The antithrombotic effect of aprotinin: Actions mediated via the protease-activated receptor 1

Background: Despite aprotinin being in widespread clinical use to prevent bleeding during cardiac surgery, there remains concern that such a powerful hemostatic agent may also be prothrombotic, particularly in relation to coronary vein graft occlusion. The major thrombin receptor on platelets, protease-activated receptor 1 (PAR1) requires proteolytic cleavage to transmit activating signals. Here we have investigated the effect of aprotinin on thrombin-induced PAR 1 activation of platelets. Methods and results: Proteolysis-dependent and -independent responses of washed platelets were studied in vitro. Platelet aggregation induced by trypsin was dependent on PAR1 (inhibited by the PAR1-specific antagonist peptide, FLLRN) and was completely blocked by aprotinin at doses more than 100 KIU/mL, Aggregation in response to thrombin, 1 nmol/L, was predominantly mediated through PAR1 and was inhibited 42.6% to 86.6% (P <.05-.001) by pharmacologic doses of aprotinin (50-160 KIU/mL), Aprotinin did not inhibit the nonproteolytic agonists collagen, epinephrine, adenosine diphosphate, or phorbol 12-myristate 13-acetate, Furthermore, blockade of the thrombin response by aprotinin did not prevent subsequent platelet aggregation through collagen or epinephrine, Experiments with intraplatelet Ca2+ fluxes, which provided an earlier measure of platelet activation, placed the effect of aprotinin proximal to the PAR1 activation event. Since aprotinin did not inhibit platelet responses to the nonproteolytic PAR1 agonist peptide, SFLLRN, this implied that aprotinin acted by preventing PAR1 receptor cleavage by thrombin, Conclusions: Aprotinin inhibits thrombin-induced platelet activation by preventing proteolysis of the PAR 1 receptor. These findings argue against aprotinin being prothrombotic and suggest instead that aprotinin may have significant antithrombotic effects.