Studies on the mechanisms by which gastrin releasing peptide potentiates glucose-induced insulin secretion from mouse islets.

The mechanisms underlying the insulinotropic action of gastrin releasing peptide (GRP) were examined in normal mouse islets. GRP (100 nM) enhanced insulin secretion at glucose concentrations of > or = 11.1 mM (p < 0.05) but only in the presence of extracellular Ca2+. The insulinotropic effect of the peptide studied during perifusion at 16.7 mM glucose was transient and vanished in time. GRP stimulated, transiently, 45Ca2+ efflux from 45Ca(2+)-prelabeled islets, both in the presence and in the absence of extracellular Ca2+ (p < 0.05), suggesting that GRP releases Ca2+ from intracellular stores. Similarly, GRP increased 86Rb+ efflux from 86Rb(+)-prelabeled islets both in the presence and in the absence of extracellular Ca2+ (p < 0.001). In contrast to GRP-induced insulin secretion, the GRP-induced 86Rb+ efflux was sustained throughout the stimulation period, suggesting that increased K+ conductance may be involved in the vanishing effect of GRP on insulin secretion. Furthermore, both inhibition of protein kinase C (PKC) by staurosporine (1-10 microM) and down-regulation of PKC activity by long-term incubation with the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate inhibited GRP-stimulated insulin secretion (p < 0.05). These results indicate that GRP activates PKC by an action involving liberation of Ca2+ from Ca2+ stores. Therefore, also the influence of GRP on phosphoinositide hydrolysis was studied by means of 3H efflux from myo-[2-3H]inositol prelabeled islets. However, GRP did not stimulate the 3H efflux. In contrast, GRP-stimulated insulin secretion was abolished by an inhibitor of phospholipase D, wortmannin (1 microM). The results suggest that GRP transiently potentiates glucose-induced insulin secretion by an action mediated by PKC activated by diacylglycerol formed through activation of phospholipase D. Simultaneously, an as yet unknown mechanism liberating Ca2+ from intracellular stores is activated.