Hydrogen sulfide and oxygen sensing: implications in cardiorespiratory control

Exogenous hydrogen sulfide attenuates hyperoxia effects on neonatal mouse airways

Supplemental O2 remains a necessary intervention for many premature infants (<34 wk gestation). Even moderate hyperoxia (<60% O2) poses a risk for subsequent airway disease, thereby predisposing premature infants to pediatric asthma involving chronic inflammation, airway hyperresponsiveness (AHR), airway remodeling, and airflow obstruction. Moderate hyperoxia promotes AHR via effects on airway smooth muscle (ASM), a cell type that also contributes to impaired bronchodilation and remodeling (proliferation, altered extracellular matrix). Understanding mechanisms by which O2 initiates long-term airway changes in prematurity is critical for therapeutic advancements for wheezing disorders and asthma in babies and children. Immature or dysfunctional antioxidant systems in the underdeveloped lungs of premature infants thereby heightens susceptibility to oxidative stress from O2. The novel gasotransmitter hydrogen sulfide (H2S) is involved in antioxidant defense and has vasodilatory effects with oxidative stress. We previously showed that exogenous H2S exhibits bronchodilatory effects in human developing airway in the context of hyperoxia exposure. Here, we proposed that exogenous H2S would attenuate effects of O2 on airway contractility, thickness, and remodeling in mice exposed to hyperoxia during the neonatal period. Using functional [flexiVent; precision-cut lung slices (PCLS)] and structural (histology; immunofluorescence) analyses, we show that H2S donors mitigate the effects of O2 on developing airway structure and function, with moderate O2 and H2S effects on developing mouse airways showing a sex difference. Our study demonstrates the potential applicability of low-dose H2S toward alleviating the detrimental effects of hyperoxia on the premature lung.NEW & NOTEWORTHY Chronic airway disease is a short- and long-term consequence of premature birth. Understanding effects of O2 exposure during the perinatal period is key to identify targetable mechanisms that initiate and sustain adverse airway changes. Our findings show a beneficial effect of exogenous H2S on developing mouse airway structure and function with notable sex differences. H2S donors alleviate effects of O2 on airway hyperreactivity, contractility, airway smooth muscle thickness, and extracellular matrix deposition.

Cyano Derivatives of 7-Aminoquinoline That Are Highly Emissive in Water: Potential for Sensing Applications

6-Cyano-7-aminoquinoline (6CN-7AQ) and 3-cyano-7-aminoquinoline (3CN-7AQ) were synthesized and found to exhibit intense emission with quantum yield as high as 63 % and 85 %, respectively, in water. Conversely, their derivatives 6-cyano-7-azidoquinoline (6CN-7N₃ Q) and 3-cyano-7-azidoquinoline (3CN-7N₃ Q) show virtually no emission, which makes them suitable to be used as recognition agents in azide reactions based on fluorescence recovery. Moreover, conjugation of 6CN-7AQ with a hydrophobic biomembrane-penetration peptide PFVYLI renders a nearly non-emissive 6CN-7AQ-PFVYLI composite, which can be digested by proteinase K, recovering the highly emissive 6CN-7AQ with ∼200-fold enhancement. The result provides an effective early confirmation for RT-qPCR in viral detection.

Hydrogen sulphide sensitivity and tolerance in genetically distinct lineages of a selfing mangrove fish (Kryptolebias marmoratus)

Mangroves are critical marine habitats. High hydrogen sulphide (H2S) is a feature of these important ecosystems and its toxicity creates a challenge for mangrove inhabitants. The mangrove rivulus (Kryptolebias marmoratus) is a selfing, hermaphroditic, amphibious fish that can survive exposure to 1116 μM H2S in the wild. These fish rely on cutaneous respiration for gas and ion exchange when emerged. We hypothesized that the skin surface is fundamentally important in H2S tolerance in these mangrove fish by limiting H2S permeability. To test our hypothesis, we first disrupted the skin surface in one isogenic lineage and measured H2S tolerance and sensitivity. We increased water H2S concentration until emersion as a measure of the ability to sense and react to H2S, which we refer to as sensitivity. We then determined H2S tolerance by preventing emersion and increasing H2S until loss of equilibrium (LOE). The H2S concentration at emersion and LOE were significantly affected by disrupting the skin surface, providing support that the skin is involved in limiting H2S permeability. Capitalizing on their unique reproductive strategy, we used three distinct isogenic lineages to test the hypothesis that there would be genetic differences in H2S sensitivity and tolerance. We found significant differences in emersion concentration only among lineages, suggesting a genetic component to H2S sensitivity but not tolerance. Our study also demonstrated that external skin modifications and avoidance behaviours are two distinct strategies used to tolerate ecologically relevant H2S concentrations and likely facilitate survival in challenging mangrove habitats.

H2S-producing enzymes in anoxia-tolerant vertebrates: Effects of cold acclimation, anoxia exposure and reoxygenation on gene and protein expression

To lend insight into the potential role of the gasotransmitter hydrogen sulfide (H₂S) in facilitating anoxia survival of anoxia-tolerant vertebrates, we quantified the gene expression of the primary H₂S-synthesizing enzymes, 3-mercaptopyruvate sulfurtransferase (3MST), cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), in ventricle and brain of normoxic, anoxic and reoxygenated 21 °C- and 5 °C-acclimated freshwater turtles (Trachemys scripta) and 10 °C-acclimated crucian carp (Carassius carassius). Semi-quantitative Western blotting analysis was also conducted to assess 3MST and CBS protein abundance in ventricle and brain of 5 °C turtles and 10 °C crucian carp subjected to normoxia, anoxia and reoxygenation. We hypothesized that if H₂S was advantageous for anoxia survival, expression levels would remain unchanged or be upregulated with anoxia and/or reoxygenation. Indeed, for both species, gene and protein expression were largely maintained with anoxia exposure (24 h, 21 °C; 5 d, 10 °C; 14 d, 5 °C). With reoxygenation, 3MST expression was increased in turtle and crucian carp brain at the protein and gene level, respectively. Additionally, the effect of cold acclimation on gene expression was assessed in several tissues of the turtle. Expression levels were maintained in most tissues, but decreased in others. The maintenance of gene and protein expression of the H₂S-producing enzymes with anoxia exposure and the up-regulation of 3MST with reoxygenation suggests that H₂S may facilitate anoxic survival of the two champions of vertebrate anoxia survival. The differential effects of cold acclimation on H₂S enzyme expression may influence blood flow to different tissues during winter anoxia.

