Vasoactive drugs enhance pancreatic islet blood flow, augment insulin secretion and improve glucose tolerance in female rats

The Role of Renin-Angiotensin System in Diabetic Cardiomyopathy: A Narrative Review

Diabetic cardiomyopathy refers to myocardial dysfunction in type 2 diabetes, but without the traditional cardiovascular risk factors or overt clinical atherosclerosis and valvular disease. The activation of the renin-angiotensin system (RAS), oxidative stress, lipotoxicity, maladaptive immune responses, imbalanced mitochondrial dynamics, impaired myocyte autophagy, increased myocyte apoptosis, and fibrosis contribute to diabetic cardiomyopathy. This review summarizes the studies that address the link between cardiomyopathy and the RAS in humans and presents proposed pathophysiological mechanisms underlying this association. The RAS plays an important role in the development and progression of diabetic cardiomyopathy. The over-activation of the classical RAS axis in diabetes leads to the increased production of angiotensin (Ang) II, angiotensin type 1 receptor activation, and aldosterone release, contributing to increased oxidative stress, fibrosis, and cardiac remodeling. In contrast, Ang-(1-7) suppresses oxidative stress, inhibits tissue fibrosis, and prevents extensive cardiac remodeling. Angiotensin-converting-enzyme (ACE) inhibitors and angiotensin receptor blockers improve heart functioning and reduce the occurrence of diabetic cardiomyopathy. Experimental studies also show beneficial effects for Ang-(1-7) and angiotensin-converting enzyme 2 infusion in improving heart functioning and tissue injury. Further research is necessary to fully understand the pathophysiology of diabetic cardiomyopathy and to translate experimental findings into clinical practice.

Insulinotropic Effects of Neprilysin and/or Angiotensin Receptor Inhibition in Mice

Treatment of heart failure with the angiotensin receptor-neprilysin inhibitor sacubitril/valsartan improved glycemic control in individuals with type 2 diabetes. The relative contribution of neprilysin inhibition versus angiotensin II receptor antagonism to this glycemic benefit remains unknown. Thus, we sought to determine the relative effects of the neprilysin inhibitor sacubitril versus the angiotensin II receptor blocker valsartan on beta-cell function and glucose homeostasis in a mouse model of reduced first-phase insulin secretion, and whether any beneficial effects are additive/synergistic when combined in sacubitril/valsartan. High fat-fed C57BL/6J mice treated with low-dose streptozotocin (or vehicle) were followed for eight weeks on high fat diet alone or supplemented with sacubitril, valsartan or sacubitril/valsartan. Body weight and fed glucose levels were assessed weekly. At the end of the treatment period, insulin release in response to intravenous glucose, insulin sensitivity, and beta-cell mass were determined. Sacubitril and valsartan, but not sacubitril/valsartan, lowered fasting and fed glucose levels and increased insulin release in diabetic mice. None of the drugs altered insulin sensitivity or beta-cell mass, but all reduced body weight gain. Effects of the drugs on insulin release were reproduced in angiotensin II-treated islets from lean C57BL/6J mice, suggesting the insulin response to each of the drugs is due to a direct effect on islets and mechanisms therein. In summary, sacubitril and valsartan each exert beneficial insulinotropic, glycemic and weight-reducing effects in obese and/or diabetic mice when administered alone; however, when combined, mechanisms within the islet contribute to their inability to enhance insulin release.

Telmisartan Potentiates Insulin Secretion via Ion Channels, Independent of the AT1 Receptor and PPARγ

Angiotensin II type 1 (AT1) receptor blockers (ARBs), as antihypertensive drugs, have drawn attention for their benefits to individuals with diabetes and prediabetes. However, the direct effects of ARBs on insulin secretion remain unclear. In this study, we aimed to investigate the insulinotropic effect of ARBs and the underlying electrophysiological mechanism. We found that only telmisartan among the three ARBs (telmisartan, valsartan, and irbesartan) exhibited an insulin secretagogue role in rat islets. Independent of AT1 receptor and peroxisome proliferator-activated receptor γ (PPARγ), telmisartan exerted effects on ion channels including voltage-dependent potassium (Kv) channels and L-type voltage-gated calcium channels (VGCCs) to promote extracellular Ca²⁺ influx, thereby potentiating insulin secretion in a glucose-dependent manner. Furthermore, we identified that telmisartan directly inhibited Kv2.1 channel on a Chinese hamster ovary cell line with Kv2.1 channel overexpression. Acute exposure of db/db mice to a telmisartan dose equivalent to therapeutic doses in humans resulted in lower blood glucose and increased plasma insulin concentration in OGTT. We further observed the telmisartan-induced insulinotropic and electrophysiological effects on pathological pancreatic islets or β-cells isolated from db/db mice. Collectively, our results establish an important insulinotropic function of telmisartan distinct from other ARBs in the treatment of diabetes.

