Concentrations of oxytocin in the intercavernous sinus of mares during luteolysis: temporal relationship with concentrations of 13,14-dihydro-15-keto-prostaglandin F2 alpha.

The research investigates the relationship between oxytocin and 13,14-dihydro-15-keto prostaglandin F2 alpha (PGFM) concentrations in horses during luteolysis, by analyzing intervals of blood samples. The study indicates a potential association between high-magnitude surges of these hormones, but doubts a link between their lower magnitude pulses.

Methodology

• The reproductive tracts of nine thoroughbred mares were examined using ultrasound to determine the day of ovulation.
• The mares were fitted with intercavernous sinus cannulae a day before sample collection started.
• Intercavernous sinus blood samples were collected over a 36 hour period at 5 minute intervals, beginning at noon on either day 13, 15, or 16 after ovulation.
• Plasma concentrations of oxytocin and PGFM were determined through radioimmunoassay.

Findings

• Three high-magnitude surges (> 1 ng ml-1) of PGFM were found amongst the collected samples, along with three high-magnitude pulses of oxytocin (> 200 pg ml-1).
• In each case, it was observed that the oxytocin pulse followed or coincided with the start of the PGFM surge.
• Lower magnitude pulses of both hormones were detected periodically throughout each mare’s bleeding period.
• The average interval between these pulses was approximately 122.3 minutes for oxytocin and 121 minutes for PGFM, with no correlation detectable between the intervals amongst the different mares.

Conclusions

• The data suggests that high-magnitude surges of PGF2 alpha (measured here as PGFM, a major metabolite of PGF2 alpha) are coordinated with similar surges in oxytocin from the posterior pituitary gland.
• This indicates that PGF2 alpha might induce the secretion of oxytocin.
• Pulsatile secretion of oxytocin from the posterior pituitary gland was also observed, at a frequency of about 1 pulse every 2 hours. However, these pulses did not seem to correspond with the lower magnitude pulses of PGF2 alpha released from the uterus.

Overall, this study provides an insight into how the hormones oxytocin and PGF2 alpha behave during luteolysis in mares, which could prove beneficial in future reproductive research and healthcare in equine species.