GHRP 2, also written GHRP-2 and known by the international nonproprietary name pralmorelin, is a synthetic hexapeptide that acts as an agonist of the growth hormone secretagogue receptor type 1a (GHS-R1a), the receptor for the endogenous hormone ghrelin. It is studied in vitro as a tool for examining G protein-coupled receptor signalling, growth hormone release from pituitary cells and the interaction between secretagogue and growth hormone-releasing hormone pathways. Because it is small, stable and well defined, it is also used as an analytical reference peptide. This article covers its structure, mechanism, published research, laboratory applications and handling requirements.
What is GHRP 2?
GHRP 2 belongs to the growth hormone releasing peptide family, a group of synthetic oligopeptides that stimulate growth hormone secretion through a route distinct from that of growth hormone-releasing hormone (GHRH). Its sequence is D-Ala-D-2-naphthylalanine-Ala-Trp-D-Phe-Lys-NH2, a six-residue chain with an amidated C-terminus. The molecular formula is C45H55N9O6, and the molecular weight is approximately 818 Da for the free peptide. The acetate salt, the form commonly supplied for research, weighs more because acetate counter-ions associate with the basic groups.
Three structural features stand out. The chain contains two D-amino acids, D-alanine at the N-terminus and D-phenylalanine at position 5, along with a D-configured naphthylalanine residue, a non-natural aromatic amino acid. These features confer resistance to proteolysis and contribute to receptor binding. The tryptophan and the two aromatic D-residues create a hydrophobic cluster that matters for solubility and for light sensitivity, topics covered in the handling section below.
The family originates with the work of Cyril Bowers and colleagues, who in the early 1980s developed synthetic enkephalin-derived peptides able to release growth hormone from pituitary cells in vitro. The first widely studied hexapeptide, GHRP-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2), was reported to release growth hormone in a dose-dependent manner across several species. Medicinal chemistry then produced analogues with different potency and selectivity, and GHRP 2 emerged from that effort. It was developed in Japan, where it was assigned the nonproprietary name pralmorelin. The receptor these peptides act on was not identified until 1996, and its natural ligand, ghrelin, was found three years after that.
Mechanism of action
GHRP 2 binds GHS-R1a, a class A G protein-coupled receptor expressed in the anterior pituitary and in hypothalamic regions including the arcuate nucleus. Howard and colleagues cloned the receptor in 1996 and showed that it is the target of synthetic growth hormone secretagogues, which led them to propose that the secretagogues mimic an undiscovered endogenous hormone. That hormone was later identified as ghrelin, a 28-residue peptide carrying an n-octanoyl group on serine 3.
The signalling route differs from that of GHRH. GHS-R1a couples chiefly to Gq/11, which activates phospholipase C. This cleaves phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate and diacylglycerol. Inositol trisphosphate releases calcium from intracellular stores, and diacylglycerol activates protein kinase C. The resulting rise in intracellular calcium drives exocytosis of growth hormone granules. By contrast, GHRH acts through Gs and cyclic AMP. This difference explains one of the most reproducible observations in the field, namely that GHRP-type peptides and GHRH can act together in pituitary cell cultures.
Laboratories studying GHRP 2 can follow the pathway at several points: calcium mobilisation measured with fluorescent indicators within seconds, inositol phosphate accumulation, ERK phosphorylation and growth hormone release into the medium. The receptor shows notable constitutive activity in expression systems, which has made it a useful model for inverse agonist studies. A truncated isoform, GHS-R1b, does not appear to signal in the same way, so assay design should specify which isoform is expressed.
The peptide’s action is not confined to the pituitary in experimental systems. Hypothalamic neurons that express the receptor can be stimulated, which in animal work has been linked to changes in GHRH and somatostatin tone. Researchers should therefore be cautious when attributing an in-vivo observation solely to a direct pituitary effect.
What the research shows
The literature on growth hormone secretagogues is large. The four studies below are selected because they define the field’s understanding of the receptor, the ligand and the mechanism.
The first is Bowers and colleagues’ 1984 report in Endocrinology on GHRP-6. The group described a synthetic hexapeptide that released growth hormone from pituitary cells in a dose-dependent manner and showed activity in several species. Although it concerns the parent compound, the paper established the structure-activity logic from which GHRP 2 was later developed, including the importance of D-amino acids in the sequence.
The second is Cheng and colleagues’ 1989 study, also in Endocrinology. Working with rat primary pituitary cell cultures, the authors showed that GHRP-6 acted synergistically with growth hormone-releasing factor on hormone release and on intracellular cyclic AMP accumulation. This finding supported the view that the two peptide classes use separate signalling mechanisms, and it remains a standard reference for combination experiments in vitro.
The third is the receptor cloning paper. Howard and co-workers, publishing in Science in 1996, cloned a G protein-coupled receptor from the pituitary and hypothalamus of swine and humans and showed it to be the target of synthetic secretagogues. The authors concluded that the receptor defines a neuroendocrine pathway controlling pulsatile growth hormone release. The abstract is available on PubMed.
The fourth is the identification of the natural ligand. Kojima and colleagues, in Nature in 1999, purified a 28-residue acylated peptide from rat stomach that was specific for the secretagogue receptor. They reported that it released growth hormone in vivo and in vitro and that octanoylation at serine 3 was essential for activity. They named the peptide ghrelin. The record is on PubMed.
