The Role of Estrogen in the Regulation of Luteal Progesterone Secretion in the Rat After Day 12 of Pregnancy1

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Abstract
Pregnant rats, hypophysectomized-hysterectomized on day 12 (day 1 = insemination), secreted progesterone (P) at about 50% of the day 12 level for 3-5 days. Daily treatment with estradiol 100 .mu.g (E100) but not with E25 or E50, from day 12 to day 20, of such rats, restored P secretion until day 16 to that of intact pregnant rats; on day 17 a drastic, permanent fall occurred. E100 had no effect after ovariectomy, and did not change the metabolic clearance rate of P. This E100 effect was absent in decidual tissue (DT)-bearing, or hysterectomized pseudopregnant (PSP) rats, and in pregnant ones before day 11. When pregnant rats were hypophysectomized-hysterectomized on day 10, and were treated with rat placental luteotropin (rPL) (in the form of day 12 pregnant rat serum: PRS-12) on days 10 and 11, E100 increased P secretion above that found with either PRS-12 or E100 alone. DT-bearing PSP rats, similarly operated on day 12 and treated with PRS-12 on day 12, responded in the same way to E100. In these rats also, the E100 effect lasted for 4 days. In the day 12 hypophysectomized-hysterectomized pregnant rat, the E100 effect could not be prolonged by single treatments with PRS-12 on either days 13, 14 or 16, but when PRS-12 was given daily from day 12 to 19, P was secreted until day 20 only slightly below the day 12 level; this treatment plus E100 raised P secretion, prolonged it to day 18, and led to a marked fall by day 20 similar to that of the intact pregnant rat at term. The marked increase in P secretion between days 12 and 15 of normal pregnancy may be a response to intraluteal estrogen; the pattern of P secretion from day 12 to term may reflect the effects of both estrogen and rPL. rPL probably induces this effect by generating luteal estrogen and LH receptors. The placenta may also secrete an LH-like hormone (rCG?) which, through the LH receptors, could stimulate intraluteal estrogen production.