic atherosclerosis and plaque size and reduced aortic macrophage infiltration. These salutary actions were blocked by co-infusion of exendin(9-39), and GLP-1 (9-36) did not have the same anti-atherosclerosis effects. Native GIP(1-42) also reduced the extent of atheroslerosis, plaque size and macrophage infiltration, actions blocked by co-administration of the antagonist Pro(3)GIP, whereas GIP(3-42) alone had no effect in comparable experiments. Ex vivo, both GLP-1 and GIP treatments reduced the accumulation of oxidized cholesterol ester accumulation in peritoneal macrophages, with suppression of macrophage CD36 and ACAT-1 levels. Remarkably abundant expression of GLP-1 and GIP receptors were detected in peritoneal macrophages, aortic smooth muscle cells by RNA analyses and Western blotting. Similarly, both receptors were expressed at high levels in THP-1 cells, relative to THP-1 cells differentiated towards the macrophage lineage. Both incretins also suppressed SMC proliferation in vitro and reduced the expression of MCP1, VCAM1, ICAM1, and PAI1 in human aortic endothelial cells. Native incretins prevent the development of atherosclerotic lesions in apolipoprotein E knockout mice Diabetologia. 2011 Jul 24. [Epub ahead of print]

In contrast, Panjwani and colleagues failed to detect any attenuation of atherosclerosis progression in the thoracic or abdominal aorta of diabetic ApoE-/- mice treated with the GLP-1R agonist taspoglutide. Furthermore, no reduction of macrophage accumulation in the aorta was observed, and Glp1r expression was not detected in macrophages, although taspoglutide improved glucose tolerance, reduced hepatic fat and reduced body weight. See GLP-1 Receptor Activation Indirectly Reduces Hepatic Lipid Accumulation But Does Not Attenuate Development of Atherosclerosis in Diabetic Male ApoE−/− Mice Endocrinology 2013 Jan;154(1):127-39

Sustained GLP-1 receptor activation through use of continuously administered exendin-4 appears to reduce neointimal formation in the femoral arteries of C57/Bl6 mice following catheter-induced endothelial injury. Mice were treated with exendin-4 administered for 4 weeks by continuous minipump at a dose of 24 nmol/kg/day. Although exendin-4 did not alter metabolic parameters such as glucose, insulin, or plasma lipids, neointima formation was suppressed by 58% in damaged arteries, with no effect in control normal arteries. GLP-1R expression weas detected by Western blotting in mouse, rat and human vascular smooth muscle cells and exendin-4 suppressed the PDGF-induced proliferation of primary aortic murine SMCs in vitro, without effecting basal levels of SMC proliferation. No effect of exendin-4 on cyclic AMP, MAPK, pCREB, pAkt, p70S6kinase, or cyclin A, E, D1, or p21 or p27 was detected. Hence, exendin-4 modulates SMC growth through mechanisms that remain to be elucidated Exendin-4, a glucagon-like peptide-1 receptor agonist, reduces intimal thickening after vascular injury Biochem Biophys Res Commun. 2011 Jan 4. [Epub ahead of print]

Remarkably, GLP-1 receptor activation via continuous exendin-4 administration for 28 days reduced the development of atherosclerosis in ApoE-/- mice fed a regular chow diet independent of changes in body weight. GLP-1 receptor expression was detected in monocytes and macrophages, and Ex-4-treated mice exhibited reduced accumulation of inflammatory cells (monocytes) in the aorta. Exendin-4 also reduced the extent of LPS-induced inflammation in macrophages in vitro and attenuated expression of the CD11b receptor in human monocytes. See Inhibition of monocyte adhesion to endothelial cells and attenuation of atherosclerotic lesion by a glucagon-like peptide-1 receptor agonist, exendin-4. Diabetes. 2010 Jan 12. [Epub ahead of print]. Administration of a single injection of exenatide to human subjects results in prolonged suppression of postprandial lipids, with significant reductions in TG, apolipoproteins B-48 and CIII, RLP-cholesterol and RLP-triglyceride Exenatide suppresses postprandial elevations in lipids and lipoproteins in individuals with impaired glucose tolerance and recent onset type 2 diabetes mellitus Atherosclerosis. 2010 May 25. [Epub ahead of print]

Burgmaier used viral expression of GLP-1(7-37), GLP-1(9-37) and GLP-1(28-37) to examine whether atherosclerosis might be attenuated over 12 weeks in high fat fed apoE-/- mice. Although lesion size and number was not different, macrophage accumulation and MMP-9 expression were reduced and plaque collagen was increased in all groups expressing GLP-1-derived peptides, suggesting a role for non GLP-1R-dependent mechanisms Glucagon-like peptide-1 (GLP-1) and its split products GLP-1(9-37) and GLP-1(28-37) stabilize atherosclerotic lesions in apoe(-/-) mice Atherosclerosis. 2013 Dec;231(2):427-35

GLP-1 also reduced the advanced glycation end products (AGE)-induced generation of ROS and directly decreased the expression of RAGE, the receptor for AGE, in cultured HUVEC cells. Although these effects were modest in magnitude, they may be mediated by the GLP-1R, detected in HUVEC cells using a GLP-1R antibody from Santa Cruz Glucagon-like peptide-1 (GLP-1) inhibits advanced glycation end product (AGE)-induced up-regulation of VCAM-1 mRNA levels in endothelial cells by suppressing AGE receptor (RAGE) expression Biochem Biophys Res Commun. 2009 Dec 21. [Epub ahead of print]

GLP-1, endothelial function and blood flow

Clinical Data

Native GLP-1 modestly improves peripheral blood flow in short term studies in human subjects with type 2 diabetes Effects of glucagon-like peptide-1 on endothelial function in type 2 diabetes patients with stable coronary artery disease. Am J Physiol Endocrinol Metab. 2004 Dec;287(6):E1209-15.

Ceriello and colleagues studied the actions of acute GLP-1 infusion in normal subjects and in subjects with T2DM before and after 2 months of strict glycemic control with insulin. Under hyperglycemic clamp conditions, nitrotyrosine and 8-iso-PGF2a increased and flow mediated vasodilation decreased, in both normal and diabetic sibjects. FMD was modestly improved during the GLP-1 infusion in non-diabetic and diabetic subjects;GLP-1 infusion also reduced the relative increase in nitrotyrosine and 8-iso-PGF2a (markers of oxidative stress). GLP-1 infusion continued to exert modest improvements in FMD, and nitrotyrosine and 8-iso-PGF2a after 2 months of improved metabolic control and a repeat hyperglycemic clamp. The confounding effects of increased plasma levels of insulin seen during the GLP-1 infusion complicate interpretation of the data. The Possible Protective Role of Glucagon-Like Peptide 1 on Endothelium During the Meal and Evidence for an "Endothelial Resistance" to Glucagon-Like Peptide 1 in Diabetes  Diabetes Care. 2011 Jan 27. [Epub ahead of print]

Tesauro et al examined the effects of native GLP-1 and GLP-1(9-36), 20 pmol/minute, on forearm bloodflow (FBF) responses to acetylcholine and and sodium nitroprusside in human subjects with obesity-related metabolic syndrome (Mets). Control subjects were obese without MetS. Forearm blood flow responses were studied in the presence or absence of concomitant intrarterial insulin and constant levels of glucose. No systemic effects on HR or BP were observed. Forarm insulin levels rose from 16-198 uU/ml during the insulin infusion and GLP-1 levels rose from 8.3-22.3 pM. GLP-1 enhanced the vasodilator responses to insulin in subjects with MetS but not in normal subjects, whereas GLP-1(9-36) had no effect. In the presence of vitamin C (to reduce oxidtive stress) and hyperinsulinemia, GLP-1 had no effect to enhance FBF. In contrast, GLP-1 alone in the absence of hyperinsulinemia had no effect on FBF. GLP-1 also had no effect on forearm glucose uptake. Effects of GLP-1 on Forearm Vasodilator Function and Glucose Disposal During Hyperinsulinemia in the Metabolic Syndrome Diabetes Care. 2013 Mar;36(3):683-9

The effects of degradation-resistant GLP-1R agonists on blood flow in humans vary widely among studies, however "negative" results predominate, especially in trials using a randomized blinded design that control for changes in glucose, body weight etc.