Mangrove Fishes Rely on Emersion Behavior and Physiological Tolerance to Persist in Sulfidic Environments

Hydrogen sulfide (H ₂ S) is a potent respiratory toxin that makes sulfidic environments tolerable to only a few organisms. We report the presence of fishes ( Kryptolebias marmoratus , Poecilia orri , Gambusia sp., and Dormitator maculatus ) in Belizean mangrove pools with extremely high H ₂ S concentrations (up to 1,166 μM) that would be lethal for most fishes. Thus, we asked whether the three most prevalent species ( Kryptolebias , Poecilia , and Gambusia ) persist in sulfidic pools because they are exceptionally H ₂ S tolerant and/or because they can leave water (emerse) and completely avoid H ₂ S. We show that both physiological tolerance and emersion behavior are important. Kryptolebias demonstrated high H ₂ S tolerance, as they lost equilibrium significantly later than Poecilia and Gambusia during H ₂ S exposure ( 1,188 ± 21 μM H ₂ S). However, the fact that all species lost equilibrium at an ecologically relevant [H ₂ S] suggests that physiological tolerance may suffice at moderate H ₂ S concentrations but that another strategy is required to endure higher concentrations. In support of the avoidance behavior hypothesis, H ₂ S elicited an emersion response in all species. Kryptolebias was most sensitive to H ₂ S and emersed at H ₂ S concentrations 52% and 34% lower than Poecilia and Gambusia , respectively. Moreover, H ₂ S exposure caused Kryptolebias to emerse more frequently and spend more time out of water compared to control conditions. We suggest that physiological H ₂ S tolerance and emersion behavior are complementary strategies. The superior H ₂ S tolerance and amphibious capability of Kryptolebias may explain why this species was more prevalent in H ₂ S-rich environments than other local fishes.

Synthesis, Metabolism, and Signaling Mechanisms of Hydrogen Sulfide: An Overview

In addition to nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H₂S) has recently emerged as the novel gasotransmitter involved in the regulation of the nervous system, cardiovascular functions, inflammatory response, gastrointestinal system, and renal function. H₂S is synthesized from L-cysteine and/or L-homocysteine by cystathionine β-synthase, cystathionine γ-lyase, and cysteine aminotransferase together with 3-mercaptopyruvate sulfurtransferase. In addition, H₂S is enzymatically metabolized in mitochondria by sulfide:quinone oxidoreductase, persulfide dioxygenase, and sulfite oxidase to thiosulfate, sulfite, and sulfate which enables to regulate its level by factors such as oxygen pressure, mitochondria density, or efficacy of mitochondrial electron transport. H₂S modifies protein structure and function through the so-called sulfuration or persulfidation, that is, conversion of cysteine thiol (-SH) to persulfide (-SSH) groups. This, as well as other signaling mechanisms, is partially mediated by more oxidized H₂S-derived species, polysulfides (H₂S_n). In addition, H₂S is able to react with reactive oxygen and nitrogen species to form other signaling molecules such as thionitrous acid (HSNO), nitrosopersulfide (SSNO^-), and nitroxyl (HNO). All H₂S-synthesizing enzymes are expressed in the vascular wall, and H₂S has been demonstrated to regulate vascular tone, endothelial barrier permeability, angiogenesis, vascular smooth muscle cell proliferation and apoptosis, and inflammatory reaction. H₂S-modifying therapies are promising approach for diseases such as arterial hypertension, diabetic angiopathy, and atherosclerosis.

Acute oxygen sensing by the carotid body: a rattlebag of molecular mechanisms

The molecular underpinnings of the oxygen sensitivity of the carotid body Type I cells are becoming better defined as research begins to identify potential interactions between previously separate theories. Nevertheless, the field of oxygen chemoreception still presents the general observer with a bewildering array of potential signalling pathways by which a fall in oxygen levels might initiate Type I cell activation. The purpose of this brief review is to address five of the current oxygen sensing hypotheses: the lactate-Olfr 78 hypothesis of oxygen chemotransduction; the role mitochondrial ATP and metabolism may have in chemotransduction; the AMP-activated protein kinase hypothesis and its current role in oxygen sensing by the carotid body; reactive oxygen species as key transducers in the oxygen sensing cascade; and the mechanisms by which H2 S, reactive oxygen species and haem oxygenase may integrate to provide a rapid oxygen sensing transduction system. Over the previous 15 years several lines of research into acute hypoxic chemotransduction mechanisms have focused on the integration of mitochondrial and membrane signalling. This review places an emphasis on the subplasmalemmal-mitochondrial microenvironment in Type I cells and how theories of acute oxygen sensing are increasingly dependent on functional interaction within this microenvironment.

NO, CO and H2 S: What about gasotransmitters in fish and amphibian heart?

The gasotransmitters nitric oxide (NO), carbon monoxide (CO), and hydrogen sulphide (H₂ S), long considered only toxicant, are produced in vivo during the catabolism of common biological molecules and are crucial for a large variety of physiological processes. Mounting evidence is emerging that in poikilotherm vertebrates, as in mammals, they modulate the basal performance of the heart and the response to stress challenges. In this review, we will focus on teleost fish and amphibians to highlight the evolutionary importance in vertebrates of the cardiac control elicited by NO, CO and H₂ S, and the conservation of the intracellular cascades they activate. Although many gaps are still present due to discontinuous information, we will use examples obtained by studies from our and other laboratories to illustrate the complexity of the mechanisms that, by involving gasotransmitters, allow beat-to-beat, short-, medium- and long-term cardiac homoeostasis. By presenting the latest data, we will also provide a framework in which the peculiar morpho-functional arrangement of the teleost and amphibian heart can be considered as a reference tool to decipher cardiac regulatory networks which are difficult to explore using more conventional vertebrates, such as mammals.

