Hydrogen sulfide (H₂S), a colorless, flammable and water-soluble gas with the characteristic odor of rotten eggs, has been known for decades because of its toxicity and as an environmental hazard 12. Inhibition of mitochondrial respiration – more potent than that of cyanide 3 – resulting from blockade of cytochrome c oxidase is the main mechanism of H₂S toxicity 45. During recent years, however, H₂S has been recognized as an important signaling molecule of the cardiovascular system, the inflammatory system and the nervous system. Alongside nitric oxide (NO) and carbon monoxide, therefore, H₂S is now known as the third endogenous gaseotransmitter 16.

Since H₂S is a small ubiquitous gaseous diffusible molecule, its putative interest for intensive care research is obvious. Consequently, inhibitors of its endogenous production as well as compounds that donate H₂S have been studied in various models of shock resulting from hemorrhage 789, ischemia/reperfusion 101112131415161718, endotoxemia 192021, bacterial sepsis 22232425 and nonmicrobial inflammation 26272829 – which, however, yielded rather controversial data with respect to the proinflammatory or anti-inflammatory properties of H₂S. The present article reviews the current literature on the therapeutic potential of H₂S, with a special focus on clinically relevant studies in – if available – large animal models.

In mammals, H₂S is synthesized from the sulfur-containing amino acid L-cysteine by either cystathionine-β-synthase or cystathionine-γ-lyase, both using pyridoxal 5'-phosphate (vitamin B₆) as a cofactor 303132. This synthesis results in low micromolar H₂S levels in the extracellular space, which can be rapidly consumed and degraded by various tissues. Similarly to NO and carbon monoxide, H₂S is a lipophilic compound that easily permeates cell membranes without using specific transporters. Via direct inhibition, NO as well as carbon monoxide are involved in the regulation of cystathionine-β-synthase, but not cystathionine-γ-lyase, which can be activated by lipopolysaccharide (LPS) 16.

There are three known pathways of H₂S degradation: mitochondrial oxidation to thiosulfate, which is further converted to sulfite and sulfate; cytosolic methylation to dimethylsulfide; and sulfhemoglobin formation after binding to hemoglobin 6. Similar to NO and carbon monoxide, H₂S can also bind to hemoglobin – which was therefore termed the common sink for the three gaseous transmitters 33. Consequently, saturation with one of these gases might lead to enhanced plasma concentrations and, subsequently, to biological effects of the other gases 1. Table 1 summarizes the physicochemistry of H₂S in mammalian tissues.

H₂S exerts its effects in biological systems through a variety of interrelated mechanisms (for a review see 1). Our current knowledge of the biology of H₂S predominantly stems from in vitro studies in various cell and isolated organ systems, either using cystathionine-γ-lyase inhibitors such as D,L-propargylglycine (PAG) and β-cyanoalanine, or administration of H₂S gas or H₂S donors such as sodium disulfide (Na₂S) and sodium hydrogen sulfide (NaHS). While high (high micromolar to millimolar) levels are invariably accompanied with cytotoxic effects 34 – which result from free radical generation, glutathione deletion, intracellular iron release and pro-apoptotic action through both the death receptor and mitochondrial pathways 35 – lower (low micromolar) levels have been shown to exert either cytoprotective (antinecrotic or antiapoptotic) effects 1011121336 or proapoptotic properties 373839, depending on the cell type and on the experimental conditions.

Cytochrome c oxidase, a component of the oxidative phosphorylation machinery within the mitochondrium, is one intracellular target of H₂S 45. Both the toxic effects of H₂S as well as the induction of a so-called "suspended animation" 4041 are referred to in this inhibition of mitochondrial respiration 4243, and thus may represent a possible mechanism for the regulation of cellular oxygen consumption 44.

