Inhibin secretion in the mare: localization of inhibin alpha, betaA, and betaB subunits in the ovary.

The research investigated the sources of inhibin and estradiol-17beta hormones during the estrous cycle in mares. The authors specifically looked into the location of inhibin subunits and aromatase in the ovary and investigated their correlation with hormonal changes within the estrous cycle.

Methodology

• The research team used immunohistochemistry to determine the location of the inhibin alpha, betaA, and betaB subunits, as well as an enzyme called aromatase, in the ovary. This technique uses antibodies to visually tag specific proteins, allowing researchers to see their distribution.
• They also measured the concentrations of inhibin, estradiol-17beta, progesterone, LH (luteinizing hormone), and FSH (follicle-stimulating hormone) in the blood of mares during the estrous cycle to understand how these varied over time.

Findings and Interpretation

• Inhibin alpha subunits were found in the granulosa cells of both small and large follicles and in the theca interna cells of large follicles. In contrast, inhibin betaA and betaB subunits were localized in granulosa cells and theca interna cells of only large follicles.
• Aromatase, necessary for the formation of estradiol, was found only in the granulosa cells of large follicles, indicating that these are the primary sites of estradiol-17beta production.
• Plasma concentrations of inhibin began to increase 9 days before ovulation, maintaining high levels until two days before ovulation. A significant increase was observed during ovulation, followed by a sharp decline, staying low throughout the luteal phase.
• Plasma estradiol-17beta followed a similar trend to inhibin, suggesting a positive correlation between the two throughout the estrous cycle. However, FSH concentrations were inversely related to inhibin and estradiol-17beta, presumably as a result of feedback regulation in the hormonal system.

Conclusion

• A key conclusion drawn from the research is that the dimeric inhibin (a hormone regulating FSH secretion) is mainly secreted by granulosa and theca cells of large follicles. The smaller follicles seem to play a role in secreting inhibin alpha subunit only.
• Estradiol-17beta, an estrogen steroid hormone, was found to be secreted only by the granulosa cells of the larger follicles.

This research is crucial as it sheds light on the hormonal mechanisms at play during the reproductive cycle in mares, potentially contributing to the improved management of equine reproduction.