The potential role of platelet PAI-1 in t-PA mediated clot lysis of platelet rich plasma

Abstract

The potential role of platelets in platelet rich plasma clot lysis induced by tissue plasminogen activator (t-PA) was investigated. At the various concentrations of both single chain t-PA (sct-PA) and two chain t-PA (tct-PA) (1.5nM, 3nM, and 6nM), we compared the t-PA mediated lysis time of platelet rich plasma clot (PRP-clot) with that of platelet poor plasma clot (PPP-clot). At the concentrations ranged from 1.5 to 6 nM of both types of t-PA, the clot lysis time of PRP-clot was longer than that of PPP-clot. This elongation was more significant in the tct-PA induced clot lysis than that in the sct-PA induced clot lysis. At the concentration of 3nM of tct-PA, the lysis time of PRP-clot was longer by a factor of 30% in comparison with that of PPP-clot. When the release and the aggregation of platelets were blocked by prostaglandin E1 (PGE1) and theophylline in this experiment, the lysis time of PRP-clot was essentially the same as that of PPP-clot. We then measured the antigen levels of total PAI-1 and t-PA-PAI-1 complex in the lyzed solutions of PRP-clot and PPP-clot to analyse the possible effect of plasminogen activator inhibitor-1 (PAI-1) present in platelets. Most of PAI-1 in a PPP-clot lyzed sample existed as t-PA-PAI-1 complex. In the lyzed solution of PRP-clot, however, the antigen levels of both total PAI-1 and t-PA-PAI-1 complex were significantly higher than those in PPP-clot, and larger amounts of PAI-1 existed as free PAI-1 which possesses activity. These data suggest that at least certain amounts of PAI-1 in platelets exist as an active form and inhibits t-PA activity resulting in the prolongation of the clot lysis time. Activation of platelets, therefore, seems to play an important role in the platelet rich plasma clot lysis induced by t-PA.