Activation of potassium-dependent ATP channels is another major mechanism of H₂S, which in turn causes vasodilation, preconditioning against ischemia/reperfusion injury and myocardial protection 45. Various findings support this concept 1646: potassium-dependent ATP channel blockers (sulfonylurea derivates – for example, glibenclamide) attenuated the H₂S-induced vasodilation both in vivo and in vitro 4748, and stimulation of potassium-dependent ATP channels was demonstrated in the myocardium, pancreatic β cells, neurons and the carotid sinus 6. Moreover, glibenclamide reversed the otherwise marked Na₂S-related increase of the hepatic arterial buffer response capacity that counteracts reduction of portal venous flow, whereas PAG decreased this compensatory mechanism 49.

An endothelium-dependent effect seems to contribute to these vasodilatory properties: in human endothelial cells, H₂S caused direct inhibition of the angiotensin-converting enzyme 50, and, finally, H₂S can enhance the vasorelaxation induced by NO 5152. The interaction between H₂S and NO with respect to vascular actions is, however, fairly complex: low H₂S concentrations may cause vasoconstriction as a result of an attenuated vasorelaxant effect of NO due to scavenging of endothelial NO and formation of an inactive nitrosothiol 525354. The local oxygen concentration apparently assumes importance for the vasomotor properties of H₂S as well 55: while H₂S had vasodilator properties at 40 μM oxygen concentration (that is, an oxygen partial pressure of approximately 30 mmHg), it exerted vaso-constrictor effects at a 200 μM oxygen concentration (that is, aan oxygen partial pressure of approximately 150 mmHg) 56. Finally, the H₂S-related inhibition of oxidative phosphorylation also contributes to the vasodilatation 57.

Owing to its SH group that allows reduction of disulfide bonds and radical scavenging, H₂S also exerts biological effects as an antioxidant 9, in particular as an endogenous peroxynitrite scavenger 58, which is consistent with its cytoprotective effects in various cell-based experiments 5960. In this context the effect of H₂S on intracellular signal pathways assumes particular importance: in LPS-stimulated macrophages, pretreatment with physically dissolved gaseous H₂S or the H₂S-donor NaHS was affiliated with diminished activation of the nuclear transcription factor NF-κB and inhibition of the inducible isoform of the NO synthase. This effect coincided with increased expression of heme oxygenase-1, and co-incubation with carbon monoxide mimicked the cytoprotection exerted by H₂S 61.

Conflicting data are available on the effects of H₂S on other intracellular signal transduction pathways; for example, the mitogen-activated protein kinase pathway and the phosphatidyinositol-3-kinase/Akt pathway 206162636465. Depending on the cell lines used, both inhibitory 20 and activating 366164 effects on p38 mitogen-activated protein kinase were reported, whereas H₂S seems not to affect the stress-activated protein kinase c-Jun N-terminal kinase 6165. In contrast, activation of the extracellular signal-regulated kinase 1/2 pathway has been implicated in the H₂S-related ischemic preconditioning 48, both its proinflammatory 6365 and anti-inflammatory 2061 effects, as well as in the induction of apoptosis 62. While the influence of H₂S on extracellular signal-regulated kinase seems to be rather comprehensible 25, studies exploring the effect on downstream pathways result in conflictive statements.

Jeong and colleagues reported that H₂S enhances NO production and inducible NO synthase expression by potentiating IL-1β-induced NF-κB in vascular smooth muscle cells 63, which is consistent with the H₂S-induced NF-κB activation and subsequent proinflammatory cytokine production in IFNγ-primed monocytes 65. Nevertheless, any H₂S effect on NF-κB and its transcription-regulated mediators (for example, inducible NO synthase, cytokines and apoptotic factors) may be cell-type dependent and stimulus dependent. In fact, in addition to the above-mentioned decreased NF-κB activation and inducible NO synthase expression in LPS-stimulated macrophages 61, H₂S administration also attenuated inducible NO synthase expression, NO production, as well as TNFα secretion in microglia exposed to LPS 20.

