Cloning of glycerol-3-phosphate dehydrogenase cDNAs from two fish species and effect of temperature on enzyme expression in rainbow smelt (Osmerus mordax)

Rainbow smelt (Osmerus mordax) can accumulate extreme levels of glycerol in their blood during winter. Low temperatures are required for glycerol accumulation in smelt blood and the enzyme glycerol-3-phosphate dehydrogenase (GPDH) has been suggested to play a role in glycerol production/concentration in this species. In the present study, cDNA sequences encoding glycerol-3-phosphate dehydrogenase (GPDH) from rainbow smelt and Atlantic salmon (Salmo salar) were cloned. The encoded GPDH protein sequences were very similar to one another (88% identity). Using RT-PCR, GPDH mRNA was detected in skin, gill, heart, head kidney, brain and liver from both salmon and smelt obtained in December. However, GPDH was not detected in salmon intestine and spleen or in smelt intestine. Examination of GPDH expression in smelt liver during February by Northern blotting revealed temperature regulation. Elevation of the temperature resulted in a significant decrease in liver GPDH transcript level. Serum glycerol levels decreased concomitantly. These findings suggest a role for GPDH in the accumulation of glycerol in smelt at low temperatures.

Effects of chronic food restriction and exercise training on the recovery of cardiac function following ischemia

Clinical and experimental data suggest that exercise training (ET) and food restriction (FR) improve cardiovascular function. However, the effects of long-term FR or FR in combination with ET on the recovery of cardiac function following ischemia have not been determined. Male Wistar rats were assigned to ad libitum-fed, FR, ad libitum-exercise, and FR-exercise groups. Mechanical function of isolated working hearts was assessed in response to increases in afterload resistance and following global no-flow ischemia. At low workload, there was a significant FR effect on aortic flow as well as an interaction between FR and ET on systolic pressure. These effects remained when hearts were subjected to increases in aortic afterload resistance. During reperfusion of ischemic hearts, there was a significant FR effect on aortic flow and systolic pressure and a significant ET effect on diastolic pressure. An interaction between FR and ET on heart rate was also seen during reperfusion. In terms of percent recovery of heart function following ischemia, FR continued to affect aortic flow, and we observed an interaction between FR and ET on aortic flow. Our results clearly indicate that the myocardium from the FR animal or the FR, exercise-trained rat is more resistant to ischemia.

Anoxic performance of the american eel (Anguilla rostrata L.) heart requires extracellular glucose

The importance of extracellular glucose in the maintenance of performance of the heart of the American eel (Anguilla rostrata Le Sueur (L.) Under anoxia was assessed under a variety of experimental conditions. Ventricular strips, electrically paced at 36 bpm, in N(2)-gassed medium maintained the imposed pace rate and generated approximately 25% of the initial twitch force of contraction for at least 60 min when glucose was present in the medium. But ventricular strips challenged without glucose in the medium failed to maintain the pacing rate within 5-10 min. Isolated and intact, perfused hearts maintained pressure and followed an imposed pace rate of 24 bpm for at least 2 hr, under anoxic conditions, if glucose was present in the medium. But without glucose in the medium isolated hearts failed within 30 min. Endogenous glycogen stores were utilized in hearts perfused with medium containing NaCN to impair oxidative phosphorylation. The presence of glucose in the medium did not protect against glycogen mobilization. The data indicate that exogenous glucose is necessary to maintain performance under anoxia at high workloads and physiological Ca(2+) levels. Finally, ventricular strips treated with NaCN and forced to contract at 24 bpm lost 70% of initial twitch force. Increasing extracellular Ca(2+) concentration stepwise from 1.5 to 9.5 mM restored twitch force to approximately 50% of the initial level and this response was not dependent on exogenous glucose. However, glucose was required to maintain resting tension even under normoxic conditions in the face of a Ca(2+) challenge.

