Role of calcitriol in the management of osteoporosis.

Ovariohysterectomized Beagle dogs provide a useful model for the study of bone loss following cessation of ovarian function. Data obtained in our laboratory suggest that the sequence of events occurring after cessation of such function proceeds in two phases. The first, or "initiation," phase begins within 1 month after ovariohysterectomy (OHX). Although the number of osteoclasts is unchanged, there is an increase in trabecular separation, accompanied by a dramatic decrease in cancellous bone volume, which suggests an apparent dramatic hyperresorption. This early phase is followed by a "maintenance" phase, which commences in the second month and is related to an osteoblastic insufficiency. In another experiment, administration of 1,25(OH)2D3 corrected bone loss occurring after OHX in Beagle dogs. However, a positive balance between increased bone cell activity and decreased bone cell number was not maintained. This calls for new strategies to separate the positive effect of 1,25(OH)2D3 on bone cell activity from the negative effect on bone cell number.