GHRP-6 vs IGF-1 LR3 (Long R3 IGF-1)

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.

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-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.

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-6, Growth hormone secretagogue (GHRP); IGF-1 LR3 (Long R3 IGF-1), Modified IGF-1 analog
• Length: GHRP-6, 6 amino acids; IGF-1 LR3 (Long R3 IGF-1), not recorded
• Molecular weight: GHRP-6, 873.0 g/mol; IGF-1 LR3 (Long R3 IGF-1), not recorded
• Molecular formula: GHRP-6, C46H56N12O6; 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.