Ramona A. Silvestre

Inhibitory effect of rat amylin on the insulin responses to glucose and arginine in the perfused rat pancreas.

Amylin, a 37-amino acid polypeptide, is the main component of amyloid deposits in the islets of Langerhans, and has been identified in the B-cell secretory granules. We have investigated the effect of rat amylin on the insulin and glucagon release by the isolated, perfused rat pancreas. Amylin infusion at 750 nM, markedly reduced unstimulated insulin release (ca. 50%, P less than 0.025), whereas it did not modify glucagon output. At the same concentration, amylin also blocked the insulin response to 9 mM glucose (ca. 80%, P less than 0.025) without affecting the suppressor effect of glucose on glucagon release. The inhibitory effect of amylin on glucose-induced insulin secretion was confirmed by lowering the amylin concentration (500 nM) and increasing the glucose stimulus (11 mM); again, no effect of amylin on glucagon release was observed. Finally, amylin, at 500 nM, reduced the insulin response to 3.5 mM arginine (ca. 40%, P less than 0.025) without modifying the secretion of glucagon elicited by this amino acid. It can be concluded that, in the rat pancreas, the inhibitory effect of homologous amylin on unstimulated insulin secretion, as well as on the insulin responses to metabolic substrates (glucose and arginine), favours the concept of this novel peptide as a potential diabetogenic agent.

Amylin inhibits glucose-induced insulin secretion in a dose-dependent manner. Study in the perfused rat pancreas.

Islet amyloid polypeptide (IAPP), also called amylin, has been localized in the B-cell secretory granule and is co-secreted with insulin. We have investigated the effect of synthetic amidated rat amylin on the insulin release evoked by 9 mM glucose in the isolated, perfused rat pancreas. Amylin, in a range of 75 nM-75 pM, significantly inhibited this insulin response in a dose-dependent manner. The correlation between the logarithm of amylin concentrations and the percentages of inhibition was highly significant (r = 0.98, P < 0.01). The lowest effective amylin concentration tested (75 pM) is within the range of amylin levels reported for the effluent of the perfused rat pancreas. Finally, pre-infusion of the rat pancreas with a high amylin concentration (75 nM) did not alter the insulin response to glucose, thus excluding a toxic effect of amylin on the B-cell. These observations support the concept that amylin plays a role in the control of insulin secretion.

Effect of obestatin on insulin, glucagon and somatostatin secretion in the perfused rat pancreas.

Obestatin is a 23-amino acid peptide derived from preproghrelin, purified from stomach extracts and detected in peripheral plasma. In contrast to ghrelin, obestatin has been reported to inhibit appetite and gastric motility. However, these effects have not been confirmed by some groups. Obestatin was originally proposed to be the ligand for GPR39, a receptor related to the ghrelin receptor subfamily, but this remains controversial. Obestatin and GPR39 are expressed in several tissues, including pancreas. We have investigated the effect of obestatin on islet cell secretion in the perfused rat pancreas. Obestatin, at 10 nM, inhibited glucose-induced insulin secretion, while at 1 nM, it potentiated the insulin response to glucose, arginine and tolbutamide. The potentiated effect of obestatin on glucose-induced insulin output was not observed in the presence of diazoxide, an agent that activates ATP-dependent K(+) channels, thus suggesting that these channels might be sensitive to this peptide. Obestatin failed to significantly modify the glucagon and somatostatin responses to arginine, indicating that its stimulation of insulin output is not mediated by an alpha- or delta-cell paracrine effect. Our results allow us to speculate about a role of obestatin in the control of beta-cell secretion. Furthermore, as an insulinotropic agent, its potential antidiabetic effect may be worthy of investigation.

26RFa, a novel orexigenic neuropeptide, inhibits insulin secretion in the rat pancreas.

26RFa is a novel orexigenic neuropeptide identified as the endogenous ligand of the orphan G protein-coupled receptor GPR103. GPR103 shares sequence identity with the receptors for neuropeptide-Y and galanin, two peptides known to inhibit insulin secretion. We have investigated the effect of 26RFa on insulin and glucagon secretion in the perfused rat pancreas. 26RFa dose-dependently reduced glucose-induced insulin release, inhibited the insulin responses to both arginine and exendin-4 and did not affect glucagon output. The inhibitory effect of 26RFa on exendin-4-induced insulin secretion was not observed in pancreata from pertussis toxin-treated rats, thus suggesting that 26RFa may inhibit insulin secretion, at least in part, via a pertussis toxin-sensitive G(i) protein coupled to the adenylyl cyclase system.

Somatostatin, insulin and glucagon secretion by the perfused pancreas from the cysteamine-treated rat

In rats, administration of a single dose of cysteamine (300 mg/kg, intragastrically) induces a depletion of pancreatic somatostatin content (approximately 60%) without modifying pancreatic insulin or glucagon content. In perfused pancreases from cysteamine-treated rats, there was a lack of somatostatin response to glucose, arginine or tolbutamide. In the absence of stimulated somatostatin release, the secretory responses of insulin and glucagon to glucose, to arginine, and to tolbutamide were not significantly different from those observed in pancreases from control rats. Our data do not support the concept that pancreatic somatostatin plays a major role in the control of insulin and glucagon release.

