Uptake and inactivation of thrombin on the subendothelium: comparisons with endothelium.

One aspect of the non-thrombogenicity of endothelium is the property to bind and inhibit thrombin. Loss of the endothelial layer gives rise to a thrombogenic surface. The present study concerns uptake and inactivation of thrombin on endothelium as well as subendothelium. Both the immediate subendothelial layer and the media were investigated. Binding and inhibition of thrombin was assayed using both a synthetic chromogenic substrate and fibrinogen. In the latter case thrombin enzymatic activity was assessed by measuring liberation of fibrinopeptide A. There was a linear relationship between thrombin in the solution and thrombin enzymatic activity recovered on the surface on both endothelium and media. 30-50 per cent of the surface bound thrombin measured with the chromogenic substrate was also active towards fibrinogen. Preincubation of endothelium with plasma gave rise to an increased loss of thrombin from the solution at the same time as less thrombin was recovered on the surface. This indicates increased inactivation. Increased inactivation could also be obtained by pretreatment with antithrombin (AT) but not with AT-depleted plasma. The same general pattern was seen on the subendothelium but not on the media. It is concluded that inhibition of thrombin occurs on the endothelium and can be augmented by prior binding of AT to the surface. This effect although less pronounced can also be obtained on the subendothelium. The deeply injured vessel is much less capable of inhibiting thrombin.