Studies on the mechanism of action of 12-deoxyphorbolphenylacetate, a potent platelet aggregating tigliane ester

12-Deoxyphorbolphenylacetate (12-DOPP) induced human platelet aggregation which was dependent upon the presence of divalent cations, the intracellular level of cyclic-AMP and an intact microtubular system, in common with other aggregating agents. However platelet secretion and thromboxane (Tx)B 2 synthesis did not contribute to 12-DOPP induced platelet aggregation as neither the Tx/endoperoxide antagonists pinane TxA 2 and trimethoquinone, the thromboxane synthetase inhibitors clotrimazole and 9, 11, aza-prosta-5-13-dienoic acid nor the cyclo-oxygenase inhibitor indomethacin inhibited 12-DOPP induced aggregation. Furthermore the free radical scavengers aminopyrine, thioanisole and butylated hydroxy-toluene, the lipoxygenase inhibitor phenidone and the leucotrienes B and C antagonist FPL 55712 failed to modify 12-DOPP-induced aggregation. Compounds which are thought to act as phospholipase inhibitors (bromophenacylbromide, mepacrine and propranolol as well as imipramine, desmethylimipramine, promethazine and trifluoperazine) which have been shown to selectively bind calmodulin, were found to be effective inhibitors of 12-DOPP induced aggregation. Aggregation induced by 12-DOPP involves a direct effect upon platelets followed by the release of unknown substances, probably phospholipid products. The released substances were shown to induce further aggregation of platelets. The aggregation induced by 12-DOPP is thus distinct from that induced by ADP, collagen, adrenaline and prostaglandin endoperoxides.