Ionized calcium and bone turnover in the estrogen-deficient rat

The long-term effects of high bone resorption on blood ionized calcium and calciotropic hormone levels following oophorectomy in 6-month-old Sprague-D awley female rats were investigated. Fasting urine and blood samples were c ollected from 16 sham and 16 oophorectomized (oophx) rats preoperatively an d up to 130 days postoperatively. From 50 days postoperation, daily injecti ons of 17-beta estradiol (E-2) (20 mu g/kg body weight) were administered s ubcutaneously to eight of the oophx rats. Urine hydroxyproline excretion (O HPrE) and serum osteocalcin were significantly elevated (P < 0.001) as a re sult of oophorectomy and normalized within 6 days of E-2 replacement. Urine deoxypyridinoline and total serum alkaline phosphatase were significantly elevated (P < 0.001) following oophorectomy and suppressed to control level s after 37 days of E-2 replacement. Blood ionized calcium was significantly reduced in oophx rats (P < 0.001) compared with sham rats and was normaliz ed by E-2 replacement at 55 days posttreatment. Serum 1,25 dihydroxyvitamin D (1,25(OHD)(2)D-3) was significantly elevated (P < 0.001) in oophx rats a nd again was normalized by E-2 at 55 days posttreatment. Serum parathyroid hormone (PTH) was unaffected by oophorectomy. These data indicate that desp ite increased bone resorption following oophorectomy, blood ionized calcium levels are decreased. The increased bone turnover in oophx rats was rapidl y suppressed by E-2 replacement before ionized calcium levels were normaliz ed, suggesting a direct effect of estrogen on the modulation of bone cell a ctivity. The depression of blood ionized calcium levels following oophorect omy, which is not mediated by calciotropic hormones, suggests an effect of estrogen on intestinal calcium absorption, renal handling of calcium, or a combination of both.