Distinct effect of diazoxide on insulin secretion stimulated by protein kinase A and protein kinase C in rat pancreatic islets

Protein kinase activation is known to stimulate glucose-induced insulin sec retion in the presence of diazoxide. Diazoxide opens the ATP-sensitive K+ c hannel and inhibits FAD-linked glycerophosphate dehydrogenase activity in a concentration-dependent manner. In the present study, we examined the effe ct of lower (100 muM) and higher (250 muM) concentrations of diazoxide on i nsulin release by protein kinase A (PKA) and protein kinase C (PKC) activat ion. Forced depolarization by a high potassium concentration, augmented the intracellular Ca2+ concentration ([Ca2+](i)) similarly in the presence of both concentrations of diazoxide. Under this condition, 250 muM diazoxide i nhibited insulin release enhanced by PKA activation but not that by PKC. Un der a basal concentration of [Ca2+](i), PKC activation elicited glucose-ind uced insulin secretion at 100 and 250 muM diazoxide, while PKA activation d id so only at 100 muM. These augmentations were completely inhibited by man noheptulose, a glucokinase inhibitor. Glyceraldehyde, in place of glucose, enhanced insulin secretion by PKC activation under both concentrations of d iazoxide. On the other hand, it did not affect PKA-stimulated insulin relea se under either conditions, but in the case of 100 muM, glucose augmented t he insulin secretion in the presence of glyceraldehyde and db-cAMP concentr ation-dependently. These data suggest that insulin release stimulated by PK A and PKC activation under diazoxide is dependent on glucose metabolism, an d that a signal derived from proximal steps in glycolysis may be necessary for the secretion by PIA activation. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.