The effects of phorbol 12-myristate 13-acetate (PMA) on catecholamine secretion and protein phosphorylation from intact and digitonin-treated chromaffin cells were investigated. PMA (10-300 nM), an activator of protein kinase C, caused a slow Ca2+-dependent release of catecholamine from intact chromaffin cells that was potentiated by the Ca2+ ionophore ionomycin. PMA also enhanced secretion induced by Ba2+. In cells with plasma membranes rendered permeable by digitonin to Ca2+, ATP, and protein, PMA (100 nM) enhanced Ca2+-dependent secretion approximately 70% at 0.5 microM Ca2+ and 30% at 10 microM Ca2+. PMA enhanced the maximal response to Ca2+ approximately 25% and decreased the Ca2+ concentration required for half-maximal secretion approximately 30%. The effects of PMA on chromaffin cells were associated with a 2- to 3-fold increase in the phosphorylation of a 56-kDa protein that may be tyrosine hydroxylase. Other proteins were phosphorylated to a lesser extent. The experiments suggest that PMA increases protein kinase activity and secretion in chromaffin cells and raise the possibility that protein kinase C modulates catecholamine secretion in chromaffin cells.
