The roles of inhibin and related peptides in gonadal function

Inhibin A and B are dimeric proteins capable of suppressing FSH both in vit ro and in vivo. The principal form in the male is inhibin B which is produc ed in the testis and circulates to inhibit pituitary FSH secretion. Activin A, B and AB are dimeric proteins that share the same beta subunits with th e inhibins but, in contrast, stimulate FSH secretion. Although activin A ci rculates, castration does not lead to a decrease in serum concentrations, i ndicating that the testis is not the major source of activin A. In the circ ulation, the activins are bound to a structurally unrelated binding protein , follistatin, that neutralizes the biological actions of these proteins. T he subunits of the inhibins/activins as well as follistatin are also produc ed locally within the pituitary and their levels can be modulated by testos terone and gonadotrophin releasing hormone as well as by autocrine mechanis ms. Consequently, the output of FSH is dependent of the balance between loc al processes and the circulating feedback exerted by testosterone and inhib in. There is increasing data to support the local gonadal production of not only inhibin but also activin and follistatin by both germ cells and somat ic cells such as the Sertoli cells. Evidence is accumulating to support the concept that these proteins exert local regulatory mechanisms in the testi s. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.