Further studies on the luteotropic action of estrogen in rats.

A single injection of 10μg/rat of estradiol benzoate (EB) at 10.00 hr on the day of estrus (designated as Day 1) in 4-day cyclic rats caused a marked increase in ovarian secretion of progesterone and a concomitant decrease in 20α-hydroxypregn-4-en-3-one secretion for over a week. Hypophysectomy on Day 3 completely diminished the secretion of progesterone on Day 5. The treatment with EB induced the surgy releases of prolactin and LH on the evening of Day 2 or 3, but no further release was seen thereafter. In pseudopregnant rats, a marked release of prolactin occurred between midnight and early morning, but any nocturnal release of neither prolactin, LH, nor FSH was seen in the EB treated rats so far examined on Day 4 to 5. Single injections with prolactin, LH and/or FSH on the evening of the first day of diestrus in cycling rats failed to sustain the luteal function. Pretreatment with progesterone, given an hour prior to the EB treatment, completely blocked the release of gonadotropins expected on Day 2 without reducing the elevated secretion of progesterone in the EB treated rats. These results suggest that the gonadotropins released on Day 2 can not be mediators for the luteotropic action of estrogen. Since hypophysectomy after EB treatment reduced ovarian secretion of progesterone to the basal level, it is presumed that estrogen maintains luteal function by acting on the luteal tissue in the presence of hypophyseal factors.