The regulation and importance of glucose uptake in the isolated Atlantic cod heart: rate-limiting steps and effects of 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.

Taurine depletion impairs cardiac function and affects tolerance to hypoxia and high temperatures in brook char (Salvelinus fontinalis)

Physiological and environmental stressors can cause osmotic stress in fish hearts, leading to a reduction in intracellular taurine concentration. Taurine is a β-amino acid known to regulate cardiac function in other animal models but its role in fish has not been well characterized. We generated a model of cardiac taurine deficiency (TD) by feeding brook char (Salvelinus fontinalis) a diet enriched in β-alanine, which inhibits cardiomyocyte taurine uptake. Cardiac taurine levels were reduced by 21% and stress-induced changes in normal taurine handling were observed in TD brook char. Responses to exhaustive exercise and acute thermal and hypoxia tolerance were then assessed using a combination of in vivo, in vitro and biochemical approaches. Critical thermal maximum was higher in TD brook char despite significant reductions in maximum heart rate. In vivo, TD brook char exhibited a lower resting heart rate, blunted hypoxic bradycardia and a severe reduction in time to loss of equilibrium under hypoxia. In vitro function was similar between control and TD hearts under oxygenated conditions, but stroke volume and cardiac output were severely compromised in TD hearts under severe hypoxia. Aspects of mitochondrial structure and function were also impacted in TD permeabilized cardiomyocytes, but overall effects were modest. High levels of intracellular taurine are required to achieve maximum cardiac function in brook char and cardiac taurine efflux may be necessary to support heart function under stress. Taurine appears to play a vital, previously unrecognized role in supporting cardiovascular function and stress tolerance in fish.

Phenotypic stress response does not influence the upper thermal tolerance of male Atlantic salmon (Salmo salar)

Fish can be identified as either low responders (LR) or high responders (HR) based on post-stress cortisol levels and whether they exhibit a proactive or reactive stress coping style, respectively. In this study, male Atlantic salmon (Salmo salar) from 17 families reared at 9 °C were repeatedly exposed to an acute handling stress over a period of four months, with plasma cortisol levels measured at 1 h post-stress. Fish were identified as either LR or HR if the total Z-score calculated from their cortisol responses fell into the lower or upper quartile ranges, respectively; with intermediate responders (IR) classified as the remainder. Salmon characterized as LR, IR or HR were then subjected to an incremental thermal challenge, where temperature was raised at 0.2 °C day^-1 from their acclimation temperature (12 °C) to mimic natural sea-cage farming conditions during the summer in Newfoundland. Interestingly, feed intake remained high up to 22 °C, while previous studies have shown a decrease in salmon appetite after ∼16-18 °C. After the first three mortalities were recorded at elevated temperature, a subset of LR and HR salmon were exposed to another acute handling stress event at 23.6 °C. Basal and post-stress measurements of plasma cortisol, glucose and lactate did not differ between stress response phenotypes at this temperature. In the end, the average incremental thermal maximum (IT_Max) of LR and HR fish was not different (25.1 °C). In comparison, the critical thermal maximum (CT_Max; temperature increased at 2 °C h^-1) of the remaining IR fish that had been held at 12 °C was 28.5 °C. Collectively, these results: 1) show that this population of Atlantic salmon is very thermally tolerant, and further question the relevance of CT_Max in assessing responses to real-world temperature changes; and 2) indicate that characterization of stress phenotype at 9 °C is not predictive of their stress response or survival at high temperatures. Therefore, selection of fish based on phenotypic stress response at low temperatures may not be beneficial to incorporate into Atlantic salmon breeding programs, especially if the goal is to improve growth performance and survival at high temperatures in sea-cages.

Intracellular taurine deficiency impairs cardiac contractility in rainbow trout (Oncorhynchus mykiss) without affecting aerobic performance

Taurine is a non-proteinogenic sulfonic acid found in high concentrations inside vertebrate cardiomyocytes and its movement across the sarcolemmal membrane is critical for cell volume regulation. Taurine deficiency is rare in mammals, where it impairs cardiac contractility and leads to congestive heart failure. In fish, cardiac taurine levels vary substantially between species and can decrease by up to 60% in response to environmental change but its contribution to cardiac function is understudied. We addressed this gap in knowledge by generating a taurine-deficient rainbow trout (Oncorhynchus mykiss) model using a feed enriched with 3% β-alanine to inhibit cellular taurine uptake. Cardiac taurine was reduced by 17% after 4 weeks with no effect on growth or condition factor. Taurine deficiency did not affect routine or maximum rates of O₂ consumption, aerobic scope, or critical swimming speed in whole animals but cardiac contractility was significantly impaired. In isometrically contracting ventricular strip preparations, the force-frequency and extracellular Ca²⁺-sensitivity relationships were both shifted downward and maximum pacing frequency was significantly lower in β-alanine fed trout. Cardiac taurine deficiency reduces sarcoplasmic reticular Ca²⁺-ATPase activity in mammals and our results are consistent with such an effect in rainbow trout. Our data indicate that intracellular taurine contributes to the regulation of cardiac contractility in rainbow trout. Aerobic performance was unaffected in β-alanine-fed animals, but further study is needed to determine if more significant natural reductions in taurine may constrain performance under certain environmental conditions.

Thermal variation near the thermal optimum does not affect the growth, metabolism or swimming performance in wild Atlantic salmon Salmo salar

Typically, laboratory studies on the physiological effects of temperature are conducted using stable acclimation temperatures. Nonetheless, information extrapolated from these studies may not accurately represent wild populations living in thermally variable environments. The aim of this study was to compare the growth rate, metabolism and swimming performance of wild Atlantic salmon exposed to cycling temperatures, 16-21°C, and stable acclimation temperatures, 16, 18.5, 21°C. Growth rate, metabolic rate, swimming performance and anaerobic metabolites did not change among acclimation groups, suggesting that within Atlantic salmon's thermal optimum range, temperature variation has no effect on these physiological properties.

