The vascular endothelium as an inhibitor of thrombin

Uptake and inhibition of thrombin on the endothelium of porcine aorta was studied in vitro. The thrombin was labelled with ¹²⁵J and its enzyme activity was measured with an amidolytic assay. After exposure to the aorta both the enzyme activity and radioactivity disappeared from the solution and were recovered as surface bound activities. The rate of inhibition of the surface bound thrombin was determined in presence or absence of plasma. In presence of plasma the endothelially confined enzymatic activity was rapidly inhibited, no enzymatic activity was recovered in plasma. The surface bound radioactivity, however, decreased slowly and was recovered in plasma. In absence of plasma only slow inhibition took place. It is concluded that thrombin taken up on the endothelium is rapidly inhibited there by an interaction with plasma and then released in an inactivated state. An alternative conclusion, based on the anticoagulant and antithrombotic actions of the endothelial fibrin lining is briefly discussed.