Growth Hormone Secretagogues

The traditional concept of a neuroendocrine mechanism for regulation of growth hormone (GH) secretion is based in large part on the work of Roger Guillemin. The work of Dr. Guillemin, who was awarded the 1977 Nobel Prize in Physiology and Medicine, supported the view that quantita- tive change in GH secretion was the net result of pituitary stimulation and inhibition by the hypothalamic neurohormones, GH releasing hormone (GHRH), and somatostatin (somatotropin release inhibiting factor; SRIF), respectively. During the 1970s, another endocrine research pioneer, Dr. Cyril Bowers, discovered that structural modification of enkephalin re- sulted in a family of peptides with GH releasing properties. These com- pounds, simply called GH releasing peptide (GHRP), were originally thought to mimic GHRH. However, upon subsequent investigation they were found to supplement the activity of the natural hormone through a different mechanism. Nearly two decades after their discovery, the differ- ences between GHRP and GHRH have been described by many different laboratories throughout the world. The complementary GH secretagogues have different binding sites, second messengers, and effects on gene expres- sion. Based on these differences, it has been suggested that expansion of the original two hormone mechanisms for GH regulation to include a third molecule may be appropriate, even though the naturally occurring ana- logue of GHRP has not yet been identified. Despite our lack of knowledge concerning the natural product mimicked by GHRP, clinical development of the new family of GH secretagogues for diagnostic and therapeutic purposes has begun in earnest.