A possible dual mechanism of the anovulatory action of antiprogesterone RU486 in the rat.

The purpose of these experiments was to investigate the mechanism of the anovulatory action of ant iprogesterone RU486 (RU486) in rats by studying its effects onfollicular growth, secretion of gonadotropins and ovarian steroids, and ovulation. Rats with 4-day estrous cycles received injections (s.c.) of either 0.2 ml oil or 0.1, 1, or 5 mg of RU486 at 0800 and 1600 h on metestrus, diestrus, and proestrus. At the same times, they were bled by jugular venipuncture to determine serum concentrations of luteinizing hormone (LH). follicle-stimulqting hormone (FSH), / 7f3-es:radiol (E), and progesterone (P). On the morning of the day after proestrus, ovulation and histologicalfeatures of the ovary were recorded. Rats from each group were killed on each day of ovarian cycle to assess follicular development. Rats treated similarly were decapitated at the time of the ovulatory LH surge and blood was collected to measure LH. The serum levels of LH increased and those of FSH decreased during diestrus in rats treated with R U486.