Regulation of antioxidant defense in response to heat stress in Siberian sturgeon (Acipenser baerii)

Siberian sturgeon (Acipenser baerii) is one of the most important breeding species of sturgeon in the world. The objective of the present study was to assess the effect of short-term gradual heating up (1 degrees C/8 h) and recovery changes on antioxidant defense systems in the liver, kidneys, gills, and serum of the Siberian sturgeon, and to examine related gene expression levels and histological changes in the liver and gills. When the water temper-ature reached to 33 degrees C (about 88 h), LOE (loss of equilibrium) appeared in individuals. Heat stress affected the metabolic mode and oxidative stress parameters in a tissue-dependent manner. Expression of HSP family genes such as heat shock cognate 70 (hsc70), heat shock protein 70 (hsp70), and heat shock protein 90 (hsp90) were elevated in the liver and gills of two heating groups, and these returned to normal levels after 24 h of recovery. Hsf1 expression was not significantly different between the control and heating groups, but was lower in the liver and higher in the gills after 24 h recovery. Heat stress resulted in clear histological changes in the liver, including the hepatic lobules, showing vacuolization and loss of cell boundaries and asymmetric shapes; the gills showing increasing numbers of vacuoles, thinning of basal cells, epithelial cell denaturation, degeneration, and necrosis. After 24 h of recovery, the liver and gills in the active group recovered better than those in the LOE group. Of four antioxidant enzymes (superoxidase dismutase (SOD), catalase (CAT), lactate dehydrogenase (LDH), gluta-thione peroxidase (GPX)), only LDH activity varied significantly, decreasing in the liver, increasing in the kidneys of heating groups, and recovering to control levels after 24 h. Total antioxidant capacity (T-AOC) also did not change significantly in the three tissues. Both plasma cortisol and lactate were significantly influenced by thermal stress, and returned to control levels within 24 h. MDA (malondialdehyde) remained at a relatively stable level after heat stress and recovery. Rapid cooling after enduring heat stress allowed the Siberian sturgeon to recover quickly. After 24 h of recovery, nine LOE and eleven active recovery individuals were alive and able to feed, accounting for 32.7% and 36.5% of the experimental groups, respectively. These results indicate that Si-berian sturgeon not only has a strong antioxidant system and adaptive capacity to short-term gradual heating, but also has the capacity to quickly recover from heat stress, with physiological parameters returning to pre-stress levels after 24 h.