Ventilatory and metabolic effects of exogenous hydrogen sulfide

Acute H₂S intoxication produces an increase in ventilation followed by a fatal central apnea. The sites of mediation of H₂S induced hyperpnea and apnea have been investigated since the early 20th century in various animal models. Hyperpnea is mediated by the arterial chemoreceptors, an effect that can be reproduced by injecting a solution of H₂S at very high concentrations (high millimolar range), while the fatal apnea, which typically occurs above 1000ppm in humans, appears to result from the cessation of the activity of the medullary respiratory neurons. More recently, moderate levels of exogenous H₂S (20-80ppm) have been shown to reduce, within minutes, the metabolic rate, akin to hypoxia-induced hypometabolism. This response appears to be specific to small sized mammals. The pathway through which low levels of inhaled H₂S could exert such a powerful effect may be very relevant to the physiological mechanisms controlling non-ATP " metabolic" production. Finally, endogenous H₂S, produced from cysteine, has been proposed to transduce the effects of hypoxia in the carotid bodies. H₂S remains a mysterious gas: it is labile, difficult/impossible to properly measure in vivo, its oxidation can take place in most tissues including the blood, and it can affect multiple cellular pathways. The demarcation between effects reflecting a putative physiological function and those related to H₂S poisoning remains however to be established.