H2S concentrations in the arterial blood during H2S administration in relation to its toxicity and effects on breathing

Role of cystathionine-β-synthase and hydrogen sulfide in down syndrome

Down syndrome (DS) is a genetic condition where the person affected by it is born with an additional - full or partial - copy of chromosome 21. DS presents with characteristic morphological features and is associated with a wide range of biochemical alterations and maladaptations. Cystathionine-β-synthase (CBS) - one of the key mammalian enzymes responsible for the biogenesis of the gaseous transmitter hydrogen sulfide (H2S) - is located on chromosome 21, and people with DS exhibit a significant upregulation of this enzyme in their brain and other organs. Even though 3-mercaptopyruvate sulfurtransferase - another key mammalian enzyme responsible for the biogenesis of H2S and of reactive polysulfides - is not located on chromosome 21, there is also evidence for the upregulation of this enzyme in DS cells. The hypothesis that excess H2S in DS impairs mitochondrial function and cellular bioenergetics was first proposed in the 1990s and has been substantiated and expanded upon over the past 25 years. DS cells are in a state of metabolic suppression due to H2S-induced, reversible inhibition of mitochondrial Complex IV activity. The impairment of aerobic ATP generation in DS cells is partially compensated by an upregulation of glycolysis. The DS-associated metabolic impairment can be reversed by pharmacological CBS inhibition or CBS silencing. In rodent models of DS, CBS upregulation and H2S overproduction contribute to the development of cognitive dysfunction, alter brain electrical activity, and promote reactive gliosis: pharmacological inhibition or genetic correction of CBS overactivation reverses these alterations. CBS can be considered a preclinically validated drug target for the experimental therapy of DS.

Codelivery of Dual Gases with Metal-Organic Supramolecular Cage-Based Microenvironment-Responsive Nanomedicine for Atherosclerosis Therapy

Atherosclerosis (AS) is a common cause of coronary heart disease and stroke. The delivery of exogenous H2S and in situ production of O2 within atherosclerotic plaques can help suppress inflammatory cell infiltration and alleviate disease progression. However, the uncontrolled release of gas donors hinders achieving effective drug concentrations and causes toxic effects. Herein, diallyl trisulfide (DATS)-loaded metal-organic cage (MOC)-68-doped MnO2 nanoparticles are developed as a microenvironment-responsive nanodrug with the capacity for the in situ co-delivery of H2S and O2 to inflammatory cells within plaques. This nanomedicine exhibited excellent monodispersity and stability and protected DATS from degradation in the circulation. In vitro studies showed that the nanomedicine reduced macrophage polarization toward an inflammatory phenotype and inhibited the formation of foam cells, while suppressing the expression of NOD-like receptor thermal protein domain associated protein 3 (NLRP3) and interleukin-1β. In a mouse model of ApoE-/- genotype, the nanomedicine reduces the plaque burden, inflammatory infiltration, and hypoxic conditions within the plaques. Furthermore, the treatment process and therapeutic effects can be monitored by magnetic resonance image (MRI), in real time upon Mn2+ release from the acidic- and H2O2- microenvironment-responsive MnO2 nanoparticles. The DATS-loaded MOC-68-doped MnO2-based nanodrug holds great promise as a novel theranostic platform for AS.

Nascent transcription reveals regulatory changes in extremophile fishes inhabiting hydrogen sulfide-rich environments

Regulating transcription allows organisms to respond to their environment, both within a single generation (plasticity) and across generations (adaptation). We examined transcriptional differences in gill tissues of fishes in the Poecilia mexicana species complex (family Poeciliidae), which have colonized toxic springs rich in hydrogen sulfide (H2S) in southern Mexico. There are gene expression differences between sulfidic and non-sulfidic populations, yet regulatory mechanisms mediating this gene expression variation remain poorly studied. We combined capped-small RNA sequencing (csRNA-seq), which captures actively transcribed (i.e. nascent) transcripts, and messenger RNA sequencing (mRNA-seq) to examine how variation in transcription, enhancer activity, and associated transcription factor binding sites may facilitate adaptation to extreme environments. csRNA-seq revealed thousands of differentially initiated transcripts between sulfidic and non-sulfidic populations, many of which are involved in H2S detoxification and response. Analyses of transcription factor binding sites in promoter and putative enhancer csRNA-seq peaks identified a suite of transcription factors likely involved in regulating H2S-specific shifts in gene expression, including several key transcription factors known to respond to hypoxia. Our findings uncover a complex interplay of regulatory processes that reflect the divergence of extremophile populations of P. mexicana from their non-sulfidic ancestors and suggest shared responses among evolutionarily independent lineages.

Sulfur-Element containing metabolic pathways in human health and crosstalk with the microbiome

In humans, methionine derived from dietary proteins is necessary for cellular homeostasis and regeneration of sulfur containing pathways, which produce inorganic sulfur species (ISS) along with essential organic sulfur compounds (OSC). In recent years, inorganic sulfur species have gained attention as key players in the crosstalk of human health and the gut microbiome. Endogenously, ISS includes hydrogen sulfide (H2S), sulfite (SO32-), thiosulfate (S2O32-), and sulfate (SO42-), which are produced by enzymes in the transsulfuration and sulfur oxidation pathways. Additionally, sulfate-reducing bacteria (SRB) in the gut lumen are notable H2S producers which can contribute to the ISS pools of the human host. In this review, we will focus on the systemic effects of sulfur in biological pathways, describe the contrasting mechanisms of sulfurylation versus phosphorylation on the hydroxyl of serine/threonine and tyrosine residues of proteins in post-translational modifications, and the role of the gut microbiome in human sulfur metabolism.

Treatment of life-threatening H2S intoxication: Lessons from the trapping agent tetranitrocobinamide

We sought to evaluate the efficacy of trapping free hydrogen sulfide (H2S) following severe H2S intoxication. Sodium hydrosulfide solution (NaHS, 20 mg/kg) was administered intraperitoneally in 69 freely moving rats. In a first group (protocol 1), 40 rats were randomly assigned to receive saline (n = 20) or the cobalt compound tetranitrocobinamide (TNCbi) (n = 20, 75 mg/kg iv), one minute into coma, when free H2S was still present in the blood. A second group of 27 rats received TNCbi or saline, following epinephrine, 5 min into coma, when the concentration of free H2S has drastically decreased in the blood. In protocol 1, TNCbi significantly increased immediate survival (65 vs 20 %, p < 0.01) while in protocol 2, administration of TNCbi led to the same outcome as untreated animals. We hypothesize that the decreased efficacy of TNCbi with time likely reflects the rapid spontaneous disappearance of the pool of free H2S in the blood following H2S exposure.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Effects of fentanyl overdose-induced muscle rigidity and dexmedetomidine on respiratory mechanics and pulmonary gas exchange in sedated rats

The objective of our study was to establish in sedated rats the consequences of high-dose fentanyl-induced acute muscle rigidity on the mechanical properties of the respiratory system and on the metabolic rate. Doses of fentanyl that we have previously shown to produce persistent rigidity of the muscles of the limbs and trunk in the rat (150 -300 microg/kg iv), were administered in 23 volume-controlled mechanically ventilated and sedated rats. The effects of a low dose of the FDA approved central alpha-2 agonist, dexmedetomidine (3 microg/kg iv), which has been suggested to oppose fentanyl-induced muscle rigidity, were determined after fentanyl administration. Fentanyl produced a significant decrease in Crs in the 23 rats that were studied. In 13 rats, an abrupt response occurred within 90 seconds, consisting in rapid rhythmic contractions of most skeletal muscles, that were replaced by persistent tonic/tetanic contractions leading a significant decrease of Crs (from 0.51 ± 0.11 ml/cmH₂O to 0.36 ± 0.08 ml/cmH₂O, 3 minutes after fentanyl injection). In the other 10 animals, a Crs progressively decreased to 0.26 ± 0.06 ml/cmH₂O at 30 minutes. There was a significant rise in V̇O₂ during muscle tonic contractions (from 8.48 ± 4.31 to 11.29 ± 2.57 ml/min), which contributed to a significant hypoxemia, despite ventilation being held constant. Dexmedetomidine provoked a significant and rapid increase in Crs towards baseline levels, while decreasing the metabolic rate and restoring normoxemia. We propose that the changes in respiratory mechanics and metabolism produced by opioid-induced muscle rigidity contribute to fentanyl lethality.

