AIM:Numerous studies have shown that H2 S serves as an acute oxygen sensor in a variety of cells. We hypothesize that H2 S also serves in extended oxygen sensing. METHODS:Here, we compare the effects of extended exposure (24-48 hours) to varying O2 tensions on H2 S and polysulphide metabolism in human embryonic kidney (HEK 293), human adenocarcinomic alveolar basal epithelial (A549), human colon cancer (HTC116), bovine pulmonary artery smooth muscle, human umbilical-derived mesenchymal stromal (stem) cells and porcine tracheal epithelium (PTE) using sulphur-specific fluorophores and fluorometry or confocal microscopy. RESULTS:All cells continuously produced H2 S in 21% O2 and H2 S production was increased at lower O2 tensions. Decreasing O2 from 21% to 10%, 5% and 1% O2 progressively increased H2 S production in HEK293 cells and this was partially inhibited by a combination of inhibitors of H2 S biosynthesis, aminooxyacetate, propargyl glycine and compound 3. Mitochondria appeared to be the source of much of this increase in HEK 293 cells. H2 S production in all other cells and PTE increased when O2 was lowered from 21% to 5% except for HTC116 cells where 1% O2 was necessary to increase H2 S, presumably reflecting the hypoxic environment in vivo. Polysulphides (H2 Sn , where n = 2-7), the key signalling metabolite of H2 S also appeared to increase in many cells although this was often masked by high endogenous polysulphide concentrations. CONCLUSION:These results show that cellular H2 S is increased during extended hypoxia and they suggest this is a continuously active O2 -sensing mechanism in a variety of cells.

Reactive sulfur species (RSS) such as H2S, HS•, H2Sn, (n = 2-7) and HS2•- are chemically similar to H2O and the reactive oxygen species (ROS) HO•, H2O2, O2•- and act on common biological effectors. RSS were present in evolution long before ROS, and because both are metabolized by catalase it has been suggested that "antioxidant" enzymes originally evolved to regulate RSS and may continue to do so today. Here we examined RSS metabolism by Cu/Zn superoxide dismutase (SOD) using amperometric electrodes for dissolved H2S, a polysulfide-specific fluorescent probe (SSP4), and mass spectrometry to identify specific polysulfides (H2S2-H2S5). H2S was concentration- and oxygen-dependently oxidized by 1μM SOD to polysulfides (mainly H2S2, and to a lesser extent H2S3 and H2S5) with an EC50 of approximately 380μM H2S. H2S concentrations > 750μM inhibited SOD oxidation (IC50 = 1.25mM) with complete inhibition when H2S > 1.75mM. Polysulfides were not metabolized by SOD. SOD oxidation preferred dissolved H2S over hydrosulfide anion (HS-), whereas HS- inhibited polysulfide production. In hypoxia, other possible electron donors such as nitrate, nitrite, sulfite, sulfate, thiosulfate and metabisulfite were ineffective. Manganese SOD also catalyzed H2S oxidation to form polysulfides, but did not metabolize polysulfides indicating common attributes of these SODs. These experiments suggest that, unlike the well-known SOD-mediated dismutation of two O2•- to form H2O2 and O2, SOD catalyzes a reaction using H2S and O2 to form persulfide. These can then combine in various ways to form polysulfides and sulfur oxides. It is also possible that H2S (or polysulfides) interact/react with SOD cysteines to affect catalytic activity or to directly contribute to sulfide metabolism. Our studies suggest that H2S metabolism by SOD may have been an ancient mechanism to detoxify sulfide or to regulate RSS and along with catalase may continue to do so in contemporary organisms.

