Exploring Iodide and Hydrogen Sulfide as ROS Scavengers to Delay Acute Rejection in MHC-Defined Vascularized Composite Allografts.

Vascularized composite allografts (VCA) face ischemic challenges due to their limited availability. Reperfusion following ischemia triggers oxidative stress and immune reactions, and scavenger molecules could mitigate ischemia-reperfusion injuries and, therefore, immune rejection. We compared two scavengers in a myocutaneous flap VCA model. In total, 18 myocutaneous flap transplants were performed in Major histocompatibility complex (MHC)-defined miniature swine. In the MATCH group (n = 9), donors and recipients had minor antigen mismatch, while the animals were fully mismatched in the MISMATCH group (n = 9). Grafts were pretreated with saline, sodium iodide (NaI), or hydrogen sulfide (H2S), stored at 4 °C for 3 h, and then transplanted. Flaps were monitored until clinical rejection without immunosuppression. In the MATCH group, flap survival did not significantly differ between the saline and hydrogen sulfide treatments (p = 0.483) but was reduced with the sodium iodide treatment (p = 0.007). In the MISMATCH group, survival was similar between the saline and hydrogen sulfide treatments (p = 0.483) but decreased with the sodium iodide treatment (p = 0.007). Rhabdomyolysis markers showed lower but non-significant levels in the experimental subgroups for both the MATCH and MISMATCH animals. This study provides insightful data for the field of antioxidant-based approaches in VCA and transplantation.

Iodine Redistribution During Trauma, Sepsis, and Hibernation: An Evolutionarily Conserved Response to Severe Stress.

OBJECTIVE: We performed these studies to learn how iodine in the form of free iodide behaves during stress. DESIGN: Prospective observational trial using samples obtained from human trauma patients and retrospective observational study using remnant samples from human sepsis patients and arctic ground squirrels. Preclinical interventional study using hind-limb ischemia and reperfusion injury in mice. SETTING: Level I trauma center emergency room and ICU and animal research laboratories. SUBJECTS: Adult human sepsis and trauma patients, wild-caught adult arctic ground squirrels, and sexually mature laboratory mice. INTERVENTIONS: Ischemia and reperfusion injury was induced in mice by temporary application of tourniquet to one hind-limb. Iodide was administered IV just prior to reperfusion. MEASUREMENTS AND MAIN RESULTS: Free iodide was measured using ion chromatography. Relative to iodide in plasma from normal donors, iodide was increased 17-fold in plasma from trauma patients and 26-fold in plasma from sepsis patients. In arctic ground squirrels, iodide increases over three-fold during hibernation. And during ischemia/reperfusion injury in mice, iodide accumulates in ischemic tissue and reduces both local and systemic tissue damage. CONCLUSIONS: Iodide redistributes during stress and improves outcome after injury. Essential functions of iodide may have contributed to its evolutionary selection and be useful as a therapeutic intervention for human patients.

Iodide Improves Outcome After Acute Myocardial Infarction in Rats and Pigs.

OBJECTIVES: In this study, we tested whether iodide would reduce heart damage in rat and pig models of acute myocardial infarction as a risk analysis for a human trial. DESIGN: Prospective blinded and randomized laboratory animal investigation. SETTING: Animal research laboratories. SUBJECTS: Sexually mature rats and pigs. INTERVENTIONS: Acute myocardial infarction was induced by temporary ligation of the coronary artery followed by reperfusion. Iodide was administered orally in rats or IV in rats and pigs just prior to reperfusion. MEASUREMENTS AND MAIN RESULTS: Damage was assessed by blood cardiac troponin and infarct size; heart function was determined by echocardiography. Blood peroxide scavenging activity was measured enzymatically, and blood thyroid hormone was determined using radioimmune assay. Iodide administration preserved heart function and reduced blood cardiac troponin and infarct size by approximately 45% in pigs and approximately 60% in rats. Iodide administration also increased blood peroxide scavenging activity and maintained thyroid hormone levels. CONCLUSIONS: Iodide administration improved the structure and function of the heart after acute myocardial infarction in rats and pigs.

