GHRP-6, HGH Fragment 176-191 and IGF-1 LR3 Across the Somatotropic Axis: A Research Review of the Secretagogue, the Hormone Fragment, the Effector Analogue and the Catalogue Nomenclature Behind "ghrp6", "frag 176-191" and "igf1lr3 peptide" Labels

Last updated: September 17, 2026 | Reading time: 12 min read

Three of the most-searched compound families in research-peptide catalogues sit on a single physiological axis, yet they are almost never reviewed together. At the top is GHRP-6, the 1984 hexapeptide that first showed the pituitary could be driven to release growth hormone through a receptor other than the GHRH receptor. In the middle is the growth hormone molecule itself, represented in catalogues not as the 191-residue protein but as its C-terminal fragment, hgh fragment 176-191. At the bottom is the downstream effector, insulin-like growth factor-I, represented by the engineered analogue LR3 IGF-I, which catalogues render in a dozen orthographic forms from igf 1 lr3 peptide to igf1lr3 peptide. Each compound has a dedicated review on this site; none supplies the axis-level crosswalk that explains why the three co-occur. This review positions each compound on the somatotropic axis, reads the primary literature for each, groups the catalogue vocabulary into a single nomenclature table with an axis-position column, and states plainly where the protocol-style strings fall outside the scope of a research-use-only document. All content is provided strictly for research reference; the materials discussed are not for human or veterinary use, and nothing here describes administration to any organism.

Why One Axis Produces Three Catalogue Families

Every tier of the somatotropic axis has been reduced to a synthetic or recombinant peptide obtainable as a lyophilised solid. The classical account, formalised as the somatomedin hypothesis, holds that pituitary GH drives somatic growth largely by inducing IGF-I in the liver, and that circulating IGF-I mediates most of GH's anabolic action; the 2001 re-statement of that hypothesis by Le Roith and colleagues qualified it with liver-specific Igf1 deletion data showing normal postnatal growth despite a large fall in circulating IGF-I, which shifted attention to locally produced, autocrine and paracrine IGF-I [11].

The first tier arose from a different tradition. In 1984 Bowers and colleagues reported that a hexapeptide derived by iterative modification of Met-enkephalin released GH from pituitary cells in vitro and in rats in vivo, with no opioid activity and no dependence on the GHRH receptor [1]. The compound was named GHRP-6, and its receptor was cloned twelve years later by Howard and colleagues, who identified a G-protein-coupled receptor expressed in pituitary and hypothalamus that bound synthetic secretagogues and whose existence implied an undiscovered endogenous ligand [2]. Kojima and colleagues supplied that ligand in 1999: ghrelin, a 28-residue acylated peptide from the stomach [3]. GHRP-6 is thus a pharmacological ancestor of a hormone rather than an analogue of one.

The second tier is the odd one. Growth hormone itself is a 22-kDa protein, not a peptide in the catalogue sense, and it is rarely listed by research-peptide suppliers. What is listed is a sixteen-residue segment of its C-terminus. The rationale traces to Monash University studies that dissected GH into functional domains. Wu and Ng showed in 1993 that the synthetic sequence 177-191 reproduced the antilipogenic action of intact hGH on rat adipose tissue without the hormone's lipolytic effect [6]; subsequent work with the tyrosine-extended fragment designated AOD9401 and then AOD9604 characterised lipolytic and antilipogenic activity in rodent adipose assays and in obese-mouse models [7][8]. The catalogue label hgh fragment 176-191 (and its short forms fragment 176-191 and frag 176-191) is thus a domain-mapping artefact: a piece of a hormone studied for one of that hormone's several activities.

Tier 1: GHRP-6 and the Two-Receptor Pharmacology of a Synthetic Secretagogue

GHRP-6 carries two D-amino acids (D-Trp², D-Phe⁵) and a C-terminal amide, protease-resistance features conserved across the later GHRPs [1][5]. At GHS-R1a it acts as a full agonist, coupling through G_q/11 to phospholipase C and intracellular calcium release in somatotrophs; the receptor is also expressed in the arcuate and ventromedial hypothalamus, where secretagogue action is thought to modulate GHRH and somatostatin tone, which is why the peptide's in vivo GH response exceeds what pituitary cells alone would predict [1][2]. In modern nomenclature GHRP-6 is a ghrelin-receptor agonist [3].

