Effects Of Chronic Heat Stress On The Glucose And Lipid Metabolism In Broilers And The Alleviative Effects Of Taurine

The genetic selection for broilers in past decades has enhanced the performance and feed efficiency to a large extent,but modern broilers seem to become more sensitive to environmental challenges,especially the high temperature.Heat stress is one of the most serious problems encountered in the poultry industry,and has been reported to result in low growth performance,poor carcass characteristics,inferior meat quality,and even high mortality of broilers.The decreased growth and increased fat deposition observed in chronic heat-stressed chickens seems contradictory and suggests that chronic heat-stressed broilers initiate some special metabolic changes,especially the change in energy-substance metabolism.However,the metabolic changes of chronic heat-stressed broilers are intricate and multifactorial,and the regulatory mechanisms remain unclear.In addition,taurine(2-aminoethanesulfonic acid)is a sulfur-containing,β-amino acid found in high concentration in all animal tissues.Taurine has various biological functions and its many functions are closely related to stress alleviation including osmoregulation,anti-inflammation,cell membrane stabilization,anti-oxidation,and neuromodulation.Therefore,the present study used metabonomics method to find the key pathways changed by chronic heat stress and focused on the glucose and lipid metabolism in the breast muscle and liver in Arbor Acres(AA)broilers,and this study aimed to elucidate the effects and related mechanisms of chronic heat stress on the glucose and lipid metabolism in broilers.The objective of this study also was to investigate the alleviative effects of taurine supplementation on the negative effects of chronic heat stress in broilers.1.Effects of chronic heat stress on the growth performance and serum metabolic profile in broilersThis study was to investigate the effects of chronic heat stress on the growth performance and serum metabolic profile in broilers and find the key metabolic pathways changed by chronic heat stress.Experiment 1:A total of 144 male AA broilers at 28 d of age were selected and randomly allocated to 3 groups with 6 cages per group and 8 birds per cage.Broilers in the normal control(NC)group were kept at 22℃ ambient temperature(thermoneutral)and provided ad libitum access to feed;broilers in the heat stress(HS)group were reared at 32℃ and provided ad libitum access to feed;broilers in the pair-fed(PF)group were maintained at 22℃ and received an amount of feed equal to the feed consumed by the HS group on the previous day.The experimental period was 14 d.The results showed that the cloacal temperature of the HS group was significantly higher than that of the NC and PF groups after 2 h,3 d,7 d and 14 d of heat exposure(P<0.05).After 14 d of heat exposure,the average daily gain(ADG)and average daily feed intake(ADFI)of the HS group were significantly lower than those of the NC group(P<0.05),and the feed/gain ratio(F/G)was significantly higher than that of the NC and PF groups(P<0.05).The results of metabolomics showed that 78 and 34 metabolites with significantly different levels between the HS and NC groups and between the HS and PF groups were identified,respectively.The most-impacted pathways that were identified by the discriminating metabolites between the HS and NC groups were the glycine,serine and threonine metabolism,glyoxylate and dicarboxylate metabolism,taurine and hypotaurine metabolism,biosynthesis of unsaturated fatty acids,thiamine metabolism,aminoacyl-tRNA biosynthesis,butanoate metabolism,arginine and proline metabolism,phenylalanine metabolism,fatty acid biosynthesis,cysteine and methionine metabolism and citrate(TCA)cycling pathways.The most-responsible pathways identified by the discriminating metabolites between the HS and PF groups were the citrate(TCA)cycling,tyrosine metabolism,alanine,aspartate and glutamate metabolism,methane metabolism,butanoate metabolism,glyoxylate and dicarboxylate metabolism,neuroactive ligand-receptor interaction,phenylalanine metabolism,primary bile acid biosynthesis,insulin signalling pathway,gap junction and ATP-binding cassette(ABC)transporter pathways.2.Effects of chronic heat stress on the carcass characteristics and the glucose and lipid metabolism of the liver in broilersThis study was to investigate the effects and mechanisms of chronic heat stress on the carcass characteristics and the glucose and lipid metabolism of the liver in broilers.This experiment design was the same with the Experiment 1.The results showed that,after 7 d of heat exposure,the percentage of abdominal fat,the levels of glucose,triglyceride(TG),corticosterone,insulin,very low density