Resistance of normal endothelium to damage by thrombin.

We examined the effect of locally infused thrombin on rabbit neck veins, using autologous [111In]indium-labeled platelets on the vessel wall. However, when the animals were pre-treated with aspirin (10 mg/kg), there was a marked change in the ratio of radioactive counts between control and treated segments, consistent with platelet deposition on the walls of thrombin-treated segments. Thrombin-treated and control veins were also examined by transmission and scanning electronmicroscopy. Although occasional clumps of platelets were seen adhering to the vessel wall in both control and treated segments, the endothelium was essentially intact. There was no evidence of substantial denudation of the endothelium, and overall there was little morphological difference between control and treated segments. It is concluded that normal endothelium is not damaged by thrombin, and that venous thrombi develop by a direct effect of thrombin on platelets and fibrinogen, and not by thrombin-mediated damage to the vein wall.