Maintenance of resting tension in the american eel (Anguilla rostrata L.) heart is dependent upon exogenous fuel and the sarcoplasmic reticulum

The relationship between extracellular glucose and management of cell Ca(2+) in the heart of the American eel (Anguilla rostrata) was indirectly assessed by monitoring the performance of isolated ventricular strips at 20 degrees C. Twitch force increased in ventricular strips under specific conditions of 30 bpm pacing and an extracellular Ca(2+) challenge from 1.5 to 9.5 mM. The response was independent of any exogenous metabolic fuel in the medium. Resting tension was maintained when glucose was available, but in the absence of a metabolic fuel, resting tension increased in response to the increase in extracellular Ca(2+) level. When ventricular strips were treated with iodoacetate to inhibit glycolysis, a Ca(2+) challenge resulted in a decrease in twitch force in association with an approximately equivalent increase in resting tension even in the presence of exogenous glucose. However, when pyruvate (5 mM) was substituted as a metabolic fuel, twitch force increased as a function of extracellular Ca(2+), and resting tension was maintained in the presence of iodoacetate. Therefore, there is a need for an extracellular fuel but not a specific metabolic requirement for glucose to maintain the performance characteristics, which are presumably related to the management of intracellular Ca(2+) levels. Ventricular strips were treated with ryanodine to inhibit Ca(2+) release and uptake by the sarcoplasmic reticulum (SR). Ryanodine treatment impaired postrest potentiation at high extracellular Ca(2+) levels. In the presence of ryanodine, the protective effect of glucose on the increase in resting tension in the face of an extracellular Ca(2+) challenge was eliminated. Considered together, the results reveal that the heart of the American eel has a requirement for an extracellular fuel to manage intracellular Ca(2+) at high Ca(2+) loads, and that the SR plays a role in the beat-to-beat regulation of Ca(2+) at a frequency of 30 bpm, high Ca(2+) load, and 20 degrees C.

Activity levels of enzymes of energy metabolism in heart and red muscle are higher in north-temperate-zone than in Amazonian teleosts

Fish living in the Amazon basin typically have body temperatures of about 30°C, whereas freshwater fishes of the north-temperate zone are eurythermal, with typical body temperatures of 10-20°C in summer. Enzyme activity levels in heart and red muscle of Amazonian species, which display various physiological mechanisms for dealing with hypoxic conditions, were compared with those in north-temperate-zone species. Five Amazonian species (acará-açu (Astronotus ocellatus), acari-bodó (Lipossarcus pardalis), tambaqui (Colossoma macropomum), tamoatá (Hoplosternum littorale), and pirarucu (Arapaima gigas)) and four north-temperate-zone species (American eel, bullhead, yellow perch, and rainbow trout) were studied. The Amazonian species included obligate and facultative air breathers. Activities of key indicator enzymes associated with carbohydrate metabolism, fatty acid metabolism, the citric acid cycle, and the electron-transport chain were determined. There was no obvious correlation between cardiac enzyme activity levels and the potential ability of fish to maintain blood oxygen levels in hypoxic water or the capacity of isolated heart preparations to survive anoxia. In heart, activity levels of hexokinase, phosphofructokinase, pyruvate kinase, lactate dehydrogenase, citrate synthase, cytochrome oxidase, and β-hydroxyacyl CoA dehydrogenase were about twice as high in north-temperate-zone species than in Amazonian species. Activities of red-muscle enzymes, especially those associated with aerobic fatty acid metabolism, were significantly higher in comparable north-temperate-zone species relative to Amazonian species. Increased enzyme activity levels in north-temperate-zone species relative to Amazonian species is considered to be an adaptation to generally lower body temperatures. This finding is consistent with earlier comparisions of Antarctic and north-temperate-zone species and with the results of studies of cold acclimation within north-temperate-zone fishes.

Anoxic cardiac performance in Amazonian and north-temperate-zone teleosts

Amazonian fish face periodic conditions of hypoxic water not commonly encountered by most other species. We examined the impact of simulated anoxia on cardiac performance of Amazonian species relative to north-temperate-zone species to assess whether the ability to maintain anoxic heart performance is related to either respiratory strategy or geographic location. Four Amazonian species (acará-açu (Astronotus ocellatus), acari-bodó (Lipossarcus pardalis), tambaqui (Colossoma macropomum), and tamoatá (Hoplosternum littorale)) and four north-temperate-zone species (American eel (Anguilla rostrata), bullhead (Ictalurus punctatus), yellow perch (Perca flavescens), and rainbow trout (Oncorhynchus mykiss)) were studied. The Amazonian species display a range of respiratory modes from obligate gill-breather to facultative air-breather. The performance of isolated ventricular strips under conditions of impaired oxidative phosphorylation was monitored under the same conditions of temperature (25°C) and contraction frequency (30 bpm) for all species. In all cases, ventricular strips showed a decrease in the force of contraction with sodium cyanide (NaCN) treatment. Ventricular strips from two Amazonian and one north-temperate-zone species showed signs of recovery from NaCN treatment. There was considerable diversity in the ability of ventricular strips to develop force under conditions of impaired oxidative phosphorylation that was not related to the ability to obtain oxygen from the air or to geographic location. However, there was a major difference between Amazonian and north-temperate-zone species in the ability of ventricular strips to maintain resting tension under NaCN treatment. Resting tension increased under conditions of simulated anoxia in ventricular strips from all north-temperate-zone species, but there was no change in resting tension in ventricular strips from Amazonian species. We propose that hearts from the Amazonian species are better able to reduce cytoplasmic calcium levels, possibly through a more highly developed sarcoplasmic reticulum.