The discovery of hydrogen sulfide (H 2 S) as an important biological signalling molecule has resulted in the elucidation of many complex biochemical and physiological functions of the molecule. One of its key physiological functions is the modulation of vascular tone. Our lab has shown potent effects of hypoxia on blood vessels from a wide range of craniates that is mimicked almost exactly by the application of exogenous H 2 S. Based on these data, we propose that H 2 S is a key transducer of hypoxia signalling and may be the elusive “oxygen sensor”. However, we hypothesise that some of the responses attributed to H 2 S may in fact be due to polysulfides (S n ) which form spontaneously upon dissolution. We hypothesised that vascular responses to H 2 S and polysulfides would mimic hypoxia responses in vascular smooth muscle. In the current study, we aimed to test this hypothesis by examining the effects of H 2 S, the mitochondrial H 2 S donor AP39 and polysulfides on isolated bovine pulmonary arteries and porcine coronary arteries. Paired vascular segments (with or without endothelium) were mounted inside Radnotti organ baths filled with Krebs‐Hanseleit Ringer (21% O 2 /5% CO 2 ; 37°C). Vascular responses were recorded in response to hypoxia (1%) or cumulative doses of H 2 S (Na 2 S), K 2 Sn, H 2 S 2 , H 2 S 3 , H 2 S 4 and AP39. Hypoxia caused a rapid and potent two‐phase constriction (50% of max) followed by dilation (25% of Max) of bovine pulmonary arteries. In contrast, hypoxia caused a small transient constriction (10% of max) followed by sustained dilation (30% of max) of porcine coronary arteries. Consistent with the effects of exogenous H 2 S application, K 2 S n , H 2 S 2 , H 2 S 3 , H 2 S 4 caused a constriction‐dilation in bovine pulmonary arteries of a similar magnitude to hypoxia responses. In contrast, AP39 caused a potent dilation only of pulmonary arteries. Application of all sulfides dilated porcine coronary arteries in a similar manner to hypoxia with responses also of a similar magnitude. Removal of the endothelium did not affect vascular responses in any preparation (P>0.05, F‐test). The current data support the hypothesis that H 2 S, and possibly polysulfides, act as an “oxygen‐sensor” in oxygen‐sensitive vessels since the physiological responses observed to hypoxia were mimicked by sulfide treatments. The data also suggest that polysulfides may be responsible for vasoactivity in response to exogenous H 2 S application. Future work will elucidate intracellular signalling events common to hypoxia and sulfide treatments on exposure to both H 2 S and polysulfides.

Catalase is well-known as an antioxidant dismutating H2O2 to O-2 and H2O. However, catalases evolved when metabolism was largely sulfur-based, long before O2 and reactive oxygen species (ROS) became abundant, suggesting catalase metabolizes reactive sulfide species (RSS). Here we examine catalase metabolism of H2Sn, the sulfur analog of H2O2, hydrogen sulfide (H2S) and other sulfur-bearing molecules using H2S-specific amperometric electrodes and fluorophores to measure polysulfides (H2Sn; SSP4) and ROS (dichlorofluorescein, DCF). Catalase eliminated H2Sn, but did not anaerobically generate H2S, the expected product of dismutation. Instead, catalase concentration-and oxygen-dependently metabolized H2S and in so doing acted as a sulfide oxidase with a P50 of 20 mmHg. H2O2 had little effect on catalase-mediated H2S metabolism but in the presence of the catalase inhibitor, sodium azide (Az), H2O2 rapidly and efficiently expedited H2S metabolism in both normoxia and hypoxia suggesting H2O2 is an effective electron acceptor in this reaction. Unexpectedly, catalase concentration-dependently generated H2S from dithiothreitol (DTT) in both normoxia and hypoxia, concomitantly oxidizing H2S in the presence of O-2. H2S production from DTT was inhibited by carbon monoxide and augmented by NADPH suggesting that catalase heme-iron is the catalytic site and that NADPH provides reducing equivalents. Catalase also generated H2S from garlic oil, diallyltrisulfide, thioredoxin and sulfur dioxide, but not from sulfite, metabisulfite, carbonyl sulfide, cysteine, cystine, glutathione or oxidized glutathione. Oxidase activity was also present in catalase from Aspergillus niger. These results show that catalase can act as either a sulfide oxidase or sulfur reductase and they suggest that these activities likely played a prominent role in sulfur metabolism during evolution and may continue do so in modern cells as well. This also appears to be the first observation of catalase reductase activity independent of peroxide dismutation.