Impact of a novel phosphoinositol-3 kinase inhibitor in preventing mitochondrial DNA damage and damage-associated molecular pattern accumulation: Results from the Biochronicity Project.

BACKGROUND: Despite improvements in the management of severely injured patients, development of multiple organ dysfunction syndrome (MODS) remains a morbid complication of traumatic shock. One of the key attributes of MODS is a profound bioenergetics crisis, for which the mediators and mechanisms are poorly understood. We hypothesized that metabolic uncoupling using an experimental phosphoinositol-3 kinase (PI3-K) inhibitor, LY294002 (LY), may prevent mitochondrial abnormalities that lead to the generation of mitochondrial DNA (mtDNA) damage and the release of mtDNA damage-associated molecular patterns (DAMPs). METHODS: Sixteen swine were studied using LY, a nonselective PI3-K inhibitor. Animals were assigned to trauma only (TO, n = 3), LY drug only (LYO, n = 3), and experimental (n = 10), trauma + drug (LY + T) groups. Both trauma groups underwent laparotomy, 35% hemorrhage, severe ischemia-reperfusion injury, and protocolized resuscitation. A battery of hemodynamic, laboratory, histological, and bioenergetics parameters were monitored. Mitochondrial DNA damage was determined in lung, liver, and kidney using Southern blot analyses, whereas plasma mtDNA DAMP analysis used polymerase chain reaction amplification of a 200-bp sequence of the mtDNA D-loop region. RESULTS: Relative to control animals, H + I/R (hemorrhage and ischemia/reperfusion) produced severe, time-dependent decrements in hepatic, renal, cardiovascular, and pulmonary function accompanied by severe acidosis and lactate accumulation indicative of bioenergetics insufficiency. The H-I/R animals displayed prominent oxidative mtDNA damage in all organs studied, with the most prominent damage in the liver. Mitochondrial DNA damage was accompanied by accumulation of mtDNA DAMPs in plasma. Pretreatment of H + I/R animals with LY resulted in profound metabolic suppression, with approximately 50% decreases in O2 consumption and CO2 production. In addition, it prevented organ and bioenergetics dysfunction and was associated with a significant decrease in plasma mtDNA DAMPs to the levels of control animals. CONCLUSIONS: These findings show that H + I/R injury in anesthetized swine is accompanied by MODS and by significant mitochondrial bioenergetics dysfunction, including oxidative mtDNA damage and accumulation in plasma of mtDNA DAMPs. Suppression of these changes with the PI3-K inhibitor LY indicates that pharmacologically induced metabolic uncoupling may comprise a new pharmacologic strategy to prevent mtDNA damage and DAMP release and prevent or treat trauma-related MODS. LEVEL OF EVIDENCE: Therapeutic study, level III.

Inducing metabolic suppression in severe hemorrhagic shock: Pilot study results from the Biochronicity Project.

