Testosterone administration to mares during estrus: duration of estrus and diestrus and concentrations of LH and FSH in plasma.

This research investigated the impact of testosterone on the duration of estrus and diestrus, and the concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in mares. It found that testosterone administration during estrus did not affect the duration or hormone concentrations during this period, although FSH levels were higher during mid-diestrus in the testosterone-treated group.

Methodology

• The researchers conducted an experiment on ten mares—five were treated with 20 micrograms/kg of body weight of testosterone daily during estrus (the portion of the sexual cycle when the mare is receptive and can become pregnant) whilst the remaining five mares (control group) were given safflower oil on the same schedule.
• All mares were monitored for signs of estrus and jugular blood samples were taken daily throughout one complete estrous cycle to precisely track the hormonal changes.
• A newly developed radioimmunoassay was used to measure FSH concentrations from the plasma obtained from the blood samples. This immunoassay used anti-ovine (sheep) FSH serum and radioiodinated equine (horse) FSH.

Findings

• Administration of testosterone during estrus was found to have no impact on the duration of estrus, diestrus (the period between two estral cycles), or the total length of the cycle.
• The same observation was made for FSH concentrations during estrus: there was no noticeable difference between the testosterone group and the control group.
• However, during mid-diestrus, FSH concentrations in the testosterone-treated mares were found to be significantly higher than those in the control group. The chance of this occurring at random was less than 5% (P less than .05), implying a high degree of statistical certainty.
• There were no changes in the timing and magnitude of LH peaks between the testosterone group and the control group. Similarly, the day when increased progesterone levels were first observed remained unchanged by the testosterone treatment.

Conclusion

• From the data collected and analyzed, the researchers deduced a model for FSH secretion in mares. In this model, ovarian androgens (like testosterone) cause an accumulation of FSH in the pituitary gland during estrus. This build-up of hormone then helps trigger the surges in FSH secretion observed during the diestrus stage of the mare’s sexual cycle.