ONE-DAY ESTROUS-CYCLE SHORTENING INDUCED BY ANTIPROGESTAGEN RU486 ADMINISTRATION IN PROESTRUS TO 4-DAY CYCLIC RATS

Administration of 4 mg of the antiprogestagen RU486 to 4-day cyclic ra ts in proestrus, which blocks proestrous and diestrous progesterone ac tions, induced a one day shortening of the ovarian cycle and a reducti on of the ovulation rate in the following cycle. These effects were no t present when RU486 was administered in estrus or metestrus. RU486 in jections either in proestrus or estrus increased the serum levels of L H and 17 beta-estradiol during metestrus. However, only rats injected with RU486 in proestrus presented a 24 hour advancement of the preovul atory surge of gonadotropins and a lack of the LH-inhibiting effect of exogenous estradiol. These results suggest that, in 4-day cyclic rats , the secretion of progesterone by the corpora lutea during diestrous phase retards the follicular development by lowering the serum concent rations of LH, whereas progesterone secretion by the preovulatory foll icles in proestrus regulates the estrous cycle length by antagonizing the desensitization of the pituitary to the estrogen negative feedback on LH secretion.