Phosphatidic acid releases calcium from a platelet membrane fraction in vitro

A platelet membrane fraction which actively sequesters calcium in the presence of ATP was prepared and the influence of phosphatidic acid evaluated. At 10--60 micrograms/ml phosphatidic acid caused a concentration dependent release of calcium from the membrane fraction. The calcium was released from inside the vesicles, since release occurred in the presence of EGTA used to bind calcium outside the membrane vesicles. Aspirin failed to inhibit release of calcium by phosphatidic acid. Our results may explain, in part, the prostaglandin and thromboxane independent calcium release which occurs in response to certain aggregating agents. Thus, phosphatidic acid, or a metabolite, may have an important role intracellularly in platelets in promoting calcium movement.