Response of plasma bone markers to a single intramuscular administration of calcitriol in dairy cows

To elucidate the effects of an exogenous calcitriol (1,25-dihydroxyvitamin D 3) on plasma bone markers, the formation item osteocalcin (OC), undercarboxylated OC (ucOC) and bone-specific alkaline phosphatase (BALP), and the resorption parameter tartrate-resistant acid phosphatase isoform 5b (TRAP5b) and hydroxyproline (HYP) were measured in conjunction with plasma calcitriol and calcium (Ca) concentrations in dairy cows receiving calcitriol or its vehicle according to a 2 × 2 crossover design. Calcitriol (0.5 μg/kg, i.m.) increased significantly its plasma level during 6 h to day 2 and plasma Ca concentration during 12 h to day 7 compared to the vehicle. Also, plasma OC and ucOC started to rise from day 3 and 1, respectively, and remained elevated until day 7. No change in plasma BALP, TRAP5b or HYP associated with calcitriol treatment was noted. These results demonstrate that exogenous calcitriol stimulates osteoblasts to biosynthesise OC, a determinant of the bone formation in cows.