In the context of these contradictory findings, the doses of the H₂S donors administered may assume particular importance. Even the physiologically relevant concentrations 3664 might have to be reconsidered due to overestimation of basal H₂S levels: murine plasma sulfide levels are reported between 10 and 34 μM 2122, and are increased up to 20 to 65 μM after endotoxin injection 21 or cecal ligation and puncture 22. A reduction of plasma sulfide concentration from 50 μM to ~25 μM, finally, was reported in patients with coronary heart disease 1, whereas plasma sulfide levels increased from 44 to 150 μM in patients with sepsis 21. It should be noted, however, that the distinct techniques used by various groups to determine sulfide levels may account for the marked variability in the baseline values reported. The various derivatization methods, which are inherent to the analytic procedures, are likely to liberate sulfide from its bound forms so that the exact amount of free and bioavailable sulfide may be lower than frequently reported 66. In fact, Mitsuhashi and colleagues reported that the blood sulfite concentrations (that is, the product of mitochondrial sulfide oxidation) were 3.75 ± 0.88 μM only in patients with pneumonia (versus 1.23 ± 0.48 μM in healthy control individuals) 67. Infusing 2.4 and 4.8 mg/kg/hour in anesthetized and mechanically ventilated pigs over 8 hours resulted in maximum blood sulfide levels of 2.0 and 3.5 μM, respectively (baseline levels 0.5 to 1.2 μM) in our experiments 16.

Suspended animation is a hibernation-like metabolic status characterized by a marked yet reversible reduction of energy expenditure, which allows nonhibernating species to sustain environmental stress, such as extreme changes in temperature or oxygen deprivation 4168.

In landmark work, the Roth's group provided evidence that inhaled H₂S can induce such a suspended animation 4041: in awake mice, breathing 80 ppm H₂S caused a dose-dependent reduction of both the respiratory rate and the heart rate as well as of oxygen uptake and carbon dioxide production, which was ultimately associated with a drop in body core temperature to levels ~2°C above ambient temperature 40. All these effects were completely reversible after H₂S washout, and thereafter animals presented with a totally normal behavior. A follow-up study confirmed these observations, and the authors demonstrated using telemetry and echocardiography that the bradycardia-related fall in cardiac output coincided with an unchanged stroke volume and blood pressure. These physiologic effects of inhaled H₂S were present regardless of the body core temperature investigated (27°C and 35°C) 69.

It is noteworthy that anesthesia may at least partially blunt the myocardial effect of inhaled H₂S. In mechanically ventilated mice instrumented with left ventricular pressure volume conductance catheters and assigned to 100 ppm inhaled H₂S, we found that hypothermia alone (27°C) but not normothermic H₂S inhalation (38°C) decreased the cardiac output due to a fall in heart rate, whereas both the stroke volume as well as the parameters of systolic and diastolic function remained unaffected (Table 2) 70. Interestingly, inhaled H₂S in combination with hypothermia, however, was concomitant with the least stimulation of oxygen flux induced by addition of cytochrome c during state 3 respiration with combined complex I and complex II substrates (Figure 1) 71. Since stimulation by cytochrome c should not occur in intact mitochondria, this finding suggests better preservation of mitochondrial integrity under these conditions 72.

In good agreement with the concept that a controlled reduction in cellular energetic expenditure would allow maintenance of ATP homoeostasis 41 and thus of improving outcome during shock states due to preserved mitochondrial function 7374, the group of Roth and colleagues subsequently demonstrated that pretreatment with inhaled H₂S (150 ppm) for only 20 minutes markedly prolonged survival without any apparent detrimental effects for mice exposed to otherwise lethal hypoxia (5% oxygen) 75 and for rats undergoing lethal hemorrhage (60% of the calculated blood volume over 40 minutes) 8. It is noteworthy that in the latter study the protective effect was comparable when using either inhaled H₂S or a single intravenous bolus of Na₂S 75: parenteral sulfide administration has a number of practical advantages (ease of administration, no need for inhalation delivery systems, no risk of exposure to personnel, no issues related of the characteristic odor of H₂S gas) and, in particular, avoids the pulmonary irritant effects of inhaled H₂S, which can be apparent even at low inspiratory gaseous concentrations 76. Finally, it is noteworthy that hypothermia is not a prerequisite of H₂S-related cytoprotection during hemorrhage: the H₂S donor NaHS improved hemodynamics, attenuated metabolic acidosis, and reduced oxidative and nitrosative stress in rats subjected to controlled hemorrhage at a mean blood pressure of 40 mmHg (Figure 2) 9.