ACE2 and energy metabolism: the connection between COVID-19 and chronic metabolic disorders

The renin-angiotensin system (RAS) has currently attracted increasing attention due to its potential function in regulating energy homeostasis, other than the actions on cellular growth, blood pressure, fluid, and electrolyte balance. The existence of RAS is well established in metabolic organs, including pancreas, liver, skeletal muscle, and adipose tissue, where activation of angiotensin-converting enzyme (ACE) - angiotensin II pathway contributes to the impairment of insulin secretion, glucose transport, fat distribution, and adipokines production. However, the activation of angiotensin-converting enzyme 2 (ACE2) - angiotensin (1-7) pathway, a novel branch of the RAS, plays an opposite role in the ACE pathway, which could reverse these consequences by improving local microcirculation, inflammation, stress state, structure remolding, and insulin signaling pathway. In addition, new studies indicate the protective RAS arm possesses extraordinary ability to enhance brown adipose tissue (BAT) activity and induces browning of white adipose tissue, and consequently, it leads to increased energy expenditure in the form of heat instead of ATP synthesis. Interestingly, ACE2 is the receptor of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is threating public health worldwide. The main complications of SARS-CoV-2 infected death patients include many energy metabolism-related chronic diseases, such as diabetes. The specific mechanism leading to this phenomenon is largely unknown. Here, we summarize the latest pharmacological and genetic tools on regulating ACE/ACE2 balance and highlight the beneficial effects of the ACE2 pathway axis hyperactivity on glycolipid metabolism, as well as the thermogenic modulation.

Emerging Roles of Vascular Endothelium in Metabolic Homeostasis

Our understanding of the role of the vascular endothelium has evolved over the past 2 decades, with the recognition that it is a dynamically regulated organ and that it plays a nodal role in a variety of physiological and pathological processes. Endothelial cells (ECs) are not only a barrier between the circulation and peripheral tissues, but also actively regulate vascular tone, blood flow, and platelet function. Dysregulation of ECs contributes to pathological conditions such as vascular inflammation, atherosclerosis, hypertension, cardiomyopathy, retinopathy, neuropathy, and cancer. The close anatomic relationship between vascular endothelium and highly vascularized metabolic organs/tissues suggests that the crosstalk between ECs and these organs is vital for both vascular and metabolic homeostasis. Numerous reports support that hyperlipidemia, hyperglycemia, and other metabolic stresses result in endothelial dysfunction and vascular complications. However, how ECs may regulate metabolic homeostasis remains poorly understood. Emerging data suggest that the vascular endothelium plays an unexpected role in the regulation of metabolic homeostasis and that endothelial dysregulation directly contributes to the development of metabolic disorders. Here, we review recent studies about the pivotal role of ECs in glucose and lipid homeostasis. In particular, we introduce the concept that the endothelium adjusts its barrier function to control the transendothelial transport of fatty acids, lipoproteins, LPLs (lipoprotein lipases), glucose, and insulin. In addition, we summarize reports that ECs communicate with metabolic cells through EC-secreted factors and we discuss how endothelial dysregulation contributes directly to the development of obesity, insulin resistance, dyslipidemia, diabetes mellitus, cognitive defects, and fatty liver disease.

Pancreatic Blood Flow with Special Emphasis on Blood Perfusion of the Islets of Langerhans

The pancreatic islets are more richly vascularized than the exocrine pancreas, and possess a 5- to 10-fold higher basal and stimulated blood flow, which is separately regulated. This is reflected in the vascular anatomy of the pancreas where islets have separate arterioles. There is also an insulo-acinar portal system, where numerous venules connect each islet to the acinar capillaries. Both islets and acini possess strong metabolic regulation of their blood perfusion. Of particular importance, especially in the islets, is adenosine and ATP/ADP. Basal and stimulated blood flow is modified by local endothelial mediators, the nervous system as well as gastrointestinal hormones. Normally the responses to the nervous system, especially the parasympathetic and sympathetic nerves, are fairly similar in endocrine and exocrine parts. The islets seem to be more sensitive to the effects of endothelial mediators, especially nitric oxide, which is a permissive factor to maintain the high basal islet blood flow. The gastrointestinal hormones with pancreatic effects mainly influence the exocrine pancreatic blood flow, whereas islets are less affected. A notable exception is incretin hormones and adipokines, which preferentially affect islet vasculature. Islet hormones can influence both exocrine and endocrine blood vessels, and these complex effects are discussed. Secondary changes in pancreatic and islet blood flow occur during several conditions. To what extent changes in blood perfusion may affect the pathogenesis of pancreatic diseases is discussed. Both type 2 diabetes mellitus and acute pancreatitis are conditions where we think there is evidence that blood flow may contribute to disease manifestations. © 2019 American Physiological Society. Compr Physiol 9:799-837, 2019.

