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Function of abnormal corpora lutea in vivo after GnRH-induced ovulation in the anoestrous ewe

Summary. Anoestrous Romney Marsh ewes with and without progesterone treatment (+P, –P) were treated with small-dose (250 ng) multiple injections of GnRH at 2-h intervals for 48 h. Animals were slaughtered on Days 4, 5, 7 and 11 after the end of GnRH treatment and luteal function was assessed by the measurement of daily plasma progesterone concentrations.

In all animals which ovulated (29/32, 91%) peripheral progesterone concentrations rose to 0·5–1·0 ng/ml within 3 days of the end of GnRH treatment. In 7/7 (100%) +P animals and 5/22 (23%) –P animals, progesterone concentrations continued to rise and were maintained at levels > 1·5 ng/ml until slaughter. In the remaining –P animals, plasma progesterone concentrations declined to reach basal levels by Day 5. Corpora lutea recovered from these animals showed signs of premature regression on Day 5 and were fully regressed by Day 7. Progesterone priming delayed the occurrence of the LH surge which occurred 39·1 ± 3·6 h after the end of GnRH treatment in the +P animals compared to 20·2 ± 1·74 h (P < 0·001) in the –P animals in which luteal function was abnormal and 22·4 ± 4·35 h in the –P animals in which luteal function was normal.

These results show that abnormal luteal function occurs in the majority of GnRH-treated ewes in the absence of progesterone pretreatment. It is characterized by a transient rise in plasma progesterone concentration to 0·5–1·0 ng/ml 3 days after GnRH treatment, which then declines to basal values between Days 4 and 5, coincident with the rapid premature regression of this abnormal corpus luteum which is complete by Day 7.

Keywords: GnRH; abnormal luteal function; sheep; anoestrus

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