Differential changes in inhibin A, activin A, and total alpha-subunit levels in granulosa and thecal layers of developing preovulatory follicles in the chicken

Accumulating evidence implicates inhibins and activins as endocrine and local regulators of follicular development in mammals, and it was recently confirmed that inhibin/activin alpha and betaA 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 postovulatory 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 alpha-subunit (ir-alpha) was also measured by heterologous RIA (Monash assay). Inhibin A and ir-alpha 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-alpha 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-alpha 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-alpha support the hypothesis that these proteins subserve regulatory roles during preovulatory follicular development in the hen.