Role of inhibin-related peptides as intragonadal regulators

Evidence for intragonadal roles of the inhibin-related peptides, activin, inhibin, alpha N peptide of inhibin (alpha N) and follicle-stimulating hormone suppressing protein (FSP) or follistatin, is reviewed. It is proposed that activin, FSP and alpha N act as intragonadal regulators to influence folliculogenesis, ovulation and luteinization, and that inhibin primarily influences folliculogenesis by its peripheral effects on the secretion of FSH and LH by the pituitary gland, with few, if any, effects directly within the ovarian follicle. We propose that activin promotes follicular differentiation and delays the onset of premature luteinization, whereas FSP promotes atresia of non-dominant follicles and luteinization of dominant follicles, and is therefore antagonistic to activin. We hypothesize that alpha N, a post-translational processing product of the alpha subunit of inhibin, acts locally in the follicle to facilitate ovulation.