THE CONTRIBUTION OF ENDOGENOUS UROKINASE (UK) AND TISSUE PLASMINOGEN ACTIVATOR (t-PA) TO SPONTANEOUS CLOT LYSIS IN PLASMA
Spontaneous lysis of 125I-labeled clots was measured in order to study the relative contributions of pro-UK, t-PA and contact activation. Clots were made up from 0.3 ml plasma, platelet rich plasma (PRP) or 2% human fibrinogen (Kabi) and suspended in plasma (3 ml) prepared from blood freshly collected onto citrate. The clots were preincubated to inactivate any residual thrombin which could inactivate pro-UK. Antibiotics were added to suppress bacterial growth. Lysis was approximately linear and went to completion with plasma clots in 12-16 d. and with fibrinogen clots in 6-10 d. Spontaneous lysis was inhibited by aprotinin at >200 KlU/ml and by antibody to t-PA. When the latter was added at t 2-5 d., clot lysis was arrested at each time point suggesting that t-PA was predominantly responsible for the lysis and not just for its initiation. Addition of antibody to UK or immunodepletion of plasma had little effect on spontaneous lysis of plasma or of fibrinogen clots, but retarded lysis of PRP clots. Only after ≥20 ng/ml of pro-UK were added to plasma, was a dose-responsive acceleration of clot lysis observed. The addition of antibodies to t-PA inhibited clot lysis by the added pro-UK (20-80 ng/ml) and greatly prolonged the lag phase of clot lysis by higher concentrations of pro-UK (100-200 ng/ml). Contact activation by dextran sulfate (1-5 μM) had no effect on spontaneous clot lysis but potentiated the effect of > 20 ng/ml added pro-UK. It was concluded that 1) The UK content of resting plasma (3-5 ng/ml) contributes little to lysis of platelet-free clots. 2) The presence of physiological concentrations of t-PA is essential for lysis by endogenous UK and markedly potentiates fibrinolysis by added pro-UK (20-200 ng/ml). 3) For endogenous UK to contribute to fibrinolysis, some amplification of its effect by a cofactor other than t-PA seems to be required. The results indicate augmentation of endogenous pro-UK induced fibrinolysis by platelets and that contact activation stimulates clot lysis by added pro-UK.