Ovarian responses in gonadotrophin-releasing hormone-treated anoestrous ewes: follicular and endocrine correlates with luteal outcome

The relationships between the development of antral follicles (growing from 3 to greater than or equal to5 mm diameter), hormone secretion (luteinizin g hormone (LH), follicle-stimlating hormone (FSH), oestradiol and progester one), ovulation and the formation of luteal structures in response to gonad otrophin-releasing hormone (GnRH) were examined in 24 anoestrous Western Wh ite Face ewes (May-July). Ewes were monitored by transrectal ovarian ultras onography for 34 days, commencing 15 days before the administration of GnRH . Following treatment with GnRH, 83% (20/24) of ewes ovulated. Twenty-five per cent of all ewes (6/24) subsequently had normal (full-life span) corpor a lutea (CL), 37% (9/24) had inadequate CL, 17% (4/24) had both normal and inadequate CL, 17% (including three of four anovular ewes and one ewe with inadequate CL) formed luteinized follicles and only 4% (1/24) did not ovula te or produce any luteal structure. None of the variables of follicular gro wth (follicles reaching greater than or equal to5 mm diameter) differed bet ween follicles that either ovulated or failed to ovulate and there was no e vident correlation between the age or stage of development of ovulatory siz ed antral follicles and the type of luteal structure formed, except for lut einized unovulated follicles; these follicles all emerged within 3 days of GnRH injection. Mean serum concentrations of FSH and oestradiol before trea tment did not differ (P >0.05) between ewes with different ovarian response s, but peaks of fluctuations in serum concentrations of FSH in daily sample s were higher in ewes that produced normal CL compared with ewes with inade quate CL. After GnRH treatment. oestradiol secretion was higher in ewes tha t formed luteinzed unovulated follicles than in all ewes with inadequate CL (P <0.05). The peak concentration of the GnRH-induced LH surge was higher and the interval from GnRH to peak LH discharge was shorter in ewes with in adequate CL compared with ewes that had normal CL after ovulation (P <0.05) . In conclusion, ovulatory sized antral follicles at a similar stage of the ir life span can give rise to either normal or inadequate CL and a proporti on of these follicles do not ovulate in response to GnRH in seasonally anoe strous ewes. This suggests differences in ovarian follicular responsiveness to gonadotrophic stimuli. Both the amplitude of episodic elevations in dai ly serum FSH concentrations and the characteristics of the pre-ovulatory LH surge may be important for luteogenesis following ovulation.