Effect of phytase on growth performance, phytate degradation and gene expression of myo-inositol transporters in the small intestine, liver and kidney of 21 day old broilers

Effect of phytase on growth performance, phytate degradation and gene expression of myo-inositol... Abstract An experiment was conducted to evaluate phytase supplementation on growth, phytate degradation, and the gene expression of myo-inositol transporters in 21-day old broilers. Ross 308, male broilers (n = 240) were assigned to one of four diets, with 10 pens/diet and six birds/pen from day one to 21. The diets consisted of a negative control (NC) formulated to meet or exceed Ross 308 nutrient requirements, with the exception of calcium (Ca) and available P (avP), which were reduced by 0.16 and 0.15%, respectively. The NC diet was supplemented with 0, 500, 1,500, or 4,500 units/kg of phytase (FTU) to create four experimental diets. On day 21, all birds per pen were euthanized to obtain digesta and tissue samples for phytate degradation and gene expression. Data were analyzed as an analysis of variance using the fit model platform in JMP v 13.0. The model included phytase and significant means were separated using orthogonal linear and quadratic contrasts. Phytase supplementation increased gain (linear, P < 0.05). Phytate (iP6; quadratic, P < 0.05), phytate ester (iP5, iP4, iP3; quadratic, P < 0.05), and inositol (linear, P < 0.05) concentration in the gizzard was influenced by phytase supplementation. Phytate concentration decreased (linear, P < 0.05), iP5 or iP4 concentration increased and then decreased (quadratic, P < 0.05), and inositol concentration increased (quadratic, P < 0.05) in the ileal digesta as phytase supplementation increased in the diet. There was a tendency for the gene expression of the H+-dependent myo-inositol transporter, HMIT, to increase (linear, P < 0.05) in the ileum as phytase dose increased. Gene expression of the sodium-dependent myo-inositol transporter, SMIT2, increased in the jejunum (quadratic, P < 0.05) as phytase dose increased. Intestinal alkaline phosphatase expression increased (linear, P < 0.05) in the ileum as phytase supplementation increased in the diet. The influence of phytase on phytate, phytate esters, and inositol may influence intestinal alkaline phosphatase activity and the gene expression of myo-inositol transporters in the small intestine. INTRODUCTION Data evaluating the efficacy of phytase in poultry nutrition, to liberate phytate-bound phosphorus are readily available and span a period of more than 50 years (Nelson, 1967; Dersjant-Li et al., 2015). Recent interest in supplementing poultry diets with higher doses of phytase, sometimes referred to as “superdoses” of phytase, has led to further understanding of phytate hydrolysis and reported benefits in feed conversion (Walk et al., 2013, 2014). These benefits are thought to be predominantly associated with the near complete destruction of phytate (iP6) and lower phytate esters (iP5, iP4, iP3) in the proximal gastrointestinal tract, alleviation of their antinutritional properties (Bedford and Walk, 2016), and the provision of myo-inositol (Walk et al., 2014; Cowieson et al., 2015; Lee and Bedford, 2016). Myo-inositol is considered an essential constituent of cellular phosphoinositides and is involved in many cellular functions, such as insulin sensitivity, lipid metabolism, and cell survival, structure, and growth (Huber, 2016). Myo-inositol can be synthesized in the body from glucose, released from cellular phospholipids, and absorbed in the intestinal tract from the diet (Huber, 2016). Free myo-inositol can be actively transported with high efficiency via three co-transport systems, two are sodium dependent (SMIT1 or SLC5A3 and SMIT2 or SLC5A11) and one is proton dependent (HMIT or SLC2A13; Aouameur et al., 2007). Using rabbits and rats, previous studies have demonstrated that the expression of each cotransport system is variable between the tissues; SMIT1 is primarily expressed in the brain and renal medulla, SMIT2 is expressed in the brain, intestine, and renal cortex and HMIT is predominantly expressed in the brain (Aouameur et al., 2007; Huber, 2016) with lower levels found in white and brown adipose tissues and the kidney (Mueckler and Thorens, 2013). The location and expression of these cotransport systems in the various tissues may indicate the importance of myo-inositol on cellular metabolism and function. Evaluation of the expression of myo-inositol cotransport systems in tissues may help to further elucidate the beneficial effects of myo-inositol provision through phytate destruction from superdoses of phytase. Therefore, the objective of this trial was to determine the influence of superdoses of phytase on broiler performance, mineral digestibility, specifically Ca, P, Na and K, the concentration of iP6, iP5, iP4, iP3, iP2, and myo-inositol in the gizzard and ileum, and the expression of the myo-inositol cotransporters in the kidney, liver, and small intestine in 21-day old broilers. MATERIALS AND METHODS All animal care procedures used in this experiment were approved by the Scotland's Rural College Animal Experiment Committee (SRUC) before initiation of the experiment. Animals and Management Practices Two-hundred and forty male Ross 308 commercial broiler chicks were obtained and allocated to four dietary treatments in a randomized complete block design with six chicks per cage and 10 replicate cages per treatment. Birds were housed in thermostatically controlled brooder battery cages with raised-wire floors with a lighting program of 23L:1D from hatch to day 7 and 14L:10D for the remainder of the 21-day trial. Temperature in the battery cages was maintained at 32°C for the first day of the study and decreased to 21°C by day 21. Experimental Diets Chicks were fed one of four dietary treatments that consisted of a low Ca and and available P (avP) basal diet supplemented with 0, 500, 1,500, or 4,500 units/kg of phytase (FTU) (Table 1). The phytase was a third generation microbial phytase (Quantum Blue, AB Vista, Marlborough, Wiltshire, UK) with an expected activity of 5,000 FTU/g. All diets were formulated to meet Ross 308 nutrient recommendations, with the exception of Ca and avP, which were reduced by 0.16 and 0.15%, respectively (Table 1). Titanium dioxide was included in all diets at 0.5% as an indigestible marker to permit calculation of nutrient digestibility by the index method. Access to feed and water was provided ad libitum throughout the 21-d feeding period. Feed was fed in mash form via a feed trough and water was provided via a nipple and cup drinker. Table 1. Formulated and analyzed nutrient composition of the experimental diets (%, as fed). Ingredient  Basal diet   Wheat  61.15   Soybean meal  30.04   Soya oil  4.83   Salt  0.32   Limestone  0.92   Dicalcium phosphate  1.05   Sodium bicarbonate  0.15   Lysine HCl  0.14   DL-Methionine  0.24   Threonine  0.07   Vitamin and trace minerals premix1  0.50   Inert or phytase  0.09   TiO marker  0.50   Total  100.00  Formulated nutrient composition   Crude protein  21.50   ME, kcal/kg  3,100.00   Dry matter  87.30   Ca  0.80   P  0.55   Available P  0.30   Phytate P  0.23   Digestible Met + Cys  0.84   Digestible Lys  1.10   Digestible Thr  0.73   Digestible Val  0.84   Sodium  0.18   Chloride  0.28  Analyzed nutrient composition   Crude protein  22.2   Calcium  0.83   Total phosphorus  0.50   Sodium  0.18  Ingredient  Basal diet   Wheat  61.15   Soybean meal  30.04   Soya oil  4.83   Salt  0.32   Limestone  0.92   Dicalcium phosphate  1.05   Sodium bicarbonate  0.15   Lysine HCl  0.14   DL-Methionine  0.24   Threonine  0.07   Vitamin and trace minerals premix1  0.50   Inert or phytase  0.09   TiO marker  0.50   Total  100.00  Formulated nutrient composition   Crude protein  21.50   ME, kcal/kg  3,100.00   Dry matter  87.30   Ca  0.80   P  0.55   Available P  0.30   Phytate P  0.23   Digestible Met + Cys  0.84   Digestible Lys  1.10   Digestible Thr  0.73   Digestible Val  0.84   Sodium  0.18   Chloride  0.28  Analyzed nutrient composition   Crude protein  22.2   Calcium  0.83   Total phosphorus  0.50   Sodium  0.18  1Supplied the following per kilogram of diet: vitamin A, 5,484 IU; vitamin D3, 2,643 ICU; vitamin E, 11 IU; menadione sodium bisulfite, 4.38 mg; riboflavin, 5.49 mg; d-pantothenic acid, 11 mg; niacin, 44.1 mg; choline chloride, 771 mg; vitamin B12, 13.2 μg; biotin, 55.2 μg; thiamine mononitrate,2.2 mg; folic acid, 990 μg; pyridoxine hydrochloride, 3.3 mg; I, 1.11 mg; Mn, 66.06 mg; Cu, 4.44 mg; Fe, 44.1 mg; Zn, 44.1 mg; Se, 250 μg. View Large Measurements Chicks were weighed and randomly allotted such that average initial group weights were distributed similarly across dietary treatments. Birds were monitored daily for morbidity and mortality throughout the study. Dead or culled birds were recorded and these values were used to adjust FI and FCR according to the number of bird days. At the end of the 21-day feeding period, all birds and feeders were weighed to determine BWG, FI, and calculate FCR. Collection and Analyses On day 21, four birds per cage were euthanized by injection of pentobarbital and gizzard and ileal digesta were collected by gently flushing the entire gizzard contents and the terminal ileum (30 cm proximal to the ileo-cecal junction) with deionized water. The digesta samples were pooled per section per cage and immediately frozen (−20°C) for later analysis. Frozen gizzard and ileal digesta samples were lyophilized and ground using a 1 mm screen prior to mineral and phytate ester analyses. For the quantification of inositol phosphates in gizzard and ileal digesta, freeze dried samples