Role of the Preimplantation Embryo in the Timing of LH-Dependent Progesterone Secretion from the Rat Corpus Luteum

Abstract

The effect of the progestational uterus and preimplantation embryo on the time of appearance of LH-dependent progesterone secretion from pregnant and pseudopregnant (PSP) rat corpora lutea (CL) was studied. Two groups of experiments were performed: (1) pregnant and PSP rats were hysterectomized between Days 3 and 5 (Day 1 = vaginal estrus) and subsequently monitored for changes in serum progesterone levels following an injection of LH antiserum (LHAS) or normal horse serum (NHS) vehicle on Day 9 and, (2) Day 4 embryos were transferred to Day 3 PSP uteri; each rat was subsequently hysterectomized on Day 4 of PSP and treated with LHAS or NHS on Day 9. Progesterone levels were monitored as an index of luteal vitality. Progesterone secretion was not altered by NHS treatment in any group. LHAS treatment did not induce luteolysis in Day 3 pregnant rats or in PSP rats hysterectomized on Days 3 or 4. However, in rats hysterectomized on Day 4 or 5 of pregnancy, or hysterectomized on Day 5 of PSP, luteolysis occurred following LHAS treatment. In rats where embryo transfer occurred on Day 3 of PSP and hysterectomy on Day 4, a Day 9 injection of LHAS induced luteolysis. The results of these studies demonstrate that the embryo (i.e., morula to early blastocyst) is capable of influencing the rat CL to become critically dependent on LH for progesterone secretion. The presence of the progestational uterus in the PSP rat also has this ability, but the PSP uterus must remain in situ until Day 5, whereas an embryo-containing uterus exercises its influence by Day 4. It is hypothesized that the embryo may have an indirect effect on the induction of LH-dependent progesterone secretion from the rat CL.