Effects of vitamin D2 analogs on calcium metabolism in vitamin D-deficient rats and in MC3T3-E1 osteoblastic cells

The effects of 1α-hydroxyvitamin D₂ on calcium metabolism in vivo and of 1α,25-dihydroxyvitamin D₂, which is an active metabolite of 1α-hydroxyvitamin D₂, on bone metabolism in vitro was studied and compared with that of 1α-hydroxyvitamin D₃ or 1α,25-dihydroxyvitamin D₃. 1α-Hydroxyvitamin D₂ and 1α-hydroxyvitamin D₃ was equally potent in stimulating intestinal calcium transport by using the everted sac method and of calcium mobilization from bone in vitamin D-deficient rats. On the other hand, the hypercalcemic activity of 1α-hydroxyvitamin D₂ was much lower than that of 1α-hydroxyvitamin D₃ in normal mice and rats. 1α,25-Dihydroxyvitamin D₂ and 1α,25-dihydroxyvitamin D₃ stimulated alkaline phosphatase activity in ostoblastic MC3T3-E1 cells and bone resorption in newborn mouse calvaria maintained in organ culture. These results show that 1α-hydroxyvitamin D₂ as well as 1α-hydroxyvitamin D₃ promote calcium absorption and may accelerate bone remodelling via direct action on osteoblasts. In addition, they suggest that 1α-hydroxyvitamin D₂ may be more useful than 1α-hydroxyvitamin D₃ for the treatment of senile osteoporosis, because hypercalcemia is one of the major side effects of 1α-hydroxyvitamin D₃.