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In the heart, ACh activates the ACh-activated K(+) current (I(K,ACh)) via the M(2) muscarinic receptor. The relationship between desensitization of I(K,ACh) and internalization of the M(2) receptor has been studied in rat atrial cells. On application of the stable muscarinic agonist carbachol for 2 h, I(K,ACh) declined by approximately 62% with time constants of 1.5 and 26.9 min, whereas approximately 83% of the M(2) receptor was internalized from the cell membrane with time constants of 2.9 and 51.6 min. Transfection of the cells with beta-adrenergic receptor kinase 1 (G protein-receptor kinase 2) and beta-arrestin 2 significantly increased I(K,ACh) desensitization and M(2) receptor internalization during a 3-min application of agonist. Internalized M(2) 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 beta-arrestin 2-dependent internalization of the M(2) receptor into clathrin-coated vesicles could play a major role in I(K,ACh) desensitization.

Adverse remodeling after myocardial infarction (MI) determines the progression of heart failure. Failing hearts are characterized by downregulation of beta-adrenergic receptor (beta-AR) signaling in part because of increased beta-AR kinase 1 activity. Our previous studies have shown that overexpression of the phosphoinositide kinase (PIK) domain of phosphoinositide 3-kinase (PI3K), prevents beta-AR downregulation and enhances adrenergic agonist responsiveness by inhibiting the targeting of PI3K to the beta-AR complex. To investigate whether preventing beta-AR downregulation in the heart ameliorates cardiac function post-MI, transgenic mice with cardiac-specific overexpression of the PIK domain peptide (TgPIK) underwent left coronary artery ligation and were subsequently followed by serial echocardiography at 4, 8, 12, 16, and 20 wk. Despite having similar infarction sizes, TgPIK mice showed better systolic function, less cardiac dilatation, and improved hemodynamic response to dobutamine compared with littermate controls after MI. To test that displacement of PI3K from the beta-AR complex, but not the total loss of PI3K-gamma, is critical for amelioration of cardiac function, mice lacking the PI3K-gamma (PI3K-gamma-KO) underwent MI, and their cardiac function was assessed 20 wk post-MI. Serial echocardiographic measurements showed severe reduction in contractile performance in PI3K-gamma-KO compared with TgPIK mice. Furthermore, significant beta-AR downregulation and desensitization were only seen in infarcted wild-type and PI3K-gamma-KO mice and not in TgPIK mice. Together, these results demonstrate that adverse remodeling of the ventricle after MI can be attenuated by a strategy that prevents recruitment of PI3K to the plasma membrane and restores normal beta-AR function.