title: Plasma inhibin A in heifers: relationship with follicle dynamics, gonadotropins, and steroids during the estrous cycle and after treatment with bovine follicular fluid.
abstract: The relationship between follicle growth and plasma inhibin A, FSH, LH, estradiol (E), and progesterone was investigated during the normal bovine estrous cycle and after treatment with steroid-free bovine follicular fluid (bFF) to arrest follicle development. In the first study, four heifers were monitored over three prostaglandin (PG)-synchronized cycles. Blood was collected every 2-8 h, and ovaries were examined daily by ultrasonography. Inhibin A was measured using a modified enzyme-linked immunosorbent assay that employed a new monoclonal antibody against the alpha subunit of bovine inhibin. Plasma inhibin A ( approximately 50 pg/ml before luteolysis) rose steadily during the induced follicular phase (P < 0.05) to a peak ( approximately 125 pg/ml) coincident with the preovulatory E/LH/FSH surge. After ovulation, inhibin A fell sharply (P < 0.05) to a nadir ( approximately 55 pg/ml) coincident with the secondary FSH rise. During the next 3 days, inhibin A increased to approximately 90 pg/ml in association with growth of the new dominant follicle (DF). Plasma E also rose twofold during this period, whereas FSH fell by approximately 50%. Inhibin A was negatively correlated with FSH (r = -0.37, P < 0.001) and positively correlated with E (r = 0.49, P < 0.0001). Observations on eight cycles (two cycles/heifer), in which growth of the ovulatory DF was monitored from emergence to ovulation, showed that the first-wave DF (DF1) ovulated in three cycles and the second-wave DF (DF2) in five cycles. After PG, plasma inhibin A and E increased similarly in both groups, with concomitant falls in FSH. In the former group, the restricted ability of DF1 to secrete both inhibin A and E was restored after luteolysis. Results indicate that dynamic changes in the secretion of both E and inhibin A from the DF contribute to the fall in FSH during the follicular phase and to the generation and termination of the secondary FSH surge, both of which play a key role in follicle selection. In the second study, bFF (two dose levels) was administered to heifers (n = 3-4) for 60 h starting from the time of DF1 emergence. Both doses suppressed FSH (P < 0.05) and blocked DF1 growth to the same extent (P < 0.01), although inhibin A levels were only marginally raised by the lower dose (not significant compared to controls). The high bFF dose raised (P < 0.001) inhibin A to supraphysiological levels ( approximately 1 ng/ml). A large "rebound" rise in FSH occurred within 1 day of stopping both treatments, even though the inhibin A level in the high-dose bFF group was still approximately threefold higher than that in controls. This indicates that desensitization of gonadotropes to inhibin negative feedback is a contributory factor, together with reduced ovarian output of E, in generation of the post-bFF rebound in FSH.
authors.affiliation: School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading RG6 6AJ, United Kingdom.
authors.forename: E C
authors.initials: EC
authors.lastname: Bleach
authors.forename: R G
authors.initials: RG
authors.lastname: Glencross
authors.forename: S A
authors.initials: SA
authors.lastname: Feist
authors.forename: N P
authors.initials: NP
authors.lastname: Groome
authors.forename: P G
authors.initials: PG
authors.lastname: Knight
mesh_terms: D000818:Animals; D000911:Antibodies, Monoclonal; D002417:Cattle / Q000097:blood / Q000502:physiology*; D004591:Electrophoresis, Polyacrylamide Gel / Q000662:veterinary; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods* / Q000592:standards; D004958:Estradiol / Q000097:blood / Q000378:metabolism; D004971:Estrus / Q000502:physiology*; D004973:Estrus Synchronization; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood / Q000378:metabolism; D015571:Follicular Fluid / Q000502:physiology*; D006728:Hormones / Q000097:blood* / Q000378:metabolism; D006801:Humans; D007265:Inhibins / Q000097:blood / Q000502:physiology*; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism; D051379:Mice; D006080:Ovarian Follicle / Q000378:metabolism / Q000502:physiology*; D011374:Progesterone / Q000097:blood / Q000378:metabolism; D011863:Radioimmunoassay / Q000662:veterinary
chemical_list: D000911:Antibodies, Monoclonal; D006728:Hormones; D011374:Progesterone; D004958:Estradiol; D007265:Inhibins; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone