Measurement and function of turnover markers in sheep and pig bone
Osteocalcin, which is produced by the osteoblast, and the urinary pyridinium compounds (pyridinoline and deoxypyridinoline), which are derived from collagen, are widely used as markers of bone turnover. Osteocalcin was extracted from bone in 20% formic acid and separated using a linear 4-60% acetonitrile gradient containing 0.1% TFA at a flow rate of 1ml/min. The standard curve was linear up to 15 μg of osteocalcin injected, with inter- and intra-assay coefficients of variation of 6.9 and 8.8% respectively while recovery of osteocalcin added to bone extracts averaged 102.7 ± 6.16 %. Having developed this assay it was then used in a series of experiments designed to study the biological function of osteocalcin in bone. Plasma osteocalcin levels decreased with age and showed large between-animal variations; the variability over 24 h was also large but there was no evidence of consistent circadian rhythm. In bone, changes in osteocalcin levels tended to parallel those for calcium whereas pyridinium crosslink levels tended to increase with age. Neither were sufficiently sensitive to detect differences in bone turnover in lambs of the same age but growing at different rates though osteocalcin levels in blood and in bone were sensitive to P-deficiency in sheep but not in pigs and there was little evidence indicating that osteocalcin plays any direct role in the mineralisation process. In separate studies adult sheep were treated with a bone antiresorptive agent, Ibandronate, and its effects on the metabolism and excretion of the pyridinium crosslinks was examined. At rates which have been shown to be effective in reducing bone resorption in humans this compound had little effect on the overall rate of excretion of these crosslinks in these sheep but did alter the proportions excreted in free or in peptide bound form.