Nitric oxide-mediated modulation of the endothelin-1 signalling pathway in the human cardiovascular system

1 We studied the ability of nitric oxide (NO) to physiologically antagonize endothelin-1 (ET-1) induced constrictions in human internal mammary artery (IMA). We also investigated the hypothesis that NO interacts directly with ET-receptor binding in human heart and aorta. 2 ET-1 potently contracted IMA (EC50 6.86 nM, 95% CI: 3.5-13.4 nM; n = 12). The constrictor response to 10 nM ET-1 was fully reversed by the NO-donor diethylamine NONOate (DEA/NO; EC50 2.0 muM, 95% CI: 0.8-4.8 muM; n = 5). The guanylate cyclase inhibitor ODQ (100 muM) reduced the response to DEA/NO but did not abolish it (EM, 50.9 +/- 8.5% in the presence of ODQ; 113.0 +/- 8.4%, control). 3 The increase in cyclic GMP by 30 muM DEA/NO was abolished in the presence of 100 muM ODQ (n = 6) 4 In saturation binding experiments the NO-donor Diethyltriamine NONOate (DETA/NO; 1 mM) caused a 90% reduction in maximum binding of [I-125]-ET-1 in human heart, without affecting the affinity. This reduction in binding was abolished by haemoglobin. Pre-incubating either the radiolabel or the tissue with NO-donors did not reduce binding. A similar effect was observed in aortic smooth muscle. 5 We have shown that DEA/NO is able to reverse ET-1-induced contractions in the human vasculature. The binding studies suggest a direct interaction between NO and the ET receptor or receptor-ligand complex in human ventricular and aortic tissue. NO is released continuously in vivo, thus this apparent modification of PT-receptor binding may provide an additional mechanism by which NO counter-balances the effects of ET.