Restricted presence of the growth hormone-releasing hormone receptor to somatotropes in rat and human pituitaries

Specific binding of growth hormone-releasing hormone (GHRH) to its plasma m embrane receptor represents the first step of cellular signals leading to e xocytotic GH secretion in the anterior pituitary. The GHRH receptor (GHRH-R ) has been cloned and belongs to the secretin/glucagon/vasoactive intestina l peptide subfamilly of G-protein-coupled receptors. To study its character istics in rat and human pituitaries and examine its cellular and subcellula r localization, a site-directed polyclonal antibody recognizing the C-termi nal portion 392-404 of the rat and human GHRH-R was used. Immunohistochemis try was performed on paraffin-embedded pituitary sections while ultrastruct ural immunocytology was done on frozen and Lowicryl-resin-embedded ultrathi n sections. GHRH-R-like immunoreactivity was restricted to somatotropes and colocalized with GH in both rat and human tissues. No signal was detected in gonadotropes, lactotropes, corticotropes and thyrotropes. At the subcell ular level, gold particles were associated with the plasma membrane (observ ed on ultrathin frozen sections), secretory granule membrane, cytoplasmic m atrix, nuclear membra ne and nuclear matrix. In the nucleus, gold particles were mainly observed at the junction between eu- and heterochromatin. The highest density of labeling was observed in the cytoplasm (55 vs. 45% in th e nucleus), mainly in secretory granules (59% of cytoplasmic labeling) and the plasma membrane. These results support the hypothesis that GHRH-mediate d actions in the pituitary are specific to somatotropes and that GHRH-R iso forms and/or ligand-receptor complexes are involved in intracellular traffi cking, recycling processes and nuclear functions.