Endogenous Tissue Factor Pathway Inhibitor Modulates Thrombus Formation in an in vivo Model of Rabbit Carotid Artery Stenosis and Endothelial Injury.

Background—Tissue factor pathway inhibitor (TFPI) is the sole known inhibitor of the extrinsic coagulation pathway of physiological importance; however, its role in modulating thrombosis in vivo is still unclear. Methods and Results—Intravascular thrombosis was initiated by placing an external constrictor around endothelially injured rabbit carotid arteries (n510). Carotid blood flow velocity was measured by a Doppler flow probe. After placement of the constrictor, cyclic flow reductions (CFRs), due to recurrent thrombosis, developed at the site of stenosis. Transstenotic TFPI plasma activity was measured in blood samples before induction of CFRs and after 30, 60, and 180 minutes of CFRs. TFPI plasma activity distal to the site of thrombosis was significantly lower than the corresponding proximal values at 30, 60, and 180 minutes of CFRs. In addition, a progressive decrease in TFPI plasma activity was observed in both the proximal and the distal samples, indicating consumption of TFPI during thrombus formation. In 10 additional rabbits, CFRs were abolished by administration of aspirin (10 mg/kg). In the animals in which aspirin abolished CFRs, endogenous TFPI was depleted by a bolus of a polyclonal antibody against rabbit TFPI, and the effects on restoration of CFRs were monitored. In 5 of 6 animals in which aspirin abolished CFRs, depletion of endogenous TFPI activity caused full restoration of CFRs. Conclusions—The data of the present study support the involvement of endogenous TFPI in the process of thrombus formation in vivo and its active role in modulating arterial thrombosis.