Research Use Only. The information presented here is for scientific and educational purposes. These compounds are not intended for human consumption, self-administration, or therapeutic use.
Introduction
Insulin-like growth factor 1 (IGF-1) sits near the center of skeletal muscle growth biology, and IGF-1 LR3 (Long R3 IGF-1) is a modified analog that laboratories use to probe that biology in controlled models. Named for an arginine substitution at position 3 and a 13-residue extension at the N-terminus, IGF-1 LR3 is studied for how it engages the IGF-1 receptor (IGF-1R) while largely escaping the IGF binding proteins that normally sequester native IGF-1. This article surveys what preclinical and in-vitro literature describes about the peptide and the satellite-cell and hypertrophy pathways it is investigated within. Rejuven8 supplies IGF-1 LR3 strictly as research material.
The muscle angle here is specific. Satellite cells are the resident stem cells that repair and enlarge muscle fibers, and IGF-1 signaling is one of the most studied inputs into their activation, proliferation, and fusion. Everything below is framed around laboratory research use only, not for human consumption. No claim of human benefit, dosing, or performance is made or implied. The aim is to describe how IGF-1 LR3 appears in the scientific record on myogenesis and muscle hypertrophy in cell and animal systems.
Molecular Identity of IGF-1 LR3
Structure and modifications
IGF-1 LR3 is a single-chain polypeptide of 83 amino acids, extending the 70-residue native IGF-1 sequence. Two changes define it. First, an arginine substituted for glutamic acid at position 3 (the “R3”) reduces affinity for the IGF binding proteins. Second, a 13-amino-acid extension peptide at the N-terminus (the “Long”) further alters binding behavior and clearance in culture. Together these raise the approximate molecular weight to around 9.1 kDa and, in most reported systems, leave a larger fraction of the ligand free to interact with IGF-1R.
Reported attributes
The values below reflect commonly reported laboratory attributes and are provided for comparison only.
| Attribute | Reported value |
|---|---|
| Common name | IGF-1 LR3 (Long R3 IGF-1) |
| Class | Recombinant IGF-1 analog (growth-factor polypeptide) |
| Sequence length | 83 amino acids |
| Approximate molecular weight | ~9.1 kDa (approx 9111 Da) |
| Primary receptor target | IGF-1 receptor (IGF-1R) |
| Defining modifications | Arg at position 3; 13-residue N-terminal extension |
| IGFBP affinity | Reduced relative to native IGF-1 |
Satellite Cells and the IGF-1 Signaling Axis
What satellite cells do
Satellite cells reside between the basal lamina and the sarcolemma of muscle fibers and are marked by the transcription factor Pax7. In their quiescent state they are dormant. Mechanical load, injury, or growth-factor signaling can push them into an activated state, after which they proliferate as myoblasts (expressing MyoD and Myf5), then differentiate (marked by myogenin) and fuse, donating new myonuclei to repairing or enlarging fibers. This activation-to-fusion sequence is the cellular backbone of most muscle regeneration and hypertrophy models.
Where IGF-1 fits
In cultured myoblasts and primary satellite cells, IGF-1 has been reported to stimulate both proliferation and differentiation, a dual role documented extensively by Florini and colleagues. Because IGF-1 LR3 resists IGFBP sequestration, laboratories often adopt it as a stable, consistently available agonist in these systems, where free-ligand exposure is a key experimental variable. For broader context on how growth-factor and repair peptides are studied side by side, see the recovery and tissue-repair overview.
IGF-1 LR3 in Muscle Hypertrophy Research Models
Intracellular signaling
Engagement of IGF-1R triggers receptor autophosphorylation and recruits adaptor proteins, activating two principal branches. The PI3K/Akt/mTOR pathway is associated in models with increased protein synthesis, and the Ras/MAPK/ERK pathway is associated with proliferation. Akt activation is also reported to suppress the FoxO transcription factors that drive the atrophy-associated ligases atrogin-1 (MAFbx) and MuRF1. In cell and animal work, this combination of anabolic drive and reduced proteolytic signaling is what links IGF-1 signaling to fiber enlargement.
Model systems
Common preclinical and in-vitro platforms include the C2C12 myoblast line, primary satellite-cell cultures, and rodent overload or transgenic models. Classic work by Musaro and Rosenthal on localized IGF-1 (mIGF-1) transgene expression, and by Barton-Davis and Sweeney on IGF-1 gene delivery, described muscle hypertrophy and preserved regenerative capacity in aged rodent muscle. These are animal and cell studies that characterize mechanism, not human outcomes. Researchers comparing growth-hormone-axis tools sometimes reference secretagogue peptides such as CJC-1295 and Ipamorelin in adjacent experiments, though those act upstream on GH release rather than directly on IGF-1R.
