Most people think of peptides that act on glucagon-like peptide-1 (GLP-1) receptors purely in terms of blood sugar or appetite. A study published in Naunyn-Schmiedeberg's Archives of Pharmacology took a different angle entirely, asking what happens when a GLP-1 receptor agonist is applied directly to living human atrial tissue and then a second compound, muscarine, is added on top.
The results showed that muscarine, a compound derived from certain mushrooms that mimics the neurotransmitter acetylcholine, reduced the force of heart muscle contractions in tissue that had first been exposed to GLP-1 receptor stimulation. That finding placed GLP-1 receptor activity alongside several other well-known signaling pathways in producing a condition where muscarine can weaken the heart's squeeze.
This kind of bench research does not tell clinicians what to prescribe or patients what to do. What it does is map out the molecular territory of the human heart more precisely, one signaling pathway at a time.
Where the tissue came from
The research team worked with human atrial preparations, or HAP, meaning small strips of heart muscle taken from the upper chambers of the heart. These samples were collected during open-heart surgery performed for severe coronary artery disease. Using living human tissue rather than animal models is considered a significant advantage in cardiac research because species differences in receptor distribution and signaling can be large.
Once obtained, the tissue strips were suspended in organ baths, kept in a controlled environment, and electrically stimulated at a steady rate of one beat per second. Force of contraction, or FOC, was measured under isometric conditions, meaning the tissue length was held constant while researchers recorded how hard it pulled. This setup allowed precise, reproducible measurements of how different compounds changed contractile strength.
Muscarinic receptors in human atrial tissue
Before testing any compounds, the researchers characterized which muscarinic receptors were actually present in the tissue. Using digital polymerase chain reaction, a highly sensitive gene-expression technique, they found that the M2 subtype accounted for roughly 96 percent of total muscarinic receptor messenger RNA in these samples. The M1 and M4 subtypes contributed less than 4 percent combined.
This matters because different muscarinic receptor subtypes connect to different downstream pathways. The dominance of M2 receptors in the human atrium is consistent with established cardiac physiology, since M2 receptors are the classical brake on heart rate and contractile force in this region. Muscarine, the test compound, binds muscarinic receptors broadly, so the M2 subtype is expected to be its primary target here.
The direct and indirect inotropic effects
Researchers already knew from earlier work on acetylcholine, the body's own muscarinic agonist, that there are two distinct ways to reduce atrial contractile force. The first is a direct effect: apply the compound alone and contraction weakens. The second is an indirect effect: pre-stimulate the tissue with a compound that raises a cellular messenger called cyclic AMP, or cAMP, and then the muscarinic compound produces a much larger or more sustained reduction in force.
When muscarine was given alone to the human atrial strips in this study, it produced a transient, short-lived dip in force of contraction. That confirmed the direct effect works with muscarine as it does with acetylcholine. The more consequential part of the experiment came next.
GLP-1 receptor stimulation and the cAMP connection
The team tested six different compounds that raise cAMP through receptor-dependent means: isoprenaline acting on beta-adrenoceptors, serotonin acting on 5-HT4 receptors, histamine acting on H2 receptors, fenoldopam acting on D1 dopamine receptors, a GLP-1 receptor agonist peptide at 100 nanomolar concentration, and gastric inhibitory peptide acting on GIP receptors. Each of these compounds elevated cAMP through its own distinct receptor.
After pre-stimulation with any one of these six agents, subsequently applying muscarine produced a clear negative inotropic effect, meaning a measurable reduction in the force with which the atrial tissue contracted. The GLP-1 receptor agonist was not unique in this regard; it joined a consistent pattern seen across all six receptor types. What the finding establishes is that GLP-1 receptor activation in human atrial tissue is capable of priming the cAMP-dependent signaling environment in a way that sensitizes the tissue to muscarine's force-reducing effects.
To confirm that cAMP itself was the common denominator rather than any quirk of a specific receptor, the researchers also bypassed receptors entirely. They used forskolin to stimulate the enzyme adenylyl cyclase directly, cilostamide to block cAMP breakdown, and dibutyryl-cAMP to activate the protein kinase that cAMP normally switches on. All three of these receptor-independent interventions produced the same outcome: muscarine then significantly reduced contractile force. This chain of evidence strongly supports the conclusion that elevated intracellular cAMP, by whatever means it is achieved, sets up the conditions for muscarine's negative inotropic action.
What the authors concluded
The research team concluded that muscarine, in the presence of agents that raise cAMP or activate cAMP-dependent protein kinase, exerts genuine negative inotropic effects in human atrial tissue. They proposed that this mechanism may help explain the cardiac effects seen in muscarine intoxications in clinical settings, where patients who have ingested toxic amounts of muscarine-containing mushrooms can present with slowed, weakened heart function.
From a broader signaling standpoint, the study adds GLP-1 receptors to a growing list of receptor systems that can interact with muscarinic tone in the human atrium. The fact that GLP-1 receptor stimulation participates in this cross-talk is a reminder that peptide signaling in the heart does not occur in isolation. Each receptor pathway influences the cellular environment in ways that can alter responses to other inputs.
Limitations and research context
Several important caveats apply to this work. The tissue came from patients with severe coronary artery disease, so the receptor profile and signaling responses may differ from those in healthy hearts. The experiments were conducted ex vivo, meaning outside a living body, in isolated tissue strips. Conditions in an intact, beating heart with full circulation, nervous system input, and hormonal milieu are far more complex.
The study also does not address long-term effects, dose-response relationships across a wide range, or what happens at concentrations of any compound that would reflect normal physiological levels in people. The concentrations used, such as 100 nanomolar for the GLP-1 receptor agonist, are experimental choices intended to produce measurable receptor activation in a dish, not direct translations to any clinical scenario.
Early data of this kind is valuable precisely because it fills in mechanistic detail that later, larger studies can build on. The literature suggests that understanding how GLP-1 receptor signaling interacts with other cardiac pathways, including parasympathetic muscarinic tone, will become increasingly relevant as researchers continue to investigate the cardiovascular effects of peptides that target this receptor family.
