Relation between growth rate and metabolic organization of white muscle, liver and digestive tract in cod, Gadus morhua

Abstract

To determine whether the aerobic capacity of tissues required for growth specifically reflects growth rates, we monitored the activities of key enzymes of oxidative, glycolytic and amino acid metabolism in muscle, liver and intestine of Atlantic cod (Gadus morhua) growing at different rates. Fish were maintained individually in small tanks at 10°C and fed on rations that allowed growth rates ranging from-0.6 to 1.6% per day. The correlation between growth rate and muscle enzyme activity was pronounced for the glycolytic enzymes (LDH, PFK and PK). The activities of glycolytic enzymes were more than four times higher for fish having higher growth rates compared to those that did not grow. Mitochondrial enzyme (cytochrome c oxidase, citrate synthase and β-hydroxyacyl-CoA dehydrogenase) activities remained unchanged in fish with positive growth. The liver seems to respond to requirements of growth by an increase in size. In the liver, the activities of the enzymes of amino acid metabolism expressed as units · μg DNA-1 specifically increases with growth rate. In contrast to the two other tissues, the specific activities of mitochondrial enzymes in the intestine increased with growth rate while the relative mass of the intestine remained constant. Intestinal cytochrome c oxidase activity increased from a minimum of about 2 to more than 8 units · g intestine-1. Cytochrome c oxidase activity increased in parallel with the food conversion efficiency. This suggests that the aerobic capacity of the intestine may initially limit the rates of digestion and growth in this species.