The physiological role of glucagon-like peptide-1 in the regulation of renal function

Early treatment with GLP-1 after severe trauma preserves renal function in obese Zucker rats

Early posttrauma hyperglycemia (EPTH) is correlated with later adverse outcomes, including acute kidney injury (AKI). Controlling EPTH in the prehospital setting is difficult because of the variability in the ideal insulin dosage and the potential risk of hypoglycemia, especially in those with confounding medical comorbidities of obesity and insulin resistance. Glucagon-like peptide-1 (GLP-1) controls glucose levels in a glucose-dependent manner and is a current target in antidiabetic therapy. We have shown that after orthopedic trauma, obese Zucker rats exhibit EPTH and a later development of AKI (within 24 h). We hypothesized that GLP-1 treatment after trauma decreases EPTH and protects renal function in obese Zucker rats. Obese Zucker rats (~12 wk old) were fasted for 4 h before trauma. Soft tissue injury, fibula fracture, and homogenized bone component injection were then performed in both hind limbs to induce severe extremity trauma. Plasma glucose levels were measured before and 15, 30, 60, 120, 180, 240, and 300 min after trauma. GLP-1 (3 μg·kg^-1·h^-1, 1.5 ml/kg total) or saline was continuously infused from 30 min to 5 h after trauma. Afterwards, rats were placed in metabolic cages overnight for urine collection. The following day, plasma interleukin (IL)-6 levels, renal blood flow (RBF), glomerular filtration rate (GFR), and renal oxygen delivery (Do₂) and consumption (V̇o₂) were measured. EPTH was evident within 15 min after trauma but was significantly ameliorated during the 5 h of GLP-1 infusion. One day after trauma, plasma IL-6 was markedly increased in the trauma group and decreased in GLP-1-treated animals. RBF, GFR, and Do₂ all significantly decreased with trauma, but renal V̇o₂ was unchanged. GLP-1 treatment normalized RBF, GFR, and Do₂ without affecting V̇o₂. These results suggest that GLP-1 decreases EPTH and protects against a later development of AKI. Early treatment with GLP-1 (or its analogs) to rapidly, effectively, and safely control EPTH may be beneficial in the prehospital care of obese patients after trauma.

Cardioprotection Conferred by Sitagliptin Is Associated with Reduced Cardiac Angiotensin II/Angiotensin-(1-7) Balance in Experimental Chronic Kidney Disease

Dipeptidyl peptidase IV (DPPIV) inhibitors are antidiabetic agents that exert renoprotective actions independently of glucose lowering. Cardiac dysfunction is one of the main outcomes of chronic kidney disease (CKD); however, the effects of DPPIV inhibition on cardiac impairment during CKD progression remain elusive. This study investigated whether DPPIV inhibition mitigates cardiac dysfunction and remodeling in rats with a 5/6 renal ablation and evaluated if these effects are associated with changes in the cardiac renin-angiotensin system (RAS). To this end, male Wistar rats underwent a 5/6 nephrectomy (Nx) or sham operation, followed by an 8-week treatment period with the DPPIV inhibitor sitagliptin (IDPPIV) or vehicle. Nx rats had lower glomerular filtration rate, overt albuminuria and higher blood pressure compared to sham rats, whereas CKD progression was attenuated in Nx + IDPPIV rats. Additionally, Nx rats exhibited cardiac hypertrophy and fibrosis, which were associated with higher cardiac DPPIV activity and expression. The sitagliptin treatment prevented cardiac fibrosis and mitigated cardiac hypertrophy. The isovolumic relaxation time (IRVT) was higher in Nx than in sham rats, which was suggestive of CKD-associated-diastolic dysfunction. Sitagliptin significantly attenuated the increase in IRVT. Levels of angiotensin II (Ang II) in the heart tissue from Nx rats were higher while those of angiotensin-(1-7) Ang-(1-7) were lower than that in sham rats. This cardiac hormonal imbalance was completely prevented by sitagliptin. Collectively, these results suggest that DPPIV inhibition may delay the onset of cardiovascular impairment in CKD. Furthermore, these findings strengthen the hypothesis that a crosstalk between DPPIV and the renin-angiotensin system plays a role in the pathophysiology of cardiorenal syndromes.

The incretin pathway as a therapeutic target in diabetic kidney disease: a clinical focus on GLP-1 receptor agonists

Diabetic kidney disease (DKD) remains the main cause for chronic kidney disease (CKD) and end-stage kidney disease (ESKD) worldwide. Both CKD and ESKD lead to major increases in risk of cardiovascular disease and death in people with diabetes. Despite optimal management of lifestyle, glucose levels and hypertension, residual risk remains high, indicating that additional therapies to mitigate the burden of the disease are desired. In past decades, new treatment options for the management of diabetes have emerged, of which some have showed promising renoprotective potential. This review discusses current understanding of the renal effects of glucagon-like peptide receptor agonists and their potential use in prevention and treatment of DKD.

