Expression of inhibin alpha and inhibin/activin beta(3) subunits in the granulosa layer of the large preovulatory follicles of the hen

The expression of the mRNA for the inhibin/activin subunits (alpha and beta(A)) in the granulosa layer of the five largest preovulatory follicles of the hen was investigated. Total RNA from the granulosa layer of the F-5 (the fifth largest) to F-1 (the largest) follicles was extracted and analyzed by Northern blot analysis using homologous chicken inhibin alpha and beta(A) subunit cDNA probes. RNA loading was quantified by a cDNA probe of bovine 185 rRNA. Results showed that for the chicken inhibin alpha subunit mRNA signals (n = 3), the mean relative intensity for the F-1, F-2, F-3, and F-4 follicles was 0.50 +/- 0.10 (+/- SEM,), 0.52 +/- 0.08, 0.59 +/- 0.06, and 0.81 +/- 0.04, respectively, compared to a mean relative intensity of 1.00 (p < 0.05) for the F-5 follicle. For the beta(A) subunit mRNA signals (n = 3), the mean relative intensity for the F-5, F-4, F-3, and F-2 follicles was 0.25 +/- 0.06, 0.28 +/- 0.15, 0.40 +/- 0.17, and 0.48 +/- 0.10, respectively, compared to a mean relative intensity of 1.00 (p < 0.05) for the F-1 follicle. The inhibin alpha subunit was also estimated to be more abundantly expressed among follicles in the granulosa layer than was the beta(A) subunit. Our data indicate that the expression of inhibin alpha and beta(A) subunits is differentially regulated in the hen granulosa layer during follicular development. Expression of the alpha subunit is reduced with follicular development whereas inhibin beta(A) subunit expression is dramatically enhanced. In addition, the granulosa layer of the large preovulatory follicles may produce more inhibin alpha subunit than beta(A) subunit, and the F-1 follicle may be the primary source of the beta(A) subunit for dimeric inhibin and/or activin in the hen.