GHRP-2 vs GHRP-6

GHRP-2

GHRP-2, also known by the non-proprietary name pralmorelin, is a synthetic hexapeptide that activates the ghrelin receptor, GHS-R1a. Three of its six residues are D-amino acids, including the non-natural D-2-naphthylalanine, a design feature shared with other synthetic growth hormone-releasing peptides. These compounds were developed by screening for growth hormone release before their receptor had been identified; ghrelin, the receptor's natural ligand, was discovered afterwards. GHRP-2 has been examined in receptor and pituitary cell assays, in animal studies, and in short human endocrine studies that measured hormone secretion. Longer-term human data are sparse, and it is not selective for growth hormone, since rises in other pituitary and adrenal hormones have also been recorded.

GHRP-6

GHRP-6 is a synthetic hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, and one of the earliest growth hormone-releasing peptides to be described. It came out of structure-activity work on small synthetic peptides, carried out before anyone knew which receptor they acted on. That receptor, GHS-R1a, was later shown to respond to ghrelin, a hormone made mainly in the stomach, so GHRP-6 is now classed as a ghrelin receptor agonist. It has been used in pituitary cell and receptor assays, in a large body of animal work, and in human endocrine studies of hormone secretion. Because the same receptor takes part in the control of feeding, it has also served as a tool in appetite research.

How GHRP-2 acts

GHRP-2 has been reported to act as an agonist at GHS-R1a, a G protein-coupled receptor present on somatotroph cells of the anterior pituitary and on neurons in the hypothalamus. In somatotrophs the receptor couples mainly to Gq and phospholipase C, and the rise in intracellular calcium that follows triggers growth hormone secretion. This route is separate from that of growth hormone-releasing hormone (GHRH), which acts through its own receptor and cyclic AMP. Part of the response is thought to be hypothalamic, through effects on GHRH neurons and on somatostatin signalling. In human studies, modest increases in ACTH, cortisol and prolactin have been reported alongside growth hormone.

How GHRP-6 acts

GHRP-6 is reported to bind and activate GHS-R1a, a G protein-coupled receptor that signals mainly through Gq, phospholipase C and intracellular calcium. On pituitary somatotrophs this signalling triggers growth hormone release. The receptor is also expressed in the hypothalamus, including on neuropeptide Y and agouti-related peptide neurons of the arcuate nucleus, which rodent studies have linked to the control of food intake; that distribution is the basis for the peptide's use in feeding research. Small increases in ACTH, cortisol and prolactin have been reported with it in addition to growth hormone. Interactions with somatostatin signalling have been proposed but are less clearly defined.

The records side by side

• Type: GHRP-2, Growth hormone secretagogue (GHRP); GHRP-6, Growth hormone secretagogue (GHRP)
• Length: GHRP-2, 6 amino acids; GHRP-6, 6 amino acids
• Molecular weight: GHRP-2, 818.0 g/mol; GHRP-6, 873.0 g/mol
• Molecular formula: GHRP-2, C45H55N9O6; GHRP-6, C46H56N12O6

Reading a comparison

A comparison places two records side by side; it does not merge them. Each description above is drawn from that compound's own record, and the two were not necessarily characterised by the same methods, at the same time, or to the same level of detail. Before comparing any attribute, it is worth establishing that both records describe the substance you think they describe — names are not identifiers, and two entries sharing a name can differ in salt form, stereochemistry or the convention used to state a mass. Where a figure carries no method beside it, the comparison is between two reported numbers rather than between two measurements.