Interrelationship among insulin, glucagon and somatostatin secretory responses to exendin-4 in the perfused rat pancreas.

We have investigated the effect of exendin-4 on insulin, glucagon and somatostatin output in the perfused rat pancreas. At 9 mM glucose, exendin-4 potentiated the insulin and somatostatin responses to arginine and reduced the glucagon response to this amino acid. Thus, this reduction might be thought to be paracrine-mediated through the concomitant increase in insulin and somatostatin concentrations. At 3.2 mM glucose, exendin-4 did not affect insulin secretion, reduced glucagon release and stimulated somatostatin output. Furthermore, exendin-4 reduced glucagon secretion as induced by a glucose decline (from 11 to 3.2 mM) without affecting insulin or somatostatin responses. In summary, exendin-4 stimulated insulin and somatostatin secretion and reduced glucagon release. The glucagonostatic effect of exendin-4 was observed under conditions in which insulin and somatostatin were not affected, thus indicating that exendin-4, per se, inhibits A-cell secretion. Indeed, an additional glucagonostatic effect of exendin-4, mediated by its stimulation of insulin and/or somatostatin secretion, cannot be ruled out.

Pancreastatin inhibits insulin secretion as induced by glucagon, vasoactive intestinal peptide, gastric inhibitory peptide, and 8-cholecystokinin in the perfused rat pancreas☆

Pancreastatin is a 49-amino acid straight chain molecule isolated from porcine pancreatic extracts. In the perfused rat pancreas, this peptide has been shown to inhibit unstimulated insulin release and the insulin responses to glucose, arginine, and tolbutamide. To further explore the influence of pancreastatin on islet cell secretion, the effect of synthetic porcine pancreastatin (a 2-micrograms priming dose, followed by constant infusion at a concentration of 15.7 nmol/L) was studied on the insulin, glucagon, and somatostatin responses to 1 nmol/L vasoactive intestinal peptide (VIP), 1 nmol/L gastric inhibitory peptide (GIP), and 1 nmol/L 26 to 33 octapeptide form of cholecystokinin (8-CCK). The effect of pancreastatin on the insulin and somatostatin secretion elicited by glucagon (20 nmol/L) was also examined. Pancreastatin infusion consistently reduced the insulin responses to VIP, GIP, and 8-CCK without modifying glucagon or somatostatin release. It also inhibited the insulin release but not the somatostatin output induced by glucagon. These observations broaden the spectrum of pancreastatin as an inhibitor of insulin release. The finding that pancreastatin does not alter glucagon or somatostatin secretion supports the concept that it influences the B cell directly, and not through an A cell or D cell paracrine effect.

Inhibition of insulin release by amylin is not mediated by changes in somatostatin output

We have investigated the effect of a high concentration (750 nM) of synthetic amidated rat amylin on unstimulated somatostatin and insulin secretion as well as on the response of these hormones to arginine. Amylin consistently reduced insulin output but it did not significantly modify somatostatin release. These findings indicate that the inhibitory effect of amylin on insulin secretion is not mediated by a D-cell paracrine effect.

Reversal of the inhibitory effects of calcitonin gene-related peptide (CGRP) and amylin on insulin secretion by the 8–37 fragment of human CGRP

The 8-37 fragment of human calcitonin gene-related peptide [(8-37)hCGRP] antagonizes the effects of calcitonin gene-related peptide (CGRP) and amylin in a number of tissues. We have studied the influence of (8-37)hCGRP on the effects of both CGRP and amylin on insulin secretion. In the perfused rat pancreas, homologous CGRP and amylin, at 75 pM, exerted comparable inhibitory effects on the insulin response to 9 mM glucose (ca. 70%; P < 0.025). These effects were antagonized by (8-37)hCGRP (1 microM). Our results suggest that CGRP and amylin act on the B-cell, at least in part, through a common receptor.

Stimulatory effect of xenin-8 on insulin and glucagon secretion in the perfused rat pancreas

Xenin is a 25-amino acid peptide of the neurotensin/xenopsin family identified in gastric mucosa as well as in a number of tissues, including the pancreas of various mammals. In healthy subjects, plasma xenin immunoreactivity increases after meals. Infusion of the synthetic peptide in dogs evokes a rise in plasma insulin and glucagon levels and stimulates exocrine pancreatic secretion. The latter effect has also been demonstrated for xenin-8, the C-terminal octapeptide of xenin. We have investigated the effect of xenin-8 on insulin, glucagon and somatostatin secretion in the perfused rat pancreas. Xenin-8 stimulated basal insulin secretion and potentiated the insulin response to glucose in a dose-dependent manner (EC(50)=0.16 nM; R(2)=0.9955). Arginine-induced insulin release was also augmented by xenin-8 (by 40%; p<0.05). Xenin-8 potentiated the glucagon responses to both arginine (by 60%; p<0.05) and carbachol (by 50%; p<0.05) and counteracted the inhibition of glucagon release induced by increasing the glucose concentration. No effect of xenin-8 on somatostatin output was observed. Our observations indicate that the reported increases in plasma insulin and glucagon levels induced by xenin represent a direct influence of this peptide on the pancreatic B and A cells.