Angiotensin II binding sites in the rat pancreas and their modulation after sodium loading and depletion

Role of angiotensin II in cellular entry and replication of dengue virus

Previous studies have demonstrated the relevance of several soluble molecules in the pathogenesis of dengue. In this regard, a possible role for angiotensin II (Ang II) in the pathophysiology of dengue has been suggested by the observation of a blockade of Ang II in patients with dengue, increased expression of molecules related to Ang II production in the plasma of dengue patients, increased expression of circulating cytokines and soluble molecules related to the action of Ang II, and an apparent relationship between DENV, Ang II effects, and miRNAs. In addition, in ex vivo experiments, the blockade of Ang II AT1 receptor and ACE-1 (angiotensin converting enzyme 1), both of which are involved in Ang II production and its function, inhibits infection of macrophages by DENV, suggesting a role of Ang II in viral entry or in intracellular viral replication of the virus. Here, we discuss the possible mechanisms of Ang II in the entry and replication of DENV. Ang II has the functions of increasing the expression of DENV entry receptors, creation of clathrin-coated vesicles, and increasing phagocytosis, all of which are involved in DENV entry. This hormone also modulates the expression of the Rab5 and Rab7 proteins, which are important in the endosomal processing of DENV during viral replication. This review summarizes the data related to the possible involvement of Ang II in the entry of DENV into cells and its replication.

Angiotensin II and post-streptococcal glomerulonephritis

BACKGROUND

Post-streptococcal glomerulonephritis (PSGN) is a consequence of the infection by group A beta-hemolytic streptococcus. During this infection, various immunological processes generated by streptococcal antigens are triggered, such as the induction of antibodies and immune complexes. This activation of the immune system involves both innate and acquired immunity. The immunological events that occur at the renal level lead to kidney damage with chronic renal failure as well as resolution of the pathological process (in most cases). Angiotensin II (Ang II) is a molecule with vasopressor and pro-inflammatory capacities, being an important factor in various inflammatory processes. During PSGN some events are defined that make Ang II conceivable as a molecule involved in the inflammatory processes during the disease.

CONCLUSION

This review is focused on defining which reported events would be related to the presence of this hormone in PSGN.

Angiotensin II and dengue

Dengue is a disease caused by a flavivirus that is transmitted principally by the bite of an Aedes aegypti mosquito and represents a major public-health problem. Many studies have been carried out to identify soluble factors that are involved in the pathogenesis of this infection. Cytokines, soluble factors, and oxidative stress have been reported to be involved in the development of severe disease. Angiotensin II (Ang II) is a hormone with the ability to induce the production of cytokines and soluble factors related to the inflammatory processes and coagulation disorders observed in dengue. However, a direct involvement of Ang II in this disease has not been demonstrated. This review primarily summarizes the pathophysiology of dengue, the role of Ang II in various diseases, and reports that are highly suggestive of the involvement of this hormone in dengue.

Angiotensin II and human obesity. A narrative review of the pathogenesis

Angiotensin II (Ang II) is a hormone and the main effector of the renin-angiotensin system (RAS). This peptide has crucial pathophysiologi-cal effects on hypertension, cardiac hypertrophy, endothelial proliferation, in-flammation and tissue remodelling through G protein-coupled receptors. The pro-inflammatory role of Ang II has been reported in various inflammatory pro-cesses. Obesity is linked to a chronic inflammatory process which in turn is the cause of some of its morbidities. Ang II is related to the comorbidities related to the comorbidities of obesity, which include alterations in the heart, kid-ney, hypertension and coagulation. In this regard, activation of AT1 receptors by Ang II can induce an inflammatory process mediated by the transcription factor NF-kB, triggering inflammation in various systems that are related to the comorbidities observed in obesity. The aim of this review was to highlight the pro-inflammatory effects of Ang II and the alterations induced by this hor-mone in various organs and systems in obesity. The search was done since 1990 through Medline, EMBASE and PubMed, using the keywords: angiotensin II; an-giotensin II, obesity; angiotensin II, kidney, obesity; angiotensin II, coagulation, obesity; angiotensin II, inflammation, obesity; angiotensin II, adipose tissue, obesity; angiotensin II, hypertension, obesity; angiotensin II, insulin resistance, obesity; angiotensin II, adiponectin, leptin, obesity; angiotensin II, COVID-19, obesity. Angiotensin II through its interaction with its AT1 receptor, can induce alterations in diverse systems that are related to the comorbidities observed in obesity. Therapeutic strategies to decrease the production and action of Ang II could improve the clinical conditions in individuals with obesity.