Pleiotropic Effects of GLP-1 and Analogs on Cell Signaling, Metabolism, and Function

The incretin hormone Glucagon-Like Peptide-1 (GLP-1) is best known for its "incretin effect" in restoring glucose homeostasis in diabetics, however, it is now apparent that it has a broader range of physiological effects in the body. Both in vitro and in vivo studies have demonstrated that GLP-1 mimetics alleviate endoplasmic reticulum stress, regulate autophagy, promote metabolic reprogramming, stimulate anti-inflammatory signaling, alter gene expression, and influence neuroprotective pathways. A substantial body of evidence has accumulated with respect to how GLP-1 and its analogs act to restore and maintain normal cellular functions. These findings have prompted several clinical trials which have reported GLP-1 analogs improve cardiac function, restore lung function and reduce mortality in patients with obstructive lung disease, influence blood pressure and lipid storage, and even prevent synaptic loss and neurodegeneration. Mechanistically, GLP-1 elicits its effects via acute elevation in cAMP levels, and subsequent protein kinase(s) activation, pathways well-defined in pancreatic β-cells which stimulate insulin secretion in conjunction with elevated Ca2+ and ATP. More recently, new studies have shed light on additional downstream pathways stimulated by chronic GLP-1 exposure, findings which have direct relevance to our understanding of the potential therapeutic effects of longer lasting analogs recently developed for clinical use. In this review, we provide a comprehensive description of the diverse roles for GLP-1 across multiple tissues, describe downstream pathways stimulated by acute and chronic exposure, and discuss novel pleiotropic applications of GLP-1 mimetics in the treatment of human disease.

Renal tubular effects of prolonged therapy with the GLP-1 receptor agonist lixisenatide in patients with type 2 diabetes mellitus

Glucagon-like peptide-1 (GLP-1) receptor agonists (RAs) are well-established glucose-lowering drugs for type 2 diabetes mellitus (T2DM) management. Acute GLP-1RA administration increases urinary excretion of sodium and other electrolytes. However, the renal tubular effects of prolonged GLP-1RA treatment are largely unknown. In this secondary analysis of a randomized trial, we determined the renal tubular effects of 8-wk treatment with 20 μg lixisenatide, a short-acting (prandial) GLP-1RA, versus titrated once-daily insulin glulisine in 35 overweight T2DM-patients on stable insulin glargine background therapy (age: 62 ± 7 yr, glycated hemoglobin: 8.0 ± 0.9%, estimated glomerular filtration rate: >60 ml·min^-1·1.73 m^-2). After a standardized breakfast, lixisenatide increased absolute and fractional excretions of sodium, chloride, and potassium and increased urinary pH. In contrast, lixisenatide reduced absolute and fractional excretions of magnesium, calcium, and phosphate. At week 8, patients treated with lixisenatide had significantly more phosphorylated sodium-hydrogen exchanger isoform 3 (NHE3) in urinary extracellular vesicles than those on insulin glulisine treatment, which suggested decreased NHE3 activity in the proximal tubule. A rise in postprandial blood pressure with lixisenatide partly explained the changes in the urinary excretion of sodium, potassium, magnesium, and phosphate and the changes in urinary pH. In conclusion, lixisenatide affects postprandial urinary excretion of several electrolytes and increases urinary pH compared with insulin glulisine in T2DM patients after 8 wk of treatment. This is most likely explained by a drug-induced rise in blood pressure or direct inhibitory effects on NHE3 in the proximal tubule.

Glucagon-like peptide-1 acutely affects renal blood flow and urinary flow rate in spontaneously hypertensive rats despite significantly reduced renal expression of GLP-1 receptors

Glucagon‐like peptide‐1 (GLP‐1) is an incretin hormone increasing postprandial insulin release. GLP‐1 also induces diuresis and natriuresis in humans and rodents. The GLP‐1 receptor is extensively expressed in the renal vascular tree in normotensive rats where acute GLP‐1 treatment leads to increased mean arterial pressure (MAP) and increased renal blood flow (RBF). In hypertensive animal models, GLP‐1 has been reported both to increase and decrease MAP. The aim of this study was to examine expression of renal GLP‐1 receptors in spontaneously hypertensive rats (SHR) and to assess the effect of acute intrarenal infusion of GLP‐1. We hypothesized that GLP‐1 would increase diuresis and natriuresis and reduce MAP in SHR. Immunohistochemical staining and in situ hybridization for the GLP‐1 receptor were used to localize GLP‐1 receptors in the kidney. Sevoflurane‐anesthetized normotensive Sprague–Dawley rats and SHR received a 20 min intrarenal infusion of GLP‐1 and changes in MAP, RBF, heart rate, dieresis, and natriuresis were measured. The vasodilatory effect of GLP‐1 was assessed in isolated interlobar arteries from normo‐ and hypertensive rats. We found no expression of GLP‐1 receptors in the kidney from SHR. However, acute intrarenal infusion of GLP‐1 increased MAP, RBF, dieresis, and natriuresis without affecting heart rate in both rat strains. These results suggest that the acute renal effects of GLP‐1 in SHR are caused either by extrarenal GLP‐1 receptors activating other mechanisms (e.g., insulin) to induce the renal changes observed or possibly by an alternative renal GLP‐1 receptor.

