Effects of actinomycin D and estrogen priming on decidual growth in ovariectomized mice

Ovariectomized mice were primed for 2 days with estradiol and/or actinomycin D. In order to evaluate the effects of these treatments on endometrial cell proliferation, colchicine and [3H]-thymidine were administered shortly before killing groups of animals at days 4 and 5 after priming (the latter groups received 500 micrograms progesterone plus 10 ng estradiol 24 h before killing). The same priming treatments were administered 3 days before starting hormonal treatment eliciting uterine sensitivity to decidualization (incuded by intrauterine oil injection). The comparison of labeling and mitotic indices and of decidual tissue weights between experimental groups showed that under such conditions: (1) actinomycin D only partly inhibits the effects of estrogen priming: the increase in cell division obtained on day 4 after priming remains unchanged in all three endometrial components while the increase in stromal mitotic activity at day 5 and further decidual growth are reduced, (2) since the inhibition rate of these parameters is greater in non-primed than in primed animals, it appears that estrogen priming counteracts the antagonistic action of actinomycin D.