Effects Of Excessive Vitamin A On Calcium And Phosphorous Metabolism In Broilers And Underlying Mechanisms

The present study was conducted to investigate the effects of excessive VA on calcium and phosphorous metabolism in broilers and underlying mechanisms. The first four experiments were conducted using the experimental technique in vivo. 192 1-day-old AA male broiler chickens were randomly allotted into 4 treatments with 6 replicates in each treatment, with 8 chickens in each replicate. The trial period is 42d. Broilers were fed four levels of dietary VA (1500, 3000, 15000, and 45000 IU/kg) in a single factorial arrangement. The basal diet was mainly composed of corn and soybean. The objectives were to elucidate the effects of excess VA on growth performance and VA and VE metabolism, bone growth, hormones, enzyme activity and calcium-binding-protein (CaBP)content related to calcium and phosphorus metabolism. The effects of excess vitamin A on CaBP gene expression were also studied by using RT-PCR. The experiment 5 was conducted in vitro to demonstrate the effects of excess VA on calcium and phosphorous metabolism and CaBP gene expression of osteoblasts. The culture medium was DEM/F-12 with 20% fetal bovine serum, in which 8 levels of VA (0.0, 0.2, 0.6, 1.0, 2.0, 5.0, 10.0 and 20.0μg/ml, respectively) were added.Under the present experimental conditions , the results showed as follows:The diets containing VA 15000 and 45000 IU/kg individually would result in significant decreases in growth rates during the late growth period, and the growth rate had the tendency to be decreased during the whole growth period. At the meantime, VA concentrations in liver and kidney were significantly increased, and the concentrations of VE in serum, kidney and liver were significantly decreased. While dietary VA level was 15000 IU/kg, VA concentrations in liver and kidney were significantly increased.There was a obvious influence of excess VA on bone growth and strength. The weight, calcium and mineral density (BMD)of tibia were significantly decreased while the level of dietary VA was 45000 IU/kg, and tibia ash and length had the tendency to be decreased. While dietary VA level was 15000 IU/kg ,the indexes above were also decreased slightly.While the dietary VA wae excess, the concentrations of CaBP in serum , CaBP gene expression in small intestine and tibia tissues were decreased, and the alkaline phosphatase (AKP) activity and BGP concentration in serum were significantly decreased, and hormone CT and VD3 concentrations were depressed and PTH content was increased.The results in vitro showed that excess VA would result in a decrease of concentrations of BGP and AKP activity of culture liquids , and the concentrations of CaBP culture liquids and CaBP gene expression of osteoblasts also tended to decrease.The addition of 15000 IU/kg VA would probably be excessive for broilers, and the addition of 45000 IU/kg VA would be excessive for broilers, and the addition of 3000 IU/kg VA would be appropriate for broilers considering of the growth performance, VA and VE metabolism, bone growth and strength, and calcium and phosphorous metabolisms in broilers.The weight, calcium and mineral density (BMD)of tibia were more sensitive than tibia ash and length in reflecting bone growth and metabolisms of calcium and phosphorous. AKP activity in serum could reflect more sensitively the bone growth and the metabolisms of calcium and phosphorous than CT concentration in serum. PTH concentration in serum would be less sensitive than CT in reflecting bone growth and metabolisms of calcium and phosphorous.