What Is a GHRP? Growth Hormone Releasing Peptides Explained | QSC Research
QSC RESEARCH GUIDE
What Is a GHRP? Growth Hormone Releasing Peptides Explained
Growth hormone releasing peptides (GHRPs) are synthetic peptides that stimulate GH release from the anterior pituitary by activating the growth hormone secretagogue receptor 1a (GHSR-1a) — also known as the ghrelin receptor. This guide covers the GHRP class, the distinction from GHRHs, individual compound profiles, and research design considerations.
GHRP vs GHRH — Two Independent GH Release Pathways
Why the combination is greater than the sum of parts
GHRP (Gq/Ca²⁺) and GHRH (Gs/cAMP) activate two independent signalling cascades in somatotrophs. The Ca²⁺ and cAMP signals converge on the GH secretory machinery with synergistic effect — producing GH release substantially above what either pathway alone can achieve. This is the mechanistic basis for the standard CJC-1295 + ipamorelin combination in GH axis research.
GHRP Compound Comparison
Compound
Potency
Cortisol/ACTH
Prolactin
Appetite
Unique feature
Ipamorelin
Moderate
Minimal
Minimal
Minimal
Highest selectivity — GH only
GHRP-2
High
Moderate
Moderate
Moderate
Balance of potency and selectivity
GHRP-6
High
Moderate
Moderate
High
Strong ghrelin axis — appetite research
Hexarelin
Highest
High
High
Moderate
CD36 cardiac receptor activation
MK-677 (ibutamoren)
High
Moderate
Moderate
High
Oral bioavailability — GHSR-1a agonist
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GHSR-1a — The Ghrelin Receptor
GHSR-1a is a constitutively active GPCR with high baseline activity even without ligand:
GHSR-1a property
Research implication
Constitutive activity (~50% max without ligand)
Inverse agonists can suppress baseline GH secretion — distinct from competitive antagonists
Two isoforms: GHSR-1a and GHSR-1b
GHSR-1a is the functional receptor; GHSR-1b is truncated and non-signalling — dimerises with GHSR-1a
Expressed in pituitary, hypothalamus, vagus, cardiac, adipose
Multiple peripheral roles beyond GH release
Acylated ghrelin binds GHSR-1a; des-acyl ghrelin does not
Acylation is required for GH-stimulating activity — des-acyl has GHSR-1a independent metabolic effects
D-amino acid substitutions in GHRPs
All GHRPs contain D-amino acid substitutions (e.g., D-Nal in ipamorelin, D-Phe in GHRP-6). D-amino acids are resistant to proteolytic degradation — dramatically extending peptide half-life vs L-amino acid peptides. This is a standard strategy for improving SPPS peptide pharmacokinetics. The D-amino acid substitutions also confer specific GHSR-1a binding geometry that natural ghrelin (all L-amino acids + acylation) achieves differently.
GHRPs (ipamorelin, GHRP-2, hexarelin) act on GHSR-1a (ghrelin receptor) via Gq/Ca²⁺ signalling. GHRHs (CJC-1295, sermorelin) act on the GHRH receptor via Gs/cAMP signalling. Both ultimately stimulate GH release but through independent intracellular pathways — explaining their synergistic effect when combined.
Why is ipamorelin considered the most selective GHRP?
Ipamorelin produces selective GHSR-1a-mediated GH release with minimal co-activation of cortisol, prolactin, or appetite pathways. GHRP-2 and GHRP-6 produce equivalent or higher GH but with significant cortisol and prolactin co-elevation. Hexarelin additionally activates CD36 and causes strong HPA co-activation. Ipamorelin’s selectivity profile makes it the preferred GHRP when GH axis effects need to be studied without confounds.
What is GHSR-1a constitutive activity and why does it matter?
GHSR-1a has approximately 50% of its maximum signalling activity even without ligand binding — one of the highest constitutive activities of any GPCR. This means neutral competitive antagonists only partially suppress GHSR-1a signalling; inverse agonists are needed for full suppression. For research using GHSR-1a antagonists as controls, the constitutive activity must be accounted for.
Can GHRPs cause desensitisation?
Repeated high-dose GHSR-1a activation can cause receptor internalisation and desensitisation. This is more pronounced with hexarelin (highest efficacy) than ipamorelin (moderate efficacy). Pulsatile dosing (mimicking physiological GH pulse patterns) reduces desensitisation compared to sustained or high-frequency dosing.
