Impaired parasympathetic heart rate control in mice with a reduction of functional G protein βγ-subunits

Acetylcholine released on parasympathetic stimulation slows heart rate through activation of muscarinic receptors on the sinus nodal cells and subsequent opening of the atrial muscarinic potassium channel (K-ACh). K-ACh is directly activated by G protein betagamma-subunits. To elucidate the physiological role of Gbetagamma for the regulation of heart rate and electrophysiological function in vivo, we created transgenic mice with a reduced amount of membrane-bound Gbeta protein by overexpressing nonprenylated Ggamma(2)-subunits in their hearts using the alpha-myosin heavy chain promoter. At baseline and after muscarinic stimulation with carbachol, heart rate and heart rate variability were determined with electrocardiogram telemetry in conscious mice and in vivo intracardiac electrophysiological studies in anesthetized mice. Reduction of the amount of functional Gbetagamma protein by >50% caused a pronounced blunting of the carbachol-induced bradycardia as well as the increases in time- and frequency-domain indexes of heart rate variability and baroreflex sensitivity that were observed in wild types. In addition, sinus node recovery time and inducibility of atrial arrhythmias were reduced in transgenic mice. Our data demonstrate in vivo that Gbetagamma plays a crucial role for parasympathetic heart rate control, sinus node automaticity, and atrial arrhythmia vulnerability.