Luteotropic Role of Estrogen in Early Pregnancy in the Rat*

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Abstract
The role of estrogen and PRL in luteal cell function in early pregnancy (between days 6–8) was examined. On day 6 post coitum, rats were injected sc with LH antiserum or were hypophysectomized and hysterectomized and treated with PRL (250 or 500 μg/day), estradiol (100 μg/day), PRL and estradiol together, or the appropriate vehicle. Within 48 h after injection of LH antiserum, intraluteal concentrations of estradiol decreased to 1.1 ± 0.2 pg/mg (compared to 3.3 ± 0.3 pg/mg in control animals treated with normal horse serum), while serum progesterone decreased by only 22%. The rats remained pregnant. Within 24 h after hypophysectomy-hysterectomy, serum progesterone was 43 ± 9 ng/ml and the total luteal content of estradiol receptor (cytosolic plus nuclear) decreased by 75% compared with control sham-operated animals. Treatment with estradiol alone (100 μg/day) further decreased serum progesterone and the concentration of estradiol receptor in the corpora lutea. PRL partially maintained progesterone secretion (56 ± 10 ng/ml on day 7 and 34 ± 8 ng/ml on day 8 compared to 87 ± 8 and 91 ± 12 ng/ml in sham-operated animals) and sustained the luteal content of estradiol receptor. The combination of PRL and estradiol maintained the serum progesterone concentration and the luteal content of the estradiol receptor at the levels found in sham-operated animals. These results suggest 1) that in early pregnancy (days 6–8) progesterone production is partially maintained by LH action, but the dependency on LH is less acute than that between days 8–11 of pregnancy; 2) that the luteotropic complex in early pregnancy is formed principally by PRL and estrogen; 3) that PRL maintains the luteal estrogen receptor content; and 4) that the luteotropic or luteolytic action of estradiol appears to be related to the availability of estradiol receptor in the corpora lutea. (Endocrinology106: 1584, 1980)