Role of internalization of M2 muscarinic receptor via clathrin-coated vesicles in desensitization of the muscarinic K⁺ current in heart

n the heart, ACh activates the ACh-activated K+ current (IK,ACh) via the M2 muscarinic receptor. The relationship between desensitization of IK,ACh and internalization of the M2 receptor has been studied in rat atrial cells. On application of the stable muscarinic agonist carbachol for 2 h, IK,ACh declined by ∼62% with time constants of 1.5 and 26.9 min, whereas ∼83% of the M2 receptor was internalized from the cell membrane with time constants of 2.9 and 51.6 min. Transfection of the cells with β-adrenergic receptor kinase 1 (G protein-receptor kinase 2) and β-arrestin 2 significantly increased IK,ACh desensitization and M2 receptor internalization during a 3-min application of agonist. Internalized M2 receptor in cells exposed to carbachol for 2 h was colocalized with clathrin and not caveolin. It is concluded that a G protein-receptor kinase 2- and β-arrestin 2-dependent internalization of the M2 receptor into clathrin-coated vesicles could play a major role in IK,ACh desensitization.