Effects and interactions of 24R,25(OH)2D3 and 1,25(OH)2D3 on bone.

The effects of various combinations of therapy with 1 alpha,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24R,25-dihydroxycholecalciferol (24R,25(OH)2D3) on structural and dynamic parameters of bone were evaluated in 40 chicks raised on a vitamin D-deficient diet from time of hatching and supplemented with the dihydroxylated metabolites. The results showed that: 1) the maintenance of volumetric density of bone is dependent on the presence of 1,25(OH)2D3, 2) lack of 1,25(OH)2D3 is associated with an increase in the number of osteocytes per unit volume of bone, most probably due to decreased amounts of bone formed by each osteoblast before becoming an osteocyte, 3) adequate quantities of either 24R,25(OH)2D3 or 1,25(OH)2D3 are needed to prevent accumulation of osteoid or the production of endosteal fibrosis, and 4) maintenance of normal tetracycline label width requires both hydroxylated compounds with one of them in sufficient amounts. The data of this study demonstrate that the integrity of certain parameters of bone structure could be maintained only with 1,25(OH)2D3, others with either dihydroxylated metabolites, and still others with a combination of both. These data underscore the biological activity of 24R,25(OH)2D3 by demonstrating its effectiveness on bone.