Exenatide Improves Hypertension in a Rat Model of the Metabolic Syndrome

Glucagon-like peptide-1 receptor agonists and sodium-glucose cotransporter 2 inhibitors, anti-diabetic drugs in heart failure and cognitive impairment: potential mechanisms of the protective effects

Heart failure and cognitive impairment emerge as public health problems that need to be addressed due to the aging global population. The conditions that often coexist are strongly related to advancing age and multimorbidity. Epidemiological evidence indicates that cardiovascular disease and neurodegenerative processes shares similar aspects, in term of prevalence, age distribution, and mortality. Type 2 diabetes increasingly represents a risk factor associated not only to cardiometabolic pathologies but also to neurological conditions. The pathophysiological features of type 2 diabetes and its metabolic complications (hyperglycemia, hyperinsulinemia, and insulin resistance) play a crucial role in the development and progression of both heart failure and cognitive dysfunction. This connection has opened to a potential new strategy, in which new classes of anti-diabetic medications, such as glucagon-like peptide-1 receptor (GLP-1R) agonists and sodium-glucose cotransporter 2 (SGLT2) inhibitors, are able to reduce the overall risk of cardiovascular events and neuronal damage, showing additional protective effects beyond glycemic control. The pleiotropic effects of GLP-1R agonists and SGLT2 inhibitors have been extensively investigated. They exert direct and indirect cardioprotective and neuroprotective actions, by reducing inflammation, oxidative stress, ions overload, and restoring insulin signaling. Nonetheless, the specificity of pathways and their contribution has not been fully elucidated, and this underlines the urgency for more comprehensive research.

Role of GLP-1 Receptor Agonist in Diabetic Cardio-renal Disorder: Recent Updates of Clinical and Pre-clinical Evidence

Cardiovascular complications and renal disease is the growing cause of mortality in patients with diabetes. The subversive complications of diabetes such as hyperglycemia, hyperlipidemia and insulin resistance lead to an increase in the risk of myocardial infarction (MI), stroke, heart failure (HF) as well as chronic kidney disease (CKD). Among the commercially available anti-hyperglycemic agents, incretin-based medications appear to be safe and effective in the treatment of type 2 diabetes mellitus (T2DM) and associated cardiovascular and renal disease. Glucagon-like peptide 1 receptor agonists (GLP-1RAs) have been shown to be fruitful in reducing HbA1c, blood glucose, lipid profile, and body weight in diabetic patients. Several preclinical and clinical studies revealed the safety, efficacy, and preventive advantages of GLP-1RAs against diabetes- induced cardiovascular and kidney disease. Data from cardio-renal outcome trials had highlighted that GLP-1RAs protected people with established CKD from significant cardiovascular disease, lowered the likelihood of hospitalization for heart failure (HHF), and lowered all-cause mortality. They also had a positive effect on people with end-stage renal disease (ESRD) and CKD. Beside clinical outcomes, GLP-1RAs reduced oxidative stress, inflammation, fibrosis, and improved lipid profile pre-clinically in diabetic models of cardiomyopathy and nephropathy that demonstrated the cardio-protective and reno-protective effect of GLP-1RAs. In this review, we have focused on the recent clinical and preclinical outcomes of GLP-1RAs as cardio-protective and reno-protective agents as GLP-1RAs medications have been demonstrated to be more effective in treating T2DM and diabetes-induced cardiovascular and renal disease than currently available treatments in clinics, without inducing hypoglycemia or weight gain.

Glucagon-like peptide-1 (GLP-1) receptor agonists and their cardiovascular benefits-The role of the GLP-1 receptor

Cardiovascular outcome trials revealed cardiovascular benefits for type 2 diabetes mellitus patients when treated with long‐acting glucagon‐like peptide‐1 (GLP‐1) receptor agonists. In the last decade, major advances were made characterising the physiological effects of GLP‐1 and its action on numerous targets including brain, liver, kidney, heart and blood vessels. However, the effects of GLP‐1 and receptor agonists, and the GLP‐1 receptor on the cardiovascular system have not been fully elucidated. We compare results from cardiovascular outcome trials of GLP‐1 receptor agonists and review pleiotropic clinical and preclinical data concerning cardiovascular protection beyond glycaemic control. We address current knowledge on GLP‐1 and receptor agonist actions on the heart, vasculature, inflammatory cells and platelets, and discuss evidence for GLP‐1 receptor‐dependent versus independent effects secondary of GLP‐1 metabolites. We conclude that the favourable cardiovascular profile of GLP‐1 receptor agonists might expand their therapeutic use for treating cardiovascular disease even in non‐diabetic populations.

