Effects Of Dietary Vitamin C On Growth Performance, The Health Status Of Intestinal, Body And Gill, And Flesh Quality Of Young Grass Carp(Ctenopharyngodon Idella) And The Mechanisms

This study investigated the effects of dietary vitamin C (VC) on the growth performance, the health status of intestinal, body and gill, and flesh quality of young grass carp(Clenopharyngodon idella) and the mechanisms. In order to investigate the efftects of VC on the growth performance and flesh quality of young grass carp, we selected 540 young grass carp (264.37 ± 0.66 g), then divided into six treatments and fed six diets with graded levels of VC (2.9,44.2,89.1.133.8.179.4 and 224.5 mg/kg diet) for 10 weeks. To study the relationship between dietary VC and fish intestinal, body and gill health status and its mechanisms, we selected 30 young grass carp per treatment, and divided into two groups, then conducted two challenge trails with Aeromonas hydrophila infection for 14 days, and Flavobacterhtm Columnare infection for 3 days, respectively, after the feeding trial.Results of feeding trial indicated that optimal VC increased specific growth rate, percent weight gain (PWG), feed intake and feed efficiency (P<0.05). suggesting that optimal VC could increase growth performance of young grass carp. Meanwhile, optimal VC increased fish muscle nutrient (protein, lipid, essential amino acids, flavor amino acids and PUFA) contents, pH value, shear force and hydroxyproline content (P<0.05). and decreased cooking lose, lactate content, and cathepsin B and L activities (P<0.05). suggesting that optimal VC could increase fish flesh quality. Optimal VC increased fish flesh quality may related to reduction oxidative injury, which may be partly due to the elevated expression levels and activities of antioxidant enzymes such as copper/zinc superoxide dismutase (CuZnSOD) by up-regulating of NF-E2-related factor 2 (Nrf2) mRNA level and down-regulating the of Kelch-like ECH-associating protein 1 (Keapl) mRNA level (P< 0.05). Results of challenge trails indicated that optimal VC increased disease resistance (enteritis, skin lesion and gill rot) (P<0.05), which may relate to the enhancement of the intestinal, body and gill health status. The enhancement of the intestinal, body and gill health status may related to the increased of intestinal, head kidney, spleen, skin and gill immune function. Our investigation observed that optimal VC increased lysozyme and acid phosphatase (except skin) activities (P<0.05), as well as the complement 3 (C3) and C4 contents (P<0.05), up-regulated the mRNA levels of antimicrobial peptides mRNA levels (P<0.05) in young grass carp, suggesting that VC increased nonspecific immunity of young grass carp. Meanwhile, VC down-regulated major pro-inflammatory cytokines tumor necrosis factor a (except PI and gill), interferon ?2, interleukin (IL) 1? (IL-1?) (except intestinal) and IL-8 mRNA levels(P<0.05), and up-regulated major anti-inflammatory cytokines IL-10 and transforming growth factor ?1 (except skin) mRNA levels (P<0.05) of young grass carp, which may related to the down-regulating of nuclear factor ?B p65 (NF-?B p65) and target of rapamycin signaling (P<0.05), suggest that VC inhibited young grass carp inflammatory response. Further study found that the improvement of the intestinal, body and gill health status is related to the maintenance of the cell structural integrity and the structural integrity of intercellular:(1) optimal VC improved the ROS scavenging ability by increasing nonenzymatic compound GSH and vitamin C contents (P<0.05), and the activities and mRNA levels of antioxidant enzymes such as CuZnSOD (P<0.05). The increased of antioxidant enzymes mRNA levels may correlated to the up-regulation of Nrf2 signaling (P<0.05). Meanwhile, optimal VC down-regulated caspase-3,-8,-9 mRNA levels (P<0.05), indicating that VC could inhibite young grass carp cell apoptosis. These observations indicated that optimal VC could maintain the cell structural integrity offish. (2) optimal VC up-regulated ZO and Claudin-b (except gill) mRNA levels, and down-regulated Claudin-12,-15 by down-regulating signal molecule myosin light-chain kinase (MLCK) (except spleen) mRNA level (P<0.05), indicating that VC could maintain structural integrity of intercellular. These results demonstrated that, optimal VC could maintain the structural integrity of intestinal, head kidney, spleen, skin and gill by increasing antioxidant capacity, anti-apoptosis ability, tight junction protein of intercellular.Together, VC could increase growth performance, flesh quelity and disease resistance. Optimal VC increased fish flesh quality may relate to reduction oxidative injury, which may be partly due to the elevated expression levels and activities of antioxidant enzymes such as CuZnSOD by up-regulating of Nrf2 signaling. The increased of disease resistance is related to the enhancement of the health status of intestinal, body and gill. Briefly, VC increased antibacterial ability by increasing the activities and contents of major antibacterial compounds, and up-regulating the mRNA levels of antibacterial peptides. Meanwhile, VC decreased inflammatory injury by up-regulating anti-inflammatory cytokines and down-regulating pro-inflammatory cytokines and the related signal molecule NF-?B p65 RNA levels. VC decreased oxidative injury by increasing the activities and mRNA levels of antioxidant enzymes such as CuZnSOD, which may be involved in Nrf2 signaling pathway, and maintained the structural integrity of cell by inhibiting cell apoptosis, which may be in involved in down-regulating caspase-3,-8,-9 mRNA levels. VC affected tight junction protein by regulating MLCK signaling, maintained the structural integrity of intercellular. Base on PWG and disease resistance (enteritis, skin lesion and gill rot), the requirements of young grass carp (264-1031 g) for vitamin C was 92.8,124.0,122.9 and 127.0 mg/kg diet.