Secretion of rat placental lactogen by the fetal placenta and its inhibitory effect on prolactin surges.

The various products of conception were examined for their ability to secrete rat placental lactogen (rPL), cause normal termination of the diurnal and nocturnal prolactin (Prl) surges and maintain progesterone secretion. Serum rPL, highest on Day 12, was measured by Nb2 lymphoma cell bioassay. Surgery was performed on Day 8, leaving only the uterus in aborted animals, the decidua in the decidua intact animals, or the placenta and decidua in the fetectomized animals. In control rats, rPL levels were elevated by the afternoon of Day 8, the last day the diurnal Prl surge is seen. By Day 10, the last day of the nocturnal surge, rPL levels were extremely high. Fetectomized animals exhibited both the diurnal and nocturnal surges for 2 additional days while rPL secretion was only slightly above baseline levels. Both the decidua-intact and aborted animals continued both daily Prl surges with barely detectable rPL levels. Progesterone secretion was maintained through Day 14 only in the control animals. It is concluded the rPL is secreted by the fetal trophoblastic cells, not the decidua, and that the presence of the fetus is necessary for secretion of normal amounts of rPL and maintenance of pregnancy. Further support is given to the hypothesis that it is rPL which terminates the Prl surges at midpregnancy.