Introduction

Oxygen is vital for life and plays a key role for the physiological homeostasis of the human body. It is known to be one of the strongest electron acceptors and has one of its main functions in the electron transport chain producing adenosine 5'-triphosphate (ATP) and heat, so important for energy expenditure and thermoregulation. Yet, some important mechanisms of oxygen functions are not completely delineated and oxygen research is an active scientific field in basic medical research, experimental investigations as well as in clinical research. In clinical medicine both hyperoxia and hypoxia are in focus. This presentation will mainly be dealing with reactions to low oxygen tensions.

There is a well-balanced sensory function to maintain oxygen tension in blood within narrow margines. The normal but small oxygen variations in blood are sufficient to trigger activities in the carotid body in order to create afferent impulses via the sinus nerve and the IX^th cranial nerve to the central nervous system. Together with pH variations or carbo dioxide (CO₂), oxygen is an important factor for regulation of breathing.

In this communication some of the most highlighted mechanisms for the sensing of low oxygen tensions will be discussed and opinions on oxygen utilization will be presented with regard to plasma membrane passage of oxygen and intracellular electron transport. In addition, effects of different anesthetic agents on oxygen sensing and utilization will be commented upon.

Oxygen Sensing

Oxygen is sensed by most mammalian cells. Glomus cells in the carotid body, neuroepithelial body cells in the airways, smooth muscle cells in the systemic or pulmonary circulation, and chromaffin cells have been reported as typical cells having specific oxygen sensing capacities. In 1995, Wang, Semenza et al. discovered the hypoxia inducible factor 1 (HIF-1) as a key oxygen regulator in mammalian cells [11. Wang GL, Jiang BH, Rue EA, Semenza GL (1995) Hypoxia inducible factor 1 is a basic-helix-loophelix-PAS heterodimer regulated by cellular O2 tension. Proc Natl Acad Sci USA; 92: 5510-5514.]. HIF-1 is a heterodimeric protein that is composed of HIF-1α and HIF-1β subunits with the HIF-1α subunit being responsible for the response to hypoxia [11. Wang GL, Jiang BH, Rue EA, Semenza GL (1995) Hypoxia inducible factor 1 is a basic-helix-loophelix-PAS heterodimer regulated by cellular O2 tension. Proc Natl Acad Sci USA; 92: 5510-5514.].

Sensing oxygen is purposeful and serves various specific functions. One mode of action is to initiate afferent neuronal activity which requires increased cytosolic Ca²⁺concentrations in order to release transmitters from synaptic vesicles for postsynaptic impulse generation [22. Weir EK, López-Barneo J, Buckler KJ, Archer SL (2005) Acute oxygen-sensing mechanisms. N Engl J Med 353: 2042-2055.]. Another kind of action is linked to HIF-1, which in the normoxic state is produced in a prolyl-hydroxylase regulated reaction in the presence of Fe²⁺ and α-ketoglutarate. HIF-1 reacts with von Hippel-Lindau protein, which labels HIF-1 for ubiquitination and protein degradation. During hypoxia, prolyl-hydroxylation is inhibited and HIF-1α accumulates. Via this mechanism all nucleotide cells in the body can sense and respond to hypoxia [33. Semenza GL (2011) Oxygen sensing, homeostasis, and diseases. N Engl J Med 365: 537-547.]. Hence, both calcium and HIF based mechanisms are most prominent for protective reactions against low oxygen tensions. The calcium generated neuronal response is usually described as a quick, acute, response that is set in action within seconds whereas HIF related responses are slower, chronic, activated after several minutes to hours [22. Weir EK, López-Barneo J, Buckler KJ, Archer SL (2005) Acute oxygen-sensing mechanisms. N Engl J Med 353: 2042-2055.,33. Semenza GL (2011) Oxygen sensing, homeostasis, and diseases. N Engl J Med 365: 537-547.] (Figure 1).

