RT Journal Article SR Electronic T1 The Third Intracellular Loop and the Carboxyl Terminus of β2-Adrenergic Receptor Confer Spontaneous Activity of the Receptor JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 1048 OP 1058 DO 10.1124/mol.64.5.1048 VO 64 IS 5 A1 Khalid Chakir A1 Yang Xiang A1 Dongmei Yang A1 Sheng-Jun Zhang A1 Heping Cheng A1 Brian K. Kobilka A1 Rui-Ping Xiao YR 2003 UL http://molpharm.aspetjournals.org/content/64/5/1048.abstract AB It is well established that the β2-adrenergic receptor (β2-AR) exhibits a robust ligand-independent activity, whereas this property is considerably weaker in the closely related β1-AR subtype. To identify the potential domain(s) of β2-AR responsible for the spontaneous receptor activation, we created three chimeras in which the third intracellular loop (β1/β2-Li3) or the carboxyl terminus (β1/β2-CT) or both domains (β1/β2-Li3CT) of β1-AR are replaced by the corresponding parts of the β2-AR. Using adenoviral gene transfer, we individually expressed these β1/β2-AR chimeras in mouse cardiomyocytes lacking both native β1-AR and β2-AR (β1/β2 double knockout), and examined their possible spontaneous activities. Overexpression of these β1/β2-AR chimeras markedly elevated basal cAMP accumulation and myocyte contractility in the absence of agonist stimulation compared with those infected by a control adenovirus expressing β-galactosidase or an adenovirus expressing wild type β1-AR. These effects were fully reversed by a β2-AR inverse agonist, (±)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI 118,551; 5 × 10-7 M), regardless of inhibition of Gi with pertussis toxin, but not by a panel of β1-AR antagonists, including [2-(3-carbamoyl-4-hydroxyphenoxy)-ethylamino]-3-[4-(1-methyl-4-trifluormethyl-2-imidazolyl)-phenoxy]-2-propanolmethanesulfonate (CGP20712A), betaxolol, bisoprolol, and metoprolol. Furthermore, we have shown that the C-terminal postsynaptic density 95/disc-large/ZO-1 (PDZ) motif of β1-AR is not responsible for the lack of β1-AR spontaneous activation, although it has been known that the β1-AR PDZ motif prevents the receptor from undergoing agonist-induced trafficking and Gi coupling in cardiomyocytes. Taken together, the present results indicate that both the third intracellular loop and the C terminus are involved in β2-AR spontaneous activation and that either domain seems to be sufficient to confer the receptor spontaneous activity.