Effects Of Exogenous Ammonia Nitrogen On Growth,Blood Biochemistry And Muscle Protein Metabolism Of Juvenile Grass Carp (Ctenopharyngodon Idella)

Ammonia nitrogen is one of the common nitrogen metabolites in water environment.Due to toxic effects of high concentration of ammonia nitrogen,it has become one of the main restrictive environmental factors in fish farming,especially in industrialized circulating water farming.At present,a large number of studies are focused on the acute or chronic toxicity of ammonia nitrogen to fish.However,there are relative few of studies on detoxification mechanism of fish under exogenous ammonia treatment and its effects on growth and muscle quality.Grass carp(Ctenopharyngodon idella),as the largest freshwater fish breed in China,is often subjected to ammonia nitrogen stress under high-density aquaculture mode.In this study,grass carp was used as the research object,and ammonia nitrogen was used as the influencing factor to study the effects of exogenous ammonia nitrogen on growth,blood biochemistry and muscle protein metabolism of grass carp.A control group(0 mg/L)and four ammonia nitrogen treatment groups(1,3,6,9 mg/L)were set in the experiment,and three parallel tanks were set in each group.After 56 days of exposure,the results were as follows:(1)Exogenous ammonia nitrogen in a certain range promoted the growth of grass carp,increased muscle crude protein content and decreased muscle crude fat content.During the whole experiment,grass carp in all treatment groups had good health status except a few deaths occurred,and the feed intake was good.After 56 days of culture experiment,the weight gain rate and specific gain rate of juvenile grass carp in all ammonia treatment groups were elevated significantly compared to the control group(P < 0.05),and no significant differences on the two parameters were detected among ammonia treatment groups.Ammonia nitrogen treatment at the concentrations of 3,6 and 9 mg/L significantly reduced the feed coefficient of grass carp(P < 0.05),but didn’t affect the feeding rate in relative to the control.The results of muscle composition showed that content of crude protein significantly increased and crude fat content decreased in dorsal muscles of grass carp treated with 3,6 and 9 mg/L ammonia nitrogen(P < 0.05).At the same time,the ammonia excretion rate of grass carp was significantly increased at 6 mg/L ammonia treatment group compared with the control group(P < 0.05).(2)With the increase of ammonia nitrogen concentration,the blood ammonia level of grass carp increased significantly in a dose-dependent manner(P < 0.05).Treatment with3,6 and 9 mg/L ammonia nitrogen significantly enhanced the activity of glutamate dehydrogenase in the serum of grass carp(P < 0.05).Similarly,serum glutamine level and glutamine synthetase activity were significantly increased in fish treated with 6 and 9 mg/L ammonia nitrogen(P < 0.05).Serum urea nitrogen levels in grass carp were also significantly increased in a dose-dependent manner,and the activity of aminoformyl phosphate synthase 1 was significantly increased in 9 mg/L ammonia treatment group(P <0.05).In contrast,serum aspartate aminotransferase activity of grass carp was significantly reduced under 6 and 9 mg/L ammonia nitrogen treatment,and glutamic-pyruvic transaminase activity was also significantly reduced under 3,6 and 9 mg/L ammonia nitrogen treatment(P < 0.05).The results showed that under the action of exogenous ammonia nitrogen,grass carp detoxified ammonia through three pathways: increasing nitrogen excretion,converting ammonia nitrogen into other low-toxic or non-toxic nitrogen-containing metabolites(glutamine,urea),and elevating protein synthesis and reducing protein decomposition.(3)Exogenous ammonia nitrogen treatment significantly affected the genes related to protein synthesis metabolism in the dorsal muscle of grass carp(P < 0.05),but had no significant effect on the gene eif2α related to e IF2B-e IF2 pathway.Compared with the control group,the expression levels of TORC1 pathway related genes(igf-1,igf-1r,pi3 k,akt,tor)in dorsal muscle of grass carp were up-regulated in ammonia treatment groups(P< 0.05).Treatment with 3,6 and 9 mg/L ammonia nitrogen significantly decreased the expression level of 4e-bp1(P < 0.05),but didn’t affect the expression of gene s6k1.On the other hand,exogenous ammonia nitrogen significantly down-regulated the expression of genes related to the protein catabolic ubiquitin-protease system and autophagy-lysosomal pathway(murf1,psma2,beclin1,atg5,atg16l1,lc3b)in the dorsal muscle of grass carp(P< 0.05),but had no significant effect on the expression of gene cast related to the troponin system.The results showed that exogenous ammonia nitrogen treatment promoted protein synthesis through PI3K/Akt/TORC1 pathway in grass carp,and reduced protein degradation through ubiquitin-protease system and autophagy system.