Like most of the TGFp superfamily, activin signals through two types of closely related transmembrane serine/ threonine kinase receptors (designated type I and type II), each represented by two isoforms (ActRIA/ActRIB and ActRIIA/ActRIIB, respectively). Since a specific inhibin receptor has not yet been identified, and inhibin can bind to the activin type II receptor, it is currently thought that inhibin acts as a naturally occurring antagonist of ac-tivin, although recent evidence suggests that inhibin may have a separate signal transduction pathway as well. buy ventolin inhalers
Inhibin is a gonadal hormone that inhibits the synthesis and release of pituitary FSH, while both activin and inhibin regulate growth, proliferation, and differentiation through paracrine and/or autocrine actions in a variety of cell types including the gonads. Both activin and inhibin modulate rat ovarian folliculogenesis in vivo, while in vitro, activin has been shown to modulate FSH receptor and aromatase induction, inhibin production, and proliferation in granulosa cells. More recently, activin was found to influence follicular organization in vitro. When administered with FSH, activin was found to induce formation of follicular structures from monolayer cultures of immature rat follicles. Interestingly, this induction required the presence of an oocyte, suggesting that activin, acting directly or indirectly, induces oocytes to produce a follicle-organizing factor.