GHRP-2 and GHRP-6 are synthetic hexapeptides that stimulate growth hormone (GH) release from the pituitary. They belong to the growth hormone-releasing peptide (GHRP) family, which predates the discovery of the receptor they act on and of ghrelin, the natural hormone for that receptor. Both remain widely used research tools for studying the GH axis. This article compares their structures, receptor pharmacology and published selectivity data, and explains where ipamorelin fits as a comparator.
These compounds are supplied by BioSim for laboratory research only. They are not for human or veterinary use, and nothing here is guidance on administration to people.
Origins of the GHRP Family
GHRP-6 was one of the first small synthetic peptides shown to release GH. In the original characterization, the hexapeptide His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 released GH from rat pituitary cells at concentrations of 1 to 10 ng/mL and was active in rats, monkeys, lambs and calves. It did not release LH, FSH, TSH or prolactin, and somatostatin (SRIF-14 and SRIF-28) inhibited its GH response (Bowers et al., 1984). In perifused rat pituitary cells, GHRP and the hypothalamic releasing factor GRF-44 both stimulated GH release with a similar overall pattern, although GRF-44 was about 60 times more potent. Continuous infusion of either peptide led to desensitization (Badger et al., 1984).
GHRP-2 (D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2) was developed later in the same family. It replaces several residues with non-natural D-amino acids and the bulky aromatic 2-naphthylalanine, giving a more potent analog.
The Receptor: GHS-R1a and Ghrelin
For more than a decade, GHRPs were used without a known receptor. In 1996 a G protein-coupled receptor for GH secretagogues was cloned from pituitary and hypothalamus (Howard et al., 1996). The authors noted that the receptor’s existence implied an undiscovered natural hormone. The receptor is now called GHS-R1a.
Three years later, ghrelin was identified as its natural ligand: a 28-amino-acid peptide from the stomach in which serine 3 carries an n-octanoyl group, a modification essential for its activity (Kojima et al., 1999). Ghrelin has since been linked to many processes beyond GH release, including systemic energy metabolism, gut motility and gastric acid secretion, sleep/wake rhythm, learning and memory, and glucose metabolism (Müller et al., 2015). GHRP-2 and GHRP-6 act on the same receptor, so they can activate some of these pathways as well. That matters when choosing a comparator and interpreting results.
Potency, Efficacy and Selectivity Data
The most useful direct comparison comes from the study that introduced ipamorelin, which tested GHRP-6 and GHRP-2 side by side (Raun et al., 1998):
- In anaesthetised rats, ipamorelin released GH with potency and efficacy comparable to GHRP-6.
- In conscious swine, GHRP-2 was more potent than GHRP-6 but produced a lower maximal GH response. Ipamorelin’s potency and maximal response were very similar to those of GHRP-6.
- None of the three secretagogues affected FSH, LH, prolactin or TSH.
- GHRP-6 and GHRP-2 both increased plasma ACTH and cortisol. Ipamorelin did not raise ACTH or cortisol beyond the levels seen with GHRH, even at more than 200-fold its ED50 for GH release.
These comparisons come from one study in specific animal models. Potency and efficacy depend on species, route and assay, so read them as relative comparisons rather than fixed values.
Side-by-Side Comparison
| Property | GHRP-2 | GHRP-6 | Ipamorelin (comparator) |
|---|---|---|---|
| Sequence | D-Ala-D-2-Nal-Ala-Trp-D-Phe-Lys-NH2 | His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 | Aib-His-D-2-Nal-D-Phe-Lys-NH2 |
| Length | Hexapeptide | Hexapeptide | Pentapeptide |
| Receptor | GHS-R1a | GHS-R1a | GHS-R1a |
| Relative potency in swine | Highest | Similar to ipamorelin | Similar to GHRP-6 |
| Maximal GH response in swine | Lower | Higher | Similar to GHRP-6 |
| ACTH/cortisol response | Increased | Increased | Not different from GHRH |
| Key reference | Raun et al., 1998 | Bowers et al., 1984 | Raun et al., 1998 |
Desensitization and Study Design
GHRPs show desensitization: in perifused rat pituitary cells, hourly pulses gave consistent GH responses, while continuous infusion led to desensitization (Badger et al., 1984). For in vitro and animal work, this means:
- Timing matters. Pulsed or intermittent exposure gives different results from continuous exposure. Record both the timing and the duration of exposure.
