Growth Hormone Secretagogues: GHRH Analogues and Ghrelin Receptor Ligands
Compounds described collectively as growth hormone secretagogues act through two entirely separate receptor systems. Conflating them obscures the pharmacology.
The GHRH receptor pathway
Growth-hormone-releasing hormone is a hypothalamic peptide that acts on the GHRH receptor, a class B GPCR on pituitary somatotrophs, stimulating synthesis and release of growth hormone. Sermorelin corresponds to the first twenty-nine residues of GHRH, the minimal fragment retaining full activity. Tesamorelin and CJC-1295 are GHRH analogues3 modified for greater stability, with the DAC variant of CJC-1295 additionally engineered for albumin binding and extended exposure.
The ghrelin receptor pathway
The growth hormone secretagogue receptor, GHS-R1a, is the receptor for ghrelin1, a stomach-derived peptide. It is a class A GPCR coupling primarily through Gq and phospholipase C, raising intracellular calcium. Ipamorelin and GHRP-2 are synthetic ligands at this receptor. Because the pathway is distinct from GHRH signalling, these compounds are frequently studied alongside GHRH analogues to examine whether the two produce additive or synergistic effects on secretion.
Selectivity differences
Within ghrelin receptor ligands, selectivity varies meaningfully. Some earlier compounds in this class show measurable activity affecting cortisol and prolactin secretion, whereas ipamorelin was specifically characterised as more selective2 for growth hormone release with comparatively little effect on those other axes. Selectivity profiles of this kind are a principal reason particular compounds are chosen as research tools.
Why pulsatility matters
Endogenous growth hormone is secreted in pulses under opposing control from GHRH and somatostatin, with feedback from insulin-like growth factor 1. Secretagogues act within this regulated system rather than bypassing it, so their effects depend on the existing state of the axis. This physiological context is central to interpreting any experimental result, and it distinguishes secretagogue pharmacology from direct hormone administration.
Evidence status
Receptor-level pharmacology in this area is well established. Downstream consequences of sustained secretagogue exposure in various model systems remain an active area of investigation.
References
- Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656–660. doi:10.1038/45230
- Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552–561. doi:10.1530/eje.0.1390552
- Falutz J, Allas S, Blot K, et al. Metabolic effects of a growth hormone-releasing factor in patients with HIV (tesamorelin). N Engl J Med. 2007;357(23):2359–2370. doi:10.1056/NEJMoa072375
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