Effects Of Tryptophan On Growth Performance,antioxidant Function And Intestinal Health Of Triploid Rainbow Trout(Oncorhynchus Mykiss) In Low Fish Meal Feed

As one of the essential amino acids for the growth of fish,tryptophan is not only widely involved in the synthesis of protein,but also can regulate the feeding behavior of fish and enhance the immune function of fish to ensure the healthy growth of fish.Triploid rainbow trout(Oncorhynchus Mykiss)with an initial body weight of 9.6±0.7 g was used as the research object in this experiment.The effects of low fish meal diet supplemented with different levels of tryptophan on the growth,fish body composition,antioxidant enzyme activities in serum and liver,intestinal digestive enzyme activities,intestinal tissue section morphology,immune capacity and gene expression of triploid rainbow trout were investigated.In this experiment,fish meal,soybean meal and beer yeast were used as the main protein sources,soybean oil and fish oil were used as the main fat sources,and secondary powder was used as the main sugar source to prepare an isonitrogenous and isoenergetic diet with 41.01% crude protein and 11.37% crude lipid levels,which was used as the control group(tryptophan level was 0.28%).The diets of other treatment groups were supplemented with 0.25%,0.50%,0.75% and 1.00% L-tryptophan,respectively,and the tryptophan levels in the diets were 0.28%,0.52%,0.80%,1.06% and 1.35%,respectively.In this experiment,450 triploid rainbow trout were divided into 5 groups(G1-G5 group)with 3replicates in each group and 30 trout in each replicate.The experiment was fed twice a day for 56 days.The main research contents are as follows:(1)The effects of different levels of tryptophan in low fish meal diets on the growth of triploid rainbow troutWith the dietary tryptophan level increasing,the specific growth rate,weight gain rate,and protein efficiency ratio of triploid rainbow trout were firstly increased and then decreased,and reached the highest values in 0.80% and 1.06% groups(P<0.05).According to the quadratic regression analysis,the requirement for tryptophan of triploid rainbow trout is 0.98%(2.39% of dietary protein)under the condition of a low fish meal diet.With the increase of tryptophan supplemental level,the crude protein content and ash content of whole triploid rainbow trout showed a trend of increasing at first and then decreasing,and the crude protein content in the1.06% group reached the highest value and was significantly higher than that in the control group(P<0.05).Whole body crude lipid content reached the highest value in the 1.35% group,but there was no significant difference among all treatment groups.In conclusion,when the tryptophan level reaches 0.98% in a low fish meal diet,it can obviously promote the growth of triploid rainbow trout.(2)Effects of different levels of tryptophan supplementation in low fish meal diets on serum and liver antioxidant capacity of triploid rainbow troutWith the supplementation of tryptophan,the superoxide dismutase(SOD)activity in serum and liver of triploid rainbow trout reached the highest value at 0.28% and 1.06%,respectively(P>0.05).The serum catalase(CAT)content of triploid rainbow trout was firstly increased and then decreased and reached the highest value when the tryptophan concentration reached 0.80%(P<0.05).The 0.52% and 1.06% groups were significantly higher than that of 1.35% groups(P<0.05).The liver catalase(CAT)in the 1.35% group was significantly higher than that in the control group(P<0.05),and the catalase(CAT)in the liver gradually increased with the tryptophan supplemental level increasing,and reached the highest value in 1.35% group and was significantly higher than that in other treatment groups(P<0.05).The serum malondialdehyde(MDA)content of triploid rainbow trout was firstly decreased and then increased with the increase of tryptophan supplemental level,and reached the lowest value in the 1.06% group and was significantly lower than that in the control group(P<0.05).The liver malondialdehyde(MDA)content in the 0.80%group reached the lowest level(P>0.05).The results of this experiment showed that the increase of dietary tryptophan content did not significantly change the SOD activity in the liver and serum of triploid rainbow trout.However,the MDA content in serum and hepatopancreas of triploid rainbow trout was firstly decreased and then increased with the dietary tryptophan content increasing.The serum MDA content in the 1.06% group was the lowest.The MDA content in the liver was the lowest in the 0.80% group.