The relationship between long bone growth rate and duodenal calcium transport in female rats

Studies were carried out to determine the relationship between long bone growth and duodenal calcium (Ca) transport in female rats and the regulation of these two parameters by ovarian hormones. Female rats were ovariectomized (ovx) at 6 weeks of age. Some animals were implanted with silastic implants containing either estradiol or progesterone at the time of ovx. Studies were carried out 3 weeks later when the rats were 9 weeks old. Ovx resulted in an increase in long bone growth rate and duodenal Ca transport without any alteration in circulating levels of parathyroid hormone or 1,25-dihydroxyvitamin D [1,25-(OH)2D]. Animals receiving estradiol exhibited decreased long bone growth rate and duodenal Ca transport relative to ovx animals. These animals were mildly hypercalcemic and had lower levels of 1,25-(OH)2D than ovx or intact animals. The results of these studies suggest that the effects of ovarian hormone status on duodenal Ca transport are more closely related to long bone growth rate than to circulating levels of 1,25-(OH)2D. Further studies are required to determine whether the two parameters are coregulated by some as-yet-unidentified factor or whether bone growth is able to emit some signal, directly or indirectly, to increase duodenal Ca transport.