Haemoglobin function in vertebrates: evolutionary changes in cellular regulation in hypoxia

Hypoxia-acclimation adjusts skeletal muscle anaerobic metabolism and burst swim performance in a marine fish

Red drum, Sciaenops ocellatus, are a marine teleost native to the Gulf of Mexico that routinely experiences periods of low oxygen (hypoxia). Recent work has demonstrated this species has the capacity to improve aerobic performance in hypoxia through respiratory acclimation. However, it remains unknown how hypoxia acclimation impacts anaerobic metabolism in red drum, and the consequences of exhaustive exercise and recovery. Juvenile fish were acclimated to normoxia (n = 15, DO 90.4 ± 6.42 %) or hypoxia (n = 15, DO 33.6 ± 7.2 %) for 8 days then sampled at three time points: at rest, after exercise, and after a 3 h recovery period. The resting time point was used to characterize the acclimated phenotype, while the remaining time points demonstrate how this phenotype responds to exhaustive exercise. Whole blood, red muscle, white muscle, and heart tissues were sampled for metabolites and enzyme activity. The resting phenotype was characterized by lower pHe and changes to skeletal muscle ATP. Exhaustive exercise increased muscle lactate, and decreased phosphocreatine and ATP with no effect of acclimation. Interestingly, hypoxia-acclimated fish had higher pHe and pHi than control in all exercise time points. Red muscle ATP was lower in hypoxia-acclimated fish versus control at each sample period. Moreover, acclimated fish increased lactate dehydrogenase activity in the red muscle. Hypoxia acclimation increased white muscle ATP and hexokinase activity, a glycolytic enzyme. In a gait-transition swim test, hypoxia-acclimated fish recruited anaerobic-powered burst swimming at lower speeds in normoxia compared to control fish. These data suggest that acclimation increases reliance on anaerobic metabolism, and does not benefit recovery from exhaustive exercise.

The physiological significance of plasma-accessible carbonic anhydrase in the respiratory systems of fishes

Carbonic anhydrase (CA) activity is ubiquitously found in all vertebrate species, tissues and cellular compartments. Most species have plasma-accessible CA (paCA) isoforms at the respiratory surfaces, where the enzyme catalyzes the conversion of plasma bicarbonate to carbon dioxide (CO2) that can be excreted by diffusion. A notable exception are the teleost fishes that appear to lack paCA at their gills. The present review: (i) recapitulates the significance of CA activity and distribution in vertebrates; (ii) summarizes the current evidence for the presence or absence of paCA at the gills of fishes, from the basal cyclostomes to the derived teleosts and extremophiles such as the Antarctic icefishes; (iii) explores the contribution of paCA to organismal CO2 excretion in fishes; and (iv) the functional significance of its absence at the gills, for the specialized system of O2 transport in most teleosts; (v) outlines the multiplicity and isoform distribution of membrane-associated CAs in fishes and methodologies to determine their plasma-accessible orientation; and (vi) sketches a tentative time line for the evolutionary dynamics of branchial paCA distribution in the major groups of fishes. Finally, this review highlights current gaps in the knowledge on branchial paCA function and provides recommendations for future work.

Impact of the novel chlorinated polyfluorinated ether sulfonate, F-53B, on gill structure and reproductive toxicity in zebrafish

6:2 Chlorinated polyfluorinated ether sulfonate, commonly known as F-53B, is widely used as a mist suppressant in various industries and is frequently detected in the environment. Despite its prevalent presence, the adverse effects of F-53B are not well understood and require future investigation. This study utilized zebrafish embryos and adults to examine the toxic effects of F-53B. Our findings revealed that F-53B impaired gill structure and increased erythrocyte numbers in adult zebrafish. Notably, F-53B demonstrated a higher sensitivity for inducing mortality (LC50 at 96 h) in adult zebrafish compared to embryos. Additionally, F-53B disrupted the expression of critical steroidogenic genes and hindered sex hormone production, which negatively affecting egg production. In conclusion, this study underscores the detrimental impact of F-53B on gill structure and reproductive toxicity in zebrafish, providing valuable insights into its overall toxicity.

Testing Green Tea Extract and Ammonium Salts as Stimulants of Physical Performance in a Forced Swimming Rat Experimental Model

The study of drugs of natural origin that increase endurance and/or accelerate recovery is an integral part of sports medicine and physiology. In this paper, decaffeinated green tea extract (GTE) and two ammonium salts-chloride (ACL) and carbonate (ACR)-were tested individually and in combination with GTE as stimulants of physical performance in a forced swimming rat experimental model. The determined parameters can be divided into seven blocks: functional (swimming duration); biochemistry of blood plasma; biochemistry of erythrocytes; hematology; immunology; gene expression of slow- and fast-twitch muscles (m. soleus, SOL, and m. extensor digitorum longus, EDL, respectively); and morphometric indicators of slow- and fast-twitch muscles. Regarding the negative control (intact animals), the maximum number of changes in all blocks of indicators was recorded in the GTE + ACR group, whose animals showed the maximum functional result and minimum lactate values on the last day of the experiment. Next, in terms of the number of changes, were the groups ACR, ACL, GTE + ACL, GTE and NaCl (positive control). In general, the number of identified adaptive changes was proportional to the functional state of the animals of the corresponding groups, in terms of the duration of the swimming load in the last four days of the experiment. However, not only the total number but also the qualitative composition of the identified changes is of interest. The results of a comparative analysis suggest that, in the model of forced swimming we developed, GTE promotes restoration of the body and moderate mobilization of the immune system, while small doses of ammonium salts, especially ammonium carbonate, contribute to an increase in physical performance, which is associated with satisfactory restoration of skeletal muscles and the entire body. The combined use of GTE with ammonium salts does not give a clearly positive effect.

ROS formation, mitochondrial potential and osmotic stability of the lamprey red blood cells: effect of adrenergic stimulation and hypoosmotic stress

Semi-anadromous animals experience salinity fluctuations during their life-span period. Alterations of environmental conditions induce stress response where catecholamines (CA) play a central role. Physiological stress and changes in external and internal osmolarity are frequently associated with increased production of reactive oxygen species (ROS). In this work, we studied the involvement of the cAMP/PKA pathway in mediating catecholamine-dependent effects on osmoregulatory responses, intracellular production of ROS, and mitochondrial membrane potential of the river lamprey (Lampetra fluviatilis, Linnaeus, 1758) red blood cells (RBCs). We also investigated the role of hypoosmotic shock in the process of ROS production and mitochondrial respiration of RBCs. For this, osmotic stability and the dynamics of the regulatory volume decrease (RVD) following hypoosmotic swelling, intracellular ROS levels, and changes in mitochondrial membrane potential were assessed in RBCs treated with epinephrine (Epi, 25 μM) and forskolin (Forsk, 20 μM). Epi and Forsk markedly reduced the osmotic stability of the lamprey RBCs whereas did not affect the dynamics of the RVD response in a hypoosmotic environment. Activation of PKA with Epi and Forsk increased ROS levels and decreased mitochondrial membrane potential of the lamprey RBCs. In contrast, upon hypoosmotic shock enhanced ROS production in RBCs was accompanied by increased mitochondrial membrane potential. Overall, a decrease in RBC osmotic stability and the enhancement of ROS formation induced by β-adrenergic stimulation raises concerns about stress-associated changes in RBC functions in agnathans. Increased ROS production in RBCs under hypoosmotic shock indicates that a decrease in blood osmolarity may be associated with oxidative damage of RBCs during lamprey migration.

