Effect of vitamin D metabolites on bone metabolism in a rat model of postmenopausal osteoporosis.

A rat model of postmenopausal osteoporosis was introduced, using ovariectomized rats on a low Ca diet. CT treatment of these animals for one month prevented the decrease in both mineral contents and physical properties of the femoral bone. Treatment of the animals with 1,25(OH)2D3 was effective in increasing bone mineral contents and maintaining positive mineral balance, but did not increase the physical tolerance of bones. In contrast, 24,25(OH)2D3 increased the breaking force of the femoral bone, with minimal effect on bone mineral contents and mineral balance. These results suggest that 1,25(OH)2D3 and 24,25(OH)2D3 act differently on the matrix phase and mineral phase of bones, but that they act together to maintain mineral balance and structural integrity of bones. The mechanism of how these vitamin D metabolites affect bone metabolism remain to be clarified.