Reversal of angiotensin ll-induced β-cell dedifferentiation via inhibition of NF-κb signaling

Background

Type 2 diabetes mellitus (T2DM) is characterized by pancreatic β-cell failure, which arises from metabolic stress and results in β cell dedifferentiation, leading to β-cell death. Pathological activation of the renin–angiotensin system (RAS) contributes to increase cell stress, while RAS intervention reduces the onset of T2DM in high-risk populations and promotes insulin secretion in rodents. In this study, we investigated whether and how RAS induces β-cell dedifferentiation and the mechanism underlying this process.

Methods

In vitro, with the methods of quantitative real-time reverse transcriptase-PCR (qRT-PCR) and western blotting, we examined the change of cell identity-related gene expression, progenitor like gene expression, cellular function, and nuclear factor kappa b (NF-κb) signaling activity in β cell lines after exposure to angiotensin II (AngII) and disruption of RAS. In vivo, parallel studies were performed using db/db mice. Related protein expression was detected by Immunofluorescence analysis.

Result

Activation of RAS induced dedifferentiation and impaired insulin secretion, eventually leading to β-cell failure. Mechanistically, Angll induced β-cell dedifferentiation via NF-κb signaling, while treatment with lrbesartan and sc-514 reversed the progenitor state of β cells.

Conclusion

The present study found that RAS might induce β-cell dedifferentiation via angiotensin II receptor type 1 activation, which was promoted by NF-κb signaling. Therefore, blocking RAS or NF-kb signaling efficiently reversed the dedifferentiated status of β cells, suggesting a potential therapy for patients with type 2 diabetes.

Electronic supplementary material

The online version of this article (10.1186/s10020-018-0044-3) contains supplementary material, which is available to authorized users.

Contrast-enhanced ultrasound measurement of pancreatic blood flow dynamics predicts type 1 diabetes progression in preclinical models

In type 1 diabetes (T1D), immune-cell infiltration into the islets of Langerhans (insulitis) and β-cell decline occurs many years before diabetes clinically presents. Non-invasively detecting insulitis and β-cell decline would allow the diagnosis of eventual diabetes, and provide a means to monitor therapeutic intervention. However, there is a lack of validated clinical approaches for specifically and non-invasively imaging disease progression leading to T1D. Islets have a denser microvasculature that reorganizes during diabetes. Here we apply contrast-enhanced ultrasound measurements of pancreatic blood-flow dynamics to non-invasively and predictively assess disease progression in T1D pre-clinical models. STZ-treated mice, NOD mice, and adoptive-transfer mice demonstrate altered islet blood-flow dynamics prior to diabetes onset, consistent with islet microvasculature reorganization. These assessments predict both time to diabetes onset and future responders to antiCD4-mediated disease prevention. Thus contrast-enhanced ultrasound measurements of pancreas blood-flow dynamics may provide a clinically deployable predictive marker for disease progression in pre-symptomatic T1D and therapeutic reversal.

Sodium-glucose cotransporter-2 inhibition and acidosis in patients with type 2 diabetes: a review of US FDA data and possible conclusions

Objective

To evaluate whether adverse event reports to the US Food and Drug Administration on incidents of ketoacidosis from use of sodium glucose cotransport inhibitors (SGLT2 inhibitors) provide insight into ways this new class of drugs is being prescribed with other antihyperglycemic agents; to examine possible mechanisms to explain ketoacidosis.

Design and methods

Reports of adverse events concerned to SGLT2 inhibitors, namely, empagliflozin, dapagliflozin, and canagliflozin were obtained under the Freedom of Information Act for 5 years ending in August 31, 2015. The data were evaluated for incidents of ketoacidosis by looking for keywords such as diabetic ketoacidosis, ketoacidosis, lactic acidosis, acidosis, and metabolic acidosis. Results were tabulated individually for empagliflozin (n=260 adverse event reports), dapagliflozin (n=520), and canagliflozin (n=2159). Adverse events were categorized according to age, gender, and insulin use.

Results

There were 46, 144, and 450 reports of ketoacidosis concerned with the use of empagliflozin, dapagliflozin, and canagliflozin, respectively. The use of SGLT2 inhibitors was not strictly limited to patients with type 2 diabetes but was cut across categories of insulin use, including a total of 172 cases of SGLT2-related ketoacidosis in individuals above the age of 40 who were not on insulin.

Conclusion

Further studies should focus to detect pleiotropic effects of SGLT2 inhibitors, particularly with other oral antihyperglycemic drugs or insulin. A review of the literature suggests that patients with type 2 diabetes with low C-peptide level may be at increased risk of ketoacidosis, particularly if they are on statins and diuretics due to hypokalemia and impaired release of insulin. More studies are warranted to further clarify these mechanisms.