Spectroscopic study of cyclen-based 19F NMR probe for detection of hydrogen sulfide

A cyclen-based ¹⁹F NMR probe (F-cyclen) for hydrogen sulfide (H₂S) has been prepared and evaluated for its complex formation ability with Cu²⁺ ions and responsivity to H₂S. F-Cyclen was readily synthesized by reacting cyclen with 4-(trifluoromethyl)benzyl bromide. Visible absorption spectrophotometry showed that, same as the original cyclen, F-cyclen formed a 1:1 complex with Cu²⁺ ions. The ¹⁹F NMR signal of F-cyclen at 16.5 ppm gradually decreased in intensity with increasing CuCl₂ concentration, with trifluoromethane sulfonic acid sodium salt (TFMSNa) used as an internal standard (0 ppm). When the Cu²⁺-F-cyclen complex was subjected to an increasing concentration of Na₂S (as H₂S donor), its corresponding ¹⁹F NMR signal of F-cyclen at 16.5 ppm gradually increased in intensity. The regression curve between the ¹⁹F NMR signal intensity ratio of F-cyclen to TFMSNa and Na₂S concentration showed good linearity (r = 0.986) over the Na₂S concentration range of 25-150 μM.

Hydrogen Sulfide Produced by Gut Bacteria May Induce Parkinson's Disease

Several bacterial species can generate hydrogen sulfide (H2S). Study evidence favors the view that the microbiome of the colon harbors increased amounts of H2S producing bacteria in Parkinson's disease. Additionally, H2S can easily penetrate cell membranes and enter the cell interior. In the cells, excessive amounts of H2S can potentially release cytochrome c protein from the mitochondria, increase the iron content of the cytosolic iron pool, and increase the amount of reactive oxygen species. These events can lead to the formation of alpha-synuclein oligomers and fibrils in cells containing the alpha-synuclein protein. In addition, bacterially produced H2S can interfere with the body urate metabolism and affect the blood erythrocytes and lymphocytes. Gut bacteria responsible for increased H2S production, especially the mucus-associated species of the bacterial genera belonging to the Desulfovibrionaceae and Enterobacteriaceae families, are likely play a role in the pathogenesis of Parkinson's disease. Special attention should be devoted to changes not only in the colonic but also in the duodenal microbiome composition with regard to the pathogenesis of Parkinson's disease. Influenza infections may increase the risk of Parkinson's disease by causing the overgrowth of H2S-producing bacteria both in the colon and duodenum.

Uremic Toxin Indoxyl Sulfate Impairs Hydrogen Sulfide Formation in Renal Tubular Cells

Hydrogen sulfide (H₂S) was the third gasotransmitter to be recognized as a cytoprotectant. A recent study demonstrated that exogenous supplementation of H₂S ameliorates functional insufficiency in chronic kidney disease (CKD). However, how the H₂S system is impaired by CKD has not been elucidated. The uremic toxin indoxyl sulfate (IS) is known to accumulate in CKD patients and harm the renal tubular cells. This study therefore treated the proximal tubular cells, LLC-PK₁, with IS to see how IS affects H₂S formation. Our results showed that H₂S release from LLC-PK₁ cells was markedly attenuated by IS when compared with control cells. The H₂S donors NaHS and GYY-4137 significantly attenuated IS-induced tubular damage, indicating that IS impairs H₂S formation. Interestingly, IS downregulated the H₂S-producing enzymes cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), and 3-mercaptopyruvate sulfurtransferase (3-MST), and these effects could be reversed by inhibition of the IS receptor, aryl hydrocarbon receptor (AhR). As transcription factor specificity protein 1 (Sp1) regulates the gene expression of H₂S-producing enzymes, we further showed that IS significantly decreased the DNA binding activity of Sp1 but not its protein expression. Blockade of AhR reversed low Sp1 activity caused by IS. Moreover, exogenous H₂S supplementation attenuated IS-mediated superoxide formation and depletion of the cellular glutathione content. These results clearly indicate that IS activates AhR, which then attenuates Sp1 function through the regulation of H₂S-producing enzyme expression. The attenuation of H₂S formation contributes to the low antioxidant defense of glutathione in uremic toxin-mediated oxidative stress, causing tubular cell damage.

Buffering Adaptive Immunity by Hydrogen Sulfide

T cell-mediated adaptive immunity is designed to respond to non-self antigens and pathogens through the activation and proliferation of various T cell populations. T helper 1 (Th1), Th2, Th17 and Treg cells finely orchestrate cellular responses through a plethora of paracrine and autocrine stimuli that include cytokines, autacoids, and hormones. Hydrogen sulfide (H₂S) is one of these mediators able to induce/inhibit immunological responses, playing a role in inflammatory and autoimmune diseases, neurological disorders, asthma, acute pancreatitis, and sepsis. Both endogenous and exogenous H₂S modulate numerous important cell signaling pathways. In monocytes, polymorphonuclear, and T cells H₂S impacts on activation, survival, proliferation, polarization, adhesion pathways, and modulates cytokine production and sensitivity to chemokines. Here, we offer a comprehensive review on the role of H₂S as a natural buffer able to maintain over time a functional balance between Th1, Th2, Th17 and Treg immunological responses.

An Updated Insight Into Molecular Mechanism of Hydrogen Sulfide in Cardiomyopathy and Myocardial Ischemia/Reperfusion Injury Under Diabetes

Cardiovascular diseases are the most common complications of diabetes, and diabetic cardiomyopathy is a major cause of people death in diabetes. Molecular, transcriptional, animal, and clinical studies have discovered numerous therapeutic targets or drugs for diabetic cardiomyopathy. Within this, hydrogen sulfide (H₂S), an endogenous gasotransmitter alongside with nitric oxide (NO) and carbon monoxide (CO), is found to play a critical role in diabetic cardiomyopathy. Recently, the protective roles of H₂S in diabetic cardiomyopathy have attracted enormous attention. In addition, H₂S donors confer favorable effects in myocardial infarction, ischaemia-reperfusion injury, and heart failure under diabetic conditions. Further studies have disclosed that multiplex molecular mechanisms are responsible for the protective effects of H₂S against diabetes-elicited cardiac injury, such as anti-oxidative, anti-apoptotic, anti-inflammatory, and anti-necrotic properties. In this review, we will summarize the current findings on H₂S biology and pharmacology, especially focusing on the novel mechanisms of H₂S-based protection against diabetic cardiomyopathy. Also, the potential roles of H₂S in diabetes-aggravated ischaemia-reperfusion injury are discussed.

