The role of dietary chromium supplementation in relieving heat stress of juvenile blunt snout bream Megalobrama amblycephala

The present study assessed the role of dietary chromium (Cr) supplementation in relieving heat stress (HS) of juvenile blunt snout bream Megalobrama amblycephala. The supplemented Cr contents by chromium picolinate (Cr-Pic) was 0 mg/kg (control group), 0.4 mg/kg, 1.6 mg/kg and 12.0 mg/kg, respectively. The fish continued to be fed four diets at suitable temperatures (26 °C) for 2 weeks, and then the temperature was then heated up to 33 °C through thermo-regulated system. The results showed that Cr supplementation had no significant effect on the immune indices and antioxidant indices before HS (P > 0.05). However, Cr supplementation played an important role in relieving HS. After HS, compared with the control group, 1.6 mg/kg and 12.0 mg/kg Cr supplementation groups significantly lowered the plasma glucose level and aspartate transaminase (AST) activity (P < 0.05), and 0.4 mg/kg and 1.6 mg/kg Cr supplementation groups significantly lowered alanine aminotransferase (ALT) activity (P < 0.05). 0.4 mg/kg and 1.6 mg/kg supplementation groups significantly improved hepatic total superoxide dismutase (T-SOD) activity (P < 0.05). Furthermore, 0.4mg/kg-12.0 mg/kg Cr supplementation groups significantly improved the activities of hepatic glutathione peroxidase (GPx) and catalase (CAT) and lowered hepatic malondialdehyde (MDA) level in liver (P < 0.05). The mRNA levels of hepatic copper zinc superoxide dismutase (Cu/Zn-SOD), CAT and GPx were significantly improved in 0.4mg/kg-12.0 mg/kg supplementation Cr groups (P < 0.05), however, there was no significant variation of hepatic manganese superoxide dismutase (Mn-SOD) mRNA levels under different levels of supplementation (P > 0.05). Significantly lower mRNA levels of hepatic pro-inflammatory cytokines observed in 0.4mg/kg-12.0 mg/kg Cr supplementation groups including tumour necrosis factor-α (TNF-α), interleukin 1β (IL-1β) and interleukin 8 (IL-8) (P < 0.05), and 0.4mg/kg-12.0 mg/kg Cr supplementation significantly improved the relative expressions of hepatic heat shock protein 70 (HSP70) and heat shock protein 90 (HSP90) (P < 0.05). The present study indicated that dietary Cr supplementation might have no significant effect on immune capacity and antioxidant capacity under normal physiological conditions, whereas it played an important role in relieving HS.