Protein kinase C-dependent and -independent inhibition of Ca2+ influx by phorbol ester in rat pancreatic beta-cells

Phorbol esters were used to investigate the action of protein kinase C (PKC ) on insulin secretion from pancreatic beta -cells. Application of 80 nM ph orbol 12-myristate 13-acerate (PMA). a PKC-activating phorbol ester, had li ttle effect on glucose (15 mM)-induced insulin secretion from intact rat is lets. In islets treated with bisindolylmaleimide (BIM), a PKC inhibitor, PM A significantly reduced the glucose-induced insulin secretion. PMA decrease d the level of intracellular Ca2+ concentration ([Ca2+](i)) elevated by the glucose stimulation when tested in isolated rat beta -cells. This inhibito ry effect of PMA was not prevented by BIM. PMA inhibited glucose-induced ac tion potentials, and this effect was not prevented by BIM. Further, 4 alpha -phorbol 12,13-didecanoate (4 alpha -PDD), a non-PKC-activating phorbol es ter, produced an effect similar to PMA. In the presence of nifedipine. the glucose stimulation produced only depolarization. and PMA applied on top of glucose repolarized the cell. When applied at the resting state. PMA hyper polarized beta -cells with an increase in the membrane conductance. Recorde d under the voltage-clamp condition. PMA reduced the magnitude of Ca2+ curr ents through L-type Ca2+ channels. BIM prevented the PMA inhibition of the Ca2+ currents. These results suggest that activation of PKC maintains gluco se-stimulated insulin secretion in pancreatic beta -cells, defeating its ow n inhibition of the Ca2+ influx through L-type Ca2+ channels. PKC-independe nt inhibition of electrical excitability by phorbol esters was also demonst rated. (C) 2001 Published by Elsevier Science Inc.