Irace and colleagues assessed brachial blood flow (FMD) in a total of 20 subjects with T2DM after 16 weeks of treatment with either exenatide or glimepiride, added on to metformin. The study was non-randomized, and observational in nature, with A1c at inclusion ranging from 7.5-10%. Patients on exenatide were predominantly newly diagnosed diabetics, whereas SU-treated patients had T2DM for a mean of 9.3 years. The SU patients were ~5 kg heavier, BMI increased during the study and they had less improvement of A1c levels over 6 weeks. Baseline FMDs were comparable across groups and end of study FMS increased in both groups after 16 weeks, with greater improvement in subjects treated with exenatide. Exenatide improves endothelial function assessed by flow mediated dilation technique in subjects with type 2 diabetes: results from an observational research Diab Vasc Dis Res. 2013 Jan;10(1):72-7

Kelly et al assessed endothelial function in a randomized open label trial of 50 non-diabetic individuals with abdominal obesity and either IGT, IFG, or increased A1c (prediabetes). Subjects were treated with metformin or twice daily exenatide for 3 months. No significant change in digital reactive hyperemia (RHI, a nitric oxide dependent parameter), CRP, or other biomarkers, was detected at the end of 3 months in either treatment group. Effects of exenatide vs. metformin on endothelial function in obese patients with pre-diabetes: a randomized trial Cardiovasc Diabetol. 2012 Jun 8;11:64. doi: 10.1186/1475-2840-11-64

Hopkins et al carried out a 6 month study using either exenatide twice daily (9) or liraglutide once daily (2) in 11 subjects with obesity and T2DM, on stable OADs for 3 months prior to enrollment, mean duration of diabetes 8 years. Subjects lost weight (mean 5 kg) and improved A1c (by 1.4%), and reduced total abdominal fat but not visceral fat (MRI). However, there were no significant differences in FMD, GTN (brachial artery endothelium-independent vasodilation following glyceryl trinitrate (GTN)) or in carotid IMT. Effects of 6 months glucagon-like peptide-1 (GLP-1) receptor agonists treatment on endothelial function in patients type 2 diabetes mellitus Diabetes Obes Metab. 2013 Mar 1. doi: 10.1111/dom.12089

                         

Cardiac function and heart failure

Acute short term studies:In February 2004, a pilot study reported the effect of acute GLP-1 administration in 10 human subjects with LV dysfunction and acute MI following angioplasty. Native GLP-1 was administered as a  72-hour infusion at a rate of  1.5 pmol/kg per minute. Echocardiograms were obtained after reperfusion and after the completion of the GLP-1 infusion. GLP-1 significantly improved LVEF (from 29 + 2% to 39 + 2%, P<0.01), global wall motion score indexes (1.94 + 0.11 to 1.63+0.09, P<0.01), and regional wall motion score indexes (2.53 + 0.08 to 2.02 + 0.11, P<0.01) compared with control subjects. The benefits of GLP-1 were independent of AMI location or history of diabetes. GLP-1 administration was accompanied by a significant decrease in plasma glucose and free fatty acids and was associated with reduced mortality and duration of hospital stay. See Effects of Glucagon-Like Peptide-1 in Patients With Acute Myocardial Infarction and Left Ventricular Dysfunction After Successful Reperfusion. Circulation. 2004 Mar 2;109(8):962-5. 

A pilot study of continuous GLP-1 infusion examined the effect of GLP-1 on cardiac function in human subjects with heart failure. A 5 week infusion of GLP-1 significantly improved LV function and short term exercise capacity in both diabetic and non-diabetic subjects with Class II/IV heart failure. See Glucagon-like peptide-1 infusion improves left ventricular ejection fraction and functional status in patients with chronic heart failure. J Card Fail. 2006 Dec;12(9):694-9. However, a larger study of GLP-1 in heart failure carried out by Amylin Pharmaceuticals did not show a positive benefit.

The preliminary intriguing positive results of the above human CHF studies have not yet been widely replicated. Halbirk et al studied the metabolic and cardiovascular consequences of a 48 hr GLP-1 infusion in non-diabetic subjects with stable CHF (secondary to CAD), ejection fraction less than 40%, NYHA Class II-III. The same patients were infused with GLP-1 or placebo in a random order 2 weeks apart. GLP-1 reduced blood glucose, increased plasma insulin, was associated with modest increases in heart rate and blood pressure but had no significant impact on an extensive panel of hemodynamic parameters studied at rest and with exercise. As a result of hypoglycemia, the infusion rate was reduced to 0.7 pmol/kg/min after the first 3 patients, however hypoglycemia was reported by 8 patients. Hence these findings do not support a beneficial effect of acute GLP-1 infusion in patients with CHF Cardiovascular and Metabolic Effects of 48-Hour Glucagon-like Peptide 1 Infusion in Compensated Chronic Heart Failure Patients. Am J Physiol Heart Circ Physiol. 2010 Jan 15. [Epub ahead of print]

The ability of GLP-1 to control blood glucose while minimizing the risk of hypoglycemia, taken together with the putative inotropic and cardioprotective actions of GLP-1, has fostered efforts directed at understanding the potential role of GLP-1 therapy in subjects with established heart disease. A randomized study of conventional therapy vs continuous GLP-1 infusion in 20 human subjects undergoing CABG demonstrated that perioperative GLP-1 infusion beginning 12 hours before CABG and continuing for 48 hours resulted in improved glycemic control, less use of inotropic and vasoactive infusions and fewer arrhythmias in GLP-1-treated patients. See Effect of Glucagon-Like Peptide-1 (GLP-1) on Glycemic Control and Left Ventricular Function in Patients Undergoing Coronary Artery Bypass Grafting. Am J Cardiol. 2007 Sep 1; 100(5):824-9. Epub 2007 Jun 14

Consistent with clinical findings that GLP-1 therapy may reduce blood pressure, a 3-h infusion of GLP-1 after an iv 9.9-g salt load produced a dose-dependent increase in urinary sodium excretion in healthy subjects whereas in obese men, the GLP-1 infusion significantly increased urinary sodium excretion, decreased  urinary H(+) secretion and reduced the glomerular filtration rate. See Glucagon-like Peptide 1 induces natriuresis in healthy subjects and in insulin-resistant obese men. J Clin Endocrinol Metab. 2004 Jun;89(6):3055-61

The effects of a single dose of exenatide or placebo were examined in 28 subjects with IGT or T2DM in conjunction with a high fat meal. Endothelial function, assessed by tonometry, was higher after exenatide; whether this change reflects concomitant changes in glucose, triglycerides, other metabolic parameters, or a direct action of exenatide, cannot be determined.  Improvement of Postprandial Endothelial Function after a Single Dose of Exenatide in Individuals with Impaired Glucose Tolerance and Recent Onset Type 2 Diabetes Mellitus.  Diabetes Care. 2010 Mar 3. [Epub ahead of print]

Lepore and colleagues examined the actions of a long-acting high molecular weight GLP-1R agonist, albiglutide, over 12 weeks in human, non-diabetic overweight and obese subjects with NYHA class II–III HF and EF < 40%. Albiglutide therapy 30 mg once weekly (n = 27) did not produce improvements in LVEF, 6 min walk time, cardiac efficiency; LV structure and function, quality of life scores, or myocardial glucose or oxygen utilization were similarly not improved Effects of the Novel Long-Acting GLP-1 Agonist, Albiglutide, on Cardiac Function, Cardiac Metabolism, and Exercise Capacity in Patients With Chronic Heart Failure and Reduced Ejection Fraction JACC Heart Fail. 2016 Jul;4(7):559-66

The actions of liraglutide in human subjects with HF were examined in a randomized, double-blinded, placebo-controlled clinical trial, the (FIGHT) study: Functional Impact of GLP-1 for Heart Failure Treatment. Effects of Liraglutide on Clinical Stability Among Patients With Advanced Heart Failure and Reduced Ejection Fraction: A Randomized Clinical Trial JAMA Aug 2;316(5):500-8. doi: 10.1001/jama.2016.10260 Study subjects (60% with diabetes, 80% male, 8 year history of HF) with LVEF26%, NYHA class II–III HF (29%/65%); 85% of subjects had a recent history of acute hospitalization for heart failure syndrome (AHFS) within the preceding 12 months. Study subjects were treated for 180 days with placebo (n = 155) or liraglutide (1.8 mg daily, n = 146). Liraglutide did not improve time to death, time to HF hospitalization, or time-averaged changes in levels of (BNP). Comparable numbers of patients (12%, liraglutide; 11%, placebo) died, and 34% of patients in the liraglutide treatment group versus 28% of patients in the placebo group were re-hospitalized for HF.


