Gastric inhibitory polypeptide and splanchnic blood perfusion: augmentation of the islet blood flow increase in hyperglycemic rats

Potential for Gut Peptide-Based Therapy in Postprandial Hypotension

Postprandial hypotension (PPH) is an important and under-recognised disorder resulting from inadequate compensatory cardiovascular responses to meal-induced splanchnic blood pooling. Current approaches to management are suboptimal. Recent studies have established that the cardiovascular response to a meal is modulated profoundly by gastrointestinal factors, including the type and caloric content of ingested meals, rate of gastric emptying, and small intestinal transit and absorption of nutrients. The small intestine represents the major site of nutrient-gut interactions and associated neurohormonal responses, including secretion of glucagon-like peptide-1, glucose-dependent insulinotropic peptide and somatostatin, which exert pleotropic actions relevant to the postprandial haemodynamic profile. This review summarises knowledge relating to the role of these gut peptides in the cardiovascular response to a meal and their potential application to the management of PPH.

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.

Effects of GIP on regional blood flow during normoglycemia and hyperglycemia in anesthetized rats

The incretin hormone glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-stimulated insulin secretion, and affects β-cell turnover. This study aimed at evaluating if some of the beneficial effects of GIP on glucose homeostasis can be explained by modulation of islet blood flow. Anesthetized Sprague-Dawley rats were infused intravenously with different doses of GIP (10, 20, or 60 ng/kg*min) for 30 min. Subsequent organ blood flow measurements were performed with microspheres. In separate animals, islets were perfused ex vivo with GIP (10-6 -10-12  mol/L) during normo- and hyperglycemia and arteriolar responsiveness was recorded. The highest dose of GIP potentiated insulin secretion during hyperglycemia, but had no effect in normoglycemic rats. The highest GIP concentration decreased blood perfusion of whole pancreas, pancreatic islets, duodenum, colon, liver and kidneys. The decrease in blood flow was unaffected by ganglion blockade or adenosine receptor inhibition. In contrast to this, in single perfused islets GIP induced a dose-dependent arteriolar dilation. Thus, high doses of GIP exert a direct dilatory effect on islet arterioles in isolated islets, but induce a generalized vasoconstriction in splanchnic organs, including the whole pancreas and islets, in vivo. The latter effect is unlikely to be mediated by adenosine, the autonomic nervous system, or endothelial mediators.

Vascular Biology of Glucagon Receptor Superfamily Peptides: Mechanistic and Clinical Relevance

Regulatory peptides produced in islet and gut endocrine cells, including glucagon, glucagon-like peptide-1 (GLP-1), GLP-2, and glucose-dependent insulinotropic polypeptide, exert actions with considerable metabolic importance and translational relevance. Although the clinical development of GLP-1 receptor agonists and dipeptidyl peptidase-4 inhibitors has fostered research into how these hormones act on the normal and diseased heart, less is known about the actions of these peptides on blood vessels. Here we review the effects of these peptide hormones on normal blood vessels and highlight their vascular actions in the setting of experimental and clinical vascular injury. The cellular localization and signal transduction properties of the receptors for glucagon, GLP-1, GLP-2, and glucose-dependent insulinotropic polypeptide are discussed, with emphasis on endothelial cells and vascular smooth muscle cells. The actions of these peptides on the control of blood flow, blood pressure, angiogenesis, atherosclerosis, and vascular inflammation are reviewed with a focus on elucidating direct and indirect mechanisms of action. How these peptides traverse the blood-brain barrier is highlighted, with relevance to the use of GLP-1 receptor agonists to treat obesity and neurodegenerative disorders. Wherever possible, we compare actions identified in cell lines and primary cell culture with data from preclinical studies and, when available, results of human investigation, including studies in subjects with diabetes, obesity, and cardiovascular disease. Throughout the review, we discuss pitfalls, limitations, and challenges of the existing literature and highlight areas of controversy and uncertainty. The increasing use of peptide-based therapies for the treatment of diabetes and obesity underscores the importance of understanding the vascular biology of peptide hormone action.

