ACTIVIN-LIKE PEPTIDES IN SOMATOTROPHS AND ACTIVIN STIMULATION OF GROWTH-HORMONE RELEASE IN GOLDFISH

Activin and inhibin, dimeric protein hormones originally isolated from mammalian gonads, are involved in the regulation of pituitary gonadot ropin secretion. Using domain-specific antibodies against activin and inhibin alpha, beta A, and beta B subunits, the present study demonstr ates that immunoreactive activin and inhibin subunits, especially beta A, exist in goldfish pituitary. Immunocytochemical staining with anti -gonadotropin-II and anti-growth hormone showed that the pituitary cel ls containing immunoreactive activin beta subunits are somatotrophs. T his is different from the situation in mammals where it is the gonadot rophs that produce activin molecules within the pituitary. The stainin g with anti-beta B was overall weak compared to that with anti-beta A, but both appear to localize in the same cells. Strong immunostaining with the anti-inhibin alpha subunit was also observed in the goldfish pituitary; however, the immunoreactivity is dissociated from those of beta A and beta B, and mainly associated with nerve fibers in the neur ointermediate lobe. Based on this evidence, it is suggested that the g oldfish pituitary predominantly produced activin-like molecules. Both porcine activin and inhibin stimulate growth hormone release from peri fused goldfish pituitary fragments. Taken together with our previous f indings that porcine activin stimulates gonadotropin-II release in gol dfish, and the fact that the somatotrophs and gonadotrophs are in clos e contact with each other in the goldfish pituitary, it is hypothesize d that somatotroph-derived activin may exert paracrine actions on the adjacent gonadotrophs to stimulate gonadotropin release and autocrine actions on somatotrophs to stimulate growth hormone secretion. This al so provides a mechanism for communication between these two pituitary Cell types. (C) 1994 Academic Press, Inc.