The muscarinic acetylcholine receptor M2, also known as the cholinergic receptor, muscarinic 2, is a muscarinic acetylcholine receptor that in humans is encoded by the CHRM2 gene. Multiple alternatively spliced transcript variants have been described for this gene. M2 muscarinic receptors act via a Gi type receptor, which causes a decrease in cAMP in the cell, generally leading to inhibitory-type effects. They appear to serve as autoreceptors. In addition, they modulate G protein-coupled inwardly-rectifying potassium channels. In the heart, this contributes to a decreased heart rate. They do so by the Gβγ subunit of the G protein; Gβγ shifts the open probability of K+ channels in the membrane of the cardiac pacemaker cells, which causes an outward current of potassium, effectively hyperpolarizing the membrane, which slows down the heart rate. Few highly selective M2 agonists are available at present, although there are several non-selective muscarinic agonists that stimulate M2, and a number of selective M2 antagonists are available.