The effects of 1,25-dihydroxycholecalciferol on embryonic bone in vitro: a biochemical and histological study.

Explants from embryonic rat and mouse calvaria were cultivated in the presence of different concentrations of 1, 25-dihydroxycholecalciferol (1, 25-(OH)2D3). The bone resorbing effects of the vitamin D3 metabolite were evaluated by measuring the release of calcium, phosphate, lactate and citrate into the culture medium after 24 h of cultivation. The influence on bone morphology was studied using embryonic mouse radii, in which histological phenomena in the bony, cartilagenous and connective tissue compartments were observed. Both kinds of experiments show that 1, 25-(OH)2D3 has effects on embryonic bone which are typical for high concentrations of parathyroid hormone (PTH). However, the maximal effects on calcium release and on histology are 2.5 times less than those of PTH. In addition, some of the histological features such as the effects on epiphyseal cartilage and on osteoclasts were not observed. 1, 25-(OH)2D3 in concentrations up to 2 X 10(-9) M does not affect basal or PTH-stimulated cAMP levels in embryonic rat calvaria. In a concentration of 1 X 10(-8) M, however, a significant decrease in PTH-stimulated cAMP production was found. It is concluded from these in vitro experiments that (1) 1, 25-(OH)2D3 stimulates bone resorption, and (2) in 1, 25-(OH)2D3-induced bone resorption cAMP is apparently not involved as a second messenger.