Hydrogen sulfide as an oxygen sensor in trout gill chemoreceptors

Abstract

O2chemoreceptors elicit cardiorespiratory reflexes in all vertebrates, but consensus on O2-sensing signal transduction mechanism(s) is lacking. We recently proposed that hydrogen sulfide (H2S) metabolism is involved in O2sensing in vascular smooth muscle. Here, we examined the possibility that H2S is an O2sensor in trout chemoreceptors where the first pair of gills is a primary site of aquatic O2sensing and the homolog of the mammalian carotid body. Intrabuccal injection of H2S in unanesthetized trout produced a dose-dependent bradycardia and increased ventilatory frequency and amplitude similar to the hypoxic response. Removal of the first, but not second, pair of gills significantly inhibited H2S-mediated bradycardia, consistent with the loss of aquatic chemoreceptors. mRNA for H2S-synthesizing enzymes, cystathionine β-synthase and cystathionine γ-lyase, was present in branchial tissue. Homogenized gills produced H2S enzymatically, and H2S production was inhibited by O2, whereas mitochondrial H2S consumption was O2dependent. Ambient hypoxia did not affect plasma H2S in unanesthetized trout, but produced a Po2-dependent increase in a sulfide moiety suggestive of increased H2S production. In isolated zebrafish neuroepithelial cells, the putative chemoreceptive cells of fish, both hypoxia and H2S, produced a similar ∼10-mV depolarization. These studies are consistent with H2S involvement in O2sensing/signal transduction pathway(s) in chemoreceptive cells, as previously demonstrated in vascular smooth muscle. This novel mechanism, whereby H2S concentration ([H2S]) is governed by the balance between constitutive production and oxidation, tightly couples tissue [H2S] to Po2and may provide an exquisitely sensitive, yet simple, O2sensor in a variety of tissues.