Potential Role of Regulator of G-Protein Signaling 5 in the Protection of Vagal-Related Bradycardia and Atrial Tachyarrhythmia

Abstract

Background: The regulator of G-protein signaling 5 (Rgs5), which functions as the regulator of G-protein-coupled receptor (GPCR) including muscarinic receptors, has a potential effect on atrial muscarinic receptor-activated IKA ch current.

Methods and results: In the present study, hearts of Rgs5 knockout (KO) mice had decreased low-frequency/high-frequency ratio in spectral measures of heart rate variability. Loss of Rgs5 provoked dramatically exaggerated bradycardia and significantly (P<0.05) prolonged sinus nodal recovery time in response to carbachol (0.1 mg/kg, intraperitoneally). Compared to those from wild-type (WT) mice, Langendorff perfused hearts from Rgs5 KO mice had significantly (P<0.01) abbreviated atrial effective refractory periods and increased dominant frequency after administration of acetylcholine (ACh; 1 μmol/L). In addition, whole patch clamp analyses of single atrial myocytes revealed that the ACh-regulated potassium current (IKA ch) was significant increased in the time course of activation and deactivation (P<0.01) in Rgs5 KO, compared to those in WT, mice. To further determine the effect of Rgs5, transgenic mice with cardiac-specific overexpression of human Rgs5 were found to be resistant to ACh-related effects in bradycardia, atrial electrophysiology, and atrial tachyarrhythmia (AT).

Conclusion: The results of this study indicate that, as a critical regulator of parasympathetic activation in the heart, Rgs5 prevents vagal-related bradycardia and AT through negatively regulating the IKA ch current.

Keywords: IKAch; atrial tachyarrhythmia; bradycardia; regulator of G‐protein signaling 5.

Figure 1

A, Representative Western blots of murine Rgs5 protein in atrial tissue from WT and RGS5 KO mice; (B) Representative Western blots of human Rgs5 protein in atrial tissue from 4 lines of TG and WT mice. C, Representative Western blot of human Rgs5 protein from different tissue of TG mice as indicated (a, lung; b, muscle; c, atria; d, ventricle; e, kidney; f, spleen; g, liver; h, brain). KO indicates knockout mice; TG, transgenic mice; WT, wild‐type mice.

Figure 2

A, Mean RR interval of conscious mice were assessed by 24‐hour ECG recording. (B) Effect of CCh (0.1mg/kg, IP) on heart rate in conscious mice. C, Representative power spectrum density plot of heart rate variability (HRV). Cut‐off frequencies divided power spectrum into 3 main parts: very‐low‐frequency (VLF) power between 0.0 and 0.4 Hz; low‐frequency (LF) power between 0.4 and 1.5 Hz; and high‐frequency (HF) power between 1.5 and 4.0 Hz. D, Photograph for implantation of telemetry ECG. E through G, Frequency‐domain measures of HRV by normalized (n) HF, nLF, and LF/HF ratio. *P<0.05 vs WT and Rgs5 TG by ANOVA; ^# P<0.05 vs WT and Rgs5 KO by ANOVA. Eight conscious mice were used in each group. CCh indicates carbachol; ECG, electrocardiogram; KO, knockout mice; TG, transgenic mice; WT, wild‐type mice.

Figure 3

PES and burst pacing induced atrial tachyarrhythmias (ATs) in isolated hearts from WT (n=15), Rgs5 KO (n=15), and Rgs5 TG (n=10) mice during administration of 1 μmol/L of ACh. A, More‐sustained rapid atrial activity (duration >30 seconds) were observed in Rgs5 KO hearts. B, Rgs5 TG hearts showed nonsustained rapid atrial activity. ACh indicates acetylcholine; ATA, atrial tachyarrhythmia; KO, knockout mice; PES, programmed electric stimuli; TG, transgenic mice; WT, wild‐type mice.

Figure 4

Frequency analysis determined with bipolar electrogram recordings in WT (n=15), Rgs5 KO (n=15), and Rgs5TG (n=10) mice. A through C, Dominant frequency (DF) of AT in the fast Fourier transform was analyzed at baseline and ACh state. D, Mean±SE DF values at baseline, ACh, and washout states were compared among the 3 groups. *P<0.05 vs WT and Rgs5 TG by ANOVA; ^# P<0.05 vs WT and Rgs5 KO by ANOVA. ACh indicates acetylcholine; AT, atrial tachyarrhythmia; BS, baseline; KO, knockout mice; TG, transgenic mice; WS, washout; WT, wild‐type mice.

Figure 5

A, Representative I_kAch recordings from WT (n=10), Rgs5 KO (n=8), and Rgs5 TG (n=8) atrial myocytes. B, Photograph of isolated cell from mouse atrium. C, Current density (peak current/cell capacitance) and (D) half‐maximal activation/deactivation time constant (t_1/2) analysis of I_KACh obtained from WT, Rgs5 KO, and Rgs5 TG mice. E through G, mRNA expression of Kir3.1 and Kir3.4 was determined in atria of mice. Relative abundance was calculated with value of WT as reference of 100%. *P<0.05 vs WT and Rgs5 TG by ANOVA; ^# P<0.05 vs WT and Rgs5 KO by ANOVA. KO indicates knockout mice; TG, transgenic mice; WT, wild‐type mice.