Selective neurohormonal interactions in islet cell secretion in the isolated perfused human pancreas

Non-glucose modulators of insulin secretion in healthy humans: (dis)similarities between islet and in vivo studies

Optimal metabolic homeostasis requires precise temporal and quantitative control of insulin secretion. Both in vivo and in vitro studies have often focused on the regulation by glucose although many additional factors including other nutrients, neurotransmitters, hormones and drugs, modulate the secretory function of pancreatic β-cells. This review is based on the analysis of clinical investigations characterizing the effects of non-glucose modulators of insulin secretion in healthy subjects, and of experimental studies testing the same modulators in islets isolated from normal human donors. The aim was to determine whether the information gathered in vitro can reliably be translated to the in vivo situation. The comparison evidenced both convincing similarities and areas of discordance. The lack of coherence generally stems from the use of exceedingly high concentrations of test agents at too high or too low glucose concentrations in vitro, which casts doubts on the physiological relevance of a number of observations made in isolated islets. Future projects resorting to human islets should avoid extreme experimental conditions, such as oversized stimulations or inhibitions of β-cells, which are unlikely to throw light on normal insulin secretion and contribute to the elucidation of its defects.

GIP's involvement in the pathophysiology of type 2 diabetes

During the past four decades derangements in glucose-dependent insulinotropic polypeptide (GIP) biology has been viewed upon as contributing factors to various parts of the pathophysiology type 2 diabetes. This overview outlines and discusses the impaired insulin responses to GIP as well as the effect of GIP on glucagon secretion and the potential involvement of GIP in the obesity and bone disease associated with type 2 diabetes. As outlined in this review, it is unlikely that the impaired insulinotropic effect of GIP occurs as a primary event in the development of type 2 diabetes, but rather develops once the diabetic state is present and beta cells are unable to maintain normoglycemia. In various models, GIP has effects on glucagon secretion, bone and lipid homeostasis, but whether these effects contribute substantially to the pathophysiology of type 2 diabetes is at present controversial. The review also discusses the substantial uncertainty surrounding the translation of preclinical data relating to the GIP system and outline future research directions.

Glucagon-like peptide 1 (GLP-1)

BACKGROUND

The glucagon-like peptide-1 (GLP-1) is a multifaceted hormone with broad pharmacological potential. Among the numerous metabolic effects of GLP-1 are the glucose-dependent stimulation of insulin secretion, decrease of gastric emptying, inhibition of food intake, increase of natriuresis and diuresis, and modulation of rodent β-cell proliferation. GLP-1 also has cardio- and neuroprotective effects, decreases inflammation and apoptosis, and has implications for learning and memory, reward behavior, and palatability. Biochemically modified for enhanced potency and sustained action, GLP-1 receptor agonists are successfully in clinical use for the treatment of type-2 diabetes, and several GLP-1-based pharmacotherapies are in clinical evaluation for the treatment of obesity.

SCOPE OF REVIEW

In this review, we provide a detailed overview on the multifaceted nature of GLP-1 and its pharmacology and discuss its therapeutic implications on various diseases.

MAJOR CONCLUSIONS

Since its discovery, GLP-1 has emerged as a pleiotropic hormone with a myriad of metabolic functions that go well beyond its classical identification as an incretin hormone. The numerous beneficial effects of GLP-1 render this hormone an interesting candidate for the development of pharmacotherapies to treat obesity, diabetes, and neurodegenerative disorders.

Cholinergic signaling mediates the effects of xenin-25 on secretion of pancreatic polypeptide but not insulin or glucagon in humans with impaired glucose tolerance

We previously demonstrated that infusion of an intestinal peptide called xenin-25 (Xen) amplifies the effects of glucose-dependent insulinotropic polypeptide (GIP) on insulin secretion rates (ISRs) and plasma glucagon levels in humans. However, these effects of Xen, but not GIP, were blunted in humans with type 2 diabetes. Thus, Xen rather than GIP signaling to islets fails early during development of type 2 diabetes. The current crossover study determines if cholinergic signaling relays the effects of Xen on insulin and glucagon release in humans as in mice. Fasted subjects with impaired glucose tolerance were studied. On eight separate occasions, each person underwent a single graded glucose infusion- two each with infusion of albumin, Xen, GIP, and GIP plus Xen. Each infusate was administered ± atropine. Heart rate and plasma glucose, insulin, C-peptide, glucagon, and pancreatic polypeptide (PP) levels were measured. ISRs were calculated from C-peptide levels. All peptides profoundly increased PP responses. From 0 to 40 min, peptide(s) infusions had little effect on plasma glucose concentrations. However, GIP, but not Xen, rapidly and transiently increased ISRs and glucagon levels. Both responses were further amplified when Xen was co-administered with GIP. From 40 to 240 min, glucose levels and ISRs continually increased while glucagon concentrations declined, regardless of infusate. Atropine increased resting heart rate and blocked all PP responses but did not affect ISRs or plasma glucagon levels during any of the peptide infusions. Thus, cholinergic signaling mediates the effects of Xen on insulin and glucagon release in mice but not humans.

