Sulphaemoglobin formation in fish: a comparison between the haemoglobin of the sulphide-sensitive rainbow trout (Oncorhynchus Mykiss) and of the sulphide-tolerant common carp (Cyprinus Carpio)

The effect of temperature on haemoglobin-oxygen binding affinity in regionally endothermic and ectothermic sharks

Haemoglobin (Hb)-O2 binding affinity typically decreases with increasing temperature, but several species of ectothermic and regionally endothermic fishes exhibit reduced Hb thermal sensitivity. Regionally endothermic sharks, including the common thresher shark (Alopias vulpinus) and lamnid sharks such as the shortfin mako shark (Isurus oxyrinchus), can maintain select tissues and organs warmer than ambient temperature by retaining metabolic heat with vascular heat exchangers. In the ectothermic bigeye thresher shark (Alopias superciliosus), diurnal movements above and below the thermocline subject the tissues, including the blood, to a wide range of operating temperatures. Therefore, blood-O2 transport must occur across internal temperature gradients in regionally endothermic species, and over the range of environmental temperatures encountered by the ectothermic bigeye thresher shark. While previous studies have shown temperature-independent Hb-O2 affinity in lamnid sharks, including shortfin mako, the Hb-O2 affinity of the common and bigeye thresher sharks is unknown. Therefore, we examined the effect of temperature on whole-blood Hb-O2 affinity in common thresher shark and bigeye thresher shark. For comparison, analyses were also conducted on the shortfin mako shark and two ectothermic species, blue shark (Prionace glauca) and spiny dogfish (Squalus acanthias). Blood-O2 binding affinity was temperature independent for common thresher shark and shortfin mako shark, which should prevent internal temperature gradients from negatively affecting blood-O2 transport. Blue shark and spiny dogfish blood-O2 affinity decreased with increasing temperature, as expected, but bigeye thresher shark blood exhibited both a reduced temperature dependence and a high Hb-O2 affinity, which likely prevents large changes in environment temperature and low environmental oxygen from affecting O2 uptake.

Seeing in the swamp: hydrogen sulfide inhibits eye metabolism and visual acuity in a sulfide-tolerant fish

In fish, vision may be impaired when eye tissue is in direct contact with environmental conditions that limit aerobic ATP production. We hypothesized that the visual acuity of fishes exposed to hydrogen sulfide (H₂S)-rich water would be altered owing to changes in cytochrome c oxidase (COX) activity. Using the H₂S-tolerant mangrove rivulus (Kryptolebias marmoratus), we showed that a 10 min exposure to greater than or equal to 200 µM of H₂S impaired visual acuity and COX activity in the eye. Visual acuity and COX activity were restored in fish allowed to recover in H₂S-free water for up to 1 h. Since K. marmoratus are found in mangrove pools with H₂S concentrations exceeding 1000 µM, visual impairment may impact predator avoidance, navigation and foraging behaviour in the wild.

Factors influencing the antioxidant effect of phospholipase A2 against lipid oxidation promoted by trout hemoglobin and hemin in washed muscle

The antioxidant effect of porcine pancreatic phospholipase A2 (PLA2) was previously demonstrated. Understanding how PLA2 inhibits lipid oxidation promoted by hemoglobin (Hb) is important for its applications in muscle foods. Effects of enzyme dose, pH, and calcium ion on the ability of PLA2 to inhibit trout hemoglobin-mediated lipid oxidation were investigated in washed cod muscle (WCM). Results indicated that PLA2 required calcium ion for both the hydrolyzing activity and the antioxidant effect. The abilities of PLA2 to inhibit lipid oxidation and suppress oxidation of Hb to form methemoglobin and ferryl hemoglobin were pH-dependent. The lag phase before lipid oxidation enters the exponential phase reciprocally shortened as more hemin was bound to the insoluble matrix of WCM. However, PLA2 was able to inhibit lipid oxidation without preventing the interaction between hemin and the insoluble matrix of the washed muscle.

