Hydrogen sulfide (H₂S) is rapidly emerging as a biologically significant signaling molecule. Studies published before 2000 report low or undetectable H₂S (usually as total sulfide) levels in blood or plasma, whereas recent work has reported sulfide concentrations between 10 and 300 μM, 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 H₂S gas and turnover of exogenous H₂S gas in real time in blood from numerous animals, including lamprey, trout, mouse, rat, pig, and cow. We found that, contrary to recent reports, H₂S 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 H₂S could transiently increase, we measured oxygen-dependent H₂S production by trout hearts in vitro and in vivo. H₂S 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 H₂S production was inversely related to Po₂, whereas we failed to detect H₂S 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 H₂S in IPC, i.e., oxygen sensing, but they fail to provide any evidence that H₂S signaling is mediated by the circulation.

The chemical versatility of sulfur and its abundance in the prebiotic Earth as reduced sulfide (H₂S) implicate this molecule in the origin of life 3.8 billion years ago and also as a major source of energy in the first seven-eighths of evolution. The tremendous increase in ambient oxygen ∼600 million years ago brought an end to H₂S as an energy source, and H₂S-dependent animals either became extinct, retreated to isolated sulfide niches, or adapted. The first 3 billion years of molecular tinkering were not lost, however, and much of this biochemical armamentarium easily adapted to an oxic environment where it contributes to metabolism and signaling even in humans. This review examines the role of H₂S in evolution and the evolution of H₂S metabolism and signaling.

SUMMARY 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.

Significance: Hydrogen sulfide (H ₂ S) is garnering increasing interest as a biologically relevant signaling molecule. The effects of H ₂ S have now been observed in virtually every organ system and numerous physiological processes. Recent Advances: These studies have not only opened a new field of “gasotransmitter” biology, they have also led to the development of synthetic H ₂ S “donating” compounds with the potential to be parlayed into a variety of therapeutic applications. Critical Issues: Often lost in the exuberance of this new field is a critical examination or understanding of practical aspects of H ₂ S chemistry and biology. This is especially notable in the areas of handling and measuring H ₂ S, evaluating biosynthetic and metabolic pathways, and separating physiological from pharmacological responses. Future Directions: This brief review describes some of the pitfalls in H ₂ S chemistry and biology that can lead or have already led to misleading or erroneous conclusions. The intent is to allow individuals entering or already in this burgeoning field to critically analyze the literature and to assist them in the design of future experiments. Antioxid. Redox Signal. 17, 32–44.