Jorsa et al studied the effects of liraglutide (1.8 mg daily) vs. placebo in human subjects (90% male) with and without diabetes and pre-existing HF, EF < 45%, NYHA class I–III, and eGFR > 30 mL/min/1.73 m2 over 24 weeks. Subjects were on stable medical regimens for treatment of heart disease. Most subjects (65%) had ischemic heart disease and NYHA class I (38%) or class II (46%) HF, and 35% had T2D. Baseline LVEF was 33.7 versus 35.4 in liraglutide-vs placebo-treated subjects. No significant treatment-related differences in LVEF were noted after 24 weeks of treatment. HR was significantly increased (6.1 BPM)
and serious cardiac events, including arrhythmias and acute coronary syndrome, were more common in subjects treated with liraglutide.


In contrast to the studies of GLP-1R agonists in patients with a history of heart failure (HF) and reduced ejection fraction, a large database analysis of subjects with diabetes and a pre-existing history of HF, treated with multiple different antidiabetic agents did not reveal any relationship between therapy with GLP-1R agonists and the risk of hospitalization for HF A Multicenter Observational Study of Incretin-based Drugs and Heart FailureN Engl J Med. 2016 Mar 24;374(12):1145-54 xx

 

GLP-1 and the cardiovascular system

Preclinical studies

Analysis of direct GLP-1 actions on cardiac muscle cells was studied using cultures of rat cardiac myocytes. Although GLP-1increased intracellular cAMP in cardiac myocytes, in contrast to the positive inotropic actions of isoproterenol,GLP-1induced a decrease in contraction amplitude with no change in intracellular calcium transit. Furthermore, both isoproterenol andGLP-1produced an intracellular acidosis. Hence, these findings demonstrate that coupling of cardiomyocyte GLP-1R signaling to cAMP generation produces distinct downstream signaling events when compared to adrenergic agonists. See Glucagon-Like Peptide-1 Increases cAMP but Fails to Augment Contraction in Adult Rat Cardiac Myocytes. Circ Res. 2001 Aug 31;89(5):445-452. Similar direct prosurvival actions of GLP-1 were demonstrated using HL-1 murine atrial cardiomyocyte cells incubated with staurosporine. GLP-1 reduced apoptosis independent of the ambient glucose concentration in either a wortmannin-, U0126- and Rp-cAMP sensitive manner, implicating the PI3K and ERK1/2 pathways, depending on the specific molecular marker of apoptosis examined. GLP-1 also reduced palmitate- and ceramide-induced apoptosis. Whether these actions are mediated by the classical GLP-1 receptor was not addressed in these studies ANTIAPOPTOTIC EFFECTS OF GLP-1 IN MURINE HL-1 CARDIOMYOCYTES Am J Physiol Heart Circ Physiol. 2011 Apr;300(4):H1361-72

Wohlfart and colleagues reported novel findings challenging conventional concepts of how GLP-1R agonists produce cardioprotection. The degradation-resistant GLP-1R agonist lixisenatide significantly reduced infarct size (by 36%) in LAD-ligated rat hearts (45 min) ex vivo which were re-perfused for 120 minutes; lixisenatide was perfused from 10 minutes after LAD occlusion to the end of the 120 minute reperfusion period. No significant changes in coronary flow or rate pressure product were observed with lixisenatide. A second study involved transient LAD ligation, followed by reperfusion and 10 weeks of lixisenatide, ramipril, or plaebo, in Wistar rats. Body weight was significantly (~10%) lower, however LVEDP was higher and plasma BNP and lung weights were lower in rats treated with ramipril or lixisenatide. Lixisenatide directly reduced markers of apoptosis in isolated rat ventricular cardiomyocytes, and lixisenatide increased fractional shortning in ventricular CMs from WT rats and mice and Glp1r-/- mice. Glp1r mRNA transcripts were not detected by PCR in RNA from isolated cardiomyocytes nor in RNA isolated from the rat heart. Cardioprotective effects of lixisenatide in rat myocardial ischemia-reperfusion injury studies J Transl Med. 2013 Mar 28;11(1):84.

Younce and colleagues examined the direct cytoprotective effects of exendin-4 in primary cultures of neonatal rat ventricular cardiomyocytes. Exendin-4 reduced high glucose(HG)-induced cell death through mechanisms sensitive to Ex(9-39) and H-89, and reduced levels of cleaved caspase 3. HG induced ROS, asessed by measuring DHR123, and ROS generation was not reduced by exendin-4. However, hydrogen peroxide and HG-induction of ER stress markers CHOP, PDI and Grp 78 was reduced by Ex-4. Intriguingly, Ex-4 attenuated ER stress and cell death after thapsigargin, but not after tunicamycin. Exendin-4 prevented the decrease in SERCA2a mRNA and protein levels observed in NRVM cultured in high glucose and activated phosphorylation of PLN, a protein that may enhance SERCA2a function Exendin-4 attenuates high glucose-induced cardiomyocyte apoptosis via inhibition of endoplasmic reticulum stress and activation of SERCA2a Am J Physiol Cell Physiol. 2013 Jan 9.

The effects of GLP-1 in pigs with ventricular fibrillation was examined by Dokken and colleagues. A 4 hrs infusion of GLP-1 or placebo was commenced immediately after return of circulation in resuscitated animals. GLP-1 treatment increased cororary flow reserve and transiently cardiac output, but had no effect on survival. Glucagon-like peptide-1 (GLP-1) attenuates post-resuscitation myocardial microcirculatory dysfunction  Resuscitation. 2010 Mar 25. [Epub ahead of print] Dokken also examined various parameters of neutrophil activation in a rat ischemia-reperfusion model. Pretreatment with GLP-1 reduced infarct size. Induction of CD11b expression induced by fMLP was attenuated in rats receiving GLP-1 therapy, and PMN accumulation was reduced in post-ischemic injury. GLP-1 (1 mM) also directly decreased CD11b expression in whole blood in vitro.  Glucagon-like Peptide-1 (GLP-1), Immediately Prior to Reperfusion, Decreases Neutrophil Activation and Reduces Myocardial Infarct Size in Rodents Horm Metab Res. 2011 Feb 28. [Epub ahead of print].

