Adenosine Acts as a GHS-R Agonist, Binding the Same Receptor as MK-0677
Background
Growth hormone secretagogues (GHSs) are synthetic compounds that stimulate GH release across multiple species, including humans, and work through the GHS-R (growth hormone secretagogue receptor). Despite widespread interest in GHS-R agonism for GH deficiency and metabolic disorders, the endogenous hypothalamic ligands for this receptor remained poorly characterised at the time of this study. Identifying native GHS-R agonists is important because it could reveal the physiological pathway that GHS compounds like MK-0677 co-opt to drive GH release, opening routes to more selective or naturally-informed therapeutics. The gap between synthetic GHS pharmacology and endogenous hypothalamic signalling was the central motivation for this receptor-ligand discovery effort.
Study Design
Investigators screened for hypothalamic GHS-R agonists using cells expressing recombinant human GHS-R as the primary model. Calcium response assays were used to assess functional agonism, comparing transfected versus nontransfected cells as controls. Adenosine and the A1-selective agonist N6-R-phenylisopropyladenosine (R-PIA) were tested for calcium mobilisation. Radioligand binding experiments compared membrane preparations from GHS-R-expressing cells versus nontransfected cells, with MK-0677 as the reference compound. Antagonism by [D-Arg1, D-Phe5, D-Trp7,9, D-Leu11]-substance P was assessed to confirm GHS-R specificity. Separately, GH release was measured from rat pituitary cells in vitro.
Results
Adenosine induced calcium responses in GHS-R-transfected cells with an EC50 of 50 nM, while R-PIA was substantially more potent at an EC50 of 0.5 nM. Neither compound produced any calcium response in nontransfected control cells, establishing receptor-dependency. Binding experiments showed adenosine and MK-0677 bound GHS-R-expressing membranes with nearly identical Bmax values: 2.6 ± 0.1 × 10⁻¹⁰ mol/mg protein for adenosine and 2.0 ± 0.3 × 10⁻¹⁰ mol/mg protein for MK-0677, with no detectable binding to nontransfected membranes.
Adenosine, R-PIA, and the synthetic GHS compound MK-0677 were all inhibited by the
GHS-Rantagonist [D-Arg1, D-Phe5, D-Trp7,9, D-Leu11]-substance P at similar IC50 values, strongly confirming shared receptor occupancy atGHS-R.
Despite robust receptor-level activity, neither adenosine nor R-PIA triggered GH release from rat pituitary cells in vitro, indicating that GHS-R activation alone may be insufficient to drive downstream pituitary GH secretion under these conditions.
Key Findings
- Adenosine activated recombinant human
GHS-Rwith EC50 = 50 nM in transfected cells. - R-PIA (A1 adenosine receptor agonist) activated
GHS-Rwith EC50 = 0.5 nM. - Adenosine Bmax = 2.6 ± 0.1 × 10⁻¹⁰ mol/mg protein; MK-0677 Bmax = 2.0 ± 0.3 × 10⁻¹⁰ mol/mg protein.
- GHS-R antagonist [D-Arg1,D-Phe5,D-Trp7,9,D-Leu11]-substance P blocked all three agonists at similar IC50 values.
- Neither adenosine nor R-PIA induced GH release from rat pituitary cells in vitro.
Why It Matters
Adenosine — an endogenous purine nucleoside — is now a demonstrated GHS-R agonist, fundamentally reshaping the view of what constitutes a natural GHS-R ligand and suggesting that purinergic signalling intersects with the GH axis. For clinicians and researchers working with GHS-R-targeting compounds like MK-0677, this raises important questions about whether adenosine-modulating conditions (exercise, hypoxia, caffeine intake) could interact with GHS pharmacology. The disconnect between GHS-R activation and pituitary GH release in vitro hints that central hypothalamic circuitry, not the pituitary GHS-R alone, may be required for full GH secretion, complicating simple receptor-occupancy models of GH therapeutics. Translation to clinical protocol design is early-stage; these are in vitro and cell-based findings with no human dosing data yet.
TitrateLab articles covering MK-677
Written by TitrateLab, separately from the study summarized above.
