EFFECTS OF SUPRA-PHYSIOLOGICAL CONCENTRATIONS OF PROGESTERONE ON THE PATTERNS OF LH SECRETION AND OVARIAN ANTRAL FOLLICULAR WAVE DYNAMICS IN CYCLIC EWES

Abstract

In the ewe, ovarian antral follicles emerge or grow from a pool of 2–3 mm follicles in a wave like pattern, reaching ≥5mm in diameter before regression or ovulation. There are 3 or 4 such follicular waves during each estrous cycle. Each wave is preceded by a peak in serum FSH concentrations. LH is secreted in a pulsatile manner (1–6 pulses per 6h) but the role of pulsed LH secretion in follicle wave emergence and growth is unclear. Progesterone exerts a negative feedback action on LH secretion. The purpose of the present study was to examine the effect of supra-physiological concentrations of progesterone on LH secretion and antral follicular dynamics. Starting on day 4 of the estrous cycle, six ewes (treatment group) received subcutaneous progesterone-releasing implants for 10 days. Six ewes (control group) underwent sham surgery. Daily transrectal ovarian ultrasonography and blood sampling was performed on all ewes throughout the estrous cycle. Blood samples were also taken every 12 min for 6h on day 3, day 10 and day 16 of the estrous cycle to characterize the LH secretory pattern. Subcutaneous progesteronereleasing implants resulted in supra-physiological concentrations of progesterone in treatment ewes compared to control ewes (5.3 ± 0.4 vs. 2.6 ± 0.2; P<0.05). During the period of implant insertion, the number of LH pulses (0.50 ± 0.2 vs. 1.50 ± 0.2) was lower in treatment ewes compared to control ewes (P<0.05). There was no difference in the number of FSH peaks or the amplitude of the peaks between the treatment and control ewes (P>0.05). Mean serum estradiol and FSH concentrations did not differ between groups (P>0.05). All ewes had normal follicular wave emergence prior to and during implant insertion. There was no difference in the number of follicles 1-2mm, 2mm, 3mm, 4mm, and ≥5mm in diameter between treatment and control ewes (P>0.05). The estrous cycle length tended to be longer in treatment ewes compared to control ewes (19.3 ± 0.6 vs. 18.0 ± 0.4; P=0.1). There was no difference in the ovulation rate between the treatment and control ewes (P>0.05). We concluded that the subcutaneous progesterone releasing implants suppressed pulsatile LH secretion and tended to prolong the estrous cycle length, but did not affect FSH and ovarian antral follicular wave characteristics. Suppressing LH pulse frequency would not appear to affect ovarian antral follicular wave dynamics in cyclic ewes. (platform)