1alpha, 25 - (OH)_2 D_3 - 26, 23 - lactone stimulates bone formation.

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23(S), 25(R)-1, 25-dihydroxyvitamin D_3-26, 23-lactone(1, 25-lactone), a physiological metabolite of 1, 25-dihydroxyvitamin D_3(1, 25-(OH)_2D_3), has been shown to have unique actions different'from those of 1, 25-(OH)_2D_3. In contrast to 1, 25-(OH)_2D_3, 1, 25-lactone causes a significant reduction in serum Ca level, stimulates collagen production in osteoblastic cell lines, and inhibits bone resorption induced by 1, 25-(OH)_2D_3. We examined the effect of 1, 25-lactone on ectopic bone formation using a bone-inducing factor and on osteporosis induced by sciatic neurotomy combined with ovariectomy in adult female rats.1, 25-lactone increased[ ^3H]proline uptake during chondrogenesis and ^<85>Sr uptake during bone formation. Examination using soft X-ray apparatus showed the size and weight of induced bone increased significantly in 1, 25-lactone group by this compound 3 weeks after implantation. Furthermore, small dosis of 1, 25-lactone significantly increased bone formation rate, and prevented the development of osteoporosis. These results suggest that the 1, 25-lactone stimulates bone formation under in vivo conditions.