Plasma gonadotrophin and ovarian responses to physiological infusions of oestradiol were examined in anoestrous ewes with and without progesterone priming. Intravenous infusion of oestradiol (1 μg/h) for 24 h followed at 96 h by a second infusion for 12 h (low dose) resulted in simultaneous increases in LH and FSH of magnitude and duration similar to those of ewes at natural oestrus. Infusions of 3 μg/h for 16 h, and again at 96 h for 4 h (high dose) resulted in similar gonadotrophin peaks. Endoscopy revealed that only two out of eight ewes had ovulated despite increased ovarian activity and surges of plasma gonadotrophins.
Other anoestrous ewes were primed with 25 mg progesterone daily for 12·5 days before similar infusions of oestradiol on day 15. In the presence of very low plasma progesterone concentrations (< 1·0 ng/ml) at the onset of infusion, there was no pituitary response to the infusion of a low dose of oestradiol but the gonadotrophins were released after the second high-dose infusion.
Progesterone inhibited the positive feedback on both gonadotrophins but not the negative feedback on FSH. Ovulation occurred in all four ewes which had been progesterone-primed and infused with a high dose of oestradiol.
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Online ISSN: 1479-6805
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