Effects Of Dietary Isoleucine On Flesh Quality And Gill Barrier Function Of Young Grass Carp(Ctenopharyngodon Idella)

A total of 540 young grass carp, with an average initial weight of 256.8 (SD 3.5) g, were fed diets containing graded levels of Ile, namely,3.8 (control),6.6,9.3,12.5, 15.2 and 18.5 g/kg diet for 8 weeks. Compared with the control group, Ile supplementation increased (P< 0.05):(1) percent weight gain (PWG) and feed intake; (2) muscle protein content, fat content, collagen content, shear force and pH value; (3) anti-superoxide anion capacity, anti-hydroxyl radical capacity, Cu/Zn-SOD and GPx activities, as well as glutathione (GSH) content in the muscle; (4) relative mRNA expression of Cu/Zn-SOD, GPx, GCL, Nrf2, TOR, S6K1 and CK2 in the muscle. Conversely,Ile supplementation decreased (P< 0.05):(1) plasma ammonia concentration; (2) muscle cooking loss; (3) malondialdehyde and protein carbonyl content in the muscle; (4) relative mRNA expression of Keap1 in the muscle.A copper (0.7 mg Cu/L of water) challenge experiment was performed after the growth experiment. The results of the challenge experiment showed that Ile supplementation increased (P< 0.05):(1) gill GSH content and the activities of ASA, AHR, Cu/Zn-SOD, CAT, GPx, GR and GST; (2) the relative mRNA expression of Cu/Zn-SOD、CA、GPx、GR and GST in the gill of fish; (3) the relative mRNA expression of Claudin-3, Claudin-b, Claudin-c, Occludin, ZO-1, IL-10 and TGF-β in the gill of fish; (4) the relative mRNA expression of signaling molecules including Nrf2、TOR、S6K1 and IκB in the gill of fish. In contrast, Ile supplementation decreased (P< 0.05):(1) the gill PC and MDA content; (2) relative mRNA expression of Claudin-12, IL-1, IL-8, TNF-α, Keap1 and NF-κB in the gill of fish. However, dietary Ile did not have a significant effect on the gill Claudin-15 mRNA expression (P>0.05).In conclusion, isoleucine could improve the growth, flesh quality and muscle antioxidant ability of young grass carp; the improved flesh quality may be ascribed to the enhanced muscle antioxidant ability. In addition, isoleucine could also improve gill barrier function partly through elevating antioxidant ability and tight junction. Besides, the dietary Ile requirement of young grass carp (253.4-660.8 g) based on PWG was 12.2 g/kg diet or 39.3 g/kg of dietary protein.