Spatiotemporal regulation of hydrogen sulfide signaling in the kidney

Hydrogen sulfide (H2S) has long been recognized as a putrid, toxic gas. However, as a result of intensive biochemical research in the past two decades, H2S is now considered to be the third gasotransmitter alongside nitric oxide (NO) and carbon monoxide (CO) in mammalian systems. H2S-producing enzymes are expressed in all organs, playing an important role in their physiology. In the kidney, H2S is a critical regulator of vascular and cellular function, although the mechanisms that affect (sub)cellular levels of H2S are not precisely understood. H2S modulates systemic and renal blood flow, glomerular filtration rate and the renin-angiotensin axis through direct inhibition of nitric oxide synthesis. Further, H2S affects cellular function by modulating protein activity via post-translational protein modification: a process termed persulfidation. Persulfidation modulates protein activity, protein localization and protein-protein interactions. Additionally, acute kidney injury (AKI) due to mitochondrial dysfunction, which occurs during hypoxia or ischemia-reperfusion (IR), is attenuated by H2S. H2S enhances ATP production, prevents damage due to free radicals and regulates endoplasmic reticulum stress during IR. In this review, we discuss current insights in the (sub)cellular regulation of H2S anabolism, retention and catabolism, with relevance to spatiotemporal regulation of renal H2S levels. Together, H2S is a versatile gasotransmitter with pleiotropic effects on renal function and offers protection against AKI. Unraveling the mechanisms that modulate (sub)cellular signaling of H2S not only expands fundamental insight in the regulation of functional effects mediated by H2S, but can also provide novel therapeutic targets to prevent kidney injury due to hypoxic or ischemic injury.

Hydrogen sulfide intoxication induced brain injury and methylene blue

Hydrogen sulfide (H₂S) remains a chemical hazard in the gas and farming industry. It is easy to manufacture from common chemicals and thus represents a potential threat for the civilian population. It is also employed as a method of suicide, for which incidence has recently increased in the US. H₂S is a mitochondrial poison and exerts its toxicity through mechanisms that are thought to result from its high affinity to various metallo-proteins (such as - but not exclusively- the mitochondrial cytochrome c oxidase) and interactions with cysteine residues of proteins. Ion channels with critical implications for the cardiac and the brain functions appear to be affected very early during and following H₂S exposure, an effect which is rapidly reversible during a light intoxication. However, during severe H₂S intoxication, a coma, associated with a reduction in cardiac contractility, develops within minutes or even seconds leading to death by complete electro-mechanical dissociation of the heart. If the level of intoxication is milder, a rapid and spontaneous recovery of the coma occurs as soon as the exposure stops. The risk, although probably very small, of developing long-term debilitating motor or cognitive deficits is present. One of the major challenges impeding our effort to offer an effective treatment against H₂S intoxication after exposure is that the pool of free/soluble H₂S almost immediately disappears from the body preventing agents trapping free H₂S (cobalt or ferric compounds) to play their protective role. This paper (1) presents and discusses the neurological symptoms and lesions observed in various animals models and in humans following an acute exposure to sub-lethal or lethal levels of H₂S, (2) reviews the potential interest of methylene blue (MB), a potent cyclic redox dye - currently used for the treatment of methemoglobinemia - which has potential rescuing effects on the mitochondrial activity, as an antidote against sulfide intoxication.

Hydrogen Sulfide Toxicity: Mechanism of Action, Clinical Presentation, and Countermeasure Development

INTRODUCTION

Hydrogen sulfide (H₂S) is found in various settings. Reports of chemical suicide, where individuals have combined readily available household chemicals to produce lethal concentrations of H₂S, have demonstrated that H₂S is easily produced. Governmental agencies have warned of potential threats of use of H₂S for a chemical attack, but currently there are no FDA-approved antidotes for H₂S. An ideal antidote would be one that is effective in small volume, readily available, safe, and chemically stable. In this paper we performed a review of the available literature on the mechanism of toxicity, clinical presentation, and development of countermeasures for H₂S toxicity.

DISCUSSION

In vivo, H₂S undergoes an incomplete oxidation after an exposure. The remaining non-oxidized H₂S is found in dissolved and combined forms. Dissolved forms such as H₂S gas and sulfhydryl anion can diffuse between blood and tissue. The combined non-soluble forms are found as acid-labile sulfides and sulfhydrated proteins, which play a role in toxicity. Recent countermeasure development takes into account the toxicokinetics of H₂S. Some countermeasures focus on binding free hydrogen sulfide (hydroxocobalamin, cobinamide); some have direct effects on the mitochondria (methylene blue), while others work by mitigating end organ damage by generating other substances such as nitric oxide (NaNO2).

CONCLUSION

H₂S exists in two main pools in vivo after exposure. While several countermeasures are being studied for H₂S intoxication, a need exists for a small-volume, safe, highly effective antidote with a long shelf life to treat acute toxicity as well as prevent long-term effects of exposure.

Methylene Blue Administration During and After Life-Threatening Intoxication by Hydrogen Sulfide: Efficacy Studies in Adult Sheep and Mechanisms of Action

Exposure to toxic levels of hydrogen sulfide (H2S) produces an acute cardiac depression that can be rapidly fatal. We sought to characterize the time course of the cardiac effects produced by the toxicity of H2S in sheep, a human sized mammal, and to describe the in vivo and in vitro antidotal properties of methylene blue (MB), which has shown efficacy in sulfide intoxicated rats. Infusing NaHS (720 mg) in anesthetized adult sheep produced a rapid dilation of the left ventricular with a decrease in contractility, which was lethal within about 10 min by pulseless electrical activity. MB (7 mg/kg), administered during sulfide exposure, maintained cardiac contractility and allowed all of the treated animals to recover. At a dose of 350 mg NaHS, we were able to produce an intoxication, which led to a persistent decrease in ventricular function for at least 1 h in nontreated animals. Administration of MB, 3 or 30 min after the end of exposure, whereas all free H2S had already vanished, restored cardiac contractility and the pyruvate/lactate (P/L) ratio. We found that MB exerts its antidotal effects through at least 4 different mechanisms: (1) a direct oxidation of free sulfide; (2) an increase in the pool of "trapped" H2S in red cells; (3) a restoration of the mitochondrial substrate-level phosphorylation; and (4) a rescue of the mitochondrial electron chain. In conclusion, H2S intoxication produces acute and long persisting alteration in cardiac function in large mammals even after all free H2S has vanished. MB exerts its antidotal effects against life-threatening sulfide intoxication via multifarious properties, some of them unrelated to any direct interaction with free H2S.

