Calcium release mediated by ryanodine receptors/calcium release channels (RyR channels) is required to elicit the contraction of skeletal and cardiac muscle and to induce neuronal plasticity in the hippocampus. In particular, RyR-mediated calcium-induced calcium release (CICR) is a powerful mechanism for the amplification and propagation of calcium signals initially generated by calcium entry into cells. RyR channels are especially susceptible to redox modifications by a variety of non-physiological or endogenous redox molecules, which produce significant changes on channel activity. Consequently, we have investigated the effects of redox modification of RyR channels on calcium release in skeletal and cardiac muscle and on the activation of signaling cascades and transcription factors in neurons. All experiments were done in compliance with the protocols for animal care, handling and experimentation approved by the Facultad de Medicina, Universidad de Chile. Using SR vesicles enriched in RyR1 channels isolated from rabbit skeletal muscle (Aracena et al 2003), we studied the effects of S-nitrosylation and S-glutathionylation by two endogenous redox-active agents – glutathione disulfide (GSSG) and S-nitrosoglutathione (GSNO) – on CICR. We found that S-glutathionylation of 3 cysteine residues per RyR1 channel monomer diminishes channel inhibition by magnesium while S-nitrosylation of different cysteines enhances the activation of the channel by calcium. Furthermore, we found that the transverse tubules (plasma membrane invaginations) of skeletal muscle cells possess an endogenous NAD(P)H oxidase that on incubation with NAD(P)H generates superoxide anion, which is rapidly converted into hydrogen peroxide. Activation of this enzyme significantly stimulates RyR1 channels in vitro and in skeletal muscle cells in culture. In SR vesicles isolated from dog heart muscle (Domenech et al 1998), we found that 0.1 mM NADPH enhances calcium release kinetics; addition of superoxide dismutase or catalase prevented these effects. All four NAD(P)H oxidase subunits probed (gp91, p22, p47 and p67) were found in cardiac SR vesicles, suggesting that reactive oxygen species generated by the NAD(P)H oxidase activate RyR2-mediated release. We also found that brief periods of tachycardia – which prior to prolonged coronary artery occlusion reduce infarct size (Sanchez et al 2003) – enhanced the synthesis de novo of RyR2 channels, stimulated calcium release from isolated SR vesicles and increased 2-fold the rates of NAD(P)H-dependent superoxide production compared to the controls. Noteworthy, tachycardia also increased 2-fold the association of the p47 subunit to SR vesicles. Accordingly, we propose that tachycardia enhances calcium release by promoting oxidative activation of RyR2 channels and by stimulating de novo RyR2 synthesis. In neurons, we have investigated whether oxidative activation of RyR channels promotes the phosphorylation of ERKs and CREB, which is required for the calcium-dependent gene expression associated with long lasting synaptic plasticity in the hippocampus. To induce changes in cell redox state, we added hydrogen peroxide to neurons in culture (N2a or hippocampal neurons) or to mouse hippocampal slices, or increased the Fe content of PC12 cells in culture. In all cases, hydrogen peroxide addition significantly enhanced phosphorylation of CREB and ERKs, whereas increasing cell Fe content increased cytoplasmic calcium concentration and stimulated the phosphorylation and the nuclear translocation of phospho-ERK1/2. Ryanodine (50-100 μM) significantly inhibited all these effects, suggesting that oxidative activation of RyR-mediated calcium release from intracellular stores mediates the observed enhancement of ERK/CREB phosphorylation. In summary, the present results suggest that oxidative activation of RyR channels enhances calcium release in skeletal or cardiac muscle cells and neurons. The resulting increase in cellular calcium concentration may sustain calcium-dependent gene expression during normal cellular function, as shown in neurons. Furthermore, oxidative stress, through excessive stimulation of RyR-mediated calcium release, may lead to the development of pathological conditions.