The radial artery (RA) is used as a spastic coronary bypass graft. This study was designed to investigate the mechanism of vasorelaxant effects of YC-1 (3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole), a nitric oxide (NO)-independent soluble guanylate cyclase (sGC activator, and DEA/NO (diethylamine/nitric oxide), a NO-nucleophile adduct, on the human RA. RA segments (n = 25) were obtained from coronary artery bypass grafting patients and were divided into 3-4 turn vascular rings. Using the isolated tissue bath technique, the endothelium-independent vasodilatation function was tested in vitro by the addition of cumulative concentrations of YC-1 (10(-10) to 3 x 10(-7) mol/L) and DEA/NO (10(-8) to 3 x 10(-5) mol/L) following vasocontraction by phenylephrine in the presence or absence of 10-5 mol/L ODQ (1H-(1,2,4)oxadiazole(4,3-a)quinoxalin-1-one), the selective sGC inhibitor, 10(-7) mol/L iberiotoxin, a blocker of Ca2+- activated K+ channels, or 10(-5) mol/L ODQ plus 10(-7) mol/L iberiotoxin. We also evaluated the effect of YC-1 and DEA/NO on the cGMP levels in vascular rings obtained from human radial artery (n = 6 for each drug). YC-1 (10(-10) to 3 x 10(-7) mol/L) and DEA/NO (10(-8) to 3 x 10(-5) mol/L) caused the concentration-dependent vasorelaxation in RA rings precontracted with phenylephrine (10(-5) mol/L) (n = 20 for each drug). Pre-incubation of RA rings with ODQ, iberiotoxin, or ODQ plus iberiotoxin significantly inhibited the vasorelaxant effect of YC-1, but the inhibitor effect of ODQ plus iberiotoxin was significantly more than that of ODQ and iberiotoxin alone p < 0.05). The vasorelaxant effect of DEA/NO almost completely abolished in the presence of ODQ and iberiotoxin plus ODQ, but did not significantly change in the presence of iberiotoxin alone (P > 0.05). The pEC(50) value of DEA/NO was significantly lower than those for YC-1 (p < 0.01), with no change E. x values in RA rings. In addition, YC- I -stimulated RA rings showed more elevation in cGMP than that of DEA/NO (p < 0.05). These findings indicate that YC-1 is a more potent relaxant than DEA/NO in the human RA. The relaxant effects of YC-1 could be due to the stimulation of the sGC and Ca2+- sensitive K(+)channels, whereas the relaxant effects of DEA/NO could be completely due to the stimulation of the sGC. YC-1 and DEA/NO may be effective as vasodilator for the short-term treatment of perioperative spasm of coronary bypass grafts.