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The role of gastric inhibitory polypeptide in the regulation of pancreatic endocrine secretion Verchere, Cameron Bruce


Gastric inhibitory polypeptide (GIP) has often been referred to as glucose-dependent insulinotropic polypeptide because of its potent stimulatory effect on insulin secretion. A stimulatory action of GIP on the release of other islet hormones has also been suggested in certain studies. However, attempts to investigate the interaction of GIP with pancreatic islet cells at the molecular and cellular levels have been unsuccessful, presumably due to damage to the GIP receptor in the enzymatic isolation of pancreatic islets. The experiments presented here examined the hypothesis that GIP exerts a direct influence on the secretion of islet hormones via specific receptors. This was investigated primarily through the use of in vitro preparations of islets and /3-cells made more sensitive to the actions of GIP by tissue culture. Isolated rat islets, following two days of culture, responded to physiological concentrations of GIP (1 nM) with increased insulin, glucagon and somatostatin secretion. The stimulation of islet hormone secretion was dependent upon the concentrations of both GIP and glucose. The threshold glucose concentration for 1.0 nM GIP-stimulated insulin release from cultured islets was 8.9 mM and the maximum potentiation of GIP-stimulated insulin secretion was observed in the presence of 17.8 mM glucose. In 2.75 mM glucose, below the threshold for the insulinotropic action of GIP, the presence of arginine (10 mM) was sufficient to permit the stimulatory action of GIP on the β-cell to occur, suggesting that non-glucose fuel stimuli can sensitize the β-cell to the action of GIP. In contrast, glucagon release from isolated islets was stimulated by 1.0 nM GIP only in the presence of low glucose concentrations, as glucose levels greater than 2.75 mM attenuated the stimulatory effect of GIP on the α-cell. GIP-stimulated somatostatin secretion was not clearly glucose-dependent, although the maximum stimulation of somatostatin release by GIP occurred in 17.8 mM glucose. Acetylcholine (ACh) strongly suppressed somatostatin secretion induced by 17.8 mM glucose plus 10 nM GIP. Together, these results suggest that GIP is involved not only in the regulation of insulin and glucagon secretion, as was shown in previous studies, but also that GIP may regulate pancreatic somatostatin secretion. Further, the demonstration that GIP stimulated the release of these islet hormones in vitro indicates that the peptide acts directly on islet cells and that its effects are not dependent on indirect neural or vascular pathways. GIP-stimulated islet hormone secretion appears to be strongly influenced by the presence of glucose, other fuels and neural inputs. GIP also stimulated the release of insulin from a preparation of β-cells sorted to > 98 % purity using a fluorescence-activated cell-sorter (FACS), confirming that GIP acts directly on the β-cell to exert its insulinotropic effect. The sensitivity of FACS-purified β-cells to GIP was considerably less than intact islets, but could be enhanced by the addition of 10 nM glucagon or by culturing the cells with a non- β-cell fraction obtained from the FACS that consisted of approximately 50% α-cells. These results suggested that the mechanism of GIP-stimulated insulin secretion may partly involve intra-islet interactions, possibly with the islet a-cell. Other known insulin secretagogues increased secretion from FACS-purified β-cells, including glucagon, glucagon-like peptide I (7-36-NH₂), ACh and cholecystokinin-8 (CCK-8). GIP also stimulated insulin release from a mouse tumor β-cell line (βTC3). GIP-induced insulin secretion from βTC3 cells was glucose-dependent and like FACS-purified β-cells, was found to be approximately 100-fold less sensitive than isolated islets to the stimulatory effects of GIP. A mono-iodinated, biologically active form of ¹²⁵I-GIP was purified by reverse-phase high performance liquid chromatography (HPLC) for use in radioreceptor binding studies with βTC3 cells and cultured rat islets. Binding of the HPLC-purified radioligand to βTC3 cells and cultured islets was displaced by GIP in a concentration-dependent manner starting at concentrations of GIP as low as 1 nM. ¹²⁵I-GIP could not be displaced by numerous other peptides including those structurally related to GIP. The data strongly support the existence of a specific receptor for GIP on pancreatic β-cells. A biotinylated form of GIP (B-GIP) was also produced and found to stimulate insulin release from the perfused rat pancreas and may therefore be a useful probe in future GIP receptor studies. Islets isolated from obese Zucker (fa/fa) rats were found to be more sensitive to the insulinotropic action of GIP than those of lean (Fa/?) rats. As had been observed in the perfused pancreas, the islets of obese but not lean rats responded to GIP (2.0 nM) in the presence of 4.4 mM glucose, below the normal threshold for the insulinotropic action of GIP. This could not be attributed to intra-islet interactions, since islet glucagon and somatostatin secretion from obese and lean rat islets were similar in 4.4 mM glucose plus GIP, and obese rat islets were more sensitive to the inhibitory effects of somatostatin. The results suggest the existence of a β-cell defect in obese rats, possibly at the level of the GIP receptor or intracellular signal transduction. In 8.9 mM glucose, glucagon secretion from obese but not lean rat islets was suppressed compared to 4.4 mM glucose and enhanced by the addition of 2.0 nM GIP. The data indicate the presence of multiple alterations in hormone secretion from obese Zucker rat islets which remain apparent in culture, following removal of the islets from their in situ environment. These defects may contribute to the altered metabolic and obese state of fa/fa animals. GIP-stimulated insulin secretion from the perfused rat pancreas was potently suppressed by infusion of the pancreatic neuropeptide galanin. The inhibitory action of galanin appeared to be specific for GIP-stimulated insulin secretion, since the same concentration of galanin (10 nM) had no effect on ACh- or CCK-8-stimulated insulin release and modestly suppressed the insulin response to arginine. The further demonstration that galanin suppressed GIP-induced insulin release from mixtures of FACS-purified β- and non-β-cells suggests that galanin exerts its inhibitory influence on GIP-stimulated insulin release by a direct interaction at the level of the β-cell. Porcine galanin was without effect on somatostatin secretion from the perfused rat pancreas in the presence of GIP. In summary, these studies provided clear evidence that GIP directly interacts with the pancreatic islet to stimulate the release of insulin, glucagon and somatostatin. Moreover, it was shown that the stimulation of islet hormone secretion by GIP is subject to considerable modulation by glucose and other agents. The data further showed that alterations in islet sensitivity to GIP may contribute to the metabolic disturbances which exist in obese Zucker rats. Finally, a sensitive radioreceptor assay for GIP was developed and the presence of GIP receptors on rat pancreatic islets was demonstrated.

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