Computational modeling to determine key regulators of hypoxia effects on the lactate production in the glycolysis pathway

In solid tumors, hypoxia can trigger aberrant expression of transcription factors and genes, resulting in abnormal biological functions such as altered energetic pathways in cancer cells. Glucose metabolism is an important part of this phenomenon, which is associated with changes in the functional expression of transporters and enzymes involved in the glycolysis pathway. The latter phenomenon can finally lead to the lactate accumulation and pH dysregulation in the tumor microenvironment and subsequently further invasion and metastasis of cancer cells. Having capitalized on the computational modeling, in this study, for the first time, we aimed to investigate the effects of hypoxia-induced factor-1 (HIF-1) mediated hypoxia on the magnitude of functional expression of all the enzymes and transporters involved in the glycolysis process. The main objective was to establish a quantitative relationship between the hypoxia intensity and the intracellular lactate levels and determine the key regulators of the glycolysis pathway. This model clearly showed an increase in the lactate concentration during the oxygen depletion. The proposed model also predicted that the phosphofructokinase-1 and phosphoglucomutase enzymes might play the most important roles in the regulation of the lactate production.

The endemic and endangered Maugean Skate (Zearaja maugeana) exhibits short-term severe hypoxia tolerance

The endangered and range-restricted Maugean skate (Zearaja maugeana) is subjected to large environmental variability coupled with anthropogenic stressors in its endemic habitat, Macquarie Harbour, Tasmania. However, little is known about the basic biology/physiology of this skate, or how it may respond to future environmental challenges predicted from climate change and/or increases in human activities such as aquaculture. These skate live at a preferred depth of 5-15 m where the dissolved oxygen (DO) levels are moderate (~55% air saturation), but can be found in areas of the Harbour where DO can range from 100% saturation to anoxia. Given that the water at their preferred depth is already hypoxic, we sought to investigate their response to further decreases in DO that may arise from potential increases in anthropogenic stress. We measured oxygen consumption, haematological parameters, tissue-enzyme capacity and heat shock protein (HSP) levels in skate exposed to 55% dissolved O2 saturation (control) and 20% dissolved O2 saturation (hypoxic) for 48 h. We conclude that the Maugean skate appears to be an oxyconformer, with a decrease in the rate of O2 consumption with increasing hypoxia. Increases in blood glucose and lactate at 20% O2 suggest that skate are relying more on anaerobic metabolism to tolerate periods of very low oxygen. Despite these metabolic shifts, there was no difference in HSP70 levels between groups, suggesting this short-term exposure did not elicit a cellular stress response. The metabolic state of the skate suggests that low oxygen stress for longer periods of time (i.e. >48 h) may not be tolerable and could potentially result in loss of habitat or shifts in their preferred habitat. Given its endemic distribution and limited life-history information, it will be critical to understand its tolerance to environmental challenges to create robust conservation strategies.

Functionalized silver nanoparticles depress aerobic metabolism in the absence of overt toxicity in brackish water killifish, Fundulus heteroclitus

Engineered nanomaterials (ENMs) tend to precipitate in saline waters so the majority of aquatic toxicity studies have focused on freshwaters, where bioavailability is presumed to be higher. Recent studies have illustrated that some ENM formulations are bioavailable and bioactive in salt water and that their effects are more pronounced at the physiological than biochemical level. These findings raise concerns regarding the effects of ENMs on marine organisms. Therefore, our goal was to characterize the effects of polyvinylpyrolidone-functionalized silver ENMs (nAg) on aerobic performance in the killifish (Fundulus heteroclitus), a common euryhaline teleost. Fish were exposed to 80 μg L^-1 of 5 nm nAg for 48 h in brackish water (12 ppt) and routine (ṀO_2min) and maximum (ṀO_2max) rates of oxygen consumption were quantified. Silver dissolution was minimal and nAg remained well dispersed in brackish water, with a hydrodynamic diameter of 21.0 nm, compared to 19.3 in freshwater. Both ṀO_2min and ṀO_2max were significantly lower (by 53 and 30%, respectively) in killifish exposed to nAg and a reduction in ṀO₂ variability suggested spontaneous activity was suppressed. Neither gill Na⁺/K⁺-ATPase activity, nor various other biochemical markers were affected by nAg exposure. The results illustrate that a common ENM formulation is bioactive in salt water and, as in previous studies on functionalized copper ENMs, that effects are more pronounced at the whole animal than the biochemical level.

Species-specific low plasma glucose in fish is associated with relatively high tissue glucose content and is inversely correlated with cardiac glycogen content

The relationship between plasma glucose concentration and intracellular glucose (liver, heart, brain, gill, gonad, intestine, kidney, spleen, white muscle) was determined in fish species with a range in plasma glucose (Atlantic cod, 5.06 mM; cunner, 3.8 mM; rainbow trout, 3.7 mM; lumpfish, 0.9 mM; short-horned sculpin, 0.6 mM; and winter flounder, 0.6 mM). The ratio of intracellular glucose/plasma glucose was always higher than one in liver for all species consistent with a diffusion gradient from the tissue to the plasma. In all other tissues in Atlantic cod, cunner, and rainbow trout the diffusion gradient was from the plasma to the intracellular space. In short-horned sculpin, the mean ratio in heart and white muscle exceeded one and in winter flounder the ratio was significantly greater than one at 5.97 and 2.92 for heart and muscle, respectively. The presence of an active glucose 6-phosphatase in white muscle could account for elevated amounts of free glucose. The white muscle of all species displayed phosphoenolpyruvate carboxykinase and in winter flounder the activity was as high in white muscle as in liver suggesting that gluconeogenesis may be associated with a relatively high-muscle glucose content. The glycogen content was highest in liver followed by heart with lower amounts in all other tissues. There was an inverse correlation between heart glycogen content and plasma glucose concentration when all species were included. It is contended that in species with low plasma glucose, heart glycogen is accumulated at a slow rate under normoxia, to be called upon under hypoxic conditions when the gradient for inward diffusion is unfavourable for high rates of glucose metabolism.

