A selective increase in circulating inhibin and inhibin pro-alphaC at the time of ovulation in the mare.

The relationship between a selective increase in circulating immunoreactive (ir)-inhibin and the time of ovulation was investigated in mares. Concentrations of plasma ir-inhibin were measured every 4 h during the periovulatory period. Inhibin pro-alphaC, a precursor protein of the inhibin alpha-subunit, was also measured. The changes in ir-inhibin and inhibin pro-alphaC in circulation were parallel. Concentrations of both ir-inhibin and inhibin pro-alphaC in the plasma increased at the same time when ovulatory follicles ruptured, and the peak levels of circulating ir-inhibin and inhibin pro-alphaC were maintained for 4-8 h. There was no selective increase in plasma concentrations of estradiol-17beta during the process of ovulation. These results suggest that the selective increase in ir-inhibin and inhibin pro-alphaC was caused by the absorption of follicular fluid after the rupture of ovulatory follicles. These results also suggest that the measuring of plasma concentrations of ir-inhibin or inhibin pro-alphaC in mares might be a useful method for detecting the time of ovulation.