Acute hypoxia/reoxygenation affects muscle mitochondrial respiration and redox state as well as swimming endurance in zebrafish.

Rapid fluctuations of the oxygen content of both natural and anthropogenic origin are relatively common in freshwater environments. Fish adaptation to these conditions implies tolerance of both low levels of oxygen availability and reoxygenation. Hypoxia tolerance in fish has been widely studied, but the involvement of mitochondria in the response of fish to rapid hypoxia/reoxygenation stress is less known. Zebrafish, a floodplain species, is likely facing significant changes in dissolved oxygen in its natural environment and displays a moderate ability to tolerate hypoxia. In the present study, we report the effects of an acute hypoxia/reoxygenation stress (H/R) protocol on mitochondrial functionality (respiration, complex activities, rate of H2O2 release) and redox state (level of HPs and protein oxidation) of muscle tissue. In parallel, the animal metabolic performance (routine metabolism, nitrogen excretion and swimming performance) was measured. Additionally, the recovery from H/R was tested 20 h after treatment. A significant stimulation by H/R of muscle mitochondrial respiration and H2O2 release was observed, which was only in part counteracted by stimulation of the antioxidant system, resulting in an increased level of lipid peroxides and protein carbonyls. In parallel, H/R increased the animal oxygen consumption and urea excretion rate and reduced routine activity. A significant strong reduction of endurance at 80% Ucrit was also observed. Most of the altered parameter did not recover 20 h after reoxygenation. These data indicate a significant alteration of zebrafish muscle mitochondrial state after acute H/R, associated with changes in tissue redox state and locomotor performance.

Effects of acute changes in salinity and temperature on routine metabolism and nitrogen excretion in gambusia (Gambusia affinis) and zebrafish (Danio rerio).

Acute stress may affect metabolism and nitrogen excretion as part of the adaptive response that allows animals to face adverse environmental changes. In the present paper the acute effects of different salinities and temperatures on routine metabolism, spontaneous activity and excretion of ammonia and urea were studied in two freshwater fish: gambusia, Gambusia affinis and zebrafish, Danio rerio, acclimated to 27 degrees C. The effects on gill morphology were also evaluated. Five salinities (0 per thousand, 10 per thousand, 20 per thousand, 30 per thousand and 35 per thousand) were tested in gambusia, while four salinities were used in zebrafish (0 per thousand, 10 per thousand, 20 per thousand and 25 per thousand). Each salinity acute stress was tested alone or in combination with an acute temperature reduction to 20 degrees C. In gambusia, both salinity and temperature acute stress strongly stimulated urea excretion. Routine oxygen consumption was barely affected by acute salinity or temperature stress, and was reduced by the combined effects of temperature and high salinity. Gills maintained their structural integrity in all stressing conditions; hyperplasia and hypertrophy of mitochondria-rich cells were observed. In zebrafish, temperature and salinity acute changes, both alone and in combination, scarcely affected any parameter tested. The major effect observed was a reduction of nitrogen excretion at 20 degrees C-25 per thousand; under these extreme conditions a significant structural disruption of gills was observed. These results confirm the high tolerance to acute salinity and temperature stress in gambusia, and demonstrate the involvement of urea excretion modulation in the stress response in this species.