Hypoxia Sensing in Plants: On a Quest for Ion Channels as Putative Oxygen Sensors

Over 17 million km2 of land is affected by soil flooding every year, resulting in substantial yield losses and jeopardizing food security across the globe. A key step in resolving this problem and creating stress-tolerant cultivars is an understanding of the mechanisms by which plants sense low-oxygen stress. In this work, we review the current knowledge about the oxygen-sensing and signaling pathway in mammalian and plant systems and postulate the potential role of ion channels as putative oxygen sensors in plant roots. We first discuss the definition and requirements for the oxygen sensor and the difference between sensing and signaling. We then summarize the literature and identify several known candidates for oxygen sensing in the mammalian literature. This includes transient receptor potential (TRP) channels; K+-permeable channels (Kv, BK and TASK); Ca2+ channels (RyR and TPC); and various chemo- and reactive oxygen species (ROS)-dependent oxygen sensors. Identified key oxygen-sensing domains (PAS, GCS, GAF and PHD) in mammalian systems are used to predict the potential plant counterparts in Arabidopsis. Finally, the sequences of known mammalian ion channels with reported roles in oxygen sensing were employed to BLAST the Arabidopsis genome for the candidate genes. Several plasma membrane and tonoplast ion channels (such as TPC, AKT and KCO) and oxygen domain-containing proteins with predicted oxygen-sensing ability were identified and discussed. We propose a testable model for potential roles of ion channels in plant hypoxia sensing.

Hydrogen Sulfide Reduces Recruitment of CD11b+Gr-1+ Cells in Mice With Myocardial Infarction

The present study aimed to elucidate the mechanisms by which hydrogen sulfide (H2S) attenuates left ventricular remodeling after myocardial infarction (MI). MI was created in mice by left coronary artery ligation. One group of mice received injections of the H2S donor sodium hydrosulfide (NaHS) immediately before and 1 h after ligation, while the control group received saline alone. During both the subacute and chronic stages (1 and 4 weeks postinfarction, respectively), NaHS-treated mice demonstrated attenuation of cardiac dilation in the infarcted myocardium. Furthermore, fewer CD11b+Gr-1+ myeloid cells were detected in the infarct myocardium and peripheral blood from NaHS-treated mice, while more CD11b+Gr-1+ cells remained in the spleen and bone marrow in these animals. NaHS-treated mice also exhibited reduction in cardiomyocyte apoptosis, interstitial fibrosis, cardiac hypertrophy, and pulmonary edema, as well as overall better survival rates, when compared to controls. Thus, exogenous H2S has favorable effects on cardiac remodeling after MI. These observations further support the emerging concept that H2S treatment might have therapeutic benefits in the setting of ischemia-induced heart failure.

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.

Electrochemical hydrogen sulfide biosensors

Biological application of electrochemical hydrogen sulfide sensors.

Sensing and surviving hypoxia in vertebrates

Surviving hypoxia is one of the most critical challenges faced by vertebrates. Most species have adapted to changing levels of oxygen in their environment with specialized organs that sense hypoxia, while only few have been uniquely adapted to survive prolonged periods of anoxia. The goal of this review is to present the most recent research on oxygen sensing, adaptation to hypoxia, and mechanisms of anoxia tolerance in nonmammalian vertebrates. We discuss the respiratory structures in fish, including the skin, gills, and air-breathing organs, and recent evidence for chemosensory neuroepithelial cells (NECs) in these tissues that initiate reflex responses to hypoxia. The use of the zebrafish as a genetic and developmental model has allowed observation of the ontogenesis of respiratory and chemosensory systems, demonstration of a putative intracellular O2 sensor in chemoreceptors that may initiate transduction of the hypoxia signal, and investigation into the effects of extreme hypoxia on cardiorespiratory development. Other organisms, such as goldfish and freshwater turtles, display a high degree of anoxia tolerance, and these models are revealing important adaptations at the cellular level, such as the regulation of glutamatergic and GABAergic neurotransmission in defense of homeostasis in central neurons.

Hydrogen sulfide promotes calcium uptake in larval zebrafish

Hydrogen sulfide (H2S) can act as a signaling molecule for various ion channels and/or transporters; however, little is known about its potential involvement in Ca(2+) balance. Using developing zebrafish (Danio rerio) as an in vivo model system, the present study demonstrated that acute exposure to H2S donors increased Ca(2+) influx at 4 days postfertilization, while chronic (3-day) exposure caused a rise in whole body Ca(2+) levels. The mRNA expression of Ca(2+)-transport-related genes was unaffected by H2S exposure, suggesting that posttranscriptional modifications were responsible for the altered rates of Ca(2+) uptake. Indeed, treatment of fish with the protein kinase A inhibitor H-89 abolished the H2S-mediated stimulation of Ca(2+) influx, suggesting that H2S increased Ca(2+) influx by activating cAMP-protein kinase A pathways. Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are two key enzymes in the endogenous synthesis of H2S. Using an antisense morpholino knockdown approach, we demonstrated that Ca(2+) influx was reduced in CBS isoform b (CBSb)- but not in CSE-deficient fish. Interestingly, the reduction in Ca(2+) influx in CBSb-deficient fish was observed only in fish that were acclimated to low-Ca(2+) water (i.e., 25 μM Ca(2+); control: 250 μM Ca(2+)). Similarly, mRNA expression of cbsb but not cse was increased in fish acclimated to low-Ca(2+) water. Results from whole-mount immunohistochemistry further revealed that CBSb was expressed in Na(+)-K(+)-ATPase-rich cells, which are implicated in Ca(2+) uptake in zebrafish larvae. Collectively, the present study suggests a novel role for H2S in promoting Ca(2+) influx, particularly in a low-Ca(2+) environment.

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.

Central hydrogen sulphide mediates ventilatory responses to hypercapnia in adult conscious rats

AIM

Hydrogen sulphide (H2S) is endogenously produced and plays an important role as a modulator of neuronal functions; however, its modulatory role in the central CO2 chemoreception is unknown. The aim of the present study was to assess the role of endogenously produced H2S in the ventilatory response to hypercapnia in adult conscious rats.

