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

Abstract

Protein kinase activation is known to stimulate glucose-induced insulin secretion in the presence of diazoxide. Diazoxide opens the ATP-sensitive K + channel and inhibits FAD-linked glycerophosphate dehydrogenase activity in a concentration-dependent manner. In the present study, we examined the effect of lower (100 μM) and higher (250 μM) concentrations of diazoxide on insulin release by protein kinase A (PKA) and protein kinase C (PKC) activation. Forced depolarization by a high potassium concentration, augmented the intracellular Ca 2+ concentration ([Ca 2+] i ) similarly in the presence of both concentrations of diazoxide. Under this condition, 250 μM diazoxide inhibited insulin release enhanced by PKA activation but not that by PKC. Under a basal concentration of [Ca 2+] i , PKC activation elicited glucose-induced insulin secretion at 100 and 250 μM diazoxide, while PKA activation did so only at 100 μM. These augmentations were completely inhibited by mannoheptulose, a glucokinase inhibitor. Glyceraldehyde, in place of glucose, enhanced insulin secretion by PKC activation under both concentrations of diazoxide. On the other hand, it did not affect PKA-stimulated insulin release under either conditions, but in the case of 100 μM, glucose augmented the insulin secretion in the presence of glyceraldehyde and db-cAMP concentration-dependently. These data suggest that insulin release stimulated by PKA and PKC activation under diazoxide is dependent on glucose metabolism, and that a signal derived from proximal steps in glycolysis may be necessary for the secretion by PKA activation.