NO signaling pathways in cardiac myocytes. O2−,superoxide radical; SOD; superoxide dismutase; GC, guanylate cyclase; P, phosphorylation; XOR, xanthine oxidoreductase, ONOO−, peroxynitrite; NO+, nitrosonium ion; SR, sarcoplasmic reticulum; ATP, SR Ca-ATPase; PLB, phospholamban; RyR, SR Ca release channel (see text for other abbreviations).