Dipeptidyl peptidase IV inhibitor MK-0626 attenuates pancreatic islet injury in tacrolimus-induced diabetic rats

Background

Tacrolimus (TAC)-induced pancreatic islet injury is one of the important causes of new-onset diabetes in transplant recipients. This study was performed to evaluate whether a dipeptidyl peptidase IV (DPP IV) inhibitor is effective in improving TAC-induced diabetes mellitus by reducing pancreatic islet injury.

Methods

Rats were treated with TAC (1.5 mg/kg, subcutaneously) and the DPP IV inhibitor MK-0626 (10 or 20 mg/kg, oral gavage) for 4 weeks. The effect of MK-0626 on TAC-induced diabetes was evaluated by assessing pancreatic islet function, histopathology. TAC-induced incretin dysfunction was also examined based on active glucagon-like peptide-1 (GLP-1) levels in the serum after glucose loading. The protective effect of MK-0626 was evaluated by measuring markers of oxidative stress, oxidative resistance, and apoptosis. To determine whether enhanced GLP-1 signaling is associated with these protective effects, we measured the expression of the GLP-1 receptor (GLP-1R) and the effect of the GLP-1 analog exendin-4 on cell viability and oxidative stress in isolated islets.

Results

MK-0626 treatment attenuated TAC-induced pancreatic islet dysfunction and islet morphology. TAC treatment led to a defect in active GLP-1 secretion; however, MK-0626 reversed these effects. TAC treatment increased the level of 8-hydroxy-2′-deoxyguanosine (8-OHdG), the number of apoptotic death, and the level of active caspase-3, and decreased the level of manganese superoxide dismutase and heme oxygenase-1; MK-0626 treatment reversed these changes. MK-0626 treatment restored the expression of GLP-1R, and direct administration of exendin-4 to isolated islets reduced TAC-induced cell death and 8-OHdG expression.

Conclusions

The DPP IV inhibitor MK-0626wasan effective antidiabetic agent that exerted antioxidative and antiapoptotic effects via enhanced GLP-1 signaling in TAC-induced diabetics.

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.

Aldosterone and angiotensin: Role in diabetes and cardiovascular diseases

The present review shall familiarize the readers with the role of renin-angiotensin aldosterone system (RAAS), which regulates blood pressure, electrolyte and fluid homeostasis. The local RAAS operates in an autocrine, paracrine and/or intracrine manner and exhibits multiple physiological effects at the cellular level. In addition to local RAAS, there exists a complete pancreatic RAAS which has multi-facet role in diabetes and cardiovascular diseases. Aldosterone is known to mediate hyperinsulinemia, hypertension, cardiac failure and myocardial fibrosis while angiotensin II mediates diabetes, endothelial dysfunction, vascular inflammation, hypertrophy and remodeling. As the understanding of this biology of RAAS increases, it serves to exploit this for the pharmacotherapy of diabetes and cardiovascular diseases.

Severe acute pancreatitis is associated with upregulation of the ACE2-angiotensin-(1-7)-Mas axis and promotes increased circulating angiotensin-(1-7)

BACKGROUND/OBJECTIVES

Angiotensin-converting enzyme 2 (ACE2), its product angiotensin-(1-7) and its receptor Mas may counteract the adverse effects of the ACE-angiotensin receptor II-AT(1) axis in many diseases. We examined the expression of these novel components of the rennin-angiotensin system in an experimental mouse model of severe acute pancreatitis (SAP).

METHODS

SAP was induced by six intraperitoneal injections of caerulein, and mice were sacrificed at 2, 12, 24, 48 and 72 h post disease-induction (normal control group mice were sacrificed at 2 h post disease-induction). Tissue and blood were collected for biochemical detection, gene and protein expression by qRT-PCR and western blot analysis, enzyme-linked immunosorbent assay and immunohistology detection.