The second receptor emerged from cardiovascular pharmacology. Bodart and colleagues showed in 2002 that hexarelin and related GHRPs bind the scavenger receptor CD36 in cardiac membranes and that CD36 mediates their coronary vasoactive effects independently of GHS-R1a [4]. That finding underlies the cytoprotective literature on GHRP-6 in ischaemia–reperfusion models. Raun and colleagues designed ipamorelin in 1998 specifically to retain GHS-R1a-mediated GH release while losing the ACTH, cortisol and prolactin responses that GHRP-6 and GHRP-2 produce at higher exposures [5]. In catalogue terms, ghrp 6, ghrp6, ghrp 6 peptide and ghrp-6 peptide are four spellings of one compound; ghrp peptides is the family label for GHRP-1, GHRP-2, GHRP-6, hexarelin and ipamorelin.

Tier 2: The hGH 176-191 Fragment as a Domain-Mapping Compound

Human growth hormone comprises 191 residues folded into a four-helix bundle stabilised by two disulfides, one of which (Cys¹⁸²–Cys¹⁸⁹) lies within the C-terminal segment that the fragment reproduces. The 176-191 fragment therefore carries a small disulfide loop but none of the receptor-binding surface that intact GH uses to dimerise the GH receptor, and the Monash group reported that the fragment neither competes for GH-receptor binding nor stimulates cell proliferation in their assays [7][8]. What the fragment retains, in rodent adipose tissue preparations, is the ability to reduce lipogenesis (Wu and Ng, 1993 [6]) and, for the tyrosine-extended analogue, to increase lipolysis and fat oxidation [7]. The 2001 Endocrinology study extended this to obese mice and to β₃-adrenoceptor knock-out mice, finding that chronic fragment exposure increased adipose β₃-adrenoceptor RNA in obese animals and that the chronic lipid-metabolism effects were absent in the knock-out, implicating the β₃-adrenoceptor as a required downstream element [8].

Two nomenclature points follow. First, the catalogue string hgh fragment 176-191 denotes the sixteen-residue sequence; AOD9604 is that sequence with an N-terminal tyrosine added for radio-iodination, so catalogue entries labelled AOD-9604 and entries labelled Fragment 176-191 are closely related but not identical compounds. Second, the discussion-forum string aod 9604 reddit is a sourcing-of-opinion query, not a compound name; it appears in this review's taxonomy solely as a query class, and the review does not survey or endorse forum content. The rodent data here are preclinical; no effect in any other organism is described or inferred.

Tier 3: LR3 IGF-I and the Binding-Protein Problem

IGF-I is a 70-residue single-chain peptide whose sequence, published by Rinderknecht and Humbel in 1978, shows clear homology with proinsulin [12]. In circulation and in tissue fluids it is almost entirely bound to a family of six high-affinity IGF-binding proteins, which extend its half-life, restrict its access to receptors and modulate its local activity [16]. Studying IGF-I receptor pharmacology in any binding-protein-containing system therefore confronts a sequestration problem: most added peptide is bound, not free.

Francis and colleagues addressed that problem in 1992 with a series of recombinant fusion-protein analogues expressed in Escherichia coli. The analogue that became the catalogue standard, Long R³ IGF-I, combines a Glu³→Arg substitution, which disrupts a binding-protein contact residue, with a thirteen-residue N-terminal extension derived from the expression construct [9]. The resulting 83-residue protein binds IGFBPs with markedly reduced affinity, binds the type 1 IGF receptor with near-native affinity, and is correspondingly more potent than IGF-I in cell-based assays where binding proteins are present [9]. Tomas and colleagues showed the same year that LR3 IGF-I was more potent than native IGF-I at restoring nitrogen balance in dexamethasone-treated rats, and attributed the difference to reduced binding-protein sequestration [10]. The analogue is now used chiefly as a growth-factor supplement in serum-free cell-culture media.

The catalogue nomenclature for this compound is the most fragmented of the three tiers. Suppliers and searchers join its four name tokens in every combination: igf lr3 peptide, igf 1 lr3 peptide, igf1 lr3 peptide, igf1-lr3 peptide, igf-1lr3 peptide, igf-lr3 peptide, igf1lr3 peptide, together with the noun-first forms peptides igf-lr3, peptides igf-1 lr3, peptides igf1 and peptide igf-1 lr3. All denote the same 83-residue analogue. The broader forms igf 1 peptides and igf-1 peptides encompass native IGF-I, LR3 IGF-I and the des(1-3) IGF-I truncation, and are family labels rather than compound names.

