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Abstract

To study the relative abundance and the changes of both estrogen receptor alpha (ERα) and ERβ mRNA before and after ovariectomy in major organs important to the regulation of calcium homeostasis, we compared the degree of mRNA expression of ERα to that of ERβ in rat tissues by performing competitive reverse transcription polymerase chain reaction (RT-PCR) with internal standards. Both ERα and ERβ were highly expressed in the ovary {ERα[(2.2 ± 0.33) × 107 copies/μg of total RNA] > ERβ[(1.2 ± 0.33) × 105 copies/μg of total RNA]} as we expected. The bone marrow and renal cortex were very important target organs of estrogen because ERα was highly expressed ∼2 × 105 copies/μg of total RNA, but marrow cells revealed only a very weak expression of ERβ [(0.7 ± 0.21) × 102 copies/μg of total RNA]. Both ERα and ERβ were expressed in the trabecular bone [(3.2 ± 0.56) × 103 copy/μg of RNA] and [(2.8 ± 0.21) × 102 copy/μg of RNA], respectively. However, they were not detected in the cortical bone. In the jejunum, the expression of ERα was not detectable, while ERβ was expressed very weakly [(1.1 ± 0.24) × 102 copies/μg of total RNA]. The thyroid gland expressed low copy numbers of ERβ [(6.0 ± 0.23) × 102 copies/μg of total RNA], but the parathyroid gland was negative for both ERα and ERβ mRNA. In cultured stromal cells, ERα and ERβ mRNAs were not detected after a 24-h culture; however, the rates of mRNA expression of ERα and ERβ reached ∼105 copies/μg of total RNA and ∼102 copies/μg of total RNA, respectively, after 9-, 11-, and 13-day cultures. After ovariectomy, the expression of ERα mRNA decreased abruptly in the bone marrow and renal cortex, and both ERα and ERβ were barely detected in the trabecular bone. In conclusion, ERα might be the main ER in organs important for calcium homeostasis, except in the jejunum. The mRNA expression of ERα in the bone marrow and renal cortex decreased abruptly after ovariectomy, which may partially explain why the effect of estrogen deficiency can be amplified and why trabecular bone loss is more predominant than cortical bone loss shortly after surgical or natural menopause.