RESULTS

Pancreatic ACE2 gene and protein expression, plasma and pancreatic angiotensin-(1-7) levels and Mas receptor gene and protein expression were significantly increased (p < 0.05) following SAP induction compared with the normal control group.

CONCLUSIONS

Severe acute pancreatitis is associated with upregulation of the ACE2-angiotensin-(1-7)-Mas axis and promotes increased circulating angiotensin-(1-7). These results support the presence of an ACE2-angiotensin-(1-7)-Mas axis in pancreatitis.

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.

Angiotensin II revisited: new roles in inflammation, immunology and aging

That the renin–angiotensin system (RAS) is involved in regulation of blood pressure, vasoconstriction, sodium intake and potassium excretion is well established. Studies in the last few years have however documented new roles for this molecule as a pro-inflammatory molecule and more recently as a possible pro-fibrotic agent that contributes to progressive deterioration of organ function in disease. Binding of Ang II to its receptors (in particular AT₁) mediates intracellular free radical generation that contributes to tissue damage by promoting mitochondrial dysfunction. Blocking Ang II signalling protects against neurodegenerative processes and promotes longevity in rodents. Altogether these findings open the unanticipated perspective for exploring Ang II signalling in therapeutic interventions in inflammatory diseases and aging-related tissue injury. This review extends from the discovery of Ang II and its implications in renal and cardiovascular physiology to cover the roles of the system in inflammation, tissue injury, autoimmunity, oxidative stress and aging.

Distribution of a novel binding site for angiotensins II and III in mouse tissues

A novel binding site for angiotensins II and III that is unmasked by parachloromercuribenzoate has been reported in rat, mouse and human brains. Initial studies of this binding site indicate that it is not expressed in the adrenal, liver or kidney of the rat and mouse. To determine if this binding site occurs in other mouse tissues, 8 tissues were assayed for expression of this binding site by radioligand binding assay and compared with the expression of this binding site in the forebrain. Particulate fractions of homogenates of testis, epididymis, seminal vesicles, heart, spleen, pancreas, lung, skeletal muscle, and forebrain were incubated with (125)I-sarcosine(1), isoleucine(8) angiotensin II in the presence or absence of 0.3mM parachloromercuribenzoate plus 10microM losartan and 10microM PD123319 (to saturate AT(1) and AT(2) receptors). Specific (3microM angiotensin II displaceable) high affinity binding occurred in the testis>forebrain>epididymis>spleen>pancreas>lung when parachloromercuribenzoate was present. Binding could not be reliably observed in heart, skeletal muscle and seminal vesicles. High affinity binding of (125)I-sarcosine(1), isoleucine(8) angiotensin II was observed in the absence of parachloromercuribenzoate in the pancreas on occasion. This suggests that this novel angiotensin binding site may have a functional role in these tissues.

Pharmacotherapy for acute pancreatitis

Our knowledge of acute pancreatitis is still far from complete and there is no unanimous agreement concerning the pathophysiological processes leading to typical alterations during the course of acute pancreatitis. We reviewed the paper published in the last decade on the pathophysiology and treatment of acute pancreatitis. It is difficult to translate the experimental therapeutic results into clinical practice. For example, lexipafant was efficacious in decreasing the severity and mortality of lethal pancreatitis in rats, but seems to have no effect on severe acute pancreatitis in humans. Thus, the main problem in acute pancreatitis, especially in the severe form of the disease, is the difficulty of designing clinical studies capable of giving reliable statistically significant answers regarding the benefits of the various proposed therapeutic agents previously tested in experimental settings. Thus, analgesia, supportive care, and treatment of the pulmonary and renal complications remain the cornerstones of the treatment of acute pancreatitis, especially in the severe form of the disease.

