Effect Of Different Levels Of Dietary Threonine On Growth Performance And Intestinal Health Of Growing Rabbits

Threonine(Thr)is usually classified as the essential amino acid of animal.Thr is crucial for improving immune function and antioxidant capacity,and many previous studies have demonstrated that Thr plays an important role in the function of maintaining intestinal health and relieving damage from challenge.Therefore,this study was conducted to investigate the effects of different levels of apparent total tract digestible threonine(AFDT)on growth performance,serum biochemistry,serum free amino acid,nutrient digestibility,and intestinal morphology,immunity,redox status and barrier function of tract between lipopolysaccharide(LPS)challenged and control growing rabbits,for exploring the protective effect and the possible mechanism of Thr in intestinal health on growing rabbits.A 2×3 double factors design was used in this experiment.A total of 180 35-day-old growing New Zealand White rabbits with the same genetic background and similar initial weight(808.11±12.17 g)were selected and randomly assigned to 6 treatments.Each treatment consisted of 30 replicates with one rabbit per replicate.The challenged group received an intraperitoneal injection with LPS at 100 μg/kg BW,and the unchallenged group received an equal amount of sterile saline at day 17,20.The normal feeding group and LPS challenge group were fed 3 different levels of AFDT: 0.43%,0.54% and 0.64%,analyzed basis.These levels corresponded to 100%,125% and 150% of AFDT levels recommended to growing rabbits by the Nutrition of the Rabbit(the 3rd edition).Blood and intestinal samples were collected at 22 days of the experiment,and the experiment lasted for 35 days.Digestive experiment was performed from day 4 to day 7 after feeding experiment(intraperitoneal injection with LPS at 100 μg/kg BW and an equal amount of sterile saline at day 3,6).The results were showed below:(1)LPS challenge significantly decreased the growth performance of growing rabbits(P<0.05),and inclined to increase the mortality(P=0.05)and health risk index(P=0.09)from day 17 to day 22.Compared with the rabbits fed on the AFDT 0.43% diets,those fed on the AFDT 0.64% diets had an increased average daily gain(except for those from l to 17 days of experiment),and feed intake(from day 17 to day 22 and from day 1to day 35),while a decreased(P<0.05)feed conversion ratio from day 17 to day 22(P<0.05).The feed intake(P=0.06)tended to increased,and decreased the morbidity(P=0.08)and health risk index(P=0.08)from day 1 to day 17 in the rabbits fed on the AFDT 0.64% diets compared to those on the AFDT 0.43% diets.However,there was no interaction between dietary Thr level and LPS challenge on growth performance and health condition of growing rabbits(P>0.05).(2)LPS challenge significantly increased the total cholesterol,low density lipoprotein,free Thr,phenylalanine and arginine contents in the serum of growing rabbits(P<0.05),and significantly decreased the conent of albumin and the activities of alanine aminotransferase,alkaline phosphatase,diamine oxidase in the serum(P<0.05).Compared with the rabbits fed on the AFDT 0.43% and 0.64% diets,the high-density lipoprotein levels significantly decreased in serum in the rabbits fed on the AFDT 0.54%diets(P<0.05).Compared with the rabbits fed on the AFDT 0.54% diets,the globulin contents significantly increased in serum in the rabbits fed on the AFDT 0.64% diets(P<0.05).Whereas,there was no significant interaction between dietary Thr level and LPS challenge on the content of biochemical indices,free amino acids,D-lactic acid,and the activities of diamine oxidase in the serum of growing rabbits(P>0.05).(3)LPS challenge significantly decreased the apparent digestibility of dry matter,organic matter,gross energy,nitrogen free extract,acid detergent fiber and cysteine of growing rabbits(P<0.05),but improved the digestibility of methionine(P<0.05).Rabbits fed on the AFDT 0.64% diets had a greater digestibility(P<0.05)of Thr than those on the AFDT 0.43% and 0.54% diets,and those on the AFDT 0.43% diets had a greater(P<0.05)digestibility of dry matter,nitrogen free extract,Alanine than those on the AFDT 0.54%and 0.64% diets.Whereas,there was no significant interaction between dietary Thr level and LPS challenge on proximate nutrients,fiber fractions and amino acids of growing rabbits(P>0.05).