The role of luteal oxytocin in episodic secretion of prostaglandin F2α at luteolysis in the ewe

The role of luteal oxytocin in the generation of luteolytic episodes of prostaglandin F2α at luteolysis was investigated. On day 10 of the cycle Dorset ewes underwent either surgical removal of the corpora lutea (lutectomy; n=4) or sham operation (sham; n=4). Lutectomised ewes were then administered progesterone by twice daily i.m. injection in corn oil (20 mg/day) until day 14 when treatment was ceased to simulate luteolysis. The concentration of 13, 14 dihydro-15-keto prostaglandin F2α (PGFM) was measured in peripheral blood samples collected at 20-min intervals for 8 h on days 12–16 of the cycle. Progesterone and oestradiol concentrations were similar in the two groups over the whole experimental cycle while oxytocin fell dramatically following lutectomy. No prostaglandin F2α release episodes were seen on day 12 or 13, while from days 14–16 both groups exhibited a similar episode frequency (lutectomy 0.9/ewe/8 h; sham 0.8/ewe/8 h). Analysis of episode characteristics revealed lower episode amplitude (p<0.05) but longer episode duration (p<0.05) in the lutectomy group. The results demonstrate that a normal frequency of prostaglandin F2α release episodes occurs independently of luteal oxytocin secretion. However, luteal oxytocin is involved in regulating the pattern of release, perhaps causing the release of episodes of the magnitude required for the successful completion of luteolysis.