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.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Role of the renin-angiotensin system in the endocrine pancreas: implications for the development of diabetes

Activation of the renin-angiotensin system has a pivotal role in the pathogenesis of diabetic complications. However, recent evidence suggests that it may also contribute to the development of diabetes itself. In the endocrine pancreas, all the components of an active renin-angiotensin system are present, which modulate a range of activities including local blood flow, hormone release and prostaglandin synthesis. In both types 1 and 2 diabetes, there is an up-regulation of its expression and activity in the endocrine pancreas. Whether these changes have a direct pathogenetic role or reflect a response to local stress or tissue injury remains to be established. Angiotensin-mediated increases in oxidative stress, inflammation and free fatty acids levels potentially contribute to beta-cell dysfunction in diabetes. In addition, activation of the renin-angiotensin system appears to potentiate the action of other pathogenic pathways including glucotoxicity, lipotoxicity and advanced glycation. In experimental models of type 2 diabetes, blockade of the renin-angiotensin system with angiotensin converting enzyme inhibitors or angiotensin receptor antagonists results in the improvement of islet structure and function. Moreover, the incidence of de novo diabetes appears to be significantly reduced by blockade of the renin-angiotensin system in clinical studies. At least two large controlled trials are currently underway to study the role of renin-angiotensin system in the development of diabetes. It is hoped that these studies will demonstrate the true potential of the blockade of the renin-angiotensin system for the prevention of diabetes.

Angiotensin II type 1 receptor inhibition markedly improves the blood perfusion, oxygen tension and first phase of glucose-stimulated insulin secretion in revascularised syngeneic mouse islet grafts

AIMS/HYPOTHESIS

We recently found evidence of an angiotensin-generating system in pancreatic islets. The present study investigated the effect of endogenously produced angiotensin II on microcirculation and function in transplanted islets.

MATERIALS AND METHODS

Losartan, an angiotensin II type 1 receptor inhibitor, was administered either acute intravenously to mice with 4-week-old islet renal subcapsular transplants, or added to the drinking water for the final 14 days or throughout the 4-week post-transplantation period. The graft-bearing kidney was, in some cases, dissected out and perfused in vitro to evaluate the effect of angiotensin II and losartan on glucose-stimulated insulin release from the grafts.

RESULTS

Losartan treatment throughout the 4-week post-transplantation period had negative effects on islet revascularisation as well as on islet graft insulin release. However, administration of losartan, either intravenously or orally, after the formation of a new vascular network, improved islet graft blood perfusion. PO2 in the islet transplants was also effectively improved by the losartan treatment. Graft perfusion experiments showed a markedly better first phase of glucose-stimulated insulin release in transplanted islets when exposed to losartan. In contrast, acute administration of angiotensin II decreased islet graft blood flow, PO2 and glucose-stimulated insulin release.

CONCLUSIONS/INTERPRETATION

This study shows that inhibition of the islet reninangiotensin system may be a feasible strategy to increase the blood perfusion, PO2 and function within islet grafts. Such treatment should not be initiated, however, before the islet vascular system has been formed.

Pancreatic islet renin angiotensin system: its novel roles in islet function and in diabetes mellitus

Several regulatory systems are implicated in the regulation of islet function and beta cell mass. Of great interest in this context are some endocrine, paracrine/autocrine, and intracrine regulators. These include, to name but a few, the gut peptides, growth factors, prostaglandins, and some vasoactive mediators such as nitric oxide, bradykinins, endothelins, and angiotensins. Apart from its potent vasoconstrictor actions, the renin-angiotensin system (RAS) that generates angiotensin II has several novel functions-stimulation and inhibition of cell proliferation; induction of apoptosis; generation of reactive oxygen species; regulation of hormone secretion; and proinflammatory and profibrogenic actions. In the pancreas, recent evidence supports the presence of an islet RAS, which is subject to activation by islet transplantation and diabetes. Such a local islet RAS, if activated, may drive islet fibrosis and reduce islet blood flow, oxygen tension, and insulin biosynthesis. Moreover, activation of an islet RAS may drive the synthesis of reactive oxygen species, cause oxidative stress-induced beta cell dysfunction and apoptosis, and thus contribute to the islet dysfunction seen in type 2 diabetes and after islet transplantation. Blockade of the RAS could contribute to the development of novel therapeutic strategies in the prevention and treatment of patients with diabetes and in islet transplantation.

