Calcium and vitamin D in bone metabolism: analyses of their effects with a short-term in vivo bone model in rats.

A short-term in vivo system was developed to examine simultaneously bone formation and resorption, and the effects of dietary calcium and vitamin D on these processes. In experiment 1, 25 male Long-Evans rats were each implanted with two gelatin capsules containing mineralized bone (MB) powder subcutaneously in the thorax region. At 4, 6, 8, 10 and 12 d after implantation the acid phosphatase activity (resorption) increased significantly (P less than 0.01), whereas alkaline phosphatase activity (formation) did not change. In experiment 2, both MB and demineralized bone (DB) powder were implanted on contralateral dorsal sites of the thorax in 40 male Long-Evans rats and harvested after 7, 9, 11 and 13 d. Enzyme, mineral and histological assessment indicated bone formation in DB implants with bone resorption in MB implants. In experiment 3, the effects of dietary calcium (0.2 or 1.0%) and vitamin D (cholecalciferol at 300 ng/d or 1,25-dihydroxycholecalciferol [1,25(OH)2D3] at 75 ng/d) were examined using 40 male Long-Evans rats. These rats were implanted with both DB and MB powders and the implants were harvested on d 12. Both low (0.2%) dietary calcium and 1,25(OH)2D3 stimulated resorption of MB implants. Therefore, the physiological processes of bone formation and resorption were mimicked in this system of bone powder implants. Further, dietary calcium and 1,25(OH)2D3 were shown to modulate these processes.