The muscarinic M3 receptor is a 590-amino acid, 7-transmembrane protein. RT-PCR and radioligand binding assays detected M3 expression in the bladder, gastric and intestinal smooth muscle, brain (CNS) and the vestibular system. Muscarinic M3 receptors mediate contraction of airway and bladder smooth muscles. The receptors in the urothelium/suburothelium can also modulate the release of certain factors, which in turn may affect bladder function at the efferent or afferent axis. Blockade of M3 receptors can alleviate the symptoms of OAB. M3 receptors are also expressed myocardial tissues and are of potential roles in parasympathetic control of heart function under normal physiological conditions and in heart failure, myocardial ischemia and arrhythmias.