The Critical Period and for the Nongravid Uterus to Induce Luteolysis in Pregnant Rats

The present paper demonstrates the critical time of the nongravid uterus to induce luteolysis in pregnant rats. On day 7 of pregnancy, the number of conceptuses was adjusted to one by aspirating all but the single conceptus (Asp group). The nongravid part of the uterus was then removed by partial-hysterectomy on day 7, 8, 9 or 12 of pregnancy (Asp-Ph7, Asp-Ph8, Asp-Ph9 or Asp-Ph12 group). The Asp-Ph7 group showed significantly higher serum progesterone levels than Asp group on day 15 and 17, while the progesterone levels in the Asp-Ph9 or Asp-Ph12 group remained at or below the value of day 12 on days 15 and 17. Both the Asp and Asp-Ph7 groups, which were hypophysectomized and totally hysterectomized on day 12, responded to the treatment with estradiol plus placental luteotrophins as well, and showed rather higher serum progesterone levels in the Asp group than the Asp-Ph7 group. It was concluded that the nongravid uterus would act between days 7 and 9 of pregnancy to prevent the rise in serum progesterone levels which were normally observed after day 12 of pregnancy, which would be mediated through the pituitary.