In this study we have examined the influence of perturbation of endothelial cells on the amounts of fibronectin and von Willebrand factor in their extracellular matrix and the consequences of a changed composition of the matrix on platelet adhesion. For this purpose, we have used an in vitro perfusion system with which we can investigate the interactions of platelets in flowing blood with cultured endothelial cells and their extracellular matrix (Sakariassen, K. S., P. A. M. M. Aarts, P. G. de Groot, W. P. M. Houdgk, and J. J. Sixma, 1983, J. Lab. Clin Med. 102:522-535). Treatment of endothelial cells with 0.1-1.0 U/ml thrombin for 2 h increased the reactivity of the extracellular matrix, isolated after the thrombin treatment, towards platelets by approximately 50%. The increased reactivity did not depend on de novo protein synthesis but was inhibited by 3-deazaadenosine, an inhibitor of phospholipid methylation, which also inhibits the stimulus-induced instantaneous release of von Willebrand factor from endothelial cells. However, no changes in the amounts of von Willebrand factor and fibronectin in the matrix were detected. Thrombin may change the organization of the matrix proteins, not the composition. When endothelial cells were perturbed with the phorbol ester PMA or thrombin for 3 d, the adhesion of platelets to the extracellular matrix of treated cells was strongly impaired. This impairment coincided with a decrease in the amounts of von Willebrand factor and fibronectin present in the matrix. These results indicate that, after perturbation, endothelial cells regulate the composition of their matrix, and that this regulation has consequences for the adhesion of platelets.

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