The clinical relevance of murine models may be questioned because, due to their large surface area/mass ratio, rodents can rapidly drop their core temperature 77. In fact, other authors failed to confirm the metabolic effect of inhaled H₂S in anesthetized and mechanically ventilated piglets (body weight ~6 kg) or in H₂S-sedated and spontaneously breathing sheep (body weight ~74 kg) exposed to up to 80 or 60 ppm H₂S, respectively 7879. These findings may be due to the dosing or timing of H₂S, and are in contrast to recent data from our own group: in anesthetized and mechanically ventilated swine (body weight ~45 kg) that underwent transient thoracic aortic balloon occlusion, infusing the intravenous H₂S donor Na₂S over 10 hours reduced the heart rate and cardiac output without affecting the stroke volume, thereby reducing oxygen uptake and carbon dioxide production and, ultimately, core temperature 16. The metabolic effect of H₂S coincided with an attenuation of the early reperfusion-related hyperlactatemia – suggesting a reduced need for anaerobic ATP generation during the ischemia period – and an improved noradrenaline responsiveness, indicating both improved heart function and vasomotor response to catecholamine stimulation 16.

Deliberate hypothermia is a cornerstone of the standard procedures to facilitate neurological recovery after cardiac arrest and to improve postoperative organ function after cardiac and transplant surgery. Consequently, several authors investigated the therapeutic potential of H₂S-induced suspended animation after ischemia–reperfusion injury – and H₂S protected the lung 14, the liver 12, the kidney (Figure 3) 1780, and, in particular, the heart 101113151862818283. H₂S administered prior to reperfusion therefore limited the infarct size and preserved left ventricular function in mice 10 and in swine 11.

While these findings were obtained without induction of hypothermia, preserved mitochondrial function documented by an increased complex I and complex II efficiency assumed major importance for the H₂S-induced cytoprotection 10. The important role of preserved mitochondrial integrity was further underscored by the fact that 5-hydroxydeconoate, which is referred to as a mitochondrial potassium-dependent ATP-channel blocker, abolished the anti-apoptotic effects of H₂S 18. Clearly, anti-inflammatory and anti-apoptotic effects also contributed to the improved postischemic myocardial function: treatment with H₂S was associated with reduced myocardial myeloperoxidase activity and an absence of the increase in the IL-1β levels (that is, attenuated tissue inflammation 1018), as well as complete inhibition of thrombin-induced leukocyte rolling, a parameter for leukocyte–endothelium interaction 10. Moreover, the ischemia–reperfusion-induced activation of p38 mitogen-activated protein kinase, of c-Jun N-terminal kinase and of NF-κB was also attenuated by H₂S 18. Finally, H₂S exerted anti-apoptotic effects as shown by reduced TUNEL staining 1011 and by expression of cleaved caspase-9 18, caspase-3 1011, poly-ADP-ribose-polymerase 11 and the cell death-inducing proto-oncogene c-fos 13.

Despite the promising data mentioned above, it is still a matter of debate whether H₂S is a metabolic mediator or a toxic gas 84 – particularly given the rather controversial findings on the immune function reported in various models of systemic inflammation. In fact, H₂S exerted both marked proinflammatory effects 1921222324252785 and anti-inflammatory effects 9101820282930. Studies using inhibitors of endogenous H₂S production such as PAG demonstrated pro-nounced proinflammatory effects of H₂S: PAG attenuated organ injury, blunted the increase of the proinflammatory cytokine and chemokine levels as well as the myeloperoxidase activity in the lung and liver, and abolished leukocyte activation and trafficking in LPS-induced endotoxemia 1921 or cecal ligation and puncture-induced sepsis 2223242586. In good agreement with these findings, the H₂S donor NaHS significantly aggravated this systemic inflammation 212223242586. Although similar results were found during caerulin-induced pancreatitis 2787, the role of H₂S during systemic inflammatory diseases is still a matter of debate. Zanardo and colleagues reported reduced leukocyte infiltration and edema formation using the air pouch and carrageenan-induced hindpaw edema model in rats injected with the H₂S donors NaHS and Na₂S 30. Moreover, in mice with acute lung injury induced by combined burn and smoke levels, inhalation, a single Na₂S bolus decreased tissue IL-1β increased IL-10 levels, and attenuated protein oxidation in the lung, which ultimately resulted in markedly prolonged survival 28.

