On the mechanism of coagulation inhibition on surfaces with end point immobilized heparin

A well established technique to improve blood compatibility of artificial materials for use in the circulation is to coat the surface with heparin. The present report describes the antithrombin mediated inhibition of thrombin and factor Xa by surfaces modified with end point immobilized heparin. The reaction was followed by conventional chromogenic substrate based enzyme assays as well as by immunological measurement of the enzyme inhibitor (thrombin-antithrombin) complex formation. Both enzymes were rapidly inactivated by heparin surfaces after selective presaturation with antithrombin on the immobilized high affinity heparin molecules. The thrombin inhibitory capacity was enhanced when both high and low affinity heparin were preadsorbed with inhibitor. The main part of the thrombin-antithrombin complex formed remained bound to the surface, however, without functionally blocking the activity of the high affinity sequence of the immobilized heparin. Aliquots of recalcified plasma were slowly rotated in loops of heparinized tubing to investigate whether the main thromboresistant function of the surface was exerted at the level of thrombin or by inactivation of preceding enzymes. After 1 h no visible clotting occurred and only trace amounts of thrombin (0.07 IU/ml), measured as thrombin-antithrombin complexes, had been formed. In non-heparinized loops and in the test tube plasma clotted after 20 min. The thrombin generation when clotting occurred was in the order of 10 IU/ml. It is concluded that the immobilized heparin mediates inhibition of the coagulation cascade prior to prothrombin activation.