Angiotensin II regulates islet microcirculation and insulin secretion in mice

OBJECTIVE

Angiotensin II causes potent increases in systemic and local pressure through its vasoconstrictive effect. Despite the importance of angiotensin II for local blood flow regulation, whether angiotensin II regulates the pancreatic islet microcirculation remains incompletely understood. We hypothesized that angiotensin II directly regulates the pancreatic islet microcirculation and thereby regulates insulin secretion. The aims of this study were to develop a new technique to visualize pancreatic islet hemodynamic changes in vivo and to analyze changes in islet circulation induced by angiotensin II or an angiotensin type 1 receptor blocker.

METHODS

Using an in vivo imaging method, we observed the pancreatic islet microcirculation. Various doses of angiotensin II or an angiotensin type 1 receptor blocker were injected intravenously, and changes in islet microcirculation were observed. Glucose-stimulated insulin secretion from the pancreas was measured from the hepatic portal vein.

RESULTS

We identified islet microcirculation using a fluorescent dye. Angiotensin II significantly induced blood vessel contraction in the islets in a dose-dependent manner. In contrast, the angiotensin type 1 receptor blocker induced vasodilation. Glucose-stimulated insulin secretion was decreased by angiotensin II infusion.

CONCLUSIONS

These results show that angiotensin II is involved in the regulation of pancreatic islet microcirculation and insulin secretion.

The effect of renin angiotensin system genetic variants in acute pancreatitis

OBJECTIVES

We sought association of genetic variants in the renin-angiotensin system (RAS) and vitamin D system with acute pancreatitis (AP) development and severity.

BACKGROUND

The endocrine RAS is involved in circulatory homeostasis through the pressor action of angiotensin II at its AT1 receptor. However, local RAS regulate growth and inflammation in diverse cells and tissues, and their activity may be suppressed by vitamin D. Intrapancreatic angiotensin II generation has been implicated in the development of AP.

METHODS

Five hundred forty-four white patients with AP from 3 countries (United Kingdom, 22; Germany, 136; and The Netherlands 386) and 8487 control subjects (United Kingdom 7833, The Netherlands 717) were genotyped for 8 polymorphisms of the RAS/vitamin D systems, chosen on the basis of likely functionality.

RESULTS

The angiotensin-converting enzyme I (rather than D) allele was significantly associated with alcohol-related AP when all cohorts were combined (P = 0.03). The renin rs5707 G (rather than A) allele was associated with AP (P = 0.002), infected necrosis (P = 0.025) and mortality (P = 0.046).

CONCLUSIONS

The association of 2 RAS polymorphisms with AP suggests the need for further detailed analysis of the role of RAS/vitamin D in the genesis or severity of AP, particularly given the ready potential for pharmacological manipulation of this system using existing marketed agents. However, further replication studies will be required before any such association is considered robust, particularly given the significant heterogeneity of AP causation and clinical course.

Insulin receptor substrate-2 (Irs2) in endothelial cells plays a crucial role in insulin secretion

Endothelial cells are considered to be essential for normal pancreatic β-cell function. The current study attempted to demonstrate the role of insulin receptor substrate-2 (Irs2) in endothelial cells with regard to insulin secretion. Endothelial cell-specific Irs2 knockout (ETIrs2KO) mice exhibited impaired glucose-induced, arginine-induced, and glucagon-induced insulin secretion and showed glucose intolerance. In batch incubation and perifusion experiments using isolated islets, glucose-induced insulin secretion was not significantly different between the control and the ETIrs2KO mice. In contrast, in perfusion experiments, glucose-induced insulin secretion was significantly impaired in the ETIrs2KO mice. The islet blood flow was significantly impaired in the ETIrs2KO mice. After the treatment of these knockout mice with enalapril maleate, which improved the islet blood flow, glucose-stimulated insulin secretion was almost completely restored to levels equal to those in the control mice. These data suggest that Irs2 deletion in endothelial cells leads to a decreased islet blood flow, which may cause impaired glucose-induced insulin secretion. Thus, Irs2 in endothelial cells may serve as a novel therapeutic target for preventing and ameliorating type 2 diabetes and metabolic syndrome.

Angiotensin 1-7: a peptide for preventing and treating metabolic syndrome

Angiotensin-(1-7) is one of the most important active peptides of the renin-angiotensin system (RAS) with recognized cardiovascular relevance; however several studies have shown the potential therapeutic role of Ang-(1-7) on treating and preventing metabolic disorders as well. This peptide achieves a special importance considering that in the last few decades obesity and metabolic syndrome (MS) have become a growing worldwide health problem. Angiotensin (Ang) II is the most studied component of RAS and is increased during obesity, diabetes and dyslipidemia (MS); some experimental evidence has shown that Ang II modulates appetite and metabolism as well as mechanisms that induce adipose tissue growth and metabolism in peripheral organs. Recent articles demonstrated that Ang-(1-7)/Mas axis modulates lipid and glucose metabolism and counterregulates the effects of Ang II. Based on these data, angiotensin-converting enzyme 2 (ACE2)/Ang-(1-7)/Mas pathway activation have been advocated as a new tool for treating metabolic diseases. This review summarizes the new evidence from animal and human experiments indicating the use of Ang-(1-7) in prevention and treatment of obesity and metabolic disorders.