Hematological, biochemical and oxidative responses induced by thermal shock in juvenile Tambaqui (Colossoma macropomum) and its hybrid Tambatinga (Colossoma macropomum x Piaractus brachypomus)

The effects of thermal shock on hematological, biochemical and antioxidant responses were evaluated in liver tissue of juvenile tambaqui (Colossoma macropomum) and tambatinga (♀ C. macropomum × ♂ Piaractus brachypomus). Forty juveniles of tambaqui and 40 juveniles of tambatinga, of the same age and with an initial weight of 23.3 ± 6.7 g, were randomly distributed in eight 28L circular tanks. A tank (n = 10 fish) of tambaqui and a tank (n = 10 fish) of tambatinga were then used to obtain basal data. The other animals were subjected to thermal shock with sudden temperature reduction from 28 to 18 ºC. Blood and tissue were then collected after 1, 6 and 24 h from the onset of thermal shock. No mortality was observed during the experimental period. Thermal shock increased triglyceride levels after 24 h of stress for tambaqui and reduced values for tambatinga. There was an effect on plasma glucose only for fish group (P < 0.0001) and collection time (P < 0.0001) with a peak observed for the hybrid after 6 h. The interaction of factors for SOD indicated greater activity for tambatinga at the 6 h collection and lower at basal and 1 h collections. There was an interaction for CAT (P = 0.0020) with less activity for tambatinga at 1 h. However, thermal shock and hybridization did not influence GST and TBARS levels in liver tissue. Therefore, the results suggest that the hybrid, tambatinga, is more efficient at promoting adjustments of biochemical responses and antioxidant enzymes during thermal shock.

Respiratory plasticity improves aerobic performance in hypoxia in a marine teleost

Ocean deoxygenation is a pressing concern in the face of climate change. In response to prolonged hypoxia, fishes have demonstrated the ability to dynamically regulate hemoglobin (Hb) expression to enhance oxygen (O₂) uptake. Here, we examined hypoxia-inducible Hb expression in red drum (Sciaenops ocellatus) and the subsequent implications on Hb-O₂ binding affinity and aerobic scope. Fish were acclimated to 30 % air saturation for 1 d, 4 d, 8 d, 2 w, or 6 w, and red blood cells were collected for gene expression and biochemical profiling. Hypoxia acclimation induced significant up-regulation of one Hb subunit isoform (hbα 2) relative to control by 4 d with consistent upregulation thereafter. Hematocrit increased in hypoxia, with no changes in the allosteric modulator [NTP] at any time point. Changes in Hb expression co-occurred with a reduced Root effect (~26 % in normoxia, ~14 % in hypoxia) at a physiologically relevant pH while increasing O₂ binding affinity (i.e., lower P₅₀). These changes correlated with increased maximum metabolic rate and aerobic scope relative to controls when fish were tested in hypoxia. These results demonstrate an important role for Hb multiplicity in improving O₂ affinity and maximizing respiratory performance in hypoxia.

Hematological and serum biochemical reference intervals of rainbow trout, Oncorhynchus mykiss cultured in Himalayan aquaculture: Morphology, morphometrics and quantification of peripheral blood cells

De novo reference intervals (RIs) for a total of thirty two hematological and serum biochemical attributes were established for rainbow trout (Oncorhynchus mykiss) cultured in Himalayan aquaculture system. For this purpose, long term assessment of hemato-biochemical parameters was carried over a period of one year from March 2019 to February 2020 and a total of 444 blood samples were analysed. Blood examination results were recorded systematically and reference intervals were established, notably for erythrocyte parameters: hematocrit (Hct) 29–40%, hemoglobin 8.32–12.28 g/dL, red blood cell (RBC) count 1.01–2.04 (×10⁶/mm3); leukocyte parameters (x 10³/mm3): total leukocytes 31.32–90.60, neutrophils 4.21–18.85, total lymphocytes 20.55–63.63, small lymphocytes 14.86–46.50, large lymphocytes 6.35–22.34 and monocytes 1.22–7.56; thrombocyte count 23.00–68.00 (×10⁶/mm3). RIs were also established for red blood cell indices, vital serum constituents involved in carbohydrate, protein, lipid and nitrogen metabolism including the less known, diagnostically important, serum enzymes and electrolyte concentrations. Principal component analysis revealed that certain serum components were more efficient at distinguishing between the life stages (juvenile, adult) of fish by explaining about 92.7% of variation in the whole dataset compared to the principal hematological components which explained only about 80% of the variation. Significant (P < 0.05) differences were noted for RBC count, total leukocyte count (TLC), total protein, total cholesterol and uric acid with respect to the sex of fish. Moreover, clearly differentiable morphometric and morphological attributes were also noticed among erythrocytes, leukocytes (lymphocytes, neutrophils and monocytes) and thrombocytes. To our knowledge, the present study is the first of its kind that elucidates blood chemistry of cultured rainbow trout, O. mykiss in accordance to the guidelines framed by the American society of veterinary clinical pathologists (ASVCP). RIs reported here can help monitor the fish health status by improving the use of non-lethal diagnostics in piscine medicine.

Rapid and reversible modulation of blood haemoglobin content during diel cycles of hypoxia in killifish (Fundulus heteroclitus)

We investigated whether fish can make dynamic haematological adjustments to support aerobic metabolism during repeated cycles of hypoxia-reoxygenation. Killifish were acclimated to normoxia, constant hypoxia (2 kPa O₂), or intermittent cycles of nocturnal hypoxia (12 h of normoxia: 12 h of 2 kPa O₂ hypoxia) for 28 days. Normoxia-acclimated fish were sampled in the daytime in normoxia and after exposure to a single bout of nocturnal hypoxia. Each hypoxia acclimation group were sampled at the PO₂ experienced during acclimation during both the day and night. All acclimation groups had increased blood haemoglobin content and haematocrit and reduced spleen mass during nocturnal hypoxia compared to normoxic controls. Blood haemoglobin content was negatively correlated with spleen mass at both the individual and group level. Fish acclimated to intermittent hypoxia rapidly reversed these changes during diurnal reoxygenation. The concentrations of haemoglobin, ATP, and GTP within erythrocytes did not vary substantially between groups. We also measured resting O₂ consumption rate (MO₂) and maximum MO₂ (induced by an exhaustive chase) in hypoxia in each acclimation group. Fish acclimated to intermittent hypoxia maintained higher resting MO₂ than other groups in hypoxia, comparable to the resting MO₂ of normoxia-acclimated controls measured in normoxia. Differences in resting MO₂ in hypoxia did not result from variation in O₂ transport capacity, because maximal MO₂ in hypoxia always exceeded resting MO₂. Therefore, reversible modulation of blood haemoglobin content along with metabolic adjustments help killifish cope with intermittent cycles of hypoxia in the estuarine environment.

Hypoxia inducible factor 1-α is minimally involved in determining the time domains of the hypoxic ventilatory response in adult zebrafish (Danio rerio)

In the current study, adult zebrafish (Danio rerio) were exposed to 72 h hypoxia (90 mmHg) to assess the time domains of the hypoxia ventilatory response (HVR) and the consequence on a subsequent more severe (40 mmHg) bout of acute hypoxia. Experiments were performed on wild-type fish and mutants in which one or both paralogs of hypoxia inducible factor-1α (hif-1α) were knocked out. Although there were subtle differences among the wild-type and knockout fish, resting f_V was reestablished after 2-8 h of continuous hypoxia in both groups, a striking example of hypoxic ventilatory decline (HVD). When fish were subsequently exposed to more severe hypoxia, a rapid increase in f_V was observed, the magnitude of which was independent of genotype or prior exposure history. During recovery, fish that had been exposed to 72 h of 90 mmHg hypoxia exhibited a pronounced undershoot in f_V, which was absent in the hif-1α double knockouts. Overall, the results revealed distinct time domains of the HVR in zebrafish that were largely Hif-1α-independent.

