A growth hormone releasing peptide, GHRP or ghrelin mimick, is a amino acid peptide that has been synthesized which is very closely related to GHRH. Still today, the mode of action in which GHRP’s is widely unknown. They have no structural homology with GHRH and act via specific receptors present either at the pituitary or the hypothalamic level both in animals and in humans. GHRP’s, much like other GH related peptides, go through desensitization which is non-dependent on the mode of administration. The GH-releasing effect of GHRPs does not depend on sex but undergoes age-related variations. Specific GHRP binding sites are also found on the cerebral cortex which suggests that the peptide may cross the blood-brain barrier thus having effects on the central nervous system.

The most interesting thing recently discovered about GHRPS is that their physiological interaction are largely affected by multiple factors rather then their own potency
.
GHRP’s are poorly understood. Some data suggest that the enhanced activity of GHRP’s at puberty could depend on gonadal steroids. In fact, the GH response to Hexarelin is more marked in pubertal girls than in boys (114), is enhanced by testosterone in children with constitutional delay ofgrowth. While estradiol could play a role in the increased GH-releasing activity of GHRPs at puberty, the fall in estrogen levels during the menopause does not play a role in the reduction of the somatotrope response to GHRPs. In fact, 3-month treatment with transdermal estradiol does not modify the GH response to Hexarelin in postmenopausal women.

The GH-releasing effect of GHRPs is dose-related. The GH response to 1 µg/kg of these peptides is generally higher than that elicited by 1 Mg/kg GHRH its maximal effective dose. Higher doses of GHRPs have rarely been studied but, recently, 2 Mg/kg Hexarelin have been shown to elicit a further GH rise demonstrated that 1 and 2/kg Hexarelin and GHRP-2 have the same GH-releasing effect. Also non-peptidyl GHRPs have dose-related effects but their potency is clearly lower than that of peptides.

Bowers CY. Chang J, Momany F & Folkers K. Effects of the enkephalins and enkephalin analogs on release of pituitary hormones in vitro. In MolecularEndocrinology, pp 287-292. EdI Macintyne. Amsterdam: Elsevier/North Holland Biochemical Press. 1977.

Bowers CY, Momany F. Reynolds GA & Hong A. A study on the regulation of growth hormone release from the pituitaries of rats in vitro. Endocrinology 1981 108 1071-1080.

Momany FA, Bowers CY. Reynolds GA, Chang D, Hong A & Newlander K. Design, synthesis and biological activity of peptides which release growth hormone in vitro. Endocrinology 1981 108 31-39.

Bowers CY. Momany FA. Reynolds GA & Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology 1984 114 1537-1545

Thorner MO, Bengtsson BA. Ho KKY, Albertsson-Wikland K, Christiansen JS. Faglia G et al. Diagnosis of growth hormone deficiency in adults. Journal of Clinical Endocrinology and Metabolism 1995 80 3097-3098. 178 De Boer H, Blok G & Van Der Veen EA. Clinical aspects of growth hormone deficiency in adults. Endocrine Reviews 1995 16 63-

Guida H. Final height attainment in normal children with short stature treated with growth hormone. Pediatrie Research 1993 33 (Suppl 5) 10. 180 Rudman D, Feller AG & Nagraj HS. Effects of human growth hormone in men over 60 years old. New England Journal of Medicine 1990 323 1-6.

Corpas E. Harman SM & Blackman G. Human growth hormone and human aging. Endocrine Reviews 1993 14 20-39.

Ross JRJM & Chew SL. Acquired growth hormone resistance. European Journal ofEndocrinology 1995 132 655-660.

Fazio S, Sabatini D. Capaldo B, Vigorito C, Giordamo A. Guida R et al. A preliminary study of growth hormone in the treatment of dilated cardiomyopathy. New EnglandJournal of Medicine 1996 334 100-103.