| Abstrakt: |
Results from in vitro works suggest that 1,25- and 24,25-dihydroxyvitamin D (1,25-(OH)D and 24,25-(OH)D) act on bone via different mechanisms. The present investigation was performed to study the effect of these two metabolites and of their precursor 25-hyxdroxyvitamin D (25-(OH)D) on bone cAMP content in vitro. Rats' paired half calvaria were incubated under sterile conditions with one vitamin D derivative (10 to 10 M) or with ethanol (0.005 ml for 15 min to 24 h in 1 ml medium containing 0, 0.2, 1, 2, or 3 mM calcium. In some experiments: (a) cycloheximide (10M) was added simultaneously with the vitamin D metabolites; (b) 1-84 bPTH (5 × 10 M) was added for 5 or 15 min at the end of the 24 h incubation. Calvaria were immersed in 1 ml TCA 5% 4°C and homogenized. The cAMP was extracted with diethylether and measured by a competitive protein binding assay. Results bring further evidence for a particular effect of low doses of 24,25-(OH)D (10 to 10M) and of 25-(OH)D (10 to 10M) on bone, different from that of 1,25-(OH)D: cAMP content was higher in 24,25-(OH)D- or 25-(OH)D-treated and lower in 1,25-(OH)D-treated calvaria than in ethanol-treated ones with 1 mM calcium. The 1,25-(OH)D effect persisted in calcium-free medium whereas 25-(OH)D and 24,25-(OH)D effects could not be observed with 0 mM nor with 3 mM calcium. The required duration of the preincubation (over 1 h) as well as the inhibitory action of cycloheximide may suggest an involvement of protein synthesis in the vitamin D metabolites effects. Neither 1,25-(OH)D nor 24,25-(OH)D affected the PTH-induced increase in bone cAMP content. [ABSTRACT FROM AUTHOR] |
Full text from SpringerLink