O2 sensing and signaling by gas messengers in the carotid body. cGMP, cyclic GMP; CIH, chronic intermittent hypoxia; CSE, cystathionine-γ-lyase; CSN, carotid sinus nerve; Cys-265, cysteine residue 265; HO2, heme oxygenase 2; Homocys, homocysteine; PKG, cGMP-dependent protein kinase; ROS, reactive oxygen species; sGC, soluble guanylate cyclase. O2 stimulates CO production, which inhibits H2S production, thereby inhibiting glomus cell depolarization. ROS generated by CIH inhibit HO2 activity, probably by oxidation of Cys-265, thereby increasing H2S production and CSN activity.