Subsequent studies examined the effects of acute GLP-1 administration for 4 hr in pigs following ventricular fibrillation, cardiac arrest and resuscitation. GLP-1 increased coronary flow reserve and reduced levels of 8-iso-PGF2a, a marker of reactive oxygen species, but had no significant effect on glucose, insulin, plasma SOD activity, myocardial or aortic COX-2 expression,or parameters of LV function Glucagon-like peptide-1 preserves coronary microvascular endothelial function after cardiac arrest and resuscitation: Potential anti-oxidant effects Am J Physiol Heart Circ Physiol. 2012 Dec 15

Analysis of the actions of the GLP-1R agonist liraglutide in the setting of acute coronary artery ligation demonstrated a highly significant reduction in infarct size and a marked improvement in survival in either normoglycemic or diabetic mice treated with liraglutide. Liraglutide also induced a cardioprotective gene and protein expression profile, and these cardioprotective changes were dependent on the presence of a functional GLP-1 receptor. Liraglutide directly increased cAMP and improved cardiomyocyte survival ex vivo. See The GLP-1R Agonist Liraglutide Activates Cytoprotective Pathways and Improves Outcomes Following Experimental Myocardial Infarction in Mice Diabetes published online on January 16, 2009 as 10.2337/db08-1193

Similarly, treatment of pigs subjected to experimental LAD occlusion with exendin-4 , administered 5 minutes before the onset of re-perfusion, and for 48 hrs after onset of re-perfusion (10 ug twice daily s.c) resulted in significantly reduced infarct size, increased levels of circulating insulin, improved cardiac function as measured by echocardiography, in association with increased expression of cardioprotective proteins and reduced levels of activated caspase-3. Whether exendin-4 was associated with improved survival following MI was not assessed in the current study Exenatide reduces infarct size and improves cardiac function in a porcine model of ischemia and reperfusion injury J Am Coll Cardiol. 2009 Feb 10;53(6):501-10

In contrast, the effects of liraglutide on infarct size were assessed in pigs subjected to experimental ischemia following 3 days of daily liraglutide 10 ug/kg prior to 40 minutes of ischemia and then 2.5 hrs of re-perfusion. Liraglutide treatment had no significant effect on infarct size, however heart rate was significantly increased in liraglutide-treated pigs even prior to induction of ischemia Lack of cardioprotection from subcutaneously and preischemic administered liraglutide in a closed chest porcine ischemia reperfusion model BMC Cardiovasc Disord. 2009 Jul 23;9:31.

A weight neutral dose, 30 ug/k twice daily of liraglutide given to high fat fed mice for 1 week (on HFD for 16 or 32 weeks) attenuated histological and molecular markers of fibrosis, inflammation, reduced ER stress, and improved cardiac function in an AMPK-dependent manner. Ex vivo, liraglutide attenuated palmitate-induced lipotoxicity in mouse cardiomyocytes and decreased monocyte adhesion to endothelial cells in the presence of TNF-a. See A Glucagon-Like Peptide-1 Analogue Reverses the Molecular Pathology and Cardiac Dysfunction of a Mouse Model of Obesity Circulation. 2012 Nov 27.

Bao and colleagues treated rats with albiglutide once daily for 3 days, with the last dose administered 2 hrs prior to removal of the hearts, which were then subjected to ischemia-reperfusion ex vivo (30 minutes ischemia, 24 hrs of reperfusion). Albiglutide-treated rats exhibited reduced food intake, weight loss, decreaed glucose and increased insulin levels. Infarct size was reduced and contractile function increased in albiglutide-treated rats. Albiglutide had no effect on cardiac cAMP in normal rats, but restored cAMP levels towards normal in ischemic hearts. Albiglutide also increased glucose uptake and decreased lactate output. Metabolic substrate analysis revealed increased glycolytic flux and increased CHO fuel utilization following albiglutide in the non-ischemic heart. Albiglutide increased the expression of ACSL1, CPT1B, GAPDH, IGF1R, SLC2A4, HK2, GYS1, ESSRA, HIF1A, PPARGC1A, PDHA1, ALDOC, GSK3B, AKT1, in the normal heart and ALDOC, CPT1B, IGF1R, PDHA1, GSK3B, GYS1, SLC2A1 in non-ischemic regions of the I/R hearts. Albiglutide, a Long Lasting Glucagon-Like Peptide-1 Analog, Protects the Rat Heart against Ischemia/Reperfusion Injury: Evidence for Improving Cardiac Metabolic Efficiency PLoS One. 2011;6(8):e23570. Epub 2011 Aug 26

GLP-1 also reduces apoptosis in human cadiac progenitor cells. Laviola and colleagues isolated progenior cells that expressed stem cell markers from cultured fibroblast-like cells that were identified 1 week after culturing human heart biopsy samples. More than 80% of cells were CD105+ and expressed MEF2C and Nkx2.5. Low level GLP-1R expression was detected by RNA and Western blot analysis (Santa Cruz) in human CPC cells. GLP-1 rapidly induced CREB phosphorylation and cAMP accumulation, both abolished by exendin(9-39). Hydrogen peroxide-induced apoptosis wa attenuated by treatment with GLP-1 or exendin-4, as was the H2O2-induced phosphorylation of MKK7, JNK1/3, c-jun, and MKK4. Glucagon-Like Peptide-1 Counteracts Oxidative Stress-Dependent Apoptosis of Human Cardiac Progenitor Cells by Inhibiting the Activation of the c-Jun N-terminal Protein Kinase Signaling Pathway Endocrinology. 2012 Oct 17.

Ye and colleagues reported that a low dose of exenatide 1 ug/kg, administered as a single injection 1 hr prior to 30 minutes of LAD ligation and 24h of reperfusion reduced infarct size, increased cardiac levels of cAMP, PKA activity,  and 15-Epilipoxin A4 in male db/db mice, and these cardioprotective actions of exenatide were significantly enhanced by co-administration of the phosphodiesterase -3 inhibitor cilostazol. The cardioprotective effects of Ex-4/cilostaxol were attenuated by H89. Similar cardioprotective effects of the combined therapies, sensitive to cAMP/PKA blockade, were observed in rat cardiomyocytes in vitro. Phosphodiesterase-3 Inhibition Augments the Myocardial Infarct Size Limiting Effects of Exenatide in Mice with Type-2 Diabetes Am J Physiol Heart Circ Physiol. 2012 Oct 26

Liu and colleagues generated a mouse with cardiomyocyte-specific inactivation of Dgat1, leading to increased cradiac accumulation of DAGs and ceramides and impaired ventricluar function. Twice daily admiistration of exendin-4 reduced cardiac levels of DAGs and ceramide, decreased cardiac FA and increased glucose uptake, improved fractional shortening, without significant changes in genes regulating lipid oxidation. Cardiomyocyte specific loss of diacylglycerol acyl transferase 1 (DGAT1) reproduces the abnormalities in lipids found in severe heart failure J Biol Chem. 2014 Aug 25. pii: jbc.M114.601864

Heart failure

Infusion of wildtype recombinant GLP-1 into dogs with pacing-induced heart failure produces an improvement in myocardial function, LV stroke volume, cardiac output, increased cardiac insulin sensitivity and decreased  LV end-diastolic pressure, heart rate, and systemic vascular resistance. See Recombinant Glucagon-Like Peptide-1 Increases Myocardial Glucose Uptake and Improves Left Ventricular Performance in Conscious Dogs With Pacing-Induced Dilated Cardiomyopathy. Circulation. 2004 Aug 16. The effects of GLP-1 on myocardial metabolism were further examined using the same model of DCM in dogs. After 28 days of pacing, functional DCM was characterized by reduced LV pressure, CO, LVEF, and increased LVEDP and heart rate. A 48 hr infusion of GLP-1 reduced LVEDP, increased LV dP/dt, decreased heart rate, and increased LVEF and CO. Levels of plasma insulin and glucagon were reduced, and arterial NO was increased whereas plasma norepinephrine was reduced after GLP-1 treatment; despite lower insulin levels, GLP-1 infusion increased myocardial glucose uptake and improved insulin-stimulated glucose uptake. GLP-1 had no effect on basal or stimulated myocardial adenylate cyclase activity, Akt phosphorylation, but increased GLUT-1 expression in myocardial membranes. GLP-1 also restored levels of p38 MAPkinase, increased NOS2 expression and myocardial NO uptake. A separate study administered GLP-1 for 6 hrs with or without signal transduction inhibitors. Inhibition of MAP kinase or NO activity attenuate the effects of GLP-1 on stimulation of glucose uptake in dogs with and without DCM. See GLP-1 Increases Myocardial Glucose Uptake via p38{alpha} MAP Kinase Mediated, Nitric Oxide Dependent Mechanisms in Conscious Dogs with Dilated Cardiomyopathy Circ Heart Fail. 2010 May 13. [Epub ahead of print]

