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Abstract
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 occuring 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 pmo1/mg DNA. It is concluded that progesterone can antagonize estrogen action in the fetal uterus.