Age,Phosphorus,and 25-hydroxycholecalciferol Regulate MRNA Expression Of Sodium-phosphate Cotransporter And Vitamin D Receptor In The Small Intestine Of Broiler Chickens

Vitamin D₃ is used as a feed additive to regulate calcium（Ca）and phosphorus（P）metabolism and bone development in poultry.It undergoes 25-hydroxylation in the liver to transform into 25-hydroxycholecalciferol（25-OH-D₃）and then undergoes 1α-hydroxylation in kidneys to form the final active product 1,25-dihydroxycholecalciferol [1,25-（OH）2-D₃].1,25-（OH）2-D₃ binded to two vitamin D receptors（VDR）to regulate P absorption in the small intestine of poultry which were nuclear VDR（n VDR）and membrane VDR（m VDR）.The former n VDR was located in the nucleus of intestinal cells and activates mitogen-activated protein kinase（MAPK）pathway in chickens.m VDR was located in basal-lateral membranes of intestinal cells and promoted P uptake via protein kinase C（PKC）signaling pathway in chicks.Type Ⅱb sodium-phosphate cotransporter（Na Pi-Ⅱb）was the only protein that transports P in brush-border membranes of small intestinal cells of poultry.Expressions of n VDR,m VDR,PKCα and Na Pi-Ⅱb in animal intestine are affected by age,NPP,and 25-OH-D₃.Experiment 1 Effects of age on mRNA expressions of NaPi-Ⅱb and VDR in the small intestine of chickensThis experiment was to evaluate expression regular of phosphorus metabolism correlation m RNA in the small intestine of broiler chickens at different days of age,and to select appropriate time nodes for follow-up tests.A total of 144 male Ross 308 broilers were randomly allotted to 12 cages of 12 birds per cage.The results showed as follows:（1）The highest m RNA expression levels of n VDR,m VDR,PKCα,Na Pi-Ⅱb in the duodenum and 25-hydroxylase in the liver of birds were observed at 21 days of age.（2）The m RNA expression level of 1α-hydroxylase in the kidney of birds decreased linearly with the increase of age.The m RNA expression level of Na Pi-IIa in the kidney of birds increased to highest level at 21 days of ageand remained stable at 28 to 42 days of age.Experiment 2 The differences of m RNA expressions of Na Pi-Ⅱb and VDR in the duodenum,jejunum,and ileum of broiler chickensThis experiment was to evaluate the differences in m RNA expressions of Na Pi-Ⅱb and VDR in the duodenum,jejunum,and ileum of 21-day-old broilers.A total of 72 male Ross 308 broilers were randomly allotted to 8 replicate cages of 9 birds per cage.The results showed as follows:（1）The m RNA expression levels of n VDR and Na Pi-Ⅱb were highest in duodenum of 21-day-old broilers,followed by jejunum and ileum.（2）No significant differences in the m RNA expressions of m VDR and PKCα were observed in the duodenum,jejunum,and ileum.Experiment 3 Effects of dietary phosphorus levels on m RNA expression of Na Pi-Ⅱb and VDR in the small intestine of chickensThis experiment was to evaluate effects of dietary non-phytate phosphorus（NPP）levels on m RNA expressions of Na Pi-Ⅱb and VDR in 1-to 21-day-old chickens.A total of 300 male Ross 308 broilers were randomly allotted to 5 treatments with 6 replicate cages of 10 birds per cage.Dietary NPP levels were 0.25,0.35,0.45,0.55,and 0.65%.The results showed as follows:（1）when the level of NPP increased from 0.25 to 0.45%,the growth performance and bone mineralization of broilers were significantly improved.By contrast,increasing dietary NPP levels from 0.45% to 0.65%did not affect the growth performance and bone mineralization of broilers.（2）when the NPP level of diet was 0.55%,the m RNA expression level of n VDR,m VDR and PKCα m RNA in duodenum reached the maximum value,while the level of liver 25-hydroxylase in the liver and 1α-hydroxylation in the kidney m RNA reached the lowest level.（3）when the level of NPP increased from 0.25% to 0.65%,the m RNA expression level of Na Pi-IIa and Na Pi-Ⅱb decreased linearly.Experiment 4 Effects of 25-OH-D₃ on mRNA expressions of NaPi-Ⅱb and VDR in the small intestine of chickensThis experiment was to evaluate effects of 25-OH-D₃ on m RNA expressions of 1α-hydroxylase in kidneys and n VDR,m VDR,PKCα and Na Pi-Ⅱb in the duodenum of 1-to 14-day-old broilers.A total of 240 male Ross 308 broilers were randomly allotted to 4 treatments with 6 replicate cages of 10 birds per cage.Diet 1 was the basal diet without vitamin D.Diet 2 was the control diet with 1000 IU/kg vitamin D₃.Diets 3 and 4 were the basal diet supplemented with 500 IU/kg and 1000 IU/kg 25-OH-D₃.The results showed as follows:（1）The broilers fed 500 IU/kg 25-OH-D₃ significantly improved the growth performance and bone mineralization of 1-to 14-day-old broilers.（2）The m RNA expression levels of 1α-hydroxylase in the kidney as well as n VDR,m VDR,and Na Pi-Ⅱb m RNA in the duodenum significantly increased after adding 500 IU/kg 25-OH-D₃.In summary,age quadratically affected m RNA expression levels of n VDR,m VDR,and Na Pi-Ⅱb in the duodenum and 25-hydroxylase in the liver of 7-to 42-d-old broilers,with the highest m RNA levels observed at 21,21,35,and 21 d of age,respectively.By contrast,age linearly decreased m RNA expression levels of 1α-hydroxylase in kidneys.The highest m RNA expression levels of n VDR and Na Pi-Ⅱb were observed in duodenum of 21-d-old broilers,followed jejunum,and then ileum.By contrast,no difference in m VDR m RNA expression level was observed among the three intestine segments.Dietary NPP levels quadratically affected m RNA levels of n VDR,m VDR,and 25-hydroxylase.The highest m RNA expression levels of n VDR and m VDR and lowest m RNA level of 25-hydroxylase were observed at 0.55% NPP.m RNA levels of Na Pi-IIa and Na Pi-Ⅱb linearly declined when dietary NPP levels increased from 0.25 to 0.65%.Addition of 500 IU/kg of 25-OH-D₃ increased m RNA expression levels of n VDR,m VDR,Na Pi-Ⅱb,and 1α-hydroxylase compared with those of birds fed diets without 25-OH-D₃.