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Therapeutic efficacy of 1alpha,25-dihydroxyvitamin D3 and calcium in osteopenic ovariectomized rats: evidence for a direct anabolic effect of 1alpha,25-dihydroxyvitamin D3 on bone.

It is an important question for clinical therapy of osteoporosis with vitamin D metabolites whether these compounds exert their beneficial effects on the skeleton indirectly through an increase in intestinal calcium absorption or whether there is also a major direct component of action on bone. In this study, female 6-month-old Fischer rats were either ovariectomized (OVX) or sham operated. One month before surgery, all rats were placed on a diet containing 0.25% calcium and were kept on this diet throughout the study. Beginning 3 months post-OVX, groups of OVX rats orally received vehicle, a calcium supplement, low dose (0.025 microg/kg x day) or high dose (0.1 microg/kg x day) 1alpha,25-dihydroxyvitamin D3 [1,25-(OH)2D3], or combinations of low and high dose 1,25-(OH)2D3 with the calcium supplement. By 3 months postsurgery, pretreatment OVX controls had lost 74% and 37% of tibial and vertebral cancellous bone, respectively. Two-way factorial ANOVA showed that a 3-month treatment of osteopenic OVX rats with 1,25-(OH)2D3 dose dependently increased vertebral and tibial cancellous bone mass (P < 0.001 and P = 0.021, respectively) and trabecular width (P < 0.001). Furthermore, 1,25-(OH)2D3 increased serum calcium (P = 0.028) and urinary calcium excretion (P < 0.001) and reduced serum PTH levels (P < 0.001), osteoclast numbers (P < 0.001), and urinary collagen cross-links excretion (P < 0.001). Calcium supplementation alone was without therapeutic effect, and there was no significant two-way interaction between the individual treatment effects of 1,25-(OH)2D3 and calcium on bone mass. These data indicate that the anabolic effects of 1,25-(OH)2D3 in osteopenic OVX rats are mediated through a direct activity on bone.