Effects of daily anti-estrogen treatment on uterine growth and progesterone receptor concentrations in adult rat uterus.

The effects of daily Tamoxifen treatment on adult rat uterus weight depend on the degree of estrogenic impregnation of the animal. In intact animals, Tamoxifen caused a decrease of the uterine weight. A weight increase was observed, after castration, which was dose-dependent without reaching saturation even with 500 micrograms of Tamoxifen per day. This uterine weight increase, reaching its peak during the first 24 h after the initial Tamoxifen application, is attributable to water retention. The uterine progesterone receptor (PgR) concentrations are stimulated by daily antiestrogen treatment of intact and castrated animals. This increase in the PgR levels is initially dose-dependent up to 10 micrograms of Tamoxifen per day reaching maximal levels after 4 days of treatment. The results indicate that growth and PgR induction in adult rat uterus are essentially controlled by independent mechanisms, constituting the biological basis of the synergistic effects of anti-estrogens and progestagens.