| A total of 540 young grass carp (Ctenopharyngodon idella) (295.85 ± 2.07 g) were fed six isonitrogenous diets with graded levels of leucine (7.1,8.9,11.0,13.3,15.2 and 17.1 g/kgdiet) for 8 weeks to investigate the effects of leucine on growth performance, flesh quality and intestinal immunity.The results indicated that leucine significantly improved percent weight gain (PWG), feed intake (FI) and feed efficiency (FE) and reduced plasma ammonia content (P<0.05). Flesh quality parameters such as protein content, lipid content, shear force, and hydroxyproline concentration significantly increased with dietary leucine levels up to a certain point (P<0.05). On the other hand, moisture, cooking loss, cathepsin L activitiy in the muscle showed an opposite pattern. The positive correlation existed between shear force and hydroxyproline concentration (R=0.868, P<0.05). The data also showed that leucine significantly affected relative gene expression of tumour necrosis factor a (TNF-α), interleukin 8 (IL-8), interleukin 10 (IL-10), nuclear factor kappa B (NF-κB), inhibitor of kappa B (IκB) and TOR in the mid and distal intestine, as well as transforming growth factor β (TGF-β) in all intestinal segments (P< 0.05). Furthemore, leucine has significant effect on the relative gene expression of tight junction proteins, such as claudin b,-3,-12, occludin and ZO-1, in the intestine (P< 0.05). Moreover, leucine significantly increased glutathione content, activities of copper/zinc superoxide dismutase (CuZnSOD) and glutathione peroxidase (GPx) in the muscle and intestine, as well as catalase (CAT) in the muscle (P<0.05). Meanwhile, leucine significantly up-regulated the mRNA levels of CuZnSOD and GPx in the intestine and muscle, as well as CAT in the muscle. Positive correlation existed between CuZnSOD, CAT and GPx enzyme activities and their corresponding mRNA levels in the muscle (rCuZnSOD=+0.837, P< 0.05; +0.868, P< 0.05; rGPx=+0.954, P< 0.05) and intestine (rCuZnSOD=+0.656, P= 0.157; tGPx=+0.916, P< 0.05),indicating that activities of these antioxidant enzymes were regulated at a transcriptional level. Additionally, leucine significantly regulated the mRNA expressions of nuclear factor erythroid-2 related factor 2 (Nrf2), ECH-associated protein 1 (Keapl) and target of rapamycin (TOR) in the muscle and intestine, as well as ribosomal protein S6 kinase beta-1 (S6K1) in the muscle (P <0.05). Positive correlation existed between Nrf2 and CuZnSOD, CAT, GPx in the muscle (rCuZnSOD=+0.863, P< 0.05; rCAT=+0.630, P=0.180; rGPx=+0.977, P< 0.05) and intestine (rCuZnSOD=+0.793, P=0.06; rGPx=+0.907, P< 0.05), suggesting that leucine up-regulated the relative gene expression of antioxidant enzymes, presumably through enhancing the transcription of Nrf2 gene.In conclusion, the present study demonstrated that leucine could improve the growth of young grass carp. The dietary leucine requirement for young grass carp (292-696 g) was determined by using a quadratic regression analysis based on PWG and plasma ammonia content to be 13.0 and 12.9 g kg-1 diet, corresponding to 42.4 and 42.0 g kg-1 of dietary protein, respectively. Dietary leucine supplementation improved the flesh quality and reduced the oxidative damage by improving non-enzymatic and enzymatic antioxidant capacity in fish muscle. The improvement of antioxidant enzyme activities by leucine could be result from the up-regulation of their gene transcriptions, which were correlated with signaling molecules Nrf2, Keap1, TOR and S6K1. Meanwhile, dietary leucine improved intestinal immunity of mid and distal intestine by down-regulating pro-inflammatory cytokines and up-regulating anti-inflammatory cytokines. Dietary leucine maintained the integrity of intestinal barrier through regulating the mRNA expressions of tight junction proteins and antioxidant capacity in this organ. |
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