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

Background: Despite aprotinin being in widespread clinical use to prevent b leeding during cardiac surgery, there remains concern that such a powerful hemostatic agent may also be prothrombotic, particularly in relation to cor onary vein graft occlusion. The major thrombin receptor on platelets, prote ase-activated receptor 1 (PAR1) requires proteolytic cleavage to transmit a ctivating signals. Here we have investigated the effect of aprotinin on thr ombin-induced PAR 1 activation of platelets. Methods and results: Proteolysis-dependent and -independent responses of wa shed platelets were studied in vitro. Platelet aggregation induced by tryps in 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 media ted through PAR1 and was inhibited 42.6% to 86.6% (P <.05-.001) by pharmaco logic doses of aprotinin (50-160 KIU/mL), Aprotinin did not inhibit the non proteolytic agonists collagen, epinephrine, adenosine diphosphate, or phorb ol 12-myristate 13-acetate, Furthermore, blockade of the thrombin response by aprotinin did not prevent subsequent platelet aggregation through collag en or epinephrine, Experiments with intraplatelet Ca2+ fluxes, which provid ed an earlier measure of platelet activation, placed the effect of aprotini n proximal to the PAR1 activation event. Since aprotinin did not inhibit pl atelet responses to the nonproteolytic PAR1 agonist peptide, SFLLRN, this i mplied that aprotinin acted by preventing PAR1 receptor cleavage by thrombi n, Conclusions: Aprotinin inhibits thrombin-induced platelet activation by pre venting proteolysis of the PAR 1 receptor. These findings argue against apr otinin being prothrombotic and suggest instead that aprotinin may have sign ificant antithrombotic effects.