Nitric oxide activates skeletal and cardiac ryanodine receptors

The endothelial-derived relaxing factor, nitric oxide (NO.) has been shown to depress force in smooth and cardiac muscles through the activation of guanylyl cyclase and an increase in cGMP. In fast skeletal muscle, NO (i.e. NO-related compounds) elicits a modest decrease in developed force, but in contracting muscles NO increases force by a mechanism independent of cGMP. We now demonstrate an alternative mechanism whereby NO triggers Ca2+ release from skeletal and cardiac sarcoplasmic reticulum (SR). NO delivered in the form of NO gas, NONOates (a class of sulfur-free compounds capable of releasing NO), or S-nitrosothiols (R-SNO) oxidized or transnitrosylated regulatory thiols on the release channel (or ryanodine receptor, RyR), resulting in channel opening and Ca2+ release from skeletal and cardiac SR. The process was reversed by sulfhydryl reducing agents which promoted channel closure and Ca2+ reuptake by ATP-driven Ca2+ pumps. NO did not directly alter Ca(2+)-ATPase activity but increased the open probability of RyRs reconstituted in planar bilayers and inhibited [3H]-ryanodine binding to RyRs. The formation of peroxynitrite or thiyl radicals did not account for the reversible R-SNO-dependent activation of RyRs. Ca2+ release induced by nitric oxide free radicals (NO.) was potentiated by cysteine providing compelling evidence that NO. in the presence of O2 formed nitrosylated cysteine followed by the transnitrosation of regulatory thiols on the RyR to activate the channel. These findings demonstrate direct interactions of NO derivatives with RyRs and a new fundamental mechanism to regulate force in striated muscle.