Steroid and Gonadotropin Levels During the Luteal-Follicular Shift of the Cyclic Hamster

Abstract

This study was designed to correlate ovarian and hormonal changes of cyclic hamsters during the period of luteal regression and rising dominance of antral follicles. Cyclic hamsters (8/group) were decapitated at 2 h intervals between Day 2 at 1000 h and Day 3 at 1000 h; Day 1 is defined as the day of ovulation. Serum was collected for RIA of gonadotropins and steroids. From 1 ovary, corpora lutea (CL) and the nonluteal ovary (NLO) were saved for steroid RIA and the remaining ovary was prepared for histology. The diameter of antral follicles steadily increased from 389 to 437 μm during Days 2–3, and the number/ovary increased from 3 at 1000 h Day 2 to 11 at 2400 h Day 2. However, by 1000 h Day 3 the number of antral follicles had declined to 5/ovary. The highest level of serum progesterone (P) occurred at 2000 h Day 2; thereafter, the levels declined abruptly to less than 1 ng/ml by 0400 Day 3. The concentration of luteal P decreased gradually between 2000 h Day 2 and 0400 h Day 3;2 h later there was a drastic decline correlating with an influx of leukocytes into the CL. On Day 2, although antral follicles were present, serum estradiol (E₂) was very low. However, estradiol increased at 0400 h Day 3 associated with increased concentration of E₂ and androgen in NLO. During the 24 h period, the most striking difference in serum levels of FSH, LH and prolactin was the peak of all 3 hormones at 2200 h Day 2, 4 to 6 h before any significant changes in the serum levels of P or E₂. The results demonstrate that the shift from the luteal phase to the follicular phase is a gradual process involving concomitant declining levels of P and increasing levels of E₂. The results suggest that the transitory increase of gonadotropins and/or declining levels of P are causally related to enhanced follicular secretion of E₂ and androgen.