Hydrogen sulfide (H2S) is a novel signaling molecule most recently found to be of fundamental importance in cellular function as a regulator of apoptosis, inflammation, and perfusion. Mechanisms of endogenous H2S signaling are poorly understood; however, signal transmission is thought to occur via persulfidation at reactive cysteine residues on proteins. Although much has been discovered about how H2S is synthesized in the body, less is known about how it is metabolized. Recent studies have discovered a multitude of different targets for H2S therapy, including those related to protein modification, intracellular signaling, and ion channel depolarization. The most difficult part of studying hydrogen sulfide has been finding a way to accurately and reproducibly measure it. The purpose of this review is to: elaborate on the biosynthesis and catabolism of H2S in the human body, review current knowledge of the mechanisms of action of this gas in relation to ischemic injury, define strategies for physiological measurement of H2S in biological systems, and review potential novel therapies that use H2S for treatment.

Hydrogen sulfide (H 2 S) has emerged as an important regulator of physiological processes in multiple organ systems. One of the key functions of H 2 S is the regulation of vasoactivity and consequently blood‐flow. Our lab has shown potent effects of H 2 S and other sulfides on the vasculature of numerous vertebrate species, including vasoconstriction and vasodilation of both systemic and pulmonary arteries. Remarkably we have also shown that these effects are mimicked by hypoxia in every vertebrate class, a response which appears to be endothelium independent. Based on these observations, we hypothesised that changes in gene transcription of H 2 S production and degradation pathway genes by H 2 S and polysulfides would mimic hypoxia responses in vascular smooth muscle. In the current study we aimed to determine the effects of hypoxia and sulfides on H 2 S production ( cse, cth, mpst, got1 and got2 ) and degradation ( sqrdl, suox, tst, cdo1, tstd1 and ethe1 ) pathway genes in vascular smooth muscle using RT‐qPCR. Briefly, cultured primary bovine pulmonary artery smooth muscle cells (BPASMC) were grown until 70% confluent (passage 5) and exposed to either normoxia (21%O 2 /5%CO 2 ) or hypoxia (1%O 2 /5%CO 2 ) for 12 h with or without supplemental cysteine or sulfides (H 2 S, K 2 Sn, H 2 S 2 , H 2 S 4 ) and the cells harvested for molecular analysis. Changes in gene expression in response to hypoxia were mimicked by H 2 S and polysulfides. In general, the presence of cysteine potentiated the responses to hypoxia. In the presence of cysteine, hypoxia caused a downregulation of mRNA transcript abundance of cbs and cth (P<0.001 ANOVA). In a similar way, H 2 S and H 2 S 2 treatments resulted in a downregulation of cbs . In contrast, cth and mpst were unaffected by any treatment, although the intermediaries got1 and got2 were downregulated in both hypoxia and sulfide treated BPASMC. Degradation pathway genes ( sqrdl, suox, tst ) were downregulated in response to both hypoxia and sulfides, while cdo1 was upregulated in cysteine treatments (P<0.05 ANOVA). The housekeeping gene rn28s was not affected by any treatment. We conclude that the current data support our hypothesis that hypoxia and sulfides initiate gene transcription responses which mimic each other. Future work will include examining changes in H 2 S production and degradation pathway gene expression over an acute time‐course during hypoxia.

Fluorescence spectroscopy and microscopy have been used extensively to monitor biomolecules, especially reactive oxygen species (ROS) and, more recently, reactive sulfide (RSS) species. Nearly all fluorophores are either excited by or emit light between 450 and 550 nm, which is similar to the absorbance of heme proteins and metal-centered porphyrins. Here we examined the effects of catalase (Cat), reduced and oxidized hemoglobin (Hb and metHb), albumin (alb), manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP), iron protoporphyrin IX (hemin), and copper protoporphyrin IX (CuPPIX) on the fluorescence properties of fluorescein. We also examined the effects of catalase and MnTBAP on fluorophores for ROS (dichlorofluorescein, DCF), polysulfides (3',6'-di(O-thiosalicyl) fluorescein, SSP4), and H2S (7-azido-4-methylcoumarin, AzMC) previously activated by H2O2, a mixed polysulfide (H2Sn, n = 1-7) and H2S, respectively. All except albumin concentration dependently inhibited fluorophore fluorescence and absorbed light between 450 and 550 nm, suggesting that the inhibitory effect was physical not catalytic. Catalase inhibition of fluorescein fluorescence was unaffected by sodium azide, dithiothreitol, diamide, tris(2-carboxyethyl) phosphine (TCEP), or iodoacetate, supporting a physical inhibitory mechanism. Catalase and TBAP augmented, then inhibited DCF fluorescence, but only inhibited SSP4 and AzMC fluorescence indicative of a substrate-specific catalytic oxidation of DCF and nonspecific fluorescence inhibition of all three fluorophores. These results suggest caution must be exercised when using any fluorescent tracers in the vicinity of metal-centered porphyrins.