Thermal tolerance and hypoxia tolerance are associated in blacktip reef shark (Carcharhinus melanopterus) neonates

Thermal dependence of growth and metabolism can influence thermal preference and tolerance in marine ectotherms, including threatened and data-deficient species. Here, we quantified the thermal dependence of physiological performance in neonates of a tropical shark species (blacktip reef shark, Carcharhinus melanopterus) from shallow, nearshore habitats. We measured minimum and maximum oxygen uptake rates (ṀO2), calculated aerobic scope, excess post-exercise oxygen consumption and recovery from exercise, and measured critical thermal maxima (CTmax), thermal safety margins, hypoxia tolerance, specific growth rates, body condition and food conversion efficiencies at two ecologically relevant acclimation temperatures (28 and 31°C). Owing to high post-exercise mortality, a third acclimation temperature (33°C) was not investigated further. Acclimation temperature did not affect ṀO2 or growth, but CTmax and hypoxia tolerance were greatest at 31°C and positively associated. We also quantified in vitro temperature (25, 30 and 35°C) and pH effects on haemoglobin–oxygen (Hb–O2) affinity of wild-caught, non-acclimated sharks. As expected, Hb–O2 affinity decreased with increasing temperatures, but pH effects observed at 30°C were absent at 25 and 35°C. Finally, we logged body temperatures of free-ranging sharks and determined that C. melanopterus neonates avoided 31°C in situ. We conclude that C. melanopterus neonates demonstrate minimal thermal dependence of whole-organism physiological performance across a seasonal temperature range and may use behaviour to avoid unfavourable environmental temperatures. The association between thermal tolerance and hypoxia tolerance suggests a common mechanism warranting further investigation. Future research should explore the consequences of ocean warming, especially in nearshore, tropical species.

Regulation of erythrocyte function: Multiple evolutionary solutions for respiratory gas transport and its regulation in fish

Gas transport concepts in vertebrates have naturally been formulated based on human blood. However, the first vertebrates were aquatic, and fish and tetrapods diverged hundreds of millions years ago. Water-breathing vertebrates live in an environment with low and variable O₂ levels, making environmental O₂ an important evolutionary selection pressure in fishes, and various features of their gas transport differ from humans. Erythrocyte function in fish is of current interest, because current environmental changes affect gas transport, and because especially zebrafish is used as a model in biomedical studies, making it important to understand the differences in gas transport between fish and mammals to be able to carry out meaningful studies. Of the close to thirty thousand fish species, teleosts are the most species-numerous group. However, two additional radiations are discussed: agnathans and elasmobranchs. The gas transport by elasmobranchs may be closest to the ancestors of tetrapods. The major difference in their haemoglobin (Hb) function to humans is their high urea tolerance. Agnathans differ from other vertebrates by having Hbs, where cooperativity is achieved by monomer-oligomer equilibria. Their erythrocytes also lack the anion exchange pathway with profound effects on CO₂ transport. Teleosts are characterized by highly pH sensitive Hbs, which can fail to become fully O₂ -saturated at low pH. An adrenergically stimulated Na⁺ /H⁺ exchanger has evolved in their erythrocyte membrane, and plasma-accessible carbonic anhydrase can be differentially distributed among their tissues. Together, and differing from other vertebrates, these features can maximize O₂ unloading in muscle while ensuring O₂ loading in gills.

Body condition impacts blood and muscle oxygen storage capacity of free-living beluga whales (Delphinapterus leucas)

Arctic marine ecosystems are currently undergoing rapid environmental changes. Over the past 20 years, individual growth rates of beluga whales (Delphinapterus leucas) have declined, which may be a response to climate change; however, the scarcity of physiological data makes it difficult to gauge the adaptive capacity and resilience of the species. We explored relationships between body condition and physiological parameters pertaining to oxygen (O₂) storage capacity in 77 beluga whales in the eastern Beaufort Sea. Muscle myoglobin concentrations averaged 77.9 mg g^-1, one of the highest values reported among mammals. Importantly, blood haematocrit, haemoglobin and muscle myoglobin concentrations correlated positively to indices of body condition, including maximum half-girth to length ratios. Thus, a whale with the lowest body condition index would have ∼27% lower blood (26.0 versus 35.7 ml kg^-1) and 12% lower muscle (15.6 versus 17.7 ml kg^-1) O₂ stores than a whale of equivalent mass with the highest body condition index; with the conservative assumption that underwater O₂ consumption rates are unaffected by body condition, this equates to a >3 min difference in maximal aerobic dive time between the two extremes (14.3 versus 17.4 min). Consequently, environmental changes that negatively impact body condition may hinder the ability of whales to reach preferred prey sources, evade predators and escape ice entrapments. The relationship between body condition and O₂ storage capacity may represent a vicious cycle, in which environmental changes resulting in decreased body condition impair foraging, leading to further reductions in condition through diminished prey acquisition and/or increased foraging efforts.