Acute regulation of pancreatic islet microcirculation and glycaemia by telmisartan and ramipril: discordant effects between normal and Type 2 diabetic rats

Diabetic patients are often treated with an ACEi (angiotensin-converting enzyme inhibitor) or angiotensin receptor antagonist against hypertension or albuminuria. These drugs also have a positive impact on glucose tolerance, but the mechanism for this remains elusive. Hypothesizing a positive non-additive effect, we studied whether the angiotensin receptor antagonist telmisartan or the ACEi ramipril acutely influence insulin secretion and glycaemia in vivo in healthy and Type 2 diabetic rats through effects on islet blood perfusion. Telmisartan and ramipril were injected intravenously into anaesthetized non-diabetic Wistar rats or Type 2 diabetic GK (Goto–Kakizaki) rats. In non-diabetic Wistar rats, neither whole PBF (pancreatic blood flow) nor IBF (islet blood flow) were significantly influenced by telmisartan and ramipril, alone or in combination. Renal blood flow was enhanced significantly by telmisartan and ramipril when used in combination, whereas ABF (adrenal blood flow) was not affected by any of the drugs. Telmisartan and ramipril both significantly increased serum insulin levels, but did not influence glycaemia. In Type 2 diabetic GK rats, both whole PBF and IBF were significantly decreased by telmisartan and ramipril, but only when used in combination. Renal blood flow was enhanced significantly by telmisartan and ramipril alone, but not when used in combination, whereas ABF was not affected by any of the drugs. Telmisartan and ramipril both significantly decreased serum insulin levels, and non-additively elevated blood glucose levels. In conclusion, the present study suggests that a local pancreatic RAS (renin–angiotensin system), sensitive to acute administration of telmisartan and ramipril, controls pancreatic IBF and insulin secretion and thereby has an impact on glucose tolerance. Our findings indicate unexpected significant differences in the effects of these agents on islet microcirculation, in vivo insulin secretion and glycaemia between healthy and Type 2 diabetic rats.

The renin angiotensin aldosterone system and insulin resistance in humans

Alterations in the renin angiotensin aldosterone system (RAAS) contribute to the underlying pathophysiology of insulin resistance in humans; however, individual differences in the treatment response of insulin resistance to RAAS blockade persist. Thus, understanding inter-individual differences in the relationship between the RAAS and insulin resistance may provide insights into improved personalized treatments and improved outcomes. The effects of the systemic RAAS on blood pressure regulation and glucose metabolism have been studied extensively; however, recent discoveries on the influence of local tissue RAAS in the skeletal muscle, heart, vasculature, adipocytes, and pancreas have led to an improved understanding of how activated tissue RAAS influences the development of insulin resistance and diabetes in humans. Angiotensin II (ANGII) is the predominant RAAS component contributing to insulin resistance; however, other players such as aldosterone, renin, and ACE2 are also involved. This review examines the role of local ANGII activity on insulin resistance development in skeletal muscle, adipocytes, and pancreas, followed by a discussion of the other RAAS components implicated in insulin resistance, including ACE2, Ang1-7, renin, and aldosterone.

Pancreatic angiotensin-converting enzyme 2 improves glycemia in angiotensin II-infused mice

An overactive renin-angiotensin system (RAS) is known to contribute to type 2 diabetes mellitus (T2DM). Although ACE2 overexpression has been shown to be protective against the overactive RAS, a role for pancreatic ACE2, particularly in the islets of Langerhans, in regulating glycemia in response to elevated angiotensin II (Ang II) levels remains to be elucidated. This study examined the role of endogenous pancreatic ACE2 and the impact of elevated Ang II levels on the enzyme's ability to alleviate hyperglycemia in an Ang II infusion mouse model. Male C57bl/6J mice were infused with Ang II or saline for a period of 14 days. On the 7th day of infusion, either an adenovirus encoding human ACE2 (Ad-hACE2) or a control adenovirus (Ad-eGFP) was injected into the mouse pancreas. After an additional 7-8 days, glycemia and plasma insulin levels as well as RAS components expression and oxidative stress were assessed. Ang II-infused mice exhibited hyperglycemia, hyperinsulinemia, and impaired glucose-stimulated insulin secretion from pancreatic islets compared with control mice. This phenotype was associated with decreased ACE2 expression and activity, increased Ang II type 1 receptor (AT1R) expression, and increased oxidative stress in the mouse pancreas. Ad-hACE2 treatment restored pancreatic ACE2 expression and compensatory activity against Ang II-mediated impaired glycemia, thus improving β-cell function. Our data suggest that decreased pancreatic ACE2 is a link between overactive RAS and impaired glycemia in T2DM. Moreover, maintenance of a normal endogenous ACE2 compensatory activity in the pancreas appears critical to avoid β-cell dysfunction, supporting a therapeutic potential for ACE2 in controlling diabetes resulting from an overactive RAS.

