This study was designed to investigate the mechanisms involved in the regulation of the conversion of 25‐hydroxyvitamin D3 (25‐OHD3) to 1,25‐dihydroxyvitamin D3 [1,25‐(OH)2D3] and 24,25‐dihydroxyvitamin D3 [24,25‐(OH)2D3] in primary cultures of osteoblastlike cells from neonatal mouse calvariae. These cells, when incubated with tritiated 25‐OHD3 ([3H]25‐OHD3), spontaneously synthesized [3H]24,25‐(OH)2D3 20‐50 times more efficiently than [3H]1,25‐(OH)2D3 at a rate of conversion that was substrate dependent and linear from 1 to 36 h. Gas chromatography‐mass spectrometry verified the identity of the dihydroxylated metabolites. The calcium ionophore A23187 (5 μM) consistently stimulated the synthesis of 1,25‐(OH)2D3 while suppressing the production of 24,25‐(OH)2D3. This effect was sustained for 36 h and was dose dependent for concentrations from 0.05 to 10 μM. Furthermore, A23187 stimulated cAMP production and indomethacin (50 ng/ml) blocked the A23187‐induced production of cAMP and 1,25‐(OH)2D3 but had no effect on the suppression of 24,25‐(OH)2D3 by A23187. This led to other experiments to find out whether the stimulative effect of A23187 on 1,25‐(OH)2D3 synthesis is mediated by prostaglandins or cAMP, or both. PGE2 (10−8‐10−6 M) increased the production of 1,25‐(OH)2D3 and of 24,25‐(OH)2D3. Forskolin (0.01‐10 μM) and dibutyryl cAMP (0.1‐10 mM) increased the production of both metabolites but to a lesser degree than PGE2. These data suggest that osteoblastlike cells are stimulated by A23187 to increase the synthesis of 1,25‐(OH)2D3 through mechanisms involving prostaglandins and cAMP. The synthesis of 24,25‐(OH)2D3 is suppressed by A23187 through different mechanisms.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Orthopedics and Sports Medicine