Antiandrogenic property of RU 486: enhancement of estrogen-induced uterine peroxidase activity in the rat.

The contragestational steroid RU 486 enhanced the increase in peroxidase activity produced by estradiol in estrogen-primed immature rat uteri and, like the antiandrogen flutamide, RU 486 reversed the inhibitory effect of testosterone on this estrogen-induced response. It antagonized the inhibition produced by progesterone but had no effect on peroxidase induction by itself or in unprimed immature animals. RU 486 also enhanced the effect of estradiol on the synthesis of complement component C3 in the rat uterus. The results confirm that RU 486 possesses antiandrogenic as well as antiprogestational properties. They also suggest that, in normal adult animals, the increase in peroxidase activity in the uterus in response to estrogen is not expressed fully but held in check by other endogenous steroids acting through their individual receptors.