BACKGROUND: Suspended animation-like states have been achieved in small animal models, but not in larger species. Inducing metabolic suppression and temporary oxygen independence could enhance survivability of massive injury. Based on prior analyses of key pathways, we hypothesized that phosphoinositol-3-kinase inhibition would produce metabolic suppression without worsening organ injury or systemic physiology. METHODS: Twenty swine were studied using LY294002 (LY), a nonselective phosphoinositol-3-kinase inhibitor. Animals were assigned to trauma only (TO, n = 3); dimethyl sulfoxide only (DMSO, n = 4), LY drug only (LYO, n = 3), and drug + trauma (LY + T, n = 10) groups. Both trauma groups underwent laparotomy, 35% hemorrhage, severe ischemia/reperfusion injury, and protocolized resuscitation. Laboratory, physiologic, cytokine, and metabolic cart data were obtained. Histology of key end organs was also compared. RESULTS: Baseline values were similar among the groups. Compared with the TO group, the LYO group had reversible decreases in heart rate, mean arterial pressure, cardiac output, oxygen consumption, and carbon dioxide production. Compared with TO, LY + T showed sustained decreases in heart rate (113 vs. 76, p = 0.03), mean arterial pressure (40 vs. 31 mm Hg, p = 0.02), and cardiac output (3.8 vs. 1.9 L/min, p = 0.05) at 6 hours. Metabolic parameters showed profound suppression in the LY + T group. Oxygen consumption in LY + T was lower than both TO (119 vs. 229 mL/min, p = 0.012) and LYO (119 vs. 225 mL/min, p = 0.014) at 6 hours. Similarly, carbon dioxide production was decreased at 6 hours in LY + T when compared with TO (114 vs. 191 mL/min, p = 0.043) and LYO (114 vs. 195 mL/min, p = 0.034) groups. There was no worsening of acidosis (lactate 6.4 vs. 8.3 mmol/L, p = 0.4) or other endpoints. Interleukin 6 (IL-6) showed a significant increase in LY + T when compared with TO at 6 hours (60.5 vs. 2.47, p = 0.043). Tumor necrosis factor α and IL-1ß were decreased, and IL-10 increased in TO and LY + T at 6 hours. Markers of liver and kidney injury were no different between TO and LY + T groups at 6 hours. CONCLUSIONS: Phosphoinositol-3-kinase inhibition produced metabolic suppression in healthy and injured swine without increasing end-organ injury or systemic physiologic markers and demonstrated prolonged efficacy in injured animals. Further study may lead to targeted therapies to prolong tolerance to hemorrhage and extend the "golden hour" for injured patients.

Glycogen Fuels Survival During Hyposmotic-Anoxic Stress in Caenorhabditis elegans.

Oxygen is an absolute requirement for multicellular life. Animals that are deprived of oxygen for sufficient periods of time eventually become injured and die. This is largely due to the fact that, without oxygen, animals are unable to generate sufficient quantities of energy. In human diseases triggered by oxygen deprivation, such as heart attack and stroke, hyposmotic stress and cell swelling (edema) arise in affected tissues as a direct result of energetic failure. Edema independently enhances tissue injury in these diseases by incompletely understood mechanisms, resulting in poor clinical outcomes. Here, we present investigations into the effects of osmotic stress during complete oxygen deprivation (anoxia) in the genetically tractable nematode Caenorhabditis elegans. Our findings demonstrate that nematode survival of a hyposmotic environment during anoxia (hyposmotic anoxia) depends on the nematode's ability to engage in glycogen metabolism. We also present results of a genome-wide screen for genes affecting glycogen content and localization in the nematode, showing that nematode survival of hyposmotic anoxia depends on a large number of these genes. Finally, we show that an inability to engage in glycogen synthesis results in suppression of the enhanced survival phenotype observed in daf-2 insulin-like pathway mutants, suggesting that alterations in glycogen metabolism may serve as a basis for these mutants' resistance to hyposmotic anoxia.

Aquaporins-2 and -4 regulate glycogen metabolism and survival during hyposmotic-anoxic stress in Caenorhabditis elegans.

Periods of oxygen deprivation can lead to ion and water imbalances in affected tissues that manifest as swelling (edema). Although oxygen deprivation-induced edema is a major contributor to injury in clinical ischemic diseases such as heart attack and stroke, the pathophysiology of this process is incompletely understood. In the present study we investigate the impact of aquaporin-mediated water transport on survival in a Caenorhabditis elegans model of edema formation during complete oxygen deprivation (anoxia). We find that nematodes lacking aquaporin water channels in tissues that interface with the surrounding environment display decreased edema formation and improved survival rates in anoxia. We also find that these animals have significantly reduced demand for glycogen as an energetic substrate during anoxia. Together, our data suggest that reductions in membrane water permeability may be sufficient to induce a hypometabolic state during oxygen deprivation that reduces injury and extends survival limits.

