Retraction Of Fibrin Clots By Cultured Endothelial Cells

The interactions between endothelial cells (EC) and fibrin are still poorly understood. We approached this problem by studying the ability of cultured EC to induce in vitro the retraction of fibrin clots. EC were obtained from bovine aorta and human umbilical veins by collagenase treatment and grown in Eagle MEM. At the time of the test the cells were harvested from the flask by a short trypsin-EDTA treatment and resuspended in tyrode solution. The test system involved incubation of the cell suspension in a water-bath at 37°C in the presence of cell-free plasma which was clotted by thrombin. The course of retraction was followed by measuring the diameter of the clot with a microcaliper. Retraction values were expressed after calculation of percent activity by an appropriate formula. EC were found to induce the retraction of the fibrin clot to an extent which increased with the time (1-24 h) and with the number of cells in the system (l-4×l06/ml f.c.). Fibrin clot retractile (FCR) activity of EC could not be detected at 22°C or in presence of Na2-EDTA or using mechanically disrupted cells. Moreover, using batroxobin instead of thrombin as a clotting agent, no retraction occurred; 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. Similarly to fibroblasts, EC with higher density in culture showed lower FCR, suggesting that con-fluency 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.