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Energy metabolism in carp white muscle Driedzic, William Richard

Abstract

The myotomal muscle of fish is largely composed of two tissue types, usually referred to as the red and white fibers. On the basis of histochemical and biochemical properties it is generally accepted that white muscle has a metabolism which is predominantly anaerobically based, utilizing glycogen as its fuel source and that red muscle functions largely aerobically burning fats and/or carbohydrates. In carp (Cyprinus carpio L.) the red fibers are found as a thin superficial layer below the skin and the white fibers make up the mass of the underlying myotome. Thus, in this species it is possible to rapidly obtain an homogenous sample of white muscle allowing the analysis of labile metabolites. This study investigates the control of glycolysis in white muscle and the source and function of anaerobic NH₄⁺ production by white muscle. The concentrations of key metabolites were determined in muscle before exercise and after maximal activity. During exercise there was an increase in levels of glucose-6-phosphate, fructose-6-phosphate and fructose-1-6-di-phosphate which, along with a decrease in ATP levels, could account for the increase in glucolytic flux by activation of phosphofructokinase and pyruvate kinase. It was found that after maximal activity the concentration of ATP decreased by about 65%, ADP decreased slightly, while AMP remained low and unchanged. Consequently the level of the free adenylate pool decreased. Simultaneously there was an increase in the concentration of IMP (inosine monophosphate) and NH₄⁺. The increase in IMP level and the decrease in adenylate pool were essentially in 1:1 stoichiometry, a result showing that the adenylate pool was decreased by the reaction catalyzed by 5' AMP deaminase. However, the increase in free NH₄⁺ was less than the decrease in the adenylate pool. The concentration of free amino acids was also determined in white muscle, before and after severe environmental hypoxia. During the hypoxic period the total amount of nitrogen in the amino acid pool increased and there was a tendency for an increase in the total number of free amino acids. On the basis of this study it is possible to construct a fairly comprehensive metabolic scheme for nitrogen metabolism in carp white muscle during anaerobic work. The energy required for work is ultimately derived from the hydrolysis of ATP. When ATP levels cannot be maintained the content of ADP increases and as this occurs the level of AMP also increases due to the equilibrium relationship of the adenylates. As the work load on the tissue exceeds its aerobic capabilities GTP (guanosine triphosphate) levels drop, 5' AMP deaminase is activated and the adenylate pool is decreased. NH₄⁺ released from AMP is subsequently incorporated into the free amino acid pool.

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