H2S during circulatory shock: some unresolved questions

Numerous papers have been published on the role of H2S during circulatory shock. Consequently, knowledge about vascular sulfide concentrations may assume major importance, in particular in the context of "acute on chronic disease", i.e., during circulatory shock in animals with pre-existing chronic disease. This review addresses the questions (i) of the "real" sulfide levels during circulatory shock, and (ii) to which extent injury and pre-existing co-morbidity may affect the expression of H2S producing enzymes under these conditions. In the literature there is a huge range on sulfide blood levels during circulatory shock, in part as a result of the different analytical methods used, but also due to the variable of the models and species studied. Clearly, some of the very high levels reported should be questioned in the context of the well-known H2S toxicity. As long as "real" sulfide levels during circulatory shock are unknown and/or undetectable "on line" due to the lack of appropriate techniques, it appears to be premature to correlate the measured blood levels of hydrogen sulfide with the severity of shock or the H2S therapy-related biological outcomes. The available data on the tissue expression of the H2S-releasing enzymes during circulatory shock suggest that a "constitutive" CSE expression may play a crucial role of for the maintenance of organ function, at least in the kidney. The data also indicate that increased CBS and CSE expression, in particular in the lung and the liver, represents an adaptive response to stress states.

Hydrogen sulfide: environmental factor or signalling molecule?

Hydrogen sulfide (H₂S) has traditionally been thought of as a phytotoxin, having deleterious effects on the plant growth and survival. It is now recognized that plants have enzymes which generate H₂S, cysteine desulfhydrase, and remove it, O-acetylserine lyase. Therefore, it has been suggested that H₂S is considered as a signalling molecule, alongside small reactive compounds such as hydrogen peroxide (H₂O₂) and nitric oxide (NO). Exposure of plants to low of H₂S, for example from H₂S donors, is revealing that many physiological effects are seen. H₂S seems to have effects on stomatal apertures. Intracellular effects include increases in glutathione levels, alterations of enzyme activities and influences on NO and H₂O₂ metabolism. Work in animals has shown that H₂S may have direct effects on thiol modifications of cysteine groups, work that will no doubt inform future studies in plants. It appears therefore, that instead of thinking of H₂S as a phytotoxin, it needs to be considered as a signalling molecule that interacts with reactive oxygen species and NO metabolism, as well as having direct effects on the activity of proteins. The future may see H₂S being used to modulate plant physiology in the field or to protect crops from postharvest spoilage.