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

Summary. Normal and abnormal corpora lutea were recovered from anoestrous Romney Marsh ewes on Days 3, 4, 5 and 6 after treatment with small-dose (250 ng) multiple injections of GnRH followed by a bolus injection (125 µg) with (+P) and without (–P) progesterone pretreatment and a study made of their characteristics in vitro. Plasma progesterone concentrations initially rose concurrently in all animals but abnormal luteal function occurred in 70% of the –P ewes and was defined on Day 5 when plasma progesterone concentrations declined relative to those in the +P ewes.

All corpora lutea recovered on Days 3 and 4 appeared macroscopically similar and there were no significant differences between the +P and –P groups in terms of luteal weight, progesterone content and binding of ¹²⁵I-labelled hCG on these days. However, corpora lutea from the –P animals only exhibited a decline in progesterone production in vitro on Day 4 (P < 0·01), and morphological differences became apparent on Days 5 and 6 when the abnormal corpora lutea from the –P animals also decreased in weight (P < 0·01) and progesterone content (P < 0·001). Binding of ¹²⁵I-labelled hCG increased on Day 5 in the normal corpora lutea only.

These results show that, although abnormal luteal function induced by GnRH treatment of anoestrous ewes could not be distinguished from normal corpora lutea before Day 5 by measurement of progesterone in peripheral plasma, a significant decline in progesterone production in vitro occurred on Day 4 in the abnormal corpora lutea. This was followed by significant decreases in weight and progesterone content and a failure to increase ¹²⁵I-labelled hCG binding. Abnormal corpora lutea are therefore capable of some initial growth and progesterone production, before undergoing a rapid and premature regression from Day 4, which has similar characteristics to natural luteolysis.

Keywords: abnormal luteal function; in vitro; sheep; anoestrus

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