Selenide Targets to Reperfusing Tissue and Protects It From Injury.

OBJECTIVES: Since blood selenium levels decrease after ischemia and reperfusion injury, and low blood selenium correlates with negative outcome, we designed and performed experiments to determine how selenium distribution is affected by ischemia reperfusion injury. Furthermore, we tested whether different chemical forms of selenium would affect outcome after ischemia and reperfusion injury. We also examined the metabolic effects of selenide administration. DESIGN: Laboratory investigation. SETTING: Animal research laboratory. SUBJECTS: Adult male C57BL/6 mice. INTERVENTIONS: To determine selenium localization, we administered tracer doses of radioactive selenium 75 in the form of selenite or selenide and measured blood and tissue selenium levels after ischemia and reperfusion injury. Anesthetized mice were subjected to myocardial ischemia reperfusion injury (coronary artery occlusion for 60 min followed by 5 min of reperfusion after occlusion was removed) or hindlimb ischemia reperfusion injury (left leg tourniquet for 90 min followed by 5 min reperfusion after tourniquet removal). To determine whether exogenous selenium administration could reduce ischemia reperfusion injury, we synthesized and administered sodium hydroselenide and sodium selenite solutions (0.05-2.4 mg/kg). Solutions were administered at the end of coronary artery occlusion but before reperfusion. In order to determine the metabolic effects of selenide administration, we exposed mice to hydrogen selenide gas (0-5 ppm) mixed into air (20.95% oxygen) for up to 3 hours. MEASUREMENTS AND MAIN RESULTS: In targeting assays, we measured blood and tissue selenium levels. We observed that blood selenium decreases after myocardial ischemia reperfusion and displays an inverse correlation with injury severity; selenium accumulation in heart correlates directly with injury severity. We also measured whether oxidized selenium, selenite, and reduced selenium, selenide, would target to injured heart tissue in myocardial ischemia reperfusion and injured leg muscle in a hindlimb model of ischemia reperfusion. Only selenide targets to injured tissue. We also measured damage after myocardial ischemia reperfusion injury using morphometry, neutrophil accumulation, blood cardiac troponin levels, and echocardiography and observed in all assays that selenide reduced damage to the heart; selenite was not effective. And finally, to assay metabolism, we measured oxygen consumption, carbon dioxide production, and body core temperature before, during, and after hydrogen selenide administration. All measurements indicate that selenide decreases metabolism. CONCLUSIONS: Selenide targets to reperfusing tissue and reduces reperfusion injury perhaps by affecting oxygen metabolism.

Iodide protects heart tissue from reperfusion injury.

Iodine is an elemental nutrient that is essential for mammals. Here we provide evidence for an acute therapeutic role for iodine in ischemia reperfusion injury. Infusion of the reduced form, iodide, but not the oxidized form iodate, reduces heart damage by as much as 75% when delivered intravenously following temporary loss of blood flow but prior to reperfusion of the heart in a mouse model of acute myocardial infarction. Normal thyroid function may be required because loss of thyroid activity abrogates the iodide benefit. Given the high degree of protection and the high degree of safety, iodide should be explored further as a therapy for reperfusion injury.

HIF-1 and SKN-1 coordinate the transcriptional response to hydrogen sulfide in Caenorhabditis elegans.

Hydrogen sulfide (H2S) has dramatic physiological effects on animals that are associated with improved survival. C. elegans grown in H2S are long-lived and thermotolerant. To identify mechanisms by which adaptation to H2S effects physiological functions, we have measured transcriptional responses to H2S exposure. Using microarray analysis we observe rapid changes in the abundance of specific mRNAs. The number and magnitude of transcriptional changes increased with the duration of H2S exposure. Functional annotation suggests that genes associated with protein homeostasis are upregulated upon prolonged exposure to H2S. Previous work has shown that the hypoxia-inducible transcription factor, HIF-1, is required for survival in H2S. In fact, we show that hif-1 is required for most, if not all, early transcriptional changes in H2S. Moreover, our data demonstrate that SKN-1, the C. elegans homologue of NRF2, also contributes to H2S-dependent changes in transcription. We show that these results are functionally important, as skn-1 is essential to survive exposure to H2S. Our results suggest a model in which HIF-1 and SKN-1 coordinate a broad transcriptional response to H2S that culminates in a global reorganization of protein homeostasis networks.