Variable dosing and timing make it difficult to definitely conclude on the proinflammatory and/or anti-inflammatory effects of H₂S: while the median sulfide lethal dose in rats has been described to be approximately 3 mg/kg intravenously 1, studies in the literature report on doses ranging from 0.05 to 5 mg/kg. In addition, there are only a small number of reports on continuous intravenous infusion rather than bolus administration. Finally, the role of the suspended animation-related hypothermia per se remains a matter of debate. While some studies report that spontanoues hypothermia and/or control of fever may worsen the outcome 88, other authors describe decreased inflammation 89 and improved survival after inducing hypothermia in sepsis 90.

We found in anesthetized and mechanically ventilated mice undergoing sham operation for surgical instrumentation that normothermic H₂S (100 ppm) inhalation (38°C) over 5 hours and hypothermia (27°C) alone comparably attenuated the inflammatory chemokine release (monocyte chemotactic protein-1, macrophage inflammatory protein-2 and growth-related oncogen/keratinocyte-derived chemokine) in the lung tissue. While H₂S did not affect the tissue concentrations of TNFα, combining hypothermia and inhaled H₂S significantly decreased tissue IL-6 expression (Table 3) 91.

Based on the concept that multiorgan failure secondary to shock, inflammation and sepsis may actually be an adaptive hypometabolic reponse to preserve ATP homoeostasis 92 – such as has been demonstrated for the septic heart 93 – and thus represent one of the organism's strategies to survive under stress conditions, the interest of inducing a hibernation-like suspended animation with H₂S is obvious. Investigations have currently progressed most for the treatment of myocardial ischemia 94. It must be underscored, however, that only a relatively small proportion of the published studies was conducted in clinically relevant large animal models 111695, and, furthermore, that the findings reported are controversial 167879.

Moreover, several crucial issues warrant further investigation before the clinical application of this concept. First, the role of hypothermia for any suspended animation-related organ protection is well established 96, but its impact remains a matter of debate for H₂S-related organ protection. Clearly, in the rodent studies 10121828, any cytoprotective effect was apparent without a change in core body temperature, but localized metabolic effects cannot be excluded 10. In addition, the role of any H₂S-related hypothermia remains controversial in the context of systemic inflammation 88. Second, similar to the friend and foe character of NO, no definitive conclusions can be made as to whether H₂S exerts proinflammatory or anti-inflammatory properties 1685. Finally, in addition to the question of dosing and timing (for example, bolus administration versus continuous intravenous infusion), the preferred route of H₂S administration remains to be settled: while inhaling gaseous H₂S probably allows easily titrating target blood concentrations, it is well established that this method can also directly cause airway irritation 76.

While H₂S-induced suspended animation in humans to date may still be referred to as science fiction, there are ample promising preclinical data that this approach is a fascinating new therapeutic perspective for the management of shock states that merits further investigation.

H₂S: hydrogen sulfide; IFN: interferon; IL: interleukin; LPS: lipopolysaccharide; Na₂S: sodium disulfide; NaHS: sodium hydrogen sulfide; NF: nuclear factor; NO: nitric oxide; PAG: D,L-propargylglycine; TNF: tumor necrosis factor; TUNEL: terminal deoxynucleotidyltransferase-mediated dUTP nick-end labeling.

CS is an officer and stockholder of Ikaria (Seattle, WA, USA), a company involved in the commercial development of hydrogen sulfide. PR received research grants from Ikaria. FW, PA and EC declare that they have no competing interests.

This article is part of a review series on Gaseous Mediators, edited by Peter Radermacher.

Other articles in the series can be found online at http://www.ccforum.com/series/gaseous_mediators