Synergistic functions of phorbol ester and calcium in serotonin release from human platelets

In human platelets, thrombin activates Ca²⁺-activated, phospholipid-dependent protein kinase (protein kinase C) and mobilizes Ca²⁺ concomitantly, whereas 12-O-tetradecanoylphorbol-13-acetate (TPA) may be intercalated into membranes and directly activates protein kinase C without mobilization of Ca²⁺ in sufficient quantities. A series of experiments with TPA and Ca²⁺-ionophore (A23187) indicates that activation of protein kinase C is a prerequisite requirement for release of serotonin, and that this enzyme activation and Ca²⁺ mobilization act synergistically to elicit a full cellular response. Both cyclic AMP and cyclic GMP inhibit activation of protein kinase C by prohibiting the signal-dependent breakdown of inositol phospholipid to produce diacylglycerol, but none of these cyclic nucleotides prevents the TPA-induced activation of this enzyme.