Effect of hydroxy-vitamin D3 derivatives on 45Ca release from rat fetal bones in vitro.

Several hydroxy vitamin D3 (OH-D3) derivatives were tested for biological activity by measuring 45Ca release from prelabeled rat fetal bones, in vitro. In a 72 h continuous culture, 1 alpha,25-(OH)2-D3 produced a significant effect at 10 pg/ml. A similar effect was produced by 50 ng/ml of either 25-OH-D3 or 5,6-trans-25-OH-D3, and by 500 pg of 3 deoxy-1 alpha,25-(OH)2-D3. 24R,25-(OH)2-D3 was more active than 24S,25-(OH)2-D3, and the R isomer had activity that more closely resembled the biosynthetic compound. 3-deoxy-1 alpha-OH-D3 was inactive at a concentration of up to 1 microgram/ml. Using a 24 h preincubation period with no added vitamin D3 derivative, a steep dose-response curve could be obtained with 1 alpha,25-(OH)2-D3 over a range of 2-10 pg/ml during a subsequent 96 h incubation period, and 1 alpha,25-(OH)2-D3 was found to have about 5000 times the activity of 25-OH-D3.