(3)Effects of different levels of tryptophan supplementation in low fish meal diets on the intestinal digestive capacity of triploid rainbow troutThe effects of tryptophan on intestinal digestibility of triploid rainbow trout were investigated by analyzing intestinal digestive enzyme activity and intestinal tissue section morphology.With the increase of tryptophan supplemental level,the intestinal trypsin content of triploid rainbow trout in the 0.80% group reached the highest value and was significantly higher than that in control group(P<0.05).With the increase of tryptophan supplemental level,the intestinal lipase content of triploid rainbow trout in the 1.06% group reached the highest value(64.47±15.73 U/m L)and was significantly higher than that in the control group(P<0.05).With the increase of tryptophan supplemental level,the intestinal amylase of triploid rainbow trout was firstly increased and then decreased,and the highest value was reached in the 0.52% group,which was significantly higher than that in the control group(P<0.05).With the dietary tryptophan level increasing,villus height increased(P<0.05),but the villus height decreased when the tryptophan content reached 1.06%.The striatum margin also increased at first and then decreased with the increase of tryptophan content,and the difference was significant compared with the control group(P<0.05).(4)Effects of different levels of tryptophan supplementation in low fish meal diets on intestinal gene expression of triploid rainbow troutThe effect of tryptophan on the expression of intestinal growth-related genesWith the increase of tryptophan levels in feed,Expression of TOR in the gut increased first and then decreased and reached its highest level in the 1.06% group,Between the 0.80% and1.06%(P>0.05),But both groups were significantly higher than the 1.35%(P<0.05),while the latter was significantly higher than other treatment groups.The expression of S6K1 in the gut is regulated by its upstream molecular TOR,At 1.06%,And 0.80%,there was significant higher than in the 0.28% group(P<0.05).Unlike the above,Expression of 4EBP1 in the intestine increased first and then decreased and reached the highest value in the 1.06% group.With the increase of tryptophan content GH and the expression of IGF-1 m RNA increased first and then decreased,The expression of GH in intestinal tract reached the highest value in the 1.06% group and was significantly higher than that in control group(P<0.05);Expression of IGF-1 in the gut reached its maximum at 0.80%,and 1.06% group(P>0.05),but significantly higher than the other treatment groups(P<0.05).The effect of tryptophan on the expression of intestinal immune-related genes after rainbow trout was injected with Aeromonas salmonicidaThe supplementation of tryptophan in a low fish meal diet had significant effects on the expression levels of interleukin(IL-1β,IL-6,IL-10)genes in the triploid rainbow trout intestinal tract after challenge(P<0.05).IL-1β gene expression showed a trend of increasing at first and then decreasing,and reached the highest value in the 0.80% group,and was significantly higher than that in 0.28% and 1.35% groups(P<0.05).The expression of the IL-10 gene in the 1.06% group reached the highest value and was significantly higher than that in the control group(P<0.05),but there was no significant difference in the expression of the interleukin(IL-8)gene among all treatment groups(P>0.05).The expression levels of intestinal tumor necrosis factor(TNF-α)and nuclear factor-κB(NF-κB)genes in triploid rainbow trout after challenge were significantly affected(P<0.05).The expression level of the TNF-α in the 0.80% group reached the highest value,and the lowest value in the 1.35% group was significantly lower than that in the control group(P<0.05).The expression level of the NF-κB gene was gradually decreased with the increase of tryptophan supplemental level in a low fish meal diet.To sum up,adding tryptophan to low fish meal(15%)feed had beneficial effects on the growth,digestive physiology and intestinal health of triploid rainbow trout.Secondary regression analysis with weight gain rate as marker showed that when tryptophan level was 0.98%(2.39% of feed protein),the growth promoting effect of triploid rainbow trout was more obvious.