Methylene Blue Counteracts H2S-Induced Cardiac Ion Channel Dysfunction and ATP Reduction

We have previously demonstrated that methylene blue (MB) counteracts the effects of hydrogen sulfide (H₂S) cardiotoxicity by improving cardiomyocyte contractility and intracellular Ca²⁺ homeostasis disrupted by H₂S poisoning. In vivo, MB restores cardiac contractility severely depressed by sulfide and protects against arrhythmias, ranging from bundle branch block to ventricular tachycardia or fibrillation. To dissect the cellular mechanisms by which MB reduces arrhythmogenesis and improves bioenergetics in myocytes intoxicated with H₂S, we evaluated the effects of H₂S on resting membrane potential (E_m), action potential (AP), Na⁺/Ca²⁺ exchange current (I_NaCa), depolarization-activated K⁺ currents and ATP levels in adult mouse cardiac myocytes and determined whether MB could counteract the toxic effects of H₂S on myocyte electrophysiology and ATP. Exposure to toxic concentrations of H₂S (100 µM) significantly depolarized E_m, reduced AP amplitude, prolonged AP duration at 90% repolarization (APD₉₀), suppressed I_NaCa and depolarization-activated K⁺ currents, and reduced ATP levels in adult mouse cardiac myocytes. Treating cardiomyocytes with MB (20 µg/ml) 3 min after H₂S exposure restored E_m, APD₉₀, I_NaCa, depolarization-activated K⁺ currents, and ATP levels toward normal. MB improved mitochondrial membrane potential (∆ψ_m) and oxygen consumption rate in myocytes in which Complex I was blocked by rotenone. We conclude that MB ameliorated H₂S-induced cardiomyocyte toxicity at multiple levels: (1) reversing excitation-contraction coupling defects (Ca²⁺ homeostasis and L-type Ca²⁺ channels); (2) reducing risks of arrhythmias (E_m, APD, I_NaCa and depolarization-activated K⁺ currents); and (3) improving cellular bioenergetics (ATP, ∆ψ_m).

Revisiting the physiological effects of methylene blue as a treatment of cyanide intoxication

BACKGROUND

Although methylene blue (MB) had long been proposed to counteract the effects of cyanide (CN) intoxication, research on its mechanisms of action and efficacy has been abandoned for decades. Recent studies on the benefits of MB in post-anoxic injuries have prompted us to reexamine the relevance of this historical observation.

METHODS

Our study was performed in adult male Sprague-Dawley rats and on HEK293T epithelial cells. First, the effects and toxicity of MB (0-80 mg/kg) on circulation and metabolism were established in four urethane-anesthetized rats. Then nine rats received a lethal infusion of a solution of KCN (0.75 mg/kg/min) and were treated by either saline or MB, at 20 mg/kg, a dose that we found to be innocuous in rat and to correspond to a dose of about 4 mg/kg in humans. MB was also administered 5 min after the end of a sub-lethal exposure to CN in a separate group of 10 rats. In addition, ATP/ADP ratio, ROS production, mitochondrial membrane potential (Δψm) and cellular O₂ consumption rate (OCR) were determined in HEK293T cells exposed to toxic levels of CN (200 µM for 10 min) before and after applying a solution containing MB (1-100 µM for 10 min).

RESULTS

Methylene blue was found to be innocuous up to 50 mg/kg. KCN infusion (0.75 mg/kg/min) killed all animals within 7-8 min. MB (20 mg/kg) administered at the same time restored blood pressure, cardiac contractility and limited O₂ deficit, allowing all the animals to survive, without any significant methemoglobinemia. When administered 5 min after a non-lethal CN intoxication, MB sped up the recovery of lactate and O₂ deficit. Finally, MB was able to decrease the production of ROS and restore the ATP/ADP ratio, Δψm as well as OCR of epithelial cells intoxicated by CN.

CONCLUSIONS

The present observations should make us consider the potential interest of MB in the treatment of CN intoxication. The mechanisms of the antidotal properties of MB cannot be accounted for by the creation of a cyanomethemoglobinemia, rather its protective effects appears to be related to the unique properties of this redox dye, which, depending on the dose, could directly oppose some of the consequences of the metabolic depression produced by CN at the cellular level.

Analysis of endogenous H2S and H2Sn in mouse brain by high-performance liquid chromatography with fluorescence and tandem mass spectrometric detection

Previous studies indicated that bound sulfur species (BSS), including hydrogen polysulfide (H₂S_n), have various physiological functions in mammalian cells. Although H₂S_n molecules have been considered as secondary metabolites derived from hydrogen sulfide (H₂S) based on in vitro studies or predetermined reaction formula, the physiological form of BSS and their endogenous concentration remain unclear. In the present study, we aimed to improve the usual method using monobromobimane (mBB) followed by high performance liquid chromatographic (HPLC) analysis for HS^－ for simultaneous determination of H₂S, H₂S₂, H₂S₃ and cysteine persulfide in biological samples. We demonstrated that mBB derivatization of H₂S and H₂S_n standards under alkaline conditions (pH 9.5) induced significant decreases in H₂S₂ and H₂S₃ levels and a significant increase in the H₂S level in an incubation time-dependent manner. Conversely, the derivatization of mBB adducts of H₂S₂ and H₂S₃ were stable under neutral conditions (pH 7.0), which is physiologically relevant. Therefore, we re-examined the method using mBB and applied an improved method for the evaluation of H₂S, H₂S₂, and H₂S₃ in mouse brain under physiological pH conditions. The concentrations of H₂S and H₂S₂ were 0.030 ± 0.004μmol/g protein and 0.026 ± 0.002μmol/g protein, respectively. Although the level of H₂S₃ was below the quantification limit of this method, H₂S₃ was detected in mouse brain. Using the method established here, we reveal for the first time the existence of endogenous H₂S₂ and H₂S₃ in mammalian brain tissues. H₂S₂ and H₂S₃ exert anti-oxidant activity and anti-carbonyl stress effects through the regulation of redox balance in neuronal cells. Thus, our observations provide novel insights into the physiological functions of BSS in the brain and into neuronal diseases involved in redox imbalance.

Fluorescent Probes and Selective Inhibitors for Biological Studies of Hydrogen Sulfide- and Polysulfide-Mediated Signaling

SIGNIFICANCE

Hydrogen sulfide (H₂S) plays roles in many physiological processes, including relaxation of vascular smooth muscles, mediation of neurotransmission, inhibition of insulin signaling, and regulation of inflammation. Also, hydropersulfide (R-S-SH) and polysulfide (-S-S_n-S-) have recently been identified as reactive sulfur species (RSS) that regulate the bioactivities of multiple proteins via S-sulfhydration of cysteine residues (protein Cys-SSH) and show cytoprotection. Chemical tools such as fluorescent probes and selective inhibitors are needed to establish in detail the physiological roles of H₂S and polysulfide. Recent Advances: Although many fluorescent probes for H₂S are available, fluorescent probes for hydropersulfide and polysulfide have only recently been developed and used to detect these sulfur species in living cells.

CRITICAL ISSUES

In this review, we summarize recent progress in developing chemical tools for the study of H₂S, hydropersulfide, and polysulfide, covering fluorescent probes based on various design strategies and selective inhibitors of H₂S- and polysulfide-producing enzymes (cystathionine γ-lyase, cystathionine β-synthase, and 3-mercaptopyruvate sulfurtransferase), and we summarize their applications in biological studies.

FUTURE DIRECTIONS

Despite recent progress, the precise biological functions of H₂S, hydropersulfide, and polysulfide remain to be fully established. Fluorescent probes and selective inhibitors are effective chemical tools to study the physiological roles of these sulfur molecules in living cells and tissues. Therefore, further development of a broad range of practical fluorescent probes and selective inhibitors as tools for studies of RSS biology is currently attracting great interest. Antioxid. Redox Signal. 27, 669-683.