Simultaneous GLP-1 receptor activation and angiotensin receptor blockade increase natriuresis independent of altered arterial pressure in obese OLETF rats

Obesity is associated with an inappropriately activated renin-angiotensin-aldosterone system, suppressed glucagon-like peptide-1 (GLP-1), increased renal Na⁺ reabsorption, and hypertension. To assess the link between GLP-1 and angiotensin receptor type 1 (AT₁) signaling on obesity-associated impairment of urinary Na⁺ excretion (U_NaV) and elevated arterial pressure, we measured mean arterial pressure (MAP) and heart rate by radiotelemetry and metabolic parameters for 40 days. We tested the hypothesis that stimulation of GLP-1 signaling provides added benefit to blockade of AT₁ by increasing U_NaV and further reducing arterial pressure in the following groups: (1) untreated Long-Evans Tokushima Otsuka (LETO) rats (n = 7); (2) untreated Otsuka Long-Evans Tokushima Fatty (OLETF) rats (n = 9); (3) OLETF + ARB (ARB; 10 mg olmesartan/kg/day; n = 9); (4) OLETF + GLP-1 receptor agonist (EXE; 10 µg exenatide/kg/day; n = 7); and (5) OLETF + ARB + EXE (Combo; n = 6). On day 2, U_NaV was 60% and 62% reduced in the EXE and Combo groups, respectively, compared with that in the OLETF rats. On day 40, U_NaV was increased 69% in the Combo group compared with that in the OLETF group. On day 40, urinary angiotensinogen was 4.5-fold greater in the OLETF than in the LETO group and was 56%, 62%, and 58% lower in the ARB, EXE, and Combo groups, respectively, than in the OLETF group. From day 2 to the end of the study, MAP was lower in the ARB and Combo groups than in the OLETF rats. These results suggest that GLP-1 receptor activation may reduce intrarenal angiotensin II activity, and that simultaneous blockade of AT₁ increases U_NaV in obesity; however, these beneficial effects do not translate to a further reduction in MAP.

The potential and pitfalls of GLP-1 receptor agonists for renal protection in type 2 diabetes

Glucagon-Like Peptide-1 Receptor agonists (GLP-1 RA) offer substantial benefits for the management of glucose levels in type 2 diabetes. In addition, recent data from clinical trials have demonstrated that treatment with Glucagon-Like Peptide-1 Receptor agonists (GLP-1 RA) are also able to reduce new onset macroalbuminuria. These benefits may be consistent with the known effects of GLP-1 RA on traditional risk factors for progressive kidney disease including glucose lowering, blood pressure lowering, reduced insulin levels and weight reduction. However, emerging evidence suggests that GLP-1 RA can also have direct effects in the kidney, including inhibiting NHE3-dependent sodium reabsorption in the proximal tubule. Additional effects on the intrarenal renin angiotensin system, ischaemia/hypoxia, inflammation, apoptosis and neural signalling may also contribute to renal benefits. The extent to which these effects are mediated by the GLP-1R remains to be established. Recent studies confirm that the metabolic products of GLP-1 retain important antioxidant and anti-apoptotic activities that are GLP-1 R independent. Moreover the divergent peptide sequences of the currently available GLP-1 RA may mean that divergent reno-protective efficacy could be anticipated from different GLP-1 RA on this basis. Kidney disease is an important and deadly clinical outcome, and one worth preventing. Although both experimental and clinical data now support the possibility of renoprotective effects arising from treatment with GLP-1 RA, further work is needed to optimise these effects. A logical synergism with SGLT2 inhibition also exists, and at least in the short term, this combination approach may become the most useful way to protect the kidney in type 2 diabetes.

Contrasting effects on the risk of macrovascular and microvascular events of antihyperglycemic drugs that enhance sodium excretion and lower blood pressure

