Effects Of Excessive Vitamin A On Deposition Of Calcium And Phosphorus And Expression Of Alkaline Phosphatase And Bone Gla-protein In Broiler Chickens

The present study was conducted to investigate the effects of vitamin A excess on calcium and phosphorus deposition and alkaline phosphatase and bone gla-protein mRNA expression in broiler chickens. The first three experiments were conducted by using in vivo experimental techniques. A total of 450 one-day-old Arbor Acre half male and half female broiler chickens with similar body weight were randomly allotted to five dietary treatments with six replicate cages per dietary treatment and fifteen birds per cage. The basal diet that didn't contain vitamin A was mainly composed of corn and soybean meal.The broiler chickens in the five treatments were fed the basal diets supplemented with 3000, 6000, 15000, 30000 or 60000 IU/kg vitamin A in a single factorial arrangement. The trial lasted for 42 days. The objective of the experiment 1 was to elucidate the effects of vitamin A excess on contents of Ca, P in serum and concentrations of ash, Ca and P and BMD in tibia of broilers, and further to demonstrate the negative effects of excessive vitamin A on calcium and phosphorus deposition in broiler chickens. The experiment 2 was conducted to investigate the effects of vitamin A excess on hormones related to calcium and phosphorus deposition and biochemical markers of bone formation. The experiment 3 was conducted to study the effects of vitamin A excess on relative expression of ALP mRNA and BGP mRNA in small intestine, tibia and liver, and to elucidate the molecular mechanisms for vitamin A excess to affect calcium and phosphorus deposition in broiler chickens. The experiment 4 was conducted to demonstrate the effects of different levels of vitamin A(0, 0.03, 0.10, 0.30, 1.00, 2.00, 6.00, 12.00, 24.00μg/ml) on osteoblasts in vitro by evaluating cells activity and relative expression of ALP mRNA and BGP mRNA of osteoblastic cells and further to investigate the molecular mechanisms by which vitamin A modulated calcium and phosphorus deposition.Under the present experimental conditions, the results showed as follows:①Dietary vitamin A influenced the strength and disposition of calcium and phosphours in the tibia of broilers in a dose-dependent manner(P<0.05;P<0.10). The supplementation of 3000~6000 IU/kg vitamin A in the diet improved the strength and disposition of calcium in the tibia. But the strength and disposition of calcium in the tibia tended to be suppressed when the addition of vitamin A in diet was increased to 30000~60000 IU/kg. And the tibia strength tended to be lower while 15000 IU/kg vitamin A was added in the diet. ②Dietary vitamin A influenced contents of PTH, CT, 1,25(OH)2D3, BGP and PINP as well as ALP and BAP activity in serum in a dose-dependent manner (P<0.05; P<0.10) with the supplementation of vitamin A in the diet from 3000 to 60000 IU/kg. The addition of 30000～60000 IU/kg vitamin A to the diet increased serum PTH level, and depressed the contents of CT, 1,25(OH)2D3, BGP and PINP as well as ALP and BAP activity in serum of broilers.③The dietary vitamin A influenced the relative expressions of the ALP mRNA and BGP mRNA in the tibia, liver and small intestine in a linear or quadratical dose-dependent manner (P<0.05, P<0.10). The addition of 3 000~6 000 IU/kg vitamin A to the diet showed the better effect in relative expressions of ALP mRNA and BGP mRNA in tibia, liver and the small intestine of broilers. However, the addition of 3 0000~6 0000 IU/kg vitamin A to the diet could down-regulate the relative expressions of ALP mRNA and BGP mRNA, and the addition of 15 000 IU/kg vitamin A tended to decrease in relative expressions of ALP mRNA and BGP mRNA.④With the increase of vitamin A in the culture, ALP activity and concentrations of BGP, PINP, OPN and OPG in the culture of the cranium osteoblasts of broilers decreased linearly or quadratically(P<0.05, P<0.10). This suggested that the vitamin A could promote the secretion functions and activity of cranium osteoblasts in broiler chickens in a dose-dependent manner. Adequate doses of vitamin A could promote the activity of osteoblasts, and induce differentiation and maturation. But the large doses of vitamin A was less effective.⑤With the increase of vitamin A in the culture of the osteoblasts in broilers, the relative expression of the ALP mRNA of osteoblasts decreased linearly or quadratically(P<0.05), and the relative expression of BGP mRNA of osteoblasts tended to decrease quadratically(P<0.10). This suggested that suitable dose of vitamin A up-regulated the relative expression of the ALP mRNA and BGP mRNA of the osteoblasts, and high levels of vitamin A down-regulated its expression.⑥The present results implied that the mechanisms for vitamin A excess to affect calcium and phosphorous deposition in bone of broiler chickens were probably that the excessive vitamin A depressed the relative expressions of the ALP mRNA and BGP mRNA in tissues and osteoblasts, which lowered the synthesis of the ALP and BGP and resulted in the negative effects to the despositions of the calcium and phosphorous in bone.