We investigated the changes in gene expression, receptor number, echocardiography, muscarinic/adrenergic agonist/antagonist changes in heart rate (HR) and HR biorhythm in M-2 KO mice (mice lacking the main cardioinhibitory receptors) in the left ventricle (LV) and right ventricle (RV). We can conclude that although the main cardioinhobitory receptors are not present in the heart, the function is not much affected.