Ca²⁺ related Oxygen sensing

Until the discovery of HIF-1, in 1995, research on oxygen sensing mechanisms has mainly been conducted with focus on physiological responses to acute hypoxia, as hypoxia induces critical damages in tissues and organs and jeopardizes physiological homeostasis. There are different mechanisms to accomplish the acute, calcium mediated, oxygen sensing. One of them, as described by Lopez-Barneo and colleagues [44. López-Barneo J, Lopez-Lopez JR, Urena J, Gonzalez C (1988) Chemotransduction in the carotid body: K+ current modulated by PO2 in type I chemoreceptor. Science 241: 580-582.], expresses an oxygen-sensitive potassium channel and it is now known that large-conductance Ca²⁺-sensitive potassium channels (BK_Ca channels), voltage-gated potassium channels, and TASK-like glomus-cell potassium channels are involved in arterial oxygen sensing [22. Weir EK, López-Barneo J, Buckler KJ, Archer SL (2005) Acute oxygen-sensing mechanisms. N Engl J Med 353: 2042-2055.]. The presented sequence of events is that hypoxia inhibits the potassium current in these channels which alters the cellular membrane potential promoting cellular depolarization. As a consequence, voltage-sensitive calcium channels are opened which enhances cytosolic calcium concentrations. Another route to get higher calcium concentrations is via inositol 1, 4, 5-triphosphate (IP3). IP3 is generated by phospholipase C which is a component of the plasma membrane. IP3 acts on its own receptor at the endoplasmic reticulum (ER) to open up channels for calcium to pass from the ER to the cytoplasm [55. Bickler PE (2004) Clinical perspectives: neuroprotection lessons from hypoxia-tolerant organisms. J Exp Biol 207: 3243-3249.,66. Lahiri S, Roy A, Baby SM, Hoshi T, Semenza GL, et al. (2006) Oxygen sensing in the body. Prog Biophys Mol Biol 91: 249-286.]. Again, as stated above, increased Ca²⁺ influx stimulates pre-synaptic vesicles to release transmitter substances.

There are many transmitters in the glomus cells of the carotid body and other chemoreceptor specific cells. They include ATP, gamma-aminobutyric acid (GABA), norepinephrine, dopamine, and acetylcholine (ACh). Among these transmitters, ACh and ATP seem to be important excitatory transmitters, whereas dopamine and GABA have modulatory and inhibitory functions, respectively [77. Eriksson LI (1999) The effects of residual neuromuscular blockade and volatile anesthetics on the control of ventilation. Anesth Anal 89: 243–251.,88. Nieuwenhuijs D, Sarton E, Teppema L, Dahan A (2000) Propofol for monitored anesthesia care: Implications on hypoxic control of cardiorespiratory responses. Anesthesiology 92: 46-54.].

Some inhaled anesthetics affect mostly inhibitory ion channels such as GABAA receptors, nicotinic ACh receptors, and potassium channels [99. Campagna JA, Miller KW, Forman SA (2003) Mechanisms of actions of inhaled anesthetics. N Engl J Med 348: 2110-2124.]. Both halothane and sevoflurane depress the increase of intracellular calcium caused by hypoxia-induced potassium channel closure in the carotid body type 1 cells [1010. Pandit JJ, Buckler KJ (2009) Differential effects of halothane and sevoflurane on hypoxia-induced intracellular calcium transients of neonatal rat carotid body type I cells. Br J Anaesth 103: 701–710.]. In addition, it is also known that propofol and neuromuscular blocking agents prevent chemoreceptor activity in the carotid body [1111. Ponte J, Sadler CL (1989) Effect of thiopentone, etomidate and propofol on carotid body chemoreceptor activity in the rabbit and the cat. Br J Anaesth 62: 41–45.-1313. Jonsson MM, Lindahl SG, Eriksson LI (2005) Effect of propofol on carotid body chemosensitivity and cholinergic chemotransduction. Anesthesiology 102: 110 – 116.]. Thus, common drugs, used during anesthesia, influence oxygen sensing which have implications on the function of the central neuronal control of breathing.

