CALCIUM AND A MITOCHONDRIAL SIGNAL INTERACT TO STIMULATE PHOSPHOINOSITIDE HYDROLYSIS AND INSULIN-SECRETION IN RAT ISLETS

Fuel metabolism generates multiple signals that interact to stimulate insulin secretion. These studies explored the mechanism by which fuels activate phosphoinositide (PI) hydrolysis and the role of this signal transduction pathway in fuel-stimulated insulin secretion. High potas sium (30 mM), which depolarizes the membrane and increases Ca2+ influx , caused only a transient monophasic release of insulin. In contrast, glucose (20 mM) or monomethylsuccinate (MMSucc; 10 mM) markedly stimul ated a sustained insulin secretory response, indicating that fuel meta bolism generates a signal(s) in addition to Ca2+ influx that is requir ed for a sustained secretory response. On the other hand, diazoxide, a n ATP-sensitive K+ channel activator that prevents membrane depolariza tion and Ca2+ influx in response to fuel metabolism, reduced the secre tory responses to glucose and MMSucc to baseline levels, demonstrating that Ca2+ influx was essential to fuel-stimulated insulin secretion. The further addition of high K+ bypassed the diazoxide block and resto red insulin secretory rates. The insulin secretory response to glucose or MMSucc in the presence of diazoxide and K+ was inhibited by the Ca 2+ channel antagonist nitrendipine and the protein kinase-C inhibitor staurosporine. Changes in PI hydrolysis paralleled those in insulin se cretion. High potassium alone induced only a modest 2.5-fold increase in inositol phosphate accumulation. This response was significantly le ss than that to glucose or MMSucc, which increased inositol phosphate accumulation by 6.8- or 5.2-fold, respectively. Like its effect on sec retion, diazoxide markedly reduced glucose- or MMSucc-stimulated PI hy drolysis, and this inhibition was reversed with high K+. In contrast, diazoxide had no effect on receptor-activated PI hydrolysis stimulated by 100 nm cholecystokinin (CCK), and the effects of CCK were not depe ndent on added fuel, indicating that fuel and CCK activate PI hydrolys is by distinct pathways. These findings demonstrate that mitochondrial metabolism of glucose or MMSucc generates a signal(s) that interacts with Ca2+ influx to stimulate PI hydrolysis and sustained insulin secr etion. This pathway of fuel-activated PI hydrolysis is distinct from t hat of CCK receptor-activated PI hydrolysis. These studies suggest tha t fuel-activated PI hydrolysis plays an important role in fuel-stimula ted insulin secretion.