CHARACTERIZATION OF PROTEIN KINASE-C AND ITS ROLE IN CATECHOLAMINE SECRETION FROM BOVINE ADRENAL-MEDULLARY CELLS

Protein kinase C activity towards exogenous histone was detected in a cytosolic fraction of bovine adrenal medulla. The enzyme was dependent on Ca2+ and phosphatidylserine for its activity, with half-maximal activation being achieved at .apprx. 18 .mu.M free Ca2+ and 8 .mu.g of phosphatidylserine/ml. Both diolein and 4.beta.-phorbol 12-myristate 13-acetate (TPA) decreased the Ca2+ requirement of the enzyme, half-maximal activation being obtained at .apprx. 12 .mu.M and 9 .mu.M free Ca2+ respectively in the presence of these agents. Many endogenous proteins in the adrenal-medullary cytosolic fraction were detected whose phosphorylation was dependent on the presence of both Ca2+ and phosphatidylserine. TPA stimulated catecholamine release from cultured bovine adrenal-chromaffin cell in a Ca2+-dependent manner. A23187 [calcimycin] also stimulated catecholamine secretion, and at suboptimal concentrations of TPA and A23187 a synergistic secretory response was obtained. Results are consistent with protein kinase C having a regulatory role in exocytosis in bovine adrenal chromaffin cells.