The inhibition of nitric oxide synthase by N-nitro-L-arginine methyl ester (0.03-3 mM) dose-dependently reduced nitric oxide (NO.) levels and enhanced the outward currents carried by human ether-a-gogo-related gene-1 (hERG1) K+ channels expressed in Xenopus laevis oocytes, whereas the increase in NO. levels achieved by exposure to L-arginine (0.03-10 mM) inhibited these currents. Furthermore, four NO. donors belonging to such different chemical classes as sodium nitroprusside (1-1000 mu M), 3-morpholino-sydnonimine (100-1000 mu M), (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (NOC-18; 1-300 mu M), and S-nitroso N-acetylpenicillamine (1-300 mu M) dose-dependently inhibited hERG1 outward K+ currents. By contrast, the NO. donor NOC-18 (0.3 mu M) did not affect other cloned K+ channels such as rat neuroblastoma-glioma K+ channel 2, rat delayed rectifier K+ channel 1, bovine ether-a-gogo gene, rat ether-a-gogo-related gene-2, and rat ether-a-gogo-related gene-3. The inhibitory effect of NO. donors on hERG1 K+ channels was prevented by the NO. scavengers 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and hemoglobin. The membrane permeable analog of cGMP, 8-bromo-cGMP (1 mM), failed to reproduce the inhibitory action of NO. donors on hERG1 outward currents; furthermore, the specific inhibitor of the NO.-dependent guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (50 mu M), neither interfered with outward hERG1 K+ currents nor prevented their inhibition by 0.3 mM NOC-18. Both L-arginine (10 mu M) and NOC-18 (0.3 mM) counteracted the stimulatory effect on hERG1 outward currents induced by the radical oxygen species-generating system FeSO4 (25 mu M)/ascorbic acid (50 mu M; Fe/Asc). Finally, L-arginine (10 mu M) and NOC-18 (0.3 mM) inhibited both basal and Fe/Asc (0.1 mM/0.2 mM) stimulated lipid peroxidation in X. laevis oocytes. Collectively, the present results suggest that NO., both endogenously produced and pharmacologically delivered, may exert in a cGMP-independent way an inhibitory effect on hERG1 outward K+ currents via an interaction with radical oxygen species either generated under resting conditions or triggered by Fe/Asc.