Comparative gluconeogenesis in hepatocytes from salmonid fishes

Abstract

Rates of gluconeogenic flux and substrate oxidation are assessed in isolated hepatocytes from three species of salmonid fishes: rainbow trout (Salmo gairdneri), coho salmon (Oncorhynchus kisutch), and sockeye salmon (Oncorhynchus nerka). Coho salmon displays the highest capacity for gluconeogenesis from lactate and alanine, but rates are well below those of eels. Enzyme compartmentation on isolated mitochondria shows that in trout and sockeye, phosphoenolpyruvate carboxykinase is almost entirely localized in the mitochondrion and the cytosol, respectively, while in the coho, 40% of phosphoenolpyruvate carboxykinase activity is associated with the cytosol. Freshly isolated salmonid hepatocytes are in negative glycogen balance. It is established here that at low (<250 μmol glucosyl units/g) glycogen concentrations a linear relationship exists between the rate of endogenous glucose production and the initial glycogen concentration. High rates of endogenous glycogen breakdown necessitate the use of radiotracers for determining gluconeogenic fluxes in fish hepatocytes. Rates of gluconeogenesis calculated from radiolabel experiments are compared with nonlabelled lactate and are determined not to be significantly different from each other. It is concluded that in fish hepatocytes, (i) radiotracer experiments give accurate estimates of gluconeogenesis, (ii) dilution of label at the oxalacetate level is insignificant, and, consequently, (iii) rates of 14CO2 production are a valid measure of true substrate oxidation.