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

Pancreas. 1996 Jan;12(1):48-57. doi: 10.1097/00006676-199601000-00006.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Drug Synergism
  • Female
  • Gastrin-Releasing Peptide
  • Glucose / pharmacology*
  • Insulin / metabolism*
  • Insulin Secretion
  • Islets of Langerhans / metabolism*
  • Mice
  • Peptides / pharmacology*

Substances

  • Insulin
  • Peptides
  • Gastrin-Releasing Peptide
  • Glucose