Oxytocin does not contribute to the effects of cervical dilation on progesterone secretion and embryonic development in mares

The aim of the present study was, to investigate the effects of oxytocin administration on Day 7 post-ovulation on progesterone secretion, pregnancy rate and embryonic growth in mares. Endogenous stimulation of oxytocin release was compared to the administration of native oxytocin or the long-acting oxytocin analogue carbetocin. At Day 7 after ovulation, mares had to undergo four treatments in a crossover design: (a) control, (b) oxytocin (10 IU i.v.), (c) carbetocin (280 μg i.m.) and (d) cervical dilation. On Day 13, all mares (8 of 8 mares) were pregnant on groups control, oxytocin and carbetocin and only 6 of 8 mares on group dilation. In one mare uterine fluid accumulation and uterine edema from Day 6 to 13 and early embryonic death by Day 11 occurred during dilation treatment. Another mare, which did not become pregnant during dilation treatment, developed uterine fluid accumulation and uterine edema from Day 10 to 14. Mean growth rates of the conceptuses did not differ among treatment groups and individual growth rates varied in a wide range from -0.1 to 0.8 cm per day. At Day 13, mean diameters of conceptuses yielded 1.4 ± 0.1 cm in control group, 1.5 ± 0.1 in oxytocin and carbetocin group and 1.3 ± 0.2 cm in dilation group. Secretion of progesterone was not affected by treatments. Administration of oxytocin and carbetocin caused similar maximum plasma concentrations of oxytocin, but onset and duration of peaks differed. Maximum concentrations after intramuscular application of carbetocin were obtained almost 20 min later when compared to intravenous administration of oxytocin. Duration of peaks after injection of the long-acting oxytocin analogue was more than three-fold longer than after administration of native oxytocin. In conclusion, the present study showed that single administration of oxytocin or its long-acting analogue carbetocin at Day 7 after ovulation did not affect progesterone secretion, pregnancy rate and embryonic growth. Two possible scenarios concerning the effects of cervical dilation were observed: In the majority of mares, dilation of the caudal half to two-third of the cervical lumen up to a diameter of 4.5 cm had no negative consequences on progesterone secretion and pregnancy outcome. However, cervical dilation caused uterine inflammation and subsequent luteolysis in two mares and early embryonic death in one of them. Thus, manipulation of the cervix itself seems not to have negative impact on success rates of transcervical transfer of embryos in the mare.