Effects of age and estrogen on renal vitamin D metabolism in the female rat

Young and adult, intact and ovariectomized female rats were used to study the effects of increasing age and estrogen loss on the conversion of 25-hydroxycholecalciferol [25(OH)D] to 1,25-dihydroxycholecalciferol [1,25(OH)2D] and 24,25-dihydroxycholecalciferol [24,25(OH)2D]. The recovery level of radioactivity in plasma as 1,25(OH)2D after [3H]25(OH)D administration was lower in older animals; ovariectomy reduced [3H]1,25(OH)2D recovery in both young and adult animals. 1,25(OH)2D synthesis by kidney slices was reduced in all adult animals and in ovariectomized young animals. Calcium absorption was also lower in adult animals and in young ovariectomized animals then in controls. Beta-estradiol administration increased recovery of [3H]1,25(OH)2D but not [3H]24,25(OH)2D, which is parallel to the general lack of effect of ovariectomy on 24,25(OH)2D production. Parathyroidectomy eliminated the estrogen effect on 1,25(OH)2D production. Thus, estrogen loss and advancing age reduced 1,25(OH)2D production and Ca absorption independently, the estrogen effect being mediated via the parathyroid gland.