ANTITHROMBOTIC EFFECTS OF ACTIVATED PROTEIN-C AND PROTEIN-S IN A RABBIT MODEL OF MICROARTERIAL THROMBOSIS

The antithrombotic properties of activated protein C (APC) and protein S were investigated in a rabbit model of microarterial thrombosis. The study focused on the ability of intact and thrombin-cleaved bovine protein S to potentiate the biological effects of bovine APC in vivo. Segments of the central arteries of the ears were subjected to arteriotomy, deep-vessel wall trauma, and arteriotomy suture. Five minutes before vascular reperfusion, groups of rabbits were infused with boluses of 0.1 mg/kg bovine APO alone or combined with different doses of intact (0.5, 0.1, or 0.05 mg/kg) or thrombin-cleaved (0.5 mg/kg) protein S. APC in combination with the two higher doses of protein S produced a potent antithrombotic response, as judged by assessment of vessel patency rates, while only the group receiving APC+0.5 mg/kg protein S showed significant reduction of thrombus weights as well. The biological effect depended on the active site in APC, as the antithrombotic effect was lost on pretreatment of APC with the serine protease inhibitor D-phenylalanyl-L-prolyl-L-arginine chloromethylketone. The potentiation of the APC response by protein S depended on the structural integrity of the protein, and cleavage of the thrombin-sensitive region in protein S by thrombin resulted in a loss of biological response. No hemorrhagic side effects were noted by the APC-protein S combination, and the anticoagulant response was mild, even to the highest doses of APC and protein S. The study demonstrates the specificity of the APC-protein S interaction in vivo and shows both the necessity of an active catalytic site in APC and the physiological importance of the APC-protein 5 interaction. APC appears to be a suitable antithrombotic agent when fully potentiated by its cofactor protein S, as it produces only mild plasma anticoagulation and no hemorrhagic side effects.