The aims of this study were to develop a sensitive and specific assay for bovine
inhibin A using
europium and to investigate the endocrine role of
inhibin A in various reproductive conditions by characterizing the relationship between profiles of
inhibin A, FSH, and
estradiol and follicle growth during the postpartum period, during the intact estrous cycle, and in cows with
follicular cysts. The time-resolved immunofluorometric assay (Tr-IFMA) for bovine
inhibin A, using purified polyclonal
antibodies to alpha and beta(A) subunits, was specific for bovine
inhibin A and did not cross-react with
bovine activin A,
activin AB,
activin B, pro-alphaC or human recombinant
inhibin B. The detection limit of the IFMA was 3.3 pg/ml expressed in terms of bovine 32-kDa
inhibin A. Dose-response curves of plasma samples obtained from intact and FSH-stimulated cows and cystic cows were parallel to the standard without any preassay processing of samples. Plasma
inhibin A levels increased (P < 0.01) concomitant with emergence of nonovulatory or ovulatory follicular waves during the postpartum period. In cystic cows, plasma
inhibin A was sustained at high levels for a longer period, associated with growth of persistent dominant follicles. The highest levels of
inhibin A were noted during the growth phase of normal and persistent dominant follicles; however,
inhibin A levels declined (P < 0.01) as these dominant follicles ceased to grow or ovulated. An inverse relationship between patterns of plasma
inhibin A and FSH existed during each follicular wave in the three physiologic conditions. Increases in plasma
inhibin A levels were associated with increases in plasma
estradiol levels during most follicular waves; however, there was no increase in plasma
estradiol level and no relationship between patterns of
estradiol and FSH during follicular waves observed during the early postpartum period or midluteal phase of the estrous cycle. In conclusion, the Tr-IFMA does not require pretreatment of samples and can be used for precise measurement of bovine
inhibin A without interference with free
inhibin alpha subunits.
Inhibin A, produced primarily during growth of the dominant follicle, functions as a negative feedback regulator for FSH secretion throughout the postpartum period and the estrous cycle, whereas
estradiol appears to have a minor role in regulation of FSH compared with
inhibin A, especially during the early postpartum period and midluteal phase of the estrous cycle. The results also indicate that a persistent dominant follicle sustains
inhibin A production for a longer period than the dominant follicle emerging in the estrous cycle and establishes long-term dominance by suppressing emergence of a new follicular wave.