GHRP-2 vs IGF-1 LR3 (Long R3 IGF-1)

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.

IGF-1 LR3 (Long R3 IGF-1)

IGF-1 LR3 is an 83-residue analogue of insulin-like growth factor 1, with an N-terminal extension and arginine replacing glutamate at position 3. The extension adds 13 amino acids to the 70 of native IGF-1, and the name records both changes: L for long, R3 for the arginine. Together the modifications lower its affinity for the IGF-binding proteins that normally hold most IGF-1 in bound complexes. It is produced recombinantly and is used as a supplement in serum-free media for growing mammalian cell lines. Outside cell culture, published work consists mainly of animal studies, and human data are very limited.

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 IGF-1 LR3 (Long R3 IGF-1) acts

IGF-1 LR3 is reported to activate the type 1 IGF receptor, a receptor tyrosine kinase, with an affinity similar to or somewhat lower than that of native IGF-1. Its defining property is weak binding to the IGF-binding proteins, which in serum and conditioned culture media normally sequester most IGF-1. With less of it held in complexes, more is free to reach the receptor, which is the usual explanation for its higher apparent activity in cell culture. Receptor activation engages insulin receptor substrate proteins and the PI3K–Akt–mTOR and Ras–MAPK pathways, which govern protein synthesis, proliferation and survival in the cell types studied.

The records side by side

• Type: GHRP-2, Growth hormone secretagogue (GHRP); IGF-1 LR3 (Long R3 IGF-1), Modified IGF-1 analog
• Length: GHRP-2, 6 amino acids; IGF-1 LR3 (Long R3 IGF-1), not recorded
• Molecular weight: GHRP-2, 818.0 g/mol; IGF-1 LR3 (Long R3 IGF-1), not recorded
• Molecular formula: GHRP-2, C45H55N9O6; IGF-1 LR3 (Long R3 IGF-1), not recorded

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.