Anoxia and Re-oxygenation at Colder Temperatures in Goldfish (Carassius auratus): Effects on Oxidative Stress and Metabolites

Under the ice of ponds and lakes, goldfish (Carassius auratus) can survive weeks of anoxia at low temperatures by conserving ATP and by reducing their metabolic rate and relying on liver glycogen stores to meet their ATP demands using glycolysis. The following period of re-oxygenation is also known to be damaging in most vertebrates because it can lead to the generation of reactive oxygen species (ROS) that can cause oxidative damage to lipids, proteins and nucleic acids. Anoxia tolerant vertebrates such as the goldfish and crucian carp (Carassius carassius) accumulate reduced molecules with anoxia, overwhelming the electron transport system, producing ROS, and thus oxidative stress during re-oxygenation. Primary antioxidant defenses and pathways regulating ROS production are thought to minimize oxidative stress. In natural, temperate environments, fishes normally experience anoxia and re-oxygenation at colder temperatures. Yet relatively few studies have directly addressed how important cold-water temperatures are in limiting oxidative damage to goldfish during anoxia and re-oxygenation. In this study, the effects of cold-water acclimation (4°C) on the physiological responses of goldfish to anoxia and re-oxygenation were studied. Goldfish were acclimated to 4 °C or 14°C for 3 months, followed by 96 h or 24 h of anoxia exposure and a 12 h period of re-oxygenation. Tissues (liver, muscle, blood) were collected for analysis of antioxidant enzyme activities, oxidative damage, glycogen stores, and metabolites. Temperature acclimation resulted in 1.5-fold greater superoxide dismutase (SOD) activity and 2.2-fold lower glutathione peroxidase (GPx) activity in the normoxic liver at 4°C. Anoxia exposure resulted in no oxidative damage or changes in antioxidant enzyme activity in the liver or muscle at either temperature. Notably, liver glycogen did not significantly decrease during anoxia at 4°C, but a 70% decrease in liver glycogen was observed following 24 h of anoxia at 14°C and sustained through 12 h of re-oxygenation. Both blood glucose and lactate were elevated following anoxia at both temperatures, consistent with a greater reliance on anaerobic glycolysis. Significant anoxic ethanol accumulation occurred in the blood, which took longer to eliminate at 4°C during subsequent re-oxygenation. These findings support the hypothesis that reduced metabolic rates at cold temperatures reduce energy demands and the accumulation of anaerobic metabolites. In conclusion, oxidative stress in goldfish during re-oxygenation is prevented by mechanisms that do not involve changes in antioxidant enzyme activity, but recovery from anoxia is faster at colder temperatures. The robustness of goldfish to periods of anoxia and re-oxygenation, especially at colder, more environmentally relevant temperatures, represent adaptations that have allowed them to occupy niches that are unsuitable for other freshwater fishes.