The mechanism(s) by which cells detect hypoxia and transduce this into appropriate homeostatic responses is a contentious issue. Vascular oxygen sensing is generally attributed to hypoxic generation of reactive oxygen species (ROS). However, we recently found that methods commonly employed to measure ROS cannot distinguish between ROS and reactive sulfur species (RSS) and that RSS and ROS have similar effects on intracellular redox status. Our previous studies suggest that metabolism of hydrogen sulfide (H2S) is the O2 sensor, as H2S mimics hypoxia in numerous systems and H2S metabolism is inversely coupled to O2 availability at physiological levels. In the present study we measure intracellular production of H2S and H2Sn using 7‐Azido‐4‐Methylcoumarin (AzMC) and 3,6‐DI(O‐Thiosalicyl) (SSP4), respectively in HEK‐293 cells and bovine pulmonary artery smooth muscle cells (BPASMC) under both normoxic (21% O2) and hypoxic (0.5% O2) conditions over a 48 hr period. In addition, we attempted to augment H2S and H2Sn production via the known intracellular pathways through addition of substrates (to increase production) and inhibitors (to decrease production). We found that hypoxia increased AzMC fluorescence by 50%, but did not affect SSP4 fluorescence. In normoxia, AzMC fluorescence did not change, whereas SSP4 fluorescence increased two‐fold. These results suggest that hypoxia increases intracellular H2S concentration and concomitantly prevents further the increase in H2Sn concentration. Intracellular H2S and H2Sn was not affected by either cysteine or cystine (10 and 100 uM). Both propargylglycine (10 mM), an inhibitor of cystathionine ã‐lyase and aspartate (1 mM) an inhibitor of cysteine aminotransferase reduced hypoxia stimulated AzMC fluorescence in HEK‐293 cells. Aminooxyacetate, an inhibitor of cystathionione â‐synthase, greatly increased AzMC fluorescence in HEK‐293 cells in normoxia and hypoxia. These are the first measurements of oxygen‐dependent changes in intracellular H2S and H2Sn in living cells and they support our hypothesis that RSS are directly coupled to O2 sensing.Support or Funding InformationNSF Grant IOS‐144‐6310 (KRO), NSF GRFP DGE‐131‐3583 (ERD)

Stepwise one-electron reduction of oxygen to water produces reactive oxygen species (ROS) that are chemically and biochemically similar to reactive sulfide species (RSS) derived from one-electron oxidations of hydrogen sulfide to elemental sulfur. Both ROS and RSS are endogenously generated and signal via protein thiols. Given the similarities between ROS and RSS, we wondered whether extant methods for measuring the former would also detect the latter. Here, we compared ROS to RSS sensitivity of five common ROS methods: redox-sensitive green fluorescent protein (roGFP), 2', 7'-dihydrodichlorofluorescein, MitoSox Red, Amplex Red, and amperometric electrodes. All methods detected RSS and were as, or more, sensitive to RSS than to ROS. roGFP, arguably the "gold standard" for ROS measurement, was more than 200-fold more sensitive to the mixed polysulfide H2Sn (n = 1-8) than to H2O2. These findings suggest that RSS may be far more prevalent in intracellular signaling than previously appreciated and that the contribution of ROS may be overestimated. This conclusion is further supported by the observation that estimated daily sulfur metabolism and ROS production are approximately equal and the fact that both RSS and antioxidant mechanisms have been present since the origin of life, nearly 4 billion years ago, long before the rise in environmental oxygen 600 million years ago. Although ROS are assumed to be the most biologically relevant oxidants, our results question this paradigm. We also anticipate our findings will direct attention toward development of novel and clinically relevant anti-(RSS)-oxidants.

