Endothelial cell function and thrombosis.

Healthy endothelium is a metabolically active interface between the blood and extravascular tissues. Its intimal surface is anticoagulant and antithrombotic, and it secretes a variety of molecules involved in regulating platelet function and blood coagulation. The rapid interactions between platelets, their secreted components, or thrombin and endothelial cells at sites of vessel damage ensure the local secretion of mediators such as prostacyclin and nitric oxide that limit the intravascular growth of the haemostatic plug. There is considerable evidence that a decreased ability of endothelial cells to synthesize NO contributes to the pathogenesis of arterial disease. Local deficiency of PGI2 synthesis has also been implicated in the thrombotic problems in haemolytic uraemic syndrome. Endothelium is also the source of circulating von Willebrand factor, important for efficient platelet adhesion. Chronically elevated plasma levels of vWF in a series of diseases where there is vascular pathology apparently reflect endothelial cell damage or activation, and may contribute to the prothrombotic tendency they exhibit. They may be compounded by decreased levels of the surface anticoagulant thrombomodulin, if the increased concentrations of the soluble forms of thrombomodulin detected in the circulation under similar conditions are a reflection of loss from the endothelium. Further alterations of function in a procoagulant/prothrombotic direction take place when endothelial cells are exposed to certain cytokines or lipopolysaccharide. Tissue factor synthesis is induced, thrombomodulin expression is decreased, and there is enhanced sensitivity of vWF secretion. In addition, the balance of tissue-type plasminogen activator and plasminogen activator inhibitor type I secretion is changed in favour of the latter. These processes are each likely to contribute to the occurrence of disseminated intravascular coagulation which can accompany septic shock.