Three classes of anti-hyperglycaemic medications are distinguished by their urinary sodium excretion-enhancing and blood pressure-lowering actions: long-acting glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors and sodium-glucose co-transporter-2 inhibitors. Yet, these drugs exert different effects on macrovascular risk. Glucagon-like peptide-1 receptor agonists reduce atherosclerotic thromboembolic events, but have little effect on heart failure; sodium-glucose co-transporter-2 inhibitors decrease the occurrence of heart failure, but have minimal effect on myocardial infarction and stroke; and dipeptidyl peptidase-4 inhibitors do not ameliorate either atherosclerotic thromboembolic events or heart failure. Similarly, the three classes of drugs differ in their early effects on renal function. Dipeptidyl peptidase-4 inhibitors produce a small decrease in renal function that persists for the duration of treatment, and they do not prevent serious adverse renal events. For glucagon-like peptide-1 receptor agonists, a small early decrease in renal function persists for 2 years and is superseded by a small improvement in renal function, with no effect on renal outcomes. In contrast, an initial decrease in glomerular filtration with sodium-glucose co-transporter-2 inhibitors persists for only 1 year and is superseded by a durable improvement in renal function and a reduced risk of serious adverse renal events. These differences may be related to different actions on the proximal tubular reabsorption of sodium, and thereby, on glomerular hyperfiltration. Anti-hyperglycaemic drugs that have natriuretic actions differ markedly in their ability to modulate macrovascular and microvascular risk. These contrasting profiles cannot be predicted by their effects on blood glucose or blood pressure.

The glucagon-like peptide-1 receptor agonist Exendin-4, ameliorates contrast-induced nephropathy through suppression of oxidative stress, vascular dysfunction and apoptosis independent of glycaemia

Contrast-induced nephropathy (CIN) is a leading cause of hospital-acquired acute kidney injury, particularly in diabetic patients. Previous studies have shown renoprotective effects of glucagon-like peptide-1 (GLP-1) signalling; however, its role in CIN remains unexplored. This study investigates the prophylactic effect of exendin-4, a GLP-1R agonist, against CIN in a rat model mimicking both healthy and diabetic conditions. Animals were randomly divided into 7 groups: a control sham group (n = 8), and 2 identical sets of 3 disease groups, one received exendin-4 before exposure to contrast medium (CM), while the other served as untreated control. The 3 disease groups represented diabetes (n = 8), CIN (n = 8), or diabetes and CIN combined (n = 8). Untreated groups showed deteriorating renal function as indicated by significantly higher levels of serum creatinine and blood urea nitrogen, malondialdehyde, and endothelin-1 and caspase-3 expression compared to the sham control group. This was accompanied by a significant decrease in tissue reserves of reduced glutathione, superoxide dismutase, nitrate and endothelin nitric oxide synthase as well as deteriorating renal histology. The CM-induced changes in diabetic rats indicate impaired renal function, oxidative stress, vascular dysfunction, and apoptosis, and were significance higher in intensity compared to non-diabetic rats. Pretreatment with exendin-4 ameliorated all the aforementioned CM-induced nephropathic effects independent of the glycemic state. To our knowledge, this is the first study describing the prophylactic renoprotective effects of exendin-4 against CIN. With the current pharmaceutical use of exendin-4 as a hypoglycaemic agent, the GLP-1R agonist becomes an interesting candidate for human clinical trials on CIN prevention.

Renal Effects of Incretin-Based Diabetes Therapies: Pre-clinical Predictions and Clinical Trial Outcomes

PURPOSE OF REVIEW

The purpose of this review is to correlate predictions based on pre-clinical data with outcomes from clinical trials that examine the effects of incretin-based diabetes treatments on the kidney. The incretin-based treatments include agonists of the glucagon-like peptide 1 receptor (GLP-1R) and inhibitors of the enzyme, dipeptidyl peptidase-4 (DPP-4). In addition, what is known about the incretin-based therapies will be compared to what is known about the renal effects of SGLT2 inhibitors.

RECENT FINDINGS

Large-scale clinical trials have shown that SGLT2 inhibitors reduce albuminuria and preserve estimated glomerular filtration rate (eGFR) in patients with diabetic nephropathy. A concise and plausible hemodynamic mechanism is supported by pre-clinical research on the physiology and pharmacology of SGLT2. Large-scale clinical trials have shown that incretin-based therapies mitigate albuminuria but have not shown beneficial effects on eGFR. Research on the incretin-based therapies has yielded a diverse array of direct effects throughout the body, which fuels speculation as to how these drugs might benefit the diabetic kidney and affect its function(s). But in vivo experiments have yet to confirm that the proposed mechanisms underlying emergent phenomena, such as proximal tubular fluid reabsorption, are the ones predicted by cell and molecular experiments. There may be salutary effects of incretin-based treatments on the diabetic kidney, but the system is complex and not amenable to simple explanation or prior prediction. This contrasts with the renal effects of SGLT2 inhibitors, which can be explained concisely.

