Sulfhydryl modulation of K+ channels in rat ventricular myocytes.

Oxidative stress markedly alters protein function through redox modification of sulfhydryl groups present in cysteine residues. To explore the role of redox state in modulating cardiac K+ channels, this study examined the effects of sulfhydryl modifiers on the repolarizing transient outward current (Ito) in voltage-clamped myocytes from rat ventricle. Oxidized glutathione (GSSG; 5mM), an endogenous disulfide that specifically reacts with protein sulfhydryls, decreased maximum Ito amplitude from baseline by 49% when added to the external solution (P<0.05) and by 27% during internal dialysis (P<0.05). The membrane-impermeable disulfide, 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB) did not alter Ito when added to the external solution, but it decreased current amplitude by 31% during internal dialysis (P<0.05). GSSG-mediated Ito inhibition varied in a frequency- and voltage-dependent manner, consistent with a state-dependent blocking mechanism. This phenomenon was also observed in myocytes internally dialyzed with DTNB or Cd2+, which also covalently binds to free sulfhydryls. Inhibition of Ito by GSSG was not reversed by washout alone, consistent with the stable nature of covalently-modified sulfhydryl groups. However, when myocytes pretreated with GSSG were dialyzed with the reducing agent dithiothreitol, Ito amplitude increased significantly by 42% (P<0.05). These data suggest that alpha-subunits underlying Ito, or associated proteins, have one or more sulfhydryl groups within the cytoplasmic domain that directly modulate channel activity in response to changes in cell redox state. Redox modulation of Ito channels may be an important post-translational mechanism contributing to acute changes in cardiac repolarization under conditions of oxidative stress, such as ischemia and reperfusion.