Effect of GLP-1 Receptor Agonists in the Cardiometabolic Complications in a Rat Model of Postmenopausal PCOS

Polycystic ovary syndrome (PCOS) is characterized by hyperandrogenism and ovulatory dysfunction. Women with PCOS have an elevated prevalence of cardiometabolic risk factors that worsen after menopause. Liraglutide (Lira), a glucagon-like peptide-1 receptor agonist, has shown beneficial metabolic effects in small clinic trials in reproductive-age women with PCOS. We have shown that chronic hyperandrogenemia in an experimental model of postmenopausal PCOS is associated with an adverse cardiometabolic profile and upregulation of the intrarenal renin-angiotensin system (RAS). We analyzed the effect of Lira in the cardiometabolic profile, intrarenal RAS, and blood pressure (BP) in postmenopausal PCOS. Four-week-old female Sprague Dawley rats were treated with DHT or placebo for 17 months. Lira administration during the last 3 weeks caused a bigger reduction in food intake, body weight, fat mass, and homeostasis model assessment of insulin resistance index in PCOS than in control rats. Moreover, Lira improved dyslipidemia and elevated leptin levels in PCOS. In contrast, Lira decreased intrarenal expression of RAS components only in the control group. Lira transiently increased heart rate and decreased BP in control rats. However, Lira did not modify BP but increased heart rate in PCOS. The angiotensin-converting-enzyme inhibitor enalapril abolished the BP differences between PCOS and control rats. However, Lira coadministration with enalapril further reduced BP only in control rats. In summary, Lira has beneficial effects for several cardiometabolic risk factors in postmenopausal PCOS. However, hyperandrogenemia blunted the BP-lowering effect of Lira in postmenopausal PCOS. Androgen-induced activation of intrarenal RAS may play a major role mediating increases in BP in postmenopausal PCOS.

Glucagon-like peptide-1 receptor action in the vasculature

Glucagon-like peptide-1 receptor (GLP-1R) agonists augment insulin secretion and are thus used clinically to improve glycemia in subjects with type 2 diabetes (T2D). As recent data reveal marked improvements in cardiovascular outcomes in T2D subjects treated with the GLP-1R agonists liraglutide and semaglutide in the LEADER and SUSTAIN-6 clinical trials respectively, there is growing interest in delineating the mechanism(s) of action for GLP-1R agonist-induced cardioprotection. Of importance, negligible GLP-1R expression in ventricular cardiac myocytes suggests that cardiac-independent actions of GLP-1R agonists may account for the reduced death rates from cardiovascular causes in T2D subjects enrolled in the LEADER trial. Conversely, vascular smooth muscle cells (VSMCs) appear to express the canonical GLP-1R, and GLP-1/GLP-1R agonists exhibit a number of salutary actions on the vascular endothelium that could potentially contribute to GLP-1R agonists directly improving cardiovascular outcomes in subjects with T2D. We review herein the described actions of GLP-1/GLP-1R agonists on the vascular endothelium, which include antiproliferative actions on VSMCs and endothelial cells, reductions in oxidative stress, and increases in nitric oxide generation. GLP-1 also increases microvascular recruitment and microvascular blood flow. Taken together, such actions may explain the antihypertensive and/or antiatherosclerotic actions attributed to GLP-1/GLP-1R agonists in preclinical and clinical studies. Nonetheless, further mechanistic studies are still necessary to determine the relative importance of such actions in accounting for reductions in macrovascular cardiovascular disease in human subjects with T2D treated with GLP-1R agonists.

The potential for renoprotection with incretin-based drugs

Incretin-based drugs, i.e., glucagon-like peptide-1 (GLP-1) receptor agonists and dipeptidyl peptidase-4 (DPP-4) inhibitors, are widely used for the treatment of type 2 diabetes. In addition to the primary role of incretins in stimulating insulin secretion from pancreatic β-cells, they have extra pancreatic functions beyond glycemic control. Indeed, recent studies highlight the potential beneficial effects of incretin-based therapy in diabetic kidney disease (DKD). Experimental studies using various diabetic models suggest that incretins protect the vascular endothelium from injury by binding to GLP-1 receptors, thereby ameliorating oxidative stress and the local inflammatory response, which reduces albuminuria and inhibits glomerular sclerosis. In addition, there is some evidence that GLP-1 receptor agonists and DPP-4 inhibitors mediate sodium excretion and diuresis to lower blood pressure. The pleiotropic actions of DPP-4 inhibitors are ascribed primarily to their effects on GLP-1 signaling, but other substrates of DPP-4, such as brain natriuretic peptide and stromal-derived factor-1α, may have roles. In this review, we summarize recent studies of the roles of incretin-based therapy in ameliorating DKD and its complications.

