Hypoxia-induced arterial chemoreceptor stimulation and hydrogen sulfide: too much or too little?

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.

Intracellular Mechanisms of Oxygen Sensing

The review describes molecular mechanisms for sensing oxygen levels in various compartments of animal cell. Several pathways for intracellular oxygen sensing are discussed together with details of functioning of the near-membrane and cytoplasmic pools of molecular components in hypoxic cells. The data on the role of mitochondria in cell sensitivity to a decreased oxygen content are presented. Details of mutual influence of the operational and chronic intracellular mechanisms for detecting the negative gradients of molecular oxygen concentration and their relationship with cell metabolism response to the oxidative stress are discussed.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

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).

Voltage- and receptor-mediated activation of a non-selective cation channel in rat carotid body glomus cells

A recent study showed that hypoxia activates a Ca²⁺-sensitive, Na⁺-permeable non-selective cation channel (NSC) in carotid body glomus cells. We studied the effects of mitochondrial inhibitors that increase Ca²⁺ influx via Ca²⁺ channel (Ca_v), and receptor agonists that release Ca²⁺ from endoplasmic reticulum (ER) on NSC. Mitochondrial inhibitors (NaCN, FCCP, H₂S, NO) elevated [Ca²⁺]_i and activated NSC. Angiotensin II and acetylcholine that elevate [Ca²⁺]_i via the Gq-IP₃ pathway activated NSC. However, endothelin-1 (Gq) and 5-HT (Gq) showed little or no effect on [Ca²⁺]_i and did not activate NSC. Adenosine (Gs) caused a weak rise in [Ca²⁺]_i but did not activate NSC. Dopamine (Gs) and γ-aminobytyric acid (Gi) were ineffective in raising [Ca²⁺]_i and failed to activate NSC. Store-operated Ca²⁺ entry (SOCE) produced by depletion of Ca²⁺ stores with cyclopiazonic acid activated NSC. Our results show that Ca²⁺ entry via Ca_v, ER Ca²⁺ release and SOCE can activate NSC. Thus, NSC contributes to both voltage- and receptor-mediated excitation of glomus cells.

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.

Hydrogen sulfide and hypoxia-induced changes in TASK (K2P3/9) activity and intracellular Ca(2+) concentration in rat carotid body glomus cells

Acute hypoxia depolarizes carotid body chemoreceptor (glomus) cells and elevates intracellular Ca(2+) concentration ([Ca(2+)]i). Recent studies suggest that hydrogen sulfide (H2S) may serve as an oxygen sensor/signal in the carotid body during acute hypoxia. To further test such a role for H2S, we studied the effects of H2S on the activity of TASK channel and [Ca(2+)]i, which are considered important for mediating the glomus cell response to hypoxia. Like hypoxia, NaHS (a H2S donor) inhibited TASK activity and elevated [Ca(2+)]i. To inhibit the production of H2S, glomus cells were incubated (3h) with inhibitors of cystathionine-β-synthase and cystathionine-γ-lyase (DL-propargylglycine, aminooxyacetic acid, β-cyano-L-alanine; 0.3 mM). SF7 fluorescence was used to assess the level of H2S production. The inhibitors blocked L-cysteine- and hypoxia-induced elevation of SF7 fluorescence intensity. In cells treated with the inhibitors, hypoxia produced an inhibition of TASK activity and a rise in [Ca(2+)]i, similar in magnitude to those observed in control cells. L-cysteine produced no effect on TASK activity or [Ca(2+)]i and did not affect hypoxia-induced inhibition of TASK and elevation of [Ca(2+)]i. These findings suggest that under normal conditions, H2S is not a major signal in hypoxia-induced modulation of TASK channels and [Ca(2+)]i in isolated glomus cells.

Gasotransmitter regulation of ion channels: a key step in O2 sensing by the carotid body

Carotid bodies detect hypoxia in arterial blood, translating this stimulus into physiological responses via the CNS. It is long established that ion channels are critical to this process. More recent evidence indicates that gasotransmitters exert powerful influences on O₂ sensing by the carotid body. Here, we review current understanding of hypoxia-dependent production of gasotransmitters, how they regulate ion channels in the carotid body, and how this impacts carotid body function.

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.

