The action of the neurotransmitter acetylcholine is mediated through two types of receptors, the ionotrophic nicotinic receptors and the metabotrophic muscarinic receptors. The muscarinic receptors belong to the superfamily of 7-TM G-protein-coupled receptors. Five subtypes of muscarinic receptors have been cloned and are named m1-m5.The m2 receptor is considered to be the predominant muscarinic receptor that is expressed in cardiac muscle