Using a transgenic mouse model of CHF (TG9) that expresses CRE in cardiomyocytes under the control of the cardiac myosin promoter, Kalla Vyas et al examined the effects of twice daily exendin-4 administration (40 mg/kg/day) to mice starting at 56 days of age; at 75 days of age, a subset of mice also received ritonavir. Ex-4 improved glucose tolerance assessed at 2 weeks; Ex-4 did not increase insulin levels during an IPGTT, and body weights were not reported. Ex-4 increased expression of cardiac levels of GLUT4, pAKT, and pAMPK and reduced BNP RNA expression and improved survival; After 3 weeks of treatment, Ex-4 also increased 2-deoxyglucose uptake in the left ventricle, with no effect noted on skeletal muscle 2-DG uptake. Ex-4 also increase myocardial glucose uptake in mice treated with a single dose of ritonavir. Echocardiography performed after 14 days of Ex-4 administration showed no change in the majority of parameters with the exception of fractional shortening that was improved in Ex-4-treated mice. Exenatide improves glucose homeostasis and prolongs survival in a murine model of dilated cardiomyopathy PLoS One. 2011 Feb 17;6(2):e17178

Classical GLP-1 receptor signaling also modulates the function and survival of cardiomyocytes. See Glucagon-like Peptide 1 Can Directly Protect the Heart Against Ischemia/Reperfusion Injury. Diabetes. 2005 Jan;54(1):146-51 and The Direct Effects of Glucagon-Like Peptide-1 (GLP-1) on Myocardial Contractility and Glucose Uptake in Normal and Post-Ischemic Isolated Rat Hearts. J Pharmacol Exp Ther. 2006 Feb 17; [Epub ahead of print] and Myocardial Ischaemia-reperfusion Injury is Attenuated by Intact Glucagon Like Peptide-1 (GLP-1) in the In Vitro Rat Heart and may Involve the p70s6K Pathway. Cardiovasc Drugs Ther. 2007 May 31; This latter paper suggests that GLP-1 exerts its actions in a p70 S6 kinase-dependent manner and is only cardioprotective if intact and not degraded by DPP-4, suggesting that GLP-1(9-36) amide is not responsible for cardioprotection in this isolated rat heart model of experimental ischemia. A single dose of a GLP-1:transferrin conjugate also reduced infarct size in an IR study in rabbits, whether administered immediately prior to or following ischemia as outlined in Single Dose GLP-1-Tf Ameliorates Myocardial Ischemia/Reperfusion Injury J Surg Res. 2009 Apr 16. [Epub ahead of print]

Wallner and colleagues examined the direct actions of GLP-1R agonists using atirla or ventricular muscle strips from human heart trabeculae. Right atrial appendages were isolated from patients undergoing heart surgery, left ventricular muscle was obtained from human donor hearts not suitable for heart transplantation. Exenatide and native GLP-1 but not GLP-1(9-36)amide increased atrial contractile force, effects blocked by exendin(9-39). In contrast, GLP-1R agonists had no effect on contactility of ventricular muscle strips. The inotropic effects of exenatide were attenuated by thapsigargin and H89. GLP-1R agonists also increased phosphorylation of phospholamban in atrial muscle. GLP-1R expression, assessed using PCR to detect a 100 bp PCR product, was 3-4-fold greater in atria vs. ventricle. Exenatide also modestly increased GLUT-1 and Epac2 translocation to the membrane in atrial cardiomyocytes. Exenatide exerts a PKA-dependent positive inotropic effect in human atrial myocardium: GLP-1R mediated effects in human myocardium J Mol Cell Cardiol. 2015 Sep 30. pii: S0022-2828(15)30071-7.

a-glucosidase inhibitors, cardioprotection and GLP-1

A number of oral anti-diabetic agents have been associated with increased levels of GLP-1 and GIP and hence may exert their effects in part through incretin pathways. Molecules that prevent breakdown of carbohydrates and fats (glucosidase or lipase inhibitors) are among such agents associated with raised plasma levels of GLP-1. Iwasa and colleagues treated rabbits with oral voglibose, then subjected the animals to ischemia (30 minutes)-reperfusion (48 hrs) injury. Voglibose treatment reduced infarct size, and these effects were attenuated by co-treatment with the antagonist exendin(9-39), or the signal transduction inhibitors wortmannin, L-NAME (Nomega-nitro-L-arginine methylester) or 5-HD (5-hydroxydecanoate), which inhibit GLP-1 receptors, PI3K, NOS and KATP channels, respectively. Hence these findings suggest that at least part of the cardioprotection ascribed to a-glucoseidase inhbitors may reflect increased plasma levels of GLP-1. See Anti-Diabetic Drug Voglibose is Protective Against Ischemia-Reperfusion Injury via GLP-1 Receptors and PI3 Kinase-Akt-eNOS Pathway in Rabbits  J Cardiovasc Pharmacol. 2010 Mar 24. [Epub ahead of print] In a related study using miglitol, the same group demonstrated that oral administration of miglitol significantly increased plasma levels of GLP-1 and reduced infarct size (percent area at risk) in rabbits through mechanisms that were partially dependent on GLP-1 as the effects of oral but not iv miglitol were partially attenuated by co-administration of exendin(9-39). Moreover the effects of miglitol to upregulate pI3K and pAKT were attenuated by exendin(9-39). Consistent with the importance of GLP-1 action in this model, the glucoregulatory effects of miglitol were greater when administered orally. Both stimulation of GLP-1 receptors and inhibition of glycogenolysis additively contribute to a protective effect of oral miglitol against ischaemia-reperfusion injury in rabbits Both stimulation of GLP-1 receptors and inhibition of glycogenolysis additively contribute to a protective effect of oral miglitol against ischaemia-reperfusion injury in rabbits Br J Pharmacol. 2011 Mar 22. doi: 10.1111/j.1476-5381.2011.01357.x.

GLP-1, endothelial function, coagulation and blood flow

 

Preclinical data

There is very little data examining the effects of GLP-1 on coagulation. Zhao et queries the FAERS database and hypothesize that exenatide may reduce the number of reported cardiovascular events with rosiglitazone therapy, possibly due to effects on coagulation. Treatment of db/db mice with both drugs demonstrated that exenatide reduced plasma PAI-1 levels compared to that seen with rosiglitazone alone, and increased clot formation time and decreased maximum clot firmness Systems pharmacology of adverse event mitigation by drug combinations Sci Transl Med. 2013 Oct 9;5(206):206ra140

GLP-1 may exert direct effects on the peripheral vasculature and vascular tone, as outlined in Glucagon-like peptide-1 relaxes rat conduit arteries via an endothelium-independent mechanism. Regul Pept. 2005 Feb 15;125(1-3):173-7 and Effect of Glucagon-Like Peptide-1(7-36) and Exendin-4 on the Vascular Reactivity in Streptozotocin /Nicotinamide-Induced Diabetic Rats. Pharmacology. 2005 Mar 3;74(3):119-126.   GLP-1 had no effect on basal vessel permeability but reduced LPS-induced permeability in rat mesenteric blood vessels ex vivo. These actions were partially attenuated by co-treatment with the GLP-1R antagonist exendin(9-39) and blocked using inhibitors of cAMP/PKA. See Glucagon-like Peptide-1 Protects Mesenteric Endothelium from Injury During Inflammation Peptides. 2009 Jun 25. [Epub ahead of print]. GLP-1R activation also produces favorable effects on cell survival in an islet microvascular endothelial cell (MS-1) line. GLP-1 reduced PARP-1 and iNOS expression, reduced NO production, and partially attenuated the cytotoxic effects of oxidized LDL in MS-1 cells Glucagon-like peptide 1 protects microvascular endothelial cells by inactivating the PARP-1/iNOS/NO pathway. Mol Cell Endocrinol. 2011 Mar 30. [Epub ahead of print]