Analysis of decreases in systemic arterial pressure and heart rate in response to the hydrogen sulfide donor sodium sulfide

The actions of hydrogen sulfide (H2S) on the heart and vasculature have been extensively reported. However, the mechanisms underlying the effects of H2S are unclear in the anesthetized rat. The objective of the present study was to investigate the effect of H2S on the electrocardiogram and examine the relationship between H2S-induced changes in heart rate (HR), mean arterial pressure (MAP), and respiratory function. Intravenous administration of the H2S donor Na2S in the anesthetized Sprague-Dawley rat decreased MAP and HR and produced changes in respiratory function. The administration of Na2S significantly increased the RR interval at some doses but had no effect on PR or corrected QT(n)-B intervals. In experiments where respiration was maintained with a mechanical ventilator, we observed that Na2S-induced decreases in MAP and HR were independent of respiration. In experiments where respiration was maintained by mechanical ventilation and HR was maintained by cardiac pacing, Na2S-induced changes in MAP were not significantly altered, whereas changes in HR were abolished. Coadministration of glybenclamide significantly increased MAP and HR responses at some doses, but methylene blue, diltiazem, and ivabradine had no significant effect compared with control. The decreases in MAP and HR in response to Na2S could be dissociated and were independent of changes in respiratory function, ATP-sensitive K+ channels, methylene blue-sensitive mechanism involving L-type voltage-sensitive Ca2+ channels, or hyperpolarization-activated cyclic nucleotide-gated channels. Cardiovascular responses observed in spontaneously hypertensive rats were more robust than those in Sprague-Dawley rats.NEW & NOTEWORTHY H2S is a gasotransmitter capable of producing a decrease in mean arterial pressure and heart rate. The hypotensive and bradycardic effects of H2S can be dissociated, as shown with cardiac pacing experiments. Responses were not blocked by diltiazem, ivabradine, methylene blue, or glybenclamide.

Hydrogen Sulfide in Exhaled Gases From Ventilated Septic Neonates and Children: A Preliminary Report

OBJECTIVES

There is increasing interest in hydrogen sulfide as a marker of pathologic conditions or predictors of outcome. We speculate that as hydrogen sulfide is a diffusible molecule, if there is an increase in plasma hydrogen sulfide in sepsis, it may accumulate in the alveolar space and be detected in exhaled gas. We wished to determine whether we could detect hydrogen sulfide in exhaled gases of ventilated children and neonates and if the levels changed in sepsis.

DESIGN

Prospective, observational study.

SETTING

The study was conducted across three intensive care units, pediatric, neonatal and cardiac in a large tertiary children's hospital.

PATIENTS

We studied ventilated children and neonates with sepsis, defined by having two or more systemic inflammatory response syndrome criteria and one organ failure or suspected infection. A control group of ventilated non-septic patients was also included.

INTERVENTION

A portable gas chromatograph (OralChroma; Envin Scientific, Chester, United Kingdom) was used to measure H2S in parts per billion.

MEASUREMENTS AND MAIN RESULTS

A 1-2 mL sample of expired gas was taken from the endotracheal tube and analyzed. A repeat sample was taken after 30 minutes and a further single daily sample up to a maximum of 5 days or until the patient was extubated. WBC and C-reactive protein were measured around the time of gas sampling. Each group contained 20 subjects. Levels of H2S were significantly higher in septic patients (Mann Whitney U-test; p < 0.0001) and trended to control levels over five days. C- reactive protein levels were also significantly raised (p < 0.001) and mirrored the decrease in H2S levels.

CONCLUSION

Hydrogen sulfide can be detected in expired pulmonary gases in very low concentrations of parts per billion. Significantly higher levels are seen in septic patients compared with controls. The pattern of response was similar to that of C-reactive protein.

Circulatory Failure During Noninhaled Forms of Cyanide Intoxication

Our objective was to determine how circulatory failure develops following systemic administration of potassium cyanide (KCN). We used a noninhaled modality of intoxication, wherein the change in breathing pattern would not influence the diffusion of CN into the blood, akin to the effects of ingesting toxic levels of CN. In a group of 300 to 400 g rats, CN-induced coma (CN i.p., 7 mg/kg) produced a central apnea within 2 to 3 min along with a potent and prolonged gasping pattern leading to autoresuscitation in 38% of the animals. Motor deficits and neuronal necrosis were nevertheless observed in the surviving animals. To clarify the mechanisms leading to potential autoresuscitation versus asystole, 12 urethane-anesthetized rats were then exposed to the lowest possible levels of CN exposure that would lead to breathing depression within 7 to 8 min; this dose averaged 0.375 mg/kg/min i.v. At this level of intoxication, a cardiac depression developed several minutes only after the onset of the apnea, leading to cardiac asystole as PaO2 reached value approximately 15 Torr, unless breathing was maintained by mechanical ventilation or through spontaneous gasping. Higher levels of KCN exposure in 10 animals provoked a primary cardiac depression, which led to a rapid cardiac arrest by pulseless electrical activity (PEA) despite the maintenance of PaO2 by mechanical ventilation. These effects were totally unrelated to the potassium contained in KCN. It is concluded that circulatory failure can develop as a direct consequence of CN-induced apnea but in a narrow range of exposure. In this "low" range, maintaining pulmonary gas exchange after exposure, through mechanical ventilation (or spontaneous gasping), can reverse cardiac depression and restore spontaneous breathing. At higher level of intoxication, cardiac depression is to be treated as a specific and spontaneously irreversible consequence of CN exposure, leading to a PEA.

Diffusive shunting of gases and other molecules in the renal vasculature: physiological and evolutionary significance

Countercurrent systems have evolved in a variety of biological systems that allow transfer of heat, gases, and solutes. For example, in the renal medulla, the countercurrent arrangement of vascular and tubular elements facilitates the trapping of urea and other solutes in the inner medulla, which in turn enables the formation of concentrated urine. Arteries and veins in the cortex are also arranged in a countercurrent fashion, as are descending and ascending vasa recta in the medulla. For countercurrent diffusion to occur, barriers to diffusion must be small. This appears to be characteristic of larger vessels in the renal cortex. There must also be gradients in the concentration of molecules between afferent and efferent vessels, with the transport of molecules possible in either direction. Such gradients exist for oxygen in both the cortex and medulla, but there is little evidence that large gradients exist for other molecules such as carbon dioxide, nitric oxide, superoxide, hydrogen sulfide, and ammonia. There is some experimental evidence for arterial-to-venous (AV) oxygen shunting. Mathematical models also provide evidence for oxygen shunting in both the cortex and medulla. However, the quantitative significance of AV oxygen shunting remains a matter of controversy. Thus, whereas the countercurrent arrangement of vasa recta in the medulla appears to have evolved as a consequence of the evolution of Henle’s loop, the evolutionary significance of the intimate countercurrent arrangement of blood vessels in the renal cortex remains an enigma.