Role of the sodium-hydrogen exchanger in mediating the renal effects of drugs commonly used in the treatment of type 2 diabetes

Diabetes is characterized by increased activity of the sodium-hydrogen exchanger (NHE) in the glomerulus and renal tubules, which contributes importantly to the development of nephropathy. Despite the established role played by the exchanger in experimental studies, it has not been specifically targeted by those seeking to develop novel pharmacological treatments for diabetes. This review demonstrates that many existing drugs that are commonly prescribed to patients with diabetes act on the NHE1 and NHE3 isoforms in the kidney. This action may explain their effects on sodium excretion, albuminuria and the progressive decline of glomerular function in clinical trials; these responses cannot be readily explained by the influence of these drugs on blood glucose. Agents that may affect the kidney in diabetes by virtue of an action on NHE include: (1) insulin and insulin sensitizers; (2) incretin-based agents; (3) sodium-glucose cotransporter 2 inhibitors; (4) antagonists of the renin-angiotensin system (angiotensin converting-enzyme inhibitors, angiotensin receptor blockers and angiotensin receptor neprilysin inhibitors); and (5) inhibitors of aldosterone action and cholesterol synthesis (spironolactone, amiloride and statins). The renal effects of each of these drug classes in patients with type 2 diabetes may be related to a single shared biological mechanism.

Sexual Dimorphic Pattern of Renal Transporters and Electrolyte Homeostasis

Compared with males, females have lower BP before age 60, blunted hypertensive response to angiotensin II, and a leftward shift in pressure natriuresis. This study tested the concept that this female advantage associates with a distinct sexual dimorphic pattern of transporters along the nephron. We applied quantitative immunoblotting to generate profiles of transporters, channels, claudins, and selected regulators in both sexes and assessed the physiologic consequences of the differences. In rats, females excreted a saline load more rapidly than males did. Compared with the proximal tubule of males, the proximal tubule of females had greater phosphorylation of Na+/H+ exchanger isoform 3 (NHE3), distribution of NHE3 at the base of the microvilli, and less abundant expression of Na+/Pi cotransporter 2, claudin-2, and aquaporin 1. These changes associated with less bicarbonate reabsorption and higher lithium clearance in females. The distal nephrons of females had a higher abundance of total and phosphorylated Na+/Cl- cotransporter (NCC), claudin-7, and cleaved forms of epithelial Na+ channel (ENaC) α and γ subunits, which associated with a lower baseline plasma K+ concentration. A K+-rich meal increased the urinary K+ concentration and decreased the level of renal phosphorylated NCC in females. Notably, we observed similar abundance profiles in female versus male C57BL/6 mice. These results define sexual dimorphic phenotypes along the nephron and suggest that lower proximal reabsorption in female rats expedites excretion of a saline load and enhances NCC and ENaC abundance and activation, which may facilitate K+ secretion and set plasma K+ at a lower level.

Postprandial renal haemodynamic effect of lixisenatide vs once-daily insulin-glulisine in patients with type 2 diabetes on insulin-glargine: An 8-week, randomised, open-label trial

AIM

To determine whether lixisenatide, a prandial short-acting glucagon-like peptide receptor agonist (GLP-1RA), ameliorates postprandial glomerular hyperfiltration in patients with type 2 diabetes mellitus (T2DM) compared with insulin-glulisine (iGlu).

METHODS

Postprandial renal haemodynamic effects of 8-week treatment with lixisenatide 20 µg vs once-daily titrated iGlu were measured in 35 overweight patients with T2DM inadequately controlled on insulin-glargine, with or without metformin [mean ± SD age 62 ± 7 years, HbA1c 8.0% ± 0.9%, estimated glomerular filtration rate (GFR) 85 ± 12 mL/min/1.73 m² , median (IQR) urinary albumin/creatinine ratio 1.5 (0.9-3.0) mg/mmol]. After a standardised breakfast, GFR (primary endpoint) and effective renal plasma flow (ERPF) were determined by inulin and para-aminohippuric acid renal clearance, respectively, based on timed urine sampling. Intrarenal haemodynamic functions were estimated using Gomez equations.

RESULTS

Compared with iGlu, lixisenatide did not affect GFR [+0.1 mL/min/1.73 m² (95% CI -9 to 9)], ERPF [-17 mL/min/1.73 m² (-61 to 26)], other (intra-)renal haemodynamics or renal damage markers, but increased fractional sodium excretion [+0.25% (0.09-0.41)] and urinary pH [+0.7 (0.3-1.2)]. Plasma renin, angiotensin-II and aldosterone were unchanged. Lixisenatide and iGlu reduced HbA1c similarly, by 0.8% ± 0.1% and 0.6% ± 0.1%, respectively, while postprandial glucose was lower with lixisenatide (P = .002). Compared with iGlu, lixisenatide reduced bodyweight [-1.4 kg (-2.5 to -0.2)] and increased postprandial mean arterial pressure [+9 mm Hg (4-14)].

CONCLUSION

Eight-week lixisenatide treatment does not affect postprandial (intra-)renal haemodynamics compared with iGlu when added to insulin-glargine in patients with T2DM without overt nephropathy. Prolonged lixisenatide treatment has a sustained natriuretic effect, which is in contrast to previous reports on long-acting GLP-1RA, reduces body weight and increases postprandial blood pressure compared with iGlu.

TRIAL REGISTRATION

ClinicalTrials.gov identifier NCT02276196.