were extracted with 10 mL of 0.5 M HCl for 1 h at 20°C by ultrasonication. The extracts were then centrifuged for 10 minutes at 2,200 × g, and 5 mL of the supernatant was evaporated to dryness in a vacuum centrifuge. The samples were then re-dissolved in 1 mL of distilled, deionized water by ultrasonication for 1 h at 20°C and centrifuged for 15 minutes at 18,000 × g. The resulting supernatant was filtered through a 13-mm syringe filter with a 0.45 μm membrane (GH Polypro Acrodisc, Pall Corporation, Ann Arbor, MI) and placed in a 30 kDa centrifugal filter (Microcon Ultracel YM-30, Millipore Corporation, Bedford, MA) and finally centrifuged for 30 minutes at 9,000 × g. Quantification of inositol phosphates (iP2 to iP6) was performed using high-performance ion chromatography and UV detection at 290 nm after post-column derivitization according to methods of Blaabjerg et al. (2010). Myo-inositol was determined using high-performance liquid chromatography with pulsed amperometric detection. Titanium dioxide concentrations of diet and ileal digesta were determined following the procedures of Short et al. (1996). Duplicate samples were weighed into crucibles, dried at 105°C for 24 h, and subsequently ashed at 550°C for 24 h. The ashed samples were then dissolved in 7.4 M sulfuric acid. Hydrogen peroxide (30% vol./vol.) was subsequently added to produce a yellow color with an intensity proportional to the titanium dioxide concentration in each sample. Duplicate aliquots of these sample solutions were analyzed using a UV spectrophotometer by measuring the absorbance at 410 nm. Calcium, total P, Na, and K were analyzed in the diet and ileal digesta samples using inductively coupled plasma—optical emission spectroscopy (AOAC Method 990.08; AOAC, 2006) following digestion, in turn, in concentrated HNO3 and HCl. Apparent nutrient digestibility (AND, %) was calculated according to the following equation: AND = [1− [(Mi/Mo) × (Xo/Xi)] *100,  where Mi = concentration of TiO2 (marker) of the diet sample,  Mo = concentration of TiO2 (marker) of the ileal digesta,  Xo = nutrient concentration of the ileal digesta sample,  Xi = nutrient concentration of the diet sample. The remaining 2 birds per cage were euthanized by a lethal injection of pentobarbital to permit collection of tissue samples. An incision was made below the sternum to expose the abdominal cavity as previously described (Olukosi and Dono, 2014). The entire liver and kidney and sections of the jejunum and ileum were collected from each bird, stored in RNA later, and frozen until PCR analyses. The genes analyzed in the liver and kidney were sodium/glucose cotransporter 11 (SLC5A11 or SMIT2); sodium myo-inositol cotransporter (SLC5A3 or SMIT1) and H+/myo-inositol transporter (SLC2A13 or HMIT). The genes analyzed in the intestine were the three listed previously as well as intestinal alkaline phosphatase (ALPI). RNA were extracted from the tissues and total RNA (5 μL) was reverse transcribed onto cDNA using 20 μL RT premix (PrimerDesign, Southampton, UK). The reaction was performed at 55°C for 20 min and 72°C for 10 min. The Gallus gallus gene-specific primers for all the genes of interest (Table 2) were designed by PrimerDesign (Southampton, UK). Table 2. GenBank accession number, sequences of forward and reverse primers and fragments sizes used for real-time PCR. Target  Accession  Primer sequence  Size    number    (bp)  SLC5A11  XM_0,152,9447  F: 5΄-ATGACCATCCCGTCCCTGT-3΄ R: 5΄-CCTTGGCGTGTGAGAGGTT-3΄  88  SLC5A3  000282  F: 5΄-GGCTGTACTTCGTGCTTGTAAT-3΄ R: 5΄-CCTGCCAAGAAGTAGCCACT-3΄  88  SLC2A13  XM_001,23293  F: 5΄-CATCTATGACAGTGCCTGTGTAC-3΄ R: 5΄-CTCCAGTGATGAACAGAGTGTTAAT-3΄  93  ALPI  XM_0,152,9148  F: 5΄-AGTCACTTCTCCCTGACTCTG-3΄ R: 5΄-GCCTTCTGTGTCCATGAAGC-3΄  84  GAPDH  NM_204,305  F: 5΄-CCCCA CTCCAATTTCTTC-3΄ R: 5΄-CAGATGGTGAACACTTTTATTGATG-3΄.  105  Target  Accession  Primer sequence  Size    number    (bp)  SLC5A11  XM_0,152,9447  F: 5΄-ATGACCATCCCGTCCCTGT-3΄ R: 5΄-CCTTGGCGTGTGAGAGGTT-3΄  88  SLC5A3  000282  F: 5΄-GGCTGTACTTCGTGCTTGTAAT-3΄ R: 5΄-CCTGCCAAGAAGTAGCCACT-3΄  88  SLC2A13  XM_001,23293  F: 5΄-CATCTATGACAGTGCCTGTGTAC-3΄ R: 5΄-CTCCAGTGATGAACAGAGTGTTAAT-3΄  93  ALPI  XM_0,152,9148  F: 5΄-AGTCACTTCTCCCTGACTCTG-3΄ R: 5΄-GCCTTCTGTGTCCATGAAGC-3΄  84  GAPDH  NM_204,305  F: 5΄-CCCCA CTCCAATTTCTTC-3΄ R: 5΄-CAGATGGTGAACACTTTTATTGATG-3΄.  105  SLC5A11 = SMIT2 (sodium/glucose cotransporter 11). SLC5A3 = SMIT1 (sodium/myo-inositol cotransporter). SLC2A13 = HMIT (H+/myo-inositol transporter). ALPI = intestinal alkaline phosphatase. GAPDH, glyceraldehyde 3-phosphate dehydrogenase. View Large Quantitative real-time PCR was performed using Stratagene Mx3005p (Agilent Techhonologies, UK). A total 1 μL of each primer/probe mix was combined with 10 μL Precision 2 × Mastermix and 4 μL PCR water (all from PrimerDesign, Southampton, UK) and 5 μL diluted cDNA was used in each reaction. All PCR were performed in duplicate in Stratagene PCR plates (Agilent Techhonologies, UK) under the following conditions: 95°C for 10 min, 40 cycles of 95°C for 15 s, and 60°C for 1 min. Relative target gene expression level was determined by the comparative cycle threshold (CT) method (Livak and Schmittgen, 2001). Glyceraldehyde-3-phosphate dehydrogenase gene (GAPDH) was used to normalize variations in the amount of mRNA for the target genes. The ΔCT value was calculated as the difference between the CT value of each GAPDH and the average CT value for GAPDH, this value was used to calculate GAPDH fold (i.e., ΔCT1.97). The same mathematical treatment was done for the CT value of the target genes and these values were normalized against the value for GAPDH. Statistical Analyses Cage served as the experimental unit for all parameters. Performance, apparent ileal digestibility, and phytate and phytate ester data are presented as least square means per treatment group. Gene expression data are presented as the relative fold change when compared to the housekeeping gene, GADPH. All data were analyzed as analysis of variance using the fit model platform in JMP Pro v 13.0 (SAS Institute, Cary, NC). The model included phytase and means were separated using linear and quadratic orthogonal polynomial contrasts. Statistical significance was considered when P ≤ 0.05 and trends discussed at P ≤ 0.10. RESULTS Phytase activity recovered in the experimental diets was higher than expected at <50, 907, 2,050, and 6,120 FTU/kg for 0, 500, 1,500 and 4,500 FTU/kg, respectively. Overall mortality was 5%, influenced by phytase dose (quadratic, P < 0.05; data not shown), and predominantly associated with yolk sac infection in the first week post-hatch. Analyzed total P, Ca, Na, and CP are presented in Table 1 and were within the expected levels for all the diets. Overall feed intake or feed conversion ratio were not influenced by phytase dose (Table 3). Body weight gain from hatch to 21-days post-hatch increased (linear, P < 0.05) as phytase dose increased from 0 to 4,500 FTU/kg (Table 3). Table 3. Growth performance of broilers fed phytase from hatch to 21-days post-hatch. Phytase, FTU/kg  Feed intake, g  Weight gain, g  FCR, g:g  0  938.0  581.6  1.635  500  887.3  603.0  1.500  1,500  978.3  641.3  1.552  4,500  949.0  675.1  1.410  SEM  30.6  22.6  0.09  P-values   Phytase  0.483  0.012  0.351    Linear  0.251  0.003  0.151    Quadratic  0.729  0.783  0.969  Phytase, FTU/kg  Feed intake, g  Weight gain, g  FCR, g:g  0  938.0  581.6  1.635  500  887.3  603.0  1.500  1,500  978.3  641.3  1.552  4,500  949.0  675.1  1.410  SEM  30.6  22.6  0.09  P-values   Phytase  0.483  0.012  0.351    Linear  0.251  0.003  0.151    Quadratic  0.729  0.783  0.969  Means are based on 6 birds per pen and 10 replicate pens per diet. View Large Apparent ileal digestibility of dry matter (linear, P < 0.05) and Ca (quadratic, P < 0.05) decreased and P (quadratic, P < 0.05) and Na (quadratic, P < 0.05) increased as phytase dose increased from 0 to 4,500 FTU/kg (Table 4). Increasing phytase dose from 0 to 4,500 FTU/kg decreased (quadratic, P < 0.05) the concentration of iP6 and iP5 in the gizzard digesta (Table 5). The concentration of iP4, iP3, and iP2 in the gizzard digesta increased and then decreased (all quadratic, P < 0.05) as phytase supplementation in the diet increased from 0 to 4,500 FTU/kg (Table 5). Inositol concentration increased (linear, P < 0.05) in the gizzard digesta as phytase supplementation increased in the diet (Table 5). In the ileal digesta, the concentration of iP6 (linear, P < 0.05) decreased and iP5 (quadratic, P < 0.05) and iP4 (quadratic, P < 0.05) increased and then decreased as phytase dose increased in the diets (Table 6). In contrast to the other phytate esters, the concentration of iP3 (linear, P < 0.05) and inositol (quadratic, P < 0.05) increased as phytase dose in the diet increased to 4,500 FTU/kg (Table 6). There was no effect of phytase dose on the concentration of iP2 in the ileal digesta. Table 4. Apparent ileal nutrient digestibility of broilers fed phytase from hatch to 21-days post-hatch. Phytase, FTU/kg  Dry matter, %  Ca, %  P, %  K, %  Na, %  Na, g/kg DMI1  0  73.60  69.87  69.41  88.48  −15.98  0.25  500  71.24  56.82  66.95  86.15  −27.66  0.25  1500  68.92  56.26  73.95  86.08  −19.30  0.24  4500  70.24  59.43  81.42  85.89  −0.20  0.21  SEM  0.97  1.79  1.83  1.08  7.31  0.02  P-values   Phytase  0.005  <0.001  <0.001  0.196  0.038  0.314    Linear  0.006  <0.001  <0.001  0.136  0.091  0.191    Quadratic  0.066  <0.001  0.010  0.329  0.043  0.423  Phytase, FTU/kg  Dry matter, %  Ca, %  P, %  K, %  Na, %  Na, g/kg DMI1  0  73.60  69.87  69.41  88.48  −15.98  0.25  500  71.24  56.82  66.95  86.15  −27.66  0.25  1500  68.92  56.26  73.95  86.08  −19.30  0.24  4500  70.24  59.43  81.42  85.89  −0.20  0.21  SEM  0.97  1.79  1.83  1.08  7.31  0.02  P-values   Phytase  0.005  <0.001  <0.001  0.196  0.038  0.314    Linear  0.006  <0.001  <0.001  0.136  0.091  0.191    Quadratic  0.066  <0.001  0.010  0.329  0.043  0.423  Means are based on 4 birds per pen and 10 replicate pens per diet. View Large Table 5. Phytate, phytate esters and inositol concentration (umol/g DM) in the gizzard digesta of broilers fed phytase from hatch to 21-days post hatch. Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  0.936  1.448  0.319  0.920  1.918  4.296  8.902  500  1.353  1.807  0.753  1.845  0.683  0.463  5.290  1,500  1.542  1.761  0.765  0.610  0.030  0.063  3.228  4,500  2.305  1.635  0.635  0.386  0.014  0.050  2.719  SEM  0.090  0.072  0.066  0.143  0.159  0.227  0.298  P-values   Phytase  <0.001  0.003  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.161  0.004  <0.001  <0.001  <0.001  <0.001    Quadratic  0.062  0.002  <0.001  <0.001  <0.001  <0.001  <0.001  Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  0.936  1.448  0.319  0.920  1.918  4.296  8.902  500  1.353  1.807  0.753  1.845  0.683  0.463  5.290  1,500  1.542  1.761  0.765  0.610  0.030  0.063  3.228  4,500  2.305  1.635  0.635  0.386  0.014  0.050  2.719  SEM  0.090  0.072  0.066  0.143  0.159  0.227  0.298  P-values   Phytase  <0.001  0.003  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.161  0.004  <0.001  <0.001  <0.001  <0.001    Quadratic  0.062  0.002  <0.001  <0.001  <0.001  <0.001  <0.001  Means are based on 4 birds per pen and 10 replicate pens per diet. 1Inositol bisphosphate. 2Inositol triphosphate. 3Inositol tetraphosphate. 4Inositol pentaphosphate. 5Inositol hexakisphosphate (phytate, phytic acid). 6Sum of iP2 to iP6 concentration. View Large Table 6. Phytate, phytate esters and inositol concentration (umol/g DM) in the ileal digesta of broilers fed phytase from hatch to 21-days post hatch. Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  6.820  7.269  0.231  0.820  2.704  30.325  41.349  500  8.307  7.529  0.499  2.055  4.076  23.519  37.677  1,500  11.489  7.040  0.726  2.504  2.933  12.475  25.678  4,500  15.594  7.341  0.796  1.553  0.560  2.757  12.927  SEM  0.644  0.345  0.082  0.271  0.322  1.973  2.503  P-values   Phytase  <0.001  0.950  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.754  <0.001  0.032  <0.001  <0.001  <0.001    Quadratic  0.050  0.953  0.238  <0.001  <0.001  0.466  0.078  Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  6.820  7.269  0.231  0.820  2.704  30.325  41.349  500  8.307  7.529  0.499  2.055  4.076  23.519  37.677  1,500  11.489  7.040  0.726  2.504  2.933  12.475  25.678  4,500  15.594  7.341  0.796  1.553  0.560  2.757  12.927  SEM  0.644  0.345  0.082  0.271  0.322  1.973  2.503  P-values   Phytase  <0.001  0.950  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.754  <0.001  0.032  <0.001  <0.001  <0.001    Quadratic  0.050  0.953  0.238  <0.001  <0.001  0.466  0.078  Means are based on 4 birds per pen and 10 replicate pens per diet. 1Inositol bisphosphate. 2Inositol triphosphate. 3Inositol tetraphosphate. 4Inositol pentaphosphate. 5Inositol hexakisphosphate (phytate, phytic acid). 6Sum of iP2 to iP6 concentration. View Large The relative changes in the gene expression of inositol transporters and intestinal alkaline phosphatase in the jejunum and ileum are presented in Table 7. In the jejunum, increasing phytase dose from 0 to 4,500 FTU/kg up-regulated the relative expression of SLC5A11 (quadratic, P < 0.05), tended to up-regulate the relative expression of SLC5A3 (quadratic, P = 0.10), and there was a tendency (P < 0.10) for phytase to up-regulate the relative expression of SLC2A13. There was no effect of phytase dose on the relative expression of iALP in the jejunum. In the ileum, the effect of phytase dose approached significance (P < 0.06) towards an up-regulation of the relative expression of SLCA13 (linear, P < 0.05) and significantly increased the expression of iALP (linear, P < 0.05). There was no effect of phytase dose on the relative expression of SLC5A11 or SLC5A3 in the ileum. The effect of phytase dose on gene expression of myo-inositol transporters in the liver and kidney were not significant (Table 8). Table 7. Expression of genes in the small intestine mucosa of broilers fed phytase from hatch to 21-days post hatch.   >Jejunum  >Ileum  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  iALP4  SLC5A111  SLC5A32  SLC2A133  iALP4  0  0.661  0.923  0.962  0.949  1.218  0.994  0.975  0.932  500  1.147  0.804  0.834  1.041  1.344  0.846  0.837  1.463  1,500  3.094  1.455  1.476  1.140  1.096  0.636  0.975  1.036  4,500  2.890  1.232  1.331  1.159  1.128  1.139  1.452  2.295  SEM  0.38  0.20  0.17  0.16  0.17  0.11  0.12  0.23  P-values   Phytase  0.002  0.149  0.090  0.703  0.760  0.263  0.055  0.027    Linear  0.003  0.236  0.314  0.521  0.897  0.206  0.007  0.009    Quadratic  0.017  0.101  0.112  0.703  0.731  0.146  0.965  0.265    >Jejunum  >Ileum  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  iALP4  SLC5A111  SLC5A32  SLC2A133  iALP4  0  0.661  0.923  0.962  0.949  1.218  0.994  0.975  0.932  500  1.147  0.804  0.834  1.041  1.344  0.846  0.837  1.463  1,500  3.094  1.455  1.476  1.140  1.096  0.636  0.975  1.036  4,500  2.890  1.232  1.331  1.159  1.128  1.139  1.452  2.295  SEM  0.38  0.20  0.17  0.16  0.17  0.11  0.12  0.23  P-values   Phytase  0.002  0.149  0.090  0.703  0.760  0.263  0.055  0.027    Linear  0.003  0.236  0.314  0.521  0.897  0.206  0.007  0.009    Quadratic  0.017  0.101  0.112  0.703  0.731  0.146  0.965  0.265  Means are based on 2 birds per pen and 10 replicate pens per diet. 1SLC5A11 = SMIT2 (sodium/glucose cotransporter 11). 2SLC5A3 = SMIT1 (sodium/myo-inositol cotransporter). 3SLC2A13 = HMIT (H+/myo-inositol transporter). 4iALP = intestinal alkaline phosphatase. View Large Table 8. Expression of genes in the kidney and liver of broilers fed phytase from hatch to 21-days post hatch.   >Kidney  >Liver  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  SLC5A111  SLC5A32  SLC2A133  0  0.971  0.936  1.026  0.948  1.116  1.148  500  1.195  0.708  0.887  0.943  1.157  0.826  1,500  0.912  0.819  0.870  1.396  0.936  0.967  4,500  0.641  0.734  0.744  1.160  0.820  0.924  SEM  0.20  0.09  0.09  0.16  0.13  0.12  P-values   Phytase  0.410  0.234  0.174  0.247  0.334  0.379    Linear  0.170  0.390  0.044  0.846  0.103  0.481    Quadratic  0.921  0.516  0.565  0.069  0.528  0.443    >Kidney  >Liver  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  SLC5A111  SLC5A32  SLC2A133  0  0.971  0.936  1.026  0.948  1.116  1.148  500  1.195  0.708  0.887  0.943  1.157  0.826  1,500  0.912  0.819  0.870  1.396  0.936  0.967  4,500  0.641  0.734  0.744  1.160  0.820  0.924  SEM  0.20  0.09  0.09  0.16  0.13  0.12  P-values   Phytase  0.410  0.234  0.174  0.247  0.334  0.379    Linear  0.170  0.390  0.044  0.846  0.103  0.481    Quadratic  0.921  0.516  0.565  0.069  0.528  0.443  Means are based on 2 birds per pen and 10 replicate pens per diet. 1SLC5A11 = SMIT2 (sodium/glucose cotransporter 11). 2SLC5A3 = SMIT1 (sodium/myo-inositol cotransporter). 3SLC2A13 = HMIT (H+/myo-inositol transporter). View Large DISCUSSION Mortality was 5% and higher than expected due to yolk sac infections and also the small number of chicks in each pen. Each pen contained 6 chicks with 10 replicate pens per diet and the loss of only 3, 3, 6, and 0 birds in the diets containing 0, 500, 1,500, or 4,500 FTU/kg of phytase, respectively, resulted in the quadratic effect of diet on mortality. Growth performance of broilers was 30 to 39% below Ross 308 standards (Ross 308 Broiler Performance Objectives, 2012) and this may be associated with the causative agent of the yolk sac infection slowing growth in the first week post-hatch and the use of mash diets (Kilburn and Edwards, 2001). Body weight gain and P digestibility increased as phytase supplementation increased and this has been previously reported in low avP diets supplemented with 0 to 12,500 FTU/kg (Karadas et al., 2010) or 0 to 24,000 FTU/kg (Cowieson et al., 2006) of phytase indicating further benefits in nutrient digestibility and growth are attainable with higher doses of phytase. Contradictory to previously published research (Walk et al., 2013, 2014), there was no significant effect of phytase dose on FCR in the current trial. Numeric improvements in FCR were noted however, with the highest dose of phytase improving feed efficiency by approximately 14%. Mechanisms by which high doses of phytase elicit beneficial effects on performance are proposed to be related to 1) destruction of the anti-nutritive effects of phytate with generation of more soluble lower phytate esters and 2) generation of myo-inositol (Cowieson et al., 2011). Phytate, phytate ester, and inositol concentrations in the gizzard and ileal digesta in the current experiment would partially support the above proposed mechanisms of superdosing. For example, in the gizzard and ileal digesta the concentration of iP6 decreased and the concentration of iP5 and iP4 increased and then decreased, while iP3 and myo-inositol concentration increased as phytase supplementation increased in the diet and this has been previously reported (Walk et al., 2014; Beeson et al., 2017). Using in vitro models, other authors have reported that phytate, as well as the lower phytate esters, have the capacity to bind minerals and to interfere with pepsin activity, particularly as pH increases, as summarized by Bedford and Walk (2016). While phytate is considered a more potent anti-nutrient than the lower esters, the anti-nutritive