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Two experiments were conducted during the anoestrous period in Border Leicester x Merino ewes with ovarian autotransplants to study the effects of a single 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 ewes. In 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 androstenedione secretion increased progressively after the GnRH-induced LH pulse and reached maximum rates of secretion between 60 and 90 min before decreasing 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). In Expt 1, the injection of progesterone 72 h before the challenge with GnRH had no effect on the maximum rate of oestradiol secretion from the autotransplanted ovary. However, in Expt 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 oestradiol and androstenedione between 60 and 90 min after GnRH injection. These data show that pretreatment of anoestrous sheep with progesterone can suppress LH-stimulated steroid secretion from the ovary and indicate that progesterone may have a direct effect on oestrogenic follicles in sheep.

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