Gonadotropin Control of Inhibin Secretion and the Relationship to Follicle Type

Inhibin is secreted in two distinct heterodimeric forms, A and B, but the mechanism for the differential control of these two forms is unclear. To evaluate the relationship between secretion of inhibin forms and folliculogenesis, the effects of gonadotro-pins on inhibin concentrations were studied in parallel with ste-reological enumeration of ovarian follicle types in gonadotropin-deficient hypogonadal (hpg) female mice treated with recom-binant human FSH (10 IU/day), hCG (1 IU/day), or both for 20 days. Treatment with FSH alone significantly increased blood concentrations of both inhibin A and inhibin B, whereas hCG alone had no effect on either inhibin. The combination of FSH and hCG further increased the concentration of inhibin A but had no effect on the concentration of inhibin B beyond that of FSH. The number of primordial follicles per ovary was signifi-cantly reduced in FSH-treated hpg mice, but was not affected by hCG treatment. Antral follicles were absent in the untreated hpg mice, present following treatment with FSH, and were pres-ent in only limited numbers following hCG treatment alone. Pre-ovulatory follicles were observed only in the wild-type and com-bined FSH and hCG treatment groups. These results demonstrate that secretion of both inhibins is associated with the presence of antral follicles. Inhibin A secretion is increased by the pres-ence of preovulatory follicles, whereas the concentration of in-hibin B is not affected. The observed effects of gonadotropins on inhibin A and B secretion may be explained by corresponding gonadotropin effects on follicle development. follicle, follicle number, follicle-stimulating hormone, gonadotro-pins, hpg, human chorionic gonadotropin, inhibin, ovary, stere-olog