Furthermore, in a recent article, Takahashi et al. reported yet another interesting mechanism related to oxygen sensing. Transient receptor potential cation channel, member A1 (TRPA1), which is a membrane bound channel capable of detecting intra- and extra-cellular environments, is activated both by hyperoxia and hypoxia through mechanisms involving cystein oxidation and proline hydroxylation, respectively [1414. Takahashi N, Kuwaki T, Kiyonaka S, Numata T, Kozai D, et al. (2011) TRPA1 underlies a sensing mechanism for O2. Nat Chem Biol 7: 701-711.]. During hypoxia the prolyl hydroxylation is decreased which activates the TRPA1 channel. Hence, Takahashi el al's study indicates that there are possibilities also for TRPA1 to exist as a cell membrane bound oxygen sensor. It will be of great interest to, in the near future, follow the development of further functional mechanisms related dTRPA1.

1. 

   View Full Image

   Figure 1:

   There are two main oxygen sensing mechanisms, one via Ca²⁺ signaling (acute) and another (chronic) based on the Hypoxia Inducible Factor 1 (HIF-1). During normoxia prolyl hydroxylase (PH) functions normally and HIF-1 reacts with von Hippel Lindau protein to tag for degradation of HIF-1. PH is inhibited during hypoxia (see also text).
   HIF; hypoxic inducible factor 1, PH; prolyl-hydroxylase, vHL; von Hippel-Lindau protein, IP3Rs; inositol 1, 4, 5-triphosphate receptors, TM; transmitter
   

   ×

   

   Figure 1: There are two main oxygen sensing mechanisms, one via Ca²⁺ signaling (acute) and another (chronic) based on the Hypoxia Inducible Factor 1 (HIF-1). During normoxia prolyl hydroxylase (PH) functions normally and HIF-1 reacts with von Hippel Lindau protein to tag for degradation of HIF-1. PH is inhibited during hypoxia (see also text).
   HIF; hypoxic inducible factor 1, PH; prolyl-hydroxylase, vHL; von Hippel-Lindau protein, IP3Rs; inositol 1, 4, 5-triphosphate receptors, TM; transmitter

   Close

HIF-1 related Oxygen Sensing

It is obvious from the above regarding TRPA1 and hypoxia that prolyl hydroxylation has a key role involved in oxygen sensing. It was Semenza et al. [11. Wang GL, Jiang BH, Rue EA, Semenza GL (1995) Hypoxia inducible factor 1 is a basic-helix-loophelix-PAS heterodimer regulated by cellular O2 tension. Proc Natl Acad Sci USA; 92: 5510-5514.] who discovered HIF-1 and Ratcliff et al. who described the importance of prolyl hydroxylation [1515. Stolze IP, Tian YM, Appelhoff RJ, Turley H, Wykoff CC, et al. (2004) Genetic analysis of the role of the asparaginyl hydroxylase factor inhibiting hypoxia-inducible factor (FIH) in regulating hypoxia-inducible factor (HIF) transcriptional target genes. J Biol Chem 279: 42719-42725.]. In the normoxic state, HIF-1 is produced in a prolyl-hydroxylase regulated reaction in the presence of Fe²⁺ and alpha-ketoglutarate. HIF-1 reacts with von Hippel-Lindau protein, which labels HIF-1 for ubiquitination and protein degradation [16Semenza GL (2012) Hypoxia-inducible factors in physiology and medicine. Cell 148: 399-408.]. During hypoxia, the prolyl-hydroxylation is inhibited and HIF-1α accumulates. It is most likely so that all nucleotide cells in the body can sense and respond to hypoxia via this mechanism.

As pointed out above this is a fairly slow responding system that takes minutes to hours. HIF-1 regulates the expression of more than hundred genes that mediate important adaptive physiological responses such as angiogenesis, erythropoiesis, and glycolysis [22. Weir EK, López-Barneo J, Buckler KJ, Archer SL (2005) Acute oxygen-sensing mechanisms. N Engl J Med 353: 2042-2055.] (Figure 1).

