Role of prostaglandin F-2 and oxytocin in the regression of GnRH-induced abnormal corpora lutea in anoestrous ewes

Abstract

Anoestrous Romney Marsh ewes with (+P) and without (-P) progesterone pretreatment were induced to ovulate by multiple low-dose injection of GnRH followed by a bolus injection of GnRH. Luteal function was assessed by twice daily measurement of plasma progesterone. Animals were slaughtered on Days 3 or 5 after the end of GnRH treatment and CL and endometrium were recovered. In all Day-5 ewes, blood samples were collected at 30-min intervals for 8 h on Days 3 and 5 for measurement of PGFM and oxytocin. At slaughter 92% of the Group +P ewes had ovulated compared with 54% of the Group -P ewes. The ovaries of some of the Group -P ewes only contained luteinized cysts either alone or in association with CL. In the ewes that ovulated, progesterone profiles were normal in all Group +P ewes, whereas Group -P ewes had 'normal' or 'abnormal' profiles in which plasma progesterone was declining prematurely. All of the CL from ewes with abnormal progesterone profiles were associated with follicular cysts, and were significantly smaller and with a lower progesterone content on Day 5. PGFM levels decreased (P less than 0.05) between Days 3 and 5 in ewes in Groups +P and -P with 'normal' CL but increased (P less than 0.01) in Group -P ewes with 'abnormal' CL. Oxytocin levels were lower in Group -P ewes with 'abnormal' CL on Day 5, than in 'normal' ewes in Groups -P (P less than 0.01) or +P (P less than 0.05). In 3/5 Day-5 ewes with 'abnormal' CL there was a clear association between a major peak of oxytocin and a rise in PGFM during the frequent sampling period on Day 3 or Day 5, and endometrial oxytocin binding sites were present at slaughter. This suggests that the premature regression of 'abnormal' CL occurs via the normal luteolytic mechanism. Although ewes in Groups +P and -P with 'normal' CL had similar progesterone profiles, plasma oxytocin was significantly higher (P less than 0.05) in the Group -P ewes and oxytocin binding sites were present only in this group, suggesting that progesterone pretreatment can influence the production of both oxytocin and its receptor.