Prothrombin is activated on vascular endothelial cells by factor Xa and calcium.

Abstract

Vascular endothelial cells derived from adult bovine aorta (ABAE) treated with factor Xa and Ca were found to activate prothrombin. In contrast, nonvascular cells (human foreskin fibroblasts, bovine corneal endothelial cells or human fetal lung cells) had either no or very little effect on prothrombin activation. In the presence of 6 .times. 105 ABAE cells, 20 ng of factor Xa converted 90 .mu.g of prothrombin into 80 U of thrombin after 45 min at 37.degree. C. Exogenous factor V was not required for prothrombin activation, but thrombin generation was enhanced 2- to 4-fold by the addition of factor V (500-2,500 ng/ml). Treatment of ABAE cells with anti-bovine factor V IgG markedly inhibited prothrombin activation by factor Xa and Ca. In cells grown in serum-free medium for 3 mo., the amount of factor V activity was equivalent to that found in cells grown with serum, which suggests that these cells probably synthesize factor V. Sparse ABAE cells increased prothrombin activation by factor Xa 6-fold compared to activation in confluent cells. Although previous thrombin treatment of ABAE cells did not enhace prothrombin activation, addition of dansyl arginine-4-ethyl piperidine amide markedly inhibited activation of 125I-labeled prothrombin by factor Xa, indicating that thrombin formation is necessary for optimal prothrombin activation. Aortic endothelium may provide a physiologically important surface for activation of prothrombin as well as a mechanism for optimal formation of clots at sites of vascular injury.