Characterization of protein kinase C and its role in catecholamine secretion from bovine adrenal-medullary cells

1. 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 approx. 18 μM free Ca²⁺ and 8 μg of phosphatidylserine/ml. Both diolein and 4β-phorbol 12-myristate 13-acetate (TPA) decreased the Ca²⁺ requirement of the enzyme, half-maximal activation being obtained at approx. 12 μM and 9 μM free Ca²⁺ respectively in the presence of these agents. 2. Many endogenous proteins in the adrenal-medullary cytosolic fraction were detected whose phosphorylation was dependent on the presence of both Ca²⁺ and phosphatidylserine. 3. TPA stimulated catecholamine release from cultured bovine adrenal-chromaffin cells in a Ca²⁺-dependent manner. A23187 also stimulated catecholamine secretion, and at suboptimal concentrations of TPA and A23187 a synergistic secretory response was obtained. 4. These results are consistent with protein kinase C having a regulatory role in exocytosis in bovine adrenal chromaffin cells.