Role of nitric oxide in isoprenaline and sodium nitroprusside-induced relaxation in human hand veins

Abstract

Aims. Recent reports, largely in animal models, have suggested that either inhibition of nitric oxide (NO) synthase or endothelium removal in arteries inhibits the response to isoprenaline, a β-adrenoceptor agonist, and also enhances the response to sodium nitroprusside, a nitrovasodilator. This in vivo study was designed to determine whether N(G)-monomethyl-L-arginine (L-NMMA), an inhibitor of NO synthesis, influences relaxation of human hand veins mediated by isoprenaline or by sodium nitroprusside. Methods. Using the dorsal hand vein technique, full dose-response curves to bradykinin (0.27-278 ng min-1, n = 6), isoprenaline (2.12-271 ng min-1, n = 8) and sodium nitroprusside (0.01-634 ng min-1, n = 7) were generated on separate occasions before and after L-NMMA co-infusion (50 μg min-1). Results. In veins preconstricted with the α1-adrenoceptor-selective agonist phenylephrine, the three vasodilators induced maximal responses (E(max)) of 119 ± 35, 72 ± 18 and 103 ± 17%, respectively. L-NMMA inhibited relaxation to bradykinin by 64% (P = 0.014) but did not influence relaxation induced by isoprenaline. The sensitivity to sodium nitroprusside was significantly enhanced by L-NMMA co-infusion (concentration shift of 2.3, P = 0.031). Conclusions. We conclude that in human veins, spontaneously released NO does not play a major role in isoprenaline-induced relaxation. Our results also suggest that the effects of sodium nitroprusside in this vascular bed may be attenuated by endothelium-derived NO.