Sulfhydryl oxidation overrides Mg2+ inhibition of calcium-induced calcium release in skeletal muscle triads

We studied the effect of oxidation of sulfhydryl (SH) residues on the inhibition by Mg2+ of calcium-induced calcium release (CICR) in triad-enriched sarcoplasmic reticulum vesicles isolated from rabbit skeletal muscle. Vesicles were either passively or actively loaded with calcium before eliciting CICR by dilution at pCa 4.5-4.4 in the presence of 1.2 mM free [ATP] and variable free [Mg2+]. Native triads exhibited a significant inhibition of CICR by Mg2+, with a K-0.5 approximate to 50 mu M. Partial oxidation of vesicles with thimerosal produced a significant increase of release rate constants and initial release rates at all [Mg2+] tested (up to 1 mM), and shifted the K-0.5 value for Mg2+ inhibition to 101 or 137 mu M in triads actively or passively loaded with calcium, respectively. Further oxidation of vesicles with thimerosal completely suppressed the inhibitory effect of [Mg2+] on CICR, yielding initial rates of CICR of 2 mu mol/(mg x s) in the presence of 1 mM free [Mg2+]. These effects of oxidation on CICR were fully reversed by SH reducing agents. We propose that oxidation of calcium release channels, by decreasing markedly the affinity of the channel inhibitory site for Mg2+, makes CICR possible in skeletal muscle.