Disruption of the periovulatory LH surge by a transient increase in circulating 17beta-estradiol at the time of ovulation in mares.

This research article investigates the correlation between ovulation, levels of Luteinizing Hormone (LH), and transient increases in 17beta-estradiol in mares. The study suggests that a rise in estradiol at the time of ovulation may disrupt and decrease the surge of LH, with the source of estradiol likely derived from the follicular fluid released during ovulation.

Study Design and Methodology

• The study involved nine mares whose preovulatory follicle approached or exceeded 32mm in diameter. At this stage, they were treated with human chorionic gonadotropin, a hormone that stimulates ovulation and the release of estradiol.
• Examinations to detect ovulation and blood collection were performed at 2-hour intervals. To allow for methodical analysis, each mare’s results were retrospectively centralized to the time of ovulation, marked as Hour 0.

Findings

• Concentrations of LH were found to increase significantly 18 hours before ovulation, then decrease noticeably between ovulation and 6 hours afterwards. Following this, concentrations once again rose after the 12th hour.
• Progressive reduction in estradiol levels between Hours -30 and 24 was temporarily interrupted by a surge in estradiol 2 hours prior to and during ovulation, followed by a marked decrease two hours after ovulation.

Conclusion

• The study found a significant correlation between ovulation, a transient increase in estradiol levels, and a temporary disruption and subsequent fall in LH concentration. This suggests that a surge in estradiol at the time of ovulation disrupts the rise of LH.
• From prior research suggesting the negative effect of estradiol on LH, the decrease in LH concentrations is therefore attributed to the surge in estradiol.
• The source of estradiol is likely due to rapid absorption into the bloodstream from estradiol-rich follicular fluid discharged into the abdomen during ovulation.