Sleep apnea, which is the periodic cessation of breathing during sleep, is a major health problem affecting over 10 million people in the United States and is associated with several sequelae, including hypertension and stroke. Clinical studies suggest that abnormal carotid body (CB) activity may be a driver of sleep apnea. Because gaseous molecules are important determinants of CB activity, aberrations in their signaling could lead to sleep apnea. Here, we report that mice deficient in heme oxygenase-2 (HO-2), which generates the gaseous molecule carbon monoxide (CO), exhibit sleep apnea characterized by high apnea and hypopnea indices during rapid eye movement (REM) sleep. Similar high apnea and hypopnea indices were also noted in prehypertensive spontaneously hypertensive (SH) rats, which are known to exhibit CB hyperactivity. We identified the gaseous molecule hydrogen sulfide (HS) as the major effector molecule driving apneas. Genetic ablation of the HS-synthesizing enzyme cystathionine-γ-lyase (CSE) normalized breathing in HO-2 mice. Pharmacologic inhibition of CSE with l-propargyl glycine prevented apneas in both HO-2 mice and SH rats. These observations demonstrate that dysregulated CO and HS signaling in the CB leads to apneas and suggest that CSE inhibition may be a useful therapeutic intervention for preventing CB-driven sleep apnea.

The role of cystathionine-γ-lyase (CSE) derived HS 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 (pO = 37 ± 3 mmHg) increased the CB sensory nerve activity and elevated HS levels in rats. In contrast, anoxia (pO = 5 ± 4 mmHg) produced only a modest CB sensory excitation with no change in HS levels. DL-propargylglycine (DL-PAG), a blocker of CSE, inhibited hypoxia but not anoxia-evoked CB sensory excitation and [Ca] 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 [Ca] 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 [Ca] 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% N (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 HS contributes to CB response to hypoxia but not to that of anoxia.

We previously reported that cystathionine‐γ‐lyase (CSE)‐derived H2S mediates carotid body (CB) response to hypoxia (Hx) (Peng et al., PNAS, 2010). A recent study showed that CB Type I cell responses to anoxia (Ax) were unaffected by CSE inhibitors (Kim et al. Respir Physiology, 2015). We re‐examined the role of CSE‐derived H2S in CB sensory and Type I cell responses to Hx and Ax in adult Sprague‐Dawley rats, wild‐type and CSE knockout (KO) mice. Hx (pO2=37± 3mmHg) produced robust sensory excitation, and increased H2S levels in the CB; whereas Ax (pO2 =5±4 mmHg) produced only a weak sensory excitation and had no effect on H2S levels. Ax evoked a robust [Ca2+]i response of Type I cells as compared to hypoxia. CSE inhibitor DL‐propargylglycine (DL‐PAG, 50μM) inhibited Hx but not Ax‐evoked sensory and Type I responses. Higher concentration (300μM) of DL‐PAG alone, or in combination with aminooxyacetic acid (300μM), non‐selectively blocked [Ca2+]i responses of Type I cells to Ax and decreased cell viability. CB sensory excitation and [Ca2+]i elevation in Type I cells were unaffected in CSE null mice as compared to wild‐type (Wt) mice. Breathing stimulation by Hx was absent in CSE KO mice, whereas depressed breathing by Ax was unaffected. These observations suggest that CSE‐derived H2S contributes to carotid body response to “physiologically” relevant hypoxia but not to anoxia.Support or Funding InformationSupported by National Institutes of Health grants P01‐HL‐90554This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.