Hb adaptation to hypoxia in high-altitude fishes: Fresh evidence from schizothoracinae fishes in the Qinghai-Tibetan Plateau

Uncovering the genetic basis of hypoxic adaptation is one of the most active research areas in evolutionary biology. Among air-breathing vertebrates, modifications of hemoglobin (Hb) play a pivotal role in mediating an adaptive response to high-altitude hypoxia. However, the relative contributions in water-breathing organisms are still unclear. Here, we tested the Hb concentration of fish at different altitudes. All species showed species-specific Hb concentration, which has a non-positive correlation with altitude. Moreover, we investigated the expression of Hb genes by the RNA-seq and quantitative real-time PCR (qRT-PCR), and Hb composition by two-dimensional electrophoresis (2-DE). The results showed that the multiple Hb genes and isoforms are co-expressed in schizothoracinae fishes endemic to the Qinghai-Tibetan Plateau (QTP). Phylogenetic analyses of Hb genes indicated that the evolutionary relationships are not easily reconciled with the organismal phylogeny. Furthermore, evidence of positive selection was found in the Hb genes of schizothoracinae fishes through the selection pressure analysis. We demonstrated that positively selected sites likely facilitated the functional divergence of Hb isoforms. Taken together, this study indicated that the long-term maintenance of high Hb concentration may be a disadvantage for physiologically acclimating to high altitude hypoxia. Meanwhile, the genetically based modification of Hb-O₂ affinity in schizothoracinae fishes might facilitate the evolutionary adaptation to Tibetan aqueous environments.

Constant and intermittent hypoxia modulates immunity, oxidative status, and blood components of red seabream and increases its susceptibility to the acute toxicity of red tide dinoflagellate

Hypoxia is an increasing threat to aquatic ecosystems and its impact on economically and ecologically important marine fish species needs to be studied. Especially, the consequences of hypoxia when occurring along with harmful algal blooms (HABs) are currently not well documented. In this study, we investigated the effect of constant and intermittent (daily and weekly) hypoxia on respiration, immunity, hematological parameters, and oxidative status of red seabream for 2, 4, and 6 weeks. Under constant and daily intermittent hypoxia, respiration rate significantly increased in 2 weeks compared to the control. Constant and daily intermittent hypoxia caused significant decreases in the activity of alternative complement pathway, lysozyme, and the level of total immunoglobulin (Ig), as well as significant increases in the concentrations of cortisol, hemoglobin, red blood cells, and white blood cells. A significantly higher level of malondialdehyde was measured for all hypoxia-exposed groups, indicating lipid peroxidation and oxidative stress. At 4 and 6 week, the level of glutathione and enzymatic activities of glutathione reductase and glutathione peroxidase were significantly decreased after constant and daily intermittent hypoxia challenge. The enzymatic activities of superoxide dismutase and catalase were significantly increased at 2 and 4 weeks, but they were decreased after 6 weeks by constant and daily intermittent hypoxia. Constant and daily intermittent hypoxia with subsequent non-toxin producing dinoflagellate Cochlodinium polykrikoides treatment significantly reduced the respiration rate in 3 and 24 h exposure and survival rate of red seabream. Taken together, the red seabream can be vulnerable to HABs under hypoxia condition through inhibition of immunity and antioxidant defense ability. Our findings are helpful in better understanding of molecular and physiological effects of hypoxia, which can be used in aquaculture and fisheries management.

Molecular and cellular responses to long-term sound exposure in peled (Coregonus peled)

This research examined the impacts of acoustic stress in peled (Coregonus peled Gmelin, 1788), a species commonly cultivated in Russia. This study presents a comparative analysis of the macula sacculi and otoliths, as well as primary hematological and secondary telomere stress responses, in control and sound-exposed peled. The authors measured the effects of long-term (up to 18 days) exposure to a 300 Hz tone at mean sound pressure levels of 176-186 dB re 1 μPa (SPL_pk-pk); the frequency and intensity were selected to approximate loud acoustic environments associated with cleaning equipment in aquaculture settings. Acoustic exposure resulted in ultrastructure changes to otoliths, morphological damage to sensory hair cells of the macula sacculi, and a gradual decrease in the number of functionally active mitochondria in the red blood cells but no changes to telomeres. Changes were apparent following at least ten days of acoustic exposure. These data suggest that acoustic exposure found in some aquaculture settings could cause stress responses and auditory damage to peled and, potentially, other commercially important species. Reducing sound levels in fish rearing facilities could contribute to the formation of effective aquaculture practices that mitigate noise-induced stress in fishes.

Response of juvenile Lophiosilurus alexandri to osmotic and thermic shock

The objective of the present study was to evaluate the physiological responses of juvenile Lophiosilurus alexandri submitted to osmotic and thermic shock. Thirty juveniles were used for each test, of which 10 were not subjected to stress and remained in normal conditions (fresh water at 28.0 °C). The others were submitted to stress shock (saline water of 10.0 g of salt/L or water cooled to 18.0 °C). Blood samples were taken at 0 h (no exposure to the stress factor) and 1 h and 24 h after the tests. At 24 h, the survivorship was 100% in both tests. In both the osmotic and thermic shock tests, cortisol and glucose levels were higher at 1 h but then decreased after 24 h. Lactate dehydrogenase showed differences in the temperature test, but there was no difference between 1 and 24 h after exposure to osmotic shock (P > 0.05). The difference was recorded in blood gas variables (pH, PvCO₂, PvO₂, hemoglobin, sO₂, BE, tCO₂, HCO₃^-, and stHCO₃^-) and electrolytes (Na⁺, Ca⁺⁺, nCa⁺⁺, and K⁺) in both experiments. With regard to hematology and blood biochemistry, exposure to thermal shock did not affect (P > 0.05) ALP, total plasma protein, hematocrit, and ALT and AST at 1 h and 24 h. ALP and total protein in the blood of fish submitted to the osmotic shock were lowest (P < 0.05) at 24 h. Leukocyte and erythrocyte counts exhibited differences after osmotic shock, in contrast to erythrocyte counts of the temperature test, which did not change in 24 h (P > 0.05). Juveniles of L. alexandri were able to reestablish the main indicators of stress (cortisol, glucose), while the others (hematological, biochemical, and gasometric) varied in compensation for normal physiological reestablishment.

Sublethal exposure to copper supresses the ability to acclimate to hypoxia in a model fish species

Hypoxia is one of the major threats to biodiversity in aquatic systems. The association of hypoxia with nutrient-rich effluent input into aquatic systems results in scenarios where hypoxic waters could be contaminated with a wide range of chemicals, including metals. Despite this, little is known about the ability of fish to respond to hypoxia when exposures occur in the presence of environmental toxicants. We address this knowledge gap by investigating the effects of exposures to different levels of oxygen in the presence or absence of copper using the three-spined sticklebacks (Gasterosteus aculeatus) model. Fish were exposed to different air saturations (AS; 100%, 75% and 50%) in combination with copper (20 μg/L) over a 4 day period. The critical oxygen level (P_crit), an indicator of acute hypoxia tolerance, was 54.64 ± 2.51% AS under control conditions, and 36.21 ± 2.14% when fish were chronically exposed to hypoxia (50% AS) for 4 days, revealing the ability of fish to acclimate to low oxygen conditions. Importantly, the additional exposure to copper (20 μg/L) prevented this improvement in P_crit, impairing hypoxia acclimation. In addition, an increase in ventilation rate was observed for combined copper and hypoxia exposure, compared to the single stressors or the controls. Interestingly, in the groups exposed to copper, a large increase in variation in the measured P_crit was observed between individuals, both under normoxic and hypoxic conditions. This variation, if observed in wild populations, may lead to selection for a tolerant phenotype and alterations in the gene pool of the populations, with consequences for their sustainability. Our findings provide strong evidence that copper reduces the capacity of fish to respond to hypoxia by preventing acclimation and will inform predictions of the consequences of global increases of hypoxia in water systems affected by other pollutants worldwide.