METHODS

Cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) inhibitors (aminooxyacetate: AOA and propargylglycine: PAG respectively) and a H2S donor (sodium sulphide: Na2S) were microinjected into the fourth ventricle (4V). Ventilation (V̇(E)), oxygen consumption (V̇O2) and body temperature were recorded before (room air) and during a 30-min CO2 exposure (hypercapnia, 7% CO2). Endogenous H2S levels were measured in the nucleus tractus solitarius (NTS).

RESULTS

Microinjection of Na2S (H2S donor), AOA (CBS inhibitor) or PAG (CSE inhibitor) did not affect baseline of the measured variables compared to control group (vehicle). In all experimental groups, hypercapnia elicited an increase in V̇(E). However, AOA microinjection, but not PAG, attenuated the ventilatory response to hypercapnia (P < 0.05), whereas Na2S elicited a slight, not significant, enhancement. Moreover, endogenous H2S levels were found higher in the NTS after hypercapnia (P < 0.05) compared to room air (normoxia) condition.

CONCLUSION

There are a few reports on the role of gaseous transmitters in the control of breathing. Importantly, the present data suggest that endogenous H2S via the CBS-H2S pathway mediates the ventilatory response to hypercapnia playing an excitatory role.

Carbon monoxide, hydrogen sulfide, and nitric oxide as signaling molecules in the gastrointestinal tract

Carbon monoxide (CO) and hydrogen sulfide (H2S) used to be thought of simply as lethal and (for H2S) smelly gaseous molecules; now they are known to have important signaling functions in the gastrointestinal tract. CO and H2S, which are produced in the gastrointestinal tract by different enzymes, regulate smooth muscle membrane potential and tone, transmit signals from enteric nerves, and can regulate the immune system. The pathways that produce nitric oxide, H2S, and CO interact; each can inhibit and potentiate the level and activity of the other. However, there are significant differences between these molecules, such as in half-lives; CO is more stable and therefore able to have effects distal to the site of production, whereas nitric oxide and H2S are short lived and act only close to sites of production. We review their signaling functions in the luminal gastrointestinal tract and discuss how their pathways interact. We also describe other physiological functions of CO and H2S and how they might be used as therapeutic agents.

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.

The role of hydrogen sulphide in the control of breathing in hypoxic zebrafish (Danio rerio)

The current study investigated the role of hydrogen sulphide (H2S) in oxygen sensing, intracellular signalling and promotion of ventilatory responses to hypoxia in adult and larval zebrafish (Danio rerio). Both larval and adult zebrafish exhibited a dose-dependent increase in ventilation to sodium sulphide (Na2S), an H2S donor. In vertebrates, cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) are enzymes that catalyse the endogenous production of H2S. In adult zebrafish, inhibition of both CBS and CSE with aminooxyacetate (AOA) and propargyl glycine (PPG) blunted or abolished the hypoxic hyperventilation, and the addition of Na2S to the water partially rescued the effects of inhibiting endogenous H2S production. In zebrafish larvae (4 days post-fertilization), gene knockdown of either CBS or CSE using morpholinos attenuated the hypoxic ventilatory response. Furthermore, the intracellular calcium concentration of isolated neuroepithelial cells (NECs), which are putative oxygen chemoreceptors, increased significantly when these cells were exposed to 50 μm Na2S, supporting a role for H2S in Ca(2+)-evoked neurotransmitter release in these cells. Finally, immunohistochemical labelling showed that NECs dissociated from adult gill contained CBS and CSE, whereas cutaneous NECs in larval zebrafish expressed only CSE. Taken together, these data show that H2S can be produced in the putative oxygen-sensing cells of zebrafish, the NECs, in which it appears to play a pivotal role in promoting the hypoxic ventilatory response.

Regulation of cellular gas exchange, oxygen sensing, and metabolic control

Cells must continuously monitor and couple their metabolic requirements for ATP utilization with their ability to take up O2 for mitochondrial respiration. When O2 uptake and delivery move out of homeostasis, cells have elaborate and diverse sensing and response systems to compensate. In this review, we explore the biophysics of O2 and gas diffusion in the cell, how intracellular O2 is regulated, how intracellular O2 levels are sensed and how sensing systems impact mitochondrial respiration and shifts in metabolic pathways. Particular attention is paid to how O2 affects the redox state of the cell, as well as the NO, H2S, and CO concentrations. We also explore how these agents can affect various aspects of gas exchange and activate acute signaling pathways that promote survival. Two kinds of challenges to gas exchange are also discussed in detail: when insufficient O2 is available for respiration (hypoxia) and when metabolic requirements test the limits of gas exchange (exercising skeletal muscle). This review also focuses on responses to acute hypoxia in the context of the original "unifying theory of hypoxia tolerance" as expressed by Hochachka and colleagues. It includes discourse on the regulation of mitochondrial electron transport, metabolic suppression, shifts in metabolic pathways, and recruitment of cell survival pathways preventing collapse of membrane potential and nuclear apoptosis. Regarding exercise, the issues discussed relate to the O2 sensitivity of metabolic rate, O2 kinetics in exercise, and influences of available O2 on glycolysis and lactate production.

Regulation of cystathionine γ-lyase in mammalian cells by hypoxia

Hydrogen sulfide (H2S), an endogenous signaling molecule in mammalian cells, shows a variety of biological effects. Cystathionine γ-lyase (CSE) is a key enzyme in the trans-sulfuration pathway responsible for the production of endogenous H2S. Whether CSE expression is regulated by hypoxia in mammalian cells remains largely unknown. This study revealed that these regulatory effects changed with time at transcriptional and post-transcriptional levels. Hypoxia regulated CSE expression in mammalian cells in a complex manner; CSE transcription went through a down-regulation and recovery period, while CSE mRNA and protein levels increased during hypoxia. Taken together, the results suggest that CSE can respond to hypoxia through transcriptional and post-transcriptional regulation, and CSE expression can be up-regulated by hypoxia to a certain extent. Therefore, the up-regulation of CSE expression during hypoxia may be useful for increasing the production and concentration of H2S in mammalian cells and indirectly protecting cells from hypoxia.