The role of the pancreatic renin-angiotensin system in acinar digestive enzyme secretion and in acute pancreatitis

The pancreas contains a local renin-angiotensin system (RAS), which is subject to activation by experimental pancreatitis. In the exocrine pancreas, angiotensin II receptor subtypes AT1 and AT2 have been localized in the pancreatic ducts, blood vessels and acinar cells. We hypothesize that local RAS activities may have a potential role in regulating pancreatic acinar digestive enzyme secretion. The present study was designed to elucidate firstly the existence of RAS components in pancreatic acinar cells and their regulation by acute pancreatitis. Secondly, the differential roles of AT1 and AT2 receptors in controlling digestive enzyme secretion from dispersed functional pancreatic acini were also investigated. The mRNA levels of RAS components were assessed by semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR). Acinar secretions were assayed by the measurement of alpha-amylase and lipase activities. Induction of acute pancreatitis was achieved by hyperstimulation of two intraperitoneal (i.p.) injections of cerulein (50 microg/kg/h). Results from RT-PCR showed that the mRNA levels of the major RAS components (angiotensinogen, AT1 and AT2 receptors) were expressed in isolated rat pancreatic acinar cells, and they were upregulated during pancreatitis. Exogenous addition of angiotensin II could stimulate a dose-dependent release of digestive enzymes from the acinar cells. Administration of the selective AT1 receptor antagonist losartan significantly inhibited the acinar digestion enzyme secretion in both normal and pancreatitis-induced acini. However, a specific AT2 receptor blocker PD123319 did not exhibit such a suppressive effect. These data indicate the existence of an acinar RAS in the pancreas of potential importance in the physiological regulation of digestive enzyme secretion. The differential actions of AT1 and AT2 receptors and their upregulation may have clinical relevance to the pathogenesis and management of acute pancreatitis.

Immunohistochemical colocalization of type II angiotensin receptors with somatostatin in rat pancreas

Earlier studies indicate that binding sites of type II angiotensin (AT2) receptors are detected all over the pancreas, as well as in the pancreatic exocrine cell line AR4-2J. However, lack of corresponding functional AT2 receptor responses can be detected in the exocrine pancreas. The aim of present study is to determine the protein expression of AT2 receptors in the pancreas by probing with an AT2 receptor-specific antibody, and to examine the role of AT2 receptors in the regulation of pancreatic endocrine hormone release. In Western protein analysis of adult rat tissues, expression of AT2 receptor-immunoreactive bands of 56, 68, and 78 kDa was detected in the adrenal, kidney, liver, salivary glands, and pancreas. In adult rat pancreas, strong immunoreactivity was detected on cells that were located at the outer region of Langerhans islets. Immunohistochemical studies indicated that AT2 receptors colocalized with somatostatin-producing cells in the endocrine pancreas. Consistent with the findings in adult pancreas, abundant expression of AT2 receptors was also detected in immortalized rat pancreatic endocrinal cells lines RIN-m and RIN-14B. To examine the role of AT2 receptors on somatostatin secretion in the pancreas, angiotensin-stimulated somatostatin release from pancreatic RIN-14B cells was studied by an enzyme immunoassay in the absence or presence of various subtype-selective angiotensin analogues. There was a basal release of somatostatin from RIN-14B cells at a rate of 8.72 +/- 4.21 ng/10(6) cells (n = 7). Angiotensin II (1 nM-10 microM) stimulated a biphasic somatostatin release in a dose-dependent manner with an apparent EC50 value of 49.3 +/- 25.9 nM (n = 5), and reached maximal release at 1 microM angiotensin II (982 +/- 147.34% over basal secretion; n = 5). Moreover, the AT2 receptor-selective angiotensin analogue, CGP42112, was 1000 times more potent than the AT1 receptor-selective angiotensin analogue, losartan, in inhibiting angiotensin II-stimulated somatostatin release. These results suggest that angiotensin may modulate pancreatic hormone release via regulation of somatostatin secretion.

Evidence for a local angiotensin-generating system and dose-dependent inhibition of glucose-stimulated insulin release by angiotensin II in isolated pancreatic islets

AIMS/HYPOTHESIS

A local angiotensin-generating system has been found in the exocrine pancreas. This study aimed, primarily, to investigate the existence of a local angiotensin-generating system in the pancreatic islets and, secondly, to elucidate its role in regulating insulin secretion.

METHODS

Real-time RT-PCR and western blot were used to investigate if angiotensin-generating components are present in the mouse pancreatic islets, which are subject to regulation by islet transplantation. The localisation of AT1-receptors in islets was investigated by immunohistochemistry. Batch-type incubations of isolated islets were applied for studying the influence of angiotensin II on the glucose-stimulated insulin release, glucose oxidation and (pro)insulin, and total protein biosynthesis.

