Progesterone-like relaxant effect of RU 486 in the rat myometrium

The antihormone RU 486 is characterized by its antiprogesterone and antiglucocorticoid activities. In this work the likelihood of a non-genomic effect for this compound was assessed. Thus, RU 486 was compared with progesterone and the 5 beta-progestin pregnanolone, for its ability to modify the uterine contractility of the rat. An outstanding relaxant effect elicited by RU 486 was observed, slightly higher than that produced by progesterone but lower than pregnanolone. Moreover, calcium promoted contractions were antagonized by RU 486, in the same way as the endogenous steroids. The data suggest the capability of RU 486 to block the calcium channels. It is concluded that a non-genomic effect of RU 486 is produced before its journey into the cell for its genomic action.