Efficacy and safety of lixisenatide in patients with type 2 diabetes and renal impairment

AIMS

This post hoc assessment evaluated the efficacy and safety of once-daily, prandial glucagon-like peptide-1 receptor agonist lixisenatide in patients with type 2 diabetes (T2D) and normal renal function (estimated glomerular filtration rate ≥90 mL/min), or mild (60-89 mL/min) or moderate (30-59 mL/min) renal impairment.

METHODS

Patients from 9 lixisenatide trials in the GetGoal clinical trial programme were categorized by baseline creatinine clearance: normal renal function (lixisenatide n = 2094, placebo n = 1150); renal impairment (mild: lixisenatide n = 637, placebo n = 414; moderate: lixisenatide n = 122, placebo n = 68). Meta-analyses of placebo-adjusted mean differences between baseline renal categories were performed for efficacy and safety outcomes.

RESULTS

HbA1c, 2-hour postprandial plasma glucose and fasting plasma glucose were comparably reduced in lixisenatide-treated patients with normal renal function, and mild and moderate renal impairment. The most common adverse events (AEs) in all renal function categories were gastrointestinal (GI), predominantly nausea and vomiting. A 14% higher incidence of GI AEs and a 10% higher incidence of nausea and vomiting were seen with mild impairment vs normal function (P = .003 for both), but no significant differences were observed between the mild and moderate impairment categories (P = .99 and P = .57, respectively), or between the moderate impairment and normal categories (P = .16 and P = .65, respectively). Additionally, the incidence of hypoglycaemia was similar in all categories.

CONCLUSIONS

This study demonstrates that baseline renal status does not affect efficacy outcomes in lixisenatide- vs placebo-treated patients, and that no lixisenatide dose adjustment is required for patients with T2D with mild or moderate renal impairment.

Activation and Inhibition of Sodium-Hydrogen Exchanger Is a Mechanism That Links the Pathophysiology and Treatment of Diabetes Mellitus With That of Heart Failure

The mechanisms underlying the progression of diabetes mellitus and heart failure are closely intertwined, such that worsening of one condition is frequently accompanied by worsening of the other; the degree of clinical acceleration is marked when the 2 coexist. Activation of the sodium-hydrogen exchanger in the heart and vasculature (NHE1 isoform) and the kidneys (NHE3 isoform) may serve as a common mechanism that links both disorders and may underlie their interplay. Insulin insensitivity and adipokine abnormalities (the hallmarks of type 2 diabetes mellitus) are characteristic features of heart failure; conversely, neurohormonal systems activated in heart failure (norepinephrine, angiotensin II, aldosterone, and neprilysin) impair insulin sensitivity and contribute to microvascular disease in diabetes mellitus. Each of these neurohormonal derangements may act through increased activity of both NHE1 and NHE3. Drugs used to treat diabetes mellitus may favorably affect the pathophysiological mechanisms of heart failure by inhibiting either or both NHE isoforms, and drugs used to treat heart failure may have beneficial effects on glucose tolerance and the complications of diabetes mellitus by interfering with the actions of NHE1 and NHE3. The efficacy of NHE inhibitors on the risk of cardiovascular events may be enhanced when heart failure and glucose intolerance coexist and may be attenuated when drugs with NHE inhibitory actions are given concomitantly. Therefore, the sodium-hydrogen exchanger may play a central role in the interplay of diabetes mellitus and heart failure, contribute to the physiological and clinical progression of both diseases, and explain certain drug-drug and drug-disease interactions that have been reported in large-scale randomized clinical trials.

GLP-1 and the kidney: from physiology to pharmacology and outcomes in diabetes

The gastrointestinal tract - the largest endocrine network in human physiology - orchestrates signals from the external environment to maintain neural and hormonal control of homeostasis. Advances in understanding entero-endocrine cell biology in health and disease have important translational relevance. The gut-derived incretin hormone glucagon-like peptide 1 (GLP-1) is secreted upon meal ingestion and controls glucose metabolism by modulating pancreatic islet cell function, food intake and gastrointestinal motility, amongst other effects. The observation that the insulinotropic actions of GLP-1 are reduced in type 2 diabetes mellitus (T2DM) led to the development of incretin-based therapies - GLP-1 receptor agonists and dipeptidyl peptidase 4 (DPP-4) inhibitors - for the treatment of hyperglycaemia in these patients. Considerable interest exists in identifying effects of these drugs beyond glucose-lowering, possibly resulting in improved macrovascular and microvascular outcomes, including in diabetic kidney disease. As GLP-1 has been implicated as a mediator in the putative gut-renal axis (a rapid-acting feed-forward loop that regulates postprandial fluid and electrolyte homeostasis), direct actions on the kidney have been proposed. Here, we review the role of GLP-1 and the actions of associated therapies on glucose metabolism, the gut-renal axis, classical renal risk factors, and renal end points in randomized controlled trials of GLP-1 receptor agonists and DPP-4 inhibitors in patients with T2DM.

