Aerobic and anaerobic contributions to energy metabolism in perfused isolated sea raven (Hemitripterus americanus) hearts

The relative contribution of aerobic and anaerobic metabolism to ATP production was assessed in sea raven (Hemitripterus americanus) hearts. The problem was approached by measuring the rates of oxygen consumption and lactate production by perfused isolated hearts performing mechanical work. In the experimental preparation aerobic metabolism could account for essentially all of the ATP synthesized; as such, the organization of metabolism in this fish heart appears similar to reptilian and mammalian hearts under conditions of adequate oxygen availability.

The fish heart as a model system for the study of myoglobin

A model is presented for myoglobin study based upon naturally occurring differences in myocardial myoglobin content in fish. The sea raven (Hemitripterus americanus) and the ocean pout (Macrozoarces americanus) have heart myoglobin contents of approx. 65 and 5 nmol/g wet wt, respectively. The maximal activities of enzymes associated with energy metabolism are similar in the two hearts. Isolated perfused hearts performed with similar efficiencies based upon similar rates of work, oxygen consumption and lactate production. Under normoxic perfusion conditions both hearts met 98% of the ATP demand by oxidative mechanisms. Myoglobin-rich sea raven hearts performed significantly better than myoglobin-poor ocean pout hearts under conditions of hypoxia and glycolytic blockage. The performance of sea raven hearts was impaired during hypoxia by decreasing the content of functional myoglobin with hydroxylamine. No effect upon performance was observed with the ocean pout heart. The data provide the first evidence that myoglobin plays a role in the maintenance of contractility in heart under hypoxic conditions.

Activities of enzymes in cardiac energy metabolism

The activities of enzymes of fat and carbohydrate energy metabolism were determined in hearts from a number of species. Activities of citrate synthase and 3-hydroxyacyl CoA dehydrogenase were lower in hearts from ectothermic animals than rats or pigeons. However, activities of hexokinase and phosphofructokinase in the ectothermic hearts were equal to or greater than the activities in the endothermic hearts. It thus appears that the hearts from the ectothermic species have a greater dependence upon a glucose-based metabolism than hearts from endothermic animals.

Rates of glucose and lactate oxidation by the perfused isolated trout (Salvelinus fontinalis) heart

Isolated brook trout (Salvelinus fontinalis) hearts were perfused with buffer containing either [U-14C]glucose or [U-14C]lactate and the rate of 14CO2 production monitored. Lactate oxidation rates were higher than glucose oxidation rates when either substrate was provided individually. When glucose and lactate were offered simultaneously glucose oxidation was markedly inhibited whereas there was only a minor inhibitory effect upon lactate oxidation. It appears that the fish heart preferentially utilizes lactate as a metabolic fuel under aerobic conditions.

Mechanical and metabolic response of the perfused isolated fish heart to anoxia and acidosis

Sea raven (Hemitriperus americanus) and ocean pout (Macrozoarces americanus) hearts were excised and perfused in an isolated system. The contractile response of hearts perfused with buffer equilibrated with 100% O2, 100% N2, or 99% O2: 1% CO2 was monitored and work was calculated from mean pressure development times cardiac output. Hearts subjected to either anoxia or hypercapnic acidosis could not sustain mechanical performance as well as hearts perfused with buffer equilibrated with 100% O2. The decrease in mechanical function was not due to an unavailability of ATP. Alternatively it is suggested that contractile failure is related to a decrease in intracellular pH.

Alterations in energy metabolism associated with the transition from water to air breathing in fish

Hoplias malaharicus and Hoplerythrinus unitaeniatus are two closely related species which live in poorly oxygenated water. The former is an obligate water breather, whereas the latter has the additional capacity to respire atmospheric oxygen through a highly vascularized swim bladder. Hoplerythrinus has greater lipid stores than Hoplias and probably places a greater dependence upon this metabolic fuel as an energy source than Hoplias. In contrast with this, Hoplias has greater heart and liver glycogen reserves than Hoplerythrinus. Hoplias hearts perfused in vitro with buffer containing cyanide continued to beat eight times longer than Hoplerythrinus hearts. The performance of both types of hearts was markedly increased when glucose was added to the perfusate; however, the Hoplias hearts still continued to function better than the Hoplerythrinus hearts. It is concluded that the Hoplias heart is better adapted to function anaerobically than the Hoplerythrinus heart.

Integration of lipid utilization with Krebs cycle activity in muscle

This essay illustrates the ways in which beta-oxidation and the citric acid cycle interact. These included: 1) competition for CoASH, 2) competition for NAD+, and 3) competition for FADH2 oxidation. By means of the above, the cell is able to maintain a precise coordination between the activation of fatty acids in the cytosol, beta-oxidation in the mitochondria, and the complete oxidation of acetyl-CoA to CO2 via the citric acid cycle throughout a wide range of energy demands and oxygen availability.

Control of energy metabolism in fish white muscle

Concentrations of key metabolites were determined in carp white muscle before exercise and after maximal activity. It was found that the concentration of ATP decreases by about 65%, ADP decreases slightly, and AMP remains unchanged. Consequently, the level of the free adenylate pool decreases. Simultaneously there is an increase in the concentration of IMP and NH4+. The increase in IMP level and the decrease in adenylate pool are essentially in 1:1 stoichiometry, a result showing that the adenylate pool is decreased by the reaction catalyzed by 5'-AMP deaminase (EC 3.5.4.6.). During exercise there is an increase in levels of glucose-6-phosphate, fructose 6-phosphate, and fructose 1,6-diphosphate that, along with the decrease in ATP levels, can account for the increase in glycolytic flux by activation of phosphofructokinase and pyruvate kinase.

Blood Lactate Levels in Free-Swimming Rainbow Trout (Salmo gairdneri) Before and After Strenuous Exercise Resulting in Fatigue

Rainbow trout (Salmo gairdneri) were exercised to fatigue in a series of 60-min stepwise increasing velocity increments. There was no increase in blood lactate concentration, serially sampled during swimming by means of indwelling dorsal and ventral aortic catheters, at velocities as high as 93% of critical velocity of individuals. The data show that under these conditions the rate of production of lactate by white muscle, at less than critical velocities, is minimal or that the rate of elimination of lactate from white muscle is equal to its rate of utilization elsewhere. Immediately following fatigue blood lactate level increases rapidly. During the recovery period there appears to be a net uptake of lactate by the gills.

The unanswered question of high anaerobic capabilities of carp white muscle

Carp white muscle is considered to have a very high anaerobic capability; however, the accumulation of lactate is often not proportional to the amount of energy which must be generated. On the basis of whole-animal studies it is known that some fish produce NH4+ and CO2 anaerobically. This is interesting in light of the concept that many invertebrate facultative anaerobes simultaneously utilize carbohydrates and amino acids with the production of a variety of end products. We tested the hypothesis that in carp white muscle (a) lactate was not the sole end product of anaerobic metabolism and (b) amino acids were utilized as an anaerobic energy source. After an anaerobic work period induced by hypoxia we could detect none of the end products known to build up in invertebrate tissues and no mobilization of the free amino acid pool. Thus, carp white muscle appears to depend solely on glycogenolysis. The low level of lactate in carp white muscle even after severe hypoxic stress may be due to the transfer of this compound out of the tissue despite poor vascularization.