Inorganic nitrite stimulates pancreatic islet blood flow and insulin secretion

Reactive nitrogen and oxygen species have been proposed to be involved in control of insulin release from the pancreatic β cell. Recent evidence suggests that the supposedly inert anions nitrate and nitrite are metabolized in blood and tissues to form nitric oxide (NO) and other bioactive nitrogen oxides. Here we present evidence for a novel stimulatory role of nitrite in influencing pancreatic islet physiology via a dual mechanism, involving both indirect enhancement (through microcirculation redistribution) and direct insulinotropic effects on the β cell. In rats, intraperitoneal injection of sodium nitrite increased pancreatic islet blood flow by 50% and serum insulin concentrations by 30%, while whole pancreatic blood flow and glycemia remained unaffected. Nitrite also dose dependently enhanced insulin secretion from rat β cells in vitro under nonstimulatory glucose concentrations. This effect was not mimicked by nitrate and was abolished by the guanylyl cyclase (GC) inhibitor ODQ and the NO scavenger cPTIO. It was also mimicked by a cyclic GMP agonist (8-CPT-cGMP) and a classical NO donor (NONOate). Interestingly, a reactive oxygen species scavenger (vitamin E analog, Trolox) abolished the insulin secretion induced by nitrite. We conclude that nitrite exerts dual stimulatory effects on pancreatic islet function, including enhancement of islet blood flow and subsequent insulin secretion in vivo and direct stimulation of insulin release in vitro. The insulinotropic effect of nitrite is cGMP-dependent and involves formation of reactive nitrogen and oxygen species.

Does interference with the renin-angiotensin system protect against diabetes? Evidence and mechanisms

Agents interfering with the renin-angiotensin system (RAS) were consistently shown to lower the incidence of type 2 diabetes mellitus (T2DM), as compared to other antihypertensive drugs, in hypertensive high-risk populations. The mechanisms underlying this protective effect of RAS blockade using angiotensin-converting enzyme inhibitors or angiotensin-receptor blockers on glucose metabolism are not fully understood. In this article, we will review the evidence from randomized controlled trials and discuss the proposed mechanisms as to how RAS interference may delay the onset of T2DM. In particular, as T2DM is characterized by β-cell dysfunction and obesity-related insulin resistance, we address the mechanisms that underlie RAS blockade-induced improvement in β-cell function and insulin sensitivity.

Early intervention with liraglutide improves glucose tolerance without affecting islet microcirculation in young Goto-Kakizaki rats

Liraglutide, an analog of glucagon-like peptide-1 (GLP-1), is an effective anti-diabetic agent with few side effects. Since native GLP-1 exerts vascular effects, we investigated changes in pancreatic islet blood flow using a non-radioactive microsphere technique, as well as insulin concentration and glucose tolerance after 17 day treatment with liraglutide in 6-week-old Goto-Kakizaki (GK) rats. Compared to saline-treated control GK rats, liraglutide limited body weight gain, decreased glycemia, improved glucose tolerance and lowered serum insulin concentration. Neither pancreatic or islet blood flow, nor pancreatic insulin content, was affected by liraglutide treatment. We conclude that early intervention with liraglutide decreases glycemia and improves glucose tolerance, thus halting the natural progression towards diabetes, without affecting islet microcirculation or pancreatic insulin content in young female GK rats.

Review article: pancreatic renin-angiotensin systems in health and disease

BACKGROUND

In addition to the circulating (endocrine) renin-angiotensin system (RAS), local renin-angiotensin systems are now known to exist in diverse cells and tissues. Amongst these, pancreatic renin-angiotensin systems have recently been identified and may play roles in the physiological regulation of pancreatic function, as well as being implicated in the pathogenesis of pancreatic diseases including diabetes, pancreatitis and pancreatic cancer.

AIM

To review and summarise current knowledge of pancreatic renin-angiotensin systems.

METHODS

We performed an extensive PubMed, Medline and online review of all relevant literature.

RESULTS

Pancreatic RAS appear to play various roles in the regulation of pancreatic physiology and pathophysiology. Ang II may play a role in the development of pancreatic ductal adenocarcinoma, via stimulation of angiogenesis and prevention of chemotherapy toxicity, as well as in the initiation and propagation of acute pancreatitis (AP); whereas, RAS antagonism is capable of preventing new-onset diabetes and improving glycaemic control in diabetic patients. Current evidence for the roles of pancreatic RAS is largely based upon cell and animal models, whilst definitive evidence from human studies remains lacking.