Tissue-Specific Orchestration of Gilthead Sea Bream Resilience to Hypoxia and High Stocking Density

Two different O₂ levels (normoxia: 75–85% O₂ saturation; moderate hypoxia: 42–43% O₂ saturation) and stocking densities (LD: 9.5, and HD: 19 kg/m³) were assessed on gilthead sea bream (Sparus aurata) in a 3-week feeding trial. Reduced O₂ availability had a negative impact on feed intake and growth rates, which was exacerbated by HD despite of the improvement in feed efficiency. Blood physiological hallmarks disclosed the enhancement in O₂-carrying capacity in fish maintained under moderate hypoxia. This feature was related to a hypo-metabolic state to cope with a chronic and widespread environmental O₂ reduction, which was accompanied by a differential regulation of circulating cortisol and growth hormone levels. Customized PCR-arrays were used for the simultaneous gene expression profiling of 34–44 selected stress and metabolic markers in liver, white skeletal muscle, heart, and blood cells. The number of differentially expressed genes ranged between 22 and 19 in liver, heart, and white skeletal muscle to 5 in total blood cells. Partial Least-Squares Discriminant Analysis (PLS-DA) explained [R2Y(cum)] and predicted [Q2Y(cum)] up to 95 and 65% of total variance, respectively. The first component (R2Y = 0.2889) gathered fish on the basis of O₂ availability, and liver and cardiac genes on the category of energy sensing and oxidative metabolism (cs, hif-1α, pgc1α, pgc1β, sirts 1-2-4-5-6-7), antioxidant defense and tissue repair (prdx5, sod2, mortalin, gpx4, gr, grp-170, and prdx3) and oxidative phosphorylation (nd2, nd5, and coxi) highly contributed to this separation. The second component (R2Y = 0.2927) differentiated normoxic fish at different stocking densities, and the white muscle clearly promoted this separation by a high over-representation of genes related to GH/IGF system (ghr-i, igfbp6b, igfbp5b, insr, igfbp3, and igf-i). The third component (R2Y = 0.2542) discriminated the effect of stocking density in fish exposed to moderate hypoxia by means of hepatic fatty acid desaturases (fads2, scd1a, and scd1b) and muscle markers of fatty acid oxidation (cpt1a). All these findings disclose the different contribution of analyzed tissues (liver ≥ heart > muscle > blood) and specific genes to the hypoxic- and crowding stress-mediated responses. This study will contribute to better explain and understand the different stress resilience of farmed fish across individuals and species.

Hypoxia enhances blood O2 affinity and depresses skeletal muscle O2 consumption in zebrafish (Danio rerio)

Zebrafish (Danio rerio) are widely used animal models. Nevertheless, the mechanisms underlying hypoxia tolerance in this species have remained poorly understood. In the present study, we have determined the effects of hypoxia on blood-O₂ transport properties and mitochondrial respiration rate in permeabilized muscle fibres of adult zebrafish exposed to either 1) a gradual decrease in O₂ levels until fish lost equilibrium (~1 h, acute hypoxia), or 2) severe hypoxia (PO₂ ∼ 15 Torr) for 48 h (prolonged hypoxia). Acute, short-term hypoxia caused an increase in hemoglobin (Hb) O₂ affinity (decrease in P₅₀), due to a decrease in erythrocyte ATP after erythrocyte swelling. No changes in isoHb expression patterns were observed between hypoxic and normoxic treatments. Prolonged hypoxia elicited additional reponses on O₂ consumption: lactate accumulated in the blood, indicating that zebrafish relied on glycolysis for ATP production, and mitochondrial respiration of skeletal muscle was overall significantly inhibited. In addition, male zebrafish had higher hypoxia tolerance (measured as time to loss of equilibrium) than females. The present study contributes to our understanding of the adaptive mechanisms that allow zebrafish, and by inference other fish species, to cope with low O₂ levels.

Glutathione peroxidase and glutathione S-transferase in blood and liver from a hypoxia-tolerant fish under oxygen deprivation

Liver enzyme activities can be employed as biomarkers, but liver can only be obtained with death of the specimen. On the other hand, blood withdrawal is a non-lethal procedure. Accordingly, the hypothesis of this study is to verify if glutathione peroxidase (GPX) and glutathione S-transferase (GST) activities in blood parallel those in the liver of the hypoxia-tolerant fish, Piaractus mesopotamicus (pacu), submitted to hypoxia conditions. GPX was assayed with H₂O₂ in cytosols from both liver and erythrocytes and exhibited no significant variation, either in erythrocytes or in liver, when comparing pacus under normoxia with those under hypoxia (42 h). GST activity with chloro-dinitrobenzene (CDNB), an artificial substrate suitable for almost all GST isoenzymes, was compared to activity with 4-hydroxy-nonenal (4-HNE), a physiological endogenous substrate. GST activity with CDNB did not change in liver or in erythrocyte cytosols in pacus under hypoxia compared to those under normoxia. On the other hand, a significant decrease in erythrocyte activity with 4-HNE was observed after 42 h of hypoxia in both erythrocytes and liver, which may be a response to increased lipid oxidation in erythrocytes. Erythrocyte GST activity was 3-fold higher with 4-HNE than with CDNB, indicating that 4-HNE is a more appropriate substrate to determine GST activity in pacu erythrocytes.

Molecular evolution of myoglobin in the Tibetan Plateau endemic schizothoracine fish (Cyprinidae, Teleostei) and tissue-specific expression changes under hypoxia

Myoglobin (Mb) is an oxygen-binding hemoprotein that was once thought to be exclusively expressed in oxidative myocytes of skeletal and cardiac muscle where it serves in oxygen storage and facilitates intracellular oxygen diffusion. In this study, we cloned the coding sequence of the Mb gene from four species, representing three groups, of the schizothoracine fish endemic to the Qinghai-Tibetan Plateau (QTP), then conducted molecular evolution analyses. We also investigated tissue expression patterns of Mb and the expression response to moderate and severe hypoxia at the mRNA and protein levels in a representative of the highly specialized schizothoracine fish species, Schizopygopsis pylzovi. Molecular evolution analyses showed that Mb from the highly specialized schizothoracine fish have undergone positive selection and one positively selected residue (81L) was identified, which is located in the F helix, close to or in contact with the heme. We present tentative evidence that the Mb duplication event occurred in the ancestor of the schizothoracine and Cyprininae fish (common carp and goldfish), and that the Mb2 paralog was subsequently lost in the schizothoracine fish. In S. pylzovi, Mb mRNA is expressed in various tissues with the exception of the intestine and gill, but all such tissues, including the liver, muscle, kidney, brain, eye, and skin, expressed very low levels of Mb mRNA (< 8.0%) relative to that of the heart. The trace levels of Mb expression in non-muscle tissues are perhaps the major reason why non-muscle Mb remained undiscovered for so long. The expression response of the Mb gene to hypoxia at the mRNA and protein levels was strikingly different in S. pylzovi compared to that found in the common carp, medaka, zebrafish, and goldfish, suggesting that the hypoxia response of Mb in fish may be species and tissue-specific. Notably, severe hypoxia induced significant expression of Mb at the mRNA and protein levels in the S. pylzovi heart, which suggests Mb has a major role in the supply of oxygen to the heart of Tibetan Plateau fish.