Several studies have reported about the effect of anesthetic drugs on HIF-1 expression and activity. In hypoxic cells with accumulation of HIF-1 both halothane and propofol inhibit the HIF-1 accumulation [17Itoh T, Namba T, Fukuda K, Semenza GL, Hirota K (2001) Reversible inhibition of hypoxia-inducible factor 1 activation by exposure of hypoxic cells to the volatile anesthetic halothane. FEBS Lett 509: 225–229.,1818. Takabuchi S, Hirota K, Nishi K, Oda S, Oda T, et al. (2004) The intravenous anesthetic propofol inhibits hypoxia-inducible factor 1 activity in an oxygen tension-dependent manner. FEBS Lett 577: 434–438.]. It is reported that isoflurane and xenon enhance HIF-1 accumulation during hypoxia [1919. Raphael J, Zuo Z, Abedat S, Beeri R, Gozal Y (2008) Isoflurane preconditioning decreases myocardial infarction in rabbits via upregulation of hypoxia inducible factor 1 that is mediated by mammalian target of rapamycin. Anesthesiology 108: 415–425.-2121. Ma D, Lim T, Xu J, Tang H, Wan Y, et al. (2009) Xenon preconditioning protects against renal ischemic-reperfusion injury via HIF-1alpha activation. J Am Soc Nephrol 4: 713–720.].

Oxygen utilization

Oxygen and passage of the plasma membrane.

Traditionally, it has been the opinion that oxygen can diffuse freely across plasma membranes [2222. Subczynski WK, Hopwood LE, Hyde JS (1992) Is the mammalian cell plasma membrane a barrier to oxygen transport? J Gen Physiol 100: 69-87.]. However, the lipid bilayer has higher viscosity than water by several times, and high oxygen permeability has not been proven. In model experiments using biological monolayer membranes, it was shown that oxygen permeability is lower than previously reported [2323. Ivanov II, Fedorov GE, Gus'kova RA, Ivanov Ki, Rubin AB (2004) Permeability of lipid membranes to dioxygen. Biochem Biophys Res Commun 322: 746-750.]. Hence, oxygen transportation across plasma or cell membranes cannot be explained by diffusion alone. It is therefore justified to ask the question if a specific oxygen channel or transport mechanism remains to be discovered.

One way to address this is whether oxygen uses established pores or channels. Currently, one of the best candidates for a collaborative function is aquaporin (AQP). AQP is a water channel protein and facilitates the transport of water across plasma membranes. In 1992, Peter Agre et al. first purified AQP1 from red blood cell membranes [2424. Preston GM, Carroll TP, Guggino WB, Agre P (1992) Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein. Science 256: 385-387.], and 13 isoforms (AQP0-12) have since been identified in humans. Among them, AQP 1, 2, 4, and 5 are primarily aquaporins, and some of them have function also as gas channels. It is especially AQP1 that have been reported as a gas channel, transporting CO₂ and nitric oxide (NO) across the plasma membrane [2525. Nakhoul NL, Davis BA, Romero MF, Boron WF (1998) Effect of expressing the water channel aquaporin-1 on the CO2 permeability of Xenopus oocytes. Am J Physiol 274: 543-548.,26]. The first suggestion that AQP1 might be a gas channel was reported in 1998, by Nakhoul NL, et al. [2525. Nakhoul NL, Davis BA, Romero MF, Boron WF (1998) Effect of expressing the water channel aquaporin-1 on the CO2 permeability of Xenopus oocytes. Am J Physiol 274: 543-548.]. The investigators measured the rate of cellular acidification caused by CO₂ influx across the cell membrane using intracellular electrodes.

