Endothelium-derived relaxing factor from cultured human endothelial cells inhibits aggregation of human platelets.

The present study was designed to investigate whether endothelium-derived relaxing factor (EDRF) produced by cultured human endothelial cells (HEC) inhibits the aggregation of human platelets. Microcarrier beads covered with HEC from umbilical veins (approximately 2 X 10(6)) or empty beads (as controls) were co-incubated in an aggregometer with washed human platelets (approximately 5 X 10(7)). Indomethacin was present throughout and no prostacyclin production (measured as 6-keto-PGF1 alpha by radioimmunoassay) could be detected. The presence of HEC markedly inhibited thrombin-induced platelet aggregation and this inhibition was further enhanced by bradykinin, a stimulator of EDRF production. The anti-platelet aggregatory effect was blocked by treating the HEC with the inhibitor of EDRF production gossypol, by treating the platelets with the inhibitor of soluble guanylate cyclase methylene blue, or by adding the EDRF scavenger oxyhemoglobin to the aggregation mixture. The antiaggregatory material was labile, since the supernatant of indomethacin-treated cultured HEC did not inhibit aggregation. In the absence of indomethacin, the prostacyclin-mediated antiaggregatory effect of HEC was not inhibited by gossypol, methylene blue or hemoglobin. These data strongly suggest that the EDRF formed by HEC is an inhibitor of platelet aggregation and may constitute an important defense mechanism against vasospasm and platelet aggregation.