Renal hemodynamic effects of glucagon-like peptide-1 agonist are mediated by nitric oxide but not prostaglandin

The incretin hormone, glucagon-like peptide-1 (GLP-1), is known for responding to dietary fat and carbohydrate. It elicits effects on pancreas, gut, and brain to stabilize blood glucose levels. We have previously reported that the GLP-1 agonist, exenatide, vasodilates the kidney and suppresses proximal reabsorption. The present study was undertaken to determine whether the renal effects of exenatide are mediated by nitric oxide (NO) and/or prostaglandins. Inulin clearance (glomerular filtration rate, GFR) and urine flow rate (UV) were measured in anesthetized rats before and during exenatide infusion (1 nmol/h iv). Animals were pretreated with cyclooxygenase (COX) inhibitor (meclofenamate), NO synthase (NOS) inhibitor (N^G-monomethyl-l-arginine, l-NMMA), NO clamp (l-NMMA + sodium nitroprusside), or placebo. Effectiveness of COX inhibition was tested by measuring urinary prostaglandin E₂ (UPGE₂). Effectiveness of NOS blockade and NO clamp was determined by urinary NO degradation products (UNOx). Exenatide increased GFR, UV, UPGE₂, and UNOx. Pretreatment with meclofenamate reduced UPGE₂ by 75% and reduced the effect of exenatide on UPGE₂ by 30% but did not modify the effects of exenatide on GFR or UV. Pretreatment with l-NMMA reduced UNOx and the impact of exenatide on GFR and UV by 50%. Pretreatment by NO clamp did not prevent UNOx from increasing during exenatide but blunted the effects of exenatide on GFR and UV. In conclusion, exenatide is a potent renal vasodilator and diuretic in the rat. These effects of exenatide are insensitive to COX inhibition but are mediated, in part, by NO.

Attenuated diuresis and natriuresis in response to glucagon-like peptide-1 in hypertensive rats are associated with lower expression of the glucagon-like peptide-1 receptor in the renal vasculature

Accumulating evidence from clinical and experimental studies indicates that the incretin glucagon-like peptide-1 (GLP-1) elicits blood-pressure lowering effects via its diuretic, natriuretic and vasodilatory properties. The present study investigated whether acute infusion of GLP-1 induces diuresis and natriuresis in spontaneously hypertensive rats (SHRs). Additionally, we examined whether GLP-1 influences the vascular reactivity of the renal arteries of normotensive and hypertensive rats and elucidated the underlying mechanisms. We found that the increase in urinary output and urinary sodium excretion in response to systemic infusion of GLP-1 for 30min in SHRs was much less pronounced than in normotensive rats. The diuretic and natriuretic actions of GLP-1 in normotensive rats were accompanied by increases in GFR and RBF and a reduction in RVR through activation of the cAMP signaling pathway. However, no changes in renal hemodynamics were observed in SHRs. Similarly, GLP-1 induced an endothelium-independent relaxation effect in the renal arteries of normotensive rats, whereas the renal vasculature of SHRs was unresponsive to this vasodilator. The absence of a GLP-1-induced renal artery vasodilator effect in SHRs was associated with lower expression of the GLP-1 receptor, blunted GLP-1-induced increases in cAMP production and higher activity and expression of the GLP-1 inactivating enzyme dipeptidyl peptidase IV relative to the renal arteries of normotensive rats. Collectively, these results demonstrate that the renal acute responses to GLP-1 are attenuated in SHRs. Thus, chronic treatment with incretin-based agents may rely upon the upregulation of GLP-1/GLP-1 receptor signaling in the kidneys of hypertensive patients and experimental models.

Angiotensin II counteracts the effects of cAMP/PKA on NHE3 activity and phosphorylation in proximal tubule cells

Binding of angiotensin II (ANG II) to the AT₁ receptor (AT₁R) in the proximal tubule stimulates Na⁺/H⁺ exchanger isoform 3 (NHE3) activity through multiple signaling pathways. However, the effects of ANG II/AT₁R-induced inihibitory G protein (G_i) activation and subsequent decrease in cAMP accumulation on NHE3 regulation are not well established. We therefore tested the hypothesis that ANG II reduces cAMP/PKA-mediated phosphorylation of NHE3 on serine 552 and, in doing so, stimulates NHE3 activity. Under basal conditions, ANG II stimulated NHE3 activity but did not affect PKA-mediated NHE3 phosphorylation at serine 552 in opossum kidney (OKP) cells. However, in the presence of the cAMP-elevating agent forskolin (FSK), ANG II blocked FSK-induced NHE3 inhibition, reduced intracellular cAMP concentrations, lowered PKA activity, and prevented the FSK-mediated increase in NHE3 serine 552 phosphorylation. All effects of ANG II were blocked by pretreating OKP cells with the AT₁R antagonist losartan, highlighting the contribution of the AT₁R/G_i pathway in ANG II-mediated NHE3 upregulation under cAMP-elevating conditions. Accordingly, G_i inhibition by pertussis toxin treatment decreased NHE3 activity both in vitro and in vivo and, more importantly, prevented the stimulatory effect of ANG II on NHE3 activity in rat proximal tubules. Collectively, our results suggest that ANG II counteracts the effects of cAMP/PKA on NHE3 phosphorylation and inhibition by activating the AT₁R/G_i pathway. Moreover, these findings support the notion that NHE3 dephosphorylation at serine 552 may represent a key event in the regulation of renal proximal tubule sodium handling by ANG II in the presence of natriuretic hormones that promote cAMP accumulation and transporter phosphorylation.