effects of these lower esters on minerals (Xu et al., 1992) and pepsin (Yu et al., 2012) requires further consideration, and continued reduction of these phytate esters with high doses of phytase may be a factor contributing to the increase in BWG and numeric improvements in FCR. In addition, the continued destruction of phytate and the lower phytate esters as phytase dose increased also resulted in significant increases in myo-inositol in the gizzard and ileal digesta. Myo-inositol is an important component of cellular phospholipids and is involved in many cellular functions including survival, structure and signaling (Huber, 2016). Previous authors have loosely correlated an increase in myo-inositol concentrations in the gizzard with significant improvements in FCR (Walk et al., 2014). Cowieson et al. (2013) reported supplementation of broiler diets with 0.15% myo-inositol resulted in significant improvements in FCR of 42-day old broilers. Others have also reported significant increases in plasma myo-inositol as phytase supplementation increased in the diet (Cowieson et al., 2015; Laird, 2016). Therefore, it is likely one of the beneficial effects of feeding high doses of phytase would be the provision of myo-inositol through phytate and phytate ester destruction. Free myo-inositol in the gastrointestinal tract is absorbed with great efficiency, 99.8% (Holub, 1986; Croze and Soulage, 2013) by an active, Na-dependent process. Sodium is transported across the brush border together with myo-inositol via SLC5A11 (SMIT2) at a ratio of 2 Na to 1 myo-inositol (Huber, 2016). In the current trial, the expression of SLC5A11 was up-regulated in the jejunum and the apparent ileal Na digestibility was significantly increased as phytase supplementation increased in the diet. In addition, at least in the jejunum, there was also a tendency toward an up-regulation of both SLC5A3 (SMIT1) and SLC2A13 (HMIT) as dietary phytase increased from 0 to 4,500 FTU/kg, but there was no effect on the gene expression of iALP. In contrast, in the ileum there was no effect of phytase on SLC5A11 (SMIT2) or SLC5A3 (SMIT1) and a tendency for a linear increase in SLC2A13 (HMIT) with a significant increase in iALP expression. These results are interesting, especially when considering previous authors reported the expression of SLC5A3 (SMIT1) and SLC2A13 (HMIT) are most noted in the brain and/or kidney with SMIT2 predominantly found in the small intestine (Aouameur et al., 2007; Mueckler and Thorens, 2013; Huber, 2016). However, species differences exist in myo-inositol transporter expression in the tissues, with SMIT2 being expressed in high concentration in the kidney of both rats and rabbits, but barely detectable in the small intestine of rabbits (Aouameur et al., 2007). In the current experiment, the expression of myo-inositol transporters was not measured in the brain and more work is needed to confirm the effects reported in herein, particularly in poultry. Regardless, a few interesting points can be discussed based on the gene expression data, specifically: 1) Myo-inositol appears to be actively transported in the small intestine and transporter expression is influenced by myo-inositol concentration, with an up-regulation of the gene expression of SMIT2 or HMIT in the jejunum and ileum as phytase dose and production of myo-inositol increased. Myo-inositol uptake in the brush border vesicles of rats has been previously reported through SMIT2 with no evidence of uptake from HMIT or SMIT1 (Aouameur et al., 2007). However, as previously mentioned, these same authors reported barely any SMIT2 detection in the rabbit intestine, indicating species differences exist and these results need to be confirmed in subsequent trials. Regardless, it would appear there is an effect of myo-inositol concentration in the intestinal lumen on the up-regulation of transporters in the jejunum of poultry. 2) In the ileum however, only HMIT expression was up-regulated as phytase dose increased. This could also be related to the concentration of myo-inositol present in the ileum but also due to the reduction of Na concentration (as depicted by an increase in Na digestibility) and the concentration and type of soluble lower phytate esters present in the ileal lumen, which subsequently resulted in an up-regulation of intestinal alkaline phosphatase (Schlemmer et al., 2009); all of which resulted in an increase in HMIT, the proton dependent myo-inositol transporter. Previous authors have reported phytase specific activity along the intestinal brush border of broilers and layers, with intestinal phytase activity decreasing from the duodenum to the ileum (Maenz and Classen, 1998). These results may be contradictory to the current trial; however specific phytase activity was not evaluated and effects cannot be compared directly. Furthermore, the iALP expression in the jejunum and the ileum of birds fed 0 FTU phytase/kg diet was 0.949 vs. 0.932, respectively and phytase had a significant effect in the ileum, suggesting the response to HMIT and iALP in the ileum was associated with lower phytate esters and the production of inositol by iALP. Interestingly, even by the terminal ileum the concentration of inositol was remarkably high (53% of the total), indicating there is a rate-limiting step in inositol absorption within the gastrointestinal tract. This is contradictory to previous estimates of free myo-inositol absorption in the human small intestine at around 99.8% (Holub, 1986; Croze and Soulage, 2013). However, the differences may be dependent on the availability of Na and H+ and the location in the GIT. For example, previous authors reported phytase supplementation significantly increased pH in the distal ileum from 6.56 in birds fed 0 FTU/kg phytase to 6.99 in birds fed 5,000 FTU/kg phytase (Walk et al., 2012). Taking the anti-log of these pH values indicates an almost 40% reduction in H+ ion concentration (2.754E-07 vs. 1.023E-07) in the ileum of broilers fed 5,000 FTU/kg of phytase compared with that of broilers fed 0 FTU/kg phytase. In effect, this means that while phytase supplementation increases myo-inositol concentration, it also creates a rate-limiting step in myo-inositol uptake by the ileum by increasing Na digestibility and reducing H+ ions which are needed for co-transport of myo-inositol. The lack of an effect of phytase dose on SMIT1 or SMIT2 support this as they are both Na dependent co-transporters. 3) Finally, notable is the non-significant effect of phytase dose on myo-inositol transporter gene expression in the kidney and liver. Both the liver and kidney play important roles in myo-inositol metabolism and de novo synthesis and the kidney is the main site of myo-inositol excretion (Holub, 1986; Lahjouji et al., 2007; Croze and Soulage, 2013). The lack of an effect of phytase dose may be indicative of a reduced need for endogenous synthesis or excretion of myo-inositol due to the provision of dietary myo-inositol. These results require further evaluation but may be indicative of the pathways and the regulation of myo-inositol provided from phytate destruction in the diet. In conclusion, supplementation of broiler diets with phytase up to 4,500 FTU/kg significantly increased weight gain and resulted in nearly complete phytate and phytate ester destruction and the significant increases in myo-inositol. This influenced and up-regulated the gene expression Na+ of H+-dependent myo-inositol transporters within the jejunum and the ileum, respectively. These results may indicate myo-inositol is predominantly taken up in the broiler proximal small intestine via a Na+-dependent transporter, whereas in the distal intestine phytate esters created from phytate destruction may up-regulate the expression of alkaline phosphatase, which in turn yields myo-inositol and increases the expression of the proton dependent myo-inositol transporter, HMIT. Data from the liver and kidney need further evaluation but may indicate complex pathways in regards to regulation of myo-inositol in tissues beyond the intestine. REFERENCES AOAC. 2006. Official methods of analysis . 18th ed. AOAC Int., Arlington, VA. PubMed PubMed  Aouameur R., Da Cal S., Bissonnette P., Coady M. J., Lapointe J.-Y.. 2007. SMIT2 mediates all myo-inositol uptake in apical membranes of rat small intestine. Am. J. Physiol. Gastrointest. Liver Physiol.  293: G1300– G1307. Google Scholar CrossRef Search ADS PubMed  Bedford M. R., Walk C. L.. 2016. Reduction of phytate to tetrakisphosphate (IP4) to triphosphate (IP3), or perhaps even lower, does not remove its antinutritive properties. 45– 52 in Phytate Destruction – Consequences for Precision Animal Nutrition . Walk C. L., Kuhn I., Stein H. H., Kidd M. T., Rodehutscord M., eds. Wageningen Academic Publishers, the Netherlands. Beeson L. A., Walk C. L., Bedford M. R., Olukosi O. A.. 2017. Hydrolysis of phytate to its lower esters can influence the growth performance and nutrient utilization of broilers with regular or super doses of phytase. Poult. Sci.  doi: 10.3382/ps/pex012. Blaabjerg K., Hanson-Mooler J., Poulsen H. D.. 2010. High-performance ion chromatography for separation and quantification of inositol phosphates in diets and digesta. J. Chromatogr. B Analyst. Technol. Biomed. Life Sci.  878: 347– 354. Google Scholar CrossRef Search ADS   Cowieson A. J., Aureli R., Guggenhuhl P., Fru-Nji F.. 2015. 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Effect of phytase on growth performance, phytate degradation and gene expression of myo-inositol transporters in the small intestine, liver and kidney of 21 day old broilers

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© 2018 Poultry Science Association Inc.
