• Jianqi Yang, Jie Huang, Biswanath Maity, Zhan Gao, Ramón A Lorca, Hjalti Gudmundsson, Jingdong Li, Adele Stewart, Paari Dominic Swaminathan, Stella-Rita Ibeawuchi, Andrew Shepherd, Ching-Kang Chen, William Kutschke, Peter J Mohler, Durga P Mohapatra, Mark E Anderson, and Rory A Fisher.
• Department of Pharmacology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
• Circ. Res. 2010 Nov 26; 107 (11): 1345-9.

RationaleParasympathetic regulation of heart rate is mediated by acetylcholine binding to G protein-coupled muscarinic M2 receptors, which activate heterotrimeric G(i/o) proteins to promote G protein-coupled inwardly rectifying K(+) (GIRK) channel activation. Regulator of G protein signaling (RGS) proteins, which function to inactivate G proteins, are indispensable for normal parasympathetic control of the heart. However, it is unclear which of the more than 20 known RGS proteins function to negatively regulate and thereby ensure normal parasympathetic control of the heart.ObjectiveTo examine the specific contribution of RGS6 as an essential regulator of parasympathetic signaling in heart.Methods And ResultsWe developed RGS6 knockout mice to determine the functional impact of loss of RGS6 on parasympathetic regulation of cardiac automaticity. RGS6 exhibited a uniquely robust expression in the heart, particularly in sinoatrial and atrioventricular nodal regions. Loss of RGS6 provoked dramatically exaggerated bradycardia in response to carbachol in mice and isolated perfused hearts and significantly enhanced the effect of carbachol on inhibition of spontaneous action potential firing in sinoatrial node cells. Consistent with a role of RGS6 in G protein inactivation, RGS6-deficient atrial myocytes exhibited a significant reduction in the time course of acetylcholine-activated potassium current (I(K)(ACh)) activation and deactivation, as well as the extent of I(K)(ACh) desensitization.ConclusionsRGS6 is a previously unrecognized, but essential, regulator of parasympathetic activation in heart, functioning to prevent parasympathetic override and severe bradycardia. These effects likely result from actions of RGS6 as a negative regulator of G protein activation of GIRK channels.

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