Relative roles of oestradiol and of the uterus in the maintenance of the corpus luteum in the pseudopregnant brown hare (Lepus europaeus).

Abstract

Brown hares were made pseudopregnant by sterile matings or PMSG-hCG treatment (day of mating or hCG injection = Day 0 of pseudopregnancy). Progesterone secretion by the CL began 3-4 days after the ovulatory stimuli, reached maximum on Days 8 to 11 and decreased thereafter to reach low levels from Day 9 to 18, depending on the female. Cauterization of all large ovarian follicles on Day 7 resulted in an immediate luteolysis in young females, but had no effect in older ones. Oestradiol capsules implanted from Day 7 to Day 46 were able to maintain progesterone secretion until at least Day 30, in intact females as well as in females with all large follicles cauterized. Hysterectomy on Day 7 or 8 was followed by an immediate drop in progesterone concentrations; oestradiol capsules implanted at the time of hysterectomy prevented the drop in progesterone values, which remained elevated until Day 38. The induction of ovulation in females hysterectomized 2 months before resulted in CL of slightly shortened life-span. The injection of PGF-2 alpha on Day 7 of pseudopregnancy was followed by an immediate luteolysis. These results suggest that oestradiol secreted by the large ovarian follicles is the main luteotrophic factor in the brown hare. In old hares, the large amount of interstitial tissue could secrete oestrogens, and thus maintain pseudopregnancy. On Day 7 of pseudopregnancy, the uterus secretes a luteotrophic substance acting either directly on the ovary, or via the pituitary, to maintain oestradiol secretion by the follicles. In long-term hysterectomized females, the CL would be able to develop independently of any trophic substance, but for a reduced duration.