Growth hormone acts partly directly and partly through IGF-1, a 70-amino-acid peptide with a structure similar to insulin. The liver releases it into the blood in response to GH, but it is also produced locally in muscle, bone and other tissues.
In circulation, almost all IGF-1 travels bound to binding proteins (IGFBPs), mainly IGFBP-3, which regulate how much is free and active. IGF-1 also closes the loop: it holds back GH release in the hypothalamus and pituitary (negative feedback). That is why it is used in research as an integrated marker of somatotropic axis activity.
IGF-1 LR3 is an analogue with 13 extra amino acids at the N-terminus and a substitution at position 3. Those changes greatly reduce its binding to IGFBPs, so it stays free and active longer than natural IGF-1.
Distinguishing between stimulating GH (secretagogues), supplying GH (somatropin) and acting directly on the IGF-1 receptor (IGF-1 LR3) is essential when comparing compounds of the somatotropic axis.
Related terms
Learn it in depth at the Peptide University
- IGF-1, its binding proteins and its variants · Module 5
- Physiology of the GH/IGF-1 axis · Module 5
- Special populations and class warnings · Module 10
- Pharmacodynamics: receptors and signaling · Module 2
- Safety profiles by class · Module 10
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Definition for educational and scientific purposes. It is not medical advice or a recommendation for use. NeoPeptidos products are sold labelled for research use only (RUO).