Effects of age and estrogen on calcium absorption in the rat.

In order to differentiate the relative effects of age and estrogen on rates of calcium absorption and serum levels of parathyroid hormone and calcitonin, the effect of oophorectomy with and without estrogen replacement (2 weeks) was studied in rats for 30- or 120-day periods. Whereas oophorectomy for 30 days resulted in a significant decrease in serum calcium and an increase in serum phosphate, no change in either calcium or phosphate was observed in the 120-day oophorectomized animals. iPTH decreased in both the 30- and 120-day oophorectomized animals although these changes were not significant at the .05 level. Whereas no significant change in basal circulating calcitonin occurred in the 30-day oophorectomized rats, it decreased significantly in the older animals following the ablative procedure. Forty-five days following estrogen deprivation, calcitonin release to a calcium secretagogue was significantly blunted. Intestinal calcium absorption decreased with age, and unlike the increments in calcium absorption observed in the younger estrogen-repleted, 30-day oophorectomized rat, no change in calcium absorption was observed when estrogens were administered to the older, 120-day oophorectomized rat. The accumulated data suggest that the effects of estrogen loss on the hormonal control of bone metabolism and calcium absorption are age dependent, and that estrogen contributes significantly to changes in calcium homeostasis observed in the maturing rat.