Production and availability of thromboplastin in endothelial cells: the effects of thrombin, endotoxin and platelets

Freshly prepared endothelial cells from umbilical cord veins were demonstrated to possess a weak but significant thromboplastin activity. Upon exposure of cultured endothelial cells to catalytically active thrombin (4.5 U/ml) or endotoxin (0.05 microgram/ml), the thromboplastin activity of the cells was increased 10-40-fold during 5 h incubation, and with the additional presence of washed, isolated platelets during the incubation, the activity was enchanced up to 170-fold. Platelets or catalytically inactivated thrombin added alone were largely ineffective as stimulants. 75-80% of the enhanced thromboplastin activity became available on the surface of the cultured cells as demonstrated by utilizing thromboplastin antibodies. Such exposed activity is suggested to be important in thrombotic situations where initial formation of thrombin may significantly accelerate the generation of thrombin by a positive feedback effect upon thromboplastin synthesis in the endothelial cells.