(4)LPS challenge significantly decreased the villus height in jejunum(P<0.05),and tended to decrease the villus height in duodenum(P=0.06)and the villus height to crypt depth ratio(P=0.09)in jejunum in growing rabbits.The results showed that increasing Thr supplementation had no effect on intestinal morphology of growing rabbits(P>0.05).There was significant interaction between dietary Thr level and LPS challenge in duodenum villus height of growing rabbits(P<0.05),the rabbits fed on the AFDT 0.64%diets alleviated the decreased the villus height in duodenum caused by LPS challenge compared with those fed on the AFDT 0.54% diets(P=0.07).(5)LPS challenge significantly reduced the goblet cell count in jejunum in growing rabbits(P<0.05).The level of secretory immunoglobulin A(P=0.06)in jejunum mucosa tended to be higher in the rabbits fed on the AFDT 0.64% diets than those on the AFDT0.43% diets.There was significant interaction between dietary Thr level and LPS challenge on the level of secretory immunoglobulin A and mucin 2 in duodenum,and on secretory immunoglobulin A and goblet cell count in ileum(P<0.05).Compared with the rabbits fed on the AFDT 0.43% diets,those on the AFDT 0.54% diets had a greater level of secretory immunoglobulin A both in duodenum and ileum,and the goblet cell count in ileum when challenged with LPS.Mucin 2 level(P=0.06)in duodenum in AFDT 0.54%group tended to be higher than in the AFDT 0.43% group.(6)LPS challenge significantly reduced the content of malondialdehyde in the duodenum(P<0.05),and significantly increased the activities of total antioxidant capacity and malondialdehyde in jejunum in growing rabbits(P<0.05).Compared with rabbits fed on the AFDT 0.43% diets,the activities of total antioxidant capacity significantly decreased in the rabbits fed on the AFDT 0.54% and 0.64% diets(P<0.05).There was significant interaction between dietary Thr level and LPS challenge on the activities of total antioxidant capacity,malondialdehyde,xanthine oxidase from buttermilk and glutathione peroxidase in ileum in growing rabbits(P<0.05).Glutathione peroxidase activity in ileum was significantly increased,while xanthine oxidase activity in ileum was decreased,by LPS challenge in the rabbits fed on the AFDT 0.54% diets than those on the AFDT 0.43% diets(P<0.05).Meanwhile,the AFDT 0.64% diets significantly alleviated the adverse effect of elevated xanthine oxidase activity in ileum caused by LPS challenge(P<0.05),and tended to increase the glutathione peroxidase activity(P=0.09)and decreased the malondialdehyde content(P=0.07)in ileum in growing rabbits.(7)LPS challenge significantly up-regulated the expression of TLR4,IL-1β m RNA in duodenum tissues,TNF-α,CLDN in jejunum tissues,and TNF-α in ileum tissues,while down-regulated JAM2,JAM3 m RNA in duodenum tissues,IL-2,OCLN,ZO-1 in jejunum tissues,and CLDN,ZO-1 in ileum tissues of growing rabbits(P<0.05).Compared with rabbits fed on the AFDT 0.43% diets,those fed on the AFDT 0.64% diets showed a down-regulated expression of TLR4,TNF-α,JAM2,JAM3 in duodenum tissues,TLR4,NF-κB,IL-1β,OCLN,JAM3 in jejunum tissues,and TLR4,My D88,NF-κB,IL-1β,JAM2,JAM3,CLDN in ileum tissues(P<0.05).There was significant interaction between dietary Thr level and LPS challenge on the expression of TLR4,My D88,NF-κB,TNF-α,IL-1β,IL-2,IL-10,ZO-1 in duodenum tissues,My D88,IL-1β,IL-10,CLDN in jejunum tissues,and TLR4,NF-κB,TNF-α,IL-1β,IL-2,IL-10,CLDN,JAM2,JAM3 in ileum tissues(P<0.05).The expression of TLR4,My D88,NF-κB,TNF-α,IL-1β,ZO-1 in duodenum tissues,and My D88,IL-1β,CLDN in jejunum tissues were significantly down-regulated,and the expression of IL-10 in ileum tissues significantly up-regulated,in the rabbits fed on the AFDT 0.64% diets compared with those fed on the AFDT 0.43%diets when challenged with LPS(P<0.05).In conclusion,these results showed that dietary Thr supplementation improved intestinal immune function,health status and growth performance in growing rabbits.LPS challenge impaired the intestinal health,compromised digestion and absorption,and ultimately reduced the growth performance.Dietary Thr supplementation relieved intestinal inflammation and oxidative stress induced by LPS challenge,improved intestinal immune function.In this experiment,the optimal level of digestible Thr in the diet for growing rabbits was 0.64% in terms of promoting growth performance,health status,antioxidant capacity and optimizing immune molecule content and gene expression.