THROMBIN PLAYS A KEY ROLE IN LATE PLATELET THROMBUS GROWTH AND/OR STABILITY - EFFECT OF A SPECIFIC THROMBIN INHIBITOR ON THROMBOGENESIS INDUCED BY AORTIC SUBENDOTHELIUM EXPOSED TO FLOWING RABBIT-BLOOD

Thrombin is involved in the pathogenesis of venous and arterial thrombosis. This study addressed the question of the relative importance of thrombin in the early and late phases of thrombogenesis. The effect of Ro 46-6240 (1.43 mg/kg bolus and 0.1 mg/kg per minute IV), a novel, selective thrombin inhibitor on thrombogenesis induced by rabbit aorta subendothelium, was measured ex vivo in a perfusion chamber model after a short (5-minute) and long (30-minute) exposure time to rabbit native blood. The role of the perfusion time was assessed at shear rates of 100/s, 650/s, and 2600/s. These shear rates mimic blood flow conditions found in veins, arteries, and small or stenosed arteries, respectively. Fibrin deposition and platelet thrombus formation on subendothelium were evaluated by microscopic morphometry. In the presence of Ro 46-6240, fibrin deposition was abolished at both perfusion times and at all shear rates. In the 5-minute experiments, thrombus height was reduced by Ro 46-6240 at shear rates of 100/s (85%) and 650/s (35%) but not at a shear rate of 2600/s, whereas thrombus area was not affected at any shear rate. In contrast, both thrombus height and thrombus area were reduced (60% to 90%) by Ro 46-6240 in the 30-minute perfusion groups at all wall shear rates. The antithrombotic effect of Ro 46-6240 after 30-minute perfusion was confirmed by the minimal increase in the pressure difference between the entrance and the exit of the perfusion chamber compared with the control groups. In summary, our data suggest that thrombin activity is not involved in platelet adhesion and in the initial platelet thrombus formation that occurs in early thrombogenesis but that thrombin is a major mediator of subsequent thrombus growth and/or stabilization.