Mitochondrial protein synthesis in rainbow trout (Oncorhynchus mykiss) heart is enhanced in sexually mature males but impaired by low temperature

Throughout the life cycle of the rainbow trout (Oncorhynchus mykiss), the heart exhibits periods of enhanced growth. Two such instances are cardiac enlargement associated with sexual maturity in males and heart growth at seasonally low environmental temperatures. Heart growth includes a parallel increase in the number of mitochondria. These natural models of heart growth have been exploited to study protein synthesis directed by the mitochondrial genome. Methods were developed to assess protein synthesis in mitochondria isolated from the heart of rainbow trout. Protein synthesis was assessed by tracking the incorporation of l-[2,6-(3)H]phenylalanine into trichloracetic-acid-precipitable protein. Amino acid incorporation into mitochondrial protein was linear with respect to time and was inhibited by chloramphenicol. Radiolabel was selectively enhanced in molecular mass fractions over the same size range as polypeptides known to be encoded by the mitochondrial genome. Protein synthesis was measured in mitochondria isolated from sexually mature animals and from animals subjected to different thermal regimes. The relative ventricular mass of sexually mature male rainbow trout was significantly greater than that of sexually mature females (0. 104+/−0.004 versus 0.087+/−0.002; mean +/− s.e.m.). Mitochondria isolated from the heart of males synthesized protein at a faster rate than mitochondria isolated from the heart of females (0.22+/−0. 02 versus 0.11+/−0.02 pmol phenylalanine mg(−)(1)protein min(−)(1)). That is, ‘male’ mitochondria are inherently predisposed to synthesize protein at faster rates. We speculate that the difference may result from higher levels of mitochondrial RNA in males than in females. Mitochondria isolated from the heart of sexually immature rainbow trout acclimated to 13 degrees C synthesized protein at the same rate at 25 degrees C (0.456+/−0.075 pmolphenylalanine mg(−)(1)protein min(−)(1)) and 15 degrees C (0.455+/−0.027 pmol phenylalanine mg(−)(1)protein min(−)(1)). However, the rate of protein synthesis was severely impaired at 5 degrees C (0.125+/−0.02 pmol phenylalanine mg(−)(1)protein min(−)(1)). Since the rate of state 3 respiration by isolated mitochondria decreased in a linear fashion over the temperature range 25 to 5 degrees C, the rate of mitochondrial protein synthesis is not directly coupled to the rate of respiration. Thermal acclimation to 5 degrees C did not result in positive thermal compensation in either the rate of protein synthesis or the rate of oxygen consumption by isolated mitochondria. In a further series of experiments, total protein synthesis and oxygen consumption were measured in isolated myocytes. The rate of oxygen consumption by myocytes remained constant over the temperature range 25 to 5 degrees C. There was no difference in the rate of total cell protein synthesis between 25 degrees C (1.73+/−0. 29 pmol phenylalanine 10(6)cells(−)(1)h(−)(1)) and 15 degrees C (2. 12+/−0.19 pmol phenylalanine 10(6)cells(−)(1)h(−)(1)), but at 5 degrees C protein synthesis was substantially impaired to approximately one-sixth of the level observed at 15 degrees C. As such, rates of total cell protein synthesis were not directly coupled to rates of respiration and were curtailed at low temperature. In vitro studies show that mitochondria isolated from the heart of sexually mature male rainbow trout are inherently different from mitochondria isolated from the heart of females such that the former are able to synthesize protein at a faster rate. The rate of mitochondrial protein synthesis does not correlate with the greater than twofold changes in rates of oxygen consumption induced by acute changes in assay temperature, suggesting that protein synthesis is not directly coupled to rates of ATP or GTP synthesis.

