NITRIC-OXIDE ACTIVATES SKELETAL AND CARDIAC RYANODINE RECEPTORS

The endothelial-derived relaxing factor, nitric oxide (NO.) has been s hown to depress force in smooth and cardiac muscles through the activa tion of guanylyl cyclase and an increase in cGMP. In fast skeletal mus cle, NO (i.e. NO-related compounds) elicits a modest decrease in devel oped force, but in contracting muscles NO increases force by a mechani sm independent of cGMP. We now demonstrate an alternative mechanism wh ereby 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 an d Ca2+ release from skeletal and cardiac SR. The process was reversed by sulfhydryl reducing agents which promoted channel closure and Ca2reuptake by ATP-driven Ca2+ pumps. NO did not directly alter Ca2+-ATPa se activity but increased the open probability of RyRs reconstituted i n planar bilayers and inhibited [H-3]-ryanodine binding to RyRs. The f ormation of peroxynitrite or thiyl radicals did not account for the re versible R-SNO-dependent activation of RyRs. Ca2+ release induced by n itric oxide free radicals (NO.) was potentiated by cysteine providing compelling evidence that NO. in the presence of O-2 formed nitrosylate d cysteine followed by the transnitrosation of regulatory thiols on th e RyR to activate the channel. These findings demonstrate direct inter actions of NO derivatives with RyRs and a new fundamental mechanism to regulate force in striated muscle.