Redox regulation of calcium release in skeletal and cardiac muscle

In skeletal and cardiac muscle cells, specific isoforms of the Ryanodine receptor channels mediate Ca2+ release from the sarcoplasmic reticulum. These channels are highly susceptible to redox modifications, which regulate channel activity. In this work, we studied the effects of Ca2+ (endogenous agonist) and Mg2+ (endogenous inhibitor) on the kinetics of Ca2+ release from sarcoplasmic reticulum vesicles isolated from skeletal or cardiac mammalian muscle. Native skeletal vesicles exhibited maximal stimulation of release kinetics by 10-20 muM [Ca2+], whereas in native cardiac vesicles, maximal stimulation of release required only 1 muM [Ca2+]. In 10 muM [Ca2+], free [Mg2+] < 0.1 mM produced marked inhibition of release from skeletal vesicles but free [Mg2+] less than or equal to 0.8 mM did not affect release from cardiac vesicles. Incubation of skeletal or cardiac vesicles with the oxidant thimerosal increased their susceptibility to stimulation by Ca2+ and decreased the inhibitory effect of Mg2+ in skeletal vesicles. Sulfhydryl-reducing agents fully reversed the effects of thimerosal. The endogenous redox species, glutathione disulfide and S-nitrosoglutathione, also stimulated release from skeletal sarcoplasmic reticulum vesicles. In 10 muM [Ca2+], S-35-nitrosoglutathione labeled a protein fraction enriched in release channels through S-glutathiolation. Free [Mg2+] I mM or decreasing free [Ca2+] to the nM range prevented this reaction. Possible physiological and pathological consequences of redox modification of release channels on Ca2+ signaling in heart and muscle cells are discussed.