Phorbol-myristate-acetate-induced platelet aggregation in the presence of inhibitors

4 beta-Phorbol-12-myristate-13-acetate (PMA) at 100 ng/ml was able to induce platelet aggregation in the presence of agents which inhibited aggregation, triggered by other agonists such as adenosine diphosphate sodium salt (ADP), thrombin and collagen. PMA induced aggregation in acid-citrate-dextrose platelet-rich plasma. 100 microM tetracaine, 5 microM bromophenacyl bromide and 0.2 mM mepacrine decreased PMA-induced aggregation by only 10% in contrast to their high inhibitory effect on other aggregation systems. However, 0.4 mM mepacrine did inhibit PMA-induced aggregation at the same rate as the other aggregation systems. 100 mg/ml vincristine slightly affected PMA-induced platelet aggregation, whereas cytochalasin B rather enhanced it. Nordihydroguaiaretic acid, 5, 8, 11, 14-eicosatetraynoic acid and p-nitrophenyl-phosphorylcholine had no effect on PMA- or collagen-induced platelet aggregation, partially inhibited aggregation triggered by ADP and strongly inhibited aggregation caused by thrombin. It is suggested that PMA exerts its effect on platelets mainly due to its ability to alter their membranes.