Effects Of Dietary Protein Supplies On Meat Quality And Intestinal Physiology Of Pigs

This study aims to investigate the effects of dietary protein supplies on meat quanlity,nitrogen excretion,pool size of amino acid,as well as intestinal physiology in pigs.In Exp.1,we designed a 2×2 factorial arrangement with two sources of dietary proteins(cottonseed meal vs.soybean meal)and two levels of crude protein(CP,12%vs.14%,as-fed basis).Seventy-two crossbred gilts(89.5 ± 0.9 kg)were allotted to one of four dietary treatments in a randomized complete block design for a period of 28 d.All diets were formulated to be isoenergetic and similar concentrations of essential amino acids(EAA)covering the nutrient requirements of pigs.The results showed that dietary crude protein level did not altered average daily gain and protein deposition(P>0.10).Gilts offered reduced protein diets had lower muscle pH45 min,N intake,N excretion,and serum urea nitrogen content,whilst improved N efficiency(P<0.05).When gilts offered cottonseed meal diets,average daily gain of gilts tended to inceased(P = 0.08),and the concentrations of total phenylalanine,tryptophan,cysteine and tyrosine(P<0.05)of the longissimus dorsi muscle were decreased.In Exp.2,we conducted two animal experiments.In the first animal experiment,seventy-two gilts weighting 29.9 ± 1.5 kg were allocated to one of the three diets that containing 14,16,or 18%CP for a 28-d trial.All diets were formulated to be isoenergetic and balanced essential amino acid(EAA).The apparent total tract digestibilities of amino acids as well as protein expression of amino acid and peptide transpoters were analyzed.In the second animal experiment,twelve barrows weighting 22.7 ± 1.3 kg were surgically fitted with catheters in the portal and jejunal veins as well as the carotid artery.Barrows were assigned to the 14 or 18%CP diet as described in the first animal experiment for seven days before and after the surgery,respectively.Amino acid concentrations as well as blood flux on the sampling sites(carotid artery,jejunal vein,and portal vein)were analyzed,and the pool size of circulating amino acids,portal vein free amino acid flux and flux rate were calculated.The results showed that protein limitation did not sacrifice body weight gain and protein deposition(P>0.10),however pancreas index was decreased(P<0.05).The apparent total tract digestibilities of lysine,methionine,and valine were linearly increased as the dietary CP level decreased,whereas those of arginine,glutamate,aspartate,glysine,serine,and tyrosine were linearly decreased(P<0.05).The pool size of circulating amino acids except for lysine and threonine,and most amino acid flux through the portal vein were reduced in pigs upon dietary protein limitation(P<0.05).Meanwhile,the expression of peptide transporter 1 was stimulated(P<0.05),but the expression of the neutral and cationic amino acid transporter systems was depressed(P<0.05).In Exp.3,sixteen barrows weighting 13.5 ± 0.5 kg were allocated alternatively to diets that containing 14 or 18%CP for a 28-d trial.Both diets were formulated to be isoenergetic and similar concentrations of EAA.We detected the proteome and mTOR signal pathway of the jejunal mucosae.The results showed that protein intake,average daily gain,and the villus length of jejunum were reduced in pigs upon dietary protein limitation(P<0.05),however,the feed conversion ratio improved(P<0.05),and neither feed intake nor net enegy intake were affected(P>0.10).We identified 5,275 quantitative proteins in which pigs suffered dietary protein limitation.A total of 202 proteins(fold change>1)were taken as differentially expressed proteins and subjected to functional and pathway enrichment analysis to reveal proteomic alterations of the jejunal mucosa.We found that protein/carbohydrate digestion,intestinal mucosal tight junction and cell adhesion molecules,and the immune response to foreign antigens were increased in the jejunal mucosa of the pigs upon protein limitation.In contrast,amino acid transport,innate and auto immunity,as well as cell proliferation and apoptosis were reduced.Furthermore,protein limitation may reduce amino acid transport and cell proliferation through the depression of mTOR pathway.In conclusion,dietary protein supplies alter meat quanlity via changing muscle amino acids profiles in pigs,and reduced protein diets are beneficial to reducing N excretion.Dietary protein limitation depresses intestinal amino acid absorption,shrinks circulating pool size of amino acids,and reduces free amino acid flux and flux rate in portal vein.Importantly,dietary protein limitation exerts profound impacts on small intestine physiology in terms of amino acid digestion and absorption,mucosa immunity,cell renewal,as well as regulation of DNA replication and gene expression in pigs.Thereinto,the mTOR signal pathway seems to play a central role in mediating intestinal physiological functions upon dietary protein limitation through sensing amino acid supply.