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

Abstract

Plasma concentrations of immunoreactive (ir)-inhibin were measured in seven pregnant mares from around Day 140 of gestation to Day 2 after parturition using a heterologous bovine-based radioimmunoassay (RIA). Concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), oestradiol-17 beta, progesterone and relaxin were also measured in the same samples. A marked increase in plasma concentrations of ir-inhibin, FSH and LH occurred between Day 220 and Day 300 of gestation but the concentrations of all three hormones returned to baseline by about Day 320 (three weeks before parturition). In contrast, circulating concentrations of the three placental hormones, oestradiol-17 beta, progesterone and relaxin, increased during the final weeks of pregnancy and then decreased markedly to basal values within two days of parturition. There was a positive correlation between circulating concentrations of ir-inhibin and FSH (r = 0.75, P < 0.01) rather than the expected negative correlation. ir-inhibin was not detected in homogenates obtained at Day 190 of pregnancy and form term placenta, but high concentrations of ir-inhibin were present in homogenates of fetal and newborn gonads. Despite the high concentrations of ir-inhibin in these homogenates, they failed to exert any suppressive bioactivity on FSH secretion by rat pituitary cells cultured in vitro. Furthermore, immunohistochemical staining revealed the presence of inhibin in the interstitial cells of equine fetal gonads at Day 190 of gestation. These findings demonstrate for the first time that high concentrations of ir-inhibin, LH and FSH are secreted into the peripheral circulation of the mare during the second half of pregnancy. However, ir-inhibin present in the plasma of pregnant mares appears to be biologically inactive. This hormone is not presumed to be of placental origin but it is proposed that either the enlarged fetal gonads or the maternal ovaries, or both of these organs, may be a source of inhibin in response to the coincident increase in circulating concentrations of LH and FSH.

Immunoreactive Insulin-Like Growth Factor in Plasma during Pre- and Post-Partum Periods of Thoroughbred Mares from which the Newborn were Removed: its Pattern, Physiological Function and Relation to Other Hormones

“…The only consistent feature of previous studies of FSH concentration occurred during the last days of pregnancy when there appeared to be an increase, beginning 30 days prior to parturition [3,7,14]. This rise in ir-IGF-I and FSH levels and the timing of the first ovulation 10-14 days later corresponds to what is seen at mid-diestrus in the mare, leading to a wave of follicular growth and ovulation 10-14 days later [10,13].…”

“…Plasma IGF-I concentrations were measured by a double-antibody RIA procedure [10] as described in a previous paper [4]. Levels of immunoreactive (ir-) inhibin, follicle stimulating hormone (FSH), luteinizing hormone (LH), equine chorionic gonadotrophin (eCG), estradiol-17β (E 2 ) and progesterone (P 4 ) in peripheral blood were measured by RIA as described in previous papers [2,4].…”

Expression and Cellular Localization of Inhibin .ALPHA.-Subunit mRNA in Equine Fetal Gonads.

“…Recently, Nambo et al [13] reported the presence of high concentrations of immunoreactive (ir)-inhibin in the plasma of pregnant mares and demonstrated that ir-inhibin was present in the equine fetal ovary, though its homogenate showed no suppressive activity on pituitary FSH secretion. This does not, however, exclude the possibility that inhibin and/or inhibin α-subunit related molecules probably produced in equine fetal gonads might be functioning as paracrine/autocrine factors.…”

Change in Circulating Follicle-Stimulating Hormone, Luteinizing Hormone, Immunoreactive Inhibin, Progesterone, Testosterone and Estradiol-17.BETA. in Fillies from Birth to 6 Months of Age

“…In equine, plasma concentrations of immunoreactive (ir) inhibin, inhibin A, inhibin pro-αC, FSH, and estradiol-17β during the estrous cycle in mares and their relationship with follicular growth have been reported [10]. Our previous studies have shown that high concentrations of ir-inhibin were present in homogenates of equine fetal ovaries at Day 190 of gestation [12]. Furthermore, immunohistochemical staining revealed the presence of inhibin α and inhibin/activin (β A and β B ) subunits in the interstitial cells of equine fetal ovaries, showing that the main sources of the large amounts of inhibins in fetal circulation are the interstitial cells of the ovary, and are not, therefore, of maternal origin [17].…”