High-Load Reovirus Infections Do Not Imply Physiological Impairment in Salmon

The recent ubiquitous detection of PRV among salmonids has sparked international concern about the cardiorespiratory performance of infected wild and farmed salmon. Piscine orthoreovirus (PRV) has been shown to create substantial viremia in salmon by targeting erythrocytes for principle replication. In some instances, infections develop into heart and skeletal muscle inflammation (HSMI) or other pathological conditions affecting the respiratory system. Critical to assessing the seriousness of PRV infections are controlled infection studies that measure physiological impairment to critical life support systems. Respiratory performance is such a system and here multiple indices were measured to test the hypothesis that a low-virulence strain of PRV from Pacific Canada compromises the cardiorespiratory capabilities of Atlantic salmon. Contrary to this hypothesis, the oxygen affinity and carrying capacity of erythrocytes were unaffected by PRV despite the presence of severe viremia, minor heart pathology and transient cellular activation of antiviral response pathways. Similarly, PRV-infected fish had neither sustained nor appreciable differences in respiratory capabilities compared with control fish. The lack of functional harm to salmon infected with PRV in this instance highlights that, in an era of unprecedented virus discovery, detection of viral infection does not necessarily imply bodily harm and that viral load is not always a suitable predictor of disease within a host organism.

Hydrogen sulphide toxicity and the importance of amphibious behaviour in a mangrove fish inhabiting sulphide-rich habitats

We investigated amphibious behaviour, hydrogen sulphide (H₂S) tolerance, and the mechanism of H₂S toxicity in the amphibious mangrove rivulus (Kryptolebias marmoratus). We found that fish emersed (left water) in response to acutely elevated [H₂S] (~ 130-200 µmol l^-1). The emersion response to H₂S may be influenced by prior acclimation history due to acclimation-induced alterations in gill morphology and/or the density and size of neuroepithelial cells (NECs) on the gills and skin. Thus, we acclimated fish to water (control), H₂S-rich water, or air and tested the hypotheses that acclimation history influences H₂S sensitivity due to acclimation-induced changes in (i) gill surface area and/or (ii) NEC density and/or size. Air-acclimated fish emersed at significantly lower [H₂S] relative to fish acclimated to control or H₂S-rich water, but exhibited no change in gill surface area or in NEC density or size in the gills or skin. Despite possessing exceptional H₂S tolerance, all fish lost equilibrium when unable to emerse from environments containing extremely elevated [H₂S] (2272 ± 46 µmol l^-1). Consequently, we tested the hypothesis that impaired blood oxygen transport (i.e., sulphemoglobin formation) causes H₂S toxicity in amphibious fishes. In vitro exposure of red blood cells to physiologically relevant [H₂S] did not cause a substantial increase in sulphemoglobin formation. We found evidence, however, for an alternative hypothesis that H₂S toxicity is caused by impaired oxidative phosphorylation (i.e., cytochrome c oxidase inhibition). Collectively, our results show that amphibious behaviour is critical for the survival of K. marmoratus in H₂S-rich environments as fish experience impaired oxidative phosphorylation when unable to emerse.

Control of air-breathing in fishes: Central and peripheral receptors

This review considers the environmental and systemic factors that can stimulate air-breathing responses in fishes with bimodal respiration, and how these may be controlled by peripheral and central chemoreceptors. The systemic factors that stimulate air-breathing in fishes are usually related to conditions that increase the O₂ demand of these animals (e.g. physical exercise, digestion and increased temperature), while the environmental factors are usually related to conditions that impair their capacity to meet this demand (e.g. aquatic/aerial hypoxia, aquatic/aerial hypercarbia, reduced aquatic hidrogenionic potential and environmental pollution). It is now well-established that peripheral chemoreceptors, innervated by cranial nerves, drive increased air-breathing in response to environmental hypoxia and/or hypercarbia. These receptors are, in general, sensitive to O₂ and/or CO₂/H⁺ levels in the blood and/or the environment. Increased air-breathing in response to elevated O₂ demand may also be driven by the peripheral chemoreceptors that monitor O₂ levels in the blood. Very little is known about central chemoreception in air-breathing fishes, the data suggest that central chemosensitivity to CO₂/H⁺ is more prominent in sarcopterygians than in actinopterygians. A great deal remains to be understood about control of air-breathing in fishes, in particular to what extent control systems may show commonalities (or not) among species or groups that have evolved air-breathing independently, and how information from the multiple peripheral (and possibly central) chemoreceptors is integrated to control the balance of aerial and aquatic respiration in these animals.

