Ca2+ requirement in hydrolysis of phosphatidylinositol-4,5-bisphosphate in human platelets

The activity of partially purified phospholipase C from human platelets was totally dependent on Ca2+, and approximately 800 microM Ca2+ was required for half-maximal activity. The enzyme hydrolyzed endogenous substrates in the order DPI greater than TPI greater than PI in a Ca2+-dependent manner. Hydrolysis of TPI in thrombin-stimulated platelets was dependent on the amount of the agonist, and it was not affected by the presence or absence of extracellular Ca2+. Hydrolysis was inhibited by preincubation with Quin-2AM in the absence of extracellular Ca2+. The intracellular Ca2+ concentration was significantly lowered below the basal level by such treatment. These observations suggested that TPI breakdown in thrombin-stimulated platelets is mediated by agonist-receptor coupling and requires at least the basal level of intracellular Ca2+.