Measurement of Estrogen Effect on Bone Turnover by ²H₂O Labeling

Estrogen loss has been known to increase bone turnover through accelerated bone resorption coupled by increased bone formation. In the present study, we measured estrogen effect on bone turnover by incorporation of ²H from ²H₂O into amino acids. At 6 weeks of age, rats were either sham-operated (sham) or ovariectomized (ovx). Two weeks after surgery, 17β-estradiol (est) was implanted subcutaneously to ovx rats. At 9 weeks of age, ²H₂O labeling started by administration of 4% ²H₂O to rats for 4 or 7 weeks in drinking water after a single intraperitonial bolus injection with 99.9% ²H₂O. Body ²H₂O enrichments were stable at approximately 3.0% over labeling period. Fractional replacements (f) of the midshaft femur were higher in the sham group (40.36 ± 4.89% vs 42.47 ± 11.22%) than the ovx (28.57 ± 9.67% vs 37.47 ± 8.34%) and est (26.57 ± 4.00% vs 30.35 ± 5.34%) groups 4 and 7 weeks after labeling, respectively. Ovariectomy-induced bone loss was observed in the trabecular bone along with a significantly increased number of osteoclasts, all of which were normalized after estradiol treatment. Taken together, our results indicate that estrogen deficiency significantly reduces the proportion of newly synthesized bone matrix as well as the total amount of bone matrix. The reduced portion of new matrix in ovx rats, presumably caused by activated osteoclastic degradation, was compensated rapidly with time. In addition, estradiol treatment protected the bone matrix by decreasing bone turnover rate.