Neuropeptides in the regulation of islet hormone secretion--localization, effects and mode of action

Neural control of the endocrine pancreas

The autonomic nervous system affects glucose metabolism partly through its connection to the pancreatic islet. Since its discovery by Paul Langerhans, the precise innervation patterns of the islet has remained elusive, mainly because of technical limitations. Using 3-dimensional reconstructions of axonal terminal fields, recent studies have determined the innervation patterns of mouse and human islets. In contrast to the mouse islet, endocrine cells within the human islet are sparsely contacted by autonomic axons. Instead, the invading sympathetic axons preferentially innervate smooth muscle cells of blood vessels. This innervation pattern suggests that, rather than acting directly on endocrine cells, sympathetic nerves may control hormone secretion by modulating blood flow in human islets. In addition to autonomic efferent axons, islets also receive sensory innervation. These axons transmit sensory information to the brain but also have the ability to locally release neuroactive substances that have been suggested to promote diabetes pathogenesis. We discuss recent findings on islet innervation, the connections of the islet with the brain, and the role islet innervation plays during the progression of diabetes.

Novel approaches to studying the role of innervation in the biology of pancreatic islets

The autonomic nervous system helps regulate glucose homeostasis by acting on pancreatic islets of Langerhans. Despite decades of research on the innervation of the pancreatic islet, the mechanisms used by the autonomic nervous input to influence islet cell biology have not been elucidated. This article discusses how these barriers can be overcome to study the role of the autonomic innervation of the pancreatic islet in glucose metabolism. It describes recent advances in microscopy and novel approaches to studying the effects of nervous input that may help clarify how autonomic axons regulate islet biology.

Arterial gastroduodenal infusion of cholecystokinin-33 stimulates the exocrine pancreatic enzyme release via an enteropancreatic reflex, without affecting the endocrine insulin secretion in pigs

OBJECTIVES

Cholecystokinin (CCK)-dependent exocrine pancreatic regulation seems to involve different pathways in different species. The aims were to explore the enteropancreatic reflex in the CCK-mediated regulation of the exocrine pancreas and to evaluate a possible involvement of this reflex in the endocrine insulin release.

METHODS

In anesthetized pigs, CCK-33 in increasing doses (4-130 pmol kg 10 min) was infused locally to the gastroduodenal artery, or systemically via the jugular vein. Also, a low CCK-33 dose (13 pmol kg) was injected to the duodenum/antrum area before and after a bilateral truncal vagotomy.

RESULTS

Cholecystokinin-33 in the physiological dose range 4 to 32 pmol kg 10 min increased protein and trypsin outputs after local infusion to the antral-duodenal area, whereas it had no effect after systemic infusion. Cholecystokinin-33 in the pharmacological dose range 64 to 130 pmol kg 10 min further increased the secretion after both local and systemic infusions. Only CCK-33 infusions in the pharmacological dose range were able to elevate the plasma insulin levels. Vagotomy had no effect on CCK-33-mediated stimulation of the enzyme release, whereas it had a significant effect on the plasma insulin level.

CONCLUSIONS

Cholecystokinin-33 in the physiological dose range 4 to 32 pmol kg 10 min stimulates the enzyme secretion but had no effect on the insulin release via a short enteropancreatic pathway in pigs.

Expression of the NK-1 receptor on islet cells and invading immune cells in the non-obese diabetic mouse

The underlying mechanistic causes of immune cell infiltration in the islets of Langerhans and beta cell failure in the non-obese diabetic (NOD) mouse is still to be completely revealed. Substance P (SP) is a substance known to have pro-inflammatory, endocrine, neuromodulatory and trophic effects, and its preferred receptor, the neurokinin receptor 1 (NK-1 R), is reported to be involved in extravasation of granulocytes and in inflammation and tissue derangement. Therefore, we have investigated the expression of NK-1 R during development of insulitis in the NOD mouse. We show that the magnitude of immunoreactivity scoring NK-1 R expression in the islets was increased in the 12-week-old NOD mouse. Expression of NK-1 R co-localized with expression of glucagon. In line with this expression pattern, we did not detect any effect of SP on glucose-induced insulin release. NK-1 R expression was particularly observed in islet cells in association with the clusters of immune cells. Expression of NK-1 R was also demonstrated in a fraction of the infiltrating B and T lymphocytes, as well as on infiltrating macrophages and dendritic cells. The observations show that the level of NK-1 R expression is increased in 12-week-old NOD mice, being correlated with the occurrence of islet mononuclear infiltration. Our data suggest that SP may act as a chemoattractant, contributing to the pathogenic mononuclear infiltration process in the NOD mouse. On the whole, the observations suggest that SP and the NK-1 R to certain extents are involved in the changes that occur during the development of insulitis in the NOD mouse.

Peptides and other neuronal markers in transplanted pancreatic islets

Functional alterations are developed in transplanted islets over time. Because islets in situ are densely innervated and isolation disconnects the endocrine organ from extrinsic nerves and from ganglia in the exocrine pancreas, it is important to examine the reinnervation of islet grafts. This review describes the patterns of appearances of intrinsic perikarya and reinnervating fibers demonstrating markers for parasympathetic, sympathetic or sensory nerve substances, most notably neuropeptides, in islet transplants. An altered innervation pattern, as compared to normal islets, develops. Presumably the expression of neuronal markers in the grafts is related to factors both in the islets and in the ectopic environment offered by the implantation organ.