Hormonal Responses to Cholinergic Input Are Different in Humans with and without Type 2 Diabetes Mellitus

Peripheral muscarinic acetylcholine receptors regulate insulin and glucagon release in rodents but their importance for similar roles in humans is unclear. Bethanechol, an acetylcholine analogue that does not cross the blood-brain barrier, was used to examine the role of peripheral muscarinic signaling on glucose homeostasis in humans with normal glucose tolerance (NGT; n = 10), impaired glucose tolerance (IGT; n = 11), and type 2 diabetes mellitus (T2DM; n = 9). Subjects received four liquid meal tolerance tests, each with a different dose of oral bethanechol (0, 50, 100, or 150 mg) given 60 min before a meal containing acetaminophen. Plasma pancreatic polypeptide (PP), glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), glucose, glucagon, C-peptide, and acetaminophen concentrations were measured. Insulin secretion rates (ISRs) were calculated from C-peptide levels. Acetaminophen and PP concentrations were surrogate markers for gastric emptying and cholinergic input to islets. The 150 mg dose of bethanechol increased the PP response 2-fold only in the IGT group, amplified GLP-1 release in the IGT and T2DM groups, and augmented the GIP response only in the NGT group. However, bethanechol did not alter ISRs or plasma glucose, glucagon, or acetaminophen concentrations in any group. Prior studies showed infusion of xenin-25, an intestinal peptide, delays gastric emptying and reduces GLP-1 release but not ISRs when normalized to plasma glucose levels. Analysis of archived plasma samples from this study showed xenin-25 amplified postprandial PP responses ~4-fold in subjects with NGT, IGT, and T2DM. Thus, increasing postprandial cholinergic input to islets augments insulin secretion in mice but not humans.

Trial Registration: ClinicalTrials.gov NCT01434901

Glucagon and type 2 diabetes: the return of the alpha cell

In normal physiology, glucagon from pancreatic alpha cells plays an important role in maintaining glucose homeostasis via its regulatory effect on hepatic glucose production. Patients with type 2 diabetes suffer from fasting and postprandial hyperglucagonemia, which stimulate hepatic glucose production and, thus, contribute to the hyperglycemia characterizing these patients. Although this has been known for years, research focusing on alpha cell (patho)physiology has historically been dwarfed by research on beta cells and insulin. Today the mechanisms behind type 2 diabetic hyperglucagonemia are still poorly understood. Preclinical and clinical studies have shown that the gastrointestinal hormone glucose-dependent insulinotropic polypeptide (GIP) might play an important role in this pathophysiological phenomenon. Furthermore, it has become apparent that suppression of glucagon secretion or antagonization of the glucagon receptor constitutes potentially effective treatment strategies for patients with type 2 diabetes. In this review, we focus on the regulation of glucagon secretion by the incretin hormones glucagon-like peptide-1 (GLP-1) and GIP. Furthermore, potential advantages and limitations of suppressing glucagon secretion or antagonizing the glucagon receptor, respectively, in the treatment of patients with type 2 diabetes will be discussed.

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.

Exogenous glucose-dependent insulinotropic polypeptide worsens post prandial hyperglycemia in type 2 diabetes

OBJECTIVE

Glucose-dependent insulinotropic polypeptide (GIP), unlike glucagon-like peptide (GLP)-1, lacks glucose-lowering properties in patients with type 2 diabetes. We designed this study to elucidate the underlying pathophysiology.

RESEARCH DESIGN AND METHODS

Twenty-two insulin-naïve subjects with type 2 diabetes were given either synthetic human GIP (20 ng x kg(-1) x min(-1)) or placebo (normal saline) over 180 min, starting with the first bite of a mixed meal (plus 1 g of acetaminophen) on two separate occasions. Frequent blood samples were obtained over 6 h to determine plasma GIP, GLP-1, glucose, insulin, glucagon, resistin, and acetaminophen levels.