Central nervous system GLP-1R-dependent signals also appear to modulate peripheral blood flow. Administration of exendin-4 into the lateral ventricles of mice reduced femoral arterial blood flow in a glucose-dependent manner via mechanisms that involve reactive oxygen species (ROS). See Brain GLP-1 regulates arterial blood flow, heart rate and insulin sensitivity. Diabetes. 2008 Jul 15. [Epub ahead of print]. GLP-1 can also regulate blood flow under experimental conditions designed to reduce normal blood flow, as in the case of ethanol injection, which reduces gastric mucosal blood flow. GLP-1 administration prevented the decrease in gastric blood flow, actions which were diminished following co-administration of a)Nitric oxide (NO) synthase inhibitor L-NAME, (30 mg/kg; s.c.), b) calcitonine gene-related peptide (CGRP) receptor antagonist CGRP-(8-37) (10mug/kg; i.p.), and c) cyclooxygenase inhibitor indomethacin (5 mg/kg; i.p.) Effect of peripherally-injected glucagon-like peptide-1 on gastric mucosal blood flow. Regul Pept. 2009 May 7. [Epub ahead of print]. In contrast, Dong and colleagues infused GLP-1 into rats and demonstrated a rapid increase in hindlimb muscle blood volume and blood flow, in a PKA-dependent manner in rats, associated with an increase in plasma NO levels. The target site (endothelial cells, smooth muscle, brain etc) for the actions of GLP-1 on muscle blood flow were not identified. Protein Kinase A Mediates Glucagon-Like Peptide 1-Induced Nitric Oxide Production and Muscle Microvascular Recruitment Am J Physiol Endocrinol Metab. 2012 Nov 27.

Cabou and colleagues examined the CNS mechanisms through which GLP-1 regulates blood flow in mice. Exendin-4 promoted translocation of PKC-d, but not the other PKC isoforms, to the plasma membrane in an exendin(9-39)-dependent manner and these actions were absent in Glp1r-/- mice. Central GLP-1R activation reduced femoral blood flow and peripheral insulin sensitivity, findings reversed by central infusion of calphostin C, a non-specific PKC inhibitor. Conversely, Glp1r-/- mice exhibited increased femoral artery blood flow and insulin sensitivity. The specific PKC-d inhibitor rottlerin reversed the effects of central exendin-4 on blood flow and insulin sensitivity. Experimental diabetes was associated with reduced blood flow and insulin sensitivity, findings that were reversed by central infusion of the antagonist exendin(9-39). Hence central GLP-1R signaling regulates blood flow and peripheral insulin sensitivity in a PKC-d-dependent manner. Brain GLP-1 Signaling Regulates Femoral Artery Blood Flow and Insulin Sensitivity Through Hypothalamic PKC-δ Diabetes published ahead of print August 1, 2011, doi:10.2337/db11-0464

The effects of GLP-1 and exendin-4 on islet blood flow were examined in rats by Wu and colleagues. The stimulatory effect of islet blood flow induced by glucose was reduced by acute GLP-1 or exendin-4 administration. GLP-1 but not exendin-4 potentiated the stimulatory effects of glucose on kidney blood flow, which was actually reduced by exendin-4. The effects of GLP-1 on islet blood flow were not modified by the NO synthetase inhibitor L-NAME. GLP-1, Exendin-4 and C-peptide regulate pancreatic islet microcirculation, insulin secretion and glucose tolerance in rats Clin Sci (Lond). 2011 Nov 7. [Epub ahead of print]

Some of the effects of GLP-1 on blood vessels may be mediated through GLP-1(9-36)-dependent mechanisms, and not through the known GLP-1 receptor. For example, intralipid-induced vascular dysfunction in rat femoral arterial rings is partially reversed by infusion of GLP-1 or GLP-1(9-36), whereas the potent GLP-1 agonist exendin-4 has no effect in the same experiments. See Endothelial dysfunction induced by triglycerides is not restored by exenatide in rat conduit arteries ex vivo Regul Pept. 2009 Jul 9. [Epub ahead of print]. In contrast, experiments using HUVEC cells demonstrated comparable effects of both exendin-4, GLP-1, and to a lesser extent, GLP-1(9-36)amide, on cell proliferation, and Akt and eNOS phosphorylation in vitro. The effects of exendin-4 on cell proliferation was abolished by multiple signal transduction inhibitors acting on different downstream pathways and several of the actions of exendin-4 and GLP-1 on HUVEC cells were attenuated by exendin(9-39). Exendin-4 stimulates proliferation of human coronary artery endothelial cells through eNOS-, PKA- and PI3K/Akt-dependent pathways and requires GLP-1 receptor. Mol Cell Endocrinol. 2010 May 7. [Epub ahead of print]

Erdogu and colleagues examined the direct cytoprotective actions of exendin-4 and native GLP-1 in human coronary artery endothelial cells. Both exendin-4 and GLP-1 reduced palmitate-induced lipoapoptosis, effects which appeared to be mediated by multiple signal transduction pathways. Inhibitors of PKA, PI3K/Akt, eNOS, p38 MAPK and JNK all reduced the actions of exendin-4. Exendin-4 protects endothelial cells from lipoapoptosis by PKA PI3K eNOS p38MAPK and JNK pathways J Mol Endocrinol. 2013 Jan 23

Wang and colleagues compared the effects of 12 week administration of vildagliptin, exendin-4 once daily, or insulin, on cardiac function and cardiac microvascular endothelial cell signal transduction in Sprague Dawley rats with STZ-induced diabetes. All rats remained hyperglycemic after the treatment, with glucose levels of 17-18 mM and none of the treatment groups exhibited weight gain relative to the untreated controls. No differences in BP or insulin resistance, assessed by clamps, were detected across groups. Both vildagliptin and exendin-4 increased diastolic LV function by echocardiography and markedly improved cardiac glucose uptake by FDG-PET scanning. Both agents also preserved microvessel integrity (scanning EM) and decreased permeability, assessed by diffusion of lanthanum nitrate. IHC (diffuse cytoplasmic staining) and Western blotting were used to detect expression of an immunoreactive GLP-1R protein in cardiac microvascular endothelial cells (CMECs) using antisera from Tocris Bioscience and Santa Cruz, for IHC and Westerns, respectively). GLP-1 suppressed high glucose-induced apoptosis, ROS and NADPH activity, and reduced Rho and Rock expression through a caMP/PKA-dependent pathway in CMECs. Glucagon-Like Peptide-1 Protects Against Cardiac Microvascular Injury in Diabetes Via a cAMP/PKA/Rho-Dependent Mechanism. Diabetes. 2013 Jan 30.

GLP-1, heart rate and the cardiac conduction system

GLP-1 administered intravenously or by ICV injection acutely increases heart rate and blood pressure in rats. See Changes in arterial blood pressure and heart rate induced by glucagon-like peptide-1-(7-36) amide in rats. Am J Physiol. 1994 Mar;266(3 Pt 1):E459-66 and Sustained expression of exendin-4 does not perturb glucose homeostasis, beta-cell mass, or food intake in metallothionein-preproexendin transgenic mice. J Biol Chem. 2000 Nov 3;275(44):34471-7. These effects can be blocked by intravenous or ICV administration of the antagonist exendin(9-39) and bilateral vagotomy blocked the cardiovascular effects of ICV, but not peripherally administered GLP-1 Neural contribution to the effect of glucagon-like peptide-1-(7-36) amide on arterial blood pressure in rats. Am J Physiol. 1999 Nov;277(5 Pt 1):E784-91.

