Interactions of oestradiol-17beta and tamoxifen in the uterus of the pregnant rat.

Measurement of the uptake and retention of a radioactive post-coital antifertility agent tamioxifen, by reproductive tissues of the rat have shown that the ovary retained more radioactivity than did any other reproductive organ. Studies have also been made of the uptake and distribution of [3H]tamoxifen and [3H]oestradiol-17beta in the uterus of the pregnant rat on days 2-6 post coitum. Twenty-four hours after administration of tamoxifen, either i.v. or orally, 40-50% of the radioactivity was in the high speed pellet, 10-20% in the nuclear fraction, and 15-30% in the cytosol. An equivalent dose of [3H]oestradiol-17beta yielded distributions of 5%, 5% and 82% respectively. Fractionation of uteri from animals given 0-2 mg tamoxifen/kg on Day 2 of pregnancy followed by [3H]oestradiol 60 min before death showed little difference in total uptake of oestradiol or distribution in the subcellular fraction on Days 4,5 and 6. Although uptake of oestradiol by uterine nuclei was reduced on Day 3 by previous administration of tamoxifen on Day 2, appreciable quantities were still bound to the nuclear receptors. Treatment of ovariectomized animals with tamoxifen at doses up to 40 mug/rat (i.e. 0-2 mg/kg) led to the accumulation of oestrogen-receptor complex in the nucleus. It is concluded that the antifertility properties of tamoxifen (under the conditions of these experiments) cannot be ascribed to the suppression of uptake and binding of oestradiol by the uterus.