The health benefits of garlic and other organosulfur-containing foods are well recognized and have been attributed to both prooxidant and antioxidant activities. The effects of garlic are surprisingly similar to those of hydrogen sulfide (H2S), which is also known to be released from garlic under certain conditions. However, recent evidence suggests that polysulfides, not H2S, may be the actual mediator of physiological signaling. In this study, we monitored formation of H2S and polysulfides from garlic oil in buffer and in human embryonic kidney (HEK) 293 cells with fluorescent dyes, 7-azido-4-methylcoumarin and SSP4, respectively and redox activity with two redox indicators redox-sensitive green fluorescent protein (roGFP) and DCF. Our results show that H2S release from garlic oil in buffer requires other low-molecular-weight thiols, such as cysteine (Cys) or glutathione (GSH), whereas polysulfides are readily detected in garlic oil alone. Administration of garlic oil to cells rapidly increases intracellular polysulfide but has minimal effects on H2S unless Cys or GSH are also present in the extracellular medium. We also observed that garlic oil and diallyltrisulfide (DATS) potently oxidized roGFP in buffer but did not affect DCF. This appears to be a direct polysulfide-mediated oxidation that does not require a reactive oxygen species intermediate. Conversely, when applied to cells, garlic oil became a significant intracellular reductant independent of extracellular Cys or GSH. This suggests that intracellular metabolism and further processing of the sulfur moieties are necessary to confer antioxidant properties to garlic oil in vivo.

In lung epithelial cells, hypoxia decreases the expression and activity of sodium-transporting molecules, thereby reducing the rate of transepithelial sodium absorption. The mechanisms underlying the sensing of hypoxia and subsequent coupling to sodium-transporting molecules remain unclear. Hydrogen sulfide (H2S) has recently been recognized as a cellular signaling molecule whose intracellular concentrations critically depend on oxygen levels. Therefore, it was questioned whether endogenously produced H2S contributes to hypoxic inhibition of sodium transport. In electrophysiological Ussing chamber experiments, hypoxia was established by decreasing oxygen concentrations in the chambers. Hypoxia concentration dependently and reversibly decreased amiloride-sensitive sodium absorption by cultured H441 monolayers and freshly dissected porcine tracheal epithelia due to inhibition of basolateral Na+/K+-ATPase. Exogenous application of H2S by the sulfur salt Na2S mimicked the effect of hypoxia and inhibited amiloride-sensitive sodium absorption by both tissues in an oxygen-dependent manner. Hypoxia increased intracellular concentrations of H2S and decreased the concentration of polysulfides. Pretreatment with the cystathionine-γ-lyase inhibitor d/l-propargylglycine (PAG) decreased hypoxic inhibition of sodium transport by H441 monolayers, whereas inhibition of cystathionine-β-synthase (with aminooxy-acetic acid; AOAA) or 3-mercaptopyruvate sulfurtransferase (with aspartate) had no effect. Inhibition of all of these H2S-generating enzymes with a combination of AOAA, PAG, and aspartate decreased the hypoxic inhibition of sodium transport by H441 cells and pig tracheae and decreased H2S production by tracheae. These data suggest that airway epithelial cells endogenously produce H2S during hypoxia, and this contributes to hypoxic inhibition of transepithelial sodium absorption.

Biologically relevant gaseous signaling molecules, O2, NO, H2O2 and H2S are often quantified with amperometric sensors (AS). Although AS are developed with reference to specificity regarding similar chemical moieties, rarely are they examined for cross-sensitivity to dissimilar species. Here, we examine the specificity of three commercial AS (O2, NO, H2O2) and an H2S AS built in-house for these signaling molecules using known amounts of premixed O2, NO [via S-Nitroso-N- acetylpenicillamine (SNAP)], and reagent grade H2O2 and Na2S using concentrations commonly cited in literature. We found that the H2S and O2 AS were sensitive only to their respective gases, whereas the H2O2 AS responded positively to both H2S and H2O2, and the NO AS responded to all of the chemicals tested. The H2O2 AS was 25 fold more sensitive to H2S than H2O2, the NO AS was 15 fold more sensitive to H2S than NO and 3 fold more sensitive to H2O2 than NO. Furthermore, the order of addition of these particular compounds altered the responses of each AS. The H2O2 AS can be used on complex samples, but only in conjunction with an H2S AS, as determination of NO and H2O2 requires significant back-calculation; and even then results are only approximate. Conclusions O2 and H2S AS are quite specific whereas there is substantial cross-sensitivity when using NO and H2O2 sensors. This can become especially problematic in tissues where multiple gaseous signaling molecules are generally present. Support: NSF DGE-131-3583 (ERD), IOS- 105-1627 (KRO), IOS- 144-6310 (KRO).