Incretin-based therapies: can we achieve glycemic control and cardioprotection?

Glucagon-like (GLP-1) is a peptide hormone secreted from the small intestine in response to nutrient ingestion. GLP-1 stimulates insulin secretion in a glucose-dependent manner, inhibits glucagon secretion and gastric emptying, and reduces appetite. Because of the short circulating half-life of the native GLP-1, novel GLP-1 receptor (GLP-1R) agonists and analogs and dipeptidyl peptidase 4 (DPP-4) inhibitors have been developed to facilitate clinical use. Emerging evidence indicates that GLP-1-based therapies are safe and may provide cardiovascular (CV) benefits beyond glycemic control. Preclinical and clinical studies are providing increasing evidence that GLP-1 therapies may positively affect CV function and metabolism by salutary effects on CV risk factors as well as via direct cardioprotective actions. However, the mechanisms whereby the various classes of incretin-based therapies exert CV effects may be mechanistically distinct and may not necessarily lead to similar CV outcomes. In this review, we will discuss the potential mechanisms and current understanding of CV benefits of native GLP-1, GLP-1R agonists and analogs, and of DPP-4 inhibitor therapies as a means to compare their putative CV benefits.

GLP-1 receptor agonists: effects on cardiovascular risk reduction

Comorbid obesity, dyslipidemia, and hypertension place patients with type 2 diabetes (T2DM) at greatly increased risk of cardiovascular (CV) disease-related morbidity and mortality. An urgent need exists for effective treatment for patients with T2DM that encompasses glycemic control, weight loss, and reduction in CV risk factors. The glucagon-like peptide-1 receptor agonists (GLP-1 RAs) liraglutide and exenatide are incretin-based antidiabetes agents. This review examines CV-associated effects of liraglutide and exenatide in animal models and clinical trials with patients with T2DM. Studies support the effectiveness of GLP-1 RAs in reducing hyperglycemia. Further, GLP-1 RAs represent a significant advance in T2DM treatment because they uniquely affect a broad array of CV risk factors through significant weight and systolic blood pressure reduction, improved lipid levels, and possibly, as shown in in vitro studies and animal models, through direct effects on cardiac myocytes and endothelium.

Is there a role for the incretin system in blood pressure regulation?

Incretin-based therapies are now well established for diabetes management and are among the frontline agents for control of hyperglycemia. In addition to their antihyperglycemic effects, evidence is emerging on the role of these agents on blood pressure regulation, cardioprotective and renoprotective properties. Because of the pleiotropic nature of these affects, these agents could offer significant benefits with regards to the cardiorenal metabolic complications that are part of the diabetes and obesity epidemic in the United States and worldwide. We review the various known mechanisms or pathways by which incretin based therapy exerts its regulation of blood pressure with emphasis on novel mechanisms such as inflammation/immunomodulation and oxidative stress.

Improved glucose control and reduced body weight in rodents with dual mechanism of action peptide hybrids

Combination therapy is being increasingly used as a treatment paradigm for metabolic diseases such as diabetes and obesity. In the peptide therapeutics realm, recent work has highlighted the therapeutic potential of chimeric peptides that act on two distinct receptors, thereby harnessing parallel complementary mechanisms to induce additive or synergistic benefit compared to monotherapy. Here, we extend this hypothesis by linking a known anti-diabetic peptide with an anti-obesity peptide into a novel peptide hybrid, which we termed a phybrid. We report on the synthesis and biological activity of two such phybrids (AC164204 and AC164209), comprised of a glucagon-like peptide-1 receptor (GLP1-R) agonist, and exenatide analog, AC3082, covalently linked to a second generation amylin analog, davalintide. Both molecules acted as full agonists at their cognate receptors in vitro, albeit with reduced potency at the calcitonin receptor indicating slightly perturbed amylin agonism. In obese diabetic Lep^ob/Lep^ob mice sustained infusion of AC164204 and AC164209 reduced glucose and glycated haemoglobin (Hb_A1c) equivalently but induced greater weight loss relative to exenatide administration alone. Weight loss was similar to that induced by combined administration of exenatide and davalintide. In diet-induced obese rats, both phybrids dose-dependently reduced food intake and body weight to a greater extent than exenatide or davalintide alone, and equal to co-infusion of exenatide and davalintide. Phybrid-mediated and exenatide + davalintide-mediated weight loss was associated with reduced adiposity and preservation of lean mass. These data are the first to provide in vivo proof-of-concept for multi-pathway targeting in metabolic disease via a peptide hybrid, demonstrating that this approach is as effective as co-administration of individual peptides.

