GHRP vs GHRH: Two Classes of Growth Hormone Secretagogue
Different receptors, different endogenous ligands, and a ten-fold difference in length — which is why they are constantly confused and constantly studied together.
Growth hormone secretagogues divide into two classes that are constantly confused, partly because their names are similar and partly because they produce a broadly similar readout. They act on different receptors, mimic different endogenous ligands, and behave differently — which is exactly why the literature so often studies them together.
The two pathways
| GHRH analogues | GHRP / ghrelin mimetics | |
|---|---|---|
| Receptor | GHRH receptor (GHRH-R) | Growth hormone secretagogue receptor (GHS-R1a) |
| Endogenous ligand | Growth hormone-releasing hormone | Ghrelin |
| Origin of the ligand | Hypothalamus | Stomach, mainly |
| Examples | Tesamorelin, CJC-1295, sermorelin | Ipamorelin, GHRP-2, GHRP-6, hexarelin |
| Typical length | 29–44 residues | 5–6 residues |
Both act on pituitary somatotrophs and both increase growth hormone release. They do it through separate receptors with separate downstream signalling, which is the reason combined administration appears so frequently in study designs: two independent inputs to one output.
Why the length difference
It is not arbitrary. GHRH is a 44-residue hormone and its receptor requires a substantial portion of that sequence — analogues are therefore long, which makes them expensive to synthesise and harder to purify.
Ghrelin is 28 residues, but the pharmacophore its receptor recognises is small, and synthetic GHRPs were developed by working down to a minimal active core. Five or six residues is enough. That is why ipamorelin is a pentapeptide and tesamorelin is a 44-mer, despite belonging to the same functional area.
The selectivity story within the GHRP class
The early GHRPs were potent but not clean. GHRP-2 and GHRP-6 release growth hormone and also raise ACTH, cortisol and prolactin — the ghrelin receptor is expressed in places that make that unavoidable at sufficient occupancy.
Ipamorelin is notable precisely because the original characterisation reported growth hormone release comparable to GHRP-6 without the accompanying ACTH and cortisol response. That separation is what makes it useful as a research tool: the GH axis can be probed with less confounding from adjacent endocrine effects.
Hexarelin sits at the other end, being highly potent and correspondingly less selective.
Practical differences that follow
- Cost. A 44-residue peptide costs far more per milligram than a pentapeptide — more coupling steps, lower yield, harder purification.
- Analytical demand. Long GHRH analogues need mass confirmation because deletion sequences are the main impurity and HPLC may not resolve them. Short GHRPs give clean, unambiguous chromatograms.
- Stability. Both classes carry engineered protease resistance, but by different means — GHRH analogues typically modify the N-terminus to block DPP-4, GHRPs typically use D-amino acids and terminal capping.
- Handling. Longer sequences have more conformational freedom and more scope for aggregation, so the standard rules matter more for the GHRH class.
Reading a compound name
The WHO stem -relin marks a releasing-hormone agonist and appears in both classes — tesamorelin and ipamorelin share it despite different receptors. The stem describes function, not target. A code of the form GHRP-n marks the ghrelin-mimetic class specifically, with the number indicating development order and nothing chemical.
Our CJC-1295 versus ipamorelin comparison covers one pairing in depth, and the naming article covers what the stems and codes mean.
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