Together these papers explain why synthetic peptides such as GHRP 2 are used as receptor probes: they were discovered before the receptor and ligand were known, and later work showed they mimic an endogenous system.
Research applications
The most common use is receptor pharmacology. Cell lines transfected with GHS-R1a, or primary pituitary cultures, are exposed to defined concentrations of the peptide, and responses are measured through calcium flux, inositol phosphate assays or growth hormone immunoassay. GHRP 2 serves as a reference agonist against which new ligands, antagonists and inverse agonists are ranked.
A second use is the study of synergy and cross-talk. Because GHRP 2 signals through a different G protein from GHRH, researchers use it alongside GHRH analogues and somatostatin to model how stimulatory and inhibitory inputs combine in somatotrophs. Experiments of this kind examine whether combined exposure produces more than additive hormone release in culture and how somatostatin modifies the response.
Neuroendocrine research is a third area, where the peptide is used to probe GHS-R1a function in hypothalamic slice preparations and neuronal cultures, as well as in animal models in which investigators study receptor distribution and signalling. Analytical chemistry is a fourth. The six-residue sequence, with its non-natural residue and hydrophobic character, makes GHRP 2 a useful test compound for reversed-phase HPLC method development, mass spectrometry and forced-degradation studies. Finally, structural and medicinal chemists use it as a template when designing peptidomimetics and non-peptide secretagogues that target the same receptor.
Purity, storage and handling
Verification starts with reversed-phase HPLC, typically monitored at 214 nm for the peptide bond and at 280 nm for the tryptophan and naphthylalanine chromophores, with purity commonly specified at 98% or higher by peak area. Because the molecule contains D-amino acids, stereochemical purity matters, and diastereomeric impurities may elute close to the main peak. Mass spectrometry should confirm the intact mass near 818 Da, and the observed value should be compared with the theoretical figure on the certificate. Net peptide content is a separate measure. A lyophilised vial contains peptide, counter-ions and water, so gross weight overstates the active amount and affects molar calculations.
Because degradation accelerates once reconstituted, batch-level HPLC verification matters more here than with more stable peptides. UK laboratories sourcing GHRP 2 should expect a batch-specific certificate of analysis and lyophilised storage at -20°C, and researchers assessing the best peptides supplier uk can use those documentation requirements as a common benchmark when comparing vendors.
A batch-specific certificate matters because impurity profiles vary with each synthesis. Deletion sequences, racemised residues and oxidised tryptophan species can all appear in a given run, and only the method details, batch number and test date on the certificate show that the stated purity applies to the vial in hand. A generic data sheet cannot give that assurance.
Lyophilised peptide is kept at -20°C in a sealed, desiccated container. Vials should reach room temperature before opening to prevent condensation. Short exposure to ambient temperature in transit is generally tolerated by dry powder. Once reconstituted, the peptide is more vulnerable. The hexapeptide has no methionine or cysteine, which removes two common degradation routes, but tryptophan oxidation, hydrolysis and adsorption to plastic surfaces still occur in solution. Reconstituted stock is normally held at 2-8°C for short periods and divided into single-use aliquots to avoid repeated freeze-thaw cycles, and low-binding tubes reduce losses at low concentrations. Because the molecule is hydrophobic, solvent choice affects dissolution, and the supplier’s recommended diluent should be followed. Stability windows depend on pH, concentration and diluent, so they should be taken from batch documentation and confirmed in-house for critical work.
Light sensitivity deserves more attention here than with many peptides. Tryptophan and naphthylalanine absorb in the near-ultraviolet and can undergo photo-oxidation, so amber vials or storage in the dark are recommended for both powder and solutions.
Frequently asked questions
What is GHRP 2 and what receptor does it act on?
GHRP 2 is a synthetic hexapeptide that acts as an agonist at the growth hormone secretagogue receptor type 1a (GHS-R1a), also called the ghrelin receptor. The receptor couples mainly to Gq/11 and raises intracellular calcium, a route distinct from the cyclic AMP pathway used by GHRH.
What is the molecular weight and sequence of GHRP-2?
The sequence is D-Ala-D-2-naphthylalanine-Ala-Trp-D-Phe-Lys-NH2, with the formula C45H55N9O6. The free peptide has a molecular weight of approximately 818 Da. The acetate salt is heavier because of the counter-ions, so batch documentation should state the salt form and net peptide content.
How should lyophilised GHRP 2 be stored?
Lyophilised material is generally stored at -20°C in a sealed, desiccated container protected from light. Vials should reach room temperature before opening. Once reconstituted, solutions are usually kept at 2-8°C for short periods as single-use aliquots, with stability limits confirmed from the supplier’s batch documentation.
What is the difference between GHRP-2 and GHRP-6?
Both act on GHS-R1a, but they differ in sequence. GHRP-6 is His-D-Trp-Ala-Trp-D-Phe-Lys-NH2, while GHRP-2 contains D-Ala and D-2-naphthylalanine in the first two positions. The substitutions change potency, selectivity and physical properties such as hydrophobicity, which matters for assay design and handling.
GHRP 2 is supplied for in-vitro laboratory research only. It is not for human or veterinary use, and it must not be administered to people or animals.