Acute hypoxic stress: Effect on blood parameters, antioxidant enzymes, and expression of HIF-1alpha and GLUT-1 genes in largemouth bass (Micropterus salmoides)

Dissolved oxygen (DO) plays a crucial role in survival, growth, and normal physiological functions of aquatic organisms. Nevertheless, the mechanisms involved in hypoxic stress and adaptation have not been fully elucidated in Largemouth bass (Micropterus salmoides). To reveal the effect of acute hypoxia on Largemouth bass, we simulated acute hypoxia (DO: 1.2 ± 0.2 mg/L) in the laboratory and analyzed physiological parameters (RBCs, Hb, SOD, CAT, NA⁺/K⁺-ATPase, GPx, and MDA) and gene expression (HIF-1alpha and GLUT-1) in Largemouth bass exposed to various durations of acute hypoxia (0, 1, 2, 4, 8, 12, and 24 h). Our results indicated that acute hypoxic exposure significantly increased RBCs but decreased Hb. In addition, antioxidant enzyme activity was enhanced significantly in the liver and muscles at the initial stage of acute hypoxic exposure, but decreased significantly in gills during the entire process of hypoxic exposure. Furthermore, the expression levels of HIF-1alpha and GLUT-1 mRNA were significantly up-regulated in Largemouth bass under acute hypoxic exposure. In conclusion, our study provides a valuable basis for further elucidation of hypoxic adaptation and facilitates husbandry for an economically valuable species.

Early exposure to chronic hypoxia induces short- and long-term regulation of hemoglobin gene expression in European sea bass (Dicentrarchus labrax)

European sea bass (Dicentrarchus labrax) inhabits coastal waters and may be exposed to hypoxia at different life stages, requiring physiological and behavioral adaptation. In the present study, we attempted to determine whether regulation of hemoglobin (Hb) gene expression plays a role in the physiological response to chronic moderate hypoxia in whole larvae and hematopoietic tissues (head kidney and spleen) of juveniles. We also tested the hypothesis that hypoxia exposure at the larval stage could induce a long-term effect on the regulation of Hb gene expression. For this purpose, D. labrax were exposed to a non-lethal hypoxic condition (40% air saturation) at the larval stage from 28 to 50 days post-hatching (dph) and/or at the juvenile stage from 196 to 296 dph. Data obtained from larvae indicate that hypoxia induced a subtype-specific regulation of Hb gene expression, with a significant decrease of MN-Hbα3, MN-Hbβ4 and MN-Hbβ5 and increase of MN-Hbα2, LA-Hbα1 and LA-Hbβ1 transcript levels. Hypoxia did not induce regulation of Hb gene expression in juveniles, except in the head kidney for those that experienced hypoxia at the larval stage. The latter exhibited a significant hypoxia-induced stimulation of MN-Hbα2, LA-Hbα1 and LA-Hbβ1 gene expression, associated with stimulation of the PHD-3 gene involved in the hypoxia-inducible factor oxygen-sensing pathway. We conclude that subtype- and stage-specific regulation of Hb gene expression plays a role in the physiological response of D. labrax to cope with hypoxia and that early exposure to low oxygen concentration has a long-term effect on this response.

Gene Duplication and Evolutionary Innovations in Hemoglobin-Oxygen Transport

During vertebrate evolution, duplicated hemoglobin (Hb) genes diverged with respect to functional properties as well as the developmental timing of expression. For example, the subfamilies of genes that encode the different subunit chains of Hb are ontogenetically regulated such that functionally distinct Hb isoforms are expressed during different developmental stages. In some vertebrate taxa, functional differentiation between co-expressed Hb isoforms may also contribute to physiologically important divisions of labor.

The influence of acute hypoxia on the functional and morphological state of the black scorpionfish red blood cells

The investigation of the mechanisms of red blood cell steadiness to the oxygen lack in tolerant teleosts is of current scientific interest. Black scorpionfish, Scorpaena porcus L., is a widespread benthal species in the Black Sea and is highly resistant to hypoxic influence. The morphological state of black scorpionfish red blood cells under acute hypoxia was assessed using DNA-binding dye SYBR Green I and fluorescent microscopy. Changes in membrane potential of mitochondria and functional activity of cells were determined by rhodamine 123 (R123) and fluorescein diacetate (FDA) fluorescence. Oxygen deficiency leads to bidirectional changes in volume of erythrocytes and their nuclei. Between 0.57 and 1.76 mg О₂ l^-1, both parameters increased on 3-12 and 7-21%, respectively. At 1.76-4.03, cells shrank on 1.5-6.0% and nucleus size decreased on 1.5-3%. Acute hypoxia induced a significant increase of R123 (12-60%) and FDA (30-184%) fluorescence. These reactions are caused by a probable decrease in erythrocyte membrane permeability.

Hemoglobin-oxygen affinity in high-altitude vertebrates: is there evidence for an adaptive trend?

In air-breathing vertebrates at high altitude, fine-tuned adjustments in hemoglobin (Hb)-O2 affinity provide an energetically efficient means of mitigating the effects of arterial hypoxemia. However, it is not always clear whether an increased or decreased Hb-O2 affinity should be expected to improve tissue O2 delivery under different degrees of hypoxia, due to the inherent trade-off between arterial O2 loading and peripheral O2 unloading. Theoretical results indicate that the optimal Hb-O2 affinity varies as a non-linear function of environmental O2 availability, and the threshold elevation at which an increased Hb-O2 affinity becomes advantageous depends on the magnitude of diffusion limitation (the extent to which O2 equilibration at the blood-gas interface is limited by the kinetics of O2 exchange). This body of theory provides a framework for interpreting the possible adaptive significance of evolved changes in Hb-O2 affinity in vertebrates that have colonized high-altitude environments. To evaluate the evidence for an empirical generalization and to test theoretical predictions, I synthesized comparative data in a phylogenetic framework to assess the strength of the relationship between Hb-O2 affinity and native elevation in mammals and birds. Evidence for a general trend in mammals is equivocal, but there is a remarkably strong positive relationship between Hb-O2 affinity and native elevation in birds. Evolved changes in Hb function in high-altitude birds provide one of the most compelling examples of convergent biochemical adaptation in vertebrates.

Changes of hemoglobin expression in response to hypoxia in a Tibetan schizothoracine fish, Schizopygopsis pylzovi

Fishes endemic to the Qinghai-Tibetan Plateau are comparatively well adapted to aquatic environments with low oxygen partial pressures (hypoxia). Here, we cloned the complete cDNA of hemoglobin (Hb) α and β from the Tibetan schizothoracine fish Schizopygopsis pylzovi, and then investigated changes in Hb mRNA and protein levels in spleen, liver and kidney in response to hypoxia. We applied severe hypoxia (4 h at PO₂ = 0.6 kPa) and moderate hypoxia (72 h at PO₂ = 6.0 kPa) to adult S. pylzovi. Changes of Hb expression under hypoxia, together with the investigations of spleen somatic index, kidney somatic index and Hb concentration in circulation, suggest that the kidney may not only serve as the erythropoietic organ, but also act as the major blood reservoir in S. pylzovi. From this perspective, the transcriptional activity of Hb in S. pylzovi, as reflected in the kidney, was turned down quickly after the onset of severe hypoxia, while under moderate hypoxia the transcriptional activity of Hb showed upregulation for a short time, but then the transcriptional machinery was turned down slowly on prolonged exposure. Notably, the changes in Hb protein levels in spleen, liver and kidney in response to severe and moderate hypoxia were not in line with the changes in mRNA levels, which are related with the blood reservoir in the kidney. Tibetan schizothoracine fish, at least S. pylzovi, show a particular response of the transcription regulation of Hb to moderate hypoxia, which is different from that of other fish species.

