M. Tebar et al., ONE-DAY ESTROUS-CYCLE SHORTENING INDUCED BY ANTIPROGESTAGEN RU486 ADMINISTRATION IN PROESTRUS TO 4-DAY CYCLIC RATS, Revista Espanola de Fisiologia, 51(2), 1995, pp. 85-91
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.