<p>Acetylcholine, the physiological agonist at all muscarinic receptors, alone (“direct effect”) or after pre-stimulation with isoprenaline (“indirect effect”), is known to reduce the force of contraction (FOC) in human atrial preparations (HAP). We hypothesized that muscarine might act similar to acetylcholine. During open heart surgery for severe coronary heart disease, HAP were obtained. Using digital polymerase chain reaction, we noted that the messenger ribonucleic acid (mRNA) of the M₁-, M₂-, and M₃-muscarinic receptors in HAP contributed to 0.09%, 96.34%, and 3.57%, respectively, of the total mRNA expression of the combined mRNA expression of the sum of the M₁-, M₂-, and M₃-muscarinic receptors. The HAP were mounted in organ baths, electrically stimulated (1&#xa0;Hz) and FOC was measured under isometric conditions. When given alone, muscarine transiently reduced FOC. After application of 1&#xa0;µM isoprenaline (to stimulate β-adrenoceptors) or 1&#xa0;µM serotonin (to stimulate 5-HT₄-serotonin receptors) or 1&#xa0;µM histamine (to stimulate H₂-histamine receptors) or 10&#xa0;µM fenoldopam (to stimulate D₁-dopamine receptors) or 100&#xa0;nM semaglutide (to stimulate glucagon-like-peptide-1 receptors) or 100&#xa0;nM gastric inhibitory peptide (to stimulate the gastric inhibitory peptide receptors) additionally applied 1&#xa0;µM muscarine reduced FOC. Interestingly, after 1&#xa0;µM forskolin (to stimulate adenylyl cyclase independent of receptor involvement), 1&#xa0;µM cilostamide (to inhibit cAMP degradation) or 100&#xa0;µM dibutyryl-cAMP (to directly stimulate the cAMP-dependent protein kinase), additionally applied 1&#xa0;µM muscarine also reduced FOC. We conclude that muscarine in the presence of several cAMP-increasing agents or a cAMP-dependent protein kinase activating agent exerts negative inotropic effects in HAP. These contractile responses might underlie negative inotropic effects in muscarine intoxications in the clinic.</p>