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Differential Changes in Inhibin A, Activin A, and Total -Subunit Levels in Granulosa and Thecal Layers of Developing Preovulatory Follicles in the Chicken¹

School of Animal and Microbial Sciences, University of Reading (T.M.L., R.T.G., F.J.C., P.G.K.), Whiteknights, Reading, United Kingdom RG6 6AJ; and the School of Biological and Molecular Sciences, Oxford Brookes University (N.P.G.), Oxford, United Kingdom OX3 OBP

Address all correspondence and requests for reprints to: Dr. P. G. Knight, School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, United Kingdom RG6 6AJ.

Accumulating evidence implicates inhibins and activins as endocrine and local regulators of follicular development in mammals, and it was recently confirmed that inhibin/activin and ßA genes are also expressed in the avian ovary. To investigate the potential involvement of these proteins in the chicken ovary, thecal and granulosa layers of the four largest follicles (F1–F4) and the most recent post-ovulatory follicle were collected from hens (10/group) killed 4, 12, and 20 h before the expected time of F1 ovulation. Inhibin A and activin A concentrations of tissue extracts (expressed per mg DNA) were measured using validated two-site enzyme-linked immunosorbent assays; total immunoreactive inhibin -subunit (ir-) was also measured by heterologous RIA (Monash assay). Inhibin A and ir- were largely confined to the granulosa layer, whereas activin A was much more abundant in the thecal layer. Granulosa inhibin A contents were similar in F4 and F3, but increased approximately 40-fold from F3–F1 (P < 0.0001). As such, the F1 granulosa layer was by far the richest source of inhibin A in the chicken ovary, but contained very little activin A. Total ir- in granulosa was much more abundant than inhibin A and increased only 3-fold from F4–F1 (P < 0.001). Activin A in both granulosa and theca showed little variation between F1 and F4 follicles (by ANOVA, P > 0.05). The inhibin A content of F1 granulosa was maximal 12 h before ovulation and had fallen approximately 6-fold (P < 0.0001) within 8 h, suggesting an inhibitory effect of the preovulatory LH surge on the F1 capacity to synthesize inhibin A. Inhibin A, activin A, and ir- were all less in the postovulatory follicle compared with F1 before ovulation (P < 0.0001). In conclusion, application of the present two-site enzyme-linked immunosorbent assays to the chicken ovary revealed 1) divergent tissue distribution of inhibin A and activin A within preovulatory follicles, and 2) differential regulation of granulosa cell production of inhibin A and activin A dimers during preovulatory follicular development. These findings of dynamic changes in inhibin A, activin A, and total ir- support the hypothesis that these proteins subserve regulatory roles during preovulatory follicular development in the hen.

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