DIFFERENTIAL CHANGES IN INHIBIN-A, ACTIVIN-A, AND TOTAL ALPHA-SUBUNITLEVELS IN GRANULOSA AND THECAL LAYERS OF DEVELOPING PREOVULATORY FOLLICLES IN THE CHICKEN

Accumulating evidence implicates inhibins and activins as endocrine an d local regulators of follicular development in mammals, and it was re cently confirmed that inhibin/activin alpha and beta A genes are also expressed in the avian ovary. To investigate the potential involvement of these proteins in the chicken ovary, thecal and granulosa layers o f the four largest follicles (F1-F4) and the most recent postovulatory follicle were collected from hens (10/group) killed 4, 12, and 20 h b efore the expected time of F1 ovulation. Inhibin A and activin A conce ntrations of tissue extracts (expressed per mg DNA) were measured usin g validated two-site enzyme-linked immunosorbent assays; total immunor eactive inhibin alpha-subunit (ir-alpha) was also measured by heterolo gous RIA (Monash assay). Inhibin A and ir-alpha were largely confined to the granulosa layer, whereas activin A was much more abundant in th e 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-alph a in granulosa was much more abundant than inhibin A and increased onl y 3-fold from F4-F1 (P < 0.001). Activin Ain both granulosa and theca showed little variation between F1 and F4 follicles (by ANOVA, P > 0.0 5). The inhibin A content of F1 granulosa was maximal 12 h before ovul ation and had fallen approximately 6-fold (P < 0.0001) within 8 h, sug gesting an inhibitory effect of the preovulatory LH surge on the F1 ca pacity to synthesize inhibin A. Inhibin A, activin A, and ir-alpha wer e all less in the postovulatory follicle compared with F1 before ovula tion (P < 0.0001). In conclusion, application of the present two-site enzyme-linked immunosorbent assays to the chicken ovary revealed 1) di vergent tissue distribution of inhibin A and activin A within preovula tory follicles, and 2) differential regulation of granulosa cell produ ction 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 subse rve regulatory roles during preovulatory follicular development in the hen.