Lack of in vitro evidence for storage of 1,25-dihydroxycholecalciferol (1,25(OH)(2)D-3) and 1,25(OH)(2)D-3 binding protein in skeletal matrix

A few studies have reported on the measurement of 1,25-dihydroxycholecalciferol (1,25(OH)(2)D-3) in bone, using chloroform/methanol extraction and radioreceptor assay. As the significance of bone 1,25(OH)(2)D-3 content was not defined in any of these reports, the objective of the current investigation was to determine whether 1,25(OH)(2)D-3 may be stored in skeletal matrix. Bone powder samples from the iliac crest were extracted in ethylacetate/cyclohexane and 1,25(OH)(2)D-3 isolated from the extract by means of Sephadex LH-20 and high pressure liquid chromatographic separation and subsequently measured by radioimmunoassay (RIA). Within the detection range of the RIA, no 1,25(OH)(2)D-3 could be measured, suggesting that 1,25(OH)(2)D-3 is not stored in skeletal matrix. Vitamin D bone concentrations previously measured may therefore have reflected plasma contamination. Consistent with this hypothesis, only traces of skeletal 1,25(OH)(2)D-3 binding protein were measured when compared with serum values. Although 1,25(OH)(2)D-3 may act as a potential local determinant of bone remodeling, there is no evidence supporting a delayed paracrine function by matrix-derived 1,25(OH)(2)D-3.