Direct cardiovascular effects of glucagon like peptide-1

Current gold standard therapeutic strategies for T2DM target insulin resistance or β cell dysfunction as their core mechanisms of action. However, the use of traditional anti-diabetic drugs, in most cases, does not significantly reduce macrovascular morbidity and mortality. Among emerging anti-diabetic candidates, glucagon like peptide-1 (GLP-1) based therapies carry special cardiovascular implications, exerting both direct as well as indirect effects. The direct cardiovascular effects of GLP-1 and its analogs remain the focus of this review.

Blood pressure-lowering effects of GLP-1 receptor agonists exenatide and liraglutide: a meta-analysis of clinical trials

AIMS

Aside from lowering blood glucose, glucagon-like peptide-1 receptor agonists (GLP-1 RAs) attract much attention because of their cardioprotective effects. The aim of this study was to assess the blood pressure-lowering effects of the GLP-1 RAs exenatide and liraglutide compared with other common drugs used to treat type 2 diabetes (T2DM) based on randomized controlled trials (RCTs) including data describing complete blood pressure (BP) changes from baseline.

METHODS

We searched the major databases for published or unpublished RCTs that had been performed in patients with T2DM and compared the effects of exenatide and liraglutide to those of other common drugs used to treat T2DM. The RCTs that included data describing BP changes between the baseline and the end of the study were selected for further analysis.

RESULTS

A total of 16 RCTs that enrolled 3443 patients in the GLP-1 RA treatment group and 2417 subjects in the control group were included in this meta-analysis. The GLP-1 RA exenatide reduced systolic blood pressure (SBP) when compared with both placebo and insulin glargine, with mean differences of -5.24 and -3.46 mmHg, respectively, and with 95% confidence intervals (CI) of -6.88 to -3.59, p < 0.00001 and -3.63 to -3.29, p < 0.00001, respectively. Meanwhile, in the exenatide-treated group, diastolic blood pressure (DBP) was reduced by -5.91 mmHg, with a 95% CI of -7.53 to -4.28, p < 0.00001 compared with the placebo group, and -0.99 mmHg with a 95% CI of -1.12 to -0.87, p < 0.00001 compared with the sitagliptin group. SBP changes in this meta-analysis were assessed in the groups treated with 1.2 or 1.8 mg liraglutide per day. In the 1.2 mg-treated group, liraglutide treatment reduced SBP compared with placebo and glimepiride treatment, with mean differences of -5.60 and -2.38 mmHg, and 95% CIs of -5.84 to -5.36, p < 0.00001 and -4.75 to -0.01, p = 0.05, respectively. In the 1.8-mg-treated group, liraglutide also reduced SBP compared with placebo and glimepiride treatment with mean differences of -4.49 and -2.62 mmHg, and a 95% CI of -4.73 to -4.26, p < 0.00001, and -2.91 to -2.33, p < 0.00001, respectively.

CONCLUSION

Treatment with the GLP-1 RAs exenatide and liraglutide reduced SBP and DBP by 1 to 5 mmHg compared with some other anti-diabetic drugs including insulin, glimepiride and placebo for patients with T2DM. GLP-1 RAs may offer an alternative therapy for these patients and will help provide extra cardiovascular benefits.

Extraglycemic effects of glp-1-based therapeutics: addressing metabolic and cardiovascular risks associated with type 2 diabetes

CONTEXT

To examine whether widespread tissue expression of the glucagon-like peptide (GLP)-1 receptor supports the possibility of differential effects of GLP-1-based therapeutics on cardiac function, blood pressure, food intake, gastric emptying, and other regulatory activities. GLP-1 receptor agonists (RAs) have demonstrated pleiotropic effects on overweight/obesity, hypertension, dyslipidemia, and cardiovascular (CV) disease. Food-regulatory effects have been demonstrated in preclinical and clinical trials, including reduced gastric motility and food intake leading to body weight reductions. Native GLP-1 and GLP-1 RAs have demonstrated cardioprotective effects in preclinical models.

EVIDENCE ACQUISITION

Using PubMed, we performed a search of the recent literature on GLP-1 and GLP-1 RAs.

EVIDENCE SYNTHESIS

Preliminary clinical data indicate native GLP-1 has beneficial effects on endothelial cell function and vascular inflammation. Native GLP-1 and GLP-1 RAs have demonstrated renoprotective and antihypertensive effects, and reductions in lipid parameters. The GLP-1 RA liraglutide has also demonstrated positive effects on such markers of endothelial dysfunction as tumor necrosis factor-α and plasminogen activator inhibitor-1.