Sensing hypoxia: physiology, genetics and epigenetics

The carotid body is a sensory organ for detecting arterial blood O2 levels and reflexly mediates systemic cardiac, vascular and respiratory responses to hypoxia. This article presents a brief review of the roles of gaseous messengers in the sensory transduction at the carotid body, genetic and epigenetic influences on hypoxic sensing and the role of the carotid body chemoreflex in cardiorespiratory diseases. Type I (also called glomus) cells, the site of O2 sensing in the carotid body, express haem oxygenase-2 and cystathionine-γ-lyase, the enzymes which catalyse the generation of CO and H2S, respectively. Physiological studies have shown that CO is an inhibitory gas messenger, which contributes to the low sensory activity during normoxia, whereas H2S is excitatory and mediates sensory stimulation by hypoxia. Hypoxia-evoked H2S generation in the carotid body requires the interaction of cystathionine-γ-lyase with haem oxygenase-2, which generates CO. Hypoxia-inducible factors 1 and 2 constitute important components of the genetic make-up in the carotid body, which influence hypoxic sensing by regulating the intracellular redox state via transcriptional regulation of pro- and antioxidant enzymes. Recent studies suggest that developmental programming of the carotid body response to hypoxia involves epigenetic changes, e.g. DNA methylation of genes encoding redox-regulating enzymes. Emerging evidence implicates heightened carotid body chemoreflex in the progression of autonomic morbidities associated with cardiorespiratory diseases, such as sleep-disordered breathing with apnoea, congestive heart failure and essential hypertension.

Oxygen deficit and H2S in hemorrhagic shock in rats

Introduction

Hemorrhagic shock induced O₂ deficit triggers inflammation and multiple organ failure (MOF). Endogenous H₂S has been proposed to be involved in MOF since plasma H₂S concentration appears to increase in various types of shocks and to predict mortality. We tested the hypothesis that H₂S increases during hemorrhagic shock associated with O₂ deficit, and that enhancing H₂S oxidation by hydroxocobalamin could reduce inflammation, O₂ deficit or mortality.

Methods

We used a urethane anesthetized rat model, where 25 ml/kg of blood was withdrawn over 30 minutes. O₂ deficit, lactic acid, tumor necrosis factor (TNF)-alpha and H₂S plasma concentrations (Siegel method) were measured before and after the bleeding protocol in control animals and animals that received 140 mg/kg of hydroxocobalamin. The ability to oxidize exogenous H₂S of the plasma and supernatants of the kidney and heart homogenates was determined in vitro.

Results

We found that withdrawing 25 ml/kg of blood led to an average oxygen deficit of 122 ± 23 ml/kg. This O₂ deficit was correlated with an increase in the blood lactic acid concentration and mortality. However, the low level of absorbance of the plasma at 670 nm (A₆₇₀), after adding N, N-Dimethyl-p-phenylenediamine, that is, the method used for H₂S determination in previous studies, did not reflect the presence of H₂S, but was a marker of plasma turbidity. There was no difference in plasmatic A₆₇₀ before and after the bleeding protocol, despite the large oxygen deficit. The plasma sampled at the end of bleeding maintained a very large ability to oxidize exogenous H₂S (high μM), as did the homogenates of hearts and kidneys harvested just after death. Hydroxocobalamin concentrations increased in the blood in the μM range in the vitamin B12 group, and enhanced the ability of plasma and kidneys to oxidize H₂S. Yet, the survival rate, O₂ deficit, H₂S plasma concentration, blood lactic acid and TNF-alpha levels were not different from the control group.

Conclusions

In the presence of a large O₂ deficit, H₂S did not increase in the blood in a rat model of untreated hemorrhagic shock. Hydroxocobalamin, while effective against H₂S in vitro, did not affect the hemodynamic profile or outcome in our model.

Role of neurotransmitter gases in the control of the carotid body in heart failure

The peripheral arterial chemoreflex, arising primarily from the carotid body in most species, plays an important role in the control of breathing and in autonomic control of cardiovascular function. The peripheral chemoreflex is enhanced in heart failure patients and animal models of heart failure and contributes to the sympathetic hyperactivity and breathing instability that exacerbates the progression of the disease. Studies in animal models have shown that carotid body chemoreceptor activity is enhanced under both normoxic and hypoxic conditions in heart failure due to disruption of local mediators that control carotid body function. This brief review highlights evidence that the alterations in the gasotransmitters, nitric oxide, carbon monoxide, and hydrogen sulfide in the carotid body contribute to the exaggerated carotid body function observed in heart failure.

