Augmentation of Cardiac Contractility Mediated by the Human β 3 -Adrenergic Receptor Overexpressed in the Hearts of Transgenic Mice

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Abstract
Background Stimulation of β1- and β2-adrenergic receptors (ARs) in the heart results in positive inotropy. In contrast, it has been reported that the β3AR is also expressed in the human heart and that its stimulation leads to negative inotropic effects. Methods and Results To better understand the role of β3ARs in cardiac function, we generated transgenic mice with cardiac-specific overexpression of 330 fmol/mg protein of the human β3AR (TGβ3 mice). Hemodynamic characterization was performed by cardiac catheterization in closed-chest anesthetized mice, by pressure-volume-loop analysis, and by echocardiography in conscious mice. After propranolol blockade of endogenous β1- and β2ARs, isoproterenol resulted in an increase in contractility in the TGβ3 mice (30%), with no effect in wild-type mice. Similarly, stimulation with the selective human β3AR agonist L-755,507 significantly increased contractility in the TGβ3 mice (160%), with no effect in wild-type mice, as determined by hemodynamic measurements and by end-systolic pressure-volume relations. The underlying mechanism of the positive inotropy incurred with L-755,507 in the TGβ3 mice was investigated in terms of β3AR–G-protein coupling and adenylyl cyclase activation. Stimulation of cardiac membranes from TGβ3 mice with L-755,507 resulted in a pertussis toxin–insensitive 1.33-fold increase in [35S]GTPγS loading and a 1.6-fold increase in adenylyl cyclase activity. Conclusions Cardiac overexpression of human β3ARs results in positive inotropy only on stimulation with a β3AR agonist. Overexpressed β3ARs couple to Gs and activate adenylyl cyclase on agonist stimulation.