Native IGF-1 compared with IGF-1 LR3
These figures reflect commonly reported laboratory attributes and are provided for comparison only.
| Attribute | Native IGF-1 | IGF-1 LR3 |
|---|---|---|
| Sequence length | 70 amino acids | 83 amino acids |
| Approx molecular weight | ~7.6 kDa | ~9.1 kDa |
| Position 3 residue | Glutamic acid | Arginine |
| N-terminal extension | None | 13 residues |
| IGFBP binding | High | Reduced |
| Reported in-vitro potency | Reference | Higher (attributed to free-ligand availability) |
Research Handling and Quality Considerations
IGF-1 LR3 is typically supplied as a lyophilized powder that laboratories reconstitute before use; general handling and diluent guidance sits in the bacteriostatic water reconstitution guide. Because the peptide is a larger growth factor, minimizing repeated freeze-thaw cycles and protecting reconstituted stock from prolonged warmth are frequently noted as ways to preserve integrity across an experiment.
Reproducibility depends on identity and purity. Batch data should be confirmed against a certificate of analysis, and the COA reading guide explains how to interpret those numbers, while published certificates document specific lots. Sourcing practices also affect results, which is covered in the guide to choosing a research peptide supplier. The full catalog of research peptides is available for laboratories building comparative studies.
Frequently Asked Questions
What is IGF-1 LR3?
IGF-1 LR3, or Long R3 IGF-1, is a modified 83-amino-acid analog of insulin-like growth factor 1. It is studied as a stable agonist of the IGF-1 receptor in cell and animal muscle research, and it is intended for research use only, not for human consumption.
How is IGF-1 LR3 different from native IGF-1?
Native IGF-1 has 70 amino acids, while IGF-1 LR3 has 83 because of an arginine substitution at position 3 and a 13-residue N-terminal extension. These changes raise its molecular weight to about 9.1 kDa and reduce its binding to IGF binding proteins in most reported systems.
How does IGF-1 LR3 relate to satellite cells in research models?
In cultured myoblasts and primary satellite cells, IGF-1 signaling has been reported to influence activation, proliferation, and differentiation. Because IGF-1 LR3 resists IGF binding proteins, laboratories use it to study these processes with a more consistently available ligand.
What signaling pathways does IGF-1 LR3 activate in muscle research?
Through the IGF-1 receptor, IGF-1 LR3 is reported to engage the PI3K/Akt/mTOR pathway linked to protein synthesis and the Ras/MAPK/ERK pathway linked to proliferation. Akt activity is also associated with suppression of atrophy-related signaling in model systems.
Why is IGF-1 LR3 used in cell culture research?
Its reduced binding to IGF binding proteins gives more predictable free-ligand exposure, which is why IGF-1 LR3 is a common reagent in myogenesis and hypertrophy research and in some bioprocessing contexts.
Is IGF-1 LR3 approved for human use?
No. IGF-1 LR3 is a research compound described here only in preclinical and in-vitro contexts. It is not intended for human consumption, self-administration, or therapeutic use.
References and Further Reading
- Musaro A, Rosenthal N, et al. Localized IGF-1 transgene expression and muscle hypertrophy and regeneration (2001). PubMed: Musaro localized IGF-1 muscle hypertrophy regeneration
- Barton-Davis ER, Sweeney HL. IGF-1 gene delivery and skeletal muscle in aging rodents. PubMed: Barton-Davis Sweeney IGF-1 skeletal muscle
- Florini JR, Ewton DZ, Coolican SA. Growth hormone and the IGF system in myogenesis. PubMed: Florini IGF system myogenesis
- Rommel C, Glass DJ. IGF-1-induced myotube hypertrophy via PI3K/Akt/mTOR. PubMed: Rommel Glass IGF-1 myotube hypertrophy Akt mTOR
- Rotwein P. IGF signaling in skeletal muscle growth and regeneration. PubMed: Rotwein IGF skeletal muscle
- Satellite cell activation, Pax7, and MyoD in muscle research. PubMed: satellite cell activation Pax7 MyoD IGF-1
- Long R3 IGF-1 as a cell-culture growth-factor reagent. PubMed: Long R3 IGF-1 cell culture
