EVIDENCE FOR DIRECT ACTION OF GASTRIN-RELEASING PEPTIDE (GRP) IN RAT LACTOTROPHS, SOMATOTROPHS AND GONADOTROPHS

Gastrin-Releasing Peptide (GRP) is the 27 amino acid mammalian homolog ue of the batracian 14 amino acid peptide bombesin. Its receptors have been characterized in pituitary cells. The aim of the present study w as to test for the presence of GRP itself in the anterior pituitary ce lls of rats by an immunocytological method, using a GRP-specific antib ody. The anterior parts of the pituitary glands were removed, and fixe d either in 2.5% glutaraldehyde, followed by 1% osmium tetroxide or in 4% paraformaldehyde. They were then frozen, and the ultrathin section s cut by cryoultramicrotomy. The presence of GRP-like immunoreactivity was detected in lactotrophs, somatotrophs and gonadotrophs, but not i n thyrotrophs or corticotrophs. In positive cells, characterized by th eir cytological features, gold particles were localized on the plasma membrane, and in the cytoplasm and nucleus. In the cytoplasm, immunore activity was detected in the cytoplasmic matrix, and in the nucleus it was found in the euchromatin in the vicinity of the heterochromatin. In vitro, in a primary culture of anterior pituitary cells, GRP (1-27) stimulated the secretion of GH, PRL and LH in a dose-dependent way, b ut did not stimulate secretion of TSH or ACTH. These results - the det ection of GRP-like immunoreactivity, and the effect of this peptide on GH, PRL and LH secretion - provide evidence for a direct action of GR P on somatotrophs, lactotrophs and gonadotophs.