PT - JOURNAL ARTICLE AU - Ying-Ying Zhou AU - Dongmei Yang AU - Wei-Zhong Zhu AU - Sheng-Jun Zhang AU - Ding-Ji Wang AU - Dan K. Rohrer AU - Eric Devic AU - Brian K. Kobilka AU - Edward G. Lakatta AU - Heping Cheng AU - Rui-Ping Xiao TI - Spontaneous Activation of β<sub>2</sub>- but Not β<sub>1</sub>-Adrenoceptors Expressed in Cardiac Myocytes from β<sub>1</sub>β<sub>2</sub> Double Knockout Mice AID - 10.1124/mol.58.5.887 DP - 2000 Nov 01 TA - Molecular Pharmacology PG - 887--894 VI - 58 IP - 5 4099 - http://molpharm.aspetjournals.org/content/58/5/887.short 4100 - http://molpharm.aspetjournals.org/content/58/5/887.full SO - Mol Pharmacol2000 Nov 01; 58 AB - Although ligand-free, constitutive β2-adrenergic receptor (AR) signaling has been demonstrated in naive cell lines and in transgenic mice overexpressing cardiac β2-AR, it is unclear whether the dominant cardiac β-AR subtype, β1-AR, shares the ability of spontaneous activation. In the present study, we expressed human β1- or β2-AR via recombinant adenoviral infection in ventricular myocytes isolated from β1β2-AR double knockout mice, creating pure β1-AR and β2-AR systems with variable receptor densities. A contractile response to a nonselective β-AR agonist, isoproterenol, was absent in double knockout mouse myocytes but was fully restored after adenoviral β1-AR or adenoviral β2-AR infection. Increasing the titer of adenoviral vectors (multiplicity of infection 10–1000) led to a dose-dependent expression of β1- or β2-AR with a maximal density of 1207 ± 173 (36-fold over the wild-type control value) and 821 ± 38 fmol/mg protein (69-fold), respectively. Using confocal immunohistochemistry, we directly visualized the cellular distribution of β1-AR and β2-AR and found that both subtypes were distributed on the cell surface membrane and transverse tubules, resulting in a striated pattern. In the absence of ligand, β2-AR expression resulted in graded increases in baseline cAMP and contractility up to 428% and 233% of control, respectively, at the maximal β2-AR density. These effects were specifically reversed by a β2-AR inverse agonist, ICI 118,551 (10−7 M). In contrast, overexpression of β1-AR, even at a greater density, failed to enhance either basal cAMP or contractility; the alleged β1-AR inverse agonist, CGP 20712A (10−6 M), had no significant effect on basal contraction in these cells. Thus, we conclude that acute β2-AR overexpression in cardiac myocytes elicits significant physiological responses due to spontaneous receptor activation; however, this property is β-AR subtype specific because β1-AR does not exhibit agonist-independent spontaneous activation.