Synthesis, characterization, and biological relevance of hydroxypyrone and hydroxypyridinone complexes of molybdenum

We have prepared a number of complexes of the type cis-MoO2L2 where L represents a hydroxypyronato or hydroxypyridinonato ligand. Both the maltol (3-hydroxy-2-methyl-4-pyrone, Hma) and kojic acid (5-hydroxy-2-hydroxymethyl-4-pyrone, Hka) complexes, cis-MoO2(ma)2 (1) and cis-MoO2(ka)2 (2), have been characterized by X-ray diffraction studies. The pyrone ligands are bound to molybdenum in a cis bidentate fashion via the deprotonated hydroxyl groups and the ketone moieties. Crystals of 1 are orthorhombic, a = 12.107 (1), b = 8.6169 (8), c = 16.472 (1) Å, Z = 4, space group Pca21, and those of 2 are monoclinic, a = 8.4591 (5), b = 16.3453 (10), c = 10.2954 (7) Å, β = 103.0320 (10)°, Z = 4, space group P21/c. Hydroxypyridinone molybdenum complexes have been prepared for both maltol and kojic acid derivatives with the substituents Me, n-Pr, CH2Ph, Ph at the ring nitrogen. Crystals of the 3-hydroxy-2-methyl-1-phenyl-4-pyridinone (Hppp) derivative, MoO2(ppp)2 (9), are monoclinic, a = 10.9476 (6), b = 13.5353 (9), c = 17.4877 (10) Å, β = 93.465 (4)°, Z = 4, space group P21/n. Initial investigations into the effects molybdenum compounds have on diabetic hearts are presented. Both Na2MoO4 (used as a control) and 1 were effective in lowering blood glucose and free fatty acid levels. Diabetic rats treated with molybdate showed significant improvements in postischemic cardiac function.Key words: molybdenum, hydroxypyrones, hydroxypyridinones, heart function.

Oxygen consumption in myoglobin-rich and myoglobin-poor isolated fish cardiomyocytes

The function of myoglobin at the cellular level was investigated by comparing O2 consumption in isolated myoglobin-rich cardiac myocytes from the sea raven (Hemitripterus americanus) and myoglobin-poor myocytes from the ocean pout (Macrozoarces americanus). O2 consumption by sea raven myocytes, 0.21 +/- 0.04 microM O2/10(6) cells.min-1, was significantly higher than O2 consumption by ocean pout myocytes, 0.10 +/- 0.07 microM O2/10(6) cells.min-1 at high PO2. O2 consumption in sea raven myocytes treated with sodium nitrite was not significantly different than that in untreated myocytes at high PO2, but it was significantly lower than controls at low PO2. O2 consumption of sea raven myocytes treated with the mitochondrial uncoupler CCCP was not significantly different from that of control myocytes at high PO2, but it was significantly greater than untreated controls at low PO2. In ocean pout preparations, O2 consumption by nitrite-treated myocytes was significantly higher than that of untreated myocytes at high PO2, but it was not different from that of controls at low PO2. CCCP-treated ocean pout myocytes had a significantly higher oxygen consumption than that of untreated myocytes at high PO2, but oxygen consumption was not different from that of controls at low PO2. The CCCP-activated O2 consumption at low PO2 was myoglobin-dependent in that CCCP alone resulted in a threefold increase in sea raven cells over controls but had no impact on sea raven cells in the presence of nitrite or ocean pout cells treated with CCCP alone. This study further supports the contention that myoglobin only plays an important role in oxygen metabolism at low extracellular PO2's.

Protein synthesis under conditions of anoxia and changing workload in ventricle strips from turtle heart

An earlier study determined that protein synthesis in isolated perfused turtle (Trachemys [= Pseudemys] scripta elegans) hearts was three-fold lower under conditions of anoxia than under conditions of normoxia. However, the earlier study did not attempt to define the role of work in the isolated perfused preparation. In this study, the effects of varying workload, as defined by changing frequency of contraction, and anoxia on protein synthesis were examined. The ventricle strip preparation allows for comparison of multiple strips from a single heart, which aids in eliminating the variability found between individuals chosen from wild populations. Ventricle strips forced to contract at 24 contractions.min-1 under anoxic conditions failed more rapidly than strips forced to contract at 24 contractions.min-1 under normoxic conditions. Protein synthesis decreased by 32% when compared to normoxic controls. When stimulation was terminated after 2 hr of contraction, the rate of protein synthesis in strips under anoxic conditions was similar to that in strips under normoxic conditions. Also, returning strips to normoxic conditions after 2 hr of anoxia restored protein synthesis to the level of the normoxic controls. A significant correlation between pacing rate and protein synthesis was found under normoxic conditions but not under anoxic conditions when strips were paced at 12, 18 and 24 contractions.min-1. Protein synthesis increased by 30% at the 18 contractions.min-1 frequency and 45% at the 24 contractions.min-1 frequency over the rate at 12 contractions.min-1 frequency. Force-frequency studies revealed that under normoxic conditions force generation did not change until above 24 contractions.min-1, but under anoxic conditions there was a significant negative inotropic effect (20% decrease in force) at 24 contractions.min-1 and fell to 50% of initial at 36 contractions.min-1. These studies indicate that, in the turtle heart, anoxia per se is not the only determinant of protein synthesis but rather that work plays an important role in protein synthesis, as in the mammalian heart.

