| Summary: | Heart rate is modulated by the opposing activities of sympathetic and parasympathetic inputs to pacemaker cardiomyocytes in the sinoatrial (SA) node. Parasympathetic activity on nodal myocytes is mediated by acetylcholine-dependent stimulation of M2 muscarinic receptors and activation of Gαi/o signalling. Although, regulators of G-protein signalling (RGS) proteins are potent inhibitors of Gαi/o signalling in many tissues, the RGS protein(s) that regulate parasympathetic tone in the SA node are unknown. Our results demonstrate that RGS4 mRNA levels are higher in the SA node compared to right atrium. Conscious freely moving RGS4-null mice showed a greater extent of bradycardia in response to parasympathetic agonists compared to wild-type animals. Moreover, anaesthetized rgs4-null mice had lower baseline heart rates and greater heart rate increases following atropine administration. Retrograde-perfused hearts from rgs4-null mice also showed enhanced negative chronotropic responses to carbachol, while isolated SA node myocytes showed greater sensitivity to carbachol-mediated reduction in the action potential firing rate. Finally, rgs4-null SA node cells showed decreased levels of G-protein-coupled inward rectifying potassium (GIRK) channel desensitization, and altered modulation of acetylcholine-sensitive potassium current (IKACh) kinetics following carbachol stimulation. Taken together, our studies establish that RGS4 plays an important role in regulating sinus rhythm by inhibiting parasympathetic signalling and IKACh activity. Following these results, we predicted that loss of RGS4 expression and function will result in increased levels of parasympathetic effector activity leading to increased susceptibility to atrial fibrillation.
Susceptibility to atrial fibrillation (AF) depends strongly on parasympathetic activity. Since RGS4 inhibits parasympathetic / M2-dependent Gαi/o signalling in the SA node, we explored whether changes in RGS4 levels altered the susceptibility of atrial fibrillation. We found that, RGS4 levels were decreased in atria of tachypaced dogs prior to their development of chronic AF. Moreover, in vivo ECG recordings of anaesthetized mice showed greater susceptibility to AF while optical mapping of isolated atrial preparations using a voltage-sensitive dye revealed greatly increased susceptibility to rotor formation when RGS4 was ablated. Consistent with altered parasympathetic signalling in the myocardium of rgs4-null mice, IKACh evoked by carbachol application were greater in isolated atrial myocytes from rgs4-null mice. These IKACh changes were, as expected, associated with marked action potential duration shortening in response to parasympathetic activation, but not to slower conduction velocities. Together, our findings establish that RGS4 protects atrial tissues from excess parasympathetic signalling that predispose to atrial fibrillation.
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