Low plasma glucose limits glucose metabolism by RBCs and heart in some species of teleosts

Within teleosts there is a species range in plasma glucose levels from undetectable to 20mM. At low plasma glucose levels the gradient from the extracellular to the intracellular space is decreased. The impact of this on glucose metabolism by RBCs and heart from species with different steady state levels of plasma glucose (Atlantic cod ~5mM; Atlantic salmon ~5mM, cunner ~1mM, lumpfish <1mM; short-horned sculpin <1mM) is the subject of this review. Under normoxia, at physiological levels of extracellular glucose, RBCs and heart produce lactate although the contribution of anaerobic metabolism to ATP production is small. Sustained lactate production from extracellular glucose appears to be the primary fate of extracellular glucose. In many cases, glycogen is not mobilized and the rate of glucose metabolism=two times the rate of lactate production. As such, alternative metabolic sources are required to fuel oxidative metabolism. Under hypoxia, hearts from Atlantic cod and rainbow trout increase rates of both glucose metabolism and lactate production, partially supported by glycogen reserves. But in lumpfish and short-horned sculpin hearts there is no change in rates of glucose metabolism. The most likely explanation is that glucose uptake is compromised in lumpfish and short-horned sculpin hearts due to a low diffusion gradient. Under these conditions rates of lactate production are well below that of Atlantic cod or rainbow trout. Energy demand must be reduced under hypoxia in lumpfish and short-horned sculpin hearts in order to maintain ATP balance.

Low levels of extracellular glucose limit cardiac anaerobic metabolism in some species of fish

There is a wide interspecific range in plasma glucose levels in teleosts from less than 0.5 to greater than 10 mmol l^-1 Here we assessed how glucose availability influences glucose metabolism in hearts of Atlantic cod (Gadus morhua), rainbow trout (Oncorhynchus mykiss), lumpfish (Cyclopterus lumpus) and short-horned sculpin (Myoxocephalus scorpius) under normoxic and hypoxic conditions. These species had plasma glucose levels of 5.1, 4.8, 0.9 and 0.5 mmol l^-1, respectively. Rates of glucose metabolism and lactate production were determined in isolated hearts perfused with medium containing physiological levels of glucose. Under normoxic conditions there was no significant difference in rates of either glucose metabolism (average 15 nmol g^-1 min^-1) or lactate production (average 30 nmol g^-1 min^-1) across species. Under hypoxia (12% of air saturation) there were significant increases in rates of glucose metabolism and lactate production in hearts from Atlantic cod (glucose-130; lactate-663 nmol g^-1 min^-1) and rainbow trout (glucose-103; lactate-774 nmol g^-1 min^-1); however, there was no change in rate of glucose metabolism in hearts from either lumpfish or short-horned sculpin and only increases in lactate production to rates much lower than the other species. Furthermore, Atlantic cod hearts perfused with medium containing low non-physiological levels of glucose (0.5 mmol l^-1) had the same rates of glucose metabolism under normoxic and hypoxic treatments. Anaerobic metabolism supported by extracellular glucose is compromised in fish with low levels of plasma glucose, which in turn may decrease performance under oxygen-limiting conditions at the whole-animal level.

Physiological effects of environmentally relevant, multi-day thermal stress on wild juvenile Atlantic salmon (Salmo salar)

The frequency of extreme thermal events in temperate freshwater systems is expected to increase alongside global surface temperature. The Miramichi River, located in eastern Canada, is a prominent Atlantic salmon (Salmo salar) river where water temperatures can exceed the proposed upper thermal limit for the species (~27°C). Current legislation closes the river to recreational angling when water temperatures exceed 20°C for two consecutive nights. We aimed to examine how natural thermal variation, representative of extreme high thermal events, affected the thermal tolerance and physiology of wild, juvenile Atlantic salmon. We acclimated fish to four thermal cycles, characteristic of real-world thermal conditions while varying daily thermal minima (16°C, 18°C, 20°C or 22°C) and diel thermal fluctuation (e.g. Δ5°C-Δ9°C). In each cycling condition, we assessed the role that thermal minima played on the acute thermal tolerance (critical thermal maximum, (CTMax)), physiological (e.g. heat shock protein 70 (HSP70), ubiquitin) and energetic (e.g. hepatic glycogen, blood glucose and lactate) status of juvenile Atlantic salmon throughout repeated thermal cycles. Exposure to 16-21°C significantly increased CTMax (+0.9°C) compared to a stable acclimation temperature (16°C), as did exposure to diel thermal fluctuations of 18-27°C, 20-27°C and 22-27°C, yet repeated exposure provided no further increases in acute thermal tolerance. In comparison to the reference condition (16-21°C), consecutive days of high temperature cycling with different thermal minima resulted in significant increases in HSP70 and ubiquitin, a significant decrease in liver glycogen, and no significant cumulative effect on either blood glucose or lactate. However, comparison between thermally taxed treatments suggested the diel thermal minima had little influence on the physiological or energetic response of juvenile salmon, despite the variable thermal cycling condition. Our results suggest that relatively cooler night temperatures in the summer months may play a limited role in mitigating physiological stress throughout warm diel cycle events.

Extracellular glucose supports lactate production but not aerobic metabolism in cardiomyocytes from both normoglycemic Atlantic cod and low glycemic short-horned sculpin

Fish exhibit a wide range of species-specific blood glucose levels. How this relates to glucose utilization is yet to be fully realized. Here, we assessed glucose transport and metabolism in myocytes isolated from Atlantic cod (Gadus morhua) and short-horned sculpin (Myoxocephalus scorpius), species with blood glucose levels of 3.7 and 0.57 mmol l(-1), respectively. Glucose metabolism was assessed by the production of (3)H2O from [2-(3)H]glucose. Glucose metabolism was 3.5- to 6-fold higher by myocytes from Atlantic cod than by those from short-horned sculpin at the same level of extracellular glucose. In Atlantic cod myocytes, glucose metabolism displayed what appears to be a saturable component with respect to extracellular glucose, and cytochalasin B inhibited glucose metabolism. These features revealed a facilitated glucose diffusion mechanism that accounts for between 30% and 55% of glucose entry at physiological levels of extracellular glucose. Facilitated glucose diffusion appears to be minimal in myocytes for short-horned sculpin. Glucose entry by simple diffusion occurs in both cell types with the same linear relationship between glucose metabolism and extracellular glucose concentration, presumably due to similarities in membrane composition. Oxygen consumption by myocytes incubated in medium containing physiological levels of extracellular glucose (Atlantic cod 5 mmol l(-1), short-horned sculpin 0.5 mmol l(-1)) was similar in the two species and was not decreased by cytochalasin B, suggesting that these cells have the capability of oxidizing alternative on-board metabolic fuels. Cells produced lactate at low rates but glycogen levels did not change during the incubation period. In cells from both species, glucose utilization assessed by both simple chemical analysis of glucose disappearance from the medium and (3)H2O production was half the rate of lactate production and as such extracellular glucose was not available for oxidative metabolism. Overall, extracellular glucose makes only a minor contribution to ATP production but a sustained glycolysis may be necessary to support Ca(2+) transport mechanisms at either the sarcoplasmic reticulum or the sarcolemmal membrane.