Ventilatory and metabolic effects of exogenous hydrogen sulfide

Acute H(2)S intoxication produces an increase in ventilation followed by a fatal central apnea. The sites of mediation of H(2)S induced hyperpnea and apnea have been investigated since the early 20th century in various animal models. Hyperpnea is mediated by the arterial chemoreceptors, an effect that can be reproduced by injecting a solution of H(2)S at very high concentrations (high millimolar range), while the fatal apnea, which typically occurs above 1000 ppm in humans, appears to result from the cessation of the activity of the medullary respiratory neurons. More recently, moderate levels of exogenous H(2)S (20-80 ppm) have been shown to reduce, within minutes, the metabolic rate, akin to hypoxia-induced hypometabolism. This response appears to be specific to small sized mammals. The pathway through which low levels of inhaled H(2)S could exert such a powerful effect may be very relevant to the physiological mechanisms controlling non-ATP "metabolic" production. Finally, endogenous H(2)S, produced from cysteine, has been proposed to transduce the effects of hypoxia in the carotid bodies. H(2)S remains a mysterious gas: it is labile, difficult/impossible to properly measure in vivo, its oxidation can take place in most tissues including the blood, and it can affect multiple cellular pathways. The demarcation between effects reflecting a putative physiological function and those related to H(2)S poisoning remains however to be established.

Inhibitory effects of hyperoxia and methemoglobinemia on H(2)S induced ventilatory stimulation in the rat

The aim of this study was to clarify, using in vitro and in vivo approaches in the rat, the site of mediation of the inhibition of H(2)S induced arterial chemoreceptor stimulation, by hyperoxia and methemoglobinemia. We first determined the ventilatory dose-response curves during intravenous injections of H(2)S. A very high dose of NaHS, i.e. 0.4 μmol (concentration: 800 μM), was needed to stimulate breathing within 1s following i.v. injection. Above this level (and up to 2.4 μmol, with a concentration of 4800 μM), a dose-dependent effect of H(2)S injection was observed. NaHS injection into the thoracic aorta produced the same effect, suggesting that within one circulatory time, H(2)S pulmonary exchange does not dramatically reduce H(2)S concentrations in the arterial blood. The ventilatory response to H(2)S was abolished in the presence of MetHb (12.8%) and was significantly depressed in hyperoxia and, surprisingly, in 10% hypoxia. MetHb per se did not affect the ventilatory response to hypoxia or hyperoxia, but dramatically enhanced the oxidation of H(2)S in vitro, with very fast kinetics. These findings suggest that, the decrease/oxidation of exogenous H(2)S in the blood is the primary effect of MetHb in vivo. In contrast, the in vitro oxidative properties of blood for H(2)S were not affected by the level of [Formula: see text] between 23 and >760 mmHg. This suggests that the inhibition of the ventilatory response to H(2)S by hyperoxia during aortic or venous injection originates within the CB and not in the blood. The implications of these results on the role of endogenous H(2)S in the arterial chemoreflex are discussed.

Effects of exogenous hydrogen sulphide on calcium signalling, background (TASK) K channel activity and mitochondrial function in chemoreceptor cells

It has been proposed that endogenous H₂S mediates oxygen sensing in chemoreceptors; this study investigates the mechanisms by which H₂S excites carotid body type 1 cells. H₂S caused a rapid reversible increase in intracellular calcium with EC₅₀ ≈ 6 μM. This [Ca²⁺]_i response was abolished in Ca-free Tyrode. In perforated patch current clamp recordings, H₂S depolarised type 1 cells from −59 to −35 mV; this was accompanied by a robust increase in [Ca²⁺]_i. Voltage clamping at the resting membrane potential abolished the H₂S-induced rise in [Ca²⁺]_i. H₂S inhibited background K⁺ current in whole cell perforated patch and reduced background K⁺ channel activity in cell-attached patch recordings. It is concluded that H₂S excites type 1 cells through the inhibition of background (TASK) potassium channels leading to membrane depolarisation and voltage-gated Ca²⁺ entry. These effects mimic those of hypoxia. H₂S also inhibited mitochondrial function over a similar concentration range as assessed by NADH autofluorescence and measurement of intracellular magnesium (an index of decline in MgATP). Cyanide inhibited background K channels to a similar extent to H₂S and prevented H₂S exerting any further influence over channel activity. These data indicate that the effects of H₂S on background K channels are a consequence of inhibition of oxidative phosphorylation. Whilst this does not preclude a role for endogenous H₂S in oxygen sensing via the inhibition of cytochrome oxidase, the levels of H₂S required raise questions as to the viability of such a mechanism.