Effects of estradiol and progesterone on circulating LH and FSH secretion, and ovarian antral follicle growth in anestrous ewes

In the ewe, ovarian antral follicles emerge or grow in a wave-like pattern and each wave is preceded by a peak in the serum FSH level. The purpose of the current study was to investigate whether in anestrous Western White Face ewes, a combination of progesterone and estradiol affects the circulating FSH peak secretion and the number of small ovarian follicles. Five ewes were treated with subcutaneous silastic rubber implants (10cm×0.47cm), containing 10% estradiol-17β w/w (controls) and 5 ewes were treated with the same estradiol implant, along with subcutaneous implants (11cm×0.48cm) containing 10% progesterone w/w for 12 days. Daily transrectal ovarian ultrasonography and blood sampling was performed from 5 days before, to 9 days after the period of implantation. Blood samples were also taken every 12min for a 6h period on day −2, 6 and 13 prior to or after implant insertion (day 0, day of implant insertion). Pulsatility in the serum LH levels was eliminated by the implants (P<0.05). During the implantation period, the serum FSH peak amplitude was lower in ewes treated with implants releasing estradiol and progesterone, compared to ewes treated with implants releasing only estradiol (P<0.05). No follicular waves emerged during implant treatment in both groups (P<0.05) and the number of serum FSH peaks did not differ during implantation, compared to before implantation. During the implantation period, the number of small follicles did not differ in ewes with implants releasing estradiol and progesterone, compared to ewes treated with implants releasing only estradiol. To conclude, supra-physiological concentrations of estradiol completely eliminated the serum LH pulsatality and suppressed the follicular wave emergence, while the FSH secretory peaks that preceded the follicular waves were not affected. Supra-physiological concentrations of estradiol-17β with physiological concentrations of progesterone decreased the serum FSH peak amplitude, eliminated the serum LH pulses, but did not decrease the size of the small follicle pool in anestrous ewes.