Sulfide Intoxication-Induced Circulatory Failure is Mediated by a Depression in Cardiac Contractility

Hydrogen sulfide (H2S) intoxication produces a rapid cardio-circulatory failure leading to cardiac arrest. In non-lethal forms of sulfide exposure, the presence of a circulatory shock is associated with long-term neurological sequelae. Our aim was to clarify the mechanisms of H2S-induced circulatory failure. In anesthetized, paralyzed, and mechanically ventilated rats, cardiac output, arterial pressure and ventricular pressures were determined while NaHS was infused to increase arterial concentration of soluble H2S (CgH2S) from undetectable to levels leading to circulatory failure. Compared to control/saline infusion, blood pressure started to decrease significantly along with a modest drop in peripheral vascular resistance (-19 ± 5%, P < 0.01), when CgH2S reached about 1 μM. As CgH2S exceeded 2-3 μM, parameters of ventricular contractility diminished with no further reduction in peripheral resistance. Whenever H2S exposure was maintained at a higher level (CgH2S over 7 μM), a severe depression of cardiac contractility was observed, leading to asystole within minutes, but with no evidence of peripheral vasoplegia. The immediate and long-term neurological effects of specifically counteracting sulfide-induced cardiac contractility depression following H2S exposure remain to be investigated.

H2S concentrations in the heart after acute H2S administration: methodological and physiological considerations

In this study, we have tried to characterize the limits of the approach typically used to determine H₂S concentrations in the heart based on the amount of H₂S evaporating from heart homogenates-spontaneously, after reaction with a strong reducing agent, or in a very acidic solution. Heart homogenates were prepared from male rats in control conditions or after H₂S infusion induced a transient cardiogenic shock (CS) or cardiac asystole (CA). Using a method of determination of gaseous H₂S with a detection limit of 0.2 nmol, we found that the process of homogenization could lead to a total disappearance of free H₂S unless performed in alkaline conditions. Yet, after restoration of neutral pH, free H₂S concentration from samples processed in alkaline and nonalkaline milieus were similar and averaged ∼0.2-0.4 nmol/g in both control and CS homogenate hearts and up to 100 nmol/g in the CA group. No additional H₂S was released from control, CS, or CA hearts by using the reducing agent tris(2-carboxyethyl)phosphine or a strong acidic solution (pH < 2) to "free" H₂S from combined pools. Of note, the reducing agent DTT produced a significant sulfide artifact and was not used. These data suggest that 1) free H₂S found in heart homogenates is not a reflection of H₂S present in a "living" heart and 2) the pool of combined sulfides, released in a strong reducing or acidic milieu, does not increase in the heart in a measurable manner even after toxic exposure to sulfide.

Is exogenous hydrogen sulfide a relevant tool to address physiological questions on hydrogen sulfide?

This review challenges the use of solutions of dissolved exogenous H2S in the literature as a tool to determine the potential physiological functions of endogenous H2S as well as its putative therapeutic applications. Our major point of contention is that solutions of dissolved H2S are used in vitro at concentrations, within the high microM range, which are above the concentrations of dissolved H2S found in blood and tissues during lethal H2S exposure in vivo. In addition, since the levels of toxicity are extremely variable among cell types, a property that is seldom acknowledged, the physiological relevance of data obtained after local or in-vitro administrations of H2S at concentrations of few microM is far from certain. Conversely, the rate of disappearance of the dissolved pool of H2S in the body (being trapped or oxidized), which we found to be at least of several micromoles/kg/min, is so rapid in vivo that if relatively low quantities of H2S, i.e. few micromoles for instance, are administered, no change in H2S concentrations in the body is to be expected, unless toxic levels are used. Protocols looking at the effects of compounds slowly releasing H2S must also resolve a similar conundrum, as their effects must be reconciled with the unique ability of the blood and tissues to get rid of H2S and the steepness of the dose-toxic effects relationship. Only by developing a comprehensive framework in which H2S metabolism and toxicity will be used as a rationale to justify any experimental approach will we be able to bring definitive evidence supporting a protective role for exogenous H2S, if any, and its putative function as an endogenous mediator.

Methylene blue counteracts H2S toxicity-induced cardiac depression by restoring L-type Ca channel activity

We have previously reported that methylene blue (MB) can counteract hydrogen sulfide (H2S) intoxication-induced circulatory failure. Because of the multifarious effects of high concentrations of H2S on cardiac function, as well as the numerous properties of MB, the nature of this interaction, if any, remains uncertain. The aim of this study was to clarify 1) the effects of MB on H2S-induced cardiac toxicity and 2) whether L-type Ca(2+) channels, one of the targets of H2S, could transduce some of the counteracting effects of MB. In sedated rats, H2S infused at a rate that would be lethal within 5 min (24 μM·kg(-1)·min(-1)), produced a rapid fall in left ventricle ejection fraction, determined by echocardiography, leading to a pulseless electrical activity. Blood concentrations of gaseous H2S reached 7.09 ± 3.53 μM when cardiac contractility started to decrease. Two to three injections of MB (4 mg/kg) transiently restored cardiac contractility, blood pressure, and V̇o2, allowing the animals to stay alive until the end of H2S infusion. MB also delayed PEA by several minutes following H2S-induced coma and shock in unsedated rats. Applying a solution containing lethal levels of H2S (100 μM) on isolated mouse cardiomyocytes significantly reduced cell contractility, intracellular calcium concentration ([Ca(2+)]i) transient amplitudes, and L-type Ca(2+) currents (ICa) within 3 min of exposure. MB (20 mg/l) restored the cardiomyocyte function, ([Ca(2+)]i) transient, and ICa The present results offer a new approach for counteracting H2S toxicity and potentially other conditions associated with acute inhibition of L-type Ca(2+) channels.

Developing effective countermeasures against acute hydrogen sulfide intoxication: challenges and limitations

Hydrogen sulfide (H2 S) is a chemical hazard in the gas and farming industry. As it is easy to manufacture from common chemicals, it has also become a method of suicide. H2 S exerts its toxicity through its high affinity with metalloproteins, such as cytochrome c oxidase and possibly via its interactions with cysteine residues of various proteins. The latter was recently proposed to acutely alter ion channels with critical implications for cardiac and brain functions. Indeed, during severe H2 S intoxication, a coma, associated with a reduction in cardiac contractility, develops within minutes or even seconds leading to death by complete electromechanical dissociation of the heart. In addition, long-term neurological deficits can develop owing to the direct toxicity of H2 S on neurons combined with the consequences of a prolonged apnea and circulatory failure. Here, we review the challenges impeding efforts to offer an effective treatment against H2 S intoxication using agents that trap free H2 S, and present novel pharmacological approaches aimed at correcting some of the most harmful consequences of H2 S intoxication.

Chemical tools for the study of hydrogen sulfide (H2S) and sulfane sulfur and their applications to biological studies

Hydrogen sulfide (H₂S) functions in many physiological processes, including relaxation of vascular smooth muscles, mediation of neurotransmission, inhibition of insulin signaling, and regulation of inflammation. On the other hand, sulfane sulfur, which is a sulfur atom with six valence electrons but no charge, has the unique ability to bind reversibly to other sulfur atoms to form hydropersulfides (R-S-SH) and polysulfides (-S-S_n-S-). H₂S and sulfane sulfur always coexist, and recent work suggests that sulfane sulfur species may be the actual signaling molecules in at least some biological phenomena. For example, one of the mechanisms of activity regulation of proteins by H₂S is the S-sulfhydration of cysteine residues (protein Cys-SSH). In this review, we summarize recent progress on chemical tools for the study of H₂S and sulfane sulfur, covering fluorescence probes utilizing various design strategies, H₂S caged compounds, inhibitors of physiological H₂S-producing enzymes (cystathionine γ-lyase, cystathionine β-synthase and 3-mercaptopyruvate sulfurtransferase), and labeling reagents. Fluorescence probes offer particular advantages as chemical tools to study physiological functions of biomolecules, including ease of use and real-time, nondestructive visualization of biological processes in live cells and tissues.

