LH pulse frequency and the emergence and growth of ovarian antral follicular waves in the ewe during the luteal phase of the estrous cycle

Ewes (n = 7) were given 200 ng of GnRH (IV) every hour for 96 h from Day 7 of the estrous cycle, to increase LH pulse frequency. Controls (n = 6) received saline. In a second study, ewes (n = 6) received subcutaneous progesterone-releasing implants for 10 days starting on Day 4 of the cycle, to decrease LH pulse frequency. Controls (n = 6) underwent sham surgery. Daily transrectal ovarian ultrasonography and blood sampling was performed on all ewes from the day of estrus to the day of ovulation at the end of the cycle of the study. At appropriate times, additional blood samples were taken every 12 minutes for 6 h and 36 min or 6 h in studies 1 and 2 respectively.The largest follicle of the follicular wave growing when GnRH treatment started, grew to a larger diameter than the equivalent wave in control ewes (P < 0.05). Mean serum estradiol and progesterone concentrations were higher but mean serum FSH concentrations were lower during GnRH treatment compared to control ewes (P < 0.05). The increased serum concentrations of estradiol and progesterone, in GnRH treated ewes, suppressed a peak in serum concentrations of FSH, causing a follicular wave to be missed. Treatment with progesterone decreased the frequency of LH pulses but did not have any influence on serum FSH concentrations or follicular waves.We concluded that waves of ovarian follicular growth can occur at LH pulse frequencies lower than those seen in the luteal phase of the estrous cycle but frequencies seen in the follicular phase, when applied during the mid-luteal phase, in the presence of progesterone, do enhance follicular growth to resemble an ovulatory follicle, blocking the emergence of the next wave.In the ewe, recent studies using transrectal ovarian ultrasonography have revealed a wave like pattern in the growth and regression of ovarian antral follicles. During the breeding season [1-4] and anestrus [5,6], 1–3 follicles emerge or continue to grow from a pool of small follicles (2–3 mm in diamet