Enzymes of energy metabolism in salmonid hearts: spongy versus cortical myocardia

Abstract

The maximal in vitro activity of key enzymes of energy metabolism from the spongy and cortical layers of adult Atlantic salmon (Salmo salar) hearts was determined. Enzymes were also measured in hearts from salmon parr and smolt, and from juvenile and adult trout (Salvelinus fontinalis). Indices of carbohydrate metabolism, hexokinase, and lactate dehydrogenase were significantly higher in the spongy than in the cortical layer of the salmon heart. Markers of aerobically based fatty acid metabolism (carnitine palmitoyl transferase, β-hydroxyacyl CoA dehydrogenase, and cytochrome oxidase) were higher in the cortical region. Although there were clear differences in metabolic organization between the two tissue layers, the absolute magnitude of the enzyme activities suggest that the large variance in oxygen delivery to the two regions does not present a major constraint on aerobic energy metabolism. In both salmon and trout, the hearts from large animals exhibited much higher enzyme activities at many loci than the hearts from small animals. The data imply that there is an increase in metabolic fuel supply on a per gram weight basis in large hearts. This is contrary to the accepted paradigm of scaling of aerobic metabolism. Myoglobin content was highly variable amongst experimental groups. High levels of myoglobin were associated with potential conditions of low extracellular oxygen availability.