Hollis B W, Conrad H R, Hibbs J W
J Nutr. 1977 Apr;107(4):606-13. doi: 10.1093/jn/107.4.606.
Plasma levels of 25-hydroxycholecalciferol, free hydroxyproline, calcium, phosphorus, and magnesium were determined in non-lactating, pregnant dairy cows injected intra-muscularly with 15 X 10(6) IU of cholecalciferol or 25 mg of 25-hydroxycholecalciferol. A lag in the conversion of cholecalciferol to 25-hydroxycholecalciferol was observed in the cows injected with cholecalciferol, while an immediate increase was observed when cows were injected with 25-hydroxycholecalciferol directly. The increased plasma levels of 25-hydroxycholecalciferol following injection of cholecalciferol were directly related to rises in plasma free hydroxy-proline, calcium, and phosphorus, while plasma magnesium was inversely related to plasma 25-hydroxycholecalciferol. Injection of 25-hydroxycholecalciferol caused an immediate increase in plasma calcium which persisted for the duration of the experiment. The biological half-life of 25-hydroxycholecalciferol in the injected cows was found to be 34 days. The data indicate the possibility of a feedback mechanism in which massive doses of cholecalciferol inhibit hydroxylation at the 25ths carbon preventing its conversion to 25-hydroxycholecalciferol until after 8 days post injection. The increase in plasma 25-hydroxycholecalciferol after 8 days resulted in increased bone resorption as indicated by plasma free hydroxyproline.
对未泌乳的怀孕奶牛肌肉注射15×10⁶国际单位的胆钙化醇或25毫克的25-羟基胆钙化醇后，测定其血浆中25-羟基胆钙化醇、游离羟脯氨酸、钙、磷和镁的水平。在注射胆钙化醇的奶牛中观察到胆钙化醇向25-羟基胆钙化醇转化的滞后现象，而直接注射25-羟基胆钙化醇的奶牛则立即出现了增加。注射胆钙化醇后血浆中25-羟基胆钙化醇水平的升高与血浆游离羟脯氨酸、钙和磷的升高直接相关，而血浆镁与血浆25-羟基胆钙化醇呈负相关。注射25-羟基胆钙化醇导致血浆钙立即升高，并在实验期间持续存在。发现注射奶牛中25-羟基胆钙化醇的生物半衰期为34天。数据表明存在一种反馈机制的可能性，即大剂量的胆钙化醇抑制第25位碳原子的羟基化，阻止其转化为25-羟基胆钙化醇，直到注射后8天。8天后血浆25-羟基胆钙化醇的增加导致骨吸收增加，如血浆游离羟脯氨酸所示。