Chemical probes for molecular imaging and detection of hydrogen sulfide and reactive sulfur species in biological systems

Hydrogen sulfide (H2S), a gaseous species produced by both bacteria and higher eukaryotic organisms, including mammalian vertebrates, has attracted attention in recent years for its contributions to human health and disease. H2S has been proposed as a cytoprotectant and gasotransmitter in many tissue types, including mediating vascular tone in blood vessels as well as neuromodulation in the brain. The molecular mechanisms dictating how H2S affects cellular signaling and other physiological events remain insufficiently understood. Furthermore, the involvement of H2S in metal-binding interactions and formation of related RSS such as sulfane sulfur may contribute to other distinct signaling pathways. Owing to its widespread biological roles and unique chemical properties, H2S is an appealing target for chemical biology approaches to elucidate its production, trafficking, and downstream function. In this context, reaction-based fluorescent probes offer a versatile set of screening tools to visualize H2S pools in living systems. Three main strategies used in molecular probe development for H2S detection include azide and nitro group reduction, nucleophilic attack, and CuS precipitation. Each of these approaches exploits the strong nucleophilicity and reducing potency of H2S to achieve selectivity over other biothiols. In addition, a variety of methods have been developed for the detection of other reactive sulfur species (RSS), including sulfite and bisulfite, as well as sulfane sulfur species and related modifications such as S-nitrosothiols. Access to this growing chemical toolbox of new molecular probes for H2S and related RSS sets the stage for applying these developing technologies to probe reactive sulfur biology in living systems.

Immediate and Long-Term Outcome of Acute H2S Intoxication Induced Coma in Unanesthetized Rats: Effects of Methylene Blue

BACKGROUND

Acute hydrogen sulfide (H2S) poisoning produces a coma, the outcome of which ranges from full recovery to severe neurological deficits. The aim of our study was to 1--describe the immediate and long-term neurological effects following H2S-induced coma in un-anesthetized rats, and 2--determine the potential benefit of methylene blue (MB), a compound we previously found to counteract acute sulfide cardiac toxicity.

METHODS

NaHS was administered IP in un-sedated rats to produce a coma (n = 34). One minute into coma, the rats received MB (4 mg/kg i.v.) or saline. The surviving rats were followed clinically and assigned to Morris water maze (MWM) and open field testing then sacrificed at day 7.

RESULTS

Sixty percent of the non-treated comatose rats died by pulseless electrical activity. Nine percent recovered with neurological deficits requiring euthanasia, their brain examination revealed major neuronal necrosis of the superficial and middle layers of the cerebral cortex and the posterior thalamus, with variable necrosis of the caudate putamen, but no lesions of the hippocampus or the cerebellum, in contrast to the typical distribution of post-ischemic lesions. The remaining animals displayed, on average, a significantly less effective search strategy than the control rats (n = 21) during MWM testing. Meanwhile, 75% of rats that received MB survived and could perform the MWM test (P<0.05 vs non-treated animals). The treated animals displayed a significantly higher occurrence of spatial search than the non-treated animals. However, a similar proportion of cortical necrosis was observed in both groups, with a milder clinical presentation following MB.

CONCLUSION

In conclusion, in rats surviving H2S induced coma, spatial search patterns were used less frequently than in control animals. A small percentage of rats presented necrotic neuronal lesions, which distribution differed from post-ischemic lesions. MB dramatically improved the immediate survival and spatial search strategy in the surviving rats.

H2S induced coma and cardiogenic shock in the rat: Effects of phenothiazinium chromophores

CONTEXT

Hydrogen sulfide (H2S) intoxication produces an acute depression in cardiac contractility-induced circulatory failure, which has been shown to be one of the major contributors to the lethality of H2S intoxication or to the neurological sequelae in surviving animals. Methylene blue (MB), a phenothiazinium dye, can antagonize the effects of the inhibition of mitochondrial electron transport chain, a major effect of H2S toxicity.

OBJECTIVES

We investigated whether MB could affect the immediate outcome of H2S-induced coma in un-anesthetized animals. Second, we sought to characterize the acute cardiovascular effects of MB and two of its demethylated metabolites-azure B and thionine-in anesthetized rats during lethal infusion of H2S.

MATERIALS AND METHODS

First, MB (4 mg/kg, intravenous [IV]) was administered in non-sedated rats during the phase of agonal breathing, following NaHS (20 mg/kg, IP)-induced coma. Second, in 4 groups of urethane-anesthetized rats, NaHS was infused at a rate lethal within 10 min (0.8 mg/min, IV). Whenever cardiac output (CO) reached 40% of its baseline volume, MB, azure B, thionine, or saline were injected, while sulfide infusion was maintained until cardiac arrest occurred.

RESULTS

Seventy-five percent of the comatose rats that received saline (n = 8) died within 7 min, while all the 7 rats that were given MB survived (p = 0.007). In the anesthetized rats, arterial, left ventricular pressures and CO decreased during NaHS infusion, leading to a pulseless electrical activity within 530 s. MB produced a significant increase in CO and dP/dtmax for about 2 min. A similar effect was produced when MB was also injected in the pre-mortem phase of sulfide exposure, significantly increasing survival time. Azure B produced an even larger increase in blood pressure than MB, while thionine had no effect.

CONCLUSION

MB can counteract NaHS-induced acute cardiogenic shock; this effect is also produced by azure B, but not by thionine, suggesting that the presence of methyl groups is a prerequisite for producing this protective effect.

High-dose hydroxocobalamin administered after H2S exposure counteracts sulfide-poisoning-induced cardiac depression in sheep

CONTEXT

Severe H2S poisoning leads to death by rapid respiratory and cardiac arrest, the latter can occur within seconds or minutes in severe forms of intoxication.

OBJECTIVES

To determine the time course and the nature of H2S-induced cardiac arrest and the effects of high-dose hydroxocobalamin administered after the end of sulfide exposure.

MATERIALS AND METHODS

NaHS was infused in 16 sedated mechanically ventilated sheep to reach concentrations of H2S in the blood, which was previously found to lead to cardiac arrest within minutes following the cessation of H2S exposure. High-dose hydroxocobalamin (5 g) or saline solution was administered intravenously, 1 min after the cessation of NaHS infusion.

RESULTS

All animals were still alive at the cessation of H2S exposure. Three animals (18%) presented a cardiac arrest within 90 s and were unable to receive any antidote or vehicle. In the animals that survived long enough to receive either hydroxocobalamin or saline, 71% (5/7) died in the control group by cardiac arrest within 10 min. In all instances, cardiac arrest was the result of a pulseless electrical activity (PEA). In the group that received the antidote, intravenous injection of 5 g of hydroxocobalamin provoked an abrupt increase in blood pressure and blood flow; PEA was prevented in all instances. However, we could not find any evidence for a recovery in oxidative metabolism in the group receiving hydroxocobalamin, as blood lactate remained elevated and even continued to rise after 1 h, despite restored hemodynamics. This, along with an unaltered recovery of H2S kinetics, suggests that hydroxocobalamin did not act through a mechanism of H2S trapping.

CONCLUSION

In this sheep model, there was a high risk for cardiac arrest, by PEA, persisting up to 10 min after H2S exposure. Very high dose of hydroxocobalamin (5 g), injected very early after the cessation of H2S exposure, improved cardiac contractility and prevented PEA.

