Response of Estradiol and Inhibin to Experimentally Reduced Luteinizing Hormone During Follicle Deviation in Mares1

Abstract

The increase in LH concentrations at the time of the decrease in FSH concentrations during follicle deviation in mares was studied to determine the role of LH in the production of estradiol and immunoreactive inhibin (ir-inhibin). Ten days after ovulation, all follicles > or =6 mm were ablated, prostaglandin F(2 alpha) was given, and either 0 mg (control group, n = 15) or 100 mg of progesterone in safflower oil (treated group, n = 16) was given daily for 14 days, encompassing the day of diameter deviation. The follicular and hormonal data were normalized to the expected day of the beginning of diameter deviation when the largest follicle first reached > or =20 mm (Day 0). The experimentally induced decrease in LH concentrations during follicle deviation beginning on Day -4 delayed and stunted the increase in circulating concentrations of ir-inhibin and estradiol beginning on Days -3 and -1, respectively, but did not alter the predeviation FSH surge and the initiation of diameter deviation between the two largest follicles. Combined for both groups, the interval to the expected day of deviation was 16.6 days after ovulation when the largest follicle was a mean of 21.6 mm. After deviation, the largest follicle started to regress in the treated group beginning on Day 1 and was associated with decreased concentrations of ir-inhibin and estradiol, and increased concentrations of FSH. The negative influence of the dominant follicle on the postdeviation decrease in FSH observed in the control group was alleviated and concentrations resurged in the treated group. Apparently this is the first in vivo evidence that the increase in LH that precedes follicle deviation has a positive effect in supporting the production of inhibin during diameter deviation. It was concluded that the increase in LH concentrations before diameter deviation played a role in the production of estradiol and inhibin by the largest follicle during deviation.

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].…”

“…Toward the end of diestrus, progesterone begins to decrease about 2 days before an increase in LH. In addition to these temporal relationships, a negative effect of progesterone on LH also has been shown, using exogenous progesterone [6,12,13]. Treatment with progesterone during early development of the ovulatory follicular wave reduced the circulating concentrations of LH.…”

“…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. The relationship of ovarian hormones to systemic FSH concentrations was studied every 4 days in an ovariectomy experiment [10].…”