Suppressive Effects of Glucose-Dependent Insulinotropic Polypeptide on Cardiac Hypertrophy and Fibrosis in Angiotensin II-Infused Mouse Models

BACKGROUND

Activation of glucose-dependent insulinotropic polypeptide receptor (GIPR) has been shown to be protective against atherosclerosis. However, effects of GIP on the heart have remained unclear. To address this question, in vitro and in vivo experiments were conducted.

METHODS AND RESULTS

In isolated mouse cardiomyocytes, GIPR mRNA was detected by reverse transcription-polymerase chain reaction, and GIP stimulation increased adenosine 3',5'-cyclic monophosphate production. In apolipoprotein E-knockout mice, infusion of angiotensin II (AngII; 2,000 ng·kg(-1)·min(-1)) significantly increased the heart weights, and co-administration of GIP (25 nmol·kg(-1)·day(-1)) reversed this increase (both P<0.01). In the left ventricular walls, GIP suppressed AngII-induced cardiomyocyte hypertrophy by 34%, apoptosis by 77%, and interstitial fibrosis by 79% (all P<0.01). Furthermore, GIP reduced AngII-induced expression of transforming growth factor-β1 (TGF-β1) and hypoxia inducible factor-1α. In wild-type mice, cardiac hypertrophy was induced by AngII to a lesser extent, and prevented by GIP. In contrast, GIP did not show any cardioprotective effect against AngII-induced cardiac hypertrophy in GIPR-knockout mice. In an in vitro experiment using mouse cardiomyocytes, GIP suppressed AngII-induced mRNA expression of B-type natriuretic peptide and TGF-β1.

CONCLUSIONS

It was demonstrated that cardiomyocytes represent a direct target of GIP action in vitro, and that GIP ameliorated AngII-induced cardiac hypertrophy via suppression of cardiomyocyte enlargement, apoptosis, and fibrosis in vivo. (Circ J 2016; 80: 1988-1997).

Pancreatic islet blood flow and its measurement

Pancreatic islets are richly vascularized, and islet blood vessels are uniquely adapted to maintain and support the internal milieu of the islets favoring normal endocrine function. Islet blood flow is normally very high compared with that to the exocrine pancreas and is autonomously regulated through complex interactions between the nervous system, metabolites from insulin secreting β-cells, endothelium-derived mediators, and hormones. The islet blood flow is normally coupled to the needs for insulin release and is usually disturbed during glucose intolerance and overt diabetes. The present review provides a brief background on islet vascular function and especially focuses on available techniques to measure islet blood perfusion. The gold standard for islet blood flow measurements in experimental animals is the microsphere technique, and its advantages and disadvantages will be discussed. In humans there are still no methods to measure islet blood flow selectively, but new developments in radiological techniques hold great hopes for the future.

Incretins

Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are the known incretin hormones in humans, released predominantly from the enteroendocrine K and L cells within the gut. Their secretion is regulated by a complex of integrated mechanisms involving direct contact for the activation of different chemo-sensors on the brush boarder of K and L cells and several indirect neuro-immuno-hormonal loops. The biological actions of GIP and GLP-1 are fundamental determinants of islet function and blood glucose homeostasis in health and type 2 diabetes. Moreover, there is increasing recognition that GIP and GLP-1 also exert pleiotropic extra-glycaemic actions, which may represent therapeutic targets for human diseases. In this review, we summarise current knowledge of the biology of incretin hormones in health and metabolic disorders and highlight the therapeutic potential of incretin hormones in metabolic regulation.

Effects of sitagliptin on blood pressure and heart rate in response to intraduodenal glucose infusion in patients with Type 2 diabetes: a potential role for glucose-dependent insulinotropic polypeptide?

AIMS

To evaluate the effects of the dipeptidyl peptidase-4 inhibitor sitagliptin on blood pressure and heart rate, measured during a previously reported study, in which the effects of sitagliptin during intraduodenal glucose infusion at the rate of 2 kcal/min on glucose homeostasis were examined in patients with Type 2 diabetes.

