Effects Of Heat Stress On Intestinal Health And HPA Axis In Growing Pigs

High environmental temperature in summer can lead to heat stress of animals and thus affect the economic efficiency of the livestock industry,mainly manifest feed intake reduction,growth and feed conversion rate slowing,disease resistance weakened and even death.The intestine is the main site of nutrient digestion and absorption,and the first barrier to external immune defense to the organism,and it is also the most likely target organ to be affected by heat stress.However,there are few studies on the effects and underlying mechanisms of heat stress on intestinal health and feed intake of growing pigs.Therefore,this paper investigated to reveal the effects and underlying mechanisms of heat stress on intestinal health and feed intake of growing pigs.In this study,a total of 24 growing pigs(Landrace ×Large White ×Duroc)weighing about 34.14 ± 0.40 kg were randomized allocated to one of three treatment groups according to body weight.The groups were that: 1)Thermal neutral group with ad libitum(TN,25±1℃);2)Heat stress group with ad libitum(HS,35±1℃);3)Pair-fed group(the same environmental temperature as TN,the same of feed intake as the HS group on the previous 1 day,PF),respectively.The study was lasted for 7 d.At the beginning of the experiment,the environmental temperature of HS group rose rapidly from 25.0 ℃ to 35.0±1.0 ℃,and then the temperature was hold for 24 hours.Serum sample was collected on the 1st and 7th day of the experiment,and pigs were slaughtered and sampled at the end of the experiment.The main results were as follows:(1)Compared with the TN group,the average daily water intake,average daily weight gain and average daily feed intake of growing pigs in HS and PF groups were significantly decreased(P<0.05),and the water-feed ratio was significantly increased(P<0.05).The content of adrenaline,cortisol and insulin-like growth factor-1 in the serum of HS group was significantly increased(P<0.05);on day 7 of the study,serum cortisol content were significantly reduced in the HS group(P<0.05).(2)Compared with the TN group,the trypsin activity in the jejunum of HS group was significantly increased(P<0.05),while the lipase activity in the jejunum and ileum was significantly decreased(P<0.05).Compared with the TN group,villi height/crypt depth,the villi height of the ileum,jejunum and duodenum,and jejunual crypt depth were decreased(P <0.05)in the HS group.Compared with the TN group,the protein expression of HSP70 in the duodenum,jejunum,ileum and cecum,ZO-1 in the duodenum and ileum,Occludin in the ileum and cecum was significantly improved(P<0.05),but Claudin-1 in the ileum and cecum was significantly decreased(P<0.05)in the HS group.(3)Compared with the TN group,the m RNA expression of Mucin 1,Mucin 2,PG1-5,TLR2,TNF-α,IL-6,IL-8 and protein expression of p65 were significantly increased(P<0.05)in the ileum mucosa of the HS group.Meanwhile,the m RNA expression of HSP27 in the ileum mucosa and liver,and the m RNA expression of HIF-1α in the ileum mucosa genes were significantly increased(P<0.05)in the HS group.However,the m RNA expression of HSP90 in hypothalamus was significantly decreased(P<0.05)in the HS group.(4)Compared with the TN group,the contents of acetic acid,propionic acid,butyric acid,valeric acid,isovaleric acid and total short-chain fatty acids in the rectal contents of the HS group were significantly reduced(P<0.05).Moreover,heat stress changed the microbial diversity.Compared with the TN group,the abundance of Proteobacteria(phylum),Gammaproteobacteria(phylum),Pseudomonadales(order),Moraxellaceae(family)and Acinetobacter(genus)were significantly increased(P<0.05)in the HS group.The abundance of Bacteroidetes(phylum),Bacteroidia(order),Bacteroidales(order)and Prevotellaceae(family)were significantly decreased(P<0.05)in the HS group.The results of functional predictive analysis showed that heat stress negatively affected nutrient metabolic pathways such as peptidases,amino acid metabolism,starch and sucrose metabolism.The metagenomic sequencing results of cecum microbes showed that the functions related with energy production and conversion,chromosome structure and dynamics,amino acid transport and metabolism,energy production and conversion,RNA processing and modification were significantly improved,and the nuclear structure function was significantly depressed in the HS group compared with the TN group.(5)Compared with the TN group,the m RNA expression of CCK,GIP,and PYY in the ileum was significantly increased(P<0.05)in the HS group,and the m RNA expression of the hypothalamus lep R,POMC,MC3 R,MC4R,and AMPKα1 was significantly decreased(P<0.05)in the HS group.Compared with the TN group,HS mainly affected hypothalamus alanine,aspartate,glutamine and arginine biosynthesis,pituitary metabolic pathway and secondary metabolite biosynthesis,and adrenal PI3K-Akt pathway.Proteomic changes in the HPA axis during heat stress suggest that heat stress mainly affects energy metabolism,immune-related pathways.In summary,acute heat stress leads feed intake reduced,and digestive and absorption functions and immune functions of the intestinal tract damaged,and leads to changes in the intestinal microbiota structure,and causes the intestinal microbiota disturbance,which seriously hazards the intestinal health of growing pigs.Reduction of feed intake due to heat stress is associated with the m RNA expression of appetite related gene in the gut and hypothalamus.