The initial action of thrombin on platelets. Conversion of phosphatidylinositol to phosphatidic acid preceding the production of arachidonic acid.

Measurements of phosphatidylinositol (PI) and phosphatidic acid (PA) by phosphorus assays and by radioactivity ([14C]arachidonate) indicate that thrombin induces the degradation of a given fraction of the total PI to PA. The maximal conversion of PI to PA represents approximately one-third of the total PI which can be degraded by thrombin. This same amount of PI is converted to PA even in the presence of 1 mM quinacrine, which completely inhibits the release of arachidonic acid from phospholipids and which reduces by two-thirds the loss of labeled PI. In this case the fall in PI is equal to the amount of PA formed. If thrombin is added to platelets previously maximally stimulated by ionophore A23187, PA is produced from PI in amounts equal to those produced by thrombin in the absence of other stimuli. Furthermore, the resynthesis of PI from PA is also unaffected by quinacrine, and thus the entire thrombin-stimulated PI-cycle is maintained. The data thus indicate the existence of a quinacrine-insensitive phospholipase C which can initially convert a given amount of PI to PA and which is closely associated to the thrombin receptor. The further breakdown of PI and production of arachidonic acid might result from the action of quinacrine-sensitive activities (i.e. phospholipase A2). The simplest scheme is one in which thrombin specifically produces an active fraction of PA which in some way results in the subsequent production of arachidonic acid from various phospholipids (including PI), perhaps by activation of quinacrine-sensitive phospholipase A2.