H₂S mediates carotid body response to hypoxia but not anoxia

The role of cystathionine-γ-lyase (CSE) derived H₂S in the hypoxic and anoxic responses of the carotid body (CB) were examined. Experiments were performed on Sprague-Dawley rats, wild type and CSE knockout mice on C57BL/6 J background. Hypoxia (pO₂ = 37 ± 3 mmHg) increased the CB sensory nerve activity and elevated H₂S levels in rats. In contrast, anoxia (pO₂ = 5 ± 4 mmHg) produced only a modest CB sensory excitation with no change in H₂S levels. DL-propargylglycine (DL-PAG), a blocker of CSE, inhibited hypoxia but not anoxia-evoked CB sensory excitation and [Ca²⁺]_i elevation of glomus cells. The inhibitory effects of DL-PAG on hypoxia were seen: a) when it is dissolved in saline but not in dimethyl sulfoxide (DMSO), and b) in glomus cells cultured for18 h but not in cells either soon after isolation or after prolonged culturing (72 h) requiring 1-3 h of incubation. On the other hand, anoxia-induced [Ca²⁺]_i responses of glomus cell were blocked by high concentration of DL-PAG (300μM) either alone or in combination with aminooxyacetic acid (AOAA; 300μM) with a decreased cell viability. Anoxia produced a weak CB sensory excitation and robust [Ca²⁺]_i elevation in glomus cells of both wild-type and CSE null mice. As compared to wild-type, CSE null mice exhibited impaired CB chemo reflex as evidenced by attenuated efferent phrenic nerve responses to brief hyperoxia (Dejours test), and hypoxia. Inhalation of 100% N₂ (anoxia) depressed breathing in both CSE null and wild-type mice. These observations demonstrate that a) hypoxia and anoxia are not analogous stimuli for studying CB physiology and b) CSE-derived H₂S contributes to CB response to hypoxia but not to that of anoxia.