Cerium oxide nanoparticles exhibit minimal cardiac and cytotoxicity in the freshwater fish Catostomus commersonii

Metal oxide nanomaterials can cause oxidative, cardiorespiratory, and osmoregulatory stress in freshwater fish. In contrast, cerium oxide nanoparticles (nCeO2) can have antioxidant effects but their aquatic toxicity has not been fully characterized. Heart rate and heart rate variability were followed in white sucker (Catostomus commersonii) acutely exposed to 1.0 mg L(-1) nCeO2 for 25 h. Malondialdehyde (MDA) was measured to assess oxidative tissue damage, and plasma cortisol, glucose, lactate, and osmolality were assessed as indicators of physiological and osmoregulatory stress. There was no MDA accumulation in gill or heart of fish exposed to nCeO2 and heart function was unchanged over the 25 h treatment. Plasma cortisol increased 6-fold but there was no change in plasma glucose or lactate. Cellular osmoregulatory toxicity was studied using an isolated red blood cell (RBC) model. In vitro exposure to 1.0 mg L(-1) nCeO2 for 1h had no effect on cell morphological parameters and did not sensitize RBCs to hemolysis under hypotonic stress. Overall, there were no indications of oxidative, cardiorespiratory, or osmoregulatory stress following acute exposure to nCeO2. Elevated plasma cortisol levels suggest that nCeO2 may exert mild toxicity to tissues outside of the cardiorespiratory system.

Extracellular glucose can fuel metabolism in RBCs from high glycemic Atlantic cod (Gadus morhua) but not low glycemic short-horned sculpin (Myoxocephalus scorpius)

Energy metabolism was assessed in RBCs from Atlantic cod and short-horned sculpin, two species that have markedly different levels of blood glucose. The objective was to determine if the level of extracellular glucose impacts rates of glucose metabolism. Blood glucose level was 2.5 mmol l-1 in Atlantic cod and 0.2 mmol l-1 in short-horned sculpin, respectively. Oxygen consumption, lactate production, and glucose utilization were measured in whole blood and related to g RBC. Glucose utilization was assessed by measuring both glucose disappearance and by the production of 3H2O from [2-3H]-glucose. RBCs from both species have an aerobic based metabolism. In Atlantic cod, extracellular glucose is sufficient to provide the sum of glucosyl equivalents to support both oxidative metabolism and lactate production. In contrast, extracellular glucose can account for only 10% of the metabolic rate in short-horned sculpin RBCs. In both species, about 70% of glucose enters the RBCs via facilitated transport. The difference in rates of extracellular glucose utilization is related to the extremely low levels of blood glucose in short-horned sculpin. In this species energy metabolism by RBCs must be supported by alternative fuels.

Limited effects of exogenous glucose during severe hypoxia and a lack of hypoxia-stimulated glucose uptake in isolated rainbow trout cardiac muscle

We examined whether exogenous glucose affects contractile performance of electrically paced ventricle strips from rainbow trout under conditions known to alter cardiomyocyte performance, ion regulation and energy demands. Physiological levels of d-glucose did not influence twitch force development for aerobic preparations (1) paced at 0.5 or 1.1 Hz, (2) at 15 or 23°C, (3) receiving adrenergic stimulation or (4) during reoxygenation with or without adrenaline after severe hypoxia. Contractile responses to ryanodine, an inhibitor of Ca(2+) release from the sarcoplasmic reticulum, were also not affected by exogenous glucose. However, glucose did attenuate the fall in twitch force during severe hypoxia. Glucose uptake was assayed in non-contracting ventricle strips using 2-[(3)H] deoxy-d-glucose (2-DG) under aerobic and hypoxic conditions, at different incubation temperatures and with different inhibitors. Based upon a lack of saturation of 2-DG uptake and incomplete inhibition of uptake by cytochalasin B and d-glucose, 2-DG uptake was mediated by a combination of facilitated transport and simple diffusion. Hypoxia stimulated lactate efflux sixfold to sevenfold with glucose present, but did not increase 2-DG uptake or reduce lactate efflux in the presence of cytochalasin B. Increasing temperature (14 to 24°C) also did not increase 2-DG uptake, but decreasing temperature (14 to 4°C) reduced 2-DG uptake by 45%. In conclusion, exogenous glucose improves mechanical performance under hypoxia but not under any of the aerobic conditions applied. The extracellular concentration of glucose and cold temperature appear to determine and limit cardiomyocyte glucose uptake, respectively, and together may help define a metabolic strategy that relies predominantly on intracellular energy stores.

Effects of fatty acid provision during severe hypoxia on routine and maximal performance of the in situ tilapia heart

The ability to maintain stable cardiac function during environmental hypoxia exposure is crucial for hypoxia tolerance in animals and depends upon the maintenance of cardiac energy balance as well as the state of the heart's extracellular environment (e.g., availability of metabolic fuels). Hypoxic depression of plasma [non-esterified fatty acids] (NEFA), an important cardiac aerobic fuel, is a common response in many species of hypoxia-tolerant fishes, including tilapia. We tested the hypothesis that decreased plasma [NEFA] is important for maintaining stable cardiac function during and following hypoxia exposure, based on the premise that continued reliance upon cardiac fatty acid metabolism under such conditions could impair cardiac function. We examined the effect of severe hypoxia exposure (PO2 < 0.2 kPa) on routine and maximum performance of the in situ perfused tilapia heart under conditions of routine (400 μmol L(-1)) and low (75 μmol L(-1)) [palmitate], which mimicked the in vivo levels of plasma [NEFA] found in normoxic and hypoxic tilapia, respectively. Under both concentrations of palmitate, the in situ tilapia heart showed exceptional hypoxic performance as a result of a high maximum glycolytic potential, confirming our previous results using a perfusate without fatty acids. We additionally provide evidence suggesting that non-contractile ATP demand is depressed in tilapia heart during hypoxia exposure. Cardiac performance during and following severe hypoxia exposure was unaffected by the level of palmitate. Thus, we conclude that hypoxic depression of plasma [NEFA] in fishes does not play a role in cardiac hypoxia tolerance.

