The Effect Of Dietary Tryptophan Supplement On Growth Performance, Flesh Quality And Intestinal Health Status Of Young Grass Carp

The present research studied the effects of dietary tryptophan (Trp) on growth performance, flesh quality, intestinal mucosal immune, barrier function and antioxidant capacity of young grass carp (Ctenopharyngodon idella), and estimated the Trp requirement of young grass carp. A total of 540 fish (with an average initial weight of 287.0±1.1 g) were randomly distributed into 6 treatment groups. Fish were fed with six different experimental diets containing graded levels of Trp at 0.7 (control),1.7,3.1,4.0, 5.2 and 6.1 g kg-1 diet for 8 weeks. At the end of the feeding trail, fish were exposed to ambient 0.7 mg L-1 copper sulfate for for 96 hours.Dietary Trp significantly enhanced the percent weight gain (PWG), feed intake and feed efficiency (P< 0.05); whereas, the plasma ammonia content (PAC) was decreased by Trp (P<0.05). Correlation analysis showed that the PWG was positively related to FI (r =+0.978, P< 0.05) and FE (r=+0.979, P< 0.05). Muscle protein and lipid content were increased by dietary Trp{P< 0.05), whereas muscle moisture content, ash content, phosphorus content and calcium content followed an opposite trend (P< 0.05). Trp increased the muscle protein content of fish may be partly through up-regulating the gene expression levels of target of rapamycin (TOR) and S6K1. Dietary Trp significantly decreased liquid loss and shear force in the muscle (P<0.05), while improved cathepsin B, cathepsin L activities and the hydroxyproline content (P<0.05). No significant difference was manifested for muscle pH among six diets (P>0.05). Malondialdehyde (MDA) and protein carbonyl (PC) content, and catalase (CAT) activity in the muscle were significantly down-regulated by Trp{P< 0.05); glutathione peroxidase activity (GPx) and glutathione content in the muscle were the lowest in fish fed the basal diet (P<0.05). Dietary Trp levels had no significant effect on copper/zinc superoxide dismutase activity in the muscle (P> 0.05). Additionally, dietary Trp up-regulated the GPx mRNA levels and down-regulated the CAT mRNA levels in the muscle (P< 0.05), which may be due to the modulation of the Nrf2/Keapl signaling pathway.After exposure to ambient copper sulfate, lysozyme, acid phosphatase activities, complement 3 contents in the proximal intestine (PI), mid intestine (MI) and distal intestine (DI) of young grass carp were significantly enhanced by Trp supplements (P< 0.05). The mRNA levels of tumor necrosis factor a (TNF-a) and interleukin 8 (IL-8) in all intestinal segments of young grass carp were decreased with increase of dietary Trp levels up to a certain point (P<0.05), whereas the interleukin 10 (IL-10) and transforming growth factor-β(TGF-β) mRNA levels were up-regulated by dietary Trp (P<0.05), suggesting optimum Trp attenuated the inflammatory response in the intestine. Dietary Trp down-regulated NF-κB and TOR mRNA levels in all intestinal segments of young grass carp (P< 0.05), while up-regulated I-κB mRNA levels (P<0.05). ZO-1, occludin, claudin-b, claudin-c and claudin-3 mRNA levels in the intestine of young grass carp elevated with dietary Trp levels up to a certain point, while claudin-12 and claudin-15 mRNA levels were significantly down-regulated by dietary Trp, suggesting optimum Trp was benefit for the improvement of intestinal mucosal barrier function. Dietary Trp significantly enhanced intestinal SOD, GPx activities and GSH content (P<0.05); whereas, the MDA and PC content in the intestine were decreased by Trp (P<0.05). The relative gene expression of SOD1, GPx and NF-E2-related factor 2 (Nrf2) in the intestine significantly up-regulated with increasing of dietary Trp up to a certain point (P<0.05), while the mRNA levels of Kelch-like-ECH-associated protein 1 (Keapl) were significantly down-regulated by Trp (P<0.05). Correlation analysis showed that the SOD and GPx activities in the intestine were positively related to their respective mRNA levels (rSoD=+0.954, P<0.05; rGPx=+0.972, P<0.05), meanwhile, the mRNA levels of SOD and GPx in the intestine was positively related to the gene expression level of Nrf2 (rsoD=+0.797, P=0.058; rGPx=+0.922, P<0.05), supporting that Trp elevated antioxidant enzyme activities may be partly through up-regulating Nrf2 gene expression to up-regulate the antioxidant enzyme mRNA levels.Collectively, appropriate Trp improves fish growth, fish flesh quality, intestinal immunity, mucosal barrier function as well as antioxidant status, and regulated the mRNA levels of related signal molecules of young grass carp. Based on the quadratic regression analysis of PWG and PAC, the Trp requirement of young grass carp (287-699 g) were 3.81 g kg-1 diet (12.7 g kg-1 protein) and 3.89 g kg-1 diet (13.0 g kg-1 protein), respectively.