Inhibin and related peptides: mechanisms of action and regulation of secretion.

The structure of inhibin is known; it consists of a heterodimer composed of one alpha and one beta subunit. The homodimer of beta A (beta A-beta A) and the heterodimer beta A-beta B, called activin A and B, respectively, stimulate the release and synthesis of FSH by gonadotrophs. Inhibin exerts effects at the hypophyseal, hypothalamic, and gonadal levels. Produced by granulosa cells in the female and by Sertoli cells in the male, inhibin synthesis is stimulated by FSH and reduced by hypophysectomy and progesterone. At present, there is no evidence for a signal from germinal cells to modify inhibin production. Inhibin secretion evolves in parallel with follicular maturation and aromatase activity, whereas luteinization arrests its production. Nevertheless, important differences in the regulation of inhibin secretion seem to exist from one species to another. Sperm inhibin levels can be correlated with spermatozoa number. Administration of inhibin to sheep induces either anovulation or an increase in the rate of ovulation depending on the scheme of treatment.