Progesterone antagonizes the effects of estradiol in the fetal uterus of guinea pig.

The antagonistic effect of progesterone on estrogen responses in the uterus of the guinea pig fetus has been demonstrated. The administration of 5 mg of progesterone to estradiol-primed pregnant guinea pigs inhibits the increase in uterine wet weight normally occurring between 1 and 2 days after estradiol treatment. Progesterone also decreased the concentration of its own receptor by as much as 51%, 4 days after treatment. Progesterone had no effect on the replenishment of estrogen receptor in the cytosol but it induced a rapid decrease in nuclear receptor since 5 days after estradiol treatment the concentration of nuclear estrogen receptor is 2.0 +/- 0.65 pmol/mg DNA while only 24h after progesterone treatment this value is 0.78 +/- 0.10 pmol/mg DNA which is not significantly different from the untreated control values of 0.68 +/- 0.12 pmol/mg DNA. It is concluded that progesterone can antagonize estrogen action in the fetal uterus.