CONCLUSION

Preliminary data suggest GLP-1 RAs could benefit type 2 diabetes patients at risk for CV comorbidities. Additional studies are needed to confirm the extraglycemic and extrapancreatic effects and determine whether outcomes will translate into beneficial effects for patient care.

Discovery and development of exenatide: the first antidiabetic agent to leverage the multiple benefits of the incretin hormone, GLP-1

INTRODUCTION

The GLP-1 receptor agonist exenatide is synthetic exendin-4, a peptide originally isolated from the salivary secretions of the Gila monster. Exenatide was developed as a first-in-class diabetes therapy, with immediate- and extended-release formulations. In preclinical diabetes models, exenatide enhanced glucose-dependent insulin secretion, suppressed inappropriately elevated glucagon secretion, slowed gastric emptying, reduced body weight, enhanced satiety, and preserved pancreatic β-cell function. In clinical trials, both exenatide formulations reduced hyperglycemia in patients with type 2 diabetes mellitus (T2DM) and were associated with weight loss.

AREAS COVERED

This article reviews the development of exenatide from its discovery and preclinical investigations, to the elucidation of its pharmacological mechanisms of action in mammalian systems. The article also presents the pharmacokinetic profiling and toxicology studies of exenatide, as well as its validation in clinical trials.

EXPERT OPINION

GLP-1 receptor agonists represent a new paradigm for the treatment of patients with T2DM. By leveraging incretin physiology, a natural regulatory system that coordinates oral nutrient intake with mechanisms of metabolic control, these agents address multiple core defects in the pathophysiology of T2DM. Studies have identified unique benefits including improvements in glycemic control and weight, and the potential for beneficial effects on the cardiometabolic system without the increased risk of hypoglycemia associated with insulin therapy. Peptide hormone therapeutics can offer significant advantages over small molecule drug targets when it comes to specificity, potency, and more predictable side effects. As exemplified by exenatide, injectable peptides can be important drugs for the treatment of chronic diseases, such as T2DM.

Glucagon-like peptide-1 receptor stimulation increases GFR and suppresses proximal reabsorption in the rat

The incretin hormone glucagon-like peptide-1 (GLP-1) is released from the gut in response to fat or carbohydrate and contributes to negative feedback control of blood glucose by stimulating insulin secretion, inhibiting glucagon, and slowing gastric emptying. GLP-1 receptors (GLP-1R) are also expressed in the proximal tubule, and possibly elsewhere in the kidney. Presently, we examined the effect of a GLP-1R agonist on single-nephron glomerular filtration rate (GFR; SNGFR), proximal reabsorption (Jprox), tubuloglomerular feedback (TGF) responses, and urine flow rate in hydropenic male Wistar and Wistar-Froemter rats. Micropuncture and whole-kidney data were obtained before and during infusion of the GLP-1 agonist exenatide (1 nmol/h iv). SNGFR and Jprox were measured by late proximal collection at both extremes of TGF activation, which was achieved by perfusing Henle's loop at 0 or 50 nl/min. Primary changes in Jprox were revealed by analysis of covariance for Jprox with SNGFR as a covariate. Effects on TGF activation were determined in a separate set of experiments by comparing early distal and late proximal collections. Exenatide increased SNGFR by 33-50%, suppressed proximal tubular reabsorption by 20-40%, doubled early distal flow rate, and increased urine flow rate sixfold without altering the efficiency of glomerulotubular balance, TGF responsiveness, or the tonic influence of TGF. This implies that exenatide is both a proximal diuretic and a renal vasodilator. Since the natural agonist for the GLP-1R is regulated by intake of fat and carbohydrate, but not by salt or fluid, the control of salt excretion by the GLP-1R system departs from the usual negative-feedback paradigm for regulating salt balance.

Natriuretic effect by exendin-4, but not the DPP-4 inhibitor alogliptin, is mediated via the GLP-1 receptor and preserved in obese type 2 diabetic mice