Mechanisms involved in hemoglobin-mediated oxidation of lipids in washed fish muscle and inhibitory effects of phospholipase A2

BACKGROUND

Hemoglobin (Hb) is a lipid oxidation promoter in fish muscle. Phospholipase A2 (PLA2; EC 3.1.1.4) is linked to an increased resistance to lipid oxidation of frozen-thawed cod fillets via an unknown mechanism. The present study aimed to investigate the mechanism of Hb-mediated lipid oxidation with a focus on ferryl Hb and methemoglobin (metHb), the pro-oxidative Hb species, and to examine how porcine pancreatic PLA2 inhibits Hb-mediated lipid oxidation in washed cod muscle (WCM). Lipid hydroperoxides (LOOHs) and thiobarbituric acid reactive substances (TBARS) were measured as primary and secondary lipid oxidation products, respectively. The formation of metHb and ferryl Hb was also monitored.

RESULTS

Ferryl Hb and metHb formed during the Hb-mediated lipid oxidation. PLA2 inhibited the formation of LOOHs and TBARS and suppressed the formation of metHb and ferryl Hb. WCM was pre-oxidized by hemin to increase the amount of LOOHs. PLA2 promoted the depletion of LOOHs in the pre-oxidized WCM with limited TBARS formation at the expense of the heme moiety of Hb.

CONCLUSION

The results of the present study suggest that ferryl Hb may play a role in Hb-mediated lipid oxidation and that PLA2 from pig pancreas may work together with Hb as a novel antioxidant with an ability to remove pre-formed LOOHs from a lipid substrate. © 2017 Society of Chemical Industry.

Factors Affecting Lipid Oxidation Due to Pig and Turkey Hemolysate

Turkey hemolysate promoted lipid oxidation in washed muscle more effectively than pig hemolysate, which was partly attributed to the greater ability of H₂O₂ that formed during auto-oxidation to oxidize the avian hemoglobin (Hb). Turkey and pig hemolysate (2.5 μM Hb) exposed to 10 μM H₂O₂ oxidized to 48% and 4% metHb, respectively. Catalase activity, which converts H₂O₂ to water, was elevated in the pig hemolysate. The larger difference in Hb oxidation when comparing turkey and pig hemolysate in washed muscle (relative to their auto-oxidation rates) suggested that lipid oxidation products facilitated formation of metHb. Turkey metHb released hemin more readily than pig metHb, which coincided with turkey metHb promoting lipid oxidation more effectively than pig metHb. Ferryl Hb was not detected during storage of turkey or pig hemolysate in washed muscle, which suggested a minor role for hypervalent forms of Hb in the oxidation of the lipids.

Life on the edge: O2 binding in Atlantic cod red blood cells near their southern distribution limit is not sensitive to temperature or haemoglobin genotype

Atlantic cod are a commercially important species believed to be threatened by warming seas near their southern, equatorward upper thermal edge of distribution. Limitations to circulatory O₂ transport, in particular cardiac output, and the geographic distribution of functionally different haemoglobin (Hb) genotypes have separately been suggested to play a role in setting thermal tolerance in this species. The present study assessed the thermal sensitivity of O₂ binding in Atlantic cod red blood cells with different Hb genotypes near their upper thermal distribution limit and modelled its consequences for the arterio-venous O₂ saturation difference, Sa–v_O2, another major determinant of circulatory O₂ supply rate. The results showed statistically indistinguishable red blood cell O₂ binding between the three HbI genotypes in wild-caught Atlantic cod from the Irish Sea (53° N). Red blood cells had an unusually low O₂ affinity, with reduced or even reversed thermal sensitivity between pH 7.4 and 7.9, and 5.0 and 20.0°C. This was paired with strongly pH-dependent affinity and cooperativity of red blood cell O₂ binding (Bohr and Root effects). Modelling of Sa–v_O2 at physiological pH, temperature and O₂ partial pressures revealed a substantial capacity for increases in Sa–v_O2 to meet rising tissue O₂ demands at 5.0 and 12.5°C, but not at 20°C. Furthermore, there was no evidence for an increase of maximal Sa–v_O2 with temperature. It is suggested that Atlantic cod at such high temperatures may solely depend on increases in cardiac output and blood O₂ capacity, or thermal acclimatisation of metabolic rate, for matching circulatory O₂ supply to tissue demand.

