Source of inhibin in ovine fetal plasma and amniotic fluid during late gestation: half-life of fetal inhibin

Post-Natal Dynamic Changes in Circulating Follicle-Stimulating Hormone, Luteinizing Hormone, Immunoreactive Inhibin, Progesterone, Testosterone and Estradiol-17β in Thoroughbred Colts until 6 Months of Age

The aim of present study was to clarify the post-natal profile of follicle-stimulating hormone (FSH), luteinizing hormone (LH), immunoreactive (ir)-inhibin, progesterone, testosterone, and estradiol-17β, and their relationships in Thoroughbred colts. Six hundred and thirty-six colts were used for the study. Single plasma samples from each animal were harvested from the blood drawn through jugular venipuncture. The subjects were born with high amounts of progesterone, testosterone, and estradiol-17β, all of which dropped significantly and remained at lower levels till the end of 6 months. FSH decreased transiently after birth until day 12 and then gradually increased to peak at day 100 which then maintained in lesser levels towards the end of the studied period. LH was highest during birth which decreased until day 26 and then increased slowly to sub-birth levels up to day 90. Animals were born with high amounts of ir-inhibin. It dropped slowly and halved by day 20 and then decreased towards rest of the studied period. The increase in FSH is negatively correlated with the declining ir-inhibin levels. The early increase in FSH can be the indication of early post-natal maturation of the hypothalamic pituitary testicular axis that ultimately might be responsible for priming the testes for future development.

Activins, inhibins and follistatins in the large domestic species

The activins and inhibins are members of the transforming growth factor-beta (TGF-beta) superfamily and, along with follistatin, a high affinity binding protein of activin, form a group of interrelated factors originally isolated for their role in regulating the release of follicle-stimulating hormone (FSH). Knowledge of their function, particularly that of activin, has expanded since being originally isolated, such that they are now regarded as important paracrine regulators in many cellular systems. This review summarizes the biology of these proteins as has been established in the large domestic animals. While the majority of data relate to the pituitary, ovary, uterus/placenta and testis, consideration is also given to emerging roles in inflammatory processes and in non-reproductive tissues or systems.

Localization and secretion of inhibins in the equine fetal ovaries

To clarify the source of inhibins in equine female fetuses, concentrations of immunoreactive (ir-) inhibin, inhibin pro-alphaC, and inhibin A in both fetal and maternal circulation and in fetal ovaries were measured. In addition, the localization of inhibin alpha and inhibin/activin beta(A), and beta(B) subunits and the expression of inhibin alpha(A) and inhibin/activin beta(A) subunit mRNA in fetal ovaries were investigated using immunohistochemistry and in situ hybridization. Concentrations of circulating ir-inhibin, inhibin pro-alphaC, and inhibin A were remarkably more elevated in the fetal than in the maternal circulation between Days 100 and 250 of gestation. Fetal ovaries contained large amounts of ir-inhibin, inhibin pro-alphaC, and inhibin A. In contrast, these inhibin forms were undetectable in both the maternal ovaries and placenta. The inhibin alpha and inhibin/activin beta(A) and beta(B) subunit proteins were localized to enlarged interstitial cells of the equine fetal ovary. Expression of inhibin alpha and inhibin/activin beta(A) subunit mRNAs were also observed in the interstitial cells. We conclude that the main source of large amounts of inhibins in fetal circulation is interstitial cells of fetal ovary and is not of maternal origin. Furthermore, these inhibins may play some important physiological roles in the development of gonads in the equine fetus.