Concerning oxygen, Jose Lopez-Barneo et al. showed, in 2007 [2626. Herrera M, Hong NJ, Garvin JL (2006) Aquaporin-1 transports NO across cell membranes. Hypertension 48: 157-164.], that AQP1 functioned as an oxygen channel. They found that HIF1-α and other oxygen dependent genes (tyrosine hydroxylase, phosphoglycerate kinase 1, and vascular endothelial growth factor mRNA) had a higher expression during hypoxia. Moreover, AQP1 mRNA expression was also upregulated in rat lung during hypoxia. In this study, cellular oxygen fluxes were not directly measured because of the technical difficulties as authors described. However, the Lopez-Barneo study strongly indicated that AQP1 is permeable for oxygen and thus could facilitate oxygen uptake into cells [2727. Echevarria M, Munoz-Cabello AM, Sanchez-Silva R, Toledo-Aral JJ, López-Barneo J (2007) Development of cytosolic hypoxia and HIF stabilization are facilitated by aquapori 1 expression. J Biol Chem 282: 30207-30215.]. In support of such a suggestion, Ivanov II et al. reported that the two AQP channel blockers, mercury chloride (HgCl2) and p-chloromercuribenzoate (PCMB), dose-dependently inhibited oxygen absorption by erythrocytes [2828. Ivanov II, Loktyushkin AV, Gus'kova RA, Vasil'ev NS, Fedorov GE, et al. (2007) Oxygen channels of erythrocyte membrane. Dokl Biochem Biophys 414: 137-140.]. Although, HgCl2 and PCMB are not AQP1 specific but inhibit almost all aquaporins, except AQP6 [2929. Yasui M, Kwon TH, Knepper MA, Nielsen S, Agre P (1999) Rapid gating and anion permeability of an intracellular aquaporin. Nature 402: 184-187.], the Ivanov et al findings indirectly support the suggestion by Lopez-Barneo et al. Future investigations regarding possible oxygen channels on plasma membranes are of great importance.

Thermogenesis and oxygen consumption

As stated above the most important role of oxygen is to accept electrons. Via the proton gradient over the mitochondrial membrane ATP is generated. There are numerous investigations about oxygen utilization in mitochondria for ATP synthase and ATP production. In this presentation it was therefore chosen to highlight aspects on the un-coupling mechanism in brown adipose tissue and how anesthesia can improve the utilization of amino acids for heat production and also to concentrate on the recent findings that H₂S induced decreasing oxygen consumption.

Thermogenesis in brown adipose tissue

Brown adipose tissue, unlike white adipose tissue, has the ability to dissipate energy by producing heat rather than storing it as triglycerides. The uncoupling protein 1 (UCP1) is expressed in brown adipose tissue and plays an important role for thermogenesis (oxygen consumption). The classic opinion is that brown adipose tissue mainly exists in neonates and young infants and in hibernating animals. In neonates and young infants the brown adipose tissue is important for temperature balance [3030. Nielesn S, Smith BL, Christensen EI, Agre P (1993) Distribution of the aquaporin CHIP in secretory and resorptive epithelia and capillary entothelia. Proc Nattl Acad Sci USA 90: 7275-7279.]. Brown-fat cells are stimulated by norepinephrine released from the sympathetic nervous system. This leads to cellular respiration in the mitochondria that is un-coupled from ATP synthesis [3131. Abreu-Rodriguez I, Sanchez Silva R, Martins AP, Soveral G, Toledo-Aral JJ, et al. (2011) Function and transcription induction of aquaporin-1 gene by hypoxia; analysis of promoter and role of Hif-1. PloS One 6: e28385.].

During the latest few years it has been proven that brown adipose tissue is also present and have an active function in adult humans. Positron emission tomography- computed tomography (PET-CT) investigations showed, in adult humans, a high uptake of 18F-FDP in the neck, in the supraclavicular region, and at classical locations for brown adipose tissue in para-aortic and paravertebral areas as well as suprarenally indicating the existence of brown adipose tissue also in adults [3232. Nederaard J, Bengtsson T, Cannon B (2011) Three years with adult human brown adipose tissue. Ann N.Y. Acad Sci 1212; 20-36.]. In 2009, three different articles supported these findings and identified the presence of brown adipose tissue in adult humans [3333. Virtanen KA, Lidell ME, Orava J, Heglind M, Westergren R, et al. (2009) Functional brown adipose tissue in healthy adults. N Eng J Med 360: 1518-1525.-3535. van Marken Lichtenbelt WD, Vanhommerig JW, Smulders NM, Drossaerts JM, Kemerink GJ, et al. (2009) Cold-activated brown adipose tissue in healthy men. N Eng J Med 360: 1500-1508.]. In these studies, investigators reported that cold-induced glucose uptake was increased in paracervical and supraclavicular adipose tissue in healthy human adults and moreover, biopsy specimens obtained from the high uptake area of 18F-FDP contained UCP1 [3636. Ohlsson KBE, Mohell N, Cannon B, Lindahl SGE, Nedergaard J. (1994) Thermogenesis in brown adipocytes is inhibited by volatile anesthetic agents. Anesthesiology 81: 176-183.].

