Uterine prostaglandin release relative to embryo collection, transfer procedures and maintenance of the corpus luteum

Summary Failure of embryo reinsertion experiments and observation of a high incidence of extended oestrous cycles following transcervical embryo recovery at Day 12.5 (ovulation = Day 0) prompted investigation of the effects of uterine lavage on cycle length and concentrations of progesterone and 15‐keto‐13, 14‐dihydroprostaglandin F 2α (PGFM) in peripheral blood. Progesterone profiles confirmed the occurrence of prolonged luteal phases after flushing on Days 12.5 and 13.5 and showed that extended cycles can occur even after flushing as early as Day 7.5. However, almost 40 per cent of the delays in return to oestrus were associated with an initial decline in luteal activity after flushing followed by a reactivation of the corpus luteum and/or the formation of a new one. Profiles of PGFM revealed variable patterns of prostaglandin F 2α release caused principally by cervical dilatation and uterine manipulation. They also showed that a measure of prostaglandin F 2α release could be obtained from paired samples taken before and after flushing. Such paired sampling was used on 113 inseminated mares flushed 6.5 to 15.5 days after ovulation. It was shown that flushing could induce prostaglandin F 2α release after Day 6.5 but probably not before. The presence of an embryo had little effect on the flush‐induced release up to Day 10.5 but its inhibitory effect had become marked by Day 11.5. Oxytocin injection on Days 10.5 or 12.5 also induced prostaglandin F 2α release. In an ovariectomised mare, uterine flushing and oxytocin injections were similarly effective in inducing prostaglandin F 2α release after treatment with oestrogen followed by progesterone, but not after oestrogen alone. This suggests a role for oxytocin in normal luteolysis in cycling mares. The fact that the embryo can alter uterine responsiveness to flushing procedures by Day 11.5 calls into question the concept of a ‘critical stage’ for pregnancy recognition in mares. Eight sham transfers in two mares between Days 5.5 and 8.5 had no effect on PGFM concentrations but it is suggested that failure of embryo reinsertion techniques might be associated with prostaglandin F 2α release provoked by uterine flushing.