Regulation of insulin release by phospholipase C activation in mouse islets: differential effects of glucose and neurohumoral stimulation

Abstract

Rat islets respond to glucose stimulation with a marked first and second 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) accounts for these differences in second phase insulin secretion in these two species. Stimulation of freshly isolated mouse and rat islets with 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 from 24 +/- 4 to 1064 +/- 93 pg/islet.min from rat islets. The addition of the muscarinic agonist carbachol (100 microM) 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 second phase insulin release from mouse islets were obtained with the addition of 500 nM of the protein kinase C activator tetradecanoyl phorbol acetate in the presence of 15 mM glucose. However, the incretin factor glucagon-like peptide-1, which elevates islet cAMP levels, had little effect on second phase insulin release in the mouse. An analysis of PLC-mediated phosphoinositide (PI) hydrolysis revealed that 15 mM glucose increased inositol phosphate (IP) accumulation 0.5-fold above baseline in mouse islets compared with 3.7-fold in rat islets. In contrast, carbachol stimulated IP accumulation 3.5-fold in both mouse and rat islets. Analysis of PLC isozymes with isozyme specific monoclonal antibodies, demonstrated that mouse islets express 14 +/- 4% of PLC-delta 1 and 18 +/- 6% of PLC-beta 1 compared with rat islets but similar amounts of the PLC-gamma 1 (117 +/- 16%). These findings suggest that the decreased second phase insulin secretory response in mouse compared 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 underexpression of specific PLC isozymes in the mouse.