Evidence for the presence of gastrin-releasing peptide immunoreactivity in rat anterior pituitary corticotrophs and lactotrophs, AtT20 cells, and GH3 cells: failure to demonstrate participation in local control of hormone release
Using antisera raised against the N-terminal (1-16) or the C-terminal part of the bombesin (BBN)-like peptide gastrin-releasing peptide (GRP) and against the C-terminus of pro-GRP, GRP- and pro-GRP-like immunoreactivity (IR) was detected by immunostaining in freshly dispersed rat anterior pituitary (AP) cells and in reaggregate AP cells cultured in serum-free medium. Depending on the antiserum used, 6.4 +/- 0.4% to 12.4 +/- 2.7% of freshly dispersed cells were immunoreactive. Solid phase preabsorption of the antisera with the respective antigens abolished the staining. GRP-IR was detectable by double immunostaining in a subpopulation of PRL cells and ACTH-containing cells. In contrast, only very few somatotrophs, gonadotrophs, and thyrotrophs were immunoreactive, and they were much smaller than the typical mature forms of these cell types. GRP- and pro-GRP-IR were also detected in the ACTH-secreting AtT20 cell line and in the PRL- and GH-secreting GH3 cell line. GRP- and pro-GRP-IR were present in AP cell aggregates maintained in culture for 4 weeks. The finding of both GRP- and pro-GRP-IR in a subpopulation of lactotrophs and corticotrophs and their persistence in culture under serum-free conditions strongly suggest that GRP-like peptides are produced in rat AP. Although the potent GRP receptor antagonist L 686,095-001C002 effectively blocks stimulation of GH and PRL release by exogenous BBN-like peptides, it failed to affect basal GH and PRL release from perifused reaggregate AP cell cultures as well as GH and/or PRL release stimulated by epinephrine, vasoactive intestinal peptide, TRH, GH-releasing factor, and angiotensin-II or PRL release inhibited by dopamine. Thus, the latter data do not support the hypothesis often suggested that peptides endogenously present in AP cells are involved in the paracrine regulation of AP hormone secretion.