Occurrence of immunoreactive activin/inhibin beta(B) in thyrotropes and gonadotropes in the bullfrog pituitary: Possible paracrine/autocrine effects of activin B on gonadotropin secretion

Occurrence of immunoreactive activin/inhibin beta(B) in the bullfrog (Rana catesbeiana) pituitary was investigated immunocytochemically by use of anti body against Xenopus activin/inhibin beta(B) subunit. Thyrotropes were demo nstrated to contain activin/inhibin beta(B)-immunoreactive substances. More over, immunoelectron microscopy revealed that in the secretory granules of thyrotropes and, to a lesser extent, in those of gonadotropes, activin/inhi bin beta(B)-immunoreactive substances were present. Based on this observati on, we investigated the effect of activin B on the release of gonadotropins from dispersed anterior pituitary cells of the bullfrog. Activin B stimula ted the release of not only follicle-stimulating hormone (FSH) but also lut einizing hormone (LH) dose dependently. Under the culture conditions used i n this experiment, inhibin B, as well as follistatin, did not affect the ba sal levels of LH and FSH, but they suppressed the activin-induced release o f these hormones. This is the first study on the effect of activin on pitui tary hormone secretion in lower tetrapods. (C) 2000 Academic Press.