Activation of the glucagon-like peptide (GLP)-1 receptor (GLP-1R) and inhibition of dipeptidyl peptidase-4 (DPP-4) are new antidiabetic strategies. The GLP-1R and DPP-4 are also expressed in the renal proximal tubular brush border, where they may regulate Na(+) reabsorption. Exendin-4 (EX4) is a naturally occurring antidiabetic polypeptide (from the saliva of the lizard Heloderma suspectum) and GLP-1R agonist; however, part of its nonglucoregulatory effects are through GLP-1R-independent mechanisms. DPP-4 cleaves and inactivates GLP-1; thus the natriuretic effect of DPP-4 inhibition may be mediated by the GLP-1R. We report that parenteral application of EX4 in wild-type mice induced a diuresis and natriuresis associated with increases in glomerular filtration rate, fractional urinary fluid and Na(+) excretion, and renal membrane expression of the Na(+)/H(+) exchanger NHE3 phosphorylated at S552 and S605, established consensus sites for cAMP-dependent PKA. These effects were absent in mice lacking the GLP-1R and independent of adenylyl cyclase 6. In comparison, parenteral application of the DPP-4 inhibitor alogliptin reduced plasma DPP-4 activity by 95% and induced a diuresis and natriuresis independent of the presence of the GLP-1R or changes in phosphorylated NHE3. The inhibitory effect on renal fluid and Na(+) reabsorption of EX4, but not alogliptin, was preserved in diabetic db/db mice and associated with a modest reduction in blood pressure. These results reveal mechanistic differences in how EX4 vs. DPP-4 inhibition induces diuresis and natriuresis under normal states, with preservation of GLP-1R-mediated, but not DPP-4 inhibitor-dependent, natriuretic mechanisms in a mouse model of obese type 2 diabetes.

The cardiovascular effects of GLP-1 receptor agonists

Glucagon-like peptide-1 receptor (GLP-1R) agonists have been shown to regulate blood glucose concentrations by mechanisms including enhanced insulin synthesis/secretion, suppressed glucagon secretion, slowed gastric emptying, and enhanced satiety. GLP-1 receptors have also been identified in the heart, kidneys, and blood vessels, leading to the hypothesis that GLP-1R agonists may affect cardiovascular function or cardiovascular disease (CVD). The aim of this literature review was to assemble and assess preclinical and clinical data of potential medical importance regarding the cardiovascular effects of GLP-1R agonists. Preclinical studies with the GLP-1R agonists GLP-1, exenatide, or liraglutide provided evidence that GLP-1R stimulation favorably affects endothelial function, sodium excretion, recovery from ischemic injury, and myocardial function in animals. Similar observations have been made in exploratory studies on GLP-1 infusion in normal subjects and patients with type 2 diabetes. Post hoc analyses of phase III studies of patients with type 2 diabetes treated with exenatide(bid or qw) or liraglutide(qd) showed that these GLP-1R agonists reduced blood pressure, an effect largely independent of weight loss, and that liraglutide slightly increased heart rate. Preliminary data also indicated that GLP-1R agonists reduced markers of CVD risk such as C-reactive protein and plasminogen activator inhibitor-1. Ongoing studies are examining the effects of administering GLP-1R agonists to patients at risk of CVD, postangioplasty patients, post-CABG patients, and patients with heart failure. Additional studies should provide meaningful data to determine whether GLP-1R agonists provide unique treatment benefits to patients at risk for or with established CVD.

Cardiovascular biology of the incretin system

Glucagon-like peptide-1 (GLP-1) is an incretin hormone that enhances glucose-stimulated insulin secretion and exerts direct and indirect actions on the cardiovascular system. GLP-1 and its related incretin hormone, glucose-dependent insulinotropic polypeptide, are rapidly inactivated by the enzyme dipeptidyl peptidase 4 (DPP-4), a key determinant of incretin bioactivity. Two classes of medications that enhance incretin action, GLP-1 receptor (GLP-1R) agonists and DPP-4 inhibitors, are used for the treatment of type 2 diabetes mellitus. We review herein the cardiovascular biology of GLP-1R agonists and DPP-4 inhibitors, including direct and indirect effects on cardiomyocytes, blood vessels, adipocytes, the control of blood pressure, and postprandial lipoprotein secretion. Both GLP-1R activation and DPP-4 inhibition exert multiple cardioprotective actions in preclinical models of cardiovascular dysfunction, and short-term studies in human subjects appear to demonstrate modest yet beneficial actions on cardiac function in subjects with ischemic heart disease. Incretin-based agents control body weight, improve glycemic control with a low risk of hypoglycemia, decrease blood pressure, inhibit the secretion of intestinal chylomicrons, and reduce inflammation in preclinical studies. Nevertheless, there is limited information on the cardiovascular actions of these agents in patients with diabetes and established cardiovascular disease. Hence, a more complete understanding of the cardiovascular risk to benefit ratio of incretin-based therapies will require completion of long-term cardiovascular outcome studies currently underway in patients with type 2 diabetes mellitus.

