Effect of FSH and cell localization on dimeric inhibin-A secretion from bovine granulosa cells in culture

We tested the hypotheses that the secretion of dimeric inhibin-h from cultu red bovine granulosa cells is stimulated by FSH, and that antral cells secr ete more inhibin-ii than do mural cells. Cells from the antral or mural com partment of follicles were cultured in defined medium in two culture system s, and dimeric inhibin-A was measured by two-site ELISA or by Western immun oblotting. In the first culture system, dimeric inhibin-A secretion decline d with time in culture, but was significantly (P<0.05) higher from antral t han from mural cells (as was total inhibin-alpha measured by RIA). The secr etion of dimeric inhibin-A and inkibin-alpha from antral but not mural cell s was responsive to FSH, In the second culture system, dimeric inhibin-A se cretion increased with time in culture, and was significantly stimulated by FSH, but FSH responsiveness was dependent on the concentrations of insulin in the culture medium. The major forms of inhibin-A secreted had molecular masses of approximately 58, 62, 103-116 and >116 kDa; the 32 kDa form was barely detectable. These different forms were all stimulated by FSH, but th e >116 and 62 kDa forms were most responsive to FSH. We conclude that (i) F SH stimulates dimeric inhibin-A secretion from bovine granulosa cells, (ii) the 62 kDa form of inhibin-A may be more responsive to FSH than the 58 kDa form, and (iii) the spatial differentiation of granulosa cell function wit hin the follicle previously observed for oestradiol secretion was also obse rved for inhibin-alpha and dimeric inhibin-A secretion.