Effects of infusion of human methemoglobin solution following hydrogen sulfide poisoning

RATIONALE

We have recently reported that infusion of a solution containing methemoglobin (MetHb) during exposure to hydrogen sulfide results in a rapid and large decrease in the concentration of the pool of soluble/diffusible H2S in the blood. However, since the pool of dissolved H2S disappears very quickly after H2S exposure, it is unclear if the ability of MetHb to "trap" sulfide in the blood has any clinical interest and relevance in the treatment of sulfide poisoning.

METHODS

In anesthetized rats, repetition of short bouts of high level of H2S infusions was applied to allow the rapid development of an oxygen deficit. A solution containing MetHb (600 mg/kg) or its vehicle was administered 1 min and a half after the end of H2S intoxication.

RESULTS

The injection of MetHb solution increased methemoglobinemia to about 6%, almost instantly, but was unable to affect the blood concentration of soluble H2S, which had already vanished at the time of infusion, or to increase combined H2S. In addition, H2S-induced O2 deficit and lactate production as well as the recovery of carotid blood flow and blood pressure were similar in treated and control animals.

CONCLUSION

Our results do not support the view that administration of MetHb or drugs-induced methemoglobinemia during the recovery phase following severe H2S intoxication in sedated rats can restore cellular oxidative metabolism, as the pool of diffusible sulfide, accessible to MetHb, disappears rapidly from the blood after H2S exposure.

Working with nitric oxide and hydrogen sulfide in biological systems

Nitric oxide (NO) and hydrogen sulfide (H2S) are gasotransmitter molecules important in numerous physiological and pathological processes. Although these molecules were first known as environmental toxicants, it is now evident that that they are intricately involved in diverse cellular functions with impact on numerous physiological and pathogenic processes. NO and H2S share some common characteristics but also have unique chemical properties that suggest potential complementary interactions between the two in affecting cellular biochemistry and metabolism. Central among these is the interactions between NO, H2S, and thiols that constitute new ways to regulate protein function, signaling, and cellular responses. In this review, we discuss fundamental biochemical principals, molecular functions, measurement methods, and the pathophysiological relevance of NO and H2S.

Hydrogen sulfide augments survival signals in warm ischemia and reperfusion of the mouse liver

BACKGROUND AND PURPOSE

Hydrogen sulfide (H2S) ameliorates hepatic ischemia and reperfusion injury (IRI), but the precise mechanism remains elusive. We investigated whether sodium hydrogen sulfide (NaHS), a soluble derivative of H2S, would ameliorate hepatic IRI, and if so, via what mechanism.

METHODS

Mice were subjected to partial warm ischemia for 75 min followed by reperfusion. Either NaHS or saline was administered intravenously 10 min before reperfusion. The liver and serum were collected 3, 6, and 24 h after reperfusion.

RESULTS

In the NaHS(-) group, severe IRI was apparent by the ALT leakage, tissue injury score, apoptosis, lipid peroxidation, and inflammation (higher plasma TNF-α, IL-6, IL-1β, IFN-γ, IL-23, IL-17, and CD40L), whereas IRI was significantly ameliorated in the NaHS(+) group. These effects could be explained by the augmented nuclear translocation of Nrf2, and the resulting up-regulation of HO-1 and thioredoxin-1. Phosphorylation of the PDK-1/Akt/mTOR/p70S6k axis, which is known to mediate pro-survival and anti-apoptotic signals, was significantly augmented in the NaHS(+) group, with a higher rate of PCNA-positive cells thereafter.

CONCLUSION

NaHS ameliorated hepatic IRI by direct and indirect anti-oxidant activities by augmenting pro-survival, anti-apoptotic, and anti-inflammatory signals via mechanisms involving Nrf-2, and by accelerating hepatic regeneration via mechanisms involving Akt-p70S6k.

In vivo interactions between cobalt or ferric compounds and the pools of sulphide in the blood during and after H2S poisoning

Hydrogen sulphide (H2S), a chemical hazard in oil and gas production, has recently become a dreadful method of suicide, posing specific risks and challenges for the first responders. Currently, there is no proven effective treatment against H2S poisoning and its severe neurological, respiratory or cardiac after-effects. We have recently described that H2S is present in various compartments, or pools, in the body during sulphide exposure, which have different levels of toxicity. The general goals of our study were to (1) determine the concentrations and kinetics of the various pools of hydrogen sulphide in the blood, i.e., gaseous (CgH2S) versus total sulphide, i.e., reacting with monobromobimane (CMBBH2S), during and following H2S exposure in a small and large mammal and (2) establish the interaction between the pools of H2S and a methemoglobin (MetHb) solution or a high dose of hydroxocobalamin (HyCo). We found that CgH2S during and following H2S infusion was similar in sedated sheep and rats at any given rate of infusion/kg and provoked symptoms, i.e., hyperpnea and apnea, at the same CgH2S. After H2S administration was stopped, CgH2S disappeared within 1 min. CMBBH2S also dropped to 2-3μM, but remained above baseline levels for at least 30 min. Infusion of a MetHb solution during H2S infusion produced an immediate reduction in the free/soluble pool of H2S only, whereas CMBBH2S increased by severalfold. HyCo (70 mg/kg) also decreased the concentrations of free/soluble H2S to almost zero; CgH2S returned to pre-HyCo levels within a maximum of 20 min, if H2S infusion is maintained. These results are discussed in the context of a relevant scenario, wherein antidotes can only be administered after H2S exposure.

Oxygen-related chemoreceptor drive to breathe during H₂S infusion

This study addresses the following question: Could the acute depression in breathing produced by hyperoxia, a reflection of the tonic drive to breathe from the arterial chemoreceptors, be accounted for by a background level of endogenous H2S? To address this question, we produced a stable but moderate increase in breathing (24±11%) via continuous infusion of low levels of H2S, in 10 spontaneously breathing urethane-sedated rats. We found that acute exposure to 100% O2 (20 tests) decreased minute ventilation (V˙(I)) from 301±51 to 210±43 ml/min within 15s in control conditions, but no additional significant drop in V˙(I) was observed during H2S induced hyperpnea. In addition, no decrease in the estimated concentrations of gaseous H2S in the arterial blood was observed during the hyperoxic tests. It is concluded that the ventilatory depression induced by high O2 appears to be limited to the tonic background peripheral chemosensory drive to breathe, but has little or no impact on the CB stimulation produced by low levels of H2S.

Hydrogen sulfide activation in hemeproteins: the sulfheme scenario

Traditionally known as a toxic gas, hydrogen sulfide (H2S) is now recognized as an important biological molecule involved in numerous physiological functions. Like nitric oxide (NO) and carbon monoxide (CO), H2S is produced endogenously in tissues and cells and can modulate biological processes by acting on target proteins. For example, interaction of H2S with the oxygenated form of human hemoglobin and myoglobin produces a sulfheme protein complex that has been implicated in H2S degradation. The presence of this sulfheme derivative has also been used as a marker for endogenous H2S synthesis and metabolism. Remarkably, human catalases and peroxidases also generate this sulfheme product. In this review, we describe the structural and functional aspects of the sulfheme derivative in these proteins and postulate a generalized mechanism for sulfheme protein formation. We also evaluate the possible physiological function of this complex and highlight the issues that remain to be assessed to determine the role of sulfheme proteins in H2S metabolism, detection and physiology.