CONCLUSIONS

The therapeutic potential for RAS antagonism, using cheap and widely available agents, and may be untapped and such roles are worthy of active investigation in diverse pancreatic disease states.

The impact of ANG II and IV on INS-1 cells and on blood glucose and plasma insulin

The impact of angiotensin (ANG) for peripheral, global effects is well known. Local ANG systems including that of the insulin-releasing beta cell are not well investigated. In insulin-secreting cell line (INS-1), AT(1) and AT(4) receptors for ANG II and IV were demonstrated by Western blots. Only small amounts of ANG II-binding sites of low affinity were observed. ANG II and SARILE displaced binding of (125)I-ANG II. ANG II and IV as well as their non-degradable analogs SARILE and Nle-ANG IV increased the glucose-induced insulin release in a bell-shaped way; the maximum effect was at approximately 1 nM. The increase was antagonized by 1 microM losartan or 10 microM divalinal (AT(1) and AT(4) receptor antagonists, respectively). The insulin release was accompanied by a (45)Ca(2+) uptake in the case of ANG II and ANG IV. Divalinal abolished the effect of ANG IV and Nle-ANG IV on this parameter. ANG IV reduced the increase in blood glucose during a glucose tolerance test with corresponding, albeit smaller effects on plasma insulin. Using confocal laser scanning microscopy, transfected insulin-regulated aminopeptidase (IRAP) with AT(4) receptors was shown to be accumulated close to the nucleus and the cytosolic membrane, whereas GLUT4 was not detectable. IRAP was inhibited by ANG IV. In conclusion, AT(1) and AT(4) receptors may be involved in diabetic homeostasis. Effects are mediated by insulin release, which is accompanied by an influx of extracellular Ca(2+). The impact of ANG IV/IRAP agonists may be worth being used as antidiabetics.

Valsartan protects pancreatic islets and adipose tissue from the inflammatory and metabolic consequences of a high-fat diet in mice

Obesity, hypertension, cardiovascular disease, and inflammation are closely associated with the rising incidence of diabetes mellitus. One pharmacological target that may have significant potential to lower the risk of obesity-related diseases is the angiotensin type 1 receptor (AT1R). We examined the hypothesis that the AT1R blocker valsartan reduces the metabolic consequences and inflammatory effects of a high-fat (Western) diet in mice. C57BL/6J mice were treated by oral gavage with 10 mg/kg per day of valsartan or vehicle and placed on either a standard chow or Western diet for 12 weeks. Western diet-fed mice given valsartan had improved glucose tolerance, reduced fasting blood glucose levels, and reduced serum insulin levels compared with mice fed a Western diet alone. Valsartan treatment also blocked Western diet-induced increases in serum levels of the proinflammatory cytokines interferon-gamma and monocyte chemotactic protein 1. In the pancreatic islets, valsartan enhanced mitochondrial function and prevented Western diet-induced decreases in glucose-stimulated insulin secretion. In adipose tissue, valsartan reduced Western diet-induced macrophage infiltration and expression of macrophage-derived monocyte chemotactic protein 1. In isolated adipocytes, valsartan treatment blocked or attenuated Western diet-induced changes in expression of several key inflammatory signals: interleukin 12p40, interleukin 12p35, tumor necrosis factor-alpha, interferon-gamma, adiponectin, platelet 12-lipoxygenase, collagen 6, inducible NO synthase, and AT1R. Our findings indicate that AT1R blockade with valsartan attenuated several deleterious effects of the Western diet at the systemic and local levels in islets and adipose tissue. This study suggests that AT1R blockers provide additional therapeutic benefits in the metabolic syndrome and other obesity-related disorders beyond lowering blood pressure.

Sardine peptide with angiotensin I-converting enzyme inhibitory activity improves glucose tolerance in stroke-prone spontaneously hypertensive rats

An enzymatic hydrolysate of sardine protein (sardine peptide, SP) derived from sardine muscle possesses angiotensin I-converting enzyme (ACE) inhibitory activity. In the present study, we investigated the effect of SP on the blood glucose levels in stroke-prone spontaneously hypertensive rats (SHRSPs). Ten-week-old SHRSPs were assigned to three groups. The control group was given tap water for 4 weeks, while the experimental groups were given water containing SP (1 g/kg/d) or an ACE inhibitor, captopril (8 mg/kg/d). Treatment with SP and captopril decreased ACE activity in the kidney, aorta, and mesentery. There were no differences in fasting blood glucose levels among the three groups, whereas SP and captopril administration significantly suppressed the increase in blood glucose after glucose loading in the control SHRSPs. No difference was observed in plasma insulin levels among the three groups. Thus treatment with captopril and ACE-inhibitory sardine peptides ameliorated the glucose tolerance of this rat strain.

