Abstract
The present study evaluated the effects of calcitriol dissolved in an oleaginous vehicle (calcitriol-OLE) on changes in plasma calcitriol, calcium and bone metabolic markers in nonpregnant, nonlactating Holstein cows. Five cows were treated intramuscularly or subcutaneously with calcitriol-OLE and oleaginous vehicle alone using a 5 × 5 Latin square design. Additionally, cows were also treated intravenously with calcitriol dissolved in an aqueous vehicle (calcitriol-AQU) for comparison. The plasma calcitriol concentrations after intramuscular and subcutaneous calcitriol-OLE administrations peaked at 24 and 12 hr, respectively, remained significantly elevated until day 3, returned to the respective control levels on day 5 and decreased significantly on day 7. In cows given intravenous calcitriol-AQU, the calcitriol levels decreased with linearity on day 1. The plasma calcium levels rose from 12 hr post-dose and peaked on day 2 for both preparations and in all three administration routes. Significantly increased calcium levels continued until day 5 in the intramuscular and intravenous routes and day 7 in the subcutaneous route. The plasma osteocalcin concentrations significantly increased from day 3 for calcitriol-OLE and from day 5 for calcitriol-AQU, whereas the bone resorption markers, tartrate-resistant acid phosphatase isoform 5b and hydroxyproline, decreased during this time. These results suggest that either intramuscular or subcutaneous injection of calcitriol-OLE extends and maintains supraphysiological calcitriol levels in the plasma and prolongs hypercalcemia. Moreover, exogenous calcitriol in normocalcemic cows increases the plasma osteocalcin concentration and decreases the plasma levels of bone resorption markers probably due to hypercalcemia.
