ACTIVATION OF HUMAN-PLATELET PHOSPHOLIPASE-C BY IONOPHORE A23187 IS TOTALLY DEPENDENT UPON CYCLO-OXYGENASE PRODUCTS AND ADP

Human platelets exposed to the Ca2+ ionophore A23187 form cyclo-oxygenase metabolites from liberated arachidonic acid and secrete dense granule substituents such as ADP. A23187 causes activation of phospholipase A2 and some stimulation of phospholipase C. In contrast to the case for thrombin, the activation of phospholipase C in response to ionophore is completely dependent upon the formation of cyclo-oxygenase products and the presence of ADP. The addition of A23187 to human platelets induces a transient drop in the amount of phosphatidylinositol 4,5-bisphosphate, a decrease in the amount of phosphatidylinositol, and the formation of diacylglycerol and phosphatidic acid. In addition, lysophosphatidylinositol and free arachidonic acid are produced. The presence of cyclo-oxygenase inhibitors or agents which remove ADP partially impairs these changes. When both types of inhibitor are present, the changes in phosphatidylinositol 4,5-bisphosphate and the formation of diacylglycerol and phosphatidic acid are blocked entirely, whereas formation of lysophosphatidylinositol and free arachidonic acid are relatively unaffected. The prostaglandin H2 analog U46619 activates phospholipase C. This stimulation is inhibited partially by competitors for ADP. Phospholipase C is not activated by Ca2+ in the platelet, and stimulation is probably totally dependent upon a receptor coupled event.