Myometrial response to prostaglandins enhanced by progesterone

The effect of estrogen and progesterone on prostaglandin-induced uterine contractions was studied in ovariectomized rats fitted with intrauterine recording balloons. The effects of the three prostaglandins, PGE₁, PGE₂, and PGF_2α, injected intravenously were very similar. Pretreatment with estradiol benzoate (1 μg per rat per day) resulted in a considerable reduction of the contractile response to the prostaglandins and was frequently associated with tachyphylactic reactions. Progesterone pretreatment (5 mg. per rat per day), on the other hand, markedly increased the uterine response to prostaglandins and no tachyphylaxis was observed. Intact pregnant rats were studied at mid-gestation; the uterine response in these rats was similar to the response in progesterone-treated rats.