Where the GHRH Analogues Sit

A fourth group of catalogue strings attaches itself to this axis: the GHRH analogues sermorelin (GHRH 1-29), CJC-1295 and tesamorelin, which act on the GHRH receptor rather than GHS-R1a and are reviewed separately on this site [13][14][17]. The misspelling seremorelin denotes sermorelin, the 29-residue N-terminal fragment of GHRH that retains full receptor activity [14]. The string tesa+ipa blend is a supplier abbreviation for a tesamorelin/ipamorelin co-formulation, pairing a GHRH-receptor agonist with a GHS-R1a agonist; the primary literature characterises each compound alone [5][17]. Two further strings — tesamorelin ipamorelin blend dosage and is cjc 1295 ipamorelin safe — ask, respectively, for a protocol value and for a safety judgement. Neither is answerable by a research-use-only reference; both sit in the declined-scope row of the taxonomy below.

Nomenclature and Catalogue Variants with Axis Position

The table groups every catalogue string discussed in this review by the compound it denotes and by the tier that compound occupies on the somatotropic axis. The axis-position column explains why strings for three chemically unrelated compounds co-occur in catalogues.

Axis position	Canonical compound	Catalogue / search strings (verbatim)	Denotation
Tier 1 — secretagogue (GHS-R1a)	GHRP-6	ghrp 6, ghrp6, ghrp 6 peptide, ghrp-6 peptide	Orthographic variants of one hexapeptide [1]
Tier 1 — secretagogue family	GHRP-1, -2, -6, hexarelin, ipamorelin	ghrp peptides	Family label for GHS-R1a peptide agonists [2][5]
Tier 1 — GHRH-receptor agonists	Sermorelin; tesamorelin/ipamorelin pairing	seremorelin, tesa+ipa blend	Misspelling; supplier abbreviation for a co-formulation [14][17]
Tier 2 — hormone fragment	hGH 176-191 / AOD9604	hgh fragment 176-191, fragment 176-191, frag 176-191	C-terminal sixteen-residue domain; AOD9604 is the Tyr-extended analogue [6][7][8]
Tier 3 — effector analogue	Long R3 IGF-I	igf lr3 peptide, igf 1 lr3 peptide, igf1 lr3 peptide, igf1-lr3 peptide, igf-1lr3 peptide, igf-lr3 peptide, igf1lr3 peptide, peptides igf-lr3, peptides igf-1 lr3, peptide igf-1 lr3, peptides igf1	Token-joining variants of one 83-residue analogue [9]
Tier 3 — effector family	IGF-I, LR3 IGF-I, des(1-3) IGF-I	igf 1 peptides, igf-1 peptides	Family labels [11][12]
Declined scope — protocol / safety / forum	—	tesamorelin ipamorelin blend dosage, is cjc 1295 ipamorelin safe, aod 9604 reddit	Protocol value, safety judgement and forum-opinion queries; not answerable by a research-use-only reference, recorded here as query classes only

