A redox O2 sensor modulates the SR Ca2+ countercurrent through voltage- and Ca(2+)-dependent Cl- channels

The activity of a relatively small Cl- (SCl) channel in the sarcoplasmic reticulum (SR) vesicles of rabbit skeletal muscle was preserved following their reconstitution into lipid bilayer. Reducing PO2 from approximately 150 to < 1 Torr in the cis-side (cytosolic) reversibly inhibited the channel activity within 2 min. The modulatory effects, deduced from reduction in Cl- current levels and in kinetic parameters of channel activation, in normoxic (PO2 approximately 150 Torr) and hypoxic (low PO2 < 1 Torr) solutions were mimicked by oxidizing and reducing agents, respectively. Cl- current transitions to the main open conductance state were increased by 100 microM of the specific sulfhydryl (SH)-oxidizing agent 4,4'-dithiodipyridine and inhibited by the SH-reducing agent glutathione (GSH) with a Hill coefficient of 8 and inhibition constant of approximately 3.1 mM. The inhibitory effects of 5 mM [GSH]cis were prevented by prior addition of 1 mM iodoacetamide, an alkylating agent, to the cytosolic side of the channel. These findings suggest that an SH-dependent mechanism (redox couple, e.g., reduced/oxidized glutathione) could be involved in the gating of the SCl channel in such a way that SH oxidation (GSSH) favors the open state of the channel, and SH reduction (GSH), which mimics the inhibitory action of low PO2, favors the closed state.