What Has Published Research Reported About GHRP-6?

What is GHRP-6?

GHRP-6 is a synthetic six-residue peptide and the best-known early member of the growth hormone releasing peptides, or GHRPs, a family of small synthetic molecules that stimulate growth hormone (GH) release from the pituitary. Its sequence, His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 with an amidated C-terminus, appears in the title of a 1989 paper by Cheng and colleagues (source 1). Bowers, Momany, Reynolds and Hong described the peptide in Endocrinology in 1984 as a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone, with activity reported both in vitro and in vivo (source 2). A 2017 review traced the family back to earlier work by Bowers on enkephalin analogs, which had shown GH-releasing activity in pituitary cultures (source 3).

Working from the sequence, the molecular formula of the free peptide comes out as C46H56N12O6, with an average mass of about 873.0 daltons and a monoisotopic mass of about 872.44 daltons. These are calculated values from the published sequence rather than figures quoted from a cited paper, but they matter for laboratories, as discussed at the end of this profile.

This Steadfast Research Group profile summarizes what the primary literature reported about GHRP-6, from its cell-culture pharmacology and early human studies to the receptor work of the 1990s. It is written for researchers reading that record and covers published observations only. Two later analogs are covered separately in the hexarelin profile and the GHRP-2 profile.

How did cell studies describe GHRP-6’s action on the pituitary?

The 1989 paper by Cheng and colleagues tested the peptide in primary rat pituitary cell cultures (source 1). The authors reported that GHRP-6 released GH in a time- and dose-dependent manner, an effect seen after a 15-minute incubation but not after 4 hours. GHRP-6 alone did not change intracellular cyclic AMP (cAMP), while GH-releasing factor (GRF) raised cAMP about threefold. When the two were present together at maximal concentrations, GH release was synergistic, and GHRP-6 potentiated the GRF-induced rise in cAMP. The authors concluded that the two peptides released GH through different receptors and discrete mechanisms (source 1).

A 1995 study took the question to human tissue. Lei, Buchfelder, Fahlbusch and Adams cultured cells from human pituitary somatotroph tumors and reported that GHRP-6 stimulated phosphatidylinositol (PI) turnover in a dose-dependent way, with GH secretion rising in parallel (source 4). The effect was visible after 15 minutes and peaked at about 2 hours. PI turnover rose in all 8 of the 8 tumors examined, by 2.1 to 7.9 fold, and the authors concluded that the GH-stimulating effect of GHRP-6 was achieved through the PI second messenger system (source 4).

Results were not identical across preparations. In partially purified sheep somatotrophs, Wu and colleagues reported in 1996 that GHRP-2 and GRF raised cAMP and released GH, whereas GHRP-6 did not raise cAMP (source 5). Calcium-channel blockade did not alter the cAMP rise produced by GHRP-2 or GRF but blocked GH release in response to all three, GHRP-2, GRF and GHRP-6 (source 5). Taken together, the cell work linked GHRP-6 to PI and calcium signaling rather than a direct cAMP rise, though the preparations differed.

Did GHRP-6 act synergistically with GHRH in humans?

The founding human study was published in 1990 by Bowers and colleagues, who studied 18 normal men (source 6). GHRP-6 was given as an intravenous bolus at three increasing amounts, and GHRH was given alone and together with GHRP-6. Mean peak GH was 1.2 ± 0.3 µg/L after placebo and 7.6 ± 2.5, 16.5 ± 4.1 and 68.7 ± 15.5 µg/L at the three ascending amounts of GHRP-6. The two lower amounts combined with GHRH stimulated GH release synergistically (source 6).

The same study asked about selectivity. Serum prolactin and cortisol rose about twofold, but only at the highest amount of GHRP-6, and LH and TSH did not change over the first hour (source 6). This fits the wording of the 1984 title, which described the peptide as acting to specifically release growth hormone (source 2), while showing that selectivity was incomplete at the highest amount.

Later studies varied the participants. Peñalva and colleagues tested GHRP-6 in adult men, in adult women at different stages of the menstrual cycle and in prepubertal children, and also examined how propranolol or clonidine given beforehand affected the response in two groups of eight men (source 7). In 1995, Micle and colleagues tested young adults (n = 9, mean age 22 years) and older adults (n = 9, mean age 59.5 years) on three occasions in random order, at least a week apart (source 8). In the young group, GHRH plus GHRP-6 produced a greater GH rise than either peptide alone, and the paper’s title reported that GH secretion after GHRP-6 or the combination did not decline in late adulthood (source 8).

Where did studies place the site of action of GHRP-6?

The pituitary experiments left open how much of the response happened in the pituitary itself. Fairhall, Mynett and Robinson addressed this in 1995 in anaesthetized guinea pigs (source 9). GHRP-6 given directly into the brain produced a large GH response, while the same response needed far more peptide when it was given intravenously. A long-acting somatostatin analogue given into the brain blocked the GH release caused by GHRP-6, yet it did not affect basal GH release or block the release triggered by intravenous GRF, which argued against leakage to the pituitary. The authors concluded that the hypothalamus is a major target for GHRP-6 in vivo (source 9).

