The influence of hirudin on the thrombin-platelet reaction was studied by determination of thrombin-induced serotonin release from blood platelets. The dissociation constant for the thrombin-hirudin complex calculated from data obtained by measuring the platelet reaction (K_i 50 pmol/l) was in accordance with the value obtained by determination of fibrinogen clotting. Prevention of the effect of thrombin on platelets required the addition of hirudin in at least 10-fold molar excess. The release of serotonin could be immediately stopped by hirudin at any time when the special conditions for the reaction of thrombin with the tight-binding inhibitor and the receptors on the platelet surface were taken into account.

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