Cultured human umbilical vein endothelial cells inhibited the platelet aggregating activity of thrombin in the absence of plasma. Using a new method in which thrombin-induced platelet aggregation was measured in the presence of endothelial cells, we showed that aspirin-treated endothelial cells inhibited platelet aggregating activity of thrombin in an incubation time- and cell number-dependent manner. This inhibitory effect of endothelial cells was partially blocked by the pretreatment of endothelial cells with monoclonal anti-thrombomodulin IgG(anti-TMIgG). It is suggested that (1) endothelial cells play a role in the clearance of thrombin by binding and inactivating this enzyme and that (2) thrombomodulin on endothelial cells, apart from its role for protein C activation, may be involved in this endothelial function.
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