Abstract
The ability to detect oxygen availability is a ubiquitous attribute of aerobic organisms. However, the mechanism(s) that transduce oxygen concentration or availability into appropriate physiological responses is less clear and often controversial. This review will make the case for oxygen-dependent metabolism of hydrogen sulfide (H2S) and polysulfides, collectively referred to as reactive sulfur species (RSS) as a physiologically relevant O2 sensing mechanism. This hypothesis is based on observations that H2S and RSS metabolism is inversely correlated with O2 tension, exogenous H2S elicits physiological responses identical to those produced by hypoxia, factors that affect H2S production or catabolism also affect tissue responses to hypoxia, and that RSS efficiently regulate downstream effectors of the hypoxic response in a manner consistent with a decrease in O2. H2S-mediated O2 sensing is then compared to the more generally accepted reactive oxygen species (ROS) mediated O2 sensing mechanism and a number of reasons are offered to resolve some of the confusion between the two.
Highlights
The Need for Oxygen SensingA key to survival for aerobic organisms is the ability to detect oxygen availability and make the necessary behavioral, physiological and/or metabolic adjustments to either ensure adequate oxygen delivery or to cope with what is available
hydrogen sulfide (H2 S) freely diffuses through cell membranes [58,59], and diffusion out of cells could theoretically contribute to H2 S inactivation, mitochondrial oxidation is far more efficient, it can be regulated, and it is O2 -dependent [60]
We initially reported that H2 S has concentration-dependent multiphasic effects on a variety of blood vessels that are virtually identical to the effects of hypoxia [22]
Summary
A key to survival for aerobic organisms is the ability to detect oxygen availability and make the necessary behavioral, physiological and/or metabolic adjustments to either ensure adequate oxygen delivery or to cope with what is available. While some form of oxygen sensing system is present in essentially all aerobic (and some anaerobic) organisms, in the interest of space this review is limited to those sensing systems employed by vertebrates to ensure adequate delivery of O2 to tissues. Additional details on hypoxia in aquatic organisms can be found in Farrell and Brauner [2]
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