Thrombin inhibition by antithrombin III on the subendothelium is explained by the isoform AT beta

Balloon injury of the rabbit aorta results in thrombin coagulant activity on the injured vessel wall that causes fibrin formation. The anticoagulant activity of both the intact and injured vessel wall has been partly explained by glycosaminoglycans with heparin-like activity that augment the activity of antithrombin III (AT). AT exists in two isoforms, alpha and beta. AT beta, which constitutes only 5% to 10% of AT in plasma, lacks one carbohydrate side chain, has higher affinity for glycosaminoglycans, and associates more readily with the subendothelium. This study evaluated whether AT can inhibit thrombin on the injured vessel wall and, if so, whether one of the isoforms is more effective then the other. The two isoforms were isolated from human plasma by heparin-Sepharose chromatography, and the purity was investigated by isoelectric focusing and crossed immunoelectrophoresis. Rabbits were subjected to balloon injury of the aorta; 3 hours after injury the aorta was excised. Thrombin coagulant activity on the aorta was measured by exposure to fibrinogen and thereafter by measuring the generation of fibrinopeptide A. Injured animals were treated with AT, AT alpha, or AT beta and were compared with control animals. AT was demonstrated on the injured vessel wall by using an immunohistochemical method. Animals receiving crude AT had significantly lower amounts of thrombin coagulant activity on the injured aortic wall than control animals, but AT alpha at a comparable dose had no effect. AT beta was given in the same dose as crude AT and also at a dose (10%) proportional to its presence in plasma. Animals receiving AT beta had significantly lower values of thrombin on the injured aortic wall than control animals. We conclude that the inhibitory effect of AT on thrombin coagulant activity on the injured vessel wall is explained by its AT beta content.