Different Functions of Phosphoinositide Turnover and Arachidonate Metabolism for Platelet Activation

Abstract

In a wide variety of cells including platelets, the hormone-induced breakdown of phosphoinositides by phospholipase C is closely associated with the mobilization of Ca²⁺ from intracellular stores (Michell, 1975). Recent studies gave new insights into the possible link between phosphoinositide breakdown and intracellular Ca²⁺mobilization: myoinositol-l,4,5-trisphosphate, the phosphodiesteratic cleavage product of phosphatidylinositol-4,5-biphosphate, mobilizes Ca²⁺ from the endoplasmatic reticulum in pancreatic acinar cells and hepatocytes (Streb et al., 1983; Joseph et al.,1984). Besides myoinositol-1,4,5-trisphosphate, two other products of the phos-pholipase-C-induced degradation of the phosphoinositides have possible functions as intracellular activators: 1,2-diacylglycerol activates the Ca²⁺, phospholipid-dependent protein kinase C, and phosphatidic acid has been implicated in Ca²⁺fluxes and membrane fusion processes (Nishizuka, 1983; Sundler and Papahadjo-poulos, 1981; Fig. 1).