Cold acclimation increases activities of mitochondrial long-chain acyl-CoA synthetase and carnitine acyl-CoA transferase I in heart of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) were acclimated to 5 or 15 °C. Hearts were excised and assayed for the activity of enzymes essential for fatty acid metabolism. The activity of long-chain acyl-CoA synthetase, the first enzyme required in either fatty acid oxidation or complex fatty acid synthesis, was increased following acclimation to low temperature. Total crude homogenates exhibited an increase in activity with either palmitate (0.037–0.047 μmol/(min∙g)), stearate (0.037–0.055 μmol/(min∙g)), or oleate (0.041–0.064 μmol/(min∙g)) as substrate. Mitochondrial preparations showed the greatest increase in activity with palmitate (0.486–0.962 nmol/(min∙g)) as substrate, whereas microsomal preparations exhibited the greatest increase in activity with oleate (0.976–1.933 nmol/(min∙g)) as substrate. The activity of carnitine acyl-CoA transferase I, which is located on the outer mitochondrial membrane and is required for fatty acid oxidation, increased following acclimation to low temperature with palmitoyl CoA (0.137–0.352 μmol/(min∙g)), stearoyl CoA (0.066–0.152 μmol/(min∙g)), or oleoyl CoA (0.137–0.224 μmol/(min∙g)) as substrate. The parallel increase in mitochondrial long-chain acyl-CoA synthetase and carnitine acyl-CoA transferase I is consistent with previous observations of an elevated capacity of heart to oxidize fatty acids as exogenous fuels following acclimation to low temperature. The increase in microsome-based long-chain acyl-CoA synthetase may contribute to heart growth at low temperature.

Decreased total ventricular and mitochondrial protein synthesis during extended anoxia in turtle heart

The turtle heart provides a model system to study the effects of anoxia on protein synthesis without the potentially confounding factor of contractile failure and decreased ATP levels. Protein synthesis, as measured by 3H-labeled phenylalanine incorporation, was studied under conditions of normoxia and anoxia in isolated perfused turtle [Trachemys (= Pseudemys) scripta elegans] hearts at 15 degrees C. Heart rate, cardiac output, and ventricular pressure development were unaffected by 2 or 3 h of anoxia. Despite the anoxia, energy levels in the heart were presumably still high, since contractility was maintained. RNA content of ventricle decreased after anoxic perfusion. Rates of total protein synthesis rates in ventricle were threefold lower under anoxia than under normoxia. These findings suggest that the total level of RNA is one determinant of protein synthesis. Incorporation of label into protein extracted from mitochondria was also assessed. The ratio of mitochondrial to whole ventricular protein synthesis was significantly lower after anoxia, revealing preferential control mechanisms under anoxia between the synthesis of total cellular protein and protein destined for mitochondria. Isolated mitochondria were still coupled after 2 or 3 h of anoxia. In effect, the mitochondria enter into a state of hypometabolism in terms of rates of ATP synthesis and protein synthesis, but functional integrity is maintained. The decrease in protein synthesis in general and mitochondrial protein synthesis in particular may represent an adaptation to allow the partitioning of the available energy resources toward mechanical function during anoxia.

Acclimation to low temperature is associated with an increase in long-chain acyl-CoA synthetase in rainbow trout (Oncorhynchus mykiss) heart

Rainbow trout (Oncorhynchus mykiss) were acclimated to 5 and 15 °C. Uptake of radiolabelled palmitate by isolated cardiomyocytes was linear for at least 65 min. Myocytes from fish acclimated to 5 °C and tested at 5 °C showed higher rates of uptake than those from fish acclimated to 15 °C and tested at 5 °C. There was no significant difference in fatty acid uptake between myocytes from fish acclimated to 5 and 15 °C and tested at their respective acclimation temperature. Acclimation temperature had no effect on levels of intracellular fatty acid binding protein or carnitine palmitoyltransferase. However, acclimation to low temperature resulted in a twofold enhancement in fatty acyl-CoA synthetase activity, which increased in a linear fashion over a 28-day period. An increase in fatty acyl-CoA synthetase, which occurs on the outer mitochondrial membrane and the endoplasmic reticulum, is possibly related to low temperature-induced changes in fatty acid oxidation and synthesis of complex lipids.