lipoprotein(VLDL)in plasma,the levels of TG,total cholesterol,acyl-CoA carboxylase(ACC)and fatty acid synthase(FAS)in the liver,and the mRNA expression of carbohydrate response element-binding protein(ChREBP),ACC,FAS,and microsomal triglyceride transfer protein(MTTP)in the liver in the HS group were significantly higher than those in the NC and PF groups(P<0.05).After 14 d of heat exposure,the percentage of abdominal fat,the organ index of liver,the level of VLDL in plasma,the levels of TG and FAS in the liver,and the mRNA expression of AMP-activated protein kinase α2(AMPKα2)in the liver in the HS group were significantly higher than those in the NC and PF groups(P<0.05).In addition,the results of histology showed that,after 7 d and 14 d of heat exposure,there were excess lipid deposition in the liver of broilers in the HS group.3.Effects of chronic heat stress on the meat quality and the glucose and lipid metabolism of the breast muscle in broilersThis study was to investigate the effects and mechanisms of chronic heat stress on the meat quality and the glucose and lipid metabolism of the breast muscle in broilers.This experiment design was the same with the Experiment 1.The results showed that,after 7 d of heat exposure,there were no significant effects from the treatments on the pH45 min,pH24 h,lightness(L*),redness(a*),yellowness(b*),drip loss,cooking loss and shear force of the breast muscle,and the activity of superoxide dismutase(SOD)and the mRNA expression of avian uncoupling protein(avUCP)of the breast muscle in broilers in the HS group were significantly lower than those in the NC and PF groups(P<0.05),but the levels of reactive oxygen species(ROS)and malondialdehyde(MDA)and the phosphorylation level of AMPKa of the breast muscle in broilers in the HS group were significantly higher than those in the NC and PF groups(P<0.05).After 14 d of heat exposure,the pH45 min and shear force of the breast muscle,the activity of citrate synthase(CS),the mRNA expression of muscular isoform of carnitine palmitoyl transferase 1 and the phosphorylation level of ACC of the breast muscle in broilers in the HS group were significantly lower than those in the NC and PF groups(P<0.05),but the L*and drip loss,lipid content,the level of lactic acid,the activities of lactic dehydrogenase and pyruvate kinase and the mRNA expression of pyruvate dehydrogenase kinase 4,ACC and FAS of the breast muscle in broilers in the HS group were significantly higher than those in the NC and PF groups(P<0.05).In addition,after 7 d and 14 d of heat exposure,some swelled mitochondria were observed in the breast muscle of broilers in the HS group.4.Effects of dietary taurine supplementation on the growth performance,carcass characteristics and the glucose and lipid metabolism of the liver in chronic heat-stressed broilersThis study was to investigate the effects and mechanisms of dietary taurine supplementation on the growth performance,carcass characteristics and the glucose and lipid metabolism of the liver in chronic heat-stressed broilers.Experiment 2:A total of 144 male AA broilers at 28 d of age were selected and randomly allocated to 3 groups with 6 cages per group and 8 birds per cage.In the normal control(NC)group,chickens were fed a basal diet and kept at 22℃ ambient temperature(thermoneutral)during the whole experiment period;in the heat stress(HS)group,chickens were fed the basal diet and subjected to constant heat stress at 32℃ ambient temperature;in the heat stress+taurine(HS+T)group,chickens were fed the basal diet supplemented with 5.00 g/kg taurine and subjected to the same constant heat stress as the HS group.The experimental period was 14 d.The results showed that cloacal temperatures of the HS and HS+T groups were significantly higher than those of the NC group after 2 h,3 d,7 d and 14 d of heat exposure(P<0.05),and there was no significant difference in the cloacal temperatures of the HS and HS+T groups at the same point in time(P>0.05).After 7 d and 14 d of heat exposure,ADG and ADFI of the HS and HS+T groups were significantly lower than those of the NC group(P<0.05),and there were no significant differences in ADG and ADFI between the HS and HS+T groups(P>0.05).After 7 d and 14 d of heat exposure,the F/G in the HS group was significantly higher than that of the NC group(P<0.05),and the F/G in the HS+T group was similar to the NC and HS groups(P>0.05).After 7 d and 14 d of heat exposure,the percentage of abdominal fat in the HS and HS+T groups was significantly higher than that of the NC group(P<0.05).Compared with the NC and HS+T groups,7 d and 14 d of heat exposure significantly decreased the percentage of breast muscle in the HS group(P<0.05)and significantly increased the organ index of liver in the HS