Endometrial oxytocin receptor and uterine prostaglandin secretion in mares during the oestrous cycle and early pregnancy.

The research investigated the relationship between oxytocin receptor concentrations in the uterine lining (endometrium) of mares and their secretion of a specific type of prostaglandin during different stages of their estrous cycle and early pregnancy. The study suggests that the presence of oxytocin receptor in the endometrium impacts the secretion of this prostaglandin in non-pregnant animals. It also indicates a potential reduction of oxytocin receptor concentrations during early pregnancy due to the conceptus (embryo and its associated tissues).

Methodology

• The researchers selected 18 mares for the study, nine of which were pregnant and nine in the cyclic phase.
• They measured the circulating concentrations of 13,14-dihydro-15-ketoprostaglandin F2 alpha (PGFM) before and after administering oxytocin on days 10, 14, and 18 after ovulation.
• They also performed endometrial biopsies with or without uterine flushing.
• The concentrations of oxytocin receptor were then measured from the endometrial biopsy samples.

Results

• The researchers found no impact of either pregnancy status or duration after ovulation on the basal PGFM concentrations.
• In non-pregnant mares, the PGFM levels increased after administering oxytocin on all studied days, with the response on day 14 being significantly higher than on days 10 and 18.
• Contrastingly, in pregnant mares, the responses to oxytocin were restricted to days 10 and 18.
• Sharp increases in PGFM concentrations occurred on day 14 in non-pregnant mares and on day 18 in pregnant mares after performing an endometrial biopsy.
• Oxytocin receptor concentrations were found to be in the range of 200-300 fmol mg-1 protein on day 10, regardless of the pregnancy status.

Interpretation of Results

• On day 14, the concentration of oxytocin receptors increased in the non-pregnant mares, while no increase was observed during pregnancy.
• The total amount of PGFM secreted after oxytocin treatment correlated with the endometrial oxytocin receptor concentrations in non-pregnant mares. This correlation was not observed in pregnant mares.
• The study, therefore, supports the hypothesis that the concentrations of oxytocin receptor in the endometrial lining dictate the secretion of uterine prostaglandin F2 alpha in non-pregnant mares.
• Interestingly, the increase in the pregnant uterus’s response to oxytocin administration or biopsy-flushing between days 14 and 18 was not due to an increase in the oxytocin receptor concentration but likely due to increased receptor sensitivity.