Vascular actions of hydrogen sulfide in nonmammalian vertebrates

Hydrogen sulfide (H2S) vasoactivity has been observed in isolated vessels from all vertebrate classes, and its effects, which include constriction, dilation, and multiphasic responses, are both species- and vessel-speciric. H2S is synthesized by mammalian and fish vessels, and because plasma H2S titers are also vasoactive in vitro, it is likely that H2S is a tonic effector of cardiovascular homeostasis in many vertebrates. Mechanisms of H2S vasoactivity in nonmammalian vertebrates have been limited to the trout where the triphasic relaxation-contraction-relaxation includes endothelium-dependent and -independent components, ATP-dependent K+ channels, and extracellular and intracellular Ca2+, all independent of cyclic GMP production. The observation that at least some H2S constrictory activity has been observed in all vertebrates except sharks suggests that H2S may have been an ancestral pressor gasotransmitter. However, the ability of H2S to serve as either (or both) an endothelium-independent constrictor or dilator, which is relatively unique among vasoregulatory molecules, is a feature that seems to have been exploited, for unknown reasons, by nearly all vertebrates. Aquatic vertebrates appear particularly vulnerable to H2S because of their intrinsically low blood pressure and the potential for increased H,S exposure from the environment.