Peripheral concentrations of inhibin A, ovarian steroids, and gonadotropins associated with follicular development throughout the estrous cycle of the sow

We investigated changes in peripheral concentrations of inhibin A, total inhibin, steroids, and gonadotropins throughout the intact estrous cycle of the sow in relation to ovarian changes determined by daily transrectal ultrasonography. All visible follicles of 3 mm or more in diameter were classified as small (≥3 and P2) levels increased (PPP≥0.1) between the patterns of plasma E2and FSH during the early luteal phase. In conclusion, measurement of plasma inhibin A levels combined with ultrasonographic examination of the ovaries revealed two periods of synchronous follicular growth during the sow's estrous cycle. The results strongly suggest that inhibin A functions as a negative feedback regulator of FSH secretion throughout the estrous cycle, whereas E2appears to influence FSH secretion only during the follicular phase.