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A B S T R A C T In studies conducted with human gelfiltered platelets, we have found: (a) that the release of serotonin and transfer of [3H]arachidonic acid from phosphatidylcholine and phosphatidylinositol to plasmalogen phosphatidylethanolamine which are associated with the activation of platelets by thrombin are both strongly dependent upon the presence of metabolic ATP; (b) that serotonin release and arachidonic acid mobilization in labeled phosphatides are promoted by the calcium ionophore A-23187 in media free of calcium ions; (c) that inhibitors of ATP synthesis, while leading to impairment of the release reaction induced by ionophore, do not inhibit ionophore-stimulated mobilization of arachidonic acid.We conclude that the activation of phospholipase A2 responsible for freeing arachidonic acid from platelet phosphatides is solely dependent upon the increased cytoplasmic levels of calcium ions promoted by either ionophore or, in an energy-dependent fashion by thrombin. Phospholipase activation is not a function of latent hydrolytic activity made available by the release reaction.

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