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 activ ity on the injured vessel wall that causes fibrin formation. The antic oagulant 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 isofo rms, alpha and beta. AT beta, which constitutes only 5% to 10% of AT i n plasma, lacks one carbohydrate side chain, has higher affinity for g lycosaminoglycans, 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 he parin-Sepharose chromatography, and the purity was investigated by iso electric focusing and crossed immunoelectrophoresis. Rabbits were subj ected to balloon injury of the aorta; 3 hours after injury the aorta w as excised. Thrombin coagulant activity on the aorta was measured by e xposure to fibrinogen and thereafter by measuring the generation of fi brinopeptide A. Injured animals were treated with AT, AT alpha, or AT beta and were compared with control animals. AT was demonstrated on th e injured vessel wall by using an immunohistochemical method. Animals receiving crude AT had significantly lower amounts of thrombin coagula nt activity on the injured aortic wall than control animals, but AT al pha 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 o f thrombin on the injured aortic wall than control animals. We conclud e that the inhibitory effect of AT on thrombin coagulant activity on t he injured vessel wall is explained by its AT beta content.