Summary
Heparin has been proposed to enhance thrombolysis by inhibiting thrombin-dependent
generation of activated TAFI (thrombin activatable fibrinolysis inhibitor), a carboxypeptidase
that inhibits fibrinolysis. We evaluated the effect of heparin in an in vitro thrombolysis
model consisting of a radiolabelled blood clot submerged in defibrinated plasma. Fibrinolysis
was induced by adding t-PA (250 ng/ml) and calcium to the plasma bath. Control experiments
indicated that thrombin generation induced by recalcification caused significant TAFI
activation and inhibited clot lysis. Heparin (up to 1 U/ml), added to the plasma bath,
failed to enhance clot lysis. Thrombin generation in the fluid phase was totally inhibited
by heparin at concentrations > 0.5 U/ml. In contrast, thrombin generation on the clot
surface was not inhibited by heparin (1 U/ml). TAFIa generation did occur in heparin-containing
samples (1 U/ml) and amounted to about 10% of TAFIa formed in control samples. This
low amount of TAFIa did exert antifibrinolytic activity as indicated by the observation
that the addition of a specific TAFIa inhibitor (PTI) along with heparin enhanced
clot lysis. Hirudin (10 µg/ml), at variance with heparin, inhibited clot-bound thrombin
and enhanced clot lysis. These data show that heparin is unable to stimulate fibrinolysis
through a TAFI-dependent mechanism, most likely because of its inefficiency in inhibiting
thrombin generation on the clot surface. Moreover, they suggest that clot-bound thrombin
plays a major role in TAFI-mediated inhibition of fibrinolysis through “localized”
TAFIa generation.
Keywords
Thrombin - thrombin inhibitors - clot lysis - carboxypeptidase
