Plasma concentrations of ir-inhibin, inhibin A, inhibin pro-alphaC, FSH, and estradiol-17beta during estrous cycle in mares and their relationship with follicular growth.

The relationship among plasma levels of immunoreactive (ir)-inhibin, inhibin A, inhibin pro-alphaC, FSH, estradiol-17beta and follicular growth were investigated during the normal estrous cycle in mares. Seven mares were used for two successive normal estrous cycles. Follicular diameters and ovulation were obtained by transrectal ultrasonography, and blood samples were collected daily by jugular venipuncture for hormonal assay. The results showed that inhibin A was inversely correlated (r = -0.59, p < 0.0001) with FSH indicating its contribution to negative feedback control of FSH secretions from the pituitary gland. Estradiol-17beta increased during the follicular phase reaching a peak (37.9 +/- 3.8 pg/mL) 2 d before ovulation. Estradiol-17beta was positively correlated (r = 0.78, p < 0.0001) with inhibin A. The high levels of inhibin A and estradiol-17beta were associated with the growth of the preovulatory dominant follicle and inversely correlated with FSH suggesting that both hormones are products of the large dominant follicles and were responsible for the decline in FSH secretion during the follicular phase of estrous cycle. In conclusion, an inverse relationship between inhibin A and FSH was clearly demonstrated indicating that inhibin A has a key role in the negative feedback control of FSH from the pituitary gland. In addition, inhibin A and estra-diol-17beta secretions were associated with the growth of the preovulatory dominant follicle and were positively correlated.