Evidence by Domain

Domain	Compound	Research model	Signal vector	Principal finding	Ref
Secretagogue discovery	GHRP-6	Rat pituitary cells; rat in vivo	GH release, dose-dependent, GHRH-receptor-independent	Novel hexapeptide releases GH specifically	[1]
Receptor identification	GHS ligands	Human and swine pituitary/hypothalamus; expression cloning	GPCR binding, calcium signalling	GHS-R cloned; endogenous ligand inferred	[2]
Endogenous ligand	Ghrelin	Rat stomach purification	GHS-R activation by acylated peptide	Ghrelin identified as GHS-R ligand	[3]
Second receptor	Hexarelin/GHRPs	Murine cardiac membranes; perfused heart	CD36 binding; coronary vasoactivity	CD36 mediates GHRP cardiac action	[4]
Selectivity ladder	Ipamorelin vs GHRP-6	Rat and swine in vivo	GH vs ACTH/cortisol/prolactin	Ipamorelin retains GH release, loses off-axis hormones	[5]
Domain mapping	hGH 177-191	Rat adipose tissue	Lipogenesis inhibition	C-terminal sequence reproduces antilipogenic action	[6]
Fragment metabolism	AOD9401/AOD9604	Rodent adipose assays; obese mice	Lipolysis, fat oxidation	Fragment retains lipid-metabolism activity without GHR binding	[7]
Downstream requirement	AOD9604	Obese and β3-AR knock-out mice	β3-adrenoceptor RNA; lipid endpoints	Chronic effects absent in knock-out	[8]
Analogue design	LR3 IGF-I	Recombinant expression; cell-based potency and binding assays	IGFBP affinity vs IGF-1R affinity	Reduced IGFBP binding raises potency	[9]
In vivo anabolic potency	LR3 IGF-I vs IGF-I	Dexamethasone-treated rats	Nitrogen balance, 3-methylhistidine	Analogue more potent than native peptide	[10]
Axis physiology	GH/IGF-I	Liver-specific Igf1 deletion mice	Circulating vs local IGF-I	Normal growth despite low circulating IGF-I	[11]
Binding-protein biology	IGFBP1-6	Review	Sequestration, half-life, local modulation	IGFBPs gate IGF access to receptors	[16]
Handling literature	All tiers	Review	Hydrolysis, oxidation, aggregation	Stability governed by sequence and storage state	[15]

Handling Considerations Common to the Three Tiers

The three compounds span an order of magnitude in size, and their stability liabilities differ accordingly. GHRP-6's D-amino acids and C-terminal amide protect it against exopeptidases, and its principal solution-phase liability is oxidation of the two tryptophan residues. The 176-191 fragment carries a reducible disulfide and, as AOD9604, an oxidisable tyrosine. LR3 IGF-I is a three-disulfide protein for which aggregation and disulfide scrambling, rather than backbone hydrolysis, dominate the degradation profile. Manning and colleagues' stability review is the standard reference for these pathways and for the principle that cold, dry lyophilised solids outlast solutions [15]. This review supplies no reconstitution or storage protocol.

Where to Source for Research

All three tiers are catalogued by research-peptide suppliers as lyophilised solids under research-use-only terms. Short Chain Aminos, BioPep, Catalyst Research and Apex Research Services list GHRP-6, hGH fragment 176-191 and LR3 IGF-I, or subsets of them, with lot-specific Certificates of Analysis. Certificate-of-Analysis identity, purity and net-peptide-content data are the verifiable attributes; this review does not evaluate or rank suppliers or products.

Frequently Asked Questions

Is ghrp 6 the same compound as ghrp6 and ghrp-6 peptide?

Yes. ghrp 6, ghrp6, ghrp 6 peptide and ghrp-6 peptide are orthographic variants of one synthetic hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys-NH₂, first described by Bowers and colleagues in 1984 [1]. The family label ghrp peptides covers the wider set of GHS-R1a peptide agonists.

What is hgh fragment 176-191, and how does it relate to AOD9604?

The fragment 176-191 (often shortened to frag 176-191) is the C-terminal sixteen-residue segment of human growth hormone, studied as a lipid-metabolism domain separable from GH's growth-promoting activity [6][7]. AOD9604 is the same sequence with an N-terminal tyrosine added for radio-iodination [8]. Both are research compounds; neither is described here for any use in an organism.

Are igf lr3 peptide, igf1-lr3 peptide and igf1lr3 peptide different products?

No. Along with igf 1 lr3 peptide, igf1 lr3 peptide, igf-1lr3 peptide, igf-lr3 peptide, peptides igf-lr3, peptides igf-1 lr3, peptide igf-1 lr3 and peptides igf1, these are token-joining variants of Long R³ IGF-I, the 83-residue analogue with an Arg³ substitution and a thirteen-residue N-terminal extension [9].

How do igf 1 peptides differ from igf-1 peptides and from LR3 specifically?

igf 1 peptides and igf-1 peptides are family labels covering native IGF-I, LR3 IGF-I and des(1-3) IGF-I. LR3 IGF-I is the member engineered for low IGF-binding-protein affinity; native IGF-I binds IGFBPs with high affinity and is largely sequestered in binding-protein-containing systems [9][16].

