The Luteotrophic Effect of Estrogen in the Rat: Prevention by Estradiol of the Luteolytic Effect of an Antiserum to Luteinizing Hormone in the Pregnant Rat¹

Pregnant rats were injected sc with either an antiserum to LH (LHAS) or with normal horse serum (NHS) on day 10 (day 1: day of insemination), and with either oil or 100 μg of estradiol-17β (El00) on days 9 through 12, 10 through 12, or 11 and 12. Although E100 induced abortion in most rats, the luteolytic effect of the LHAS, as judged by the severity of the fall in the serum progesterone level and the duration of the post-treatment vaginal diestrus, was significantly reduced by E100 treatment on days 9–12, or days 10–12, but not by E100 treatment on days 11 and 12. To see whether another form of estrogen treatment would prevent both LHASinduced luteolysis and abortion, pregnant rats were given either a cholesterol pellet containing 5 μg of estradiol (E5), or a blank cholesterol pellet, within the bursa of each ovary at 1100 h on day 11, and an sc injection of either LHAS or NHS 8 or 12 h later. Abortion occurred before day 15 in 82% of 17 rats in the blank pellet plus LHAS group, in 11% of 18 rats in the E5 plus LHAS group and in none of 9 rats in the E5 plus NHS group. Abortion or the maintenance of pregnancy fitted almost perfectly with the failure or the maintenance of progesterone secretion, respectively. The dependency on LH for the maintenance of progesterone secretion, which appears on day 8 of pregnancy or day 9 of pseudopregnancy, may thus reflect the role of LH in the regulation of intraluteal estrogen production, since estrogen, at this time, may become an essential part of the luteotrophic process.