Abstract

The inhibition of nitric oxide synthase byN-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 inXenopus 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 μM), 3-morpholino-sydnonimine (100–1000 μM), (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (NOC-18; 1–300 μM), and S-nitrosoN-acetylpenicillamine (1–300 μM) dose-dependently inhibited hERG1 outward K⁺ currents. By contrast, the NO^⋅ donor NOC-18 (0.3 mM) 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 onhERG1 outward currents; furthermore, the specific inhibitor of the NO^⋅-dependent guanylyl cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (50 μM), neither interfered with outward hERG1K⁺ currents nor prevented their inhibition by 0.3 mM NOC-18. Both l-arginine (10 mM) and NOC-18 (0.3 mM) counteracted the stimulatory effect on hERG1 outward currents induced by the radical oxygen species-generating system FeSO₄ (25 μM)/ascorbic acid (50 μM; Fe/Asc). Finally,l-arginine (10 mM) 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 onhERG1 outward K⁺ currents via an interaction with radical oxygen species either generated under resting conditions or triggered by Fe/Asc.