The effect of RU486 on progesterone and oestrogen receptor concentration in human decidua on early pregnancy

Concentrations of progesterone receptor (PR) and oestrogen receptor (ER) were measured by radioligand assay in decidual tissue of women undergoing termination of early pregnancy (amenorrhoea up to 49 days). Pregnancies were terminated by vacuum aspiration at 12 or 36 h after oral administration of placebo or antiprogestin RU486 in different doses. Treatment with RU486 decreased decidual PR content, the effect being observed at 12 h as well as at 36 h after 600 mg RU486 and at 36 h after 3 x 25 mg RU486 given at 12 h intervals. PR concentration 12 h after a single dose of 25 mg RU486 was not affected. ER content was unchanged at 12 h after RU486 but increased 36 h after 600 mg and 3 x 25 mg RU486. Our data suggest that apart from blocking progesterone action, RU486 may exert its abortifacient effect through decreasing the PR concentration. The simultaneous decrease of PR concentration and an increase of ER concentration changes the balance between them in favour of ER, which might also play a role in the abortifacient effect of RU486.