Essential Role of the m2R-RGS6-IKACh Pathway in Controlling Intrinsic Heart Rate Variability

Abstract

Normal heart function requires generation of a regular rhythm by sinoatrial pacemaker cells and the alteration of this spontaneous heart rate by the autonomic input to match physiological demand. However, the molecular mechanisms that ensure consistent periodicity of cardiac contractions and fine tuning of this process by autonomic system are not completely understood.Here we examined the contribution of the m2R-IKACh intracellular signaling pathway, which mediates the negative chronotropic effect of parasympathetic stimulation, to the regulation of the cardiac pacemaking rhythm. Using isolated heart preparations and single-cell recordings we show that the m2R-IKACh signaling pathway controls the excitability and firing pattern of the sinoatrial cardiomyocytes and determines variability of cardiac rhythm in a manner independent from the autonomic input. Ablation of the major regulator of this pathway, Rgs6, in mice results in irregular cardiac rhythmicity and increases susceptibility to atrial fibrillation. We further identify several human subjects with variants in the RGS6 gene and show that the loss of function in RGS6 correlates with increased heart rate variability. These findings identify the essential role of the m2R-IKACh signaling pathway in the regulation of cardiac sinus rhythm and implicate RGS6 in arrhythmia pathogenesis.

RGS6, but Not RGS4, Is the Dominant Regulator of G Protein Signaling (RGS) Modulator of the Parasympathetic Regulation of Mouse Heart Rate

“…Basal HR in hearts from Rgs6 Ϫ/Ϫ mice was significantly lower than measured in wild-type counterparts, consistent with published observations (14,16), whereas HR in hearts from Rgs4 Ϫ/Ϫ mice was normal ( Fig. 2A).…”

“…3). Previous studies have shown that Girk4 gene ablation completely eliminates the whole-cell inward currents induced by CCh in SAN cells, confirming the I KACh dependence of the response (10,16). No significant genotype-dependent differences in peak or steady-state CCh-induced I KACh currents were observed in response to a saturating CCh concentration (10 M) (Fig.…”

“…SAN tissue was triturated in wash solution and plated onto poly-L-lysine-coated coverslips. Coverslips containing SAN cells were transferred to a perfusion chamber, and electrophysiological recordings were conducted as described (14,16). SAN cells targeted for electrophysiological characterization were spindle-shaped and striated and, in most cases, exhibited spontaneous beating.…”

“…SAN cells targeted for electrophysiological characterization were spindle-shaped and striated and, in most cases, exhibited spontaneous beating. Most experiments used a faststep perfusion system to allow for fast application and removal of CCh (within 100 ms) and have been described previously (14,16). A small subset of experiments was done with gravity-flow perfusion of CCh (within 10 -20 s).…”

“…Indeed, Rgs6 Ϫ/Ϫ mice exhibit slower resting HR, enhanced sensitivity to M 2 R-dependent bradycardia, elevated HRV, and enhanced M 2 R-I KACh signaling characterized primarily by a prominent slowing of current deactivation (14 -16). Moreover, a loss-of-function mutation in the human RGS6 gene correlates with elevated HRV (16), and an RGS6 polymorphism correlate with altered HR recovery after exercise (17).…”