Abundance, distribution, mobility and oligomeric state of M2 muscarinic acetylcholine receptors in live cardiac muscle

► The first observation of single molecules moving on the surface of living tissue ► Only ca. 10% of murine cardiomyocytes expressed M2 receptors at appreciable levels. ► M2 density at birth was 3-fold higher than that in embryonic and adult myocytes. ► M2 receptors were primarily monomeric but formed reversible dimers. ► M2 receptors diffused 4-times faster in heart slices than in cultured myocytes.