Qualitative modification of muscle metabolic organization with thermal acclimation of rainbow trout, Oncorhynchus mykiss.

Rainbow trout (Oncorhynchus mykiss) were acclimated to 4 and 18°C and fed at rations levels which led to an equal change in mass at these temperatures during the six week acclimation. Thermal acclimation markedly modified the metabolic organization of red and white muscle. Cold-acclimated fish had activities of β-hydroxyacyl CoA dehydrogenase in both red and white muscle that were twice those of warm-acclimated fish. By contrast, the activities of cytochrome oxidase were unchanged by thermal acclimation. Thus, the capacity for β-oxidation of lipids is specifically enhanced in the muscle of cold-acclimated trout. In white muscle, citrate synthase and phosphofructokinase activities were also enhanced by cold acclimation (increases of 125% and 35% respectively), while cytochrome oxidase levels were unchanged. The non-parallel changes in the activities of mitochondrial enzymes strongly suggest that trout muscle mitochondria undergo qualitative reorganization during cold acclimation. The relative activities of mitochondrial enzymes suggest that mitochondria from red muscle have a threefold greater capacity to oxidize lipids than those from white muscle.