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0032-5791
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10.3382/ps/pex392
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Abstract

Abstract An experiment was conducted to evaluate phytase supplementation on growth, phytate degradation, and the gene expression of myo-inositol transporters in 21-day old broilers. Ross 308, male broilers (n = 240) were assigned to one of four diets, with 10 pens/diet and six birds/pen from day one to 21. The diets consisted of a negative control (NC) formulated to meet or exceed Ross 308 nutrient requirements, with the exception of calcium (Ca) and available P (avP), which were reduced by 0.16 and 0.15%, respectively. The NC diet was supplemented with 0, 500, 1,500, or 4,500 units/kg of phytase (FTU) to create four experimental diets. On day 21, all birds per pen were euthanized to obtain digesta and tissue samples for phytate degradation and gene expression. Data were analyzed as an analysis of variance using the fit model platform in JMP v 13.0. The model included phytase and significant means were separated using orthogonal linear and quadratic contrasts. Phytase supplementation increased gain (linear, P < 0.05). Phytate (iP6; quadratic, P < 0.05), phytate ester (iP5, iP4, iP3; quadratic, P < 0.05), and inositol (linear, P < 0.05) concentration in the gizzard was influenced by phytase supplementation. Phytate concentration decreased (linear, P < 0.05), iP5 or iP4 concentration increased and then decreased (quadratic, P < 0.05), and inositol concentration increased (quadratic, P < 0.05) in the ileal digesta as phytase supplementation increased in the diet. There was a tendency for the gene expression of the H+-dependent myo-inositol transporter, HMIT, to increase (linear, P < 0.05) in the ileum as phytase dose increased. Gene expression of the sodium-dependent myo-inositol transporter, SMIT2, increased in the jejunum (quadratic, P < 0.05) as phytase dose increased. Intestinal alkaline phosphatase expression increased (linear, P < 0.05) in the ileum as phytase supplementation increased in the diet. The influence of phytase on phytate, phytate esters, and inositol may influence intestinal alkaline phosphatase activity and the gene expression of myo-inositol transporters in the small intestine. INTRODUCTION Data evaluating the efficacy of phytase in poultry nutrition, to liberate phytate-bound phosphorus are readily available and span a period of more than 50 years (Nelson, 1967; Dersjant-Li et al., 2015). Recent interest in supplementing poultry diets with higher doses of phytase, sometimes referred to as “superdoses” of phytase, has led to further understanding of phytate hydrolysis and reported benefits in feed conversion (Walk et al., 2013, 2014). These benefits are thought to be predominantly associated with the near complete destruction of phytate (iP6) and lower phytate esters (iP5, iP4, iP3) in the proximal gastrointestinal tract, alleviation of their antinutritional properties (Bedford and Walk, 2016), and the provision of myo-inositol (Walk et al., 2014; Cowieson et al., 2015; Lee and Bedford, 2016). Myo-inositol is considered an essential constituent of cellular phosphoinositides and is involved in many cellular functions, such as insulin sensitivity, lipid metabolism, and cell survival, structure, and growth (Huber, 2016). Myo-inositol can be synthesized in the body from glucose, released from cellular phospholipids, and absorbed in the intestinal tract from the diet (Huber, 2016). Free myo-inositol can be actively transported with high efficiency via three co-transport systems, two are sodium dependent (SMIT1 or SLC5A3 and SMIT2 or SLC5A11) and one is proton dependent (HMIT or SLC2A13; Aouameur et al., 2007). Using rabbits and rats, previous studies have demonstrated that the expression of each cotransport system is variable between the tissues; SMIT1 is primarily expressed in the brain and renal medulla, SMIT2 is expressed in the brain, intestine, and renal cortex and HMIT is predominantly expressed in the brain (Aouameur et al., 2007; Huber, 2016) with lower levels found in white and brown adipose tissues and the kidney (Mueckler and Thorens, 2013). The location and expression of these cotransport systems in the various tissues may indicate the importance of myo-inositol on cellular metabolism and function. Evaluation of the expression of myo-inositol cotransport systems in tissues may help to further elucidate the beneficial effects of myo-inositol provision through phytate destruction from superdoses of phytase. Therefore, the objective of this trial was to determine the influence of superdoses of phytase on broiler performance, mineral digestibility, specifically Ca, P, Na and K, the concentration of iP6, iP5, iP4, iP3, iP2, and myo-inositol in the gizzard and ileum, and the expression of the myo-inositol cotransporters in the kidney, liver, and small intestine in 21-day old broilers. MATERIALS AND METHODS All animal care procedures used in this experiment were approved by the Scotland's Rural College Animal Experiment Committee (SRUC) before initiation of the experiment. Animals and Management Practices Two-hundred and forty male Ross 308 commercial broiler chicks were obtained and allocated to four dietary treatments in a randomized complete block design with six chicks per cage and 10 replicate cages per treatment. Birds were housed in thermostatically controlled brooder battery cages with raised-wire floors with a lighting program of 23L:1D from hatch to day 7 and 14L:10D for the remainder of the 21-day trial. Temperature in the battery cages was maintained at 32°C for the first day of the study and decreased to 21°C by day 21. Experimental Diets Chicks were fed one of four dietary treatments that consisted of a low Ca and and available P (avP) basal diet supplemented with 0, 500, 1,500, or 4,500 units/kg of phytase (FTU) (Table 1). The phytase was a third generation microbial phytase (Quantum Blue, AB Vista, Marlborough, Wiltshire, UK) with an expected activity of 5,000 FTU/g. All diets were formulated to meet Ross 308 nutrient recommendations, with the exception of Ca and avP, which were reduced by 0.16 and 0.15%, respectively (Table 1). Titanium dioxide was included in all diets at 0.5% as an indigestible marker to permit calculation of nutrient digestibility by the index method. Access to feed and water was provided ad libitum throughout the 21-d feeding period. Feed was fed in mash form via a feed trough and water was provided via a nipple and cup drinker. Table 1. Formulated and analyzed nutrient composition of the experimental diets (%, as fed). Ingredient  Basal diet   Wheat  61.15   Soybean meal  30.04   Soya oil  4.83   Salt  0.32   Limestone  0.92   Dicalcium phosphate  1.05   Sodium bicarbonate  0.15   Lysine HCl  0.14   DL-Methionine  0.24   Threonine  0.07   Vitamin and trace minerals premix1  0.50   Inert or phytase  0.09   TiO marker  0.50   Total  100.00  Formulated nutrient composition   Crude protein  21.50   ME, kcal/kg  3,100.00   Dry matter  87.30   Ca  0.80   P  0.55   Available P  0.30   Phytate P  0.23   Digestible Met + Cys  0.84   Digestible Lys  1.10   Digestible Thr  0.73   Digestible Val  0.84   Sodium  0.18   Chloride  0.28  Analyzed nutrient composition   Crude protein  22.2   Calcium  0.83   Total phosphorus  0.50   Sodium  0.18  Ingredient  Basal diet   Wheat  61.15   Soybean meal  30.04   Soya oil  4.83   Salt  0.32   Limestone  0.92   Dicalcium phosphate  1.05   Sodium bicarbonate  0.15   Lysine HCl  0.14   DL-Methionine  0.24   Threonine  0.07   Vitamin and trace minerals premix1  0.50   Inert or phytase  0.09   TiO marker  0.50   Total  100.00  Formulated nutrient composition   Crude protein  21.50   ME, kcal/kg  3,100.00   Dry matter  87.30   Ca  0.80   P  0.55   Available P  0.30   Phytate P  0.23   Digestible Met + Cys  0.84   Digestible Lys  1.10   Digestible Thr  0.73   Digestible Val  0.84   Sodium  0.18   Chloride  0.28  Analyzed nutrient composition   Crude protein  22.2   Calcium  0.83   Total phosphorus  0.50   Sodium  0.18  1Supplied the following per kilogram of diet: vitamin A, 5,484 IU; vitamin D3, 2,643 ICU; vitamin E, 11 IU; menadione sodium bisulfite, 4.38 mg; riboflavin, 5.49 mg; d-pantothenic acid, 11 mg; niacin, 44.1 mg; choline chloride, 771 mg; vitamin B12, 13.2 μg; biotin, 55.2 μg; thiamine mononitrate,2.2 mg; folic acid, 990 μg; pyridoxine hydrochloride, 3.3 mg; I, 1.11 mg; Mn, 66.06 mg; Cu, 4.44 mg; Fe, 44.1 mg; Zn, 44.1 mg; Se, 250 μg. View Large Measurements Chicks were weighed and randomly allotted such that average initial group weights were distributed similarly across dietary treatments. Birds were monitored daily for morbidity and mortality throughout the study. Dead or culled birds were recorded and these values were used to adjust FI and FCR according to the number of bird days. At the end of the 21-day feeding period, all birds and feeders were weighed to determine BWG, FI, and calculate FCR. Collection and Analyses On day 21, four birds per cage were euthanized by injection of pentobarbital and gizzard and ileal digesta were collected by gently flushing the