Effect of progesterone on the GnRH-induced secretion of oestradiol and androstenedione from the autotransplanted ovary of the anoestrous ewe

Two experiments were conducted during the anoestrous period in Border Leice ster x Merino ewes with ovarian autotransplants to study the effects of a s ingle injection of 20 mg progesterone on follicular steroid secretion. The aim of these experiments was to determine whether pretreatment with a 20 mg intramuscular injection of progesterone could reduce GnRH-induced ovarian steroid secretion in anoestrous awes. Ln both experiments, an injection of 150 ng GnRH induced an LH pulse in all ewes with a maximum concentration 10 min (the first post-injection sample) after injection. Oestradiol and andr ostenedione secretion increased progressively after the GnRH-induced LH pul se and reached maximum rates of secretion between 60 and 90 min before decr easing slowly to pre-injection rates at 150 min. There were no differences in the pattern of secretion of oestradiol (measured in both experiments) or androstenedione (measured only in Expt 2). Ln Expt 1, the injection of pro gesterone 72 h before the challenge with GnRH had no effect on the maximum rate of oestradiol secretion from the autotransplanted ovary. However, in E xpt 2, when progesterone was given either 36 or 60 h before GnRH, there was a significant suppression in the maximum rate of secretion of both oestrad iol and androstenedione between 60 and 90 min after GnRH injection. These d ata show that pretreatment of anoestrous sheep with progesterone can suppre ss LH-stimulated steroid secretion from the ovary and indicate that progest erone may have a direct effect on oestrogenic follicles in sheep.