Hydrogen sulfide (H2S) signaling has been implicated in physiological processes in practically all organ systems studied to date. At times the excitement of this new field has outpaced the technical expertise or practical knowledge with which to accurately assess these advancements. Recently, the myriad of proposed H2S actions has spawned interest in using indicators of H2S metabolism, especially plasma H2S concentrations, as a means of identifying a variety of pathophysiological conditions or to predict clinical outcomes. While this is a noteworthy endeavor, there are a number of contraindications to this practice at this time. First, there is little consensus regarding normal, i.e., "physiological" concentrations of H2S in either plasma or tissue. In fact, it has been shown that the methods most often employed for these measurements are associated with substantial artifact. Second, interactions, or presumed lack thereof, of H2S with other biomolecules (e.g., O2, H2O2, pH, etc.) or analytical reagents (e.g., reducing reagents, N-ethylmaleimide, phenylarsine, etc.) are often assumed but not evaluated. Third, the experimental design and/or statistical analyses may not be sufficient to justify using H2S concentration in tissue or blood as a predictive biomarker of pathophysiology. In this study, we first briefly review the problems associated with plasma and tissue H2S measurements and the associated errors and we provide some simple methods to evaluate whether the data obtained is physiologically relevant. Second we provide a brief analysis of H2S interactions with the above biomolecules. Third, we provide a statistical tool with which to determine the clinical applicability of H2S measurements. It is hoped that these points will provide a rational background for future work.

Hydrogen sulfide (H2S) is an important signaling molecule, yet it has been technically difficult to measure H2S production, metabolism or distribution at the cellular and sub‐cellular level in tissue. As H2S has been implicated in hypoxic signaling, we hypothesized that H2S produced during an induced inflammatory response would be augmented under hypoxic conditions. Intracellular H2S production was monitored via silicate nanoparticles, PEBBLEs (Photonic Explorers for Bioanalysis with Biologically Localized Embedding). PEBBLES were synthesized with a hydrophilic core that traps fluorescein mercuric acetate (FMA), an analyte‐sensitive dye that becomes irreversibly quenched when bound to sulfur moieties, and a hydrophobic shell permeable to small lipophilic molecules, such as H2S. FMA‐loaded PEBBLES are thus able to provide a historical account of intracellular H2S production. Murine bone marrow‐derived macrophages (BMDM) avidly ingest PEBBLES (BMDM+P) and remain viable as determined by confocal microscopy for an extended period (72 h). Cells incubated in normoxic conditions and treated with Lipopolysaccharide (LPS) showed a slight decrease in fluorescence intensity above the control over 48hrs, which could be blocked by the pre‐treatment with aminooxyacetic acid (AOA) and proparglycine (PPG). However, cells incubated in hypoxic conditions showed much larger decreases in fluorescence intensity, particularly when pre‐treated with LPS, and were less affected by the inhibitors. This supports the hypothesis that LPS stimulates H2S production in BMDM and that this is considerably augmented by hypoxia.Grant Funding Source: Supported by NSF DGE‐1313583 (ERD) and NSF IOS‐1051627 (KRO)