The response of Caenorhabditis elegans to hydrogen sulfide and hydrogen cyanide.

Hydrogen sulfide (H2S), an endogenously produced small molecule, protects animals from various stresses. Recent studies demonstrate that animals exposed to H2S are long lived, resistant to hypoxia, and resistant to ischemia-reperfusion injury. We performed a forward genetic screen to gain insights into the molecular mechanisms Caenorhabditis elegans uses to appropriately respond to H2S. At least two distinct pathways appear to be important for this response, including the H2S-oxidation pathway and the hydrogen cyanide (HCN)-assimilation pathway. The H2S-oxidation pathway requires two distinct enzymes important for the oxidation of H2S: the sulfide:quinone reductase sqrd-1 and the dioxygenase ethe-1. The HCN-assimilation pathway requires the cysteine synthase homologs cysl-1 and cysl-2. A low dose of either H2S or HCN can activate hypoxia-inducible factor 1 (HIF-1), which is required for C. elegans to respond to either gas. sqrd-1 and cysl-2 represent the entry points in the H2S-oxidation and HCN-assimilation pathways, respectively, and expression of both of these enzymes is highly induced by HIF-1 in response to both H2S and HCN. In addition to their role in appropriately responding to H2S and HCN, we found that cysl-1 and cysl-2 are both essential mediators of innate immunity against fast paralytic killing by Pseudomonas. Furthermore, in agreement with these data, we showed that growing worms in the presence of H2S is sufficient to confer resistance to Pseudomonas fast paralytic killing. Our results suggest the hypoxia-independent hif-1 response in C. elegans evolved to respond to the naturally occurring small molecules H2S and HCN.

Suspended animation extends survival limits of Caenorhabditis elegans and Saccharomyces cerevisiae at low temperature.

The orderly progression through the cell division cycle is of paramount importance to all organisms, as improper progression through the cycle could result in defects with grave consequences. Previously, our lab has shown that model eukaryotes such as Saccharomyces cerevisiae, Caenorhabditis elegans, and Danio rerio all retain high viability after prolonged arrest in a state of anoxia-induced suspended animation, implying that in such a state, progression through the cell division cycle is reversibly arrested in an orderly manner. Here, we show that S. cerevisiae (both wild-type and several cold-sensitive strains) and C. elegans embryos exhibit a dramatic decrease in viability that is associated with dysregulation of the cell cycle when exposed to low temperatures. Further, we find that when the yeast or worms are first transitioned into a state of anoxia-induced suspended animation before cold exposure, the associated cold-induced viability defects are largely abrogated. We present evidence that by imposing an anoxia-induced reversible arrest of the cell cycle, the cells are prevented from engaging in aberrant cell cycle events in the cold, thus allowing the organisms to avoid the lethality that would have occurred in a cold, oxygenated environment.

Hydrogen sulfide increases hypoxia-inducible factor-1 activity independently of von Hippel-Lindau tumor suppressor-1 in C. elegans.

Rapid alteration of gene expression in response to environmental changes is essential for normal development and behavior. The transcription factor hypoxia-inducible factor (HIF)-1 is well known to respond to alterations in oxygen availability. In nature, low oxygen environments are often found to contain high levels of hydrogen sulfide (H(2)S). Here, we show that Caenorhabditis elegans can have mutually exclusive responses to H(2)S and hypoxia, both involving HIF-1. Specifically, H(2)S results in HIF-1 activity throughout the hypodermis, whereas hypoxia causes HIF-1 activity in the gut as judged by a reporter for HIF-1 activity. C. elegans require hif-1 to survive in room air containing trace amounts of H(2)S. Exposure to H(2)S results in HIF-1 nuclear localization and transcription of HIF-1 targets. The effects of H(2)S on HIF-1 reporter activity are independent of von Hippel-Lindau tumor suppressor (VHL)-1, whereas VHL-1 is required for hypoxic regulation of HIF-1 reporter activity. Because H(2)S is naturally produced by animal cells, our results suggest that endogenous H(2)S may influence HIF-1 activity.

