ENDOTHELIN-1 AND AGGREGATION OF HUMAN PLATELETS IN-VITRO

Endothelin-1 (ET-1) is a potent vasoconstrictor peptide produced by endothelial cells. We investigated whether ET-1, like other potent endothelium-derived vasoactive agents, interacts directly with human platelets in vitro. Platelet-rich plasma was obtained from healthy male volunteers and incubated with ET-1 (1 muM) or vehicle (sodium chloride 154 mM) for 10 min at 37-degrees-C. Platelet aggregation was measured by the Born method, using light transmittance through the plasma sample as an index of activation. Although a significant increase in light transmittance was observed when plasma was incubated with ET-1 compared with vehicle, (3.8 +/- 0.4% versus 2.7 +/- 0.2%; n = 24; p = 0.038), this effect was small and is unlikely to be of biologic significance. To investigate the possibility that ET-1-stimulated platelet nitric oxide (NO) synthesis might be masking a direct aggregatory effect of ET-1, in a second study in six subjects N(G)-monomethyl-L-arginine (L-NMMA, 10 and 100 muM), an inhibitor of NO synthase, was preincubated with the plasma before the addition of ET-I (1 nM and 1 muM). No significant difference was observed whether samples were incubated with L-NMMA alone or with L-NMMA and ET-1. The results of this study suggest that ET-1 does not have a major direct effect as a platelet aggregating agent.