METHODS

A total of 10 people with Type 2 diabetes were studied on two different days, 30 min after oral ingestion of sitagliptin (100 mg) or placebo. Intraduodenal glucose was infused at 2 kcal/min (60 g over 120 min), and blood pressure, heart rate, plasma glucagon-like peptide-1 and glucose-dependent insulinotropic polypeptide (total and intact), glucose, insulin and glucagon responses were evaluated.

RESULTS

In response to intraduodenal glucose infusion, heart rate (treatment effect: P = 0.001) and serum insulin concentration (treatment × time interaction: P = 0.041) were higher after sitagliptin treatment than placebo, without a significant difference in blood pressure, plasma glucagon or glucose. During intraduodenal glucose infusion, there was a substantial increase in plasma total glucose-dependent insulinotropic polypeptide on both days (time effect: P < 0.001), but not in total glucagon-like peptide-1. After sitagliptin, plasma intact glucagon-like peptide-1 concentration increased slightly (treatment × time interaction: P = 0.044) and glucose-dependent insulinotropic polypeptide concentration increased substantially (treatment × time interaction: P = 0.003).The heart rate response to intraduodenal glucose was related directly to plasma intact glucose-dependent insulinotropic polypeptide concentrations (r = 0.75, P = 0.008).

CONCLUSIONS

Sitagliptin increased the heart rate response to intraduodenal glucose infusion at 2 kcal/min in people with Type 2 diabetes, which was associated with augmentation of plasma intact glucose-dependent insulinotropic polypeptide concentrations. These observations warrant further clarification of a potential role for glucose-dependent insulinotropic polypeptide in the control of the 'gut-heart' axis.

Pleiotropic actions of the incretin hormones

The insulin secretory response to a meal results largely from glucose stimulation of the pancreatic islets and both direct and indirect (autonomic) glucose-dependent stimulation by incretin hormones released from the gastrointestinal tract. Two incretins, Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), have so far been identified. Localization of the cognate G protein-coupled receptors for GIP and GLP-1 revealed that they are present in numerous tissues in addition to the endocrine pancreas, including the gastrointestinal, cardiovascular, central nervous and autonomic nervous systems (ANSs), adipose tissue, and bone. At these sites, the incretin hormones exert a range of pleiotropic effects, many of which contribute to the integration of processes involved in the regulation of food intake, and nutrient and mineral processing and storage. From detailed studies at the cellular and molecular level, it is also evident that both incretin hormones act via multiple signal transduction pathways that regulate both acute and long-term cell function. Here, we provide an overview of current knowledge relating to the physiological roles of GIP and GLP-1, with specific emphasis on their modes of action on islet hormone secretion, β-cell proliferation and survival, central and autonomic neuronal function, gastrointestinal motility, and glucose and lipid metabolism. However, it is emphasized that despite intensive research on the various body systems, in many cases there is uncertainty as to the pathways by which the incretins mediate their pleiotropic effects and only a rudimentary understanding of the underlying cellular mechanisms involved, and these are challenges for the future.

Effects of glucagon-like peptide-1-(7–36)-amide on pancreatic islet and intestinal blood perfusion in Wistar rats and diabetic GK rats

The aim of the present study was to evaluate the effects of GLP-1 [glucagon-like peptide-1-(7–36)-amide] on total pancreatic, islet and intestinal blood perfusion in spontaneously hyperglycaemic GK rats and normal Wistar rats using a microsphere technique. GK rats had hyperglycaemia and increased pancreatic and islet blood flow. Blood glucose concentrations were not affected when measured shortly (8 min) after GLP-1 administration in either GK or Wistar rats. GLP-1 had no effects on baseline pancreatic or islet blood flow in Wistar rats, but did prevent the blood flow increase normally seen following glucose administration to these animals. In GK rats, administration of GLP-1 decreased both pancreatic and islet blood flow. Glucose administration to the GK rats decreased pancreatic and islet blood flow. This decrease was not affected by pre-treatment with GLP-1. We conclude that administration of GLP-1 leads to a decrease in the augmented blood flow seen in islets of diabetic GK rats. The GLP-1-induced action on islet blood perfusion may modulate output of islet hormones and contribute to the antidiabetogenic effects of the drug in Type 2 diabetes (non-insulin-dependent diabetes).