Absence of glucose transporter 4 diminishes electrical activity of mouse hearts during hypoxia

Insulin resistance, which characterizes type 2 diabetes, is associated with reduced translocation of glucose transporter 4 (GLUT4) to the plasma membrane following insulin stimulation, and diabetic patients with insulin resistance show a higher incidence of ischaemia, arrhythmias and sudden cardiac death. The aim of this study was to examine whether GLUT4 deficiency leads to more severe alterations in cardiac electrical activity during cardiac stress due to hypoxia. To fulfil this aim, we compared cardiac electrical activity from cardiac-selective GLUT4-ablated (G4H-/-) mouse hearts and corresponding control (CTL) littermates. A custom-made cylindrical 'cage' electrode array measured potentials (Ves) from the epicardium of isolated, perfused mouse hearts. The normalized average of the maximal downstroke of Ves ( (|d Ves/dt(min)|na), which we previously introduced as an index of electrical activity in normal, ischaemic and hypoxic hearts, was used to assess the effects of GLUT4 deficiency on electrical activity. The |d Ves/dt(min)|na of G4H −/− and CTL hearts decreased by 75 and 47%, respectively (P < 0.05), 30 min after the onset of hypoxia. Administration of insulin attenuated decreases in values of |d Ves/dt(min)|na in G4H −/− hearts as well as in CTL hearts, during hypoxia. In general, however, G4H −/− hearts showed a severe alteration of the propagation sequence and a prolonged total activation time. Results of this study demonstrate that reduced glucose availability associated with insulin resistance and a reduction in GLUT4-mediated glucose transport impairs electrical activity during hypoxia, and may contribute to cardiac vulnerability to arrhythmias in diabetic patients.

Glycerol uptake is by passive diffusion in the heart but by facilitated transport in RBCs at high glycerol levels in cold acclimated rainbow smelt (Osmerus mordax)

Rainbow smelt (Osmerus mordax) is a small fish that accumulates glycerol at low winter seawater temperatures. In laboratory-held fish, glycerol concentration typically reaches 225 mM in plasma and in all cells. Glycerol uptake by the heart and red blood cells (RBCs) was assessed by tracking [(14)C(U)]glycerol into the acid-soluble pool. In fish acclimated to 9-10°C a decrease in perfusion/incubation temperature from 8 to 1°C resulted in a decrease in glycerol uptake with a Q(10) of 3.2 in heart and 2.4 in RBCs. Acclimation to ∼1.5°C did not result in an adaptive enhancement of glycerol uptake as rates were unchanged in heart and RBCs. Glycerol uptake at 1°C was by passive diffusion in heart as evidenced by a linear relationship between glycerol uptake and extracellular glycerol concentration and a lack of inhibition by phloretin. In contrast, in RBCs, glycerol uptake with respect to glycerol concentration showed two linear relationships with a transition point around 50 mM extracellular glycerol. The slope of the second phase was much steeper and eliminated with the inclusion of phloretin. In RBCs from Atlantic salmon (Salmo salar), a related species that does not accumulate glycerol, glycerol uptake showed only a single linear curve and was not inhibited by phloretin. The data imply a strong facilitated component to glycerol uptake in rainbow smelt RBCs at high glycerol concentrations. We propose this is related to cyclic changes in RBC glycerol content involving a loss of glycerol at the gill and a reaccumulation during passage through the liver.

Low O2 avoidance is associated with physiological perturbation but not exhaustion in the snapper (Pagrus auratus: Sparidae)

It is already known that the New Zealand snapper (Pagrus auratus, Sparidae) does not avoid hypoxia until reaching an oxygen partial pressure (PO(2)) of 3.1±1.2 kPa at 18 °C. Avoidance at this level of PO(2) and temperature is below the critical oxygen partial pressure of the species (P(crit)=5.8±0.6 kPa, 43.5±4.5 mmHg) and is therefore expected to result in major physiological stress. Results from the current study showed that avoidance was associated with numerous physiological perturbations, including a significant endocrine response, haematological changes, osmoregulatory disturbance and metabolic adjustments in the heart, liver and muscle. Snapper clearly experienced physiological stress at the point of avoidance but they were not however in a state of physiological exhaustion since some fuel reserves were still available. In addition to avoidance, snapper also showed a subtle reduction in swimming speed - this energy-saving response may have helped snapper minimise the physiological challenge of low O(2) residence. It is therefore concluded that snapper can reside in water below their P(crit) threshold for brief periods of time and, without any evidence of physiological exhaustion at the point of avoidance, fish should recover quickly once normoxia is selected. Lastly, with signs of anaerobic metabolism in cardiac tissue at the point of avoidance, we tentatively suggest that snapper may leave hypoxia to protect heart function.

Glucose uptake and metabolism by RBCs from fish with different extracellular glucose levels

The aim of the study was to assess if mechanisms of glucose trafficking by red blood cells (RBCs) relates to species specific extracellular glucose levels. Atlantic cod (Gadus morhua), Atlantic salmon (Salmo salar), cunner (Tautogolabrus adspersus), and short-horned sculpin (Myoxocephalus scorpius) had plasma glucose levels of 4 mmol l-1, 4.1 mmol l-1, 1.95 mmol l-1, and 0.73 mmol l-1, respectively. Glucose uptake by isolated RBCs was measured by the initial incorporation of [6-14C]-glucose and steady state glucose metabolism was determined by the production of 3H2O from [2-3H]-glucose. Saturation kinetics of glucose uptake and inhibition of both glucose uptake and metabolism by cytochalasin B and phloretin revealed that Atlantic cod, cunner, and sculpin RBCs all had a facilitated transport component to glucose trafficking. RBCs from Atlantic salmon showed a linear relationship between glucose uptake and extracellular glucose level but exhibited clear inhibition of glucose metabolism by cytochalasin B and phloretin suggesting a component of facilitated glucose transport that is more elusive to detect. The production of 3H2O was linear for at least 6 hr and as such presents a rigorous approach to measuring glycolytic rate. Steady state rates of glucose metabolism were achieved at extracellular levels of approximately 1 mmol l-1 glucose for RBCs from all species showing that within species normal extracellular glucose level is not a primary determinant of basal level of glycolysis. At physiological levels of extracellular glucose the ratio of initial glucose uptake to glucose metabolism was 1.5 to 4 for all RBCs suggesting that there is scope to increase metabolic rate without alteration of the basal glucose uptake capacity.

