Physiological response of juvenile turbot (Scophthalmus maximus. L) during hyperthermal stress

Temperature is a critical abiotic factor in aquaculture farming. Serum cortisol, glucose, respiratory frequency, hepatic glycogen, anti-oxidant enzyme activities (superoxide dismutase [SOD], catalase [CAT], and glutathione peroxidase [GPx]), malondialdehyde (MDA) content, and expression of heat shock proteins (hsp70, 90) and apoptosis-related genes (p53, caspase3, bax, bcl2) of turbot were determined at 27 degrees C under different thermal durations (0, 6, 12, 24, 48, 72, 96 h) to illustrate the underlying physiological response mechanisms. Results showed that cortisol and glucose significantly increased at 27 degrees C from 0 h to 12 h, and then dramatically decreased from 24 h to 96 h. Respiratory frequency manifested similar results to cortisol. Hepatic glycogen content, GPx and CAT activities gradually decreased in a time-dependent manner under thermal stress. However, SOD activity significantly increased at 6, 12, 24 h and then decreased until the end of thermal stress. Hepatic MDA content significantly increased from 12 h to 96 h. In addition, the apoptosis rate of hepatocyte gradually increased under thermal stress. p53, caspase3 and bcl2 mRNAs were significantly up-regulated under thermal stress within 6 h, and down-regulated from 12 h to 96 h. bax mRNA was significantly up-regulated from 0 h to 6 h, and no significant difference was observed from 6 h to 96 h. Hepatic hsp70 and hsp90 mRNAs were significantly upregulated from 6 h to 12 h, which was similar to those of serum glucose results. These results indicate that hyperthermal stress elevates respiratory frequency, serum cortisol and glucose contents, promotes hepatic glycogenolysis, impairs hepatic antioxidant capacity and induces hepatocyte apoptosis by p53-bax-bcl2 and the caspase-dependent pathways. These findings expand the current knowledge on thermal tolerance and aid the management of turbot in captivity.