Improving aeration for efficient oxygenation in sea bass sea cages. Blood, brain and gill histology

An air diffusion based system (Airx) was developed to control the dissolved oxygen levels in aquaculture sea cages. The system was introduced and then tested for 37 days in a sea bass sea cage (aerated cage). A second sea bass sea cage, without the AirX, was used as a control. Oxygen levels were measured in both cages at the start of the trial, before the AirX system was introduced, and during the working period of the AirX system. Fish samples were collected 15 days after the AirX system was introduced and at the end of the experiment. Blood smears were prepared and examined microscopically. Erythrocyte major axis, minor axis and area of fish erythrocytes were measured. Leucocyte differentiation was also examined. In the control cage, the fish had significantly larger red blood cells when compared with the red blood cells of the fish in the aerated cage. Histological examination of the gills and brain revealed no morphological differences or alterations between the two groups of fish. This study demonstrated that an air diffuser system could improve the water quality of fish farmed in sea cages and enhance sea bass physiological performance, especially if DO levels fall below 60% oxygen saturation.

Oxygenation properties and isoform diversity of snake hemoglobins

Available data suggest that snake hemoglobins (Hbs) are characterized by a combination of unusual structural and functional properties relative to the Hbs of other amniote vertebrates, including oxygenation-linked tetramer-dimer dissociation. However, standardized comparative data are lacking for snake Hbs, and the Hb isoform composition of snake red blood cells has not been systematically characterized. Here we present the results of an integrated analysis of snake Hbs and the underlying α- and β-type globin genes to characterize 1) Hb isoform composition of definitive erythrocytes, and 2) the oxygenation properties of isolated isoforms as well as composite hemolysates. We used species from three families as subjects for experimental studies of Hb function: South American rattlesnake, Crotalus durissus (Viperidae); Indian python, Python molurus (Pythonidae); and yellow-bellied sea snake, Pelamis platura (Elapidae). We analyzed allosteric properties of snake Hbs in terms of the Monod-Wyman-Changeux model and Adair four-step thermodynamic model. Hbs from each of the three species exhibited high intrinsic O2 affinities, low cooperativities, small Bohr factors in the absence of phosphates, and high sensitivities to ATP. Oxygenation properties of the snake Hbs could be explained entirely by allosteric transitions in the quaternary structure of intact tetramers, suggesting that ligation-dependent dissociation of Hb tetramers into αβ-dimers is not a universal feature of snake Hbs. Surprisingly, the major Hb isoform of the South American rattlesnake is homologous to the minor HbD of other amniotes and, contrary to the pattern of Hb isoform differentiation in birds and turtles, exhibits a lower O2 affinity than the HbA isoform.

Evaluating the Hypoxia Response of Ruffe and Flounder Gills by a Combined Proteome and Transcriptome Approach

Hypoxia has gained ecological importance during the last decades, and it is the most dramatically increasing environmental factor in coastal areas and estuaries. The gills of fish are the prime target of hypoxia and other stresses. Here we have studied the impact of the exposure to hypoxia (1.5 mg O₂/l for 48 h) on the protein expression of the gills of two estuarine fish species, the ruffe (Gymnocephalus cernua) and the European flounder (Platichthys flesus). First, we obtained the transcriptomes of mixed tissues (gills, heart and brain) from both species by Illumina next-generation sequencing. Then, the gill proteomes were investigated using two-dimensional gel electrophoresis and mass spectrometry. Quantification of the normalized proteome maps resulted in a total of 148 spots in the ruffe, of which 28 (18.8%) were significantly regulated (> 1.5-fold). In the flounder, 121 spots were found, of which 27 (22.3%) proteins were significantly regulated. The transcriptomes were used for the identification of these proteins, which was successful for 15 proteins of the ruffe and 14 of the flounder. The ruffe transcriptome dataset comprised 87,169,850 reads, resulting in an assembly of 72,108 contigs (N50 = 1,828 bp). 20,860 contigs (26.93%) had blastx hits with E < 1e-5 in the human sequences in the RefSeq database, representing 14,771 unique accession numbers. The flounder transcriptome with 78,943,030 reads assembled into 49,241 contigs (N50 = 2,106 bp). 20,127 contigs (40.87%) had a hit with human proteins, corresponding to 14,455 unique accession numbers. The regulation of selected genes was confirmed by quantitative real-time RT-PCR. Most of the regulated proteins that were identified by this approach function in the energy metabolism, while others are involved in the immune response, cell signalling and the cytoskeleton.

Integrating evolutionary and functional tests of adaptive hypotheses: a case study of altitudinal differentiation in hemoglobin function in an Andean Sparrow, Zonotrichia capensis

In air-breathing vertebrates, the physiologically optimal blood-O2 affinity is jointly determined by the prevailing partial pressure of atmospheric O2, the efficacy of pulmonary O2 transfer, and internal metabolic demands. Consequently, genetic variation in the oxygenation properties of hemoglobin (Hb) may be subject to spatially varying selection in species with broad elevational distributions. Here we report the results of a combined functional and evolutionary analysis of Hb polymorphism in the rufous-collared sparrow (Zonotrichia capensis), a species that is continuously distributed across a steep elevational gradient on the Pacific slope of the Peruvian Andes. We integrated a population genomic analysis that included all postnatally expressed Hb genes with functional studies of naturally occurring Hb variants, as well as recombinant Hb (rHb) mutants that were engineered through site-directed mutagenesis. We identified three clinally varying amino acid polymorphisms: Two in the α(A)-globin gene, which encodes the α-chain subunits of the major HbA isoform, and one in the α(D)-globin gene, which encodes the α-chain subunits of the minor HbD isoform. We then constructed and experimentally tested single- and double-mutant rHbs representing each of the alternative α(A)-globin genotypes that predominate at different elevations. Although the locus-specific patterns of altitudinal differentiation suggested a history of spatially varying selection acting on Hb polymorphism, the experimental tests demonstrated that the observed amino acid mutations have no discernible effect on respiratory properties of the HbA or HbD isoforms. These results highlight the importance of experimentally validating the hypothesized effects of genetic changes in protein function to avoid the pitfalls of adaptive storytelling.

Control of breathing and the circulation in high-altitude mammals and birds

Hypoxia is an unremitting stressor at high altitudes that places a premium on oxygen transport by the respiratory and cardiovascular systems. Phenotypic plasticity and genotypic adaptation at various steps in the O2 cascade could help offset the effects of hypoxia on cellular O2 supply in high-altitude natives. In this review, we will discuss the unique mechanisms by which ventilation, cardiac output, and blood flow are controlled in high-altitude mammals and birds. Acclimatization to high altitudes leads to some changes in respiratory and cardiovascular control that increase O2 transport in hypoxia (e.g., ventilatory acclimatization to hypoxia). However, acclimatization or development in hypoxia can also modify cardiorespiratory control in ways that are maladaptive for O2 transport. Hypoxia responses that arose as short-term solutions to O2 deprivation (e.g., peripheral vasoconstriction) or regional variation in O2 levels in the lungs (i.e., hypoxic pulmonary vasoconstriction) are detrimental at in chronic high-altitude hypoxia. Evolved changes in cardiorespiratory control have arisen in many high-altitude taxa, including increases in effective ventilation, attenuation of hypoxic pulmonary vasoconstriction, and changes in catecholamine sensitivity of the heart and systemic vasculature. Parallel evolution of some of these changes in independent highland lineages supports their adaptive significance. Much less is known about the genomic bases and potential interactive effects of adaptation, acclimatization, developmental plasticity, and trans-generational epigenetic transfer on cardiorespiratory control. Future work to understand these various influences on breathing and circulation in high-altitude natives will help elucidate how complex physiological systems can be pushed to their limits to maintain cellular function in hypoxia.

