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Ovarian Factors Inhibit and Fetal Factors Stimulate the Secretion of Rat Placental Lactogen^*

Department of Physiology and Department of Obstetrics, Gynecology, and Reproductive Sciences, Faculty of Medicine, University of Manitoba Winnipeg, Manitoba, Canada R3E0W3

Address requests for reprints to: Henry G.Friesen, M.D., Department of Physiology, Faculty of Medicine, University of Manitoba, 770 Bannatyne Avenue, Winnipeg, Manitoba, Canada R3E 0W3.

Abstract

Removal of fetuses at day 14 of gestation (Ftx14) in the pregnant rat leads to a marked suppression of serum levels of rat placental lactogen (rPL-II). One might attribute this to compromised placental growth in the absence of a fetus. However, if ovariectomy and fetectomy (Ftxu Ovxu) are carried out at the same time, a great increase in serum rPL-II levels is seen. This occurs despite a significant decrease in placental weight. When Ftx14 was performed on day 14 and Ovx was delayed 1, 2, or 3 days, the expected large increase in serum rPLII was progressively attenuated compared to that seen when Ftx and Ovx were carried out simultaneously. Daily administration of 17β-estradiol (4 µg/rat-day) to Ftx₁₄ Ovx₁₄ pregnant rats resulted in a significant suppression of rPL-II and elevation of rPRL levels, a reversal of what is seen for these hormones in untreated Ftx₁₄ Ovx₁₄ animals. To test whether 17β-estradiol was acting through rat PRL (rPRL), serum levels of rPRL were elevated in Ftx₁₃ Ovx₁₃ animals with pimozide (0.6 mg/kg), a dopamine receptor blocker. There was no effect of this treatment on rPL-II levels.

In late pregnancy (day 17) serum rPL-II levels remained high after removal of half the fetuses (1/2 Ftx), compared to the rapid fall in pregnant rats in which all fetuses were removed (Ftx₁₇). Serum levels were also elevated in 1/2 Ftx animals compared to those in which half of the fetuses and placentas were removed by hemi-hysterectomy, suggesting that the increase in rPL-II levels in 1/2 Ftx animals was due to a stimulatory effect of the remaining fetuses on all of the placentas. These results indicate that the presence of the fetus is necessary for the normally observed increase in rPL-II levels in late pregnancy. In conclusion, fetal stimulators and ovarian inhibitors influence rPL-II secretion. (Endocrinology 114: 22, 1984)

Footnotes

^* This research was supported by a grant from MRC Canada and Grant HD-07843-07 from USPHS.

Received November 15, 1983.

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