entire gizzard contents and the terminal ileum (30 cm proximal to the ileo-cecal junction) with deionized water. The digesta samples were pooled per section per cage and immediately frozen (−20°C) for later analysis. Frozen gizzard and ileal digesta samples were lyophilized and ground using a 1 mm screen prior to mineral and phytate ester analyses. For the quantification of inositol phosphates in gizzard and ileal digesta, freeze dried samples were extracted with 10 mL of 0.5 M HCl for 1 h at 20°C by ultrasonication. The extracts were then centrifuged for 10 minutes at 2,200 × g, and 5 mL of the supernatant was evaporated to dryness in a vacuum centrifuge. The samples were then re-dissolved in 1 mL of distilled, deionized water by ultrasonication for 1 h at 20°C and centrifuged for 15 minutes at 18,000 × g. The resulting supernatant was filtered through a 13-mm syringe filter with a 0.45 μm membrane (GH Polypro Acrodisc, Pall Corporation, Ann Arbor, MI) and placed in a 30 kDa centrifugal filter (Microcon Ultracel YM-30, Millipore Corporation, Bedford, MA) and finally centrifuged for 30 minutes at 9,000 × g. Quantification of inositol phosphates (iP2 to iP6) was performed using high-performance ion chromatography and UV detection at 290 nm after post-column derivitization according to methods of Blaabjerg et al. (2010). Myo-inositol was determined using high-performance liquid chromatography with pulsed amperometric detection. Titanium dioxide concentrations of diet and ileal digesta were determined following the procedures of Short et al. (1996). Duplicate samples were weighed into crucibles, dried at 105°C for 24 h, and subsequently ashed at 550°C for 24 h. The ashed samples were then dissolved in 7.4 M sulfuric acid. Hydrogen peroxide (30% vol./vol.) was subsequently added to produce a yellow color with an intensity proportional to the titanium dioxide concentration in each sample. Duplicate aliquots of these sample solutions were analyzed using a UV spectrophotometer by measuring the absorbance at 410 nm. Calcium, total P, Na, and K were analyzed in the diet and ileal digesta samples using inductively coupled plasma—optical emission spectroscopy (AOAC Method 990.08; AOAC, 2006) following digestion, in turn, in concentrated HNO3 and HCl. Apparent nutrient digestibility (AND, %) was calculated according to the following equation: AND = [1− [(Mi/Mo) × (Xo/Xi)] *100,  where Mi = concentration of TiO2 (marker) of the diet sample,  Mo = concentration of TiO2 (marker) of the ileal digesta,  Xo = nutrient concentration of the ileal digesta sample,  Xi = nutrient concentration of the diet sample. The remaining 2 birds per cage were euthanized by a lethal injection of pentobarbital to permit collection of tissue samples. An incision was made below the sternum to expose the abdominal cavity as previously described (Olukosi and Dono, 2014). The entire liver and kidney and sections of the jejunum and ileum were collected from each bird, stored in RNA later, and frozen until PCR analyses. The genes analyzed in the liver and kidney were sodium/glucose cotransporter 11 (SLC5A11 or SMIT2); sodium myo-inositol cotransporter (SLC5A3 or SMIT1) and H+/myo-inositol transporter (SLC2A13 or HMIT). The genes analyzed in the intestine were the three listed previously as well as intestinal alkaline phosphatase (ALPI). RNA were extracted from the tissues and total RNA (5 μL) was reverse transcribed onto cDNA using 20 μL RT premix (PrimerDesign, Southampton, UK). The reaction was performed at 55°C for 20 min and 72°C for 10 min. The Gallus gallus gene-specific primers for all the genes of interest (Table 2) were designed by PrimerDesign (Southampton, UK). Table 2. GenBank accession number, sequences of forward and reverse primers and fragments sizes used for real-time PCR. Target  Accession  Primer sequence  Size    number    (bp)  SLC5A11  XM_0,152,9447  F: 5΄-ATGACCATCCCGTCCCTGT-3΄ R: 5΄-CCTTGGCGTGTGAGAGGTT-3΄  88  SLC5A3  000282  F: 5΄-GGCTGTACTTCGTGCTTGTAAT-3΄ R: 5΄-CCTGCCAAGAAGTAGCCACT-3΄  88  SLC2A13  XM_001,23293  F: 5΄-CATCTATGACAGTGCCTGTGTAC-3΄ R: 5΄-CTCCAGTGATGAACAGAGTGTTAAT-3΄  93  ALPI  XM_0,152,9148  F: 5΄-AGTCACTTCTCCCTGACTCTG-3΄ R: 5΄-GCCTTCTGTGTCCATGAAGC-3΄  84  GAPDH  NM_204,305  F: 5΄-CCCCA CTCCAATTTCTTC-3΄ R: 5΄-CAGATGGTGAACACTTTTATTGATG-3΄.  105  Target  Accession  Primer sequence  Size    number    (bp)  SLC5A11  XM_0,152,9447  F: 5΄-ATGACCATCCCGTCCCTGT-3΄ R: 5΄-CCTTGGCGTGTGAGAGGTT-3΄  88  SLC5A3  000282  F: 5΄-GGCTGTACTTCGTGCTTGTAAT-3΄ R: 5΄-CCTGCCAAGAAGTAGCCACT-3΄  88  SLC2A13  XM_001,23293  F: 5΄-CATCTATGACAGTGCCTGTGTAC-3΄ R: 5΄-CTCCAGTGATGAACAGAGTGTTAAT-3΄  93  ALPI  XM_0,152,9148  F: 5΄-AGTCACTTCTCCCTGACTCTG-3΄ R: 5΄-GCCTTCTGTGTCCATGAAGC-3΄  84  GAPDH  NM_204,305  F: 5΄-CCCCA CTCCAATTTCTTC-3΄ R: 5΄-CAGATGGTGAACACTTTTATTGATG-3΄.  105  SLC5A11 = SMIT2 (sodium/glucose cotransporter 11). SLC5A3 = SMIT1 (sodium/myo-inositol cotransporter). SLC2A13 = HMIT (H+/myo-inositol transporter). ALPI = intestinal alkaline phosphatase. GAPDH, glyceraldehyde 3-phosphate dehydrogenase. View Large Quantitative real-time PCR was performed using Stratagene Mx3005p (Agilent Techhonologies, UK). A total 1 μL of each primer/probe mix was combined with 10 μL Precision 2 × Mastermix and 4 μL PCR water (all from PrimerDesign, Southampton, UK) and 5 μL diluted cDNA was used in each reaction. All PCR were performed in duplicate in Stratagene PCR plates (Agilent Techhonologies, UK) under the following conditions: 95°C for 10 min, 40 cycles of 95°C for 15 s, and 60°C for 1 min. Relative target gene expression level was determined by the comparative cycle threshold (CT) method (Livak and Schmittgen, 2001). Glyceraldehyde-3-phosphate dehydrogenase gene (GAPDH) was used to normalize variations in the amount of mRNA for the target genes. The ΔCT value was calculated as the difference between the CT value of each GAPDH and the average CT value for GAPDH, this value was used to calculate GAPDH fold (i.e., ΔCT1.97). The same mathematical treatment was done for the CT value of the target genes and these values were normalized against the value for GAPDH. Statistical Analyses Cage served as the experimental unit for all parameters. Performance, apparent ileal digestibility, and phytate and phytate ester data are presented as least square means per treatment group. Gene expression data are presented as the relative fold change when compared to the housekeeping gene, GADPH. All data were analyzed as analysis of variance using the fit model platform in JMP Pro v 13.0 (SAS Institute, Cary, NC). The model included phytase and means were separated using linear and quadratic orthogonal polynomial contrasts. Statistical significance was considered when P ≤ 0.05 and trends discussed at P ≤ 0.10. RESULTS Phytase activity recovered in the experimental diets was higher than expected at <50, 907, 2,050, and 6,120 FTU/kg for 0, 500, 1,500 and 4,500 FTU/kg, respectively. Overall mortality was 5%, influenced by phytase dose (quadratic, P < 0.05; data not shown), and predominantly associated with yolk sac infection in the first week post-hatch. Analyzed total P, Ca, Na, and CP are presented in Table 1 and were within the expected levels for all the diets. Overall feed intake or feed conversion ratio were not influenced by phytase dose (Table 3). Body weight gain from hatch to 21-days post-hatch increased (linear, P < 0.05) as phytase dose increased from 0 to 4,500 FTU/kg (Table 3). Table 3. Growth performance of broilers fed phytase from hatch to 21-days post-hatch. Phytase, FTU/kg  Feed intake, g  Weight gain, g  FCR, g:g  0  938.0  581.6  1.635  500  887.3  603.0  1.500  1,500  978.3  641.3  1.552  4,500  949.0  675.1  1.410  SEM  30.6  22.6  0.09  P-values   Phytase  0.483  0.012  0.351    Linear  0.251  0.003  0.151    Quadratic  0.729  0.783  0.969  Phytase, FTU/kg  Feed intake, g  Weight gain, g  FCR, g:g  0  938.0  581.6  1.635  500  887.3  603.0  1.500  1,500  978.3  641.3  1.552  4,500  949.0  675.1  1.410  SEM  30.6  22.6  0.09  P-values   Phytase  0.483  0.012  0.351    Linear  0.251  0.003  0.151    Quadratic  0.729  0.783  0.969  Means are based on 6 birds per pen and 10 replicate pens per diet. View Large Apparent ileal digestibility of dry matter (linear, P < 0.05) and Ca (quadratic, P < 0.05) decreased and P (quadratic, P < 0.05) and Na (quadratic, P < 0.05) increased as phytase dose increased from 0 to 4,500 FTU/kg (Table 4). Increasing phytase dose from 0 to 4,500 FTU/kg decreased (quadratic, P < 0.05) the concentration of iP6 and iP5 in the gizzard digesta (Table 5). The concentration of iP4, iP3, and iP2 in the gizzard digesta increased and then decreased (all quadratic, P < 0.05) as phytase supplementation in the diet increased from 0 to 4,500 FTU/kg (Table 5). Inositol concentration increased (linear, P < 0.05) in the gizzard digesta as phytase supplementation increased in the diet (Table 5). In the ileal digesta, the concentration of iP6 (linear, P < 0.05) decreased and iP5 (quadratic, P < 0.05) and iP4 (quadratic, P < 0.05) increased and then decreased as phytase dose increased in the diets (Table 6). In contrast to the other phytate esters, the concentration of iP3 (linear, P < 