Highlighted Article: Red blood cell oxygen binding affinity in Atlantic cod near their southern, warmer limit of distribution is largely temperature independent and not affected by functional differences between their major haemoglobin genotypes.

Magnitude of the Root effect in red blood cells and haemoglobin solutions of fishes: a tribute to August Krogh

AIM

The ability of high carbon dioxide tensions or low pH to reduce blood oxygen binding even at high oxygen tensions, first observed by August Krogh and Isabella Leitch in 1919 and now known as the Root effect, was studied in red blood cells and haemoglobin solutions of several fish species.

METHODS

Red blood cells in physiological saline were acidified at atmospheric oxygen tension by increasing carbon dioxide tensions and the percentage decrease in oxygen content was used to quantify the Root effect. Haemoglobin was incubated in air-equilibrated citrate buffers between pH 5 and 7 and the percentage decrease in oxygen saturation relative to pH 8 determined by spectral deconvolution.

RESULTS

The maximal magnitude of the Root effect in citrate-buffered haemoglobin solutions closely matched the value in blood or red blood cells of 11 vertebrates over a Root effect range between 3 and 80%. Contrary to previous reports, there was no evidence for a significant Root effect in red blood cells or haemoglobin solutions of the wels catfish, but a significant Root effect under both conditions in the Siberian sturgeon.

CONCLUSIONS

Under the conditions employed in this study, the maximal Root effect of citrate-buffered haemoglobin solutions closely resembles the maximal Root effect in red blood cells. This strengthens previous studies on the evolution of the Root effect and its role in oxygen concentration at the retina and swimbladder of a large number of fishes that were based on Root effect measurements in haemoglobin solutions.

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.

Development of hydrogen sulfide-based therapeutics for cardiovascular disease

The physiological role of the gaseous signaling molecule hydrogen sulfide (H(2)S) was first realized in the mid-1990s with the work of Abe and Kimura. Since then, it has become evident that this endogenous gas is extremely important in the homeostasis of the cardiovascular system and the pathogenesis of cardiovascular disease. Several biotechnology companies have developed and are developing H(2)S-based therapeutic compounds, and there are ongoing clinical trials investigating the therapeutic potential of H(2)S. Several organic and chemical compounds that are known H(2)S donors have the potential to be developed into effective H(2)S-based therapeutic agents. This review will provide a historical and current perspective on the role(s) of H(2)S in the cardiovascular system and the current state of development and future outlook of H(2)S-based therapies for cardiovascular disease.

Nitric oxide formation from nitrite in zebrafish

Nitrite is a potential nitric oxide (NO) donor and may have important biological functions at low concentrations. The present study tests the hypothesis that nitrite accumulation across the gills in fish will cause a massive NO production from nitrite. Zebrafish were exposed to three different nitrite levels for variable time periods, and changes in blood nitrosylhemoglobin (HbNO), methemoglobin (metHb), oxygenated hemoglobin (oxyHb) and deoxygenated hemoglobin (deoxyHb) were evaluated by spectral deconvolution. Blood HbNO (a biomarker of internal NO production) was low in controls, increased to a stable level around 3.7% of total Hb in fish exposed to 0.6 mmol l(-1) nitrite, and to 12.1% (at day 2) in fish exposed to 2 mmol l(-1) nitrite. The very high HbNO levels testify to an extensive conversion of nitrite to NO. With deoxyHb-mediated reduction of nitrite being a major NO-producing mechanism, the data reveal the significance of this mechanism, when hemoglobin cycles between full and intermediate oxygen saturations in the arterial-venous circulation. Fish exposed to 0.6 mmol l(-1) nitrite for up to 5 days could be divided into responding (with elevated metHb) and non-responding individuals. Exposure to 2 mmol l(-1) nitrite caused a time-dependent increase in metHb to 59% of total Hb within 2 days. Taking HbNO into account, the functional (potential O2 carrying) Hb was reduced to 29% at this stage. Total blood [Hb] was also significantly decreased. In spite of the reduced blood O2 capacitance, and the possibility that excess NO may inhibit mitochondrial respiration, whole animal routine oxygen consumption was not depressed.

