Characteristics and causes of the inadequate corpus luteum.

An inadequate luteal phase occurs in domestic ruminants in several physiological situations (e.g. puberty, post partum), and also following the induction of ovulation in anoestrous ewes with GnRH treatment. The induced corpora lutea (CL) initially developed, but then regressed rapidly after Day 4, unless the animals had been primed with progesterone before GnRH therapy or hysterectomized. Significant increases in prostaglandin F-2 alpha metabolite (PGFM) secretion and coincident peaks of oxytocin and PGFM occurred around the time of premature regression. Endometrial oxytocin receptors were also detectable at this time in ewes which had abnormal luteal phases, but not in ewes which had been progesterone primed and thus had normal luteal phases. This suggests that the presence or otherwise of the oxytocin receptor during the early luteal phase may be crucial in determining whether the CL has a short or normal lifespan. These results show that an inadequate luteal phase is often caused by the premature induction of luteolysis and the presence of the endometrial oxytocin receptor in necessary for this occurrence. This receptor in turn is controlled by the steroid environment to which the uterus has previously been exposed.