RESULTS

Major components, namely angiotensinogen, ACE, AT1- and AT2-receptors, were expressed in endogenous islets. AT1-receptors were localised to pancreatic beta cells. Exposure of the isolated islets to angiotensin II induced a dose-dependent inhibition of glucose-stimulated insulin release and inhibited (pro)insulin biosynthesis. This inhibitory action was fully preventable by pretreatment of the islets with losartan, an AT1-receptor antagonist. We also investigated if the expression of these components was changed after islet transplantation. Notably, a markedly increased expression of mRNA for the AT1-receptor was observed in islets retrieved from 4-week-old syngeneic islet transplants, a finding that was confirmed at the protein level.

CONCLUSION/INTERPRETATION

These data indicate the existence of an islet angiotensin-generating system of potential importance in the physiological regulation of glucose-induced insulin secretion, thus diabetes mellitus. The increased expression of the AT1-receptor in islet transplants could have relevance to islet-graft function.

Functional role, cellular source, and tissue distribution of rat elastase-2, an angiotensin II-forming enzyme

We recently described a chymostatin-sensitive elastase-2 as the major angiotensin (ANG) II-forming enzyme in the perfusate of the rat mesenteric arterial bed (MAB) with the same cDNA sequence as rat pancreatic elastase-2. The role of this enzyme in generating ANG II was examined in the rat isolated and perfused MAB. The vasoconstrictor effect elicited by ANG I and the renin substrate tetradecapeptide was only partially inhibited by captopril but abolished by the combination of captopril and chymostatin or N-acetyl-Ala-Ala-Pro-Leu-chloromethylketone (Ac-AAPL-CK; inhibitor originally developed for human elastase-2). The effect induced by [Pro11,d-Ala12]-ANG I, an ANG I-converting enzyme (ACE)-resistant biologically inactive precursor of ANG II, was blocked by chymostatin or Ac-AAPL-CK. It was also demonstrated that cultured rat mesenteric endothelial cells synthesize elastase-2 and that mRNA for this enzyme can be detected in different rat tissues such as the pancreas, MAB, lung, heart, kidney, liver, and spleen. In conclusion, the demonstration of a functional alternative pathway to ACE for ANG II generation in the rat MAB and the fact that cultured MAB endothelial cells are capable of producing and secreting elastase-2 represent strong evidence of a physiological role for this enzyme in the rat vasculature.

Local blood flow regulation in transplanted rat pancreatic islets: influence of adenosine, angiotensin II, and nitric oxide inhibition

BACKGROUND

Transplanted islets lack endothelial cells immediately after implantation and therefore depend on an adequate revascularization for their survival and function. However, the functional properties of the newly formed islet graft microvessels are largely unknown. This study aimed to investigate the blood flow regulation of transplanted pancreatic islets.

METHODS

Pancreatic islets were syngeneically transplanted beneath the renal capsule of control and streptozotocin-diabetic rats. Blood flow measurements were performed 4 weeks later using laser-Doppler flowmetry. Adenosine (0.6 mg x kg(-1) x min(-1), angiotensin II (AT II; 0.17 microg x kg(-1) x min(-1)) and the nitric oxide synthase inhibitor NG-nitro-L-arginine (25 mg/ kg) were given to each animal.

RESULTS

An increased basal blood flow and basal vascular conductance in the islet grafts, but not in the renal cortex, were seen in diabetic rats compared with control rats. Adenosine increased, and AT II decreased, the vascular conductance of the islet grafts in both nondiabetic and diabetic animals. A more pronounced circulatory response to AT II was observed in kidneys of diabetic animals, whereas there was no difference in the islet graft blood flow response between nondiabetic and diabetic animals. NG-Nitro-L-arginine decreased islet graft blood flow and vascular conductance in both nondiabetic and diabetic recipients, but the effect was more pronounced in the non-diabetic animals.

CONCLUSIONS

Islet graft blood flow was influenced by adenosine, AT II, and nitric oxide inhibition in all animals. However, diabetic animals were less dependent on nitric oxide to maintain a basal blood flow in the islet graft.

