In addition to the known hypothalamic hormone, GHRH, we speculate that there is an endogenous ligand for a novel family of GH secretagogues (peptide and nonpeptide) based on the prototypic compound, GH-releasing peptide (GHRP); these compounds which readily release GH in synergy with, and dependent on, available GHRH (as well as the reciprocal relationship). Evidence for development of this hypothesis is reported elsewhere (111-113). Based on these suppositions we have developed a diagnostic test in children and in aging subjects that attempts to discern whether there is a deficiency in endogenous GHRH and/or endogenous ligand for GHRP (111-113). The recent report of a receptor in the pituitary and hypothalamus (114) should accelerate the search for an endogenous ligand for the orally active peptide and nonpeptide GH secretagogues.
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