title: Studies on the mechanisms by which gastrin releasing peptide potentiates glucose-induced insulin secretion from mouse islets.
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.
authors.affiliation: Department of Endocrinology and Metabolism C, Aarhus University Hospital, Denmark.
authors.forename: S
authors.initials: S
authors.lastname: Gregersen
authors.forename: B
authors.initials: B
authors.lastname: Ahrén
mesh_terms: D000818:Animals; D004357:Drug Synergism; D005260:Female; D019886:Gastrin-Releasing Peptide; D005947:Glucose / Q000494:pharmacology*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D010455:Peptides / Q000494:pharmacology*
chemical_list: D007328:Insulin; D010455:Peptides; D019886:Gastrin-Releasing Peptide; D005947:Glucose