On the mechanism of plasmin-induced platelet aggregation - Implications ofthe dual role of granule ADP

Plasmin-induced platelet aggregation has been considered to be a cause of r eocclusion after thrombolytic treatment with plasminogen activators. Howeve r, little is known regarding the mechanism and regulation of plasmin-induce d platelet aggregation. In this study, we demonstrated that plasmin causes the degranulation of platelets, and that ADP released from granules plays a crucial role in the induction of platelet aggregation. This conclusion is supported by results showing that both ADP antagonists and ADPase can inhib it the effect of plasmin on platelets. We also demonstrated that pretreatme nt of platelets with ADP makes the platelets more sensitive to plasmin, and plasmin-induced platelet aggregation is, therefore, observed at lower conc entrations where no aggregation occurs in quiescent platelets. In other wor ds, it is thought that ADP potentiates the plasmin-induced aggregation. The effect of ADP was inhibited by N-6-[2-(methylthio)-ethyl]-2-(3,3,3-trifluo ropropyl)thio-5'-adenylic acid, monoanhydride with dichloromethylenebisphos phonic acid (AR-C69931), a selective antagonist for the P2T(AC) subtype of P2 receptor, but not by the P2Y1 receptor-selective antagonist adenosine 3' -phosphate 5'-phosphosulfate (A3P5PS). The P2X1 receptor agonist alpha,beta -methylene adenosine 5'-triphosphate (alpha,beta-MeATP) did not mimic the a ction of ADP. These data indicate that ADP potentiates plasmin-induced plat elet aggregation via the P2T(AC) receptor. In addition, epinephrine, a typi cal G(i) agonist against platelets, could potentiate the plasmin induced pl atelet aggregation, suggesting that the signal via the Gi protein is involv ed in potentiating the plasmin-induced platelet aggregation, ADP is secrete d from platelet granules, and concomitantly works in conjunction with plasm in in a P2T(AC) receptor-mediated manner. BIOCHEM PHARMACOL 59;11:1345-1355 , 2000. (C) 2000 Elsevier Science Inc.