What does tesa+ipa blend mean, and what about tesamorelin ipamorelin blend dosage or is cjc 1295 ipamorelin safe?

tesa+ipa blend is a supplier abbreviation for a tesamorelin/ipamorelin co-formulation combining a GHRH-receptor agonist with a GHS-R1a agonist [5][17]. The strings tesamorelin ipamorelin blend dosage and is cjc 1295 ipamorelin safe ask for a protocol value and a safety judgement respectively; both fall outside the scope of a research-use-only reference and no value or judgement is supplied.

Why is seremorelin listed with these compounds, and what is aod 9604 reddit?

seremorelin is a misspelling of sermorelin, the GHRH 1-29 fragment that acts at the GHRH receptor and is reviewed separately [14]. aod 9604 reddit is a forum-opinion query rather than a compound name; this review records it as a query class and does not survey or endorse forum content.

Works Cited

1. Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114(5):1537-1545. doi:10.1210/endo-114-5-1537
2. Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. PMID: 8688086. doi:10.1126/science.273.5277.974
3. Kojima M, Hosoda H, Date Y, Nakazato M, Matsuo H, Kangawa K. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656-660. PMID: 10604470. doi:10.1038/45230
4. Bodart V, Febbraio M, Demers A, et al. CD36 mediates the cardiovascular action of growth hormone-releasing peptides in the heart. Circ Res. 2002;90(8):844-849. doi:10.1161/01.RES.0000016164.02525.B4
5. Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PMID: 9849822. doi:10.1530/eje.0.1390552
6. Wu Z, Ng FM. Antilipogenic action of synthetic C-terminal sequence 177-191 of human growth hormone. Biochem Mol Biol Int. 1993;30(1):187-196. PMID: 8358331.
7. Heffernan MA, Jiang WJ, Thorburn AW, Ng FM. Effects of oral administration of a synthetic fragment of human growth hormone on lipid metabolism. Am J Physiol Endocrinol Metab. 2000;279(3):E501-E507. doi:10.1152/ajpendo.2000.279.3.E501
8. Heffernan M, Summers RJ, Thorburn A, et al. The effects of human GH and its lipolytic fragment (AOD9604) on lipid metabolism following chronic treatment in obese mice and β₃-AR knock-out mice. Endocrinology. 2001;142(12):5182-5189. PMID: 11713213. doi:10.1210/endo.142.12.8522
9. Francis GL, Ross M, Ballard FJ, et al. Novel recombinant fusion protein analogues of insulin-like growth factor (IGF)-I indicate the relative importance of IGF-binding protein and receptor binding for enhanced biological potency. J Mol Endocrinol. 1992;8(3):213-223. PMID: 1378742. doi:10.1677/jme.0.0080213
10. Tomas FM, Knowles SE, Owens PC, et al. Insulin-like growth factor-I (IGF-I) and especially IGF-I variants are anabolic in dexamethasone-treated rats. Biochem J. 1992;282(Pt 1):91-97. PMCID: PMC1130894. doi:10.1042/bj2820091
11. Le Roith D, Bondy C, Yakar S, Liu JL, Butler A. The somatomedin hypothesis: 2001. Endocr Rev. 2001;22(1):53-74. PMID: 11159816. doi:10.1210/edrv.22.1.0419
12. Rinderknecht E, Humbel RE. The amino acid sequence of human insulin-like growth factor I and its structural homology with proinsulin. J Biol Chem. 1978;253(8):2769-2776. PMID: 632300.
13. Teichman SL, Neale A, Lawrence B, Gagnon C, Castaigne JP, Frohman LA. Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. J Clin Endocrinol Metab. 2006;91(3):799-805. PMID: 16352683. doi:10.1210/jc.2005-1536
14. Prakash A, Goa KL. Sermorelin: a review of its use in the diagnosis and treatment of children with idiopathic growth hormone deficiency. BioDrugs. 1999;12(2):139-157. PMID: 18031173. doi:10.2165/00063030-199912020-00007
15. Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharm Res. 2010;27(4):544-575. PMID: 20143256. doi:10.1007/s11095-009-0045-6
16. Baxter RC. IGF binding proteins in cancer: mechanistic and clinical insights. Nat Rev Cancer. 2014;14(5):329-341. PMID: 24722429. doi:10.1038/nrc3720
17. Falutz J, Allas S, Blot K, et al. Metabolic effects of a growth hormone-releasing factor in patients with HIV. N Engl J Med. 2007;357(23):2359-2370. PMID: 18057338. doi:10.1056/NEJMoa072375