Evidence that vascular endothelial cells can induce the retraction of fibrin clots

Cultured endothelial cells (EC) from bovine aorta and umbilical vein induced retraction of a fibrin clot formed by addition of thrombin to cell-free plasma. Fibrin clot retractile (FCR) activity increased with time (1-24 hr) and with the number of cells in the system (1-4 x 10 ⁶/ml, final concentration), and was inhibited at 22° or in the presence of Na ₂-EDTA; moreover, no retraction occurred when batroxobin was used as a clotting agent instead of thrombin. FCR of EC thus showed many characteristics in common with platelet- and fibroblast-induced clot retraction. FCR activity of bovine EC increased with the number of subcultures, being very low in cells harvested from primary cultures. In contrast, human EC had high activity in primary cultures. Like fibroblasts, EC with a higher density in culture showed lower FCR, suggesting that confluency inhibits the cell contractile capacity. FCR could thus represent a simple in vitro test to further characterize the biology of EC and to evaluate their role in the development of fibrin thrombi.