Purinergic modulation of glucose uptake into cultured rat podocytes: effect of diabetic milieu

Extracellular purines act via P1 and P2 receptors on podocytes and may influence on their function. This action may be modified under various (patho)physiological conditions leading to development of podocytopathy. Aim of study was to investigate effects of diabetic milieu, represented by high glucose concentration (HG, 30 mM glucose) on purinergic-induced changes of 2-deoxy-D-glucose (2-DG) uptake and on extracellular purines metabolism in cultured rat podocytes. Basal 2-DG uptake was 2.7-fold enhanced in HG compared to normal glucose concentration, NG (1271 ± 86 vs. 477 ± 37 nmol/h/mg protein, P<0.001). ATP stimulated 2-DG uptake by 44 ± 4% and 29 ± 5% in NG and HG, respectively. ATP analogues, β, γ-methylene ATP and 2-methylthio ATP stimulated 2-DG uptake in range of 18-34% in NG and 16-17% in HG. Benzoylbenzoyl ATP increased 2-DG uptake about 24 ± 2% in NG however, its effect in HG reached 50 ± 1%. The antagonists of P2 receptors (suramin, reactive blue 2, PPADS) decreased basal 2-DG uptake in NG and HG; suramin and reactive blue 2 at average of 15 ± 4% in NG but in HG the effect was in following order: suramin 28 ± 3%; PPADS 20 ± 3% and RB-2 9 ± 0.9%. Extracellular adenosine concentration was higher in HG than in NG (0.48 ± 0.01 vs. 5.05 ± 0.39 μM, P < 0.05), however intracellular ATP content and extracellular ATP concentration were not affected. Neither ecto-ATPase nor ecto-5'-nucleotidase activities were affected in HG. In conclusion, diabetic milieu affects purinergic modulation of glucose transport into podocytes which may play a role in development of diabetic podocytopathy.

In situ cardiac function in Atlantic cod (Gadus morhua): effects of acute and chronic hypoxia

Recent in vivo experiments on Atlantic cod (Gadus morhua) acclimated to chronic hypoxia (6-12 weeks at 10°C; PwO2 ~8-9 kPa) revealed a considerable decrease in the pumping capacity of the heart. To examine whether this diminished cardiac performance was due to the direct effects of chronic moderate hypoxia on the myocardium (as opposed to alterations in neural and/or hormonal control), we measured the resting and maximum in situ function of hearts from normoxia- and hypoxia-acclimated cod: (1) when initially perfused with oxygenated saline; (2) at the end of a 15 min exposure to severe hypoxia (PO2 ~0.6 kPa); and (3) 30 min after the hearts had been reperfused with oxygenated saline. Acclimation to hypoxia did not influence resting (basal) in situ cardiac performance during oxygenated or hypoxic conditions. However, it caused a decrease in maximum cardiac output () under oxygenated conditions (from 49.5 to 40.3 ml min−1 kg−1; by 19%), that was due to diminished values for maximum stroke volume (VS) and scope for VS. Severe hypoxia reduced in both groups to ~20 ml min−1 kg−1, yet, the hearts of hypoxia-acclimated fish were better able to sustain this level of under hypoxia, and the recovery of (as compared with initial values under oxygenated conditions) was significantly improved (94% vs 83%). These data show that acclimation to hypoxia has a direct effect on cod myocardial function and/or physiology, and suggest that the cod heart shows some adaptations to prolonged hypoxia.

Expression levels of genes associated with oxygen utilization, glucose transport and glucose phosphorylation in hypoxia exposed Atlantic cod (Gadus morhua)

Expression level of genes associated with oxygen [cytochrome oxidase 1 (COX1) and myoglobin (Mb)] and glucose utilization [glucose transporters (GLUTs) and hexokinases (HKs)] along with metabolic indices were determined in Atlantic cod (Gadus morhua) subjected to an hypoxic challenge of <45% oxygen saturation for 24 days. There were two closely related HKs considered to be homologues of mammalian HKIs. HKIa and HKIb share 86% sequence identity and are both ubiquitously expressed. Mb was also expressed in many tissues with highest levels occurring in heart. Over the first 15 days of hypoxia there were transient increases in plasma lactate in hypoxic relative to normoxic fish associated with a significant decrease in liver glycogen. Over days 1-6, there were in ten of eleven cases, increased average (with a number of conditions being statistically significant) expression levels of GLUTs (1, 2, 4) and HKs (1a and b) in gill, heart, liver, and white muscle in hypoxic relative to normoxic fish. There were significant increases in COX1 and Mb expression levels in gill by day 24 but no changes in these aerobic indicators in heart or liver. Overall the data suggest a transient increase in genes associated with glucose utilization during the early part of the hypoxic challenge followed by alterations in gene expression in gill.

Intracellular glucose and binding of hexokinase and phosphofructokinase to particulate fractions increase under hypoxia in heart of the amazonian armored catfish (Liposarcus pardalis)

Armored catfish (Liposarcus pardalis), indigenous to the Amazon basin, have hearts that are extremely tolerant of oxygen limitation. Here we test the hypothesis that resistance to hypoxia is associated with increases in binding of selected glycolytic enzymes to subcellular fractions. Preparations of isolated ventricular sheets were subjected to 2 h of either oxygenated or hypoxic (via nitrogen gassing) treatment during which time the muscle was stimulated to contract. The bathing medium contained 5 mM glucose and was maintained at 25 degrees C. Initial experiments revealed increases in anaerobic metabolism. There was no measurable decrease in glycogen level; however, hypoxic treatment led to a twofold increase in heart glucose and a 10-fold increase in lactate content. It is suggested that the increase in heart glucose content is a result of an enhanced rate of facilitated glucose transport that exceeds the rate of phosphorylation of glucose. Further experiments assessed activities of metabolic enzymes in crude homogenates and subsequently tracked the degree of enzyme binding associated with subcellular fractions. Total maximal activities of glycolytic enzymes (hexokinase [HK], phosphofructokinase [PFK], aldolase, pyruvate kinase, lactate dehydrogenase), and a mitochondrial marker, citrate synthase, were not altered with the hypoxic treatment. A substantial portion (>/=50%) of HK is permanently bound to mitochondria, and this level increases under hypoxia. The amount of HK that is bound to the mitochondrial fraction is at least fourfold higher in hearts of L. pardalis than in rat hearts. Hypoxia also resulted in increased binding of PFK to a particulate fraction, and the degree of binding is higher in hypoxia-tolerant fish than in hypoxia-sensitive mammalian hearts. Such binding may be associated with increased glycolytic flux rates through modulation of enzyme-specific kinetics. The binding of HK and PFK occurs before any significant decrease in glycogen level.

