(A) Spatial localization of NOSs and oxidases in the cardiac myocyte. NOS3 localizes to the sarcolemmal caveolae (85, 97), where it participates in regulation of L-type Ca²⁺ channel (LTCC) currents, mediated either by cGMP formation (S64) or by S-nitrosylation of the LTCC (S13). NOS1 localizes to the SR (32), where it facilitates the SR Ca²⁺ cycle (97) by S-nitrosylation of the RyR and possibly the SR Ca²⁺ ATPase (SERCA2a) (17, 33). XOR also localizes to the SR in the cardiac myocyte; upregulation of protein or activity (caused by SR NOS1 deficiency) disrupts SNO regulation of the RyR (22). Other oxidases (e.g., NADPH oxidase) have been described in the cardiac myocyte (50), but precise signaling roles, identities, and/or subcellular localizations have not been elucidated. The mitochondria are an additional source of both O₂^– and NO, which may participate in control of mitochondrial respiration (S65–S67) and apoptosis (S41). Cyt, cytochrome; PLB, phospholamban. (B) Regulation of the RyR by S-nitrosylation. S-nitrosylation occurs at a single cysteine residue (1 of approximately 50 free thiols), which resides within a calmodulin-binding domain of the cardiac RyR (17). NO binding (shown for RyR1 of skeletal muscle) occurs in an oxygen-concentration–dependent manner and primes the channel for calmodulin regulation (18). Higher pO₂ oxidizes a small set of RyR-associated thiols that regulate the channel’s responsiveness to NO. SH, reduced thiol; S-S, oxidized thiols; CAM, calmodulin; x refers to a small set between 1 and 3.