So far, there are few studies that investigate the activities and thermogenetic effects of human adult brown adipose tissue during anesthesia. From previous studies it is known that volatile anesthetics attenuate oxygen consumption in response to norepinephrine of isolated brown fat cells, but non-volatile anesthetics such as pentobarbital, propofol, and ketamine, have not been proven to have such effects [3737. Ohlsson KBE, Lindahl SGE, Cannon B, Nedergaard J (2003) Thermogenisis inhibition in brown adipocytes is a specific property of volatile anesthetics. Anesthesiology 98: 437-448.,3838. Belani K, Sessler DI, Sessler AM, Schroeder M, Mcguire J, et al. (1993) Leg heat content continues to decrease during the core temperature plateau in human anesthetized in isoflurane. Anesthesiology 78: 856-863.]. It is also known that volatile anesthetic agents inhibit heat production, oxygen consumption, in studies of animals and infants known to have brown adipose tissue [3939. Hynson JM, Sessler DI, Moayeri A, McGuire J. (1993) Absence of nonshivering thermogenesis in anesthetized adult humans. Anesthesiology 79: 695-703.-4141. Sellden E, Brundin T, Wahren J (1994) Augmented themic effect of amino acids under general anaesthesia: amechanism useful for prevention of anaesthesia-induced hypothermia. Clin Sci 86: 611-618.]. It is likely that not only neonates and infants but also adults with brown adipose tissue will have a disturbed temperature balance during long duration surgery most likely, at least partly, due to effects on oxygen consumption in brown adipose tissue. The area is indeed open for further important clinical studies during anesthesia.

Amino acid –induced thermogenesis

It has for a long time been a dogma that amino acids during anesthesia and surgery do not contribute to heat production and recovery. Sellden et al. reported, in 1994, that amino acids induced thermogenesis and diminished hypothermia under general anesthesia [4242. Sellden E, Branstrom R, Brundin T (1996) Augmented thermic effect of amino acids under general anaesthesia occurs predominantly in exta-splanchnic tissues. Clin Sci 91: 431-439.]. They found that the energy value of an amino acid solution was 4 Watts in awake humans whereas the energy value from an identical amino acid infusion was 21 Watts during general anesthesia. Thus, this means that amino acid-induced heat production during anesthesia is five times higher, compared to in the awake state [4343. Sellden E, Lindahl SG (1998) Amino-acid-induced thermogenesis to prevent hypothermia during anesthesia is not associated with increased stress response. Anesth Analg 87: 637-640.]. In the same series of investigations it was also reported that this thermogenetic effect occurred in extra-splanchnic tissues and that it was not associated with stress response [4444. Sellden E (2002) Peri-operative amino acid administration and the metabolic response to surgery. Proc Nutr Soc 61: 337-343. ,4545. Moriyama T, Tsuneyoshi I, Omae T, Takeyama M, Kanmura Y (2008) The effect of amino-acid infusion during off-pump coronary artery bypass surgery on thermogenic and hormonal regulation. J Anesth 22: 354-360.].

As referred to above, the heat production from amino acid infusions during anesthesia occurred in extra-splanchnic tissue, most likely skeletal muscle. However, the mechanism for this heat production is still not completely understood [4141. Sellden E, Brundin T, Wahren J (1994) Augmented themic effect of amino acids under general anaesthesia: amechanism useful for prevention of anaesthesia-induced hypothermia. Clin Sci 86: 611-618.]. In one study it was reported that amino-acid infusions during general anesthesia increased plasma insulin concentrations and systemic oxygen consumption [4242. Sellden E, Branstrom R, Brundin T (1996) Augmented thermic effect of amino acids under general anaesthesia occurs predominantly in exta-splanchnic tissues. Clin Sci 91: 431-439.]. Since brown adipose tissue can be activated by insulin [4343. Sellden E, Lindahl SG (1998) Amino-acid-induced thermogenesis to prevent hypothermia during anesthesia is not associated with increased stress response. Anesth Analg 87: 637-640.], this challenges the assumption that the amino acid-induced thermogenesis might be caused by an un-coupling mechanism in skeletal muscle.

