RGS4 Regulates Parasympathetic Signaling and Heart Rate Control in the Sinoatrial Node

Abstract

Abstract-Heart rate is controlled by the opposing activities of sympathetic and parasympathetic inputs to pacemaker myocytes in the sinoatrial node (SAN). Parasympathetic activity on nodal myocytes is mediated by acetylcholinedependent stimulation of M 2 muscarinic receptors and activation of G␣ i/o signaling. Although regulators of G protein signaling (RGS) proteins are potent inhibitors of G␣ i/o signaling in many tissues, the RGS protein(s) that regulate parasympathetic tone in the SAN are unknown. Our results demonstrate that RGS4 mRNA levels are higher in the SAN compared to right atrium. Conscious freely moving RGS4-null mice showed increased bradycardic responses to parasympathetic agonists compared to wild-type animals. Moreover, anesthetized RGS4-null mice had lower baseline heart rates and greater heart rate increases following atropine administration. Retrograde-perfused hearts from RGS4-null mice showed enhanced negative chronotropic responses to carbachol, whereas SAN myocytes showed greater sensitivity to carbachol-mediated reduction in the action potential firing rate. Finally, RGS4-null SAN cells showed decreased levels of G protein-coupled inward rectifying potassium (GIRK) channel desensitization and altered modulation of acetylcholine-sensitive potassium current (I KACh ) kinetics following carbachol stimulation. Taken together, our studies establish that RGS4 plays an important role in regulating sinus rhythm by inhibiting parasympathetic signaling and I KACh activity. (Circ Res. 2008;103:527-535.)Key Words: RGS proteins Ⅲ sinoatrial node Ⅲ parasympathetic Ⅲ GIRK channels H eart rate (HR) regulation by the autonomic nervous system is integrated by specialized autorhythmic (pacemaker) cells located within the sinoatrial node (SAN). Sympathetic neurotransmitters work via G s -coupled ␤-adrenergic receptors to increase adenylyl cyclase activity, intracellular cAMP concentration and protein kinase A activity. As a result, cAMP-regulated effectors such as hyperpolarizationactivated cyclic nucleotide-gated cation (HCN) channels, delayed rectifier, and voltage-gated Ca 2ϩ channels are enlisted by sympathetic activity to increase pacemaker cell firing rate. 1,2 By contrast, vagal parasympathetic activity decreases HR via G␣ i/o -coupled cholinergic M 2 muscarinic receptors (M 2 Rs). Several effects, mediated by both G␣ i/o and G␤␥ subunits, may contribute to this reduction in HR. G␤␥ heterodimers directly activate G protein-coupled inward rectifying potassium (GIRK) channels, resulting in membrane hyperpolarization. By contrast, G␣ i/o can both modulate phosphodiesterase activity and inhibit adenylyl cyclase activity, reduce both intracellular cAMP levels and protein kinase A activity, leading to decreased depolarizing currents carried by HCN and L-type calcium channels. [2][3][4][5] Because dysregulation of parasympathetic activity occurs in heart failure, sick sinus syndrome, and selected cardiac arrhythmias, 6 it is of clinical interest to identify key molecular regulators of parasympathetic signa...

Cellular modelling: experiments and simulation to develop a physiological model of G-protein control of muscarinic K + channels in mammalian atrial cells

“…It will be interesting to use our model to investigate possible sources of constitutive K ACh channel activity-activation of K ACh channels by GTP in the absence of ACh (Ito et al 1991) and the loss of Ca 2+ -CaM regulation of RGS proteins might prove to be attractive targets for further investigation. In which context it is interesting to note that knock-out of the RGS4 protein abolished desensitization of the response to ACh in the SA node in mice (Cifelli et al 2008). A major limitation of the model presented here is that it cannot reproduce desensitization, and this after considerable effort.…”

The evolution of regulators of G protein signalling proteins as drug targets – 20 years in the making: IUPHAR Review 21

“…À/À mice to be dependent on RGS4 (Cifelli et al, 2008). This shows that RGS protein inhibitors may be used in a clinical setting alone or in conjunction with other therapies.…”

Regulator of G protein signaling 2 mediates cardiac compensation to pressure overload and antihypertrophic effects of PDE5 inhibition in mice

“…Yet other studies in hearts overexpressing RGS4 found rapid cardiac dilation and marked mortality upon pressure overload induction (11), highlighting a complex role. This was further demonstrated by the recent discovery that RGS4 regulates parasympathetic (G α[i/o] ) signaling to control heart rate in the sinoatrial node (12).…”