Bioactive follicle-stimulating hormone concentrations in plasma during the estrous cycle of the ewe

Bioactive FSH (B-FSH) concentrations in plasma were determined during the ovine estrous cycle by means of an in vitro bioassay. The concentrations of B-FSH were elevated during and after the preovulatory LH surge and were significantly (p < 0.05) lower during the late-luteal to mid-follicular phases compared with the mid-luteal phase. A significant (p < 0.05) increase in B-FSH was found about 36 h before the LH surge, at a time when the immunoreactive FSH (I-FSH) concentrations were low and unchanged. The plasma B/I ratio for FSH was relatively constant during the luteal phase; it then increased significantly (p < 0.05) before the LH surge and decreased again at the time of the LH surge itself. Pulsar analysis showed that there were 4 peaks of B-FSH throughout the estrous cycle with 2 during the luteal phase, 1 after the LH surge, and the other either during the follicular phase or associated with the LH surge. For I-FSH there were approximately 8 peaks throughout the cycle with 4 during the luteal phase, 2 after the LH surge, and 1 each during the follicular phase and the preovulatory LH surge. There was a weak negative correlation between I-FSH and immunoreactive inhibin (I-inhibin) during most of the estrous cycle, but B-FSH and the B/I ratio were only correlated (negatively) with I-inhibin in the 24 h before the preovulatory LH surge. These findings suggest that there are significant changes in the circulating isoforms of FSH during the ovine estrous cycle that may affect the growth of antral follicles developing towards ovulation.