An endogenous peptide isolated from the gut, NK-lysin, stimulates insulin secretion without changes in cytosolic free Ca2+ concentration

We have recently isolated and cloned a novel endogenous peptide from pig intestine, NK-lysin (NKL). In the present study we show that NKL (1-100 nM) potently and reversibly stimulates insulin secretion in rat pancreatic islets and in the beta-cell line HIT T15. This effect of NKL was not accompanied by changes in cytoplasmic free calcium concentration. The stimulatory activity of NKL on insulin release was also observed in permeabilized islets under Ca2+-clamped conditions. Preincubation of HIT T15 cells with NKL for 1 h or 24 h did not influence cell viability. Possible mechanisms of insulinotropic activity of NKL are discussed.

Galparan: a powerful insulin-releasing chimeric peptide acting at a novel site

Galparan is a 27-amino acid long chimeric peptide, GWTLNSAGYLLGP-INLKALAALAKKIL amide, consisting of galanin-(1-13) linked to mastoparan amide via a peptide bond to provide the mastoparan and galanin effector parts of the molecules. Galparan (10 microM) powerfully stimulates insulin secretion from isolated rat pancreatic islets in a reversible and dose-dependent manner; the stimulation is 26-fold at 3.3 mM glucose and 6-fold at 16.7 mM glucose. Galparan also enhances insulin secretion to a similar extent from islets of diabetic GK rats. The stimulatory effect of galparan on insulin release is not directly dependent on extracellular Ca2+, nor can it be explained only by changes in free cytosolic Ca2+ concentrations. Furthermore, galparan is effective in evoking insulin release in B cells depolarized by 25 mM KCl when ATP-sensitive K+ channels are kept open by diazoxide. Thus, galparan, like mastoparan, stimulates exocytosis of insulin at a distal site in the stimulus-secretion coupling of the B cell. This distal site is not identical to that used by mastoparan, as pertussis toxin pretreatment does not influence the insulinogenic effect of galparan. In conclusion, galparan evokes a large and reversible insulin secretion, acting at a yet unknown distal site and also promoting exocytosis in depolarized B cells from normal rats as well as diabetic GK rats.

Aspects of autonomic and neuroendocrine function

This article provides a brief review of aspects of autonomic and neuroendocrine function studied initially in collaboration with the late Marian Silver. The importance of the sympathetic innervation to the liver in the control of glycogenolysis was established in anaesthetised animals of various species. Otherwise the work has been carried out mainly in conscious animals under strictly physiological conditions and below behavioural threshold. Investigations of the role of the autonomic innervation to the endocrine pancreas in controlling the release of pancreatic hormones, led to the realisation that the parasympathetic innervation mediates responses to glycaemic stimuli while the sympathetic innervation mediates responses to any form of stress. Studies of adrenal medullary function have confirmed that its threshold for many forms of stress is much higher than that of other components of the sympathetic system and revealed the importance of the pattern of electrical stimulation in determining the rates of release of catecholamines, enkephalins, corticotrophin-releasing factor (CRF) and adrendocorticotrophin (ACTH). The splanchnic sympathetic innervation to the adrenal cortex also plays an important role in determining glucocorticoid output by sensitising the cells to ACTH, probably mainly by the release of vasoactive intestinal peptide (VIP) from cortical nerve terminals. Finally studies of feeding in milk-fed calves have shown that suckling is associated with a remarkable hypertension and tachycardia. These cardiovascular effects are due to a selective sympathetic discharge, which does not involve the adrenal medullae, or the release of neuropeptide Y (NPY) and, at least in the calf, can be attributed to activation of adrenoceptors.

Localization of calcitonin gene-related peptide and islet amyloid polypeptide in the rat and mouse pancreas

It was previously demonstrated that the two chemically related peptides calcitonin gene-related peptide (CGRP) and islet amyloid polypeptide (IAPP) both occur in the pancreas. We have now examined the cellular localization of CGRP and IAPP in the rat and the mouse pancreas. We found, in both the rat and the mouse pancreas, CGRP-immunoreactive nerve fibers throughout the parenchyma, including the islets, with particular association with blood vessels. CGRP-immunoreactive nerve fibers were regularly seen within the islets. In contrast, no IAPP-immunoreactive nerve fibers were demonstrated in this location. Furthermore, in rat islets, CGRP immunoreactivity was demonstrated in peripherally located cells, constituting a major subpopulation of the somatostatin cells. Such cells were lacking in the mouse islets. IAPP-like immunoreactivity was demonstrated in rat and mouse islet insulin cells, and, in the rat, also in a few non-insulin cells in the islet periphery. These cells seemed to be identical with somatostatin/CGRP-immunoreactive elements. In summary, the study shows (1) that CGRP, but not IAPP, is a pancreatic neuropeptide both in the mouse and the rat; (2) that a subpopulation of rat somatostatin cells contain CGRP; (3) that mouse islet endocrine cells do not contain CGRP; (4) that insulin cells in both the rat and the mouse contain IAPP; and (5) that in the rat, a non-insulin cell population apparently composed of somatostatin cells stores immunoreactive IAPP. We conclude that CGRP is a pancreatic neuropeptide and IAPP is an islet endocrine peptide in both the rat and the mouse, whereas CGRP is an islet endocrine peptide in the rat.