Akt/eNOS signaling and PLN S-sulfhydration are involved in H₂S-dependent cardiac effects in frog and rat

Hydrogen sulfide (H₂S) has recently emerged as an important mediator of mammalian cardiovascular homeostasis. In nonmammalian vertebrates, little is known about the cardiac effects of H₂S. This study aimed to evaluate, in the avascular heart of the frog, Rana esculenta, whether and to what extent H₂S affects the cardiac performance, and what is the mechanism of action responsible for the observed effects. Results were analyzed in relation to those obtained in the rat heart, used as the mammalian model. Isolated and perfused (working and Langendorff) hearts, Western blot analysis, and modified biotin switch (S-sulfhydration) assay were used. In the frog heart, NaHS (used as H₂S donor, 10⁻¹²/10⁻⁷ M) dose-dependently decreased inotropism. This effect was reduced by glibenclamide (KATP channels blocker), NG-monomethyl-L-arginine (NOS inhibitor), 1H-[1,2,4] oxadiazolo-[4,3-a]quinoxalin-1-one (guanylyl cyclase inhibitor), KT₅₈₂₃ (PKG inhibitor), and it was blocked by Akt1/2 (Akt inhibitor) and by detergent Triton X-100. In the rat, in addition to the classic negative inotropic effect, NaHS (10⁻¹²/10⁻⁷ M) exhibited negative lusitropism. In both frog and rat hearts, NaHS treatment induced Akt and eNOS phosphorylation and an increased cardiac protein S-sulfhydration that, in the rat heart, includes phospholamban. Our data suggest that H₂S represents a phylogenetically conserved cardioactive molecule. Results obtained on the rat heart extend the role of H₂S also to cardiac relaxation. H₂S effects involve KATP channels, the Akt/NOS-cGMP/PKG pathway, and S-sulfhydration of cardiac proteins.

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.

Hydrogen sulfide-mediated myocardial pre- and post-conditioning

Coronary artery disease is a major cause of morbidity and mortality in the Western world. Acute myocardial infarction, resulting from coronary artery atherosclerosis, is a serious and often fatal consequence of coronary artery disease, resulting in cell death in the myocardium. Pre- and post-conditioning of the myocardium are two treatment strategies that reduce the amount of cell death significantly. Hydrogen sulfide has recently been identified as a potent cardioprotective signaling molecule, which is a highly effective pre- and post-conditioning agent. The cardioprotective signaling pathways involved in hydrogen sulfide-based pre- and post-conditioning will be explored in this article.

Hydrogen sulfide mediates hypoxic vasoconstriction through a production of mitochondrial ROS in trout gills

Hypoxic pulmonary vasoconstriction (HPV) is an adaptive response that diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts. This response is important for the local matching of blood perfusion to ventilation and improves pulmonary gas exchange efficiency. HPV is an ancient and highly conserved response, expressed in the respiratory organs of all vertebrates, including lungs of mammals, birds, and reptiles; amphibian skin; and fish gills. The mechanism underlying HPV and how cells sense low Po(2) remains elusive. In perfused trout gills (Oncorhynchus mykiss), acute hypoxia, as well as H(2)S, caused an initial and transient constriction of the vasculature. Inhibition of the enzymes cystathionine-β-synthase and cystathionine-γ-lyase, which blocks H(2)S production, abolished the hypoxic response. Individually blocking the four complexes in the electron transport chain abolished both the hypoxic and the H(2)S-mediated constriction. Glutathione, an antioxidant and scavenger of superoxide, attenuated the vasoconstriction in response to hypoxia and H(2)S. Furthermore, diethyldithiocarbamate, an inhibitor of superoxide dismutase, attenuated the hypoxic and H(2)S constriction. This strongly suggests that H(2)S mediates the hypoxic vasoconstriction in trout gills. H(2)S may stimulate the mitochondrial production of superoxide, which is then converted to hydrogen peroxide (H(2)O(2)). Thus, H(2)O(2) may act as the "downstream" signaling molecule in hypoxic vasoconstriction.

Neurotransmitter profiles in fish gills: putative gill oxygen chemoreceptors

In fish, cells containing serotonin, ACh, catecholamines, NO, H(2)S, leu-5-enkephalin, met-5-enkephalin and neuropeptide Y are found in the gill filaments and lamellae. Serotonin containing neuroepithelial cells (NECs) located along the filament are most abundant and are the only group found in all fish studied to date. The presence of NECs in other locations or containing other transmitters is species specific and it is rare that any one NEC contains more than one neurochemical. The gills are innervated by both extrinsic and intrinsic nerves and they can be cholinergic, serotonergic or contain both transmitters. Some NECs are presumed to be involved in paracrine regulation of gill blood flow, while others part of the reflex pathways involved in cardiorespiratory control. There is both direct and indirect evidence to indicate that the chemosensing cells involved in these latter reflexes sit in locations where some monitor O(2) levels in water, blood or both, yet the anatomical data do not show such clear distinctions.

Carbon monoxide (CO) and hydrogen sulfide (H(2)S) in hypoxic sensing by the carotid body

Carotid bodies are sensory organs for monitoring arterial blood oxygen (O(2)) levels, and the ensuing reflexes maintain cardio-respiratory homeostasis during hypoxia. This article provides a brief update of the role of carbon monoxide (CO) and hydrogen sulfide (H(2)S) in hypoxic sensing by the carotid body. Glomus cells, the primary site of O(2) sensing in the carotid body express heme oxygenase-2 (HO-2), a CO catalyzing enzyme. HO-2 is a heme containing enzyme and has high affinity for O(2). Hypoxia inhibits HO-2 activity and reduces CO generation. Pharmacological and genetic approaches suggest that CO inhibits carotid body sensory activity. Stimulation of carotid body activity by hypoxia may reflect reduced formation of CO. Glomus cells also express cystathionine γ-lyase (CSE), an H(2)S generating enzyme. Exogenous application of H(2)S donors, like hypoxia, stimulate the carotid body activity and CSE knockout mice exhibit severely impaired sensory excitation by hypoxia, suggesting that CSE catalyzed H(2)S is an excitatory gas messenger. Hypoxia increases H(2)S generation in the carotid body, and this response was attenuated or absent in CSE knockout mice. HO inhibitor increased and CO donor inhibited H(2)S generation. It is proposed that carotid body response to hypoxia requires interactions between HO-2-CO and CSE-H(2)S systems.