Effect of exogenous cholecystokinin on islet blood flow in anesthetized rats

Although a number of studies have investigated the effect of cholecystokinin (CCK) on pancreatic blood flow and exocrine function, few have addressed the effect of CCK on islet blood flow. Here, we studied the effect of exogenous CCK on islet blood flow in anesthetized rats. Islet blood flow was measured by the color microsphere method. Bolus intravenous administration of CCK (10 microg/kg) significantly increased pancreatic and islet blood flow in control Long-Evans Tokushima Otsuka (LETO) rats, but not in Otsuka Long-Evans Tokushima Fatty (OLETF) rats lacking CCK-A receptors. Since fractional islet blood flow expressed as a percentage of whole pancreatic blood flow was decreased after CCK administration in LETO rats, the vasodilating effect of CCK appeared to be stronger in exocrine than endocrine tissue. Although vagotomy failed to alter the CCK-induced increase in pancreatic and islet blood flow, pretreatment with nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine completely prevented the increase in pancreatic and islet blood flow. Our results demonstrated that exogenous CCK is a potent vasodilator of exocrine as well as islet vasculature via CCK-A receptors, and that such action is mediated by a NO-dependent mechanism rather than by vagal mechanisms.

Secretin and pancreatic islet blood flow in anesthetized rats: increased insulin secretion with no augmentation of blood perfusion

Secretin is a stimulator of both endocrine and exocrine secretions of the pancreas, and we aimed to evaluate its effects on splanchnic blood flow in rats with a microsphere technique. Anesthetized rats were infused with secretin (0.5 or 2.0 micrograms/kg body weight/hr) for 10 minutes. Some animals were normoglycemic, whereas other received a glucose injection 3 minutes before blood flow measurements. Secretin did not affect serum insulin concentrations in normoglycemic animals but consistently led to higher insulin concentrations in the hyperglycemic rats. Total pancreatic blood flow was increased by the highest secretin dose in normoglycemic animals, whereas no effects were seen in the hyperglycemic rats. Administration of glucose caused a pronounced increase in islet and fractional islet blood flow in saline-infused animals. Secretin affected neither islet nor fractional blood flow in normoglycemic or hyperglycemic rats. Glucose administration increased duodenal blood flow in animals infused with saline and both duodenal and colonic blood flow in rats given the lowest dose of secretin. No effects on either colonic or duodenal blood perfusion were seen in animals infused with the highest dose of secretin. Secretin mainly affects blood flow to the whole pancreas and not that of the islets. Furthermore, glucose-induced insulin release can be achieved without a simultaneous increase in islet blood flow; that is these two events may be dissociated from one another.

Stimulation of intestinal glucoreceptors in rats increases pancreatic islet blood flow through vagal mechanisms

The aim of the study was to evaluate whether intestinal glucoreceptors participate in the regulation of pancreatic islet blood flow. For this purpose, anesthetized rats were infused (0.1 ml/min for 3 min) with saline, glucose, or 3-O-methylglucose directly into the duodenum. The glucose (1 mg/kg body wt) infusion rate was chosen to prevent any effects on systemic or intraportal blood glucose concentrations. Intraduodenal infusion of D-glucose increased both serum insulin concentration and islet blood flow, whereas the osmotic control substance 3-O-methylglucose had no such effects. A bilateral abdominal vagotomy performed before the infusions totally abolished both the insulin and blood flow response to glucose infusion. The absence of an increased islet blood flow in response to glucose infusion in the denervated, transplanted pancreas was a further indication of the crucial importance of the regulation of islet blood flow by the vagus nerves. It is concluded that intestinal glucoreceptors participate in the mediation of glucose-induced islet blood flow increase.