Hydrogen sulfide mediates hypoxia-induced relaxation of trout urinary bladder smooth muscle

Hydrogen sulfide as an oxygen sensor in trout gill chemoreceptors

O2 chemoreceptors 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 O2 sensing in vascular smooth muscle. Here, we examined the possibility that H2S is an O2 sensor in trout chemoreceptors where the first pair of gills is a primary site of aquatic O2 sensing 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 beta-synthase and cystathionine gamma-lyase, was present in branchial tissue. Homogenized gills produced H2S enzymatically, and H2S production was inhibited by O2, whereas mitochondrial H2S consumption was O2 dependent. 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 approximately 10-mV depolarization. These studies are consistent with H2S involvement in O2 sensing/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 PO2 and may provide an exquisitely sensitive, yet simple, O2 sensor in a variety of tissues.

Reappraisal of H2S/sulfide concentration in vertebrate blood and its potential significance in ischemic preconditioning and vascular signaling

Hydrogen sulfide (H(2)S) is rapidly emerging as a biologically significant signaling molecule. Studies published before 2000 report low or undetectable H(2)S (usually as total sulfide) levels in blood or plasma, whereas recent work has reported sulfide concentrations between 10 and 300 microM, suggesting it acts as a circulating signal. In the first series of experiments, we used a recently developed polarographic sensor to measure the baseline level of endogenous H(2)S gas and turnover of exogenous H(2)S gas in real time in blood from numerous animals, including lamprey, trout, mouse, rat, pig, and cow. We found that, contrary to recent reports, H(2)S gas was essentially undetectable (<100 nM total sulfide) in all animals. Furthermore, exogenous sulfide was rapidly removed from blood, plasma, or 5% bovine serum albumin in vitro and from intact trout in vivo. To determine if blood H(2)S could transiently increase, we measured oxygen-dependent H(2)S production by trout hearts in vitro and in vivo. H(2)S has been shown to mediate ischemic preconditioning (IPC) in mammals. IPC is present in trout and, unlike mammals, the trout myocardium obtains its oxygen from relatively hypoxic systemic venous blood. In vitro, myocardial H(2)S production was inversely related to Po(2), whereas we failed to detect H(2)S in ventral aortic blood from either normoxic or hypoxic fish in vivo. These results provide an autocrine or paracrine mechanism for myocardial coupling of hypoxia to H(2)S in IPC, i.e., oxygen sensing, but they fail to provide any evidence that H(2)S signaling is mediated by the circulation.

Hydrogen sulfide as an oxygen sensor/transducer in vertebrate hypoxic vasoconstriction and hypoxic vasodilation

How vertebrate blood vessels sense acute hypoxia and respond either by constricting (hypoxic vasoconstriction) or dilating (hypoxic vasodilation) has not been resolved. In the present study we compared the mechanical and electrical responses of select blood vessels to hypoxia and H2S, measured vascular H2S production, and evaluated the effects of inhibitors of H2S synthesis and addition of the H2S precursor, cysteine, on hypoxic vasoconstriction and hypoxic vasodilation. We found that: (1) in all vertebrate vessels examined to date, hypoxia and H2S produce temporally and quantitatively identical responses even though the responses vary from constriction (lamprey dorsal aorta; lDA), to dilation (rat aorta; rA), to multi-phasic (rat and bovine pulmonary arteries; rPA and bPA, respectively). (2) The responses of lDA, rA and bPA to hypoxia and H2S appear competitive; in the presence of one stimulus, the response to the other stimulus is substantially or completely eliminated. (3) Hypoxia and H2S produce the same degree of cell depolarization in bPA. (4) H2S is constitutively synthesized by lDA and bPA vascular smooth muscle. (5) Inhibition of H2S synthesis inhibits the hypoxic response of lDA, rA, rPA and bPA. (6) Addition of the H2S precursor, cysteine, doubles hypoxic contraction in lDA, prolongs contraction in bPA and alters the re-oxygenation response of rA. These studies suggest that H2S may serve as an O2 sensor/transducer in the vascular responses to hypoxia. In this model, the concentration of vasoactive H2S in the vessel is governed by the balance between endogenous H2S production and its oxidation by available O2.