Relative importance of diacylglycerol, phosphatidate, lysophosphatidate, inositol trisphosphate and arachidonate metabolism in platelet receptor signalling.

Activation of platelets is correlated with phospholipase C-induced degradation of phosphatidylinositol 4,5-bisphosphate and the rapid formation of 1,2-diacylglycerol and myo-inositol 1,4,5-trisphosphate. Both products are considered second messengers and they, respectively, stimulate protein kinase C and Ca2+ mobilization. Mobilization of Ca2+ leads to activation of a Ca2+/calmodulin-dependent myosin light chain kinase and phospholipases A2 which liberate arachidonic acid from phospholipids. Arachidonate is then immediately converted to active endoperoxides and thromboxanes which are released and activate further platelets again through phospholipase C. The levels of phosphatidic acid and lysophosphatidic acid are also increased following receptor-stimulated hydrolysis of the inositol phospholipids. Lysophosphatidic acid might have a direct action on the opening of Ca2+-channels.