Gender-specific regulation of pancreatic islet blood flow, insulin levels and glycaemia in spontaneously diabetic Goto-Kakizaki rats

Patients with diabetes are often treated with a statin for hyperlipidaemia and an ACE (angiotensin-converting enzyme) inhibitor or angiotensin receptor antagonist for hypertension or albuminuria. These drugs may also exert beneficial metabolic effects, causing improved glucose tolerance in patients. Gender-related differences have also been observed in the clinical responsiveness to these drugs, but the mechanism behind this is unclear. In the present study, we have investigated whether these drugs and the fatty acid palmitate influence the pancreatic microcirculation, thereby having an impact on insulin secretion and glycaemia in vivo, in spontaneously diabetic male and female Goto-Kakizaki rats. In male rats, pancreatic IBF (islet blood flow) and total PBF (pancreatic blood flow) were increased significantly by pravastatin, captopril and irbesartan. Serum insulin levels were increased by pravastatin and captopril. Palmitate suppressed pancreatic IBF and increased blood glucose. In female animals, pancreatic IBF was stimulated by captopril, candesartan and irbesartan. Total PBF was increased by captopril, candesartan and irbesartan, and by pravastatin. Palmitate suppressed pancreatic IBF and serum insulin secretion. In conclusion, the present study lends support to the view that a local pancreatic RAS (renin-angiotensin system) and pravastatin may be selectively influencing the pancreatic microcirculation and therefore affecting insulin secretion and glycaemia. NEFAs (non-esterified fatty acids) impaired pancreatic IBF, suppressed insulin secretion and increased blood glucose. Substantial gender-related differences in the vascular and metabolic responses to these drugs prevail in this animal model of diabetes.

Angiotensin receptor blockers in the treatment of NASH/NAFLD: could they be a first-class option?

Nonalcoholic fatty liver disease (NAFLD) is a condition pathogenically linked to metabolic syndrome (MS) by insulin resistance (IR), and characterized by hepatic steatosis in the absence of significant alcohol use, hepatotoxicity, and/or other known liver diseases.The principles of NAFLD therapy target IR: the key point of MS. As the renin-angiotensin system (RAS) plays a central role in IR, and subsequently in NAFLD and nonalcoholic steatohepatitis (NASH), an attempt to block the deleterious effects of RAS overexpression seems a logical target. While many potential therapies tested in NASH target only the consequences of this condition, or try to "get rid" of excessive fat, angiotensin receptor blockers (ARBs) could act as an elegant tool for adequate correction of the various imbalances that act in harmony in NASH/NAFLD. Indeed, by inhibiting RAS we can improve the intracellular insulin signaling pathway, better control adipose tissue proliferation and adipokine production, and produce more balanced local and systemic levels of various cytokines. At the same time, by controlling the local RAS in the liver we might be able to prevent at least fibrosis and also slow down the vicious cycle that links steatosis to necroinflammation. By targeting the pancreatic effects of angiotensin we should be able to preserve an adequate insulin secretion and acquire a better metabolic balance.In our opinion there are two major advantages of ARBs that make them a possible therapeutic option for treating NASH and MS: their specific antihypertensive effect, and their impact on liver fibrosis. In light of this, and based on the current evidence (including existent human studies), we can speculate that some ARBs like telmisartan, candesartan, and losartan can be beneficial in treating NASH/NAFLD and its consequences, and further larger controlled clinical trials will bring consistent data into this field.

The physiology of a local renin-angiotensin system in the pancreas

The systemic renin-angiotensin system (RAS) plays an important role in regulating blood pressure, electrolyte and fluid homeostasis. However, local RASs also exist in diverse tissues and organs, where they play a multitude of autocrine, paracrine and intracrine physiological roles. The existence of a local RAS is now recognized in pancreatic acinar, islet, duct, endothelial and stellate cells, the expression of which is modulated in response to physiological and pathophysiological stimuli such as hypoxia, pancreatitis, islet transplantation, hyperglycaemia, and diabetes mellitus. This pancreatic RAS has been proposed to have important endocrine and exocrine roles in the pancreas, regulating local blood flow, duct cell sodium bicarbonate secretion, acinar cell digestive enzyme secretion, islet beta-cell (pro)insulin biosynthesis, and thus, glucose-stimulated insulin release, delta-cell somatostatin secretion, and pancreatic cell proliferation and differentiation. It may further mediate oxidative stress-induced cell inflammation, apoptosis and fibrosis. Further exploration of this system would probably offer new insights into the pathogenesis of pancreatitis, diabetes, cystic fibrosis and pancreatic cancer formation. New therapeutic targets and strategies might thus be suggested.