C. elegans are protected from lethal hypoxia by an embryonic diapause.

At least 100 mammalian species exhibit embryonic diapause, where fertilized embryos arrest development in utero until suitable seasonal or nutritional environments are encountered. Delaying maternal investments in producing offspring allows these animals to utilize limited resources to survive while searching for better conditions and ensures that progeny are not produced when they are unlikely to survive. In addition, embryos may be protected from external environmental vicissitudes while in utero. Here we demonstrate embryonic diapause in C. elegans, and show that this diapause protects embryos from otherwise lethal hypoxia. Diapausing embryos in utero require san-1 to survive, indicating that hypoxia-induced embryonic diapause may be mechanistically related to suspended animation. Furthermore, we show that neuronal HIF-1 activity in the adult dictates the O(2) tension at which embryonic diapause is engaged. We suggest that the maternal perception of hypoxia stimulates a response to protect embryos in utero by inducing diapause, a natural form of suspended animation. This response is likely to be an important strategy to improve offspring survival in harsh conditions and allow adults to find environments more suitable for reproductive success.

Adaptive sugar provisioning controls survival of C. elegans embryos in adverse environments.

The ability to adapt to changing environmental conditions is essential to the fitness of organisms. In some cases, adaptation of the parent alters the offspring's phenotype [1-10]. Such parental effects are adaptive for the offspring if the future environment is similar to the current one but can be maladaptive otherwise [11]. One mechanism by which adaptation occurs is altered provisioning of embryos by the parent [12-16]. Here we show that exposing adult Caenorhabditis elegans to hyperosmotic conditions protects their offspring from these conditions but causes sensitivity to anoxia exposure. We show that this alteration of survival is correlated with changes in the sugar content of adults and embryos. In addition, mutations in gene products that alter sugar homeostasis also alter the ability of embryos to survive in hyperosmotic and anoxic conditions and engage in the adaptive parental effect. Our results indicate that there is a physiological trade-off between the presence of glycerol, which protects animals from hyperosmotic conditions, and glycogen, which is consumed during anoxia. These two metabolites play an essential role in the survival of worms in these adverse environments, and the adaptive parental effect we describe is mediated by the provisioning of these metabolites to the embryo.

Anoxia-induced suspended animation in budding yeast as an experimental paradigm for studying oxygen-regulated gene expression.

A lack of oxygen can force many organisms to enter into recoverable hypometabolic states. To better understand how organisms cope with oxygen deprivation, our laboratory previously had shown that when challenged with anoxia, both the nematode Caenorhabditis elegans and embryos of the zebrafish Danio rerio enter into suspended animation, in which all life processes that can be observed by light microscopy reversibly halt pending the restoration of oxygen (P. A. Padilla and M. B. Roth, Proc. Natl. Acad. Sci. USA 98:7331-7335, 2001, and P. A. Padilla, T. G. Nystul, R. A. Zager, A. C. Johnson, and M. B. Roth, Mol. Biol. Cell 13:1473-1483, 2002). Here, we show that both sporulating and vegetative cells of the budding yeast Saccharomyces cerevisiae also enter into a similar state of suspended animation when made anoxic on a nonfermentable carbon source. Transcriptional profiling using cDNA microarrays and follow-on quantitative real-time PCR analysis revealed a relative derepression of aerobic metabolism genes in carbon monoxide (CO)-induced anoxia when compared to nitrogen (N(2)) gas-induced anoxia, which is consistent with the known oxygen-mimetic effects of CO. We also found that mutants deleted for components of the mitochondrial retrograde signaling pathway can tolerate prolonged exposure to CO but not to N(2). We conclude that the cellular response to anoxia is dependent on whether the anoxic gas is an oxygen mimetic and that the mitochondrial retrograde signaling pathway is functionally important for mediating this response.

