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

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 micrograms) 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 less than 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 less than 0.01) and progesterone content (P less than 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.