Comparison of liraglutide versus other incretin-related anti-hyperglycaemic agents

The two classes of incretin-related therapies, dipeptidyl peptidase-4 (DPP-4) inhibitors and glucagon-like peptide-1 receptor agonists (GLP-1 RAs), have become important treatment options for patients with type 2 diabetes. Sitagliptin, saxagliptin, vildagliptin and linagliptin, the available DPP-4 inhibitors, are oral medications, whereas the GLP-1 RAs-twice-daily exenatide, once-weekly exenatide and once-daily liraglutide-are administered subcutaneously. By influencing levels of GLP-1 receptor stimulation, these medications lower plasma glucose levels in a glucose-dependent manner with low risk of hypoglycaemia, affecting postprandial plasma glucose more than most other anti-hyperglycaemic medications. Use of GLP-1 RAs has been shown to result in greater glycaemic improvements than DPP-4 inhibitors, probably because of higher levels of GLP-1 receptor activation. GLP-1 RAs can also produce significant weight loss and may reduce blood pressure and have beneficial effects on other cardiovascular risk factors. Although both classes are well tolerated, DPP-4 inhibitors may be associated with infections and headaches, whereas GLP-1 RAs are often associated with gastrointestinal disorders, primarily nausea. Pancreatitis has been reported with both DPP-4 inhibitors and GLP-1 RAs, but a causal relationship between use of incretin-based therapies and pancreatitis has not been established. In clinical trials, liraglutide has shown efficacy and tolerability and resulted in certain significant benefits when compared with exenatide and sitagliptin.

Effects of incretins on blood pressure: a promising therapy for type 2 diabetes mellitus with hypertension

The prevalence of type 2 diabetes mellitus (T2DM) has increased markedly worldwide. A recent epidemiological study reported that approximately 63% of T2DM patients also have high blood pressure (>130/80 mmHg), which doubles their risk of cardiac events. Of the medications used to treat T2DM, the incretins are a group of peptides that not only regulate blood glucose effectively and moderately, but also protect patients against cardiovascular events and improve several cardiovascular parameters. Here were review data from preclinical and short- and long-term clinical studies investigating the antihypertensive effects of incretins. We also elucidate four possible mechanisms underlying the antihypertensive effects of incretins. We conclude that incretins can lower blood pressure of T2DM patients, independent of weight loss, and will surely improve the prognosis of these patients.

Dipeptidyl peptidase IV inhibition attenuates blood pressure rising in young spontaneously hypertensive rats

OBJECTIVES

The present study aimed to assess the effect of the specific dipeptidyl peptidase IV (DPPIV) inhibitor sitagliptin on blood pressure and renal function in young prehypertensive (5-week-old) and adult spontaneously hypertensive rats (SHRs; 14-week-old).

METHODS

Sitagliptin (40 mg/kg twice daily) was given by oral gavage to young (Y-SHR + IDPPIV) and adult (A-SHR + IDPPIV) SHRs for 8 days. Kidney function was assessed daily and compared with age-matched vehicle-treated SHR (Y-SHR and A-SHR) and with normotensive Wistar-Kyoto rats (Y-WKY and A-WKY). Arterial blood pressure was measured in these animals at the end of the experimental protocol. Additionally, Na/H exchanger isoform 3 (NHE3) function and expression in microvilli membrane vesicles were assessed in young animals.

RESULTS

Mean arterial blood pressure of Y-SHR + IDPPIV was significantly lower than that of Y-SHR (104 ± 3 vs. 123 ± 5 mmHg, P < 0.01) and was similar to Y-WKY (94 ± 4 mmHg, P > 0.05). Compared to Y-SHR, Y-SHR + IDPPIV exhibited enhanced cumulative urinary flow and sodium excretion and decreased NHE3 activity and expression in proximal tubule microvilli. In the A-SHR, sitagliptin treatment had no significant effect on either renal function or arterial blood pressure.

CONCLUSION

Our data suggest that DPPIV inhibition attenuates blood pressure rising in young prehypertensive SHRs, partially by inhibiting NHE3 activity in renal proximal tubule.

Mechanisms mediating the diuretic and natriuretic actions of the incretin hormone glucagon-like peptide-1