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.

The liver as a central regulator of hydrogen sulfide

The liver is likely exposed to high levels of hydrogen sulfide (H2S) from endogenous hepatic synthesis and exogenous sources from the gastrointestinal tract. Little is known about the consequence of H2S exposure on the liver or hepatic regulation of H2S levels. We hypothesized that the liver has a high capacity to metabolize H2S and that H2S oxidation is decreased during sepsis, a condition in which hepatic O2 is limited and H2S synthesis is increased. Using a nonrecirculating isolated and perfused liver system, we demonstrated rapid hepatic H2S metabolism up to an infusion concentration of 200' μM H2S. Hydrogen sulfide metabolism was associated with an increase in O2 consumption from a baseline 96.7 ± 7.6 μmol O2/min/kg to 109 ± 7.4 μmol O2/min/kg at an infusion concentration of 150 μM H2S (P < 0.001). Removal of O2 from the perfusate decreased H2S clearance from a maximal 97% to only 23%. Livers isolated from rats subjected to cecal ligation and puncture (CLP) did not differ significantly from control livers in their capacity to metabolize H2S, suggesting that H2S oxidation remains a priority during sepsis. To test whether H2S induces O2 consumption in vivo, intravital microscopy was utilized to monitor the oxygen content in the hepatic microenvironment. Infusion of H2S increased the NADH/NAD+ ratio (645 gray-scale-unit increase, P = 0.035) and decreased hepatic O2 availability visualized with Ru(Phen)3(2+) (439 gray-scale-unit increase, P = 0.040). We conclude that the liver has a high hepatic capacity for H2S metabolism. Moreover, H2S oxidation consumes available oxygen and may exacerbate the tissue hypoxia associated with sepsis.

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

This brief review presents and discusses some of the important issues surrounding the theory which asserts that endogenous hydrogen sulfide (H(2)S) is the mediator of, or at least an important contributor to, hypoxia-induced arterial chemorereceptor stimulation. The view presented here is that before H(2)S can seriously be considered as a candidate for transducing the O(2)-signal in the carotid bodies (CB), fundamental contradictions need to be resolved. One of these major contradictions is certainly the discrepancy between the levels of H(2)S endogenously present in the CB during hypoxia compared to the levels required to stimulate the arterial chemoreceptors in vitro. Very small amounts of H(2)S are thought to be produced endogenously during a given level of hypoxia, yet the partial pressure of tissue H(2)S which is needed to produce an effect commensurate with that of hypoxia is thousands to millions of times higher. This review discusses this and other contradictions in light of what is known about H(2)S concentration and production in various tissues, the lessons we have learnt from the response to exogenous sulfide and the ability of the blood and the mitochondria to oxidize very large amounts of sulfide. These considerations suggest that the increased production of H(2)S in hypoxia and exogenous sulfide cannot produce the same effect on the carotid bodies and breathing. While the effects of the endogenous H(2)S on breathing remains to be established, the effects exogenous sulfide can be accounted for by its long established toxicity on cytochrome C oxidase.

Vasoactivity of the gasotransmitters hydrogen sulfide and carbon monoxide in the chicken ductus arteriosus

Besides nitric oxide (NO) and carbon monoxide (CO), hydrogen sulfide (H(2)S) is a third gaseous messenger that may play a role in controlling vascular tone and has been proposed to serve as an O(2) sensor. However, whether H(2)S is vasoactive in the ductus arteriosus (DA) has not yet been studied. We investigated, using wire myography, the mechanical responses induced by Na(2)S (1 μM-1 mM), which forms H(2)S and HS(-) in solution, and by authentic CO (0.1 μM-0.1 mM) in DA rings from 19-day chicken embryos. Na(2)S elicited a 100% relaxation (pD(2) 4.02) of 21% O(2)-contracted and a 50.3% relaxation of 62.5 mM KCl-contracted DA rings. Na(2)S-induced relaxation was not affected by presence of the NO synthase inhibitor l-NAME, the soluble guanylate cyclase (sGC) inhibitor ODQ, or the K(+) channel inhibitors tetraethylammonium (TEA; nonselective), 4-aminopyridine (4-AP, K(V)), glibenclamide (K(ATP)), iberiotoxin (BK(Ca)), TRAM-34 (IK(Ca)), and apamin (SK(Ca)). CO also relaxed O(2)-contracted (60.8% relaxation) and KCl-contracted (18.6% relaxation) DA rings. CO-induced relaxation was impaired by ODQ, TEA, and 4-AP (but not by L-NAME, glibenclamide, iberiotoxin, TRAM-34 or apamin), suggesting the involvement of sGC and K(V) channel stimulation. The presence of inhibitors of H(2)S or CO synthesis as well as the H(2)S precursor L-cysteine or the CO precursor hemin did not significantly affect the response of the DA to changes in O(2) tension. Endothelium-dependent and -independent relaxations were also unaffected. In conclusion, our results indicate that the gasotransmitters H(2)S and CO are vasoactive in the chicken DA but they do not suggest an important role for endogenous H(2)S or CO in the control of chicken ductal reactivity.

The impact of hydrogen sulfide (H₂S) on neurotransmitter release from the cat carotid body

Do cat carotid bodies (CBs) increase their release of acetylcholine and ATP in response to H(2)S? Two CBs, incubated in a Krebs Ringer bicarbonate solution at 37 ° C, exhibited a normal response to hypoxia-increased release of acetylcholine (ACh) and ATP. They were challenged with several concentrations of Na(2)S, an H(2)S donor. H(2)S, a new gasotransmitter, is reported to open K(ATP) channels. Under normoxic conditions the CBs reduced their release of ACh and ATP below control values. They responded identically to pinacidil, a well-known K(ATP) channel opener. CB glomus cells exhibited a positive immunohistochemical signal for cystathione-β-synthetase, a H(2)S synthesizing enzyme, and for a subunit of the K(ATP) channel. The data suggest that Na(2)S may have opened the glomus cells' K(ATP) channels, hyperpolarizing the cells, thus reducing their tonic release of ACh and ATP. Since during hypoxia H(2)S levels rise, the glomus cells responding very actively to hypoxia may be protected from over-exertion by the H(2)S opening of the K(ATP) channels.