RESULTS

Compared with placebo, GIP induced an early postprandial increase in insulin levels. Intriguingly, GIP also induced an early postprandial augmentation in glucagon, a significant elevation in late postprandial glucose, and a decrease in late postprandial GLP-1 levels. Resistin and acetaminophen levels were comparable in both interventions. By immunocytochemistry, GIP receptors were present on human and mouse alpha-cells. In alphaTC1 cell line, GIP induced an increase in intracellular cAMP and glucagon secretion. CONCLUSIONS; GIP, given to achieve supraphysiological plasma levels, still had an early, short-lived insulinotropic effect in type 2 diabetes. However, with a concomitant increase in glucagon, the glucose-lowering effect was lost. GIP infusion further worsened hyperglycemia postprandially, most likely through its suppressive effect on GLP-1. These findings make it unlikely that GIP or GIP receptor agonists will be useful in treating the hyperglycemia of patients with type 2 diabetes.

Immunoneutralization of somatostatin, insulin, and glucagon causes alterations in islet cell secretion in the isolated perfused human pancreas

INTRODUCTION

In this study, immunoneutralization of endogenous insulin, glucagon, and somatostatin with specific antibodies was used in an isolated perfused human pancreas (IPHP) model.

AIMS

To study intrapancreatic cellular interactions and pancreatic hormonal secretion.

METHODOLOGY

Randomized, sequential 10-minute test intervals of single-pass perfusion with each antibody were performed at 3.9 mM or 11.5 mM steady-state glucose concentrations. Somatostatin, insulin, and glucagon levels were measured in the effluent during basal and immunoneutralization intervals.

RESULTS

At 3.9 mM glucose concentration, somatostatin antibody (SS-Ab) stimulated insulin and glucagon secretion, insulin antibody (IN-Ab) inhibited glucagon secretion, and glucagon antibody (GN-Ab) stimulated insulin secretion. At 11.5 mM glucose concentration, SS-Ab stimulated insulin secretion, IN-Ab stimulated glucagon and inhibited somatostatin secretion, and GN-Ab stimulated insulin secretion.

CONCLUSION

The variation in hormonal responses to immunoneutralization during stimulated and nonstimulated glucose conditions suggests that a dynamic association exists between the pancreatic cells.

TNF but not IL-1 decreases pancreatic acinar cell survival without affecting exocrine function: a study in the perfused human pancreas

Substantial quantities of interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) are produced within the pancreatic parenchyma during acute pancreatitis. Recent evidence suggests that IL-1 beta and TNF-alpha propagate acute pancreatitis and intensify the resulting pancreatic acinar cell death. This study examines the direct effect of IL-1 beta and TNF-alpha on pancreatic acinar cells. Human pancreata (n = 6), harvested during organ procurement, were perfused ex vivo through the splenic artery using a sterile, oxygenated colloid solution. Each pancreas was perfused with either recombinant human IL-1 beta or TNF-alpha for 2 h and subsequently with the cholecystokinin analogue caerulein (positive control). Venous effluent was collected continuously and amylase and lipase were determined at 15-min intervals. Pancreatic histology was graded at baseline and following cytokine and caerulein perfusion. To examine the long-term effects of these cytokines on acinar cell viability, additional in vitro studies utilized the AR42J acinar cell line which was exposed to either IL-1 beta or TNF-alpha with survival determined daily by MTT assay. Perfusion of the human pancreas with either IL-1 beta or TNF-alpha did not alter amylase, lipase, or histology. Caerulein did induce pancreatitis as measured by increased amylase, lipase, and pancreatic histology. Survival of pancreatic acinar cells decreased when they were incubated with TNF-alpha but not IL-1 beta. Although present in large amounts within the pancreas during acute pancreatitis, IL-1 beta and TNF-alpha have no direct effect on acinar cell viability or exocrine function acutely nor do they induce pancreatitis. When present for more than 24 h, however, TNF-alpha but not IL-1 beta has a dramatic effect on acinar cell survival.

