Studies on the role of LH in the maturation of the pre-ovulatory follicle in sheep using a GnRH-antagonist

A model designed to mimic the action of LH in preovulatory follicle development was evaluated. The model involved treatment of ewes with GnRHa (50 μg kg−1 24 h−1; GnRHa) at the time of luteal regression followed by exogenous LH administered as a series of bolus injections of varying frequency and amplitude. Ovarian responses to this regimen were compared to data derived from the same animals during a normal follicular phase. In Experiment 1 four ewes with an autotransplanted ovary were treated with GnRHa followed by hourly LH (2.5 μg NIH-oLH-S25 i.v.) for 60 h at which time a further bolus of LH (25 μg i.v.) was given to mimic the LH surge. Plasma FSH profiles and the pattern and rate of secretion of oestradiol, inhibin, androstenedione and testosterone by the ovary in normal and GnRHa treated ewes were similar, but ovulation did not occur following the induced follicular phase. Experiment 2 examined the ability of a revised LH regimen to induce ovulation in normal intact ewes. Ewes (n = 11) were treated with GnRHa at the time of luteal regression followed by LH injections (1.25 μg i.v.) administered every 3 h for 12 h, every 2 h for the next 12 h and every hour for the following 36 h (3,2,1 regimen). After 60 h of LH injections the animals received an 8 h infusion of oLH (25 μg min−1) designed to mimic the pre-ovulatory LH surge. This treatment resulted in normal follicular phase patterns of LH and FSH and had no effect on the incidence (100%) or number (1.6 ± 0.1) of ovulations when compared to a normal cycle (100% and 1.8 ± 0.2). Experiment 3 examined the pattern of ovarian follicular development and hormone secretion stimulated by the 3,2,1 LH regimen in ewes with autotransplanted ovaries (n = 8). The LH regimen stimulated normal pre-ovulatory follicular development (normal 1.4 ± 0.2; induced 1.6 ± 0.2) and a normal pattern of ovarian oestradiol and androstenedione secretion. Pulse analysis showed, however, that baseline steroid secretion was lower in induced animals during the late follicular phase reflecting lower baseline LH at this stage. We conclude that administration of a GnRHa at the time of luteal regression in conjunction with frequent low amplitude pulses of exogenous LH induce follicular and hormonal patterns that closely mimic those observed following normal luteal regression and that this regimen represents a useful model to study the role of LH in the control of follicular selection.