The impact of hypoxia on in vivo glucose uptake in a hypoglycemic fish,Myoxocephalus scorpius

The mechanisms controlling carbohydrate utilization in teleost fish are poorly understood, particularly in the heart. Tissue glucose uptake and cardiovascular characteristics were measured in the short-horned sculpin, Myoxocephalus scorpius, a species exhibiting low blood glucose levels, during normoxia and hypoxia to assess the role of adenosine receptors in the control of glucose uptake and anaerobic metabolism. As expected, hypoxia exposure (300 min at 2 mg/l dissolved oxygen) resulted in a bradycardia and plasma lactate accumulation, but glucose uptake rates did not change in heart, brain, gill, spleen, and white muscle. Plasma glucose-to-intracellular glucose ratios indicated that glucose uptake was the rate-limiting step in glucose utilization. The majority of intracellular glucose was unphosphorylated, however, suggesting that hexokinase is also important in controlling the tissue glucose gradient. During hypoxia, the cholinergic blocker atropine resulted in tachycardia but did not significantly change tissue glucose uptake rates or heart and brain adenosine levels. In contrast, the combined treatment of atropine and an adenosine receptor blocker [8-( p-sulfophenyl)theophylline] during hypoxia increased heart glucose uptake to levels fivefold higher than normoxic fish, with no additive effects on cardiovascular parameters. Significant tissue lactate accumulation was observed in this group of fish, signifying that adenosine receptors may depress anaerobic metabolism, even though tissue adenosine accumulation was absent during hypoxia. White muscle accumulated glucose during normoxia, suggesting the presence of gluconeogenic pathways or active uptake mechanisms not previously described in this tissue.

Sequence of Atlantic cod (Gadus morhua) GLUT4, GLUT2 and GPDH:developmental stage expression, tissue expression and relationship to starvation-induced changes in blood glucose

cDNAs of putative glucose transporters, GLUT4 and GLUT2, were cloned from Atlantic cod (Gadus morhua). The GLUT4 cDNA encodes a 503 amino acid and the GLUT2 cDNA a 506 amino acid protein. Phylogenetic analysis, amino acid sequence alignment, and tissue distribution support categorizing them as homologues of mammalian GLUT4 and 2. GLUT4 clusters with GLUT4s from fish and other vertebrates. It shows 84% amino acid identity to GLUT4 from coho salmon and brown trout and 65% identity with other vertebrates. It is most highly expressed in heart, strongly expressed in red and white skeletal muscle and present at lower levels in gill, gonad, intestine, and kidney. GLUT2 clusters with GLUT2 from rainbow trout and other vertebrates. It shows 75% amino acid identity with rainbow trout and 62% identity with chicken GLUT2. In Atlantic cod, GLUT2 is most highly expressed in liver with lower levels noted in intestine and kidney.

Food deprivation for 2 months was used as a vehicle to monitor GLUT expression at different blood glucose levels. Starvation resulted in a decrease in blood glucose and liver glycogen that recovered following 20 days of re-feeding. GLUT4 expression in heart was decreased with starvation and increased with re-feeding. GLUT4 mRNA level in heart correlated with blood glucose. It is suggested that this relationship is related to insulin responsiveness. GLUT4 expression in white muscle increased with starvation and decreased with re-feeding. It is proposed that this is due to the necessity to maintain high levels of the glucose transporter protein in the face of starvation-associated proteolysis. GLUT2 expression in liver correlated with blood glucose, consistent with higher rates of glucose transport from liver to blood in the fed state than in the food-deprived state.

Glycerol-3-phosphate dehydrogenase (GPDH) cDNA was also cloned. It encodes a 351 amino acid protein, which is 73-90% identical to GPDH from numerous other fish species. GPDH is ubiquitously expressed. Expression in heart decreased with starvation and increased with refeeding, whereas expression in liver did not change with starvation.

In other studies, gene expression was monitored at nine time points from fertilization of eggs to larval development. GLUT4 is detectable in fertilized eggs and is fully expressed by the halfway to hatching point. GLUT2 is not evident at fertilization, is detectable at halfway to hatching, and fully expressed at hatching. GPDH expression was evident from fertilization.

Sex differences in energy metabolism and performance of teleost cardiac tissue

This study examined the effects of different oxygenation levels and substrate availability on cardiac performance, metabolism, and biochemistry in sexually immature male and female rainbow trout (Oncorhynchus mykiss). Ventricle strips were electrically paced (0.5 Hz, 14 degrees C) in hyperoxic or hypoxic Ringer solution. Our results demonstrate that 1) males sustain isometric force production (F) longer than females under hyperoxia (P O2 = 640 mmHg) with exogenous glucose present; 2) contractility is not maintained under moderate (P O2 = 130 mmHg) or severe hypoxia (P O2 = 10-20 mmHg) with glucose in either sex; however, following reoxygenation, F is higher in females compared with males; and 3) female tissue has higher lactate levels, net lactate efflux, and lactate dehydrogenase activity than males, whereas males have higher glycogen, citrate synthase, and beta-hydroxy acyl-CoA dehydrogenase activities, and greater inotropic responses to exogenous glucose and octanoate. No sex differences were detected in responsiveness to epinephrine and inhibitors of glucose transport or activities of hexokinase and pyruvate kinase. We conclude that sex differences exist in rainbow trout cardiac tissue: females appear to prefer glycolysis for ATP production, whereas males have a higher capacity for aerobic and lipid metabolism.

