Control of Corpus Luteum Function during the Second Half of Pregnancy in the Rat: A Direct Relationship between Conceptus Number and Both Serum and Ovarian Relaxin Levels*

The effects of conceptus or fetus removal on several parameters of luteal activity (serum and ovarian relaxin immunoactivity levels, serum progesterone immunoactivity levels, and corpus luteum weights) were investigated during the second half of pregnancy in the rat. Conceptuses were aspirated from the uteri of pregnant rats on day 8 (d8) of pregnancy so that they carried 0, 1, 2, 5, or 10 or more conceptuses until they were autopsied on d10, d12, d14, d16, d18, or d20 of pregnancy. A direct relationship existed between the number of conceptuses and the rate and/or degree of increase in all of the parameters of corpus luteum activity. The conceptuses had no local effect on ovarian relaxin levels or on luteal weights when the conceptus side ovary was compared with the nonconceptus side ovary. In pregnant rats in which all but two fetuses were removed (all placentas were left undisturbed), there were no significant differences on d18 or d20 in any parameter of corpus luteum activity between fetectomized and sham-operated rats. It appears, therefore, that the placenta plays the dominant role in the regulation of all of the above parameters of corpus luteum activity during the second half of pregnancy in the rat. The specific factor(s) or mechanism(s) by which the placenta promotes relaxin synthesis, progesterone secretion, and corpus luteum growth have yet to be clearly and unequivocally characterized in the rat. The fact that conceptus number and the rate and/or degree of increase of all of the parameters of luteal activity measured from d12 through d18 are directly related is consistent with the possibility that the placental support of these activities is mediated through a common mechanism during this period in the rat.