Identification of a new G-protein-linked receptor for growth hormone secretagogues

The potential application of small molecules in GH therapy has recently become a topic of increasing interest. The spiroindoline MK-0677, the benzolactam L-692,429, and the peptides, GHRP-6 and hexarelin, have been shown to possess potent and selective GH-secretory activity in several species including human. Moreover, these synthetic GH secretagogues act on a signal transduction pathway distinct from that of GHRH. A specific high affinity binding site in porcine and rat anterior pituitary membranes that mediates the activity of these secretagogues has now been identified. The binding affinity of these structurally diverse secretagogues is tightly correlated with GH-secretory activity. The binding is Mg(2+)-dependent, is inhibited by GTP-gamma-S, and is not displaced by GHRH and somatostatin. The receptor is distinct from that for GHRH and has the properties of a new G-protein-coupled receptor. It is speculated that these GH secretagogues mimic an unidentified natural hormone that regulates GH secretion in concert with GHRH and somatostatin.