Steroid-induced cardiac contractility requires exogenous glucose, glycolysis and the sarcoplasmic reticulum in rainbow trout

Recent data from our laboratory suggest that sex steroids promote contractile function in cardiac muscle of rainbow trout (Oncorhynchus mykiss Walbaum), and there are sex differences in hormone signaling and cardiac function. The current study investigated whether steroid-induced inotropism in electrically paced (0.5 Hz, 14 degrees C) ventricle strips at 90% Lmax (1) has a metabolic requirement for exogenous glucose and (2) is associated with enhanced intracellular Ca2+ storage and release from the sarcoplasmic reticulum (SR). We also explored whether sex differences exist in extracellular Ca2+ (Ca2+o) or cardiac sensitivity to Ca2+o. In the absence or at low concentrations (1 or 2 mmol l-)) of exogenous glucose, resting tension and relaxation time were increased selectively in cardiac tissue from females. Increasing glucose promoted twitch force in a bell-shaped manner, with 5 mmol l-1 representing the optimal concentration for both sexes. The positive inotropic effects of physiological concentrations of testosterone (T) and 17beta-estradiol (E2) in male and female trout ventricle strips, respectively, developed slowly (10-45 min) and were not apparent in glucose-free medium, in medium containing iodoacetate (IAA), an inhibitor of glycolysis, or medium containing 5 mmol l-) lactate or pyruvate. Male ventricle strips had increased inotropic responses to glucose and T compared with female strips exposed to glucose and E2. Furthermore, sexually maturing males showed a greater inotropic response than immature males or females. Pretreatment with ryanodine (a specific blocker of SR Ca2+ release) also eliminated the inotropic effects of sex steroids and exogenous glucose and reduced the post-rest potentiation of contractile force (a marker of SR Ca2+ storage). By contrast, the inotropic effects of epinephrine (Epi) or elevated Ca2+o were faster (developing within 1-3 min) and were not diminished by the presence or absence of glucose or by pretreatment with IAA or ryanodine. Sex differences were also found in responsiveness to caffeine (males>females) and the relationship between Ca2+ concentration and force development above baseline. The Ca2+50 was lower in female cardiac tissue than males, suggesting greater Ca2+ sensitivity, and although plasma albumin was higher in females, total and ionized plasma Ca2+ did not differ between the sexes. For the first time, our study highlights the importance of extracellular glucose, glycolytic activity and SR Ca2+ storage and release for sex steroid-induced inotropism in the trout ventricle. Conversely, the inotropes Epi and elevated [Ca2+o] do not require the presence or metabolism of exogenous glucose or the SR for signaling their positive effects on contractility. These results also demonstrate novel sex-related differences in cardiac reliance on exogenous glucose, Ca2+ sensitivity and SR function and thus should be considered in future studies.

Carbohydrate management, anaerobic metabolism, and adenosine levels in the armoured catfish,Liposarcus pardalis (castelnau), during hypoxia

The armoured catfish, Liposarcus pardalis, tolerates severe hypoxia at high temperatures. Although this species can breathe air, it also has a strong anaerobic metabolism. We assessed tissue to plasma glucose ratios and glycogen and lactate in a number of tissues under "natural" pond hypoxia, and severe aquarium hypoxia without aerial respiration. Armour lactate content and adenosine in brain and heart were also investigated. During normoxia, tissue to plasma glucose ratios in gill, brain, and heart were close to one. Hypoxia increased plasma glucose and decreased tissue to plasma ratios to less than one, suggesting glucose phosphorylation is activated more than uptake. High normoxic white muscle glucose relative to plasma suggests gluconeogenesis or active glucose uptake. Excess muscle glucose may serve as a metabolic reserve since hypoxia decreased muscle to plasma glucose ratios. Mild pond hypoxia changed glucose management in the absence of lactate accumulation. Lactate was elevated in all tissues except armour following aquarium hypoxia; however, confinement in aquaria increased armour lactate, even under normoxia. A stress-associated acidosis may contribute to armour lactate sequestration. High plasma lactate levels were associated with brain adenosine accumulation. An increase in heart adenosine was triggered by confinement in aquaria, although not by hypoxia alone.

Cloning of GLUT3 cDNA from Atlantic cod (Gadus morhua) and expression of GLUT1 and GLUT3 in response to hypoxia

A putative facilitative glucose transporter, GLUT3, cDNA was cloned from Atlantic cod. It is ubiquitously expressed, with substantial levels in kidney. The 519 aa protein has the highest sequence identity (66.3%) to grass carp GLUT3. Atlantic cod were exposed to a hypoxic challenge (45% DO2) for 24 h and the effects on GLUT1 and GLUT3 expression assessed. GLUT1 expression in gill is upregulated; however, in spleen, there is a significant decrease in both GLUT1 and GLUT3 expression. The increase in GLUT1 mRNA is considered to be associated with an increased energy demand on gill, whereas, the decrease in gene expression in spleen potentially reflects a general decrease in rates of transcription.

Sequence and expression of a constitutive, facilitated glucose transporter (GLUT1) in Atlantic cod Gadus morhua

A putative glucose transporter, GLUT1, is reported for Atlantic cod Gadus morhua. A combination of RT-PCR, RLM-RACE and genome walking were used to articulate a 4560 bp cDNA (GenBank accession number AY526497). It contains a 149 bp 5' UTR, a 1470 bp open reading frame and a 2941 bp 3' UTR. At the nucleotide level, the cod GLUT1 ORF shares 78.2% sequence identity to human GLUT1 and the deduced amino acid sequence clusters with GLUT1s from rainbow trout and carp. GLUT1 transcript is highly expressed in brain, gill, heart and kidney and expressed to a lower level in at least six other tissues. Expression is evident immediately upon fertilization of eggs. Six hours of hypoxia at 40% DO(2) did not alter expression levels in brain, gill, heart or kidney. The level of expression is not substantially altered in heart during low temperature challenge, although there is a suggestion that colder temperature could lead to lower levels of expression, consistent with the concept that the cold-acclimated heart has a reduced dependence upon glucose as a metabolic fuel. Two months of starvation did not significantly alter the level of expression of GLUT1 in heart. This is in marked contrast to the rat heart where fasting leads to a substantial decrease in GLUT1 levels. Overall, there is a ubiquitous tissue distribution of GLUT1, consistent with other species, and the level of gene expression, especially in heart, is relatively constant over a range of physiological conditions.