H₂S and oxygen consumption

Hydrogen sulfide (H₂S) has been known for a long time because of its toxicity and environmental hazard [4444. Sellden E (2002) Peri-operative amino acid administration and the metabolic response to surgery. Proc Nutr Soc 61: 337-343. ]. The main mechanism of H₂S toxicity is inhibition of mitochondrial respiration resulting from blockade of cytochrome c oxidase [4545. Moriyama T, Tsuneyoshi I, Omae T, Takeyama M, Kanmura Y (2008) The effect of amino-acid infusion during off-pump coronary artery bypass surgery on thermogenic and hormonal regulation. J Anesth 22: 354-360.].

Recently, H₂S has been the subject of a great interest as a gas with possible pharmacological potentials. In 2005, Blackstone E, et al. reported that H₂S decreased oxygen consumption and heat production which resulted in a suspended animation-like state in mice [4646. Orava J, Nuutila P, Lidell ME, Oikonen V, Noponen T, et al. (2011) Different metabolic Responses of human brown adipose tissue to activation by cold and insulin. Cell Metab 14: 272-279.]. Mice exposed to 80 ppm of H₂S, which is a safe dose for these animals, decreased their oxygen consumption by ~50%, and carbon dioxide production by ~60% during the first 5 minutes, and 6 hours later, their metabolic rates were decreased by ~90%. In parallel, H₂S caused dose-dependently decreased core body temperature.

After this report, many experimental investigations have been conducted and in a study of hemorrhagic shock, H₂S stabilized hemodynamics [4747. Szabo C (2007) Hydrogen sulphide and its therapeutic potential. Nat Rev Drug Discov 6: 917-335.,4848. Dorman DC, Moulin FJM, McManus BE, Mahle KC, James RA, et al. (2002) Cytochrome oxidase inhibition induced by acute hydrogen sulfide inhalation: correlation with tissue sulfide concentrations in the rat brain, liver, lung, and nasal epithelium. Toxicol Sci 65: 18-25.]. Several studies also reported that H₂S attenuated ischemia reperfusion injury in heart and liver [4949. Blackstone E, Morrison M, Roth MB (2005) H2S induces a suspended animation-like state in mice. Science 308: 518.-5151. Morrison ML, Blackwood JE, Lockett SL, Iwata A, Winn RK, et al. (2008) Surviving blood loss using hydrogen sulfide. J Trauma 65: 183-188.]. It was also reported that H₂S prevented sepsis-induced inflammation and protected vital organs [5252. Elrod JW, Calvert JW, Morrison J, Doeller JE, Kraus DW, et al. (2007) Hydrogen sulfide attenuates myocardial ischemia–reperfusion injury by preservation of mitochondrial function. Proc Natl Acad Sci USA 104: 15560-15565.,53]. Many studies have reported the efficacy of H₂S in animal experiments using rodent models, whereas other studies failed to show the effects, especially in larger animal models such as piglets and sheep. For future clinical applications, there are many questions that will have to be answered. It will be most interesting to follow the development of this scientific field with the goals to find correct indications for the use of H₂S, to work out therapeutic strategies and to rule out possible side effects.

In conclusion, supplying oxygen to tissues or organs is one of the most important treatments of patients in particular during surgery or critical conditions. However, in these situations, several factors like as inflammatory, oxidative stress, activation of sympathetic nerve system and renin angiotensin system disturb the balance between oxygen demand and supply. Clinicians need to recognize the mechanism of oxygen sensing, oxygen transport, and oxygen utility in the cells to maintain the homeostasis of patients in clinical conditions.