Inhibin activity in the mare and stallion

Area of Science:

• Reproductive Endocrinology
• Veterinary Medicine
• Hormone Assays

Background:

• Inhibin plays a crucial role in regulating reproductive functions.
• Accurate measurement of inhibin is essential for understanding equine reproductive physiology.
• Previous assays lacked specificity or sensitivity for equine samples.

Purpose of the Study:

• To validate a radioimmunoassay (RIA) for quantifying immunoreactive inhibin (iINH) in equine biological fluids and tissues.
• To investigate the dynamic relationship between iINH, follicle-stimulating hormone (FSH), and estradiol during the estrous cycle of mares and the annual cycle of stallions.

Main Methods:

• Developed and validated an overnight double antibody RIA using bovine inhibin standards and antiserum.
• Measured iINH concentrations in equine peripheral plasma, follicular fluid, ovarian/testicular vein plasma, and tissue extracts.
• Correlated iINH levels with FSH and estradiol during the estrous cycle (mares) and annual cycle (stallions).

Main Results:

• The RIA demonstrated parallelism and accurate recovery for equine samples.
• Significantly higher iINH concentrations were found in gonadal veins compared to peripheral plasma in both mares and stallions.
• iINH levels in mares peaked at ovulation and correlated positively with estradiol and negatively with FSH.
• Stallion iINH levels showed seasonal variation, peaking in late spring and correlating positively with FSH.

Conclusions:

• A specific and sensitive RIA for iINH was successfully validated for equine samples.
• The gonads are confirmed as the primary source of immunoreactive and bioactive inhibin in horses.
• iINH dynamics are closely linked to reproductive cycles and hormonal regulation in both mares and stallions.