25-hydroxycholecalciferol-enhanced bone maturation in the parathyroprivic state.

In vitro evidence presently favors a direct osteolytic effect of biologically active vitamin D metabolites. Studies were designed to evaluate the effect of 25-hydroxycholecalciferol (25OHD3) on bone collagen and mineral maturation in vivo and its dependence on parathyroid hormone (PTH). After treatment of sham-operated control and parathyroidectomized (PTX) mature rats with either 25OHD3 or an oil vehicle for 2 wk, tibial bone mineral-collagen maturation was quantitated by bromoform-toluene density gradient fractionation techniques. Intestinal calcium absorption was measured by in vivo 45Ca transport procedures. In contrast to the control group, the response to 25OHD3 of PTX rats was dramatic. Bone mineral and matrix maturation were both accelerated by 25OHD3 treatment without concomitant reduction in total bone mineral or collagen content or changes in the intestinal calcium absorption. These observations support the premise that biologically active vitamin D metabolites stimulate bone tissue maturation, and that PTH is not required in this regard.