ISN Forefronts Symposium 2015: Maintaining Balance Under Pressure-Hypertension and the Proximal Tubule

Renal control of effective circulating volume (ECV) is key for circulatory performance. When renal sodium excretion is inadequate, blood pressure rises and serves as a homeostatic signal to drive natriuresis to re-establish ECV. Recognizing that hypertension involves both renal and vascular dysfunction, this report concerns proximal tubule sodium hydrogen exchanger 3 (NHE3) regulation during acute and chronic hypertension. NHE3 is distributed in tall microvilli (MV) in the proximal tubule, where it reabsorbs a significant fraction of the filtered sodium. NHE3 redistributes, in the plane of the MV membrane, between the MV body, where NHE3 is active, and the MV base, where NHE3 is less active. A high-salt diet and acute hypertension both retract NHE3 to the base and reduce proximal tubule sodium reabsorption independent of a change in abundance. The renin angiotensin system provokes NHE3 redistribution independent of blood pressure: The angiotensin-converting enzyme (ACE) inhibitor captopril redistributes NHE3 to the base and subsequent angiotensin II (AngII) infusion returns NHE3 to the body of the MV and restores reabsorption. Chronic AngII infusion presents simultaneous AngII stimulation and hypertension; that is, NHE3 remains in the body of the MV, due to the high local AngII level and inflammation, and exhibits a compensatory decrease in abundance driven by the hypertension. Genetically modified mice with blunted hypertensive responses to chronic AngII infusion (due to lack of the proximal tubule AngII receptors interleukin-17A or interferon-γ expression) exhibit reduced local AngII accumulation and inflammation and larger decreases in NHE3 abundance, which improves the pressure natriuresis response and reduces the need for elevated blood pressure to facilitate circulating volume balance.

Dipeptidyl Peptidase IV Inhibition Exerts Renoprotective Effects in Rats with Established Heart Failure

Circulating dipeptidyl peptidase IV (DPPIV) activity is associated with worse cardiovascular outcomes in humans and experimental heart failure (HF) models, suggesting that DPPIV may play a role in the pathophysiology of this syndrome. Renal dysfunction is one of the key features of HF, but it remains to be determined whether DPPIV inhibitors are capable of improving cardiorenal function after the onset of HF. Therefore, the present study aimed to test the hypothesis that DPPIV inhibition by vildagliptin improves renal water and salt handling and exerts anti-proteinuric effects in rats with established HF. To this end, male Wistar rats were subjected to left ventricle (LV) radiofrequency ablation or sham operation. Six weeks after surgery, radiofrequency-ablated rats who developed HF were randomly divided into two groups and treated for 4 weeks with vildagliptin (120 mg/kg/day) or vehicle by oral gavage. Echocardiography was performed before (pretreatment) and at the end of treatment (post-treatment) to evaluate cardiac function. The fractional area change (FAC) increased (34 ± 5 vs. 45 ± 3%, p < 0.05), and the isovolumic relaxation time decreased (33 ± 2 vs. 27 ± 1 ms; p < 0.05) in HF rats treated with vildagliptin (post-treatment vs. pretreatment). On the other hand, cardiac dysfunction deteriorated further in vehicle-treated HF rats. Renal function was impaired in vehicle-treated HF rats as evidenced by fluid retention, low glomerular filtration rate (GFR) and high levels of urinary protein excretion. Vildagliptin treatment restored urinary flow, GFR, urinary sodium and urinary protein excretion to sham levels. Restoration of renal function in HF rats by DPPIV inhibition was associated with increased active glucagon-like peptide-1 (GLP-1) serum concentration, reduced DPPIV activity and increased activity of protein kinase A in the renal cortex. Furthermore, the anti-proteinuric effect of vildagliptin treatment in rats with established HF was associated with upregulation of the apical proximal tubule endocytic receptor megalin and of the podocyte main slit diaphragm proteins nephrin and podocin. Collectively, these findings demonstrate that DPPIV inhibition exerts renoprotective effects and ameliorates cardiorenal function in rats with established HF. Long-term studies with DPPIV inhibitors are needed to ascertain whether these effects ultimately translate into improved clinical outcomes.