Acute sodium depletion modifies septo-preoptic neuron sensitivities to neurohormones

Sodium (Na+) depletion induces sodium appetite to replenish Na+ loss. It appears to be a consequence of enhanced levels of aldosterone (Aldo) and angiotensin II (AII) in the plasma as well as in the brain. Mineralocorticoid pretreatment modifies the sensitivity of septo-preoptic neurons to locally applied AII and Aldo. Therefore, we investigated septo-preoptic neuronal sensitivities to AII and Aldo, as well as to the specific AII type-1 receptor (AT-1) non-peptide antagonist losartan (Los) and to the specific AII type-2 receptor (AT-2) non-peptide antagonist PD123319 after one Na+ depletion without repletion. We found that one Na+ depletion induced increases in the proportion of neurons inhibited by iontophoretic application of AII (20.5% vs. 7.8%, p=0.004) whereas, the proportion of neurons excited by Aldo was increased, (23.7% vs. 5%, p=0.001). Moreover, the proportion of neurons changing sensitivity to AII after one application of Aldo was increased in the furosemide group (44.2% vs. 20.4%, p=0.0123). The proportion of neurons inhibited by application of losartan was enhanced, (26.4% vs. 9.3%, p=0.03). No significant changes were found in response to PD123319 by itself. Moreover, there were more neurons which co-localized responses to both Los and PD123319 in the furosemide group than in the control group (29.7% vs. 8.6%, p=0.027). It is known that multidepletions induce an increased need-free sodium appetite and our present findings could well form part of the neuronal basis of this behavior.

Immunohistochemical localization of angiotensin II in the mouse pancreas

Previous studies have suggested the presence of a tissue renin-angiotensin II system in the pancreas. These studies were based on the observation of several key components of the renin-angiotensin II system using molecular biological, biochemical and pharmacological approaches. In the present study, angiotensin II was localized immunohistochemically in the mouse pancreas using an indirect immunoperoxidase-staining technique. The results showed that angiotensin II-like immunoreactivity was localized predominantly in the endothelial cells of pancreatic blood vessels and the epithelial cells of pancreatic ducts from a subgroup of the vessels and ducts. Compared with those found in the pancreatic blood vessels and ductal system, a less pronounced immunoreactivity for angiotensin II was also observed in the acinar cells and in the smooth muscle layers overlying the pancreatic ducts as well as the blood vessels. However, no angiotensin II-like immunoreactivity was detected in the islet cells. Taken together with previous findings, the present results suggest a local angiotensin II-forming system in the mouse pancreas, which may be a significant autocrine or paracrine modulator of diverse pancreatic functions, including regulation of pancreatic blood flow and pancreatic anion secretion.

Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

In urethane-anaesthetised male Wistar rats iontophoretic application of the angiotensin II (Ang II) type 1 (AT-1) receptor specific nonpeptide antagonist losartan in the septo-preoptic continuum can produce neuronal excitation of short- and long-term duration which has been interpreted as removal of tonic Ang II-induced inhibition. Mineralocorticoid pretreatment, which increases neuronal sensitivity to Ang II to enhance salt appetite, also removes this losartan-induced long-term excitation. These results suggested steroid modulation of the AT-1 receptor and a complex involvement of Ang II in salt appetite. To investigate the role of the inhibitory action of central Ang II on salt appetite, we gave intracerebroventicular injections of a single, low dose (10 ng) of losartan in normal euhydrated rats and this produced, paradoxically, a progressive increase in salt intake (1.6 +/- 0.3 ml/day to 3.5 +/- 0.9 ml/day, n = 15, P < 0.05). Treatment of these salt enhanced rats with DOCA (0.5 mg/day, s.c., for 3 days) further increased the salt appetite, but then a second i.c.v. injection of the same dose of losartan significantly inhibited the enhanced salt appetite (4.7 +/- 0.7 to 1.3 +/- 0.4, n = 9, P < 0.05). These results provide evidence for a complex action of Ang II on the At-1 receptor mediating the generation of salt appetite that appears to involve either at least two functional subtypes of this AT-1 receptor, as already suggested by previous electrophysiological experiments, or one AT-1 receptor with several activation states.