- Include a GHRH arm. GHRH acts through a different receptor. Comparing GHRP responses with GHRH, alone and together, helps separate GHS-R1a effects from general somatotroph responsiveness.
- Measure more than GH. Because GHRP-2 and GHRP-6 raised ACTH and cortisol in swine (Raun et al., 1998), endpoints such as corticosterone or cortisol, and prolactin, help define how selective the response is in your model.
- Account for metabolic effects. The ghrelin receptor system is linked to energy metabolism and gut function (Müller et al., 2015), so feeding and body-weight changes can confound longer studies. Pair-fed controls are useful.
Choosing Between Them
GHRP-6 is the classic reference compound, with the longest publication history. GHRP-2 is useful when higher potency is needed. When the study needs a GHS-R1a agonist with minimal effect on the adrenal axis, ipamorelin is the comparator most often chosen, based on the swine data above. Many labs also pair a GHS-R1a agonist with a GHRH analog such as CJC-1295 (no DAC); our ipamorelin and CJC-1295 stack guide discusses that design. More compounds are listed in our Growth and Performance category.
Handling in the Lab
Both hexapeptides contain tryptophan, which can oxidize with light and air exposure. Store lyophilized vials as labeled, protect solutions from light, aliquot stock solutions, and avoid repeated freeze-thaw cycles. Our storage and stability guide covers these steps.
Frequently Asked Questions
Do GHRP-2 and GHRP-6 work on the same receptor as ghrelin?
Yes. Both are agonists at GHS-R1a, the receptor cloned in 1996 (Howard et al., 1996) for which ghrelin was later found to be the natural ligand (Kojima et al., 1999).
Which one is more potent?
In the swine study that compared them directly, GHRP-2 was more potent than GHRP-6 but produced a lower maximal GH response (Raun et al., 1998).
Why use ipamorelin as a comparator?
In the same study, ipamorelin released GH like GHRP-6 but did not raise ACTH or cortisol beyond GHRH levels, which makes it a useful selectivity control (Raun et al., 1998).
How do I confirm the identity of my lot?
Every BioSim lot is third-party tested, with identity confirmed by mass spectrometry. Available certificates are published on our COA page.
References
- Badger TM, Millard WJ, McCormick GF, et al. (1984). The effects of growth hormone (GH)-releasing peptides on GH secretion in perifused pituitary cells of adult male rats. Endocrinology, 115(4), 1432-8. PMID: 6148232 · DOI: 10.1210/endo-115-4-1432
- Bowers CY, Momany FA, Reynolds GA, et al. (1984). On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology, 114(5), 1537-45. PMID: 6714155 · DOI: 10.1210/endo-114-5-1537
- Howard AD, Feighner SD, Cully DF, et al. (1996). A receptor in pituitary and hypothalamus that functions in growth hormone release. Science (New York, N.Y.), 273(5277), 974-7. PMID: 8688086 · DOI: 10.1126/science.273.5277.974
- Kojima M, Hosoda H, Date Y, et al. (1999). Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature, 402(6762), 656-60. PMID: 10604470 · DOI: 10.1038/45230
- Müller TD, Nogueiras R, Andermann ML, et al. (2015). Ghrelin. Molecular metabolism, 4(6), 437-60. PMID: 26042199 · DOI: 10.1016/j.molmet.2015.03.005
- Raun K, Hansen BS, Johansen NL, et al. (1998). Ipamorelin, the first selective growth hormone secretagogue. European journal of endocrinology, 139(5), 552-61. PMID: 9849822 · DOI: 10.1530/eje.0.1390552
Research These Compounds: GHRP-2 10mg, GHRP-6 10mg and Ipamorelin 10mg from BioSim Peptides: 99%+ purity, every lot third-party tested with identity confirmed by mass spectrometry, and available certificates published on our COA page. For laboratory research use only. Not for human consumption.
