The Effects Of Exogenous Bile Acid On Alleviating Heat Stress In Broiler Chickens

High-temperature environment can cause heat stress in broilers,directly impacts their feed intake and results in slow growth.Heat stress(HS)also induces negative effects such as fat accumulation,intestinal dysbacteriosis,and oxidative stress.Bile acid(BA)plays the important roles in the regulation of glucose and lipid metabolism,anti-oxidative stress,and protection of intestinal barrier function with good application prospects.To explore the roles of BA in relieving heat stress,we conducted a 2×2factorial design,with temperature(23 or 32 °C)and BA supplements(0 or 200 mg/kg).A total of 288 21-day-old AA broilers with uniform body weight were selected and distributed into 4 treatment groups,with6 replicates for each treatment,and 12 chickens for each replicate.At the end of the trial,two chickens were selected respectively from each replicate for blood and tissue samples to determine their carcass quality,meat quality,antioxidant capacity,lipid metabolism and bile acid metabolism-related indexes,and intestinal barrier-related indexes.The results were as follows.(1)Chronic heat stress model used in this experiment with a temperature of 32±0.5°C and humidity of 65%±5% for 21 days.When the ambient temperature rose to 32°C,the broilers in the HS group showed a sustained stress response,the respiratory rate and the cloaca temperature increased significantly(P<0.001).(2)Heat stress significantly reduced the final body weight(FBW),average daily gain(ADG)and average daily feed intake(ADFI)of broilers by 25.91%,35.88%,and 26.15%(P<0.001),the BA diet could significantly increase the ADG of broilers(P=0.037).Under 32℃ heat exposure,dietary supplements of BA significantly increased the ADG by 9.89%,indicating that dietary supplements of bile acids had a positive effect on the growth performance of heat-stressed broilers.(3)Heat stress tended to increase the content of malondialdehyde(MDA)in the liver.The content of MDA in the liver was decreased in the diet supplemented with BA,but did not display a statistically significant level(P>0.05).Temperature and BA had a significant interaction effect on the GSH-Px activity in the liver(P=0.045).The liver GSH-Px activity of the HS group was significantly lower than the other three groups,while the BA diet can reduce the heat response.The GSH-Px activity in the liver of broiler chickens recovered to the level of thermal neutral condition(P<0.05).(4)HS induced liver ApoB expression(P=0.019),and there was a tendency to increase SREBP-1c(P=0.080).The BA diet could significantly reduce the level of total liver triglycerides TG(P=0.007),and inhibit lipid synthesis by reducing the expression of liver SREBP-1c(P=0.002)and FAS(P=0.007).In addition,heat stress had no significant effect on the expression of genes related to liver bile acid synthesis(HMGCR,CYP7A1,CYP8B1 and CYP27A1)(P>0.05),but significantly increased the expression of BA receptor FXR in the liver(P=0.010).Diet supplemented with BA tend to reduce the expression of HMGCR(P=0.084)and CYP7A1(P=0.080)genes.(5)The BA analysis results showed that the bile acids in broiler serum,liver and intestine were mainly composed of chenodeoxycholic acid(CDCA),taurocholic acid(TCA)and taurodeoxycholic acid(TCDCA).Heat stress caused a significant decrease in the level of liver secondary BA(SBA,P=0.0498),and the content of TCDCA was significant decreased in the ileum(P=0.038).The dietary supplements of BA significantly increased the levels of TUDCA in serum,liver,and ileum(P<0.05),then significantly increased serum and liver SBA levels(P<0.001),and had no impact on the synthesis of BA in the liver(P<0.05).In addition,dietary supplements of BA increased the expression of ABST in the ileum(P<0.05)and promoted the transport capacity of BA.(6)The analysis of the diversity of the intestinal flora revealed that heat stress caused the cecal microbial community to gather than normothermia.In addition,as the main microorganism involved in the modification of BA,heat stress caused the relative abundance of Bacteroidetes to decrease from 47.80%to 38.81%.And through LEf Se multi-level species difference discriminant analysis,8 kinds of bacteria(C.Clostridia 、 O.Oscillospirales 、 F.Erysipelatoclostridiaceae 、 F.Butyricicoccaceae 、 G.Negativibacillus 、 G.Eubacterium＿brachy＿group 、 O.Monoglobales 、 G.Marvinbryantia)were preliminarily identified as candidate gut microorganisms closely related to HS,which need to be further studied.In summary,exogenous BA have the positive effects on improving heat-stressed broiler growth performance and regulating lipid metabolism,indicating that exogenous BA as a nutritional strategy have certain potential in alleviating heat stress.