Effects of platelets and plasma on fibrinolysis

The effects of various concentrations of platelets and plasma on in vitro t-PA-induced fibrinolysis were investigated. At t-PA levels between 30 and 70 ng/ml, fibrinolysis proceeded faster in platelet-rich plasma (PRP) than in platelet-poor plasma (PPP). When PRP was serially diluted with PPP and the rate of fibrinolysis compared to PPP, higher platelet counts (up to 300 x 10(9)/l) were associated with increased enhancement of lysis. However, lysis was inhibited when platelets were added to plasma diluted with buffer. Thus, platelets enhance fibrinolysis in undiluted plasma but inhibit lysis in diluted plasma. This is probably because platelets provide a catalytic surface for fibrinolysis but also release inhibitors of fibrinolysis. In undiluted plasma, there would be sufficient fibrinogen and plasminogen to overcome the effect of the platelet inhibitors, but in diluted systems, the inhibitors would predominate, retarding fibrinolysis.