An Internet Multicast Symposium Concerning the Microcirculation of the Islets of Langerhans

The Internet Globally-linked Computer System was used to conduct an international scientific symposium. The symposium was held at the VAMC-Long Beach and consisted of prepared lectures that were multicast over the Internet. The basic unit of hardware used for the Internet Multicast was the Silicon Graphics Indy Unix Workstation, which was equipped with a color video camera. The multicast required four additional pieces of software from the file transfer protocol. The multicast backbone protocol allowed for simultaneous audio and video signals (the presenter, the slides, and the videotape images of islet microcirculation studies) to be transmitted over the computer network. The faculty included 12 experts in microcirculation, who gave 15-min lectures that included a question and answer period. All lectures were received at 14 computer stations in six countries. Eleven of the faculty gave their lectures at the VAMC-Long Beach, and one gave her lecture at the Massachusetts Institute of Technology in Boston, MA. The presenter from Boston was able to receive and answer questions from the faculty at the VAMC-Long Beach. An estimated $12,000 was saved in travel, hotel, and food costs and an estimated 180 travel hours were saved by viewers who did not have to travel to the symposium. We have demonstrated that a scientific symposium can be conducted using the Internet. We propose that many of our future meetings will be organized over the computer network. This format of multiimage projections allows us to effectively communicate in a personal way with a reduction in expensive and time-consuming travel.

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

The aim of the study was to investigate how the incretin candidate hormone gastric inhibitory polypeptide (GIP) affects splanchnic blood flow, especially pancreatic islet blood flow. For this purpose, male Sprague-Dawley rats were injected intravenously with either saline or GIP (5 or 15 micrograms/kg body weight) 10 min before blood flow measurements by a microsphere technique. Furthermore, 3 min before the blood flow measurements, 1 ml of either saline or 30% D-glucose was given intravenously. All glucose-injected animals were markedly hyperglycemic (> 20 mmol/liter) at the time of the blood flow measurements. Both doses of GIP potentiated basal and glucose-stimulated insulin release. In the normoglycemic rats, the lowest dose of GIP did not affect the blood perfusion to any of the investigated organs. The highest dose of GIP decreased whole pancreatic and duodenal blood flow, whereas islet blood flow was unaffected. As a result, fractional islet blood flow was increased. In the hyperglycemic rats, where the islet blood flow was increased compared with control animals, both doses of GIP further enhanced islet blood flow. No effect on pancreatic, fractional islet, or duodenal blood flow was seen after GIP administration to hyperglycemic animals. It is concluded that administration of GIP can further augment the glucose-induced stimulation of islet blood flow. This may contribute to facilitating release of insulin from the islets.

Nitric oxide regulates insulin secretion in the isolated perfused human pancreas via a cholinergic mechanism

BACKGROUND

The purpose of this study was to determine whether nitric oxide regulates insulin secretion in the isolated perfused human pancreas.

METHODS

Single-pass perfusion was performed in four pancreata with a modified Krebs medium. Sequential 10-minute infusions (separated by 10-minute basal periods) of (1) 25 nmol/L acetylcholine, (2) 2.5 mumol/L acetylcholine, and (3) 16.7 mmol/L glucose were initially infused. Then 0.1 mumol/L of NG-monomethyl-L-arginine (NMMA) was infused during a period of 10 minutes, and steps (1) through (3) were repeated. The change in insulin secretion from basal levels during each stimulation was calculated and compared with that seen after NMMA infusion.

RESULTS

Infusion of 25 nmol/L and 2.5 mumol/L acetylcholine resulted in a significant stimulation of insulin secretion before NMMA infusion (p < 0.05) and after NMMA infusion for acetylcholine at 25 nmol/L (p < 0.05). There was a significant decrease in acetylcholine-induced insulin secretion after NMMA infusion for acetylcholine at 25 nmol/L and 2.5 mumol/L compared with before NMMA infusion (p < 0.05). Infusion of 16.7 mmol/L glucose significantly stimulated insulin secretion before and after NMMA infusion, but there was no significant difference seen with insulin secretion before and after NMMA infusion. Insulin secretion was significantly inhibited during NMMA infusion (p < 0.05).

CONCLUSIONS

These data show that infusion of the nitric oxide synthase inhibitor NMMA suppressed cholinergic-stimulated insulin secretion but did not affect glucose-stimulated insulin secretion. We conclude that nitric oxide regulates insulin secretion in the isolated perfused human pancreas.

