Study On Optimal Dietary Selenium Level,Bioavailability And Absorption Regularity In Small Intestine For Broilers Fed A Practical Diet

In this study,three series of experiments were conducted to study the optimal selenium(Se)level,the bioavailabilities of different Se sources for broilers aged 1-21 days fed a practical diet and the absorption regularity and mechanism of Se in the small intestine.A completly randomized one factorial desigh was abdopted in experiment 1 to determine the optimal dietary Se level of 1-21-d-old broilers fed a corn-soybean meal diet.384 1-d-old male broilers(n=8 replicates/diet)were fed a basal corn-soybean meal diet or the basal diet supplemented with 0.1,0.2,0.3,0.4 or 0.5 mg Se/kg in the form of Na₂Se O₃ for 21 d.Regression analysis was conducted to evaluate the optimal dietary Se level using broken-line,quadratic or asymptotic models.The activity of glutathione peroxidase(GPX)in plasma,liver,kidney and pancreas,iodothyronine deiodinase(DIO)in plasma,liver and pancreas,and thioredoxin reductase(TXNRD)in liver,kidney and pancreas,the m RNA levels of Gpx1,Gpx4,Dio1,selenoprotein(Seleno)h,Selenop and Selenou in liver,Gpx4,Dio1,Txnrd1,Txnrd2,Selenoh,Selenop and Selenou in kidney,and Gpx1,Gpx4,Selenoh and Selenou in pancreas,and the protein levels of GPX4 in liver and kidney of broilers were influenced(P<0.006)by added Se level,and increased quadratically(P<0.04)with the increase of added Se level.The estimates of optimal dietary Se levels were 0.07-0.36 mg/kg based on the fitted broken-line,quadratic or asymptotic models(P<0.0005)of the aforementioned selenoprotein expression in plasma,liver and kidney,and 0.09-0.46 mg/kg based on the fitted broken-line models(P<0.0001)of the aforementioned selenoprotein expression in pancreas.The results indicate that the optimal dietary Se levels would be0.36 mg/kg to support the full expression of selenoproteins in plasma,liver and kidney,and 0.46 mg/kg to support the full expression of selenoproteins in pancreas of broilers fed a practical corn-soybean meal diet from 1 to 21 d of age.A completly randomized 5×3+1 two factorial desigh was abdopted in experiment 2 to evaluate relative bioavailabilities of different Se sources for 1-21-d-old broilers fed a corn-soybean meal diet.634 1-d-old male Arbor Acres broilers were randomly assigned to 16 treatments with 6 replicates per treatment involving 5 Se sources(sodium selenite(SS),yeast selenium(SY),selenomethionine(SM),selenomethionine hydroxyl analogue(SO)and nano selenium(NS))and 3 added Se levels(0.15,0.30and 0.45 mg Se/kg)sharing a Se-unsupplemented control diet for 21 d.The results showed that dietary supplementated Se significantly increased Se contents in plasma,erythrocytes,liver,pancreas,kidney and breast muscle,GPX activities in plasma,liver,pancreas,kidney and breast muscle,and m RNA expression levels of Gpx1,GPx4,Selenou,Selenop and Dio1 in liver and Gpx1,GPx4 and Selenou in pancreas of 21-d-old broilers(P<0.05).The Se contents in erythrocytes,liver,pancreas and breast muscle and GPX activities in kidney and pancreas of 21-d-old broilers could be used as sensitive indexes to evaluate the bioavailabilities of different Se sources.When the response to SS was set at100%,the average relative bioavailabilities of SM,SY,SO and NS were 259%(P<0.03)?249%(P<0.01)?229%(P<0.005)and 48.4%(P<0.0001)based on the above indices,respectively.In general,the order of bioavailabilities of different Se sources to broilers was SM>SY>SO>SS>NS.Experiment 3 was conducted with broilers to study the kinetics and mechanism of Se absorption in ligated small intestinal segments.A completly randomized one factorial desigh was used in this study.Firstly,the Se absorption in duodenal,jejunal and ileal segments at different post-perfusion time points(0,20,40,60,80,100 and 120 min)were compared.Secondly,a kinetic study of Se absorption was conducted with the duodenal,jejunal and ileal loops perfused with solutions containing 0,0.0375,0.075,0.15,0.30,or 0.60?g/m L of Se as sodium selenite,and Se contents in perfusates were determined at100 min after perfusion.At last,the duodenal loops were perfused with solutions containing 0,0.15,or0.30?g/m L of Se as sodium selenite,and the proteomic analysis based on tandem mass tags(TMT)was used to screen differentially expressed proteins(DEPs)in the chick duodenal mucosa between Se O₃^2-supplemental groups and the control group,and parallel reaction monitoring(PRM)and RT-PCR techniques were performed to validate the DEPs of interest.The results indicated that there was an asymptotic increase(P<0.0001)on Se absorption after perfusion within 120 min in all the small intestinal segments,but increased linearly(P<0.0001)in less than 100 min after perfusion in duodenal and ileal segments,while more than 96.0%of the maximum Se absorption occurred at 100 min after perfusion in each small intestinal segment.There was no difference(P>0.05)in the Se absorption rate among different ligated small intestinal segments perfused with solutions containing 0.0375-0.15?g/m L of Se,however,the Se absorption rate was higher(P<0.05)in the jejunum than that in the duodenum perfused with solutions containing 0.30-0.60?g/m L of Se.The kinetic curves of Se absorption demonstrated that Se was mainly absorbed by saturated carriers in the duodenum,and the highest absorption rate was 1271 pg/min/cm;whereas the Se absorptions were mainly a non-saturated diffusion process in the jejunum and ileum,and the diffusive constants were 2107 and 1777 cm²/min,respectively.Meanwhile,we acquired a total of 4164 measurable DEPs through proteomics analysis and abtained 21DEPs according to their functions and traits.The further confirmed F1P3D8(Solute carrier family 25member 12),F1NTZ0(Alcohol dehydrogenase 6)and F1NXH1(Uncharacterized protein)by PRM and RT-PCR methods possibly participated in the carrier-mediated Se absorption process in the duodenum.The results from the present study indicate that the jejunum is the main Se absorption site,followed by ileum and duodenum,and the Se absorption is mainly a saturated carrier-mediated process in the duodenum,but mainly a non-saturated diffusion process in the jejunum and ileum of broilers.In conclusion,the optimal dietary Se levels would be 0.36 mg/kg to support the full expression of selenoproteins in plasma,liver and kidney,and 0.46 mg/kg to support the full expression of selenoproteins in pancreas of broilers fed a practical corn-soybean meal diet from 1 to 21 d of age,which were about 2-3 times higher than that recommended by NRC(1994)and 1.1-1.5 times higher than that recommended by China chicken breeding standard(2004).The bioavailability order of different Se sources for broilers was as follow:SM(259%)>SY(249%)>SO(229%)>SS(100%)>NS(48.4%).It is suggested that jejunum was the main absorption site of Se in broiler,followed by ileum and duodenum and the absorption of Se was mainly a saturated carrier-mediated process in the duodenum,while mainly an unsaturated diffusion process in jejunum and ileum in chicks.The above results are of great significance to the effective addition and utilization of Se in the diet,and the healthy and efficient production of broilers.