Placental lactogen secretion during prolonged-pregnancy in the rat: The ovary plays a pivotal role in the control of placental function

The serum of rats at mid-pregnancy contains at least 2 distinct placental lactogen (PL)-like substances tentatively termed placental lactogen-α (PL-α) and placental lactogen-β (PL-β) (Endocrinol Japan 38: 533-540, 1991). We have investigated the secretory patterns of three placental lactogens (PL-α, PL-β and placental lactogen-II) during normal pregnancy and in two prolonged-pregnancy models. Pregnancy was prolonged by the introduction of new corpora lutea by inducing ovulation on day 15 of pregnancy by successive treatments with PMSG (30 IU/rat, sc on day 12) and hCG (10 IU/rat, iv on day 14), and in the second model by progesterone implants on day 15 of pregnancy. During normal pregnancy, each of the 3 PLs exhibited only one secretory peak in the serum; PL-α and PL-β on day 12 and placental lactogen II (PL-II) on day 20. Interestingly, in the rats with new sets of corpora lutea, serum PL-α and PL-β levels began to increase again on day 18 and showed peaks on day 20 for PL-α and on day 22 for PL-β. In this model, the initiation of PL-II secretion was not affected, but high levels were maintained until day 26, when parturition occurred. In rats receiving either PMSG or hCG, the secretory patterns of the PLs were similar to those during normal pregnancy. In the progesterone-implanted rats, the second secretory peaks of PL-α appeared on day 20 and PL-β on day 22. The start of secretion of PL-II was delayed for 4 days. In the progesterone-implanted rats, ovariectomy on day 15 abolished the secretions of all 3 placental lactogens (PL-α, PL-β and PL-II). Thus, mid-pregnancy-specific PLs, PL-α and PL-β, are secreted again if the period of pregnancy is extended either by the induction of a new set of corpora lutea or by progesterone implantation, and these secretions depend on the ovary.