Two doors into the growth axis.
Every peptide that touches growth-hormone research goes through one of two receptors. Understanding which door a compound knocks on — and for how long — organizes the entire field.
The axis in one paragraph.
Growth hormone is released by the pituitary in pulses, mostly at night, under a push-pull system: the hypothalamus sends GHRH (growth-hormone-releasing hormone) to push, somatostatin to brake, and the stomach hormone ghrelin adds a second, independent push through its own receptor. Downstream, GH drives the liver to produce IGF-1, which carries much of the growth signalling to tissues. Research peptides in this space are refined versions of the two pushes — which gives the field its natural taxonomy.
Door one: the GHRH receptor.
GHRH analogues imitate the hypothalamic push. The engineering story here is entirely about duration, because native GHRH survives minutes. Sermorelin is the unmodified active fragment — GHRH(1-29), as short-lived as the parent and prized precisely for that fidelity in pulse studies. Tesamorelin is full-length GHRH with an enzyme-blocking group on its N-terminus, extending the signal to hours. CJC-1295 is the far end: its DAC linker covalently rides serum albumin, stretching GHRH-receptor activation across days. Same receptor, three deliberately different clocks.
Door two: the ghrelin receptor.
Growth-hormone secretagogues act on GHS-R1a, the ghrelin receptor — a completely separate input that converges on the same pituitary output. The class's history is a purification story: early secretagogues released GH but dragged cortisol, prolactin and appetite signalling along with it. Ipamorelin is the field's cleanest answer — a pentapeptide characterized as releasing GH with almost none of the collateral hormone activity, which is why studies wanting an uncontaminated GH pulse reach for it by default.
| Compound | Receptor | Duration character | Typical research role |
|---|---|---|---|
| Sermorelin | GHRH-R | Minutes — faithful pulse | Classical reference; pulsatility studies |
| Tesamorelin | GHRH-R | Extended — enzyme-resistant | Sustained GHRH signalling; adipose-tissue models |
| CJC-1295 | GHRH-R | Days — albumin-bound | Long-exposure designs; sustained GH/IGF-1 dynamics |
| Ipamorelin | GHS-R1a (ghrelin) | Short, selective pulse | Clean secretagogue arm; selectivity studies |
| IGF-1 LR3 | IGF-1R (downstream) | Sustained — IGFBP-resistant | Entering the cascade at its final signalling step |
Why two doors make good experiments.
Because the receptors are independent but the output converges, the axis practically begs for factorial designs: GHRH-side arm, ghrelin-side arm, both together. The literature's classic observation is synergy — the combination releasing more GH than the sum of either input alone, consistent with the two receptors engaging the pituitary through different intracellular machinery. Pairing, say, sermorelin with ipamorelin lets a study attribute effects to a specific input; adding a duration ladder on the GHRH side turns exposure time itself into a controlled variable.
The fifth compound in our growth line skips the pituitary entirely: IGF-1 LR3 enters downstream at the IGF-1 receptor, engineered to escape the binding proteins that sequester native IGF-1. For cell-culture proliferation work — where there is no pituitary in the dish — it's the tool that matters.
Practical footnotes.
All five are supplied lyophilized at independently verified purity with batch COAs. Handling: sermorelin and tesamorelin follow standard peptide discipline; CJC-1295's reactive DAC linker means prompt, thiol-free handling in solution; IGF-1 LR3, an 83-residue protein, is the least forgiving molecule in the catalog — the storage guide covers the specifics.
Common questions.
Duration at the same receptor. Sermorelin reproduces a faithful minutes-long GHRH pulse; CJC-1295's albumin anchor holds receptor activation for days. A study chooses by the temporal question it's asking.
In characterization studies it released GH without meaningful ACTH, cortisol or prolactin elevation — the side-signalling older secretagogues produce. Selectivity means downstream effects can be attributed to GH rather than stress-hormone noise.
Downstream. GH's growth signalling largely runs through IGF-1; LR3 activates the IGF-1 receptor directly and resists the binding proteins that sequester the native molecule — letting studies enter the cascade at its last step, especially in cell culture.
