Effect of number and diameter of follicles on plasma concentrations of inhibin and FSH in mares

Abstract

The role of the number of follicles and circulating immunoreactive inhibin in the decrease in plasma FSH concentrations that occurs during development of a follicular wave was studied in mares. All follicles > or = 6 mm in diameter were ablated by ultrasound-guided transvaginal aspiration of follicular fluid on day 10 after ovulation. During the subsequent wave, all follicles, the three largest follicles (three follicle group), the largest follicle (single follicle group) or no follicles were retained and the remaining follicles were ablated before they reached > 10 mm in diameter (n = 10-11 mares per group). Ablation of new follicles was continued until the day on which the largest follicle of the new wave reached 25 mm in diameter (day 18 after ovulation in the 'no follicle' group). Diameters of retained follicles were measured once a day by transrectal ultrasonography. Plasma samples were taken once a day and analysed by radioimmunoassay for concentrations of FSH and immunoreactive inhibin (includes dimeric inhibin as well as free alpha-subunit forms). Data were normalized to the day of the expected start of the decrease in plasma FSH concentrations (day 0: largest follicle 13 mm in diameter in the follicle-retained groups). A simultaneous increase in circulating concentrations of FSH (P < 0.05) and immunoreactive inhibin (P < 0.05) occurred before the largest follicle reached 13 mm in diameter, which indicates that immunoreactive inhibin produced by follicles < 13 mm in diameter did not suppress FSH. Plasma concentrations of FSH decreased (P < 0.05) and immunoreactive inhibin concentrations increased (P < 0.05) after day 0 in the follicle-retained groups. A slower decrease in FSH concentrations was associated temporally with a delay in the increase in immunoreactive inhibin concentrations in the 'single follicle' group relative to the 'three follicle' and 'all follicle' groups. All follicle-retained groups had similar plasma concentrations of FSH and immunoreactive inhibin after the expected beginning of deviation in growth rates between the two largest follicles (largest follicle 22-23 mm in diameter). These results indicated that the decrease in plasma FSH concentrations from the start of the decrease until the expected day of deviation was a function of multiple follicles of a wave and was attributable to the secretion of inhibin. Thereafter, the largest follicle alone accounted for the continued FSH suppression.

Regulation of Circulating Gonadotropins by the Negative Effects of Ovarian Hormones in Mares1

“…The ir-inhibin results in the controls are consistent with the reports [5,6] of a gradual increase in concentrations from 1 or 2 days before the largest follicle reached an estimated 13 mm until the expected beginning of deviation, followed by a plateau. The greater decrease in ir-inhibin after ovariectomy than after follicle ablation is consistent with the secretion of inhibin by follicles Ͻ10 mm in diameter.…”

“…Immunoreactive (ir) inhibin concentrations begin to increase before the beginning of the declining portion of the wave-stimulating FSH surge [5,6]. Similar results were obtained for inhibin-A [7], indicating that inhibin-A is at least one of the active forms that depresses FSH before and during deviation.…”

“…The antibody recognizes dimeric forms of inhibin, as well as free ␣ subunits [20]. Nevertheless, irinhibin is a good indicator of FSH inhibitory activity in mares, as indicated by the consistent reciprocal relationship between circulating ir-inhibin and FSH concentrations during follicular wave development [5,6]. Furthermore, during the last half of the anovulatory season and early ovulatory season, the profiles for circulating ir-inhibin [21] were similar to those for inhibin A [7].…”

“…The intra-and interassay coefficients of variation (CV) and mean sensitivity, respectively, were 2.5%, 6.3%, and 1.0 ng/ml for FSH and 5.1%, 10.3%, and 0.2 ng/ ml for LH. The concentrations of ir-inhibin were measured by a doubleantibody radioimmunoassay kit (Institute of Reproduction and Development, Monash Medical Center, Clayton, Australia) as validated and described in our laboratory for mare plasma [5]. The antibody recognizes dimeric forms of inhibin, as well as free ␣ subunits [20].…”

“…Similar results were obtained for inhibin-A [7], indicating that inhibin-A is at least one of the active forms that depresses FSH before and during deviation. The initial 2 days of the decline in the FSH surge is caused by inhibin alone, based on temporal relationships among FSH, ir-inhibin, and estradiol [5,8]. Circulating estradiol begins to increase near the beginning of deviation [6,9], but the relative contribution of estradiol to the continued decline in the FSH surge is not known.…”