Ovarian and fetal control of rat placental lactogen and prolactin secretion at midpregnancy.

Abstract

The role of the fetus and ovarian steroids in stimulating rat placental lactogen (rPL) secretion and in regulating nocturnal PRL surges was investigated. On day 10, pregnant rats were ovariectomized (O) and/or fetectomized (F). Those rats injected with steroids received 0.1 microgram estradiol benzoate (E) or 4 mg progesterone (P) in oil sc, or both (PE). Blood samples were taken by cardiac puncture at 0500 h, on days 10 through 14. OPE rats showed slightly depressed serum rPL levels on days 11 and 12 and normal growth of the conceptuses. Ovariectomized rats given P alone also showed significant growth of the conceptus, although less than the OPE rats, demonstrating the necessity for estrogen during pregnancy. PRL surges were no longer present in these groups on day 11, similar to controls. Ovariectomized rats had very low rPL levels, and the nocturnal PRL surges continued for 2 extra days. Fetectomized and OF animals also had very low rPL levels and one extra PRL surge. P counteracted the detrimental effects of fetectomy on rPL secretion; yet placental uterine growth was not greater than in F animals. Thus, increased placental mass was not always correlated with increased rPL secretion. The results show that P is necessary to maintain rPL secretion in the pregnant rat, although estradiol and P in combination is most effective. The fetus also is essential to stimulate both placental growth and rPL secretion. The inverse correlation between rPL and PRL levels on days 11 and 12 suggests that rPL normally terminates the PRL surges at midpregnancy.