GHRP-2 vs Ipamorelin

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.

Ipamorelin

Ipamorelin is a synthetic pentapeptide, Aib-His-D-2-Nal-D-Phe-Lys-NH2, that acts as an agonist at the ghrelin receptor, GHS-R1a. It was derived from earlier growth hormone-releasing peptides and is one residue shorter than GHRP-2 and GHRP-6. What set it apart in the original animal pharmacology was selectivity: growth hormone release was not accompanied by the rises in ACTH and cortisol recorded for older compounds in the class. Because ghrelin receptors are also present in the gut, it has been examined in rodent models of gastrointestinal motility, and it has been the subject of a small number of human clinical studies. Human data remain limited, and most of what is known comes from animal work.

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 Ipamorelin acts

Ipamorelin has been reported to bind GHS-R1a on somatotroph cells of the anterior pituitary, where receptor activation raises intracellular calcium and triggers growth hormone secretion. Its reported selectivity concerns other pituitary and adrenal hormones: in the animal studies that first characterised it, ACTH and cortisol did not rise appreciably, unlike with GHRP-6 or GHRP-2. The structural basis for this difference has not been fully explained. The receptor is also found in the gut wall and its nerve supply, which is the rationale for rodent studies of gastrointestinal transit using this compound.

The records side by side

• Type: GHRP-2, Growth hormone secretagogue (GHRP); Ipamorelin, Growth hormone secretagogue (GHRP)
• Length: GHRP-2, 6 amino acids; Ipamorelin, 5 amino acids
• Molecular weight: GHRP-2, 818.0 g/mol; Ipamorelin, 711.9 g/mol
• Molecular formula: GHRP-2, C45H55N9O6; Ipamorelin, C38H49N9O5

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.