0.05) and inositol (quadratic, P < 0.05) increased as phytase dose in the diet increased to 4,500 FTU/kg (Table 6). There was no effect of phytase dose on the concentration of iP2 in the ileal digesta. Table 4. Apparent ileal nutrient digestibility of broilers fed phytase from hatch to 21-days post-hatch. Phytase, FTU/kg  Dry matter, %  Ca, %  P, %  K, %  Na, %  Na, g/kg DMI1  0  73.60  69.87  69.41  88.48  −15.98  0.25  500  71.24  56.82  66.95  86.15  −27.66  0.25  1500  68.92  56.26  73.95  86.08  −19.30  0.24  4500  70.24  59.43  81.42  85.89  −0.20  0.21  SEM  0.97  1.79  1.83  1.08  7.31  0.02  P-values   Phytase  0.005  <0.001  <0.001  0.196  0.038  0.314    Linear  0.006  <0.001  <0.001  0.136  0.091  0.191    Quadratic  0.066  <0.001  0.010  0.329  0.043  0.423  Phytase, FTU/kg  Dry matter, %  Ca, %  P, %  K, %  Na, %  Na, g/kg DMI1  0  73.60  69.87  69.41  88.48  −15.98  0.25  500  71.24  56.82  66.95  86.15  −27.66  0.25  1500  68.92  56.26  73.95  86.08  −19.30  0.24  4500  70.24  59.43  81.42  85.89  −0.20  0.21  SEM  0.97  1.79  1.83  1.08  7.31  0.02  P-values   Phytase  0.005  <0.001  <0.001  0.196  0.038  0.314    Linear  0.006  <0.001  <0.001  0.136  0.091  0.191    Quadratic  0.066  <0.001  0.010  0.329  0.043  0.423  Means are based on 4 birds per pen and 10 replicate pens per diet. View Large Table 5. Phytate, phytate esters and inositol concentration (umol/g DM) in the gizzard digesta of broilers fed phytase from hatch to 21-days post hatch. Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  0.936  1.448  0.319  0.920  1.918  4.296  8.902  500  1.353  1.807  0.753  1.845  0.683  0.463  5.290  1,500  1.542  1.761  0.765  0.610  0.030  0.063  3.228  4,500  2.305  1.635  0.635  0.386  0.014  0.050  2.719  SEM  0.090  0.072  0.066  0.143  0.159  0.227  0.298  P-values   Phytase  <0.001  0.003  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.161  0.004  <0.001  <0.001  <0.001  <0.001    Quadratic  0.062  0.002  <0.001  <0.001  <0.001  <0.001  <0.001  Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  0.936  1.448  0.319  0.920  1.918  4.296  8.902  500  1.353  1.807  0.753  1.845  0.683  0.463  5.290  1,500  1.542  1.761  0.765  0.610  0.030  0.063  3.228  4,500  2.305  1.635  0.635  0.386  0.014  0.050  2.719  SEM  0.090  0.072  0.066  0.143  0.159  0.227  0.298  P-values   Phytase  <0.001  0.003  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.161  0.004  <0.001  <0.001  <0.001  <0.001    Quadratic  0.062  0.002  <0.001  <0.001  <0.001  <0.001  <0.001  Means are based on 4 birds per pen and 10 replicate pens per diet. 1Inositol bisphosphate. 2Inositol triphosphate. 3Inositol tetraphosphate. 4Inositol pentaphosphate. 5Inositol hexakisphosphate (phytate, phytic acid). 6Sum of iP2 to iP6 concentration. View Large Table 6. Phytate, phytate esters and inositol concentration (umol/g DM) in the ileal digesta of broilers fed phytase from hatch to 21-days post hatch. Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  6.820  7.269  0.231  0.820  2.704  30.325  41.349  500  8.307  7.529  0.499  2.055  4.076  23.519  37.677  1,500  11.489  7.040  0.726  2.504  2.933  12.475  25.678  4,500  15.594  7.341  0.796  1.553  0.560  2.757  12.927  SEM  0.644  0.345  0.082  0.271  0.322  1.973  2.503  P-values   Phytase  <0.001  0.950  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.754  <0.001  0.032  <0.001  <0.001  <0.001    Quadratic  0.050  0.953  0.238  <0.001  <0.001  0.466  0.078  Phytase, FTU/kg  Inositol  iP21  iP32  iP43  iP54  iP65  ∑iP6-iP26  0  6.820  7.269  0.231  0.820  2.704  30.325  41.349  500  8.307  7.529  0.499  2.055  4.076  23.519  37.677  1,500  11.489  7.040  0.726  2.504  2.933  12.475  25.678  4,500  15.594  7.341  0.796  1.553  0.560  2.757  12.927  SEM  0.644  0.345  0.082  0.271  0.322  1.973  2.503  P-values   Phytase  <0.001  0.950  <0.001  <0.001  <0.001  <0.001  <0.001    Linear  <0.001  0.754  <0.001  0.032  <0.001  <0.001  <0.001    Quadratic  0.050  0.953  0.238  <0.001  <0.001  0.466  0.078  Means are based on 4 birds per pen and 10 replicate pens per diet. 1Inositol bisphosphate. 2Inositol triphosphate. 3Inositol tetraphosphate. 4Inositol pentaphosphate. 5Inositol hexakisphosphate (phytate, phytic acid). 6Sum of iP2 to iP6 concentration. View Large The relative changes in the gene expression of inositol transporters and intestinal alkaline phosphatase in the jejunum and ileum are presented in Table 7. In the jejunum, increasing phytase dose from 0 to 4,500 FTU/kg up-regulated the relative expression of SLC5A11 (quadratic, P < 0.05), tended to up-regulate the relative expression of SLC5A3 (quadratic, P = 0.10), and there was a tendency (P < 0.10) for phytase to up-regulate the relative expression of SLC2A13. There was no effect of phytase dose on the relative expression of iALP in the jejunum. In the ileum, the effect of phytase dose approached significance (P < 0.06) towards an up-regulation of the relative expression of SLCA13 (linear, P < 0.05) and significantly increased the expression of iALP (linear, P < 0.05). There was no effect of phytase dose on the relative expression of SLC5A11 or SLC5A3 in the ileum. The effect of phytase dose on gene expression of myo-inositol transporters in the liver and kidney were not significant (Table 8). Table 7. Expression of genes in the small intestine mucosa of broilers fed phytase from hatch to 21-days post hatch.   >Jejunum  >Ileum  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  iALP4  SLC5A111  SLC5A32  SLC2A133  iALP4  0  0.661  0.923  0.962  0.949  1.218  0.994  0.975  0.932  500  1.147  0.804  0.834  1.041  1.344  0.846  0.837  1.463  1,500  3.094  1.455  1.476  1.140  1.096  0.636  0.975  1.036  4,500  2.890  1.232  1.331  1.159  1.128  1.139  1.452  2.295  SEM  0.38  0.20  0.17  0.16  0.17  0.11  0.12  0.23  P-values   Phytase  0.002  0.149  0.090  0.703  0.760  0.263  0.055  0.027    Linear  0.003  0.236  0.314  0.521  0.897  0.206  0.007  0.009    Quadratic  0.017  0.101  0.112  0.703  0.731  0.146  0.965  0.265    >Jejunum  >Ileum  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  iALP4  SLC5A111  SLC5A32  SLC2A133  iALP4  0  0.661  0.923  0.962  0.949  1.218  0.994  0.975  0.932  500  1.147  0.804  0.834  1.041  1.344  0.846  0.837  1.463  1,500  3.094  1.455  1.476  1.140  1.096  0.636  0.975  1.036  4,500  2.890  1.232  1.331  1.159  1.128  1.139  1.452  2.295  SEM  0.38  0.20  0.17  0.16  0.17  0.11  0.12  0.23  P-values   Phytase  0.002  0.149  0.090  0.703  0.760  0.263  0.055  0.027    Linear  0.003  0.236  0.314  0.521  0.897  0.206  0.007  0.009    Quadratic  0.017  0.101  0.112  0.703  0.731  0.146  0.965  0.265  Means are based on 2 birds per pen and 10 replicate pens per diet. 1SLC5A11 = SMIT2 (sodium/glucose cotransporter 11). 2SLC5A3 = SMIT1 (sodium/myo-inositol cotransporter). 3SLC2A13 = HMIT (H+/myo-inositol transporter). 4iALP = intestinal alkaline phosphatase. View Large Table 8. Expression of genes in the kidney and liver of broilers fed phytase from hatch to 21-days post hatch.   >Kidney  >Liver  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  SLC5A111  SLC5A32  SLC2A133  0  0.971  0.936  1.026  0.948  1.116  1.148  500  1.195  0.708  0.887  0.943  1.157  0.826  1,500  0.912  0.819  0.870  1.396  0.936  0.967  4,500  0.641  0.734  0.744  1.160  0.820  0.924  SEM  0.20  0.09  0.09  0.16  0.13  0.12  P-values   Phytase  0.410  0.234  0.174  0.247  0.334  0.379    Linear  0.170  0.390  0.044  0.846  0.103  0.481    Quadratic  0.921  0.516  0.565  0.069  0.528  0.443    >Kidney  >Liver  Phytase, FTU/kg  SLC5A111  SLC5A32  SLC2A133  SLC5A111  SLC5A32  SLC2A133  0  0.971  0.936  1.026  0.948  1.116  1.148  500  1.195  0.708  0.887  0.943  1.157  0.826  1,500  0.912  0.819  0.870  1.396  0.936  0.967  4,500  0.641  0.734  0.744  1.160  0.820  0.924  SEM  0.20  0.09  0.09  0.16  0.13  0.12  P-values   Phytase  0.410  0.234  0.174  0.247  0.334  0.379    Linear  0.170  0.390  0.044  0.846  0.103  0.481    Quadratic  0.921  0.516  0.565  0.069  0.528  0.443  Means are based on 2 birds per pen and 10 replicate pens per diet. 1SLC5A11 = SMIT2 (sodium/glucose cotransporter 11). 2SLC5A3 = SMIT1 (sodium/myo-inositol cotransporter). 