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Metabolic adjustments to acute hypoxia in the African lungfish and rainbow trout Dunn, Jeffrey Frank


The inter-tissue metabolic responses to hypoxia were determined in lungfish (Protopterus aethiopicus), and trout (Salmo gairdneri). Lungfish respond to hypoxia with a reduction in metabolic rate. It was intended to determine which tissue, or tissues exhibit decreased metabolic rates during hypoxia, and then compare the results with the metabolic reactions observed in trout, which are not reported to reduce metabolic rate during hypoxia. The metabolic potentials of the heart, brain, white muscle and liver in the African lungfish were estimated using enzymatic data. Metabolic effects of a 12 hr submergence were monitored using metabolite measurements. Heart was the most oxidative tissue, but also showed the greatest anaerobic potential. The brain displayed relatively low oxidative capabilities. White muscle remained almost inert. Although high energy phosphate concentrations in brain and heart did not fall during submergence, glycolysis was activated as indicated by cross-over plots, depletion of endogenous glycogen stores, and lactate accumulation. Blood-tissue lactate and glucose gradients indicated (1) that the heart and brain released lactate throughout submergence, (2) that after 12 hr of submersion the brain and heart were probably obtaining all their required glucose from the blood (3) that the liver released glucose throughout submergence, and (4) the white muscle was metabolically isolated from the rest of the body during submergence. The lack of measurable changes in white muscle metabolite concentrations coupled with the low enzyme activities leads to the suggestion that the most significant adaptation to hypoxia in these fishes may not be the capacity for increased anaerobic energy production. Instead, it is likely that the ability of the muscle to prevent the activation of glycolysis during hypoxic dysoxia is the key to the animal's survival. Histochemical and ultrastructural studies were done on the axial musculature of the lungfish. The small wedge of red coloured muscle evident upon gross examination was shown by histochemical demonstrations of lactate and succinate dehydrogenases, of adenosine triphosphatases, and of lipid to be composed of a mosaic of red and intermediate fibres. Respectively, these fibres measured 23.6 and 34.3 microns in average diameter. The bulk of the myotome is composed of white fibres having an average diameter of 67.3 microns. Mitochondrial density, capillarity and lipid content were very low for all fibres. These data suggest that the axial musculature is geared primarily for anaerobic function. The relatively large percentage of white muscle indicates that the overall metabolic rate of the axial muscle is low. The capacity of the muscle to exist with a reduced rate of ATP turnover (as was suggested above) may be related to the large proportion of white fibres present in the myotome. Tissue metabolites were measured in a hypoxia sensitive organism, the Rainbow trout (Salmo qairdneri), before and after exposure for 3 hr to inspired oxygen tensions of 20 torr (at 4°C). There were small changes in the brain but the energy status was maintained. The red muscle was the least affected. White muscle creatine phosphate was depleted. Various data indicate that the white muscle is the major user of glycolytic substrates and the major producer of lactate. The heart is stressed as indicated by a decline in glycogen, ATP, CrP, and the total adenylate pool. The liver exhibited declines in every indicator of metabolic homeostasis. The liver concentrations of glycogen did not decline. The fact that anaerobic glycolysis has been activated in the white muscle, while the muscle remains in metabolic communication with the other tissues via the blood, supports the suggestion that the metabolism of the white muscle will have a pronounced effect on the metabolic status of the whole animal. The trout is maintaining its rate of oxygen uptake while activating anaerobic glycolysis in the attempt to maintain 'normal' rates of energy utilization. The turnover rates of glucose and lactate were measured in trout subjected to the same hypoxic stress as above. Glucose turnover did not change while lactate turnover increased from 2.8 ± 0.4 µmoles/min./kg to 20.6 ± 6.8 µmoles/min./kg. The lack of increase in glucose turnover was attributed to the observation that liver glycogen concentrations do not change and so there is no increase in glucose flux. The increase in lactate turnover emphasizes the fact that anaerobic glycolysis is activated and that some tissues are oxidizing lactate. The problem of when a cell becomes hypoxic and the reactions of the cell to that stress is addressed. The cell (tissue, organ, animal) has two options if oxygen supply drops to a level which prevents oxidative metabolism from supplying all of the requirements for ATP synthesis. The cell may exhibit a decline in requirements, in which case the rate of ATP production need not be as high as in the oxidative state or, conversely, anaerobic energy production may increase in the attempt to maintain ATP production rates. The lungfish muscle appears to be capable of the former, thus preserving substrates for other tissues and reducing the rate of end-product formation. The trout white muscle, on the other hand, exerts a major influence upon the other tissues when the animal is stressed with hypoxia. The term 'energy conformer' is applied to animals which do not maintain oxygen uptake in the face of a declining supply, and which allow ATP production to decline concomittantly by not activating glycolysis to a marked degree. An energy regulator would activate glycolysis in the attempt to maintain oxidative rates of ATP production. The trout is more of an energy regulator than is the lungfish with the main difference in this capacity being in the white muscle.

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