H2S derived from organic thiol metabolism has been proposed serve as an oxygen sensor in a variety of systems because of its susceptibility to oxidation and its ability to mimic hypoxic responses in numerous oxygen-sensing tissues. Thiosulfate, an intermediate in oxidative H2S metabolism can alternatively be reduced and regenerate H2S. We propose that this contributes to the H2S-mediated oxygen-sensing mechanism. H2S formation from thiosulfate in buffers and in a variety of mammalian tissues and in lamprey dorsal aorta was examined in real time using a polarographic H2S sensor. Inferences of intracellular H2S production were made by examining hypoxic pulmonary vasoconstriction (HPV) in bovine pulmonary arteries under conditions in which increased H2S production would be expected and in mouse and rat aortas, where reducing conditions should mediate vasorelaxation. In Krebs-Henseleit (mammalian) and Cortland (lamprey) buffers, H2S was generated from thiosulfate in the presence of the exogenous reducing agent, DTT, or the endogenous reductant dihydrolipoic acid (DHLA). Both the magnitude and rate of H2S production were greatly increased by these reductants in the presence of tissue, with the most notable effects occurring in the liver. H2S production was only observed when tissues were hypoxic; exposure to room air, or injecting oxygen inhibited H2S production and resulted in net H2S consumption. Both DTT and DHLA augmented HPV, and DHLA dose-dependently relaxed precontracted mouse and rat aortas. These results indicate that thiosulfate can contribute to H2S signaling under hypoxic conditions and that this is not only a ready source of H2S production but also serves as a means of recycling sulfur and thereby conserving biologically relevant thiols.

Despite evidence that hydrogen sulfide (H2S) is an important signaling molecule involved in oxygen sensing, it has been technically difficult to measure H2S production, metabolism or distribution at the cellular and sub‐cellular level in living tissue. Here, we examine the potential for cellular H2S measurement using 150–200 nm diameter silicate nanoparticles (Photonic Explorers for Bioanalysis with Biologically Localized Embedding; PEBBLEs) and demonstrate its applicability. PEBBLES were synthesized with a hydrophilic core containing fluorescein mercuric acetate (FMA), an analyte‐sensitive dye that becomes irreversibly quenched when bound to sulfur moieties, and a hydrophobic shell that is only permeable toH2S. PEBBLES can be delivered into cells via a gene gun, or as in the present experiments, ingested by mouse bone marrow‐derived macrophages (BMDM+P). BMDM+P were exposed to a range of H2S concentrations (10 nM‐10 mM H2S as Na2S) or exposed to hypoxia for 1 h and examined by flow cytometry and confocal microscopy. Increasing H2S concentration dose‐dependently quenched FMA and exposure to 1 nM H2S was readily resolved. Furthermore, hypoxia increased intracellular H2S production. This is the first demonstration of intracellular H2S production in any tissue and clearly shows that during hypoxia intracellular H2S concentration increases. This supports our hypothesis that oxygen sensing is the balance between intrinsic intracellular H2S production and its O2‐dependent metabolism and that this is the key couple in oxygen sensing. It also affirms the validity of this method in H2S research. Support: NSF IOS 1051627

We present work looking at the two-photon optical characterization of water soluble chemical probes based on surfactant nanomicelles encapsulation and ormosil PEBBLEs as well as in-vitro two-photon imaging of oxygen and hydrogen sulfide.

Hydrogen sulfide (H2S) is a volatile gas of considerable interest as a physiologically relevant signaling molecule, but this volatility has typically been overlooked in the context of biological experiments. We examined volatility of 10 and 100μM H2S (Na2S·9H2O) in real time with polarographic electrodes in three commonly employed experimental apparatuses: 24-well tissue culture plates (WP), muscle myograph baths (MB), and the Langendorff perfused heart apparatus (LPH). H2S loss from all apparatuses was rapid and exponential, with half-times (t1/2) of 5min (WP), less than 4min (MB), and less than 0.5min (LPH). The t1/2 for H2S loss from MB bubbled with 100% oxygen was slightly longer than that for MB bubbled with 100% nitrogen; both were significantly shorter than stirred but unbubbled MB (>9min). Therefore, even without tissue, H2S rapidly disappears from buffer under a variety of experimental conditions, and this is due to volatilization, not oxidation. The inability to maintain H2S concentration, even briefly, questions the accuracy of dose–response studies and the relevance of long-term (>10min) exposure to a single treatment of H2S. These results also help to explain the discrepancy between low H2S concentrations in blood and tissues versus high concentrations of exogenous H2S required to produce physiological responses.