High concentrations of immunoreactive inhibin in the plasma of mares and fetal gonads during the second half of pregnancy.

The research investigates the concentrations of several hormones, including the immunoreactive inhibin, in pregnant mares and their relation to the gestation period. It reveals that high concentrations of these hormones are secreted into the peripheral circulation of the mare during the second half of pregnancy and discloses a unique disposition of these hormones during this period.

Hormone Measurements

• The researchers measured the concentrations of immunoreactive inhibin, luteinizing hormone (LH), follicle-stimulating hormone (FSH), oestradiol-17 beta, progesterone and relaxin in seven pregnant mares from around Day 140 of gestation to Day 2 after parturition.
• A notable increase in the plasma concentrations of immunoreactive inhibin, FSH and LH was observed between Day 220 and Day 300 of gestation, but these hormones returned to baseline by Day 320, roughly three weeks before parturition.
• Conversely, the circulating concentrations of the three placental hormones (oestradiol-17 beta, progesterone and relaxin) increased during the final weeks of pregnancy and then decreased to basal values within two days of parturition.

Correlation and Bioactivity

• A positive correlation was observed between circulating concentrations of immunoreactive inhibin and FSH, which contrasts the expected negative correlation. This suggests that the levels of these hormones impact each other, although not in the anticipated manner.
• Despite the high concentrations of immunoreactive inhibin in fetal and newborn gonads, these failed to suppress hormone activity (particularly FSH secretion) in controlled lab tests with rat pituitary cells.

Source and Role of Hormones

• Immunohistochemical staining revealed the presence of inhibin in the interstitial cells of equine fetal gonads at Day 190 of gestation. No immunoreactive inhibin was detected in homogenates from Day 190 of pregnancy or term placentas.
• The researchers proposed that the enlarged fetal gonads and/or the maternal ovaries could be sources of inhibin due to the synchronous increase in the circulating concentrations of LH and FSH in response to the high concentrations of immunoreactive inhibin.
• The findings suggest that the immunoreactive inhibin present in the plasma of pregnant mares appears to be biologically inactive, contrary to its common role in hormonal regulation. It is assumed that this hormone is not of placental origin.