The influence of somatostatin on glucagon and pancreatic polypeptide secretion in the isolated perfused human pancreas

The current study was undertaken to determine whether intraislet somatostatin regulates glucagon or pancreatic polypeptide (PP) secretion in the human pancreas. A high-affinity, high-specificity monoclonal somatostatin antibody (CURE.S6) was used to immunoneutralize somatostatin in the isolated, perfused human pancreas. Single-pass perfusion was performed in pancreata obtained from cadaveric organ donors using a modified Krebs media with either 3.9 or 12.9 mM glucose. Sequential test periods separated by basal periods were performed with infusion of either exogenous somatostatin-14 (SS-14), CURE.S6, or a combined infusion. Infusion of SS-14 did not significantly alter glucagon or PP secretion during low-glucose or high-glucose perfusion. Immunoneutralization of intraislet somatostatin with CURE.S6 resulted in a significant increase of glucagon secretion under low-glucose conditions (delta X = 15 +/- 3 pM) (p < 0.05), but did not significantly effect glucagon secretion under high-glucose conditions (delta X = -2 +/- 3 pM) (p = NS). PP secretion remained unchanged during CURE.S6 infusion. Combined infusion of SS-14 and CURE.S6 did not significantly alter glucagon or PP secretion. The data suggest that intraislet somatostatin may have an inhibitory role in the regulation of glucagon secretion during low-glucose conditions and that intraislet somatostatin does not regulate PP secretion in the isolated, perfused human pancreas.

Use of the Fab fragment for immunoneutralization of somatostatin in the isolated perfused human pancreas

The role of the somatostatin-secreting D cell in the islet remains controversial. The present study was undertaken to determine whether infusion of the Fab fragment of a highly sensitive somatostatin monoclonal antibody into the isolated, perfused human pancreas would influence insulin secretion. Single-pass perfusion was performed in pancreata obtained from cadaveric organ donors using a modified Krebs-media with 3.9 mM glucose. Sequential test periods separated by basal periods were performed with either somatostatin monoclonal antibody Fab fragment (SFab), somatostatin-14 (SS-14), or a combined infusion. Immunoneutralization of intraislet somatostatin with SFab resulted in a significant increase in both immunoreactive insulin (IRI) (1,122 +/- 497 pM) (p < 0.05) and immunoreactive C-peptide (IRC-P) secretion (146 +/- 53 pM) (p < 0.05). Infusion of SS-14 resulted in inhibition of both IRI secretion (-3,372 +/- 1,360 pM) (p < 0.05) and IRC-P secretion (-708 +/- 220 pM) (p < 0.05). Combined infusion of SFab and SS-14 reversed the inhibitory effect of exogenous SS-14 on IRI and IRC-P secretion. The data suggest that intraislet somatostatin has an inhibitory role in the regulation of B-cell secretion in the human islet and demonstrates that the Fab fragment of the somatostatin monoclonal antibody is an effective tool for immunoneutralization studies in the human pancreas. In addition, immunostaining of the donor pancreata demonstrated the presence of somatostatin-immunoreactive endocrine cells interspersed throughout the islet core and mantle. The demonstrated proximity of somatostatin-immunoreactive endocrine cells to B cells lends anatomic support to the concept that intraislet somatostatin influences insulin secretion in the human islet.

Islet cell responses to glucose in human transplanted pancreas

Postsurgery, pancreas transplantation results in alterations of carbohydrate metabolism. Additionally, immunosuppressive therapy impacts on glucose regulation. We evaluated the hormonal and metabolic responses of pancreas allografts, utilizing the hyperglycemic clamp technique coupled with the tritiated glucose methodology, in 11 volunteers who had received simultaneous pancreas-kidney transplantation (P-K) with systemic drainage. Their responses were compared with seven volunteers who had received only a kidney (K) graft and with seven normal control (C) volunteers. Although basal glucose and hepatic glucose output were similar in all three groups, basal insulin, C-peptide, glucagon, and pancreatic polypeptide were highest in the P-K group and lowest in normal subjects. During hyperglycemia, all groups showed a similar characteristic, initial complete suppression of hepatic glucose production, with recovery followed by a later suppression. Peripheral glucose uptake was similar in P-K and C subjects but decreased in K patients. Systemic insulin levels were fourfold higher in the pancreas transplant patients than in healthy subjects. Thus, under basal and hyperglycemic stimulation, 1) hepatic glucose homeostasis is regulated normally, even with pancreatic drainage into the systemic circulation; 2) overall glucose disposal is normal in P-K patients because of marked hyperinsulinemia; and 3) there is loss of tonic inhibition of endocrine pancreatic function secondary to pancreatic denervation.