AUTORADIOGRAPHIC DEMONSTRATION OF I-125 TYR(4)-BOMBESIN BINDING-SITESON RAT ANTERIOR-PITUITARY-CELLS

Because several direct effects of bombesin related peptides on pituita ry hormone release have been demonstrated, we chose to study the prese nce of bombesin binding sites in the adult male rat anterior pituitary in aggregate cell culture by autoradiographic localization of I-125-t yrosine(4)-bombesin (I-125-Tyr(4)-BBN) binding and immunocytochemical localization of the anterior pituitary hormones. When aggregates were cultured in medium without hormonal supplements, the number of cells w ith detectable I-125-Tyr(4)-BBN binding was below 1%. In cell aggregat es cultured in the presence of 1 nM estradiol (E(2)) I-125-Tyr(4)-BBN binding was detected on 5.4 +/- 0.8% of the cells after redispersion a nd on 5.8 +/- 1.1% of the cells in sections of paraffin embedded aggre gates. The binding cell types were mainly lactotrophs and somatotrophs . The binding of I-125-Tyr(4)-BBN (3 or 5 nM) was specific because it was inhibited by the addition of an excess of unlabelled Tyr(4) BBN or the bombesin receptor antagonist L686,095. In aggregates cultured in the presence of 1 nM E(2) and 4 nM dex, the percentage of cells with d etectable I-125-Tyr(4)-BBN binding was significantly lower than that i n aggregates cultured in the presence of 1 nM E(2) alone. Binding on s omatotrophes almost completely disappeared. The present data show that specific I-125-Tyr(4)-BBN binding sites are present on anterior pitui tary cells, are detectable mainly on a small subpopulation of lactotro phs and somatotrophs, and are affected by hormonal conditions. These d ata are consistent with the previously observed effects of bombesin-re lated peptides on growth hormone and prolactin release and the modulat ion of these effects by E(2) and dexamethasone and indicate that these effects are brought about by a direct action on lactotrophs and somat otrophs.