3SLC2A13 = HMIT (H+/myo-inositol transporter). View Large DISCUSSION Mortality was 5% and higher than expected due to yolk sac infections and also the small number of chicks in each pen. Each pen contained 6 chicks with 10 replicate pens per diet and the loss of only 3, 3, 6, and 0 birds in the diets containing 0, 500, 1,500, or 4,500 FTU/kg of phytase, respectively, resulted in the quadratic effect of diet on mortality. Growth performance of broilers was 30 to 39% below Ross 308 standards (Ross 308 Broiler Performance Objectives, 2012) and this may be associated with the causative agent of the yolk sac infection slowing growth in the first week post-hatch and the use of mash diets (Kilburn and Edwards, 2001). Body weight gain and P digestibility increased as phytase supplementation increased and this has been previously reported in low avP diets supplemented with 0 to 12,500 FTU/kg (Karadas et al., 2010) or 0 to 24,000 FTU/kg (Cowieson et al., 2006) of phytase indicating further benefits in nutrient digestibility and growth are attainable with higher doses of phytase. Contradictory to previously published research (Walk et al., 2013, 2014), there was no significant effect of phytase dose on FCR in the current trial. Numeric improvements in FCR were noted however, with the highest dose of phytase improving feed efficiency by approximately 14%. Mechanisms by which high doses of phytase elicit beneficial effects on performance are proposed to be related to 1) destruction of the anti-nutritive effects of phytate with generation of more soluble lower phytate esters and 2) generation of myo-inositol (Cowieson et al., 2011). Phytate, phytate ester, and inositol concentrations in the gizzard and ileal digesta in the current experiment would partially support the above proposed mechanisms of superdosing. For example, in the gizzard and ileal digesta the concentration of iP6 decreased and the concentration of iP5 and iP4 increased and then decreased, while iP3 and myo-inositol concentration increased as phytase supplementation increased in the diet and this has been previously reported (Walk et al., 2014; Beeson et al., 2017). Using in vitro models, other authors have reported that phytate, as well as the lower phytate esters, have the capacity to bind minerals and to interfere with pepsin activity, particularly as pH increases, as summarized by Bedford and Walk (2016). While phytate is considered a more potent anti-nutrient than the lower esters, the anti-nutritive effects of these lower esters on minerals (Xu et al., 1992) and pepsin (Yu et al., 2012) requires further consideration, and continued reduction of these phytate esters with high doses of phytase may be a factor contributing to the increase in BWG and numeric improvements in FCR. In addition, the continued destruction of phytate and the lower phytate esters as phytase dose increased also resulted in significant increases in myo-inositol in the gizzard and ileal digesta. Myo-inositol is an important component of cellular phospholipids and is involved in many cellular functions including survival, structure and signaling (Huber, 2016). Previous authors have loosely correlated an increase in myo-inositol concentrations in the gizzard with significant improvements in FCR (Walk et al., 2014). Cowieson et al. (2013) reported supplementation of broiler diets with 0.15% myo-inositol resulted in significant improvements in FCR of 42-day old broilers. Others have also reported significant increases in plasma myo-inositol as phytase supplementation increased in the diet (Cowieson et al., 2015; Laird, 2016). Therefore, it is likely one of the beneficial effects of feeding high doses of phytase would be the provision of myo-inositol through phytate and phytate ester destruction. Free myo-inositol in the gastrointestinal tract is absorbed with great efficiency, 99.8% (Holub, 1986; Croze and Soulage, 2013) by an active, Na-dependent process. Sodium is transported across the brush border together with myo-inositol via SLC5A11 (SMIT2) at a ratio of 2 Na to 1 myo-inositol (Huber, 2016). In the current trial, the expression of SLC5A11 was up-regulated in the jejunum and the apparent ileal Na digestibility was significantly increased as phytase supplementation increased in the diet. In addition, at least in the jejunum, there was also a tendency toward an up-regulation of both SLC5A3 (SMIT1) and SLC2A13 (HMIT) as dietary phytase increased from 0 to 4,500 FTU/kg, but there was no effect on the gene expression of iALP. In contrast, in the ileum there was no effect of phytase on SLC5A11 (SMIT2) or SLC5A3 (SMIT1) and a tendency for a linear increase in SLC2A13 (HMIT) with a significant increase in iALP expression. These results are interesting, especially when considering previous authors reported the expression of SLC5A3 (SMIT1) and SLC2A13 (HMIT) are most noted in the brain and/or kidney with SMIT2 predominantly found in the small intestine (Aouameur et al., 2007; Mueckler and Thorens, 2013; Huber, 2016). However, species differences exist in myo-inositol transporter expression in the tissues, with SMIT2 being expressed in high concentration in the kidney of both rats and rabbits, but barely detectable in the small intestine of rabbits (Aouameur et al., 2007). In the current experiment, the expression of myo-inositol transporters was not measured in the brain and more work is needed to confirm the effects reported in herein, particularly in poultry. Regardless, a few interesting points can be discussed based on the gene expression data, specifically: 1) Myo-inositol appears to be actively transported in the small intestine and transporter expression is influenced by myo-inositol concentration, with an up-regulation of the gene expression of SMIT2 or HMIT in the jejunum and ileum as phytase dose and production of myo-inositol increased. Myo-inositol uptake in the brush border vesicles of rats has been previously reported through SMIT2 with no evidence of uptake from HMIT or SMIT1 (Aouameur et al., 2007). However, as previously mentioned, these same authors reported barely any SMIT2 detection in the rabbit intestine, indicating species differences exist and these results need to be confirmed in subsequent trials. Regardless, it would appear there is an effect of myo-inositol concentration in the intestinal lumen on the up-regulation of transporters in the jejunum of poultry. 2) In the ileum however, only HMIT expression was up-regulated as phytase dose increased. This could also be related to the concentration of myo-inositol present in the ileum but also due to the reduction of Na concentration (as depicted by an increase in Na digestibility) and the concentration and type of soluble lower phytate esters present in the ileal lumen, which subsequently resulted in an up-regulation of intestinal alkaline phosphatase (Schlemmer et al., 2009); all of which resulted in an increase in HMIT, the proton dependent myo-inositol transporter. Previous authors have reported phytase specific activity along the intestinal brush border of broilers and layers, with intestinal phytase activity decreasing from the duodenum to the ileum (Maenz and Classen, 1998). These results may be contradictory to the current trial; however specific phytase activity was not evaluated and effects cannot be compared directly. Furthermore, the iALP expression in the jejunum and the ileum of birds fed 0 FTU phytase/kg diet was 0.949 vs. 0.932, respectively and phytase had a significant effect in the ileum, suggesting the response to HMIT and iALP in the ileum was associated with lower phytate esters and the production of inositol by iALP. Interestingly, even by the terminal ileum the concentration of inositol was remarkably high (53% of the total), indicating there is a rate-limiting step in inositol absorption within the gastrointestinal tract. This is contradictory to previous estimates of free myo-inositol absorption in the human small intestine at around 99.8% (Holub, 1986; Croze and Soulage, 2013). However, the differences may be dependent on the availability of Na and H+ and the location in the GIT. For example, previous authors reported phytase supplementation significantly increased pH in the distal ileum from 6.56 in birds fed 0 FTU/kg phytase to 6.99 in birds fed 5,000 FTU/kg phytase (Walk et al., 2012). Taking the anti-log of these pH values indicates an almost 40% reduction in H+ ion concentration (2.754E-07 vs. 1.023E-07) in the ileum of broilers fed 5,000 FTU/kg of phytase compared with that of broilers fed 0 FTU/kg phytase. In effect, this means that while phytase supplementation increases myo-inositol concentration, it also creates a rate-limiting step in myo-inositol uptake by the ileum by increasing Na digestibility and reducing H+ ions which are needed for co-transport of myo-inositol. The lack of an effect of phytase dose on SMIT1 or SMIT2 support this as they are both Na dependent co-transporters. 3) Finally, notable is the non-significant effect of phytase dose on myo-inositol transporter gene expression in the kidney and liver. Both the liver and kidney play important roles in myo-inositol metabolism and de novo synthesis and the kidney is the main site of myo-inositol excretion (Holub, 1986; Lahjouji et al., 2007; Croze and Soulage, 2013). The lack of an effect of phytase dose may be indicative of a reduced need for endogenous synthesis or excretion of myo-inositol due to the provision of dietary myo-inositol. These results require further evaluation but may be indicative of the pathways and the regulation of myo-inositol provided from phytate destruction in the diet. In conclusion, supplementation of broiler diets with phytase up to 4,500 FTU/kg significantly increased weight gain and resulted in nearly complete phytate and phytate ester destruction and the significant increases in myo-inositol. This influenced and up-regulated the gene expression Na+ of H+-dependent myo-inositol transporters within the jejunum and the ileum, respectively. 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Poultry ScienceOxford University Press

Published: Apr 1, 2018

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