Inhibin Secretion in the Mare: Localization of Inhibin α, βA, and βB Subunits in the Ovary1

Abstract

To determine the source of circulating inhibin and estradiol-17beta during the estrous cycle in mares, the cellular localization of the inhibin alpha, betaA, and betaB subunits and aromatase in the ovary was determined by immunohistochemistry. Concentrations of immunoreactive (ir-) inhibin, estradiol-17beta, progesterone, LH, and FSH in peripheral blood were also measured during the estrous cycle in mares. Immunohistochemically, inhibin alpha subunits were localized in the granulosa cells of small and large follicles and in the theca interna cells of large follicles, whereas inhibin betaA and betaB subunits were localized in the granulosa cells and in the theca interna cells of large follicles. On the other hand, aromatase was restricted to only the granulosa cells of large follicles. Plasma ir-inhibin concentrations began to increase 9 days before ovulation; they remained high until 2 days before ovulation, after which they decreased when the LH surge was initiated. Thereafter, a further sharp rise in circulating ir-inhibin concentrations occurred during the process of ovulation, followed by a second abrupt decline. After the decline, plasma concentrations of ir-inhibin remained low during the luteal phase. Plasma estradiol-17beta concentrations followed a profile similar to that of ir-inhibin, except during ovulation, and these two hormones were positively correlated throughout the estrous cycle. Plasma FSH concentrations were inversely related to ir-inhibin and estradiol-17beta. These findings suggest that the dimeric inhibin is mainly secreted by the granulosa cells and the theca cells of large follicles; granulosa cells of small follicles may secrete inhibin alpha subunit, and estradiol-17beta is secreted by the granulosa cells of only large follicles in mares.

Immunolocalization of Inhibin/Activin Subunits and Steroidogenic Enzymes in Equine Neonatal Ovaries

“…Granulosa cells of various sized follicles and theca cells of large follicles are the primary sources of inhibin during the estrous cycle of the mare [18][19][20]24]. Our previous studies have demonstrated 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 source of large amounts of inhibins in fetal circulation is interstitial cells of the ovary and is not of maternal origin [25].…”

Immunohistochemical Localization of Inhibin .ALPHA.-Subunit in Two Equine Granulosa-Theca Cell Tumors

“…1 and 2) as described previously [14,15]. For inhibin α-subunit, a rabbit polyclonal antibody against [Tyro30]-porcine inhibin α-chain (1-30) diluted to 1:8000 in 0.5% (w/v) casein-Tris saline (CTS) was used as the primary antibody.…”

“…In female mammals, granulosa cells within the ovarian follicles secrete inhibin [6]. In mares, plasma concentrations of immunoreactive inhibin are higher in the follicular phase of the estrous cycle than the luteal phase and show a reciprocal relationship with FSH concentrations [2,13,14,18]. Nagamine et al [14] identified inhibin α-, βB-and βA-subunits in both granulosa and theca cells of large ovarian follicles using immunohistochemistry, whereas, only inhibin α was detected in granulosa cells of small ovarian follicles.…”

“…In mares, plasma concentrations of immunoreactive inhibin are higher in the follicular phase of the estrous cycle than the luteal phase and show a reciprocal relationship with FSH concentrations [2,13,14,18]. Nagamine et al [14] identified inhibin α-, βB-and βA-subunits in both granulosa and theca cells of large ovarian follicles using immunohistochemistry, whereas, only inhibin α was detected in granulosa cells of small ovarian follicles. In a previous study, inhibin α-subunit was expressed in the granulosa cells of 6 cases of GTCT, while inhibin βA-subunits were expressed in only 2 cases out of the 6 cases, whereas, inhibin βB-subunit was not localized in any of the 6 cases [8].…”

Effect of Active Immunization of Pony Mares against Recombinant Porcine Inhibin .ALPHA. Subunit on Ovarian Follicular Development and Plasma Steroids and Gonadotropins

“…66(1): [31][32][33][34][35] 2004 The physiological mechanisms responsible for limiting one follicular development and ovulation from multiple follicles during the normal follicular phase in the mare is not yet fully understood. Attempts at inducing multiple ovulation or promoting fertility in the mare with various treatments such as equine chorionic gonadotropin (eCG), equine pituitary extract and PGF 2 α have met with limited success [2,6,10,12,17,20]. Currently, crude pituitary extracts are the most reliable method for inducing multiple ovulations in the mare and this method usually results in one to four ovulations per cycle [5], but pituitary extracts are not readily available and the regimens required are relatively intensive.…”