Historical reconstructions of evolving physiological complexity:O2 secretion in the eye and swimbladder of fishes

The ability of some fishes to inflate their compressible swimbladder with almost pure oxygen to maintain neutral buoyancy, even against the high hydrostatic pressure several thousand metres below the water surface, has fascinated physiologists for more than 200 years. This review shows how evolutionary reconstruction of the components of such a complex physiological system on a phylogenetic tree can generate new and important insights into the origin of complex phenotypes that are difficult to obtain with a purely mechanistic approach alone. Thus, it is shown that oxygen secretion first evolved in the eyes of fishes, presumably for improved oxygen supply to an avascular, metabolically active retina. Evolution of this system was facilitated by prior changes in the pH dependence of oxygen-binding characteristics of haemoglobin (the Root effect) and in the specific buffer value of haemoglobin. These changes predisposed teleost fishes for the later evolution of swimbladder oxygen secretion, which occurred at least four times independently and can be associated with increased auditory sensitivity and invasion of the deep sea in some groups. It is proposed that the increasing availability of molecular phylogenetic trees for evolutionary reconstructions may be as important for understanding physiological diversity in the postgenomic era as the increase of genomic sequence information in single model species.

Mitochondrial depolarization following hydrogen sulfide exposure in erythrocytes from a sulfide-tolerant marine invertebrate

Sulfide-tolerant marine invertebrates employ a variety of mechanisms to detoxify sulfide once it has entered their bodies, but their integumentary,respiratory epithelium and circulatory cells may still be exposed to toxic sulfide concentrations. To investigate whether sulfide exposure is toxic to mitochondria of a sulfide-tolerant invertebrate, we used the fluorescent dyes JC-1 and TMRM to determine the effect of sulfide exposure on mitochondrial depolarization in erythrocytes from the annelid Glycera dibranchiata. In erythrocytes exposed to 0.11-1.9 mmol l-1 sulfide for 1 h, the dyes showed fluorescence changes consistent with sulfide-induced mitochondrial depolarization. At the highest sulfide concentration, the extent of depolarization was equivalent to that caused by the mitochondrial uncoupler carbonyl cyanide m-chlorophenylhydrazone (CCCP). Even when induced by as little as 0.3 mmol l-1 sulfide, the depolarization was not reversible over a subsequent 5 h recovery period. The mechanism of toxicity was likely not via inhibition of cytochrome c oxidase (COX),since other COX inhibitors and other mitochondrial electron transport chain inhibitors did not produce similar effects. Furthermore, pharmacological inhibition of the mitochondrial permeability transition pore failed to prevent sulfide-induced depolarization. Finally, increased oxidation of the free radical indicators H2DCFDA and MitoSOX™ in erythrocytes exposed to sulfide suggests that sulfide oxidation increased oxidative stress and superoxide production, respectively. Together, these results indicate that sulfide exposure causes mitochondrial depolarization in cells of a sulfide-tolerant annelid, and that this effect, which differs from the actions of other COX inhibitors, may be via increased free radical damage.

Physiological responses to sulfide toxicity by the air-breathing catfish, Hoplosternum littorale (Siluriformes, Callichthyidae)

Hemolytic anemia accompanied by changes in the immunology system is one of the sulfide intoxication harmful effects on Hoplosternum littorale. Hematological parameters are considered as effective indicators of stress caused by this hydrogen sulfide. During sulfide exposure, H. littorale neither alters the methemoglobin concentration nor forms sulfhemoglobin in the presence of high levels of dissolved sulfide in the water. Cytochrome c oxidase shows little activity in the gills and blood of H. littorale when exposed to sulfide. Alternative metabolic routes are suggested through which the accumulation of pyruvate leads to the formation of an end product other than lactate.

Blood parameters and metabolites in the teleost fish Colossoma macropomum exposed to sulfide or hypoxia

Juvenile tambaqui, Colossoma macropomum, were exposed to sulfide and hypoxia for 12, 24, 48 and 96 h. Hemoglobin concentrations, red blood cell counts, and mean cell hemoglobin, were higher at 12 h in fish exposed to hypoxia. However, control fish and those exposed to sulfide and hypoxia had lower red blood cell count, hemoglobin concentration and hematocrit at 96 h. Methemoglobin was higher than in the controls, probably due to the hypoxemia induced by these stressors. Sulfhemoglobin was not detected in significant amounts in the blood of fish exposed to sulfide (in vivo), yet hemoglobin converted into sulfhemoglobin at 1-15 mM sulfide in vitro. Anaerobic metabolism seemed to be an important mechanism for adapting to sulfide exposure and blood pH returned to control values after 24 h of sulfide, preventing acidosis. The high sulfide tolerance in tambaqui is associated with its high tolerance to hypoxia.