Carbohydrate metabolism of goldfish (Carassius auratus L.)

Summary The effect of hypoxia was studied in cold (15°C) and warm (30°C) acclimated goldfish. The hypoxic thresholds, defined as the lowest sustainablePO₂ were found to be 1.6 and 4.0 kPa O₂ at, respectively, 15°C and 30°C. At these levels the fish did not loose either weight or appetite over a 2-months period. While during starvation under normonic conditions a significant weight loss and breakdown of lactate dehydrogenase (90%) was observed, no such changes were found in fed hypoxic animals. In red lateral muscle, white epaxial muscle and liver of goldfish from 4 differently acclimated groups the maximal activities were measured of: glycogen phosphorylase, hexokinase, malate dehydrogenase, glycerol-3-P dehydrogenase, glucose-6-P dehydrogenase, malic enzyme, succinate oxidase, pyruvate carboxylase, phosphoenol-pyruvate carboxykinase, fructose-bisphosphatase and glucose-6-phosphatase. Thermal compensation, according to Precht's typology, was predominantly observed in red muscle and to a lesser extent in white muscle. The liver glucose-6-P dehydrogenase showed a strong inverse response, which points to enhanced synthetic activity at the higher temperature. Hypoxia acclimation exerted weaker responses at 15°C than at 30°C. Changes in liver enzyme activities suggest depressed protein synthesis and enhanced gluconeogenesis in hypoxic animals. In muscle of 30°C-acclimated goldfish hypoxia induces a significant increase of succinate oxidase activity, indicating adaptation of the aerobic energy metabolism. The occurrence of pyruvate carboxylase, never before observed in vertebrate muscle, probably plays an important role in pyruvate catabolism. Because its action produces oxalo-acetate, the enzyme may stimulate pyruvate oxidation and thus prevent early lactate accumulation. Since all gluconeogenic enzymes were shown to be active in goldfish muscle, the possible occurrence of gluconeogenesis in muscle (albeit at low rate) must be accepted. Enzyme activities in goldfish muscle were compared with literature data for a number of other fish species. This comparison indicates that maximal glycolytic flux in goldfish muscle tissue is rather low, although muscular glycogen levels are very high. It is suggested that this is part of the gold-fish's strategy to cope with hypoxia.