1- but not 24-hydroxylation of vitamin D is required for skeletal mineralization in rats

Vitamin D metabolites prevent vertebral osteopenia in ovariectomized rats

The present study investigated the prophylactic effects of vitamin D metabolites and vitamin D metabolite combinations on static and dynamic, tetracycline-based, histomorphometric parameters in the axial skeleton of ovariectomized rats. Forty-three Fischer-344 rats (10 weeks old, 130 g each body weight, BW) were either bilaterally ovariectomized (OVX) or sham-operated (SHAM). The rats were allocated into the following groups: SHAM; OVX; OVX + 7.5 ng 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]/rat/day; OVX + 15 ng 1 alpha,24R,25-trihydroxyvitamin D3 [1,24,25-(OH)3D3]/rat/day; OVX + 75 ng 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3]/rat/day; OVX + 7.5 ng 1,25(OH)2D3/rat/day + 15 ng 1,24,25(OH)3D3/rat/day; OVX + 7.5 ng 1,25(OH)2D3/rat/day + 75 ng 24,25(OH)2D3/rat/day. The vitamin D metabolites were fed orally starting 4 weeks after surgery. Urine and blood samples were collected 12 and 16 weeks postovariectomy, respectively. Sixteen weeks after surgery, all rats were sacrificed, and the first lumbar vertebrae were processed undecalcified for histomorphometric analysis. Ovariectomy induced a highly significant reduction (P less than 0.001) of cancellous bone mass in the secondary spongiosa of the lumbar vertebral body. The bone loss in OVX rats was accompanied by a distinct elevation of all histomorphometric parameters of bone formation and resorption. 1,25(OH)2D3 and both vitamin D metabolite combinations significantly raised serum calcium levels and prevented the bone loss by inhibiting the increased bone resorption in OVX rats. In the applied dosage, 1,24,25(OH)3D3 and 24,25(OH)2D3 alone were ineffective in preserving the cancellous bone of the lumbar vertebra in OVX rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcitriol but no other metabolite of vitamin D is essential for normal bone growth and development in the rat

To determine the relative importance of different metabolites of vitamin D in bone growth and development, weanling male rat pups suckled by vitamin D-deficient mothers were given either calcitriol (1,25-dihydroxycholecalciferol) by continuous subcutaneous infusion, oral calcidiol (25-hydroxycholecalciferol), or oral 24,24-difluoro-25-hydroxycholecalciferol, a synthetic compound that can undergo 1-hydroxylation but not 24-hydroxylation, as their sole source of vitamin D for 40 d. Pups raised in the same manner, but given no vitamin D, served as controls. The three metabolites compared were given in doses that restored normal plasma calcium levels and normal increments in body weight. After in vivo double tetracycline labeling, bone histomorphometry by standard methods was performed on one femur and one tail vertebra. There were no significant differences between the three metabolite-treated groups in length, periosteal or endosteal diameter, cortical cross-sectional area, cortical porosity, osteoid thickness and volume, appositional rate and bone formation rate in the femur, or in qualitative and quantitative indices of endochondral ossification in the tail vertebra. All three groups differed markedly from the untreated controls with respect to all measurements. Collectively, the data indicate that neither calcidiol nor any 24-hydroxylated metabolite of calcidiol is needed in the rat (other than as a precursor) for longitudinal or transverse bone growth, for normal endochondral ossification, or for normal periosteal and endosteal formation, mineralization, and resorption of bone. Calcitriol was fully active with respect to each of the indices listed when given in a manner resembling its continuous endogenous production by the kidney, suggesting that previous reports of incomplete skeletal response to calcitriol result from its rapid clearance and infrequent oral administration. We demonstrated that calcitriol is the only metabolite that is both necessary and sufficient for normal bone growth and development in the rat, but our data do not indicate the extent to which its beneficial skeletal effects were mediated by direct action on bone, either of calcitriol itself or of some metabolite thereof, or by restoration of normal plasma levels of calcium and phosphate.