A 1999 study by Hewson and colleagues examined this at the cellular level. The authors recorded single cells in the arcuate, ventromedial and periventricular regions of a hypothalamic slice after bath application of GHRP-6 and asked whether the arcuate cells that responded were sensitive to somatostatin. They noted that earlier work had shown systemic GHRP-6 raising arcuate neuronal activity and inducing the c-fos gene (source 11).

The human counterpart came from Popovic and colleagues in 1995 (source 10). Twelve participants with hypothalamopituitary disconnection, a functional stalk section in which the pituitary is cut off from hypothalamic input, and 11 age- and sex-matched controls were each tested with GHRH, GHRP-6 and the two combined on separate occasions. The title reported blocked GHRP-6-induced GH secretion and no synergy with GHRH in the disconnection group, and the authors concluded that the main action of GHRP-6 was exerted at the hypothalamic level (source 10).

Read together with the pituitary cell work, the papers described effects at both sites. The cell studies showed the peptide acting on isolated somatotrophs, while the animal and human studies showed that intact hypothalamic input mattered for the response.

Which receptor does GHRP-6 act on, and how did ghrelin enter the story?

GHRP-6 was characterized for 12 years before its receptor was cloned. In 1996, Howard and colleagues reported in Science a G protein-coupled receptor from pituitary and hypothalamus that functioned in GH release and served as the target of synthetic GH secretagogues, and cells expressing it responded to GHRP-6 (source 12). Three years later, Kojima and colleagues purified the endogenous ligand from rat stomach. It was a peptide of 28 amino acids with an n-octanoyl group on serine 3, essential for activity, and it released GH in vitro and in vivo. They named it ghrelin (source 13).

Growth hormone releasing peptide (GHRP)

A small synthetic peptide that stimulates GH release; GHRP-6 was described in 1984 (source 2).

Growth hormone secretagogue receptor (GHS-R)

The G protein-coupled receptor cloned in 1996 as the target of synthetic GH secretagogues and later shown to bind ghrelin (sources 12 and 13).

Synergy

A GH response to GHRP-6 plus GHRH or GRF larger than the response to either alone (sources 1 and 6).

Phosphatidylinositol (PI) turnover

A second messenger pathway that GHRP-6 stimulated in human somatotroph tumor cells (source 4).

Hypothalamopituitary disconnection

A functional stalk section in which the pituitary is cut off from hypothalamic input (source 10).

The 1984 and 1989 papers therefore predate both the receptor and its natural ligand, and they describe GHRP-6 as a GH-releasing peptide acting through its own site. Smith and colleagues published a review titled “Peptidomimetic regulation of growth hormone secretion” in 1997, after the receptor had been cloned (source 14). A nonpeptide secretagogue described in 1995 is covered in the MK-677 profile.

What else has been reported about GHRP-6?

Not all reported activity concerned GH. In 1995, Locke, Kirgis, Bowers and Abdoh reported that GHRP-6 given into the cerebral ventricles of rats stimulated eating without affecting plasma GH responses (source 15). A 2017 review by Berlanga-Acosta and colleagues, whose corresponding author is at the Center for Genetic Engineering and Biotechnology in Havana, surveyed reports of cytoprotective effects of GHRP-6, GHRP-2 and hexarelin in cardiac, neuronal, gastrointestinal and hepatic cells, along with antifibrotic effects against fibrogenic cytokines (source 3). It was a review of published work rather than a new experiment.

What remains unresolved in the GHRP-6 literature?

The record has clear limits. The human data come from small studies published between 1990 and 1995, with 18 men in the first, 9 young and 9 older adults in the age comparison and 12 participants in the disconnection study (sources 6, 8 and 10). Bowers is an author on the founding 1984 paper, the 1990 human study and the 1996 cAMP study (sources 2, 6 and 5), so several key findings trace to overlapping investigators.

The cellular results also differed between preparations. GHRP-6 potentiated the GRF-induced cAMP rise in rat cells but did not raise cAMP in sheep cells (sources 1 and 5), and the PI turnover work used human tumor cells rather than normal pituitary (source 4). Because the early papers predate the cloned receptor, their language about a separate GHRP receptor was written before the receptor and its ligand were known (sources 12 and 13).

For laboratories, the practical issue is identity. GHRP-6 calculates to about 873.0 daltons, hexarelin, which carries one extra methyl group, to about 887.0, and GHRP-2 to about 818.0. Those gaps of roughly 14 and 55 daltons are easily resolved by mass spectrometry, and purity by HPLC together with a batch-matched Certificate of Analysis are the routine checks for closely related GHRPs.