The importance of a luteolytic effect of the pituitary in understanding the placental control of the rat corpus luteum.

In rats bearing a full complement of conceptuses (FC-bearing rats) progesterone secretion increases about 1.5 times and the corpora lutea double in weight between days 12 and 16. Reducing the number of conceptuses to one (by aspirating all but the single conceptus) at any time between days 7 and 12 of pregnancy (day 1 = insemination) caused progesterone secretion and corpus luteum weight to remain at or below the day 12 level until term. When rats bearing one conceptus were hypophysectomized on day 12, however, serum progesterone increased and fell after day 12 in a pattern almost identical to that in FC-bearing rats, except that the peak and trough occurred about 2 days later; hypophysectomy on day 15 or 18 also increased serum progesterone to values approaching those in intact FC-bearing rats. In a series of rats bearing experimentally determined conceptus numbers between 0 and 5 or naturally occurring ones of 6-16, the serum level of placental lactogen on day 12 (the day of the first peak of secretion of this hormone) was directly proportional to conceptus number over the entire range. The pattern of rat placental lactogen secretion between days 7 and 23 in FC-bearing rats included two peaks, on day 12 and on days 18-21, but in one-conceptus-bearing rats, the level was only slightly higher than the PRL level, and it remained unchanged, even by hypophysectomy, until term. Serum PRL levels bore no relation to conceptus number, were very low, and were unaffected by hypophysectomy except in the FC-bearing rats, in which lypophysectomy prevented the rise that normally occurs after day 21. These results, especially in relation to others previously reported, show that by day 12 of pregnancy, the rat pituitary, instead of merely becoming luteotropically inactive, produces a substance that inhibits further development of the corpora lutea. The results also imply that the placentas maintain luteal growth and progesterone secretion as much by suppressing this luteolytic activity as by stimulating the corpus luteum directly.