Changes in plasma 25-hydroxycholecalciferol and selected blood parameters after injection of massive doses of cholecalciferol or 25-hydroxycholecalciferol in non-lactating dairy cows.

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.