Glucagon-like peptide-1 (GLP-1) is a gut incretin hormone considered a promising therapeutic agent for type 2 diabetes because it stimulates beta cell proliferation and insulin secretion in a glucose-dependent manner. Cumulative evidence supports a role for GLP-1 in modulating renal function; however, the mechanisms by which GLP-1 induces diuresis and natriuresis have not been completely established. This study aimed to define the cellular and molecular mechanisms mediating the renal effects of GLP-1. GLP-1 (1 μg·kg(-1)·min(-1)) was intravenously administered in rats for the period of 60 min. GLP-1-infused rats displayed increased urine flow, fractional excretion of sodium, potassium, and bicarbonate compared with those rats that received vehicle (1% BSA/saline). GLP-1-induced diuresis and natriuresis were also accompanied by increases in renal plasma flow and glomerular filtration rate. Real-time RT-PCR in microdissected rat nephron segments revealed that GLP-1 receptor-mRNA expression was restricted to glomerulus and proximal convoluted tubule. In rat renal proximal tubule, GLP-1 significantly reduced Na(+)/H(+) exchanger isoform 3 (NHE3)-mediated bicarbonate reabsorption via a protein kinase A (PKA)-dependent mechanism. Reduced proximal tubular bicarbonate flux rate was associated with a significant increase of NHE3 phosphorylation at the PKA consensus sites in microvillus membrane vesicles. Taken together, these data suggest that GLP-1 has diuretic and natriuretic effects that are mediated by changes in renal hemodynamics and by downregulation of NHE3 activity in the renal proximal tubule. Moreover, our findings support the view that GLP-1-based agents may have a potential therapeutic use not only as antidiabetic drugs but also in hypertension and other disorders of sodium retention.

The exenatide analogue AC3174 attenuates hypertension, insulin resistance, and renal dysfunction in Dahl salt-sensitive rats

Background

Activation of glucagon-like peptide-1 (GLP-1) receptors improves insulin sensitivity and induces vasodilatation and diuresis. AC3174 is a peptide analogue with pharmacologic properties similar to the GLP-1 receptor agonist, exenatide. Hypothetically, chronic AC3174 treatment could attenuate salt-induced hypertension, cardiac morbidity, insulin resistance, and renal dysfunction in Dahl salt-sensitive (DSS) rats.

Methods

DSS rats were fed low salt (LS, 0.3% NaCl) or high salt (HS, 8% NaCl) diets. HS rats were treated with vehicle, AC3174 (1.7 pmol/kg/min), or GLP-1 (25 pmol/kg/min) for 4 weeks via subcutaneous infusion. Other HS rats received captopril (150 mg/kg/day) or AC3174 plus captopril.

Results

HS rat survival was improved by all treatments except GLP-1. Systolic blood pressure (SBP) was lower in LS rats and in GLP-1, AC3174, captopril, or AC3174 plus captopril HS rats than in vehicle HS rats (p < 0.05). AC3174 plus captopril attenuated the deleterious effects of high salt on posterior wall thickness, LV mass, and the ratio of LV mass to body weight (P ≤ 0.05). In contrast, GLP-1 had no effect on these cardiovascular parameters. All treatments reduced LV wall stress. GLP-1, AC3174, captopril, or AC3174 plus captopril normalized fasting insulin and HOMA-IR (P ≤ 0.05). AC3174, captopril, or AC3174 plus captopril improved renal function (P ≤ 0.05). Renal morphology in HS rats was associated with extensive sclerosis. Monotherapy with AC3174, captopril, or GLP-1 attenuated renal damage. However, AC3174 plus captopril produced the most effective improvement.

Conclusions

Thus, AC3174 had antihypertensive, cardioprotective, insulin-sensitizing, and renoprotective effects in the DSS hypertensive rat model. Furthermore, AC3174 improved animal survival, an effect not observed with GLP-1.

The synthetic GLP-I receptor agonist, exenatide, reduces intimal hyperplasia in insulin resistant rats

We studied the effect of a synthetic GLP-1 receptor agonist, exenatide, a drug approved for the treatment of type 2 diabetes, on the recovery from vascular injury in Zucker (non-diabetic) fatty rats. Exenatide 5.0 microg/kg per day or saline was administered for seven days before, and 21 days after balloon catheter mediated carotid injury. A pair feeding experiment helped differentiate between the drug itself and the known effects of the drug on decreased food intake. Body weight and glucose (weekly), carotid artery I/M ratio, aortic protein eNOS and NFkappaB-p65 were measured. Body weight gain in exenatide rats was significantly lower (53+/-5 vs. 89+/-8 g) than controls. Blood glucose did not change significantly. The I/M ratio in the exenatide group was 0.2+/-0.1 vs. 0.9+/-0.1 in controls (p<0.05). The expression of aortic eNOS was unchanged in exenatide treated rats and a small decrease seen in NFkappaB-p65 expression was not statistically significant. We conclude that exenatide attenuates intimal hyperplasia following balloon catheter induced vascular injury independently of glucose regulation and food intake. Our findings provide additional support for cardiovascular benefits of exenatide, especially in obese and pre-diabetic patients. Further research is needed to elucidate the mechanism underlying these effects.