The effect of body size on the white muscle acid–base and metabolite status was examined in rainbow trout (Oncorhynchus mykiss) ranging in length from 8 to 54cm. Following 5min of exhaustive exercise, white muscle lactate concentration was approximately doubled (approximately 32 �mol g �1) in larger fish than in smaller fish (approximately 16�mol g �1). Associated with this post-exercise increase in lactate was a nearly parallel increase in the number of metabolic protons produced by larger fish. Larger fish did not possess a greater non-bicarbonate buffering capacity or soluble protein concentration, so their mean muscle intracellular pH (pHi) decreased by approximately 0.70 units compared with a change in mean pHi of about 0.40 units in the smallest fish. The relationship between resting pHi and length was independent of size (mean pHi7.31). Concentrations of muscle energy metabolites were also determined in trout white muscle before and after exercise. Under resting conditions, larger fish possessed a twofold greater concentration of ATP (approximately 7 �mol g �1) than did smaller fish (approximately 3�mol g �1). Similarly, resting values of muscle glycogen range from about 6 �mol g �1 in the smallest fish to as high as 15 �mol g �1 in the largest fish. However, the smaller fish had higher levels (approximately 35 �mol g �1) of phosphocreatine (PCr) than the larger fish (approximately 25 �mol g �1). Following exercise, however, both ATP and glycogen concentrations remained size-dependent and increased with increases in fish length. Levels of PCr were size-independent following exercise. These results demonstrate that body size has an important influence on the acid–base and metabolic status of fish before and after exercise
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