Ionomycin can elevate intraplatelet Ca2+ and activate phospholipase A without activating phospholipase C

Human platelets exposed to ionomycin, a Ca2+ ionophore, exhibit activation of both phospholipases A2 and C. Such platelets manifest a rise in cytoplasmic Ca2+ (monitored by quin 2), a loss in phosphoinositides, formation of lysophosphatidylinositol, thromboxane B2, phosphatidic acid, and phosphorylated 47 kilodalton protein, and secretion. In the absence of thromboxane formation and secreted ADP, phospholipase C is not activated and the 47 kilodalton protein is not phosphorylated. The elevation in Ca2+ is unaffected by inhibition of cyclooxygenase and ADP. Thus, an increase in cytoplasmic Ca2+ is not sufficient to stimulate phospholipase C. Further, secretion can occur in the absence of phospholipase C activation and 47 kilodalton protein phosphorylation.