In contrast, in other similar experiments, the GLP-1R antagonist exendin(9-39) was able to block the effects of both GLP-1 and exendin-4 on heart rate and blood pressure in rats Interactions of exendin-(9-39) with the effects of glucagon-like peptide-1-(7-36) amide and of exendin-4 on arterial blood pressure and heart rate in rats  Regul Pept. 1996 Nov 14;67(1):63-8.

Intravenous administration of GLP-1/glucose in calves rapidly increased heart rate and insulin levels without any associated changes in blood pressure Cardiovascular and pancreatic endocrine responses to glucagon-like peptide-1(7-36) amide in the conscious calf. Exp Physiol. 1997 Jul;82(4):709-16

Even moderate doses of GLP-1R agonists infused at levels not sufficient to lower blood glucose result in activation of central sympathetic neurons and adrenal medullary chromaffin cells  that produce catecholamines in preclinical studies. Centrally and peripherally administered GLP-1R agonists including native GLP-1 and the lizard peptide exendin-4 dose-dependently increased blood pressure and heart rate in rats. GLP-1R activation induced c-fos expression in the adrenal medulla and neurons in autonomic control sites in the rat brain, including medullary catecholamine neurons providing input to sympathetic preganglionic neurons. Furthermore, GLP-1R agonists rapidly activated tyrosine hydroxylase transcription in AP neurons which express the GLP-1R, as shown in Glucagon-Like Peptide-1-Responsive Catecholamine Neurons in the Area Postrema Link Peripheral Glucagon-Like Peptide-1 with Central Autonomic Control Sites. J Neurosci. 2003 Apr 1;23(7):2939-2946.  

Exendin-4 also produced a dose-dependent induction in heart rate and blood pressure in conscious rats; the increase in HR but not BP was blocked by co-administration of a beta-adrenergic antagonist however the BP response appeared mediated by a-adrenergic receptors J Pharmacol Exp Ther. 2006 Feb;316(2):852-9. Mesenteric vasoconstriction and hindquarters vasodilatation accompany the pressor actions of exendin-4 in conscious rats

Conversely, heart rate was lower in 2 month old Glp1r-/- mice, which also exhibit a mild concentric left ventricular hypertrophy at a younger age in the CD1 genetic background, however HR was not significantly different in Glp1r-/- vs. Glp1r+/+ mice at 5 months of age Cardiac function in mice lacking the glucagon-like peptide-1 receptor Endocrinology 2003 Jun;144(6):2242-52. Consistent with these findings John Ussher and colleagues demonstrated that 24 hr basal heart rate was modestly but significantly lower in mice with cardiac-specific reduction of Glp1r in cardiomyocytes Inactivation of the cardiomyocyte glucagon-like peptide-1 receptor (GLP-1R) unmasks cardiomyocyte-independent GLP-1R-mediated cardioprotection Mol Metab. 2014 May 9;3(5):507-17

In subsequent studies, Laurie Baggio and John Ussher examined the mechanisms through which GLP-1R agonists increase HR in mice. Induction of HR following administration of either lixisenatide or liraglutide was attenuated by concomitant administration of the beta-blocker, propranolol. Moreover HR induction by these 2 peptides was also attenuated in Glp1rCM-/- mice, indicating a role for atrial GLP-1Rs in transduction of HR. The actions of liraglutide on induction of HR were more sustained relative to those of lixisenatide, consistent with the pharmacokinetic properties of these two peptides. Interestingly, direct infusion of GLP-1R agonists into perfused mouse hearts ex vivo, or direct application of these agents to isolated mouse atrial preparations, failed to increase HR. See The autonomic nervous system and cardiac GLP-1 receptors control heart rate in mice Molecular Metabolism 2017 Volume 6, Issue 11, Pages 1339–1349

The dual Glp1rR:Gcgr agonist oxyntomodulin also increases heart rate and blood pressure however these actions do not require a functional Glp1r Oxyntomodulin increases intrinsic heart rate in mice independent of the glucagon-like peptide-1 receptor. Am J Physiol Regul Integr Comp Physiol. 2007 Feb;292(2):R962-70.

Intravenous (iv) administration of GLP-1 (50 pmol-20 nmol) produced dose-dependent increases in heart rate in urethane-anesthetized rats Energy expenditure by intravenous administration of glucagon-like peptide-1 mediated by the lower brainstem and sympathoadrenal system. Peptides. 2005 Sep;26(9):1623-31

Intraperitoneal administration of exendin-4 significantly increased HR in WT and chronic supracollicular decerebrate (CD) rats indicating that the brainstem is sufficient and forebrain-caudal brainstem communication is not required for the observed HR responses Caudal brainstem processing is sufficient for behavioral, sympathetic, and parasympathetic responses driven by peripheral and hindbrain glucagon-like-peptide-1 receptor stimulation Endocrinology. 2008 Aug;149(8):4059-68.

Cabou and colleagues emphasize the importance of the CNS in the GLP-1R-dependent control of heart rate, blood pressure and blood flow in rodents. Only a modest increase in HR was seen in mice with central exendin-4 infusion under conditions of 3 h of hyperinsulinemic-euglycemic clamping. Central GLP-1R activation also activated vagus nerve activity Brain glucagon-like peptide-1 regulates arterial blood flow, heart rate, and insulin sensitivity Diabetes. 2008 Oct;57(10):2577-87.

In contrast, there is less data affirming the short term effect of native GLP-1 on SNS and PNS activityin humans. Bharucha and colleagues examined the effects of GLP-1 infusion at doses designed to produce supraphysiological levels in healthy non-diabetic human subjects. Under the experimental conditions employed herein, GLP-1 increases skeletal muscle sympathetic nerve activity but did not affect heart rate or cardiac sympathetic or parasympathetic activity in humans. Notably, in Figure 2, there was also no effect of GLP-1 in the fasting or postprandial state on heart rate variability, assessing measures of both sympatehtic and parasympathetic cardiac activity Effects of glucagon-like peptide-1, yohimbine, and nitrergic modulation on sympathetic and parasympathetic activity in humans Am J Physiol Regul Integr Comp Physiol. 2008 Sep;295(3):R874-80

Griffioen and colleagues examined heart rate variability in non-diabetic mice following intracerebroventricular administration of exendin-4. Acute icv exendin-4 lowered glucose and body temperature and increased HR in freely moving mice, with spectral analysis of heart rate variabilty suggesting a predominant effect on reduction of parasympatehtic nervous system activity. Chronic (28 day) icv exendin-4 lowered body weight, glucose, and body temperature, and produced a sustained elevation in HR, largely through reduction of parasympathetic inputs as assessed by spectral HRV. Ex-4 also reduced glutamergic excitatory post synaptic currents in cardiac vagal neurons and decreased glycinergic neurotransmission to cardiac vagal neurons. Hence central Ex-4 decreases spectral measures of HRV and inhibits neurotransmission to cardiac vagal neurons in the brainstem GLP-1 receptor stimulation depresses heart rate variability and inhibits neurotransmission to cardiac vagal neurons Cardiovasc Res. 2010 Sep 30

The effects of GLP-1R activation on cardiovascular electrophysiology has been studied with several clinically approved GLP-1R agonists. Administration of liraglutide once daily (0.6 mg once daily for 7 days, then 1.2 mg once daily for 7 days, then 1.8 mg daily for 7 days) in 51 healthy human subjects was not associated with significant QTc prolongation using 4 different methods of correcting QT interval for changes in heart rate, QTci (primary), QTciL, QTcF, and QTcB (secondary endponts) in a classical tQT study design. Moreover, the expected prolongation in QT interval was seen in subjects administered moxifloxacin after a 7 day washout. No significant individual QT prolongation was seen with liraglutide nor was there any concentration-dependent liraglutide-QT interval relationship. No QTc prolongation greater than 500 ms or increases greater than 60 ms was detected at any exposure of liraglutide Absence of QTc prolongation in a thorough QT study with subcutaneous liraglutide, a once-daily human GLP-1 analog for treatment of type 2 diabetes J Clin Pharmacol. 2009 Nov;49(11):1353-62

A QT study was also carried out using single dose, placebo-controlled administration of exenatide, 10 ug daily in healthy human male and female subjects who were initially screened for their tolerance to exenatide with respect to GI side effects. In study Part A, patients with nausea and vomiting were withdrawn from the study. In Part B, EKGs were done at multiple intervals from 1-10 h post dose. Patients received a single dose of either placebo, exenatide 10 ug, or moxifloxacin. The primary QT correction formula reported was QTcF (Friderica). No significant effect on QTc prolongation was detected in this study, and no significant relationship between QTc interval and the dose of exenatide was observed as presented by Linnebjerg H., et al The Effect of Exenatide on Qtc Interval on QTc Interval in Healthy Subjects Poster 597 2009 Annual Meeting of the American Diabetes Association

In non-diabetic human subjects with compenated chronic heart failure and associated ischemic heart disease, a 48 h infusion of GLP-1 produced a very modest but significant increase in heart rate and diastolic BP without significant improvements in LVEF Cardiovascular and metabolic effects of 48-h glucagon-like peptide-1 infusion in compensated chronic patients with heart failure Am J Physiol Heart Circ Physiol. 2010 Mar;298(3):H1096-102.