group(P<0.05).Compared with the NC and HS+T groups,7 d of heat exposure significantly increased the level of glucose and the activity of aspartate aminotransferase(AST)in plasma,the level of ACC and the mRNA expression of liver X receptor α(LXRα),sterol response element-binding protein lc,ACC and FAS in the liver of broilers in the HS group(P<0.05).Compared with the NC and HS+T groups,14 d of heat exposure significantly increased the level of VLDL and the activity of AST in plasma,the levels of TG,ACC and FAS in the liver(P<0.05)and significantly decreased the level of glucose in plasma of broilers in the HS group(P<0.05).In addition,after 7 d and 14 d of heat exposure,there were excess lipid deposition in the liver of broilers in the HS group.5.Effects of dietary taurine supplementation on the meat quality and antioxidant capability of the breast muscle in chronic heat-stressed broilersThis study was to investigate the effects and mechanisms of dietary taurine supplementation on the meat quality and antioxidant capability of the breast muscle in chronic heat-stressed broilers.This experiment design was the same with the Experiment 2.The results showed that,after 7 d of heat exposure,the drip loss and cooking loss of the breast muscle in broilers in the HS and HS+T groups were significantly higher than those in the NC group(P<0.05),and there were no significant effects from the treatments on the pH45 min,pH24 h,L*,a*,b*and shear force of the breast muscle(P>0.05).Compared with the NC and HS+T groups,7 d of heat exposure significantly increased the levels of ROS and MDA of the breast muscle in broilers in the HS group(P<0.05)and significantly decreased the activity of total antioxidant capacity of the breast muscle in broilers in the HS group(P<0.05).After 7 d of heat exposure,the mRNA expression of avUCP in the breast muscle of broilers in the HS and HS+T groups was significantly lower than that in the NC group(P<0.05),and the mRNA expression of CS in the breast muscle of broilers in the HS and HS+T groups was significantly higher than that in the NC group(P<0.05).After 14 d of heat exposure,the pH45 min and shear force and the mRNA expression of CS in the breast muscle of broilers in the HS group were significantly lower than those in the NC and HS+T groups(P<0.05),and the L*and drip loss and the levels of ROS and MDA in the breast muscle of broilers in the HS group were significantly higher than those in the NC and HS+T groups(P<0.05).Meanwhile,the mRNA expression of nuclear factor erythroid 2-related factor 2(Nrf2),NAD(P)H quinone dehydrogenase 1 and heme oxygenase 1 in the breast muscle of broilers in the HS+T group was significantly higher than that in the NC and HS groups after 14 d of heat exposure(P<0.05).In addition,after 7 d and 14 d of heat exposure,some swelled mitochondria were observed in the breast muscle of broilers in the HS group,and the mitochondria in the breast muscle of broilers in the the NC and HS+T groups displayed structural integrity and rich cristae in the mitochondria.As mentioned above,the conclusions are as follows:(1)Chronic heat stress can increase the cloacal temperature and respiratory rate,decrease the growth performance,and change many metabolic pathways,mainly including amino acid metabolism and energy substance metabolism and regulation pathways,in broilers.(2)Chronic heat stress can decrease the percentage of breast muscle,increase the percentage of abdominal fat and the organ index of liver,and result in lipid accumulation in the liver in broilers,which is probably because chronic heat exposure increased corticosterone secretion and resulted in hyperglycemia and hyperinsulinemia in broilers,activated LXRa pathway,and enhanced fat synthesis in the liver,and then the excessive TG cannot be transported to extrahepatic tissues and deposit in the liver.(3)Chronic heat stress in the initial period can induce oxidative damage in the breast muscle of broilers,and impact the structure and function of mitochondria,and then enhance the lipid content and glycolysis in the breast muscle,and result in poor meat quality,including lower pH and shear force and higher L*and drip loss.(4)Dietary 0.5%taurine supplementation cannot enhance the growth performance of chronic heat-stressed broilers,but can decrease fat synthesis in the liver by suppressing the LXRa pathway in chronic heat-stressed broilers,and then decrease the lipid deposition and organ index of liver,and alleviate the liver damage.(5)Dietary 0.5%taurine supplementation can alleviate oxidative damage of breast muscle in chronic heat-stressed broilers,and improve the structural integrity of the mitochondria,and then enhance the meat quality of breast muscle,resulting in higher pH45min and shear force and lower L*and drip loss.