The sheep has become a common model in orthopaedic research. However, there are marked differences in calcium and phosphorus homoeostatic mechanisms between sheep and humans or other animals widely-used like the rat concerning the site of absorption and the role of the kidney. For better characterisation of the sheep as an animal model, it is essential to collect basic data on macromineral homoeostasis and its regulation.

In the present study, we administered 1,25-dihydroxyvitamin D₃ (calcitriol; 0.5 mg/kg i.v.) to six months old, female sheep fed different levels of calcium (daily calcium intake: 2.5 vs. 10.5 g) for at least three weeks. After treatment, blood samples were taken every four hours for the next three days to determine serum concentrations of calcitriol, the bone resorption marker CrossLaps (CL), total (Ca_t) and ionised calcium (Ca²⁺) and inorganic phosphorus (P_i) (each feeding group consisting of three animals). Furthermore, Ca_t and P_i were analysed in urine samples collected during the first 12 hours after treatment (five animals per group).

Although basal concentrations were affected significantly (41.8 0.6 vs. 13.7 6.4 pg·ml^-1; mean values SEM), pharmacokinetics of calcitriol were not altered by the feeding regime. Serum half-lives ranged from 6.3 to 7.2 hours. Concentrations of CL were threefold higher in the Ca depletion group (Ca depl) than in the control group (Con) (0.902 0.160 vs. 0.329 0.030 ng·ml^-1). Only one Ca depl animal showed an increase in CL following the treatment, while there was no change or a decrease in the other animals. In both groups, Ca²⁺, Ca_t and P_i increased considerably. While there were no apparent differences in peak concentrations of P_i (3.28 0.19 vs. 2.97 0.20 mmol·l^-1), concentrations of Ca²⁺ (1.45 0.07 vs. 1.70 0.10 mmol·l^-1) and Ca_t (3.19 0.18 vs. 2.73 0.22 mmol·l^-1) of the two groups differed significantly 24 hours after application of calcitriol. Interestingly, during the first 12 hours after treatment, there were no differences in renal excretion of neither Ca_t nor P_i between the groups.

From these results it can be concluded that in contrast to numerous other species, Ca_t and P_i homoeostasis in the sheep is maintained mainly via adaptation of gastrointestinal absorption mechanisms rather than by regulation of renal excretion. Further investigation is needed concerning the role of bone metabolism.