Biochemical Correlations of Power Development and Metabolic Fuel Preferenda in Fish Hearts

Hearts from 16 species of fishes, including representatives of cyclostomes, elasmobranchs, and teleosts, were examined. As a biochemical index of maximal ATP demand, ATPase activity of crude homogenates of ventricular tissue was measured under conditions which optimize activity of myofibrils and inhibit nonphysiological mitochondrial F₁-ATPase by inclusion of oligomycin in the assay medium. The activities of nine additional enzymes from pathways of energy metabolism and concentration of myoglobin-an intracellular protein involved in storage and diffusion of oxygen-were measured. Of all possible pairwise regressions between these biochemical markers, only the relationships among ATPase, carnitine palmitoyltransferase (CPT), and hexokinase (HK) and between HK and phosphofructokinase (PFK) were significant. The inability of elasmobranch myocardium to catabolize fatty acid is confirmed. ATP yields calculated from HK and CPT and corrected to reflect physiological conditions of the tissue suggest that catabolism of either carbohydrate or fatty acid fuels can meet maximal energetic demands of the tissue. The range of power developments encompassed by hearts of ectothermic fishes, although substantial, has not resulted in selective expansion of capacity to oxidize one particular metabolic fuel.