Dietary optimal reduced glutathione improves innate immunity, oxidative stress resistance and detoxification function of grass carp (Ctenopharyngodon idella) against microcystin-LR

The present study reports on the effects of dietary glutathione (GSH) on non-specific immunity, oxidative stress resistance and detoxification in grass carp (Ctenopharyngodon idella) infected with microcystins. Six hundred and thirty juvenile grass carp with initial body weight of (5.10 +/- 0.09) g were randomly distributed into six groups with three replicates each and stocked in 18 tanks (35 fish per tank), fed six diets containing graded levels of GSH for 60 days. At the end of the feeding trial, ten fish per tank were selected and injected intraperitoneally with 200 mu g/kg body weight (BW) dose of pure microcystin-LR (MC-LR). The results showed that, at 0 h, 24 h of MC-LR injection, with the increase of dietary GSH content, serum lysozyme, alkaline phosphatase (ALP) and complement 3 (C3) levels, the activities and mRNA expression levels of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) in liver, and liver GSH content increased first and then decreased (P < 0.05). When dietary GSH content was 407.45 mg/kg diet, these parameters reached the highest levels (P < 0.05), and in the same dose of GSH group, in addition to lysozyme activity at 24 h was significantly higher than that at 0 h, the other indicators at 24 h were significantly lower than those before exposure (at 0 h) (P < 0.05). In contrast, serum cortisol and glucose, and liver reactive oxygen species (ROS) and malondialdehyde (MDA) contents decreased first and then increased (P < 0.05), when dietary GSH content was 407.45 mg/kg diet, the above parameters reached the lowest levels (P < 0.05), and in the same dose of GSH group their contents at 24 h were significantly higher than those before exposure (P < 0.05). In addition, before MC-LR exposure, the activities of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) had no significant difference of all groups (P > 0.05), liver glutathione S-transferase (GST) and its mRNA expression levels had a rising trend while liver glutathione reductase (GR) activities significantly decreased and then maintained a lower level with dietary GSH content increasing (P < 0.05). At 24 h of MC-LR injection, serum ALT and AST activities decreased at first and then increased while liver GST and GR activities and their mRNA expression levels increased first and then decreased (P < 0.05). Therefore, the basal diet supplemented with GSH 407.45 mg/kg could improve innate immunity, antioxidant stress ability and detoxification function of grass carp against MC-LR exposure. However, dietary excess GSH might promote oxidation, resulting in negative effect.