REGULATION OF INSULIN RELEASE BY PHOSPHOLIPASE-C ACTIVATION IN MOUSE ISLETS - DIFFERENTIAL-EFFECTS OF GLUCOSE AND NEUROHUMORAL STIMULATION

Rat islets respond to glucose stimulation with a marked first and seco nd phase increase in insulin secretion. In contrast, mouse islets have a similar first phase response but little second phase secretion. In these studies, we determined if activation of phospholipase C (PLC) ac counts for these differences in second phase insulin secretion in thes e two species. Stimulation of freshly isolated mouse and rat islets wi th 15 mM glucose resulted in comparable first phase insulin secretion; however, the second phase response from mouse islets was only doubled from 28 +/- 6 to 60 +/- 7 pg/islet . min compared with an increase fr om 24 +/- 4 to 1064 +/- 93 pg/islet . min from rat islets. The additio n of the muscarinic agonist carbachol (100 mu M) in the presence of 15 mM glucose, however, markedly increased second phase insulin release from mouse islets to 801 +/- 80 pg/islet . min. Similar increases in s econd phase insulin release from mouse islets were obtained with the a ddition of 500 nM of the the protein kinase C activator tetradecanoyl phorbol acetate in the presence of 15 mM glucose. However, the increti n factor glucagon-like peptide-1, which elevates islet cAMP levels, ha d little effect on second phase insulin release in the mouse. An analy sis of PLC-mediated phosphoinositide (PI) hydrolysis revealed that 15 mM glucose increased inositol phosphate (IP) accumulation 0.5-fold abo ve baseline in mouse islets compared with 3.7-fold in rat islets. In c ontrast, carbachol stimulated IP accumulation 3.5-fold in both mouse a nd rat islets. Analysis of PLC isozymes with isozyme specific monoclon al antibodies, demonstrated that mouse islets express 14 +/- 4% of PLC -delta 1 and 18 +/- 6% of PLC-beta 1 compared with rat islets but simi lar amounts of the PLC-gamma 1 (117 +/- 16%). These findings suggest t hat the decreased second phase insulin secretory response in mouse com pared with rat islets results, at least in part, from an inability of high glucose to stimulate comparable increments in PI hydrolysis. This lack of glucose responsiveness may be due to the pronounced underexpr ession of specific PLC isozymes in the mouse.