Surviving blood loss using hydrogen sulfide.

BACKGROUND: Reduced metabolic activity improves outcome in many clinical and experimental models of injury and diseases that result in insufficient blood supply. Recently, we demonstrated that inhaled hydrogen sulfide gas can be used to reversibly reduce metabolic activity in mice. We hypothesize that hydrogen sulfide will confer benefit in injuries and diseases related to insufficient blood supply. METHODS: Sprague-Dawley rats were subjected to controlled hemorrhage to remove 60% of total blood. Hydrogen sulfide was administered to rats either via airway as gas, or intravenous infusion as liquid. Outcome was assayed by survival. RESULTS: Using inhaled hydrogen sulfide gas, 75% of treated and 23% of untreated rats survived longer than 24 hours. Using intravenous administered sulfide, 67% of treated and 14% of untreated rats survived longer than 24 hours. Using log-rank analysis, p < 0.001. Surviving rats showed no functional or behavioral abnormalities. Blood chemistry analysis at the end of hemorrhage showed minor but significant differences between treated and control animals. Respirometry results show that hydrogen sulfide stabilized metabolic output during and after hemorrhage. CONCLUSION: These data indicate that sulfide can protect rats from lethal hemorrhage. Future studies are needed to analyze the mechanism of benefit as well as whether sulfide is beneficial in other models of human injury and disease.

Hydrogen sulfide increases thermotolerance and lifespan in Caenorhabditis elegans.

Hydrogen sulfide (H(2)S) is naturally produced in animal cells. Exogenous H(2)S has been shown to effect physiological changes that improve the capacity of mammals to survive in otherwise lethal conditions. However, the mechanisms required for such alterations are unknown. We investigated the physiological response of Caenorhabditis elegans to H(2)S to elucidate the molecular mechanisms of H(2)S action. Here we show that nematodes exposed to H(2)S are apparently healthy and do not exhibit phenotypes consistent with metabolic inhibition. Instead, animals exposed to H(2)S are thermotolerant and long-lived. These phenotypes require SIR-2.1 activity but are genetically independent of the insulin signaling pathway, mitochondrial dysfunction, and caloric restriction. These studies suggest that SIR-2.1 activity may translate environmental change into physiological alterations that improve survival. It is interesting to consider the possibility that the mechanisms by which H(2)S increases thermotolerance and lifespan in nematodes are conserved and that studies using C. elegans may help explain the beneficial effects observed in mammals exposed to H(2)S.

Suspended animation-like state protects mice from lethal hypoxia.

Joseph Priestley observed the high burn rate of candles in pure oxygen and wondered if people would "live out too fast" if we were in the same environment. We hypothesize that sulfide, a natural reducer of oxygen that is made in many cell types, acts as a buffer to prevent unrestricted oxygen consumption. To test this, we administered sulfide in the form of hydrogen sulfide (H2S) to mice (Mus musculus). As we have previously shown, H2S decreases the metabolic rate of mice by approximately 90% and induces a suspended animation-like state. Mice cannot survive for longer than 20 min when exposed to 5% oxygen. However, if mice are first put into a suspended animation-like state by a 20-min pretreatment with H2S and then are exposed to low oxygen, they can survive for more than 6.5 h in 5% oxygen with no apparent detrimental effects. In addition, if mice are exposed to a 20-min pretreatment with H2S followed by 1 h at 5% oxygen, they can then survive for several hours at oxygen tensions as low as 3%. We hypothesize that prior exposure to H2S reduces oxygen demand, therefore making it possible for the mice to survive with low oxygen supply. These results suggest that H2S may be useful to prevent damage associated with hypoxia.