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 3 0 degrees C, whereas freshwater fishes of the north-temperate zone are eury thermal, with typical body temperatures of 10-20 degrees C in summer. Enzym e activity levels in heart and red muscle of Amazonian species, which displ ay various physiological mechanisms for dealing with hypoxic conditions, we re compared with those in north-temperate-zone species. Five Amazonian spec ies (acara-acu (Astronotus ocellatus), acari-bodo (Lipossarcus pardalis), t ambaqui (Colossoma macropomum), tamoata (Hoplosternum littorale), and pirar ucu (Arapaima gigas)) and four north-temperate-zone species (American eel, bullhead, yellow perch, and rainbow trout) were studied. The Amazonian spec ies included obligate and facultative air breathers. Activities of key indi cator enzymes associated with carbohydrate metabolism, fatty acid metabolis m, the citric acid cycle, and the electron-transport chain were determined. There was no obvious correlation between cardiac enzyme activity levels an d 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 beta-hydr oxyacyl CoA dehydrogenase were about twice as high in north-temperate-zone species than in Amazonian species. Activities of red-muscle enzymes, especi ally those associated with aerobic fatty acid metabolism, were significantl y higher in comparable north-temperate-zone species relative to Amazonian s pecies. Increased enzyme activity levels in north-temperate-zone species re lative to Amazonian species is considered to be an adaptation to generally lower body temperatures. This finding is consistent with earlier comparisio ns of Antarctic and north-temperate-zone species and with the results of st udies of cold acclimation within north-temperate-zone fishes.