Effects And Mechanisms Of Graded Levels Of Lysine On Growth Performance And Immune Function Of Head Kidney,Spleen And Skin In On-Growing Grass Carp(Ctenopharyngodon Idella)

The present study was primarily focused on the growth performance,immune function and the possible mechanisms in the head kidney,spleen and skin of on-growing grass carp(Ctenopharyngodon idella).And further based different indices to determine the requirement of lysine in on-growing grass carp(Ctenopharyngodon idella).540 healthy grass carp(initial weight was 164.85 ± 0.79 g)were randomly put into 18 cages(6 treatments and each treatment with triplicate cages).Six iso-nitrogen diets differing in lysine concentrations(5.6,8.5,11.6,14.4,17.5 and 20.7 g/kg)were fed to grass carp 60 days.After that,grass carp were challenged by A.hydrophila for 6 days.The present study showed that appropriate level of lysine in the diet obviously elevated feed intake(FI),feed efficiency(FE),percentage weight gain(PWG),specific weight gain(SGR),the glutamate-oxaloacetate transaminase(GOT)and glutamate-pyruvate transaminase(GPT)activities in the hepatopancreas(p < 0.05),and decreased the plasma ammonia content(PAC)(p <0.05)of on-growing grass carp,indicating that lysine could improved amino acid utilization and growth performance of grass carp.Further investigation showed that(1)lysine decreased skin hemorrhage and significant decresed lesion morbidity(p < 0.05)and increased the resistance of disease in on-growing grass carp,which may be related to the improvement of immune function of head kidney,spleen and skin;(2)lysine significant icreased the m RNA levels of liver-xpressed antimicrobial peptide(LEAP)-2B,LEAP-2A,hepcidin,mucin2 and β-defensin-1 and the contents of Immunoglobulin(Ig)M,complement(C)4 and C3 and the activity of lysome in the head kidney,spleen and skin(p< 0.05),elevated antimicrobial activity of immune organs;(3)lysine down-regulated the m RNA levels of pro-inflammatory cytokines [interferon(IFN)-γ2,tumor necrosis factor(TNF)-α,interleukin(IL)-17 D,IL-15,IL-12p40,IL-8 and IL-6] and up-regulated the m RNA levels of anti-inflammatory cytokines [transforming growth factor(TGF)-β1,IL-11,IL-10 and IL-4/13A],impaired the immune response of immune organs,which were tightly correlated with down-regulated the m RNA levels of TYK2 and STAT1 and up-regulated the m RNA levels of JAK2 and the protein levels of STAT3 in the immune organs of on-growing grass carp.In summary,the appropriate level of lysine in the diet elevated growth performance and enhanced the resistance of disease.Appropriate dietary lysine level improved the resistance of disease by increased the produce of antibacterial material,down-regulated proinflammatory cytokines,up-regulated anti-inflammatory cytokines and increased the immune function of immune organs of on-growing grass carp.In addition,quadratic regression analysis of PWG,skin morbidity and the LZ activities in the spleen and head kidney,the dietary lysine requirements for on-growing grass carp were estimated to be 13.58g/kg,13.51 g/kg,14.56 g/kg and 14.18 g/kg,respectively.