Low temperature acclimation decreases rates of protein synthesis in rainbow trout (Oncorhynchus mykiss) heart

Protein synthesis was assessed in rainbow trout (Oncorhynchus mykiss) hearts perfused with medium containing (3)H phenylalanine. Isolated hearts from fish acclimated to 5° and 15°C were used as the model system, and were perfused at variable test temperatures and pH. Protein synthesis expressed as nmol PHE mg protein(-1) h(-1) was two fold higher in hearts from fish acclimated to 15°C and tested at 15°C and extracellular pH 7.6 than in hearts from fish acclimated to 5°C and tested at 5°C and extracellular pH 8.0. The prime determinant of the decreased rate of protein synthesis was thermal history. Fish acclimated to 5°C had lower levels of RNA mg protein(-1) than fish held at 15°C. There was a direct linear relationship between the rate of protein synthesis in nmol PHE mg protein(-1) h(-1) and RNA content. RNA activity (nmol PHE μg RNA(-1) h(-1) remained constant regardless of thermal history or perfusion condition. Elevated pH resulted in only a marginal decrease in protein synthesis. Test temperature had no effect on in vitro rates of protein synthesis.

Adrenergic stimulation of glycolysis without change in glycolytic enzyme binding in rainbow trout (Oncorhynchus mykiss) erythrocytes

Whole blood from rainbow trout (Oncorhynchus mykiss) was incubated in vitro with pharmacological levels of isoproterenol. The adrenergic stimulation of glycolysis in erythrocytes (RBCs) was assessed by monitoring the rate of 14C incorporation from [6-14C]-glucose into the acid-soluble RBC fraction. During a 3-h in vitro incubation, incorporation of label into the acid-soluble RBC fraction of isoproterenol-treated whole blood (0.25 ± 0.04 μmol glucose∙mL−1 RBC∙h−1) was higher than into untreated blood (0.08 ± 0.01 μmol glucose∙mL−1 RBC∙h−1). The percentage of cell membrane binding for phosphofructokinase, aldolase, and glyceraldehyde-3-phosphate dehydrogenase ranged from 17 to 35% and was not altered by adrenergic stimulation. Adrenergic stimulation activates glycolysis in rainbow trout RBCs but not through the modulation of enzyme binding.

Influence of low temperature acclimation on fate of metabolic fuels in rainbow trout (Oncorhynchus mykiss) hearts

Rainbow trout (Oncorhynchus mykiss) were acclimated to 5 and 20 °C. Oxygen consumption of isolated perfused hearts was measured at 5 or 15 °C with either glucose or palmitate as the exogenous fuel source. With glucose as the fuel there was no significant difference in oxygen consumption of hearts from either acclimation group at either temperature. With palmitate as the fuel source, hearts from fish acclimated to and tested at 5 °C had significantly higher oxygen consumption than hearts from fish acclimated to 20 °C and tested at either 5 or 15 °C. Hearts from fish both acclimated to and tested at 5 °C had a higher oxygen consumption with palmitate than when glucose was supplied. This reflects the preference for fatty acid fuels found in cold acclimated muscle tissue, and consequently the amount of oxygen required to utilize fats. Under all experimental conditions, 14CO2 production from either (6-14C)glucose or (1-14C)palmitate could account for less than 0.5% of oxygen consumption. Tissue chemical analysis showed that most of the label from (6-14C)glucose appeared in acid-soluble (glycolytic intermediates, citric acid cycle intermediates, amino acids, etc.) and lipid fractions while most of the label from (1-14C)palmitate appeared in lipid- or acid-soluble or acid precipitate (protein material) fractions. This indicates considerable dilution of exogenous fuels in endogenous pools, which could account for the discrepancy in measured O2 consumption and 14CO2 production. Glucose catabolism was little affected by either acute or chronic changes in temperature other than an increase in glucose incorporation into the glycogen pool. Hearts from fish both acclimated to and tested at 5 °C showed an increased handling of exogenous fatty acids as reflected by elevated rates of catabolism and incorporation into intracellular lipids.