Effects of Large Amounts of Vitamin D₃ Injection on Plasma Vitamin D₃ Metabolites in Lactating Cows

Abstract

Effects of injection of large amounts of vitamin D₃ (VD₃) on the concentrations of plasma 25-hydroxyvitamin D (25OHD), 1α, 25-dihydroxyvitamin D (1, 25 (OH)₂D), calcium (Ca) and inorganic phosphorus (Pi) in the lactating Holstein-Friesian cows were examined. The cows were divided into two groups, an untreated control group consisting of 4 cows and a treatment group (VD₃ group) consisting of 5 cows. Each cow in the VD₃ group received 10 million I.U. of VD₃ intramuscularly at 7 days after parturition. Blood and milk samples were taken from 0 to 15 days after the VD₃ injection. The plasma 25OHD concentration was significantly higher in the VD₃ group than that in the control group throughout the experimental period, whereas the plasma 1, 25 (OH)₂D concentration rose only transiently after the VD₃ injection. The plasma Ca concentration was also significantly higher in the VD₃ group than that in the control group throughout the experiment, while the plasma Pi concentration was higher in the VD₃ group only from the 3rd to 6th day of the experiment. These results suggest that the initial increase in the plasma Ca concentration may be due to the increase in the 1, 25 (OH)₂D level whereas the substained increase in the Ca concentration may be due to the increase in the plasma 25OHD concentration. Further, it seems likely that the plasma 1, 25 (OH)₂D concentration is suppressed by the increase in the plasma Ca concentration in the later stage of experiment and also by the negative feedback of plasma 1, 25 (OH)₂D concentration.