Spontaneous clot lysis in whole human plasma by endogenous tissue type and urokinase type plasminogen activators: Demonstration of a promoting effect by t-PA and by platelets on urokinase

Abstract

Endogenous pro-urokinase (pro-UK)/urokinase (UK) has been reported not to contribute significantly to spontaneous lysis of clotted plasma. Accordingly, its importance to physiological fibrinolysis has been questioned. However, since pro-UK was recently shown to be rapidly inactivated by thrombin, its role in fibrinolysis may have been underestimated in these experiments. To avoid thrombin inactivation of pro-UK, we studied spontaneous exolysis of clots rather than endolysis. In fresh plasma, lysis of clots prepared from fibrinogen or platelet poor plasma occurred progressively over 6–10 and 8–15 days respectively, and was inhibited by aprotinin (⩾ 200 KIU/ml) or by antibody to tissue plasminogen activator (t-PA) but not by antibody to UK. Supplementation with exogenous pro-UK indicated a threshold concentration in plasma of about 20 ng/ml before pro-UK had a measurable fibrinolytic effect against these clots. The progressive fibrinolytic effect of supplemental pro-UK (20–200 ng/ml) was abolished or inhibited by t-PA antibody, indicating that endogenous t-PA had a strong promoting effect on fibrinolysis by exogenous pro-UK. Some acceleration of clot lysis occurred after addition of dextran sulfate (1 ELM). When platelet rich plasma (PRP) clots were used, t-PA antibody inhibited but did not abolish clot lysis, but the latter was inhibited by UK antibody. Therefore, endogenous pro-UK/UK had an independent fibrinolytic effect against PRP clots. When clot lysis was studied in plasma preincubated (37°C) for up to 24 h, lysis of platelet poor clots was only slightly inhibited after 4 h incubation, and still went to completion even in plasma preincubated for 24 h. This lysis was abolished by t-PA antibody. Therefore, endogenous t-PA was surprisingly stable in plasma, having t 1 2 to inhibition of > 24 h, in contrast to the t 1 2 of exogenous t-PA in plasma which is < 2 h. It was concluded that endogenous pro-UK/UK contributes to fibrinolysis in intact plasma, but that its activity is dependent on platelets and is strongly promoted by endogenous t-PA, or more likely, by t-PA:inhibitor (PAI-1) complex.