The cardiac acetylcholine-activated, inwardly rectifying K+-channel subunit GIRK1 gives rise to an inward current induced by free oxygen radicals

Reactive oxygen species (ROS) play a crucial role in pathophysiology of the cardiovascular system. The present study was designed to analyze the redox sensitivity of G-protein-activated inward rectifier K+ (GIRK) channels, which control cardiac contractility and excitability. GIRK1 subunits were heterologously expressed in Xenopus laevis oocytes and the resulting K+ currents were measured with the two-electrode voltage clamp technique. Oxygen free radicals generated by the hypoxanthine/xanthine oxidase system led to a marked increase in the current through GIRK channels, termed superoxide-induced current (ISO). Furthermore, ISO did not depend on G-protein-dependent activation of GIRK currents by coexpressed muscarinic m2-receptors, but could also be observed when no agonist was present in the bathing solution. Niflumic acid at a concentration of 0.5 mmol/l did not abolish ISO, whereas 100 μmol/l Ba2+ attenuated ISO completely. Catalase (106 i.u./l) failed to suppress ISO, whereas H2O2 concentration was kept close to zero, as measured by chemiluminescence. Hence, we conclude that O2•− or a closely related species is responsible for ISO induction. Our results demonstrate a significant redox sensitivity of GIRK1 channels and suggest redox-activation of G-protein-activated inward rectifier K+ channels as a key mechanism in oxidative stress-associated cardiac dysfunction.