Hydrogen sulfide oxidation and the arterial chemoreflex: effect of methemoglobin

Endogenous H(2)S has been proposed to transduce the effects of hypoxia in the carotid bodies (CB). To test this hypothesis, we created a sink for endogenously produced H(2)S by inducing ∼10% methemoglobinemia via the injection of 250 mg of sodium nitrite in spontaneously breathing anaesthetized sheep. Methemoglobinemia has been shown to catalyze the oxidation of large quantities of sulfide in the blood and tissues. We found that the presence of metHb completely abolished the ventilatory stimulation induced by 10 mg NaHS (i.v.), which in control conditions mimicked the effects of breathing 6-7 tidal volumes of nitrogen, confirming the dramatic increase in the oxidative power of the blood for sulfide. The ventilatory responses to hypoxia (10% O(2)), nitrogen and hyperoxia were in no way depressed by the metHb. Our results demonstrate that the ventilatory chemoreflex is not depressed in the presence of a high oxidative capacity for sulfide and challenge the view that H(2)S transduces the effects of hypoxia in the CB.

Prenatal cardiovascular shunts in amniotic vertebrates

During amniotic vertebrate development, the embryo and fetus employ a number of cardiovascular shunts. These shunts provide a right-to-left shunt of blood and are essential components of embryonic life ensuring proper blood circulation to developing organs and fetal gas exchanger, as well as bypassing the pulmonary circuit and the unventilated, fluid filled lungs. In this review we examine and compare the embryonic shunts available for fetal mammals and embryonic reptiles, including lizards, crocodilians, and birds. These groups have either a single ductus arteriosus (mammals) or paired ductus arteriosi that provide a right-to-left shunt of right ventricular output away from the unventilated lungs. The mammalian foramen ovale and the avian atrial foramina function as a right-to-left shunt of blood between the atria. The presence of atrial shunts in non-avian reptiles is unknown. Mammals have a venous shunt, the ductus venosus that diverts umbilical venous return away from the liver and towards the inferior vena cava and foramen ovale. Reptiles do not have a ductus venosus during the latter two thirds of development. While the fetal shunts are well characterized in numerous mammalian species, much less is known about the developmental physiology of the reptilian embryonic shunts. In the last years, the reactivity and the process of closure of the ductus arteriosus have been characterized in the chicken and the emu. In contrast, much less is known about embryonic shunts in the non-avian reptiles. It is possible that the single ventricle found in lizards, snakes, and turtles and the origin of the left aorta in the crocodilians play a significant role in the right-to-left embryonic shunt in these species.

Hydrogen sulfide and cell signaling

Hydrogen sulfide (H₂S) is a gaseous mediator synthesized from cysteine by cystathionine γ lyase (CSE) and other naturally occurring enzymes. Pharmacological experiments using H₂S donors and genetic experiments using CSE knockout mice suggest important roles for this vasodilator gas in the regulation of blood vessel caliber, cardiac response to ischemia/reperfusion injury, and inflammation. That H₂S inhibits cytochrome c oxidase and reduces cell energy production has been known for many decades, but more recently, a number of additional pharmacological targets for this gas have been identified. H₂S activates K(ATP) and transient receptor potential (TRP) channels but usually inhibits big conductance Ca²(+)-sensitive K(+) (BK(Ca)) channels, T-type calcium channels, and M-type calcium channels. H₂S may inhibit or activate NF-κB nuclear translocation while affecting the activity of numerous kinases including p38 mitogen-activated protein kinase (p38 MAPK), extracellular signal-regulated kinase (ERK), and Akt. These disparate effects may be secondary to the well-known reducing activity of H₂S and/or its ability to promote sulfhydration of protein cysteine moieties within the cell.

Vascular complications of cystathionine β-synthase deficiency: future directions for homocysteine-to-hydrogen sulfide research

Homocysteine (Hcy), a cardiovascular and neurovascular disease risk factor, is converted to hydrogen sulfide (H(2)S) through the transsulfuration pathway. H(2)S has attracted considerable attention in recent years for many positive effects on vascular health and homeostasis. Cystathionine β-synthase (CBS) is the first, and rate-limiting, enzyme in the transsulfuration pathway. Mutations in the CBS gene decrease enzymatic activity, which increases the plasma Hcy concentration, a condition called hyperhomocysteinemia (HHcy). Animal models of CBS deficiency have provided invaluable insights into the pathological effects of transsulfuration impairment and of both mild and severe HHcy. However, studies have also highlighted the complexity of HHcy and the need to explore the specific details of Hcy metabolism in addition to Hcy levels per se. There has been a relative paucity of work addressing the dysfunctional H(2)S production in CBS deficiency that may contribute to, or even create, HHcy-associated pathologies. Experiments using CBS knockout mice, both homozygous (-/-) and heterozygous (+/-), have provided 15 years of new knowledge and are the focus of this review. These murine models present the opportunity to study a specific mechanism for HHcy that matches one of the etiologies in many human patients. Therefore, the goal of this review was to integrate and highlight the critical information gained thus far from models of CBS deficiency and draw attention to critical gaps in knowledge, with particular emphasis on the modulation of H(2)S metabolism. We include findings from human and animal studies to identify important opportunities for future investigation that should be aimed at generating new basic and clinical understanding of the role of CBS and transsulfuration in cardiovascular and neurovascular disease.