Molecular response of estuarine fish to hypoxia: a comparative study with ruffe and flounder from field and laboratory

On a global scale, the frequencies and magnitudes of hypoxic events in coastal and estuarine waters have increased dramatically over the past 20 years. Fish populations are suitable indicators for the assessment of the quality of aquatic ecosystems, as they are omnipresent and often comprise a variety of different lifestyles and adaption strategies. We have investigated on the molecular level the impact of hypoxia on two fish species typical of European estuaries. We monitored the expression of eleven putatively hypoxia-responsive genes by means of quantitative real-time RT-PCR in brains, gills and hearts of the ruffe (Gymnocephalus cernua) and the flounder (Platichthys flesus). We first investigated the effect of naturally occurring hypoxia in the Elbe estuary. In a second approach, expression changes in the response to hypoxia were monitored under controlled laboratory conditions. The genes that showed the strongest effect were two respiratory proteins, myoglobin and neuroglobin, as well as the apoptosis enzyme caspase 3. As previously observed in other fish, myoglobin, which was considered to be muscle-specific, was found in brain and gills as well. Comparison of field and laboratory studies showed that--with the exception of the heart of flounder--that mRNA levels of the selected genes were about the same, suggesting that laboratory conditions reflect natural conditions. Likewise, trends of gene expression changes under hypoxia were the same, although hypoxia response was more pronounced in the Elbe estuary. In general, the flounder displayed a stronger response to hypoxia than the ruffe, suggesting that the flounder is more susceptible to hypoxia. The most pronounced differences were found among tissues within a species, demonstrating that hypoxia response is largely tissue-specific. In summary, our data suggest that laboratory experiments essentially mimic field data, but additional environmental factors enhance hypoxia response in nature.

Gene duplication and the evolution of hemoglobin isoform differentiation in birds

The majority of bird species co-express two functionally distinct hemoglobin (Hb) isoforms in definitive erythrocytes as follows: HbA (the major adult Hb isoform, with α-chain subunits encoded by the α(A)-globin gene) and HbD (the minor adult Hb isoform, with α-chain subunits encoded by the α(D)-globin gene). The α(D)-globin gene originated via tandem duplication of an embryonic α-like globin gene in the stem lineage of tetrapod vertebrates, which suggests the possibility that functional differentiation between the HbA and HbD isoforms may be attributable to a retained ancestral character state in HbD that harkens back to a primordial, embryonic function. To investigate this possibility, we conducted a combined analysis of protein biochemistry and sequence evolution to characterize the structural and functional basis of Hb isoform differentiation in birds. Functional experiments involving purified HbA and HbD isoforms from 11 different bird species revealed that HbD is characterized by a consistently higher O(2) affinity in the presence of allosteric effectors such as organic phosphates and Cl(-) ions. In the case of both HbA and HbD, analyses of oxygenation properties under the two-state Monod-Wyman-Changeux allosteric model revealed that the pH dependence of Hb-O(2) affinity stems primarily from changes in the O(2) association constant of deoxy (T-state)-Hb. Ancestral sequence reconstructions revealed that the amino acid substitutions that distinguish the adult-expressed Hb isoforms are not attributable to the retention of an ancestral (pre-duplication) character state in the α(D)-globin gene that is shared with the embryonic α-like globin gene.

Physiological and proteomic responses to single and repeated hypoxia in juvenile Eurasian perch under domestication--clues to physiological acclimation and humoral immune modulations

We evaluated the physiological and humoral immune responses of Eurasian perch submitted to 4-h hypoxia in either single or repeated way. Two generations (F1 and F5) were tested to study the potential changes in these responses with domestication. In both generations, single and repeated hypoxia resulted in hyperglycemia and spleen somatic index reduction. Glucose elevation and lysozyme activity decreased following repeated hypoxia. Complement hemolytic activity was unchanged regardless of hypoxic stress or domestication level. A 2D-DIGE proteomic analysis showed that some C3 components were positively modulated by single hypoxia while C3 up- and down-regulations and over-expression of transferrin were observed following repeated hypoxia. Domestication was associated with a low divergence in stress and immune responses to hypoxia but was accompanied by various changes in the abundance of serum proteins related to innate/specific immunity and acute phase response. Thus, it appeared that the humoral immune system was modulated following single and repeated hypoxia (independently of generational level) or during domestication and that Eurasian perch may display physiological acclimation to frequent hypoxic disturbances.

Molecular Characterization and Expression of α-Globin and β-Globin Genes in the Euryhaline Flounder (Platichthys flesus)

In order to understand the possible role of globin genes in fish salinity adaptation, we report the molecular characterization and expression of all four subunits of haemoglobin, and their response to salinity challenge in flounder. The entire open reading frames of α1-globin and α2-globin genes were 432 and 435 bp long, respectively, whereas the β1-globin and β2-globin genes were both 447 bp. Although the head kidney (pronephros) is the predicted major site of haematopoiesis, real-time PCR revealed that expression of α-globin and β-globin in kidney (mesonephros) was 1.5 times higher than in head kidney. Notably, the α1-globin and β1-globin mRNA expression was higher than α2-globin and β2-globin in kidney. Expression levels of all four globin subunits were higher in freshwater- (FW-) than in seawater- (SW-)adapted fish kidney. If globins do play a role in salinity adaptation, this is likely to be more important in combating the hemodilution faced by fish in FW than the dehydration and salt loading which occur in SW.

Time domains of the hypoxic ventilatory response in ectothermic vertebrates

Over a decade has passed since Powell et al. (Respir Physiol 112:123-134, 1998) described and defined the time domains of the hypoxic ventilatory response (HVR) in adult mammals. These time domains, however, have yet to receive much attention in other vertebrate groups. The initial, acute HVR of fish, amphibians and reptiles serves to minimize the imbalance between oxygen supply and demand. If the hypoxia is sustained, a suite of secondary adjustments occur giving rise to a more long-term balance (acclimatization) that allows the behaviors of normal life. These secondary responses can change over time as a function of the nature of the stimulus (the pattern and intensity of the hypoxic exposure). To add to the complexity of this process, hypoxia can also lead to metabolic suppression (the hypoxic metabolic response) and the magnitude of this is also time dependent. Unlike the original review of Powell et al. (Respir Physiol 112:123-134, 1998) that only considered the HVR in adult animals, we also consider relevant developmental time points where information is available. Finally, in amphibians and reptiles with incompletely divided hearts the magnitude of the ventilatory response will be modulated by hypoxia-induced changes in intra-cardiac shunting that also improve the match between O(2) supply and demand, and these too change in a time-dependent fashion. While the current literature on this topic is reviewed here, it is noted that this area has received little attention. We attempt to redefine time domains in a more 'holistic' fashion that better accommodates research on ectotherms. If we are to distinguish between the genetic, developmental and environmental influences underlying the various ventilatory responses to hypoxia, however, we must design future experiments with time domains in mind.

Phenotypic plasticity and genetic adaptation to high-altitude hypoxia in vertebrates

High-altitude environments provide ideal testing grounds for investigations of mechanism and process in physiological adaptation. In vertebrates, much of our understanding of the acclimatization response to high-altitude hypoxia derives from studies of animal species that are native to lowland environments. Such studies can indicate whether phenotypic plasticity will generally facilitate or impede adaptation to high altitude. Here, we review general mechanisms of physiological acclimatization and genetic adaptation to high-altitude hypoxia in birds and mammals. We evaluate whether the acclimatization response to environmental hypoxia can be regarded generally as a mechanism of adaptive phenotypic plasticity, or whether it might sometimes represent a misdirected response that acts as a hindrance to genetic adaptation. In cases in which the acclimatization response to hypoxia is maladaptive, selection will favor an attenuation of the induced phenotypic change. This can result in a form of cryptic adaptive evolution in which phenotypic similarity between high- and low-altitude populations is attributable to directional selection on genetically based trait variation that offsets environmentally induced changes. The blunted erythropoietic and pulmonary vasoconstriction responses to hypoxia in Tibetan humans and numerous high-altitude birds and mammals provide possible examples of this phenomenon. When lowland animals colonize high-altitude environments, adaptive phenotypic plasticity can mitigate the costs of selection, thereby enhancing prospects for population establishment and persistence. By contrast, maladaptive plasticity has the opposite effect. Thus, insights into the acclimatization response of lowland animals to high-altitude hypoxia can provide a basis for predicting how altitudinal range limits might shift in response to climate change.