A modest non-significant increase ( LS mean +/- SE, 2.1 +/- 1.4 versus -0.7 +/- 1.4 beats/minute) in HR was observed in a small study of patients (28 received exenatide) with type 2 diabetes treated with twice daily exenatide on a background of metformin + a TZD for 16 weeks Effect of exenatide on heart rate and blood pressure in subjects with type 2 diabetes mellitus: a double-blind, placebo-controlled, randomized pilot study Cardiovasc Diabetol. 2010 Jan 28;9:6

Smits and colleagues examined the effects of acute intravenous exenatide administration alone or with co-administration of L-NMMA in 10 young healthy overweight males in the fasting state. Cardiac ANS tone was assessed by analysis of Heart Rate Variability (HRV). Exenatide acutely increased HR (mean maximum 6.8 bpm) and systolic BP (mean maximum 9.8 mm Hg), as well as cardiac output-co-infusion of L-NMMA had no effect on these CV parameteres. Plasma glucose levels also dereased from 4.32 to 3.47 mM during the infusions, without detectable change in plasma insulin levels. Exenatide also increased the LF/HF ratio, a indirect readout of SNS activity, independent of L-NMMA infusion. Exenatide acutely increases heart rate in parallel with augmented sympathetic nervous system activation in healthy overweight males Br J Clin Pharmacol. 2015 Nov 26. doi: 10.1111/bcp.

The DPP-4 inhibitors sitagliptin and vildagliptin have also been evaluated in QT studies. Sitagliptin was studied at 100 and 800 mg a day in a single dose 4 way cross over design with placebo and moxifloxacin. No QTc prolongation, nor any effect on heart rate, PR, RR or QRS interval was detected with 100 mg daily dosing of sitagliptin. The supratherapeutic 800 mg dose did produce a small increase in QTcf from baseline of 3.7 ms. No prolongation of QTcf beyond 450 ms was noted. A thorough QTc study to assess the effect of sitagliptin, a DPP4 inhibitor, on ventricular repolarization in healthy subjects J Clin Pharmacol. 2009 Aug;49(8):937-46

For the most part, vildagliptin had little effect on QT interval when dosed in healthy volunteers at a supratherapeutic 400 mg per day dose, however at 100 mg per day, after 5 days of dosing, the placebo adjusted mean QTcF interval exceeded 5 msec at 1 hr post dose, and the upper bounds of the two-sided 90% confidence intervals were below the 10 ms threshold at all time points except 1 and 8 hrs post dosing with the 100 mg daily dose. No absolute prolongation of QTcF beyond 450 ms was detected and no significant changes from baseline in PR or QRS interval was observed with vildagliptin. This study was completed in 2002 well before the publication of the ICH E14 tQT guidance was published in 2005. Thorough QT study of the effects of vildagliptin, a dipeptidyl peptidase IV inhibitor, on cardiac repolarization and conduction in healthy volunteers Curr Med Res Opin. 2011 May 24.

The Canadian Prescribing Information for Byetta, released January 2011, contains the following overview of EKG changes observed with Exenatide (Byetta) in clinical trials. 'Modest increases in heart rate were observed in subjects with T2DM, and a modest increase in PR and QT intervals was observed after single dosing in healthy human subjects'. A repeat QT study at higher plasma exenatide levels was requested by the FDA in its second complete response letter for the Exenatide once weekly (Bydureon) NDA and on January 26 2011, Amylin outlined its Plan for a Repeat Exenatide QT Study

 

                                  Cardiovascular biology of GLP-1(9-36)amide

Remarkably, improvements in dog cardiovascular function in a canine model of pacing induced LV dysfunction have also been demonstrated using the truncated peptide GLP-1(9-36)amide, raising the possibility that a second functional receptor for GLP-1(9-36)amide may be critical for the cardiovascular actions of native GLP-1. See Active Metabolite of GLP-1 Mediates Myocardial Glucose Uptake and Improves Left Ventricular Performance in Conscious Dogs with Dilated Cardiomyopathy. Am J Physiol Heart Circ Physiol. 2005 Dec;289(6):H2401-8.

Moreover, the available evidence suggests that the cardiovascular biology of GLP-1receptor agonists and GLP-1-derived peptides is increasingly complex and likely involves multiple distinct receptors and mechanisms. Classical GLP-1R agonists exert cardioprotective actions dependent on the knownGLP-1 receptor. However, GLP-1(9-36)amide also appears to be cardioprotective in the ischemic mouse heart when infused post-ischemia, and these actions are independent of the known GLP-1R. Moreover GLP-1(9-36) also appears to exert vasodilatory actions directly on murine blood vessels, increasing coronoary flow and vasodilation in mesenteric vessels. Furthermore, unexpectedly, even exendin-4 exerts some cardioprotective activity in a GLP-1R-independent manner. Hence, it is important to evaluate the existing GLP-1 cardiovascular literature in the context of understanding that some of the actions attributed to native GLP-1 may be due in part to actions of GLP-1(9-36)amide; See Cardioprotective and Vasodilatory Actions of Glucagon-Like Peptide 1 Receptor Are Mediated Through Both Glucagon-Like Peptide 1 Receptor–Dependent and –Independent Pathways  Circulation 2008 May 6;117(18):2340-50.

Ban and colleagues carried out studies using cell lines and mouse hearts with both exendin-4 and GLP-1(9-36) to demonstrate that these peptides exert both overlapping yet distinct actions in the cardiovascular system. Intact GLP-1 was rapidly cleaved to GLP-1(9-36) in the coronary circulation ex vivo. Both peptides activated similar cardioprotective signal transduction pathways in WT murine cardiomyocytes. GLP-1(9-36) amide, but not exendin-4 continued to exert actions in Glp1r-/- cardiomyocytes. Unexpectedly, the actions of both peptides were attenuated by the GLP-1R antagonist exendin(9-39). Moroever, GLP-1(9-36), but not exendin-4, activated cardioprotective signaling pathways in human endothelial cells. See GLP-1(9-36)amide-mediated cytoprotection is blocked by exendin(9-39) yet does not require the known GLP-1 receptor Endocrinology 2010 in press

 

 

What is the cardiac phenotype of mice born without the GLP-1 receptor? GLP-1R-/-  2-month-old mice exhibit reduced resting heart rate and elevated left ventricular (LV) end diastolic pressure and older (5 month old) mice demonstrate increased LV thickness. Although baseline hemodynamic parameters were normal, GLP-1R(-/-) mice displayed impaired LV contractility and diastolic function after insulin administration. Furthermore, LV contractility after exogenous epinephrine infusion was also reduced in GLP-1R(-/-) mice. See Cardiac function in mice lacking the glucagon-like peptide-1 receptor. Endocrinology. 2003 Jun;144(6):2242-52