Sympathetic, parasympathetic and enteric regulation of the gastrointestinal vasculature in rainbow trout (Oncorhynchus mykiss) under normal and postprandial conditions

The control of the gastrointestinal hyperemia that occurs after feeding in most animals is of fundamental importance for the subsequent absorption, metabolism and redistribution of nutrients. Yet, in fish, it has received little attention and the nature of it is far from clear. We sought to investigate the importance of extrinsic and intrinsic innervation of the gastrointestinal tract in the regulation of gastrointestinal blood flow in rainbow trout (Oncorhynchus mykiss). The contribution of the extrinsic innervation, i.e. by the sympathetic and the parasympathetic nervous system, was examined by comparing the response to the injection of a predigested nutrient diet into the proximal intestine of untreated fish with the response in fish in which the splanchnic and vagal innervation of the gut had been removed. We also injected the predigested nutrient diet into anaesthetized fish treated with tetrodotoxin that would block the intrinsic innervation of the gut (i.e. enteric nervous system). Our results confirm the notion that the sympathetic portion of the extrinsic innervation maintains the basal vascular tone, but neither the splanchnic nor the vagal innervation is fundamental to the postprandial hyperemia. However, the tetrodotoxin treatment completely abolished the postprandial hyperemia, indicating the importance of the enteric nervous system. In conclusion, it seems as though the enteric nervous system is essential to the regulation of the postprandial hyperemia, and that the extrinsic innervation is involved mainly in the regulation of gastrointestinal blood flow under normal conditions and in response to central coordination with other organs.

Interactions of multiple gas-transducing systems: hallmarks and uncertainties of CO, NO, and H2S gas biology

The diverse physiological actions of the "biologic gases," O2, CO, NO, and H2S, have attracted much interest. Initially viewed as toxic substances, CO, NO, and H2S play important roles as signaling molecules. The multiplicity of gas actions and gas targets and the difficulty in measuring local gas concentrations obscures detailed mechanisms whereby gases exert their actions, and many questions remain unanswered. It is now readily apparent, however, that heme-based proteins play central roles in gas-generation/reception mechanisms and provide a point where multiple gases can interact. In this review, we consider a number of key issues related to "gas biology," including the effective tissue concentrations of these gases and the importance and significance of the physical proximity of gas-producing and gas-receptor/sensors. We also take an integrated approach to the interaction of gases by considering the physiological significance of CO, NO, and H2S on mitochondrial cytochrome c oxidase, a key target and central mediator of mitochondrial respiration. Additionally, we consider the effects of biologic gases on mitochondrial biogenesis and "suspended animation." By evaluating gas-mediated control functions from both in vitro and in vivo perspectives, we hope to elaborate on the complex multiple interactions of O2, NO, CO, and H2S.

Mechanism of inhibition by hydrogen sulfide of native and recombinant BKCa channels

Recent evidence suggests that H(2)S contributes to activation of the carotid body by hypoxia by inhibiting K(+) channels. Here, we determine both the molecular identity of the K(+) channel target within the carotid body and the biophysical characteristics of the H(2)S-evoked inhibition by analyzing native rat and human recombinant BK(Ca) channel activity in voltage-clamped, inside-out membrane patches. Rat glomus cells express the enzymes necessary for the endogenous generation of H(2)S, cystathionine-beta-synthase and cystathionine-gamma-lyase. H(2)S inhibits native carotid body and human recombinant BK(Ca) channels with IC(50) values of around 275 microM. Inhibition by H(2)S is rapid and reversible, works by a mechanism which is distinct from that suggested for CO gas regulation of this channel and does not involve an interaction with either the "Ca bowl" or residues distal to this Ca(2+)-sensing domain. These data show that BK(Ca) is a K(+) channel target of H(2)S, and suggest a mechanism to explain the H(2)S-dependent component of O(2) sensing in the carotid body.

Hydrogen sulfide and oxygen sensing in the cardiovascular system

Vertebrate cardiorespiratory homeostasis is inextricably dependent upon specialized cells that provide feedback on oxygen status in the tissues, blood, and on occasion, environment. These "oxygen sensing" cells include chemoreceptors and oxygen-sensitive chromaffin cells that initiate cardiorespiratory reflexes, vascular smooth muscle cells that adjust perfusion to metabolism or ventilation, and other cells that condition themselves in response to episodic hypoxia. Identification of how these cells sense oxygen and transduce this into the appropriate physiological response has enormous clinical applicability, but despite intense research there is no consensus regarding the initial hypoxia-effector coupling mechanism. This review examines an alternative mechanism of oxygen sensing using oxidation of endogenously produced hydrogen sulfide (H(2)S) as the O(2)-sensitive couple. Support for this hypothesis includes the similarity of effects of hypoxia and H(2)S on a variety of tissues, augmentation of hypoxic responses by precursors of H(2)S production and their inhibition by inhibitors of H(2)S synthesis, and the rapid consumption of H(2)S by O(2) in the range of intracellular/mitochondrial Po(2). These studies also indicate that, under normoxic conditions, it is doubtful that free H(2)S has longer than a transient existence in tissue or extracellular fluid.

Vasoactivity of hydrogen sulfide in normoxic and anoxic turtles (Trachemys scripta)

Systemic vascular resistance ( Rsys) of freshwater turtles increases substantially during anoxia, but the underlying mechanisms are not fully understood. We investigated whether hydrogen sulfide (H2S), an endogenously produced metabolite believed to be an O2 sensor/transducer of vasomotor tone, contributes to the increased Rsys of anoxic red-eared slider turtles ( Trachemys scripta ). Vascular infusion of the H2S donor NaHS in anesthetized turtles at 21°C and fully recovered normoxic turtles at 5°C and 21°C revealed H2S to be a potent vasoconstrictor of the systemic circulation. Likewise, wire myography of isolated turtle mesenteric and pulmonary arteries demonstrated H2S to mediate an anoxia-induced constriction. Intriguingly, however, NaHS did not exert vasoconstrictory effects during anoxia (6 h at 21°C; 14 days at 5°C) when plasma H2S concentration, estimated from the colorimetric measurement of plasma acid-labile sulfide concentration, likely increased by ∼3- and 4-fold during anoxia at 21°C, and 5°C, respectively. Yet, blockade of endogenous H2S production by DL-propargylglycine or hydroxylamine (0.44 mmol/kg) partially reversed the decreased systemic conductance ( Gsys) exhibited by 5°C anoxic turtles. These findings suggest that the signal transduction pathway of H2S-mediated vasoactivity is either maximally activated in the systemic circulation of anoxic turtles and/or that it is oxygen dependent.