STUDIES ON THE ROLE OF LH IN THE MATURATION OF THE PREOVULATORY FOLLICLE IN SHEEP USING A GNRH-ANTAGONIST

A model designed to mimic the action of LH in preovulatory follicle de velopment was evaluated. The model involved treatment of ewes with GnR Ha (50 mu g kg(-1) 24 h(-1); GnRHa) at the time of luteal regression f ollowed by exogenous LH administered as a series of bolus injections o f varying frequency and amplitude. Ovarian responses to this regimen w ere compared to data derived from the same animals during a normal fol licular phase. In Experiment 1 four ewes with an autotransplanted ovar y were treated with GnRHa followed by hourly LH (2.5 mu g NM-oLH-S25 i .v.) for 60 h at which time a further bolus of LH (25 mu g i.v.) was g iven to mimic the LH surge. Plasma FSH profiles and the pattern and ra te of secretion of oestradiol, inhibin, androstenedione and testostero ne by the ovary in normal and GnRHa treated ewes were similar, but ovu lation did not occur following the induced follicular phase. Experimen t 2 examined the ability of a revised LH regimen to induce ovulation i n normal intact ewes. Ewes (n = 11) were treated with GnRHa at the tim e of luteal regression followed by LH injections (1.25 mu g i.v.) admi nistered every 3 h for 12 h, every 2 h for the next 12 h and every hou r 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 mu g min(-1)) designed to mimic the pre-ovulatory LH surge. This treatment resulted in norma l follicular phase patterns of LH and FSH and had no effect on the inc idence (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 patter n of ovarian follicular development and hormone secretion stimulated b y the 3,2,1 LH regimen in ewes with autotransplanted ovaries (n = 8). The LPI regimen stimulated normal pre-ovulatory follicular development (normal 1.4 +/- 0.2?: induced 1.6 +/- 0.2) and a normal pattern of ov arian 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. Wt conclude that administration of a GnRHa at the time of lute al regression in conjunction with frequent low amplitude I?pulses of e xogenous LH induce follicular and hormonal patterns that closely mimic those observed following normal luteal regression and that this regim en represents a useful model to study the role of LI-I in the control of follicular selection. (C) 1997 Elsevier Science B.V.