Changes of globin expression in the Japanese medaka (Oryzias latipes) in response to acute and chronic hypoxia

Fishes live in an aquatic environment with low or temporally changing O(2) availability. Variations in O(2) levels require many anatomical, behavioral, physiological, and biochemical adaptations that ensure the uptake of an adequate amount of O(2). Some fish species are comparatively well adapted to tolerate low O(2) partial pressure (hypoxia). The Japanese ricefish medaka (Oryzias latipes) is an important model organism for biomedical research that shows remarkable tolerance towards hypoxia. We have investigated the regulation and role of globins under hypoxia. We applied four different regimes of chronic hypoxia (24 and 48 h at PO(2) = 2 or 4 kPa) as well as acute hypoxia (2 h at PO(2) = 0.5 kPa) to adult male medaka. Changes of mRNA levels of seven globin genes (adult hemoglobin α and β, myoglobin, neuroglobin, cytoglobin 1 and 2, globin X), three hypoxia-response genes (lactate dehydrogenase b, phosphoglycerate kinase, adrenomedullin 1) and two putative reference genes (cyclophilin, acidic ribosomal phosphoprotein P0) were monitored by means of quantitative real-time reverse-transcription PCR. We observed strong upregulation of myoglobin, which is also expressed in the medaka brain, as previously demonstrated for carp, goldfish and zebrafish. The hemoglobin chains were found upregulated, whereas earlier studies found down-regulation of hemoglobin in hypoxic zebrafish. By contrast, neuroglobin mRNA was not affected by hypoxia in medaka, but had been found upregulated in zebrafish. Globin X is induced in medaka brain, but down-regulated in zebrafish. Thus, the patterns of hypoxia response of globins are strikingly different in various fish species, which can be interpreted as indication for different roles of the various globins in hypoxia response and for alternative metabolic strategies of fish species in coping with O(2) deprivation.

Genetic differences in hemoglobin function between highland and lowland deer mice

In high-altitude vertebrates, adaptive changes in blood-O(2) affinity may be mediated by modifications of hemoglobin (Hb) structure that affect intrinsic O(2) affinity and/or responsiveness to allosteric effectors that modulate Hb-O(2) affinity. This mode of genotypic specialization is considered typical of mammalian species that are high-altitude natives. Here we investigated genetically based differences in Hb-O(2) affinity between highland and lowland populations of the deer mouse (Peromyscus maniculatus), a generalist species that has the broadest altitudinal distribution of any North American mammal. The results of a combined genetic and proteomic analysis revealed that deer mice harbor a high level of Hb isoform diversity that is attributable to allelic polymorphism at two tandemly duplicated alpha-globin genes and two tandemly duplicated beta-globin genes. This high level of isoHb diversity translates into a correspondingly high level of interindividual variation in Hb functional properties. O(2) equilibrium experiments revealed that the Hbs of highland mice exhibit slightly higher intrinsic O(2) affinities and significantly lower Cl(-) sensitivities relative to the Hbs of lowland mice. The experiments also revealed distinct biochemical properties of deer mouse Hb related to the anion-dependent allosteric regulation of O(2) affinity. In conjunction with previous findings, our results demonstrate that modifications of Hb structure that alter allosteric anion sensitivity play an important role in the adaptive fine-tuning of blood-O(2) affinity.

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Natriuretic peptides in hormonal regulation of hypoxia responses

The possibility that natriuretic peptides' effects are important in hypoxia responses of vertebrates is reviewed. Both the transcription and release of natriuretic peptides are affected by oxygen tension. Furthermore, many of the effects observed in hypoxia, such as diuresis and a reduction of plasma volume, are also caused by treatment of the animal with natriuretic peptides. Also, several clinical observations about changes in natriuretic peptide levels in, e.g., sleep apnea and cyanotic congenital heart disease, are consistent with the idea that hypoxia is involved in the etiology of conditions, in which natriuretic peptide levels increase. Virtually all published information on the relationship between oxygen and natriuretic peptides is based on human studies. Because hypoxic conditions are more common in aquatic than terrestrial environments, future studies about the possible role of natriuretic peptides in hypoxia, as well as the role of hypoxia in the evolution of natriuretic peptides, including the different subtypes, should increasingly involve also aquatic organisms.

Characterization of the hemoglobins of the Australian lungfish Neoceratodus forsteri (Krefft)

We examined for the first time the hemoglobin components of the blood of the Australian lungfish, Neoceratodus forsteri and their functional responses to pH and the allosteric modulators adenosine triphosphate (ATP), guanosine triphosphate (GTP), 2,3-bisphosphoglyceric acid (BPG) and inositol hexaphosphate (IHP) at 25 degrees C. Lysates prepared from stripped, unfractionated hemolysate produced sigmoidal oxygen equilibrium curves with high oxygen affinity (oxygen partial pressure required for 50% hemoglobin saturation, p(50)=5.3 mmHg) and a Hill coefficient of 1.9 at pH 7.5. p(50) was 8.3 and 4.5 mmHg at pH 6 and 8, respectively, which corresponded to a modest Bohr coefficient (Delta log p(50)/Delta pH) of -0.13. GTP increased the pH sensitivity of oxygen binding more than ATP, such that the Bohr coefficient was -0.77 in the presence of 2 mmol L(-1) GTP. GTP was the most potent regulator of hemoglobin affinity, with concentrations of 5 mmol L(-1) causing an increase in p(50) from 5 to 19 mm Hg at pH 7.5, while the order of potency of the other phosphates was IHP>ATP>BPG. Three hemoglobin isoforms were present and each contained both alpha and beta chains with distinct molecular weights. Oxygen affinity and pH-dependence of isoforms I and II were essentially identical, while isoform III had a lower affinity and increased pH-dependence. The functional properties of the hemoglobin system of Neoceratodus appeared consistent with an active aquatic breather adapted for periodic hypoxic episodes.

Hemoglobin system of Sparus aurata: changes in fishes farmed under extreme conditions

In order to gain more knowledge on the stress responses of gilhead seabream (Sparus aurata) under extreme conditions, this study investigated the functional properties of the hemoglobin system and globin gene expression under hypoxia and low salinity. The oxygen affinity for the two hemoglobin components present inside the S. aurata erythrocyte was practically identical as was the influence of protons and organic phosphates (Root effect). The quantification of S. aurata hemoglobin fractions performed by HPLC and the data on gene expression of globin chains assayed by PCR indicate that under hypoxia and low salinity there is a change in the ratio between the two different hemoglobin components. The result indicating that the distinct hemoglobins present in S. aurata erythrocyte have almost identical functional properties, does not explain the adaptive response (expression change) following exposure of the animal to hypoxia or low salinity on the basis of their function as oxygen transporter. We hypothesize that other parallel biological functions that the hemoglobin molecule is known to display within the erythrocyte are involved in adaptive molecular mechanisms. The autoxidation-reduction cycle of hemoglobin could be involved in the response to particular living conditions.