Mechanism · 8 min read
The Growth Hormone Axis: Two Receptors, Frequently Confused
Five compounds, three receptors, and one distinction that determines whether two products are alternatives or complements.
Three points on one axis
The growth hormone axis runs from the hypothalamus to the pituitary to peripheral tissue, and the compounds in this category intervene at three different points along it.
GHRH analogs act at the top, on the GHRH receptor on pituitary somatotrophs. Ghrelin receptor agonists act on GHS-R1a, a separate receptor on the same cells. IGF-1 analogs bypass the pituitary entirely and act at the IGF-1 receptor in peripheral tissue.
Knowing which point a compound acts on tells you immediately whether it is redundant with another. Two GHRH analogs are alternatives. A GHRH analog and a ghrelin receptor agonist are not.
GHRH analogs differ mainly by sequence length
Sermorelin is GHRH(1-29), the shortest fragment retaining full activity. Modified GRF (1-29) — sold almost universally as CJC-1295 without DAC — is the same 29-residue backbone with four substitutions (D-Ala2, Gln8, Ala15, Leu27) that resist DPP-4 cleavage and plasma degradation.
Tesamorelin retains the complete 44-residue sequence with a trans-3-hexenoyl group at the N-terminus for stability. It is the longest GHRH analog in common research use.
The single most consequential naming problem in this category is the DAC distinction. CJC-1295 with DAC incorporates a maleimido group for albumin conjugation and has a substantially longer half-life. Without DAC, it does not. The two are pharmacokinetically very different molecules sold under names that differ by three characters, and supplier catalogues conflate them constantly.
CJC-1295 with and without DAC are pharmacokinetically different molecules sold under names that differ by three characters. Check the molecular weight, not the product title.
Ghrelin receptor agonism and the selectivity question
GHS-R1a is the growth hormone secretagogue receptor, and it is a different protein from the GHRH receptor despite both sitting on somatotrophs.
Ipamorelin's research value is selectivity. Earlier secretagogues such as GHRP-6 and GHRP-2 produce meaningful effects on ACTH, cortisol, and prolactin alongside GH release. Ipamorelin shows minimal effect on those axes at comparable doses in animal models, which makes it useful for isolating GH-axis effects without confounds.
That clean profile is why it remains a standard tool compound rather than a historical footnote.
Bypassing the axis entirely
IGF-1 LR3 acts at the far end. Rather than stimulating GH release, it is a recombinant IGF-1 analog acting directly at the IGF-1 receptor.
Two modifications define it: an arginine substitution at position 3, and a 13-residue N-terminal extension. The Arg3 substitution sharply reduces affinity for IGF binding proteins, which normally sequester the great majority of circulating IGF-1.
The practical consequence is a much larger free fraction and correspondingly extended activity — and it also means potency comparisons against native IGF-1 are not straightforward, because the two differ in availability as much as in intrinsic activity.
It is also considerably larger than the rest of this category at roughly 9,100 g/mol, which matters for handling: it is more sensitive to freeze-thaw cycling than short peptides and should be aliquoted at first reconstitution.
Common questions
What is the difference between CJC-1295 with and without DAC?
DAC stands for drug affinity complex — a maleimido group enabling albumin conjugation and a substantially longer half-life. Without DAC, the compound is Modified GRF (1-29), a 29-residue GHRH analog with a much shorter duration. They are pharmacokinetically different molecules; check the molecular weight on the certificate of analysis rather than relying on the product name.
Why are GHRH analogs and ghrelin receptor agonists studied together?
Because they act on different receptors on the same cells — the GHRH receptor and GHS-R1a respectively. They are complementary rather than substitutable, which is the mechanistic reason combination products exist rather than a marketing convenience.
What makes ipamorelin different from earlier secretagogues?
Selectivity. GHRP-6 and GHRP-2 produce meaningful effects on ACTH, cortisol and prolactin alongside GH release. Ipamorelin shows minimal effect on those axes at comparable doses in animal models, making it a cleaner tool for isolating GH-axis effects.
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