Biochemistry of the Growth Hormone-Releasing Peptides, Secretagogues and Ghrelin

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A general chronology of growth hormone-releasing peptide (GHRP) and ghrelin and their shared receptor is shown in Table 1 [1] [3]. The discovery of the natural hormone ghrelin appears to be the exciting beginning of a new unusual hormone system. Initially, in 1980, GH-releasing activity of the unnatural GHRPs was thought to represent the activit y of the as-yet unidentified natural hypophysiotrophic hormone GH-releasing hormone (GHRH). However, in 1982, GHRH was isolated from a pancreatic tumour and the hypothalamus and chemically identified as 44 and 40 amino acid linear peptides. Although the GH-releasing activity of GHRPs and GHRH are similar, it is also apparent that they are definitely chemically and functionally different because of the uniqueness of the action of GHRP on GH secretion in vitro as well as in vivo in a variety of animal species. In 1984, it was postulated that GHRP reflected the activity of another new hypothalamic hormone involved in the increased secretion of GH and thus was in need of isolation.