Altered sinoatrial node function and intra-atrial conduction in murine gain-of-function Scn5a+/ΔKPQ hearts suggest an overlap syndrome

Abstract

Mutations in SCN5A, the gene encoding the pore-forming subunit of cardiac Na(+) channels, cause a spectrum of arrhythmic syndromes. Of these, sinoatrial node (SAN) dysfunction occurs in patients with both loss- and gain-of-function SCN5A mutations. We explored for corresponding alterations in SAN function and intracardiac conduction and clarified possible mechanisms underlying these in an established mouse long QT syndrome type 3 model carrying a mutation equivalent to human SCN5A-ΔKPQ. Electrophysiological characterizations of SAN function in living animals and in vitro sinoatrial preparations were compared with cellular SAN and two-dimensional tissue models exploring the consequences of Scn5a+/ΔKPQ mutations. Scn5a+/ΔKPQ mice showed prolonged electrocardiographic QT and corrected QT intervals confirming long QT phenotypes. They showed frequent episodes of sinus bradycardia, sinus pause/arrest, and significantly longer sinus node recovery times, suggesting compromised pacemaker activity compared with wild-type mice. Electrocardiographic waveforms suggested depressed intra-atrial, atrioventricular node, and intraventricular conduction in Scn5a+/ΔKPQ mice. Isolated Scn5a+/ΔKPQ sinoatrial preparations similarly showed lower mean intrinsic heart rates and overall slower conduction through the SAN to the surrounding atrium than did wild-type preparations. Computer simulations of both single SAN cells as well as two-dimensional SAN-atrial models could reproduce the experimental observations of impaired pacemaker and sinoatrial conduction in terms of changes produced by both augmented tail and reduced total Na(+) currents, respectively. In conclusion, the gain-of-function long QT syndrome type 3 murine Scn5a+/ΔKPQ cardiac system, in overlap with corresponding features reported in loss-of-function Na(+) channel mutations, shows compromised SAN pacemaker and conduction function explicable in modeling studies through a combination of augmented tail and reduced peak Na(+) currents.

Fig. 1.

ECG recordings (lead II) from anesthetized wild-type (WT) mice (A, C, E, and G) and Scn5a+/ΔKPQ mice (B, D, F, and H). A–D: representative recordings of surface ECG recordings under ketamine (A and B) and avertin (C and D) anesthesia. E–H: waterfall plots (E and F) showing individual ECG recordings used to obtain averaged ECG recordings (G and H).

Fig. 2.

Electrocardiographic recordings (lead II) obtained during determinations of sinus node recovery time in the form of a resumption of atrial P waves after sequences of S1 stimuli in a burst pacing protocol in WT (A and C) and Scn5a+/ΔKPQ (B and D) mice under ketamine (A and B) and avertin (C and D) anesthesia.

Fig. 3.

Examples of lead II ECG recordings of arrhythmic events in Scn5a+/ΔKPQ mice. A: bradycardic episodes occurring before and after a period of normal sinus rhythm. Insets show that the corresponding detailed ECG waveforms were normal. bpm, beats/min. B: sinus pause occurring after a sequence of seven normal P wave deflections. C: sinus arrest showing an episode in which P waves were absent. D: persistent second-degree 2:1 atrioventricular block in which QRS complexes failed to follow alternate P waves. E: episodes of third degree atrioventricular block showing dissociation between P wave and QRS deflections.

Fig. 4.

Comparison of array recording results from typical WT (A and B) and Scn5a+/ΔKPQ (C and D) preparations, showing individual traces (A and C) obtained from the selected sequence of sites 1–8 as marked on the activation map (B and D) reflecting the slower spread of electrical activity in Scn5a+/ΔKPQ preparations. The following labels were included to orient the specimen: SVC, superior vena cava; IVC, inferior vena cava; CT, crista terminalis; SEPT, atrial septum. Points on the recording arrays in which recordings were obtained are marked as solid circles in B and D. The point showing the earliest deflection, likely representing the sinoatrial node (SAN) is marked with a star on both the electrical traces and the activation map. E: quantification of propagation maps by determining the number of recording sites as a proportion of the total number of sites in which electrical activity could be recorded in Scn5a+/ΔKPQ and WT preparations at different times after the initiation of activation demonstrating the slower spread of such activity in Scn5a+/ΔKPQ preparations.

Fig. 5.

Effects of varying persistent late Na⁺ current (I_Na,L) on SAN activity. i and ii: results of simulations from central (i) and peripheral (ii) SAN cells. A–F: corresponding effects on action potentials (APs; A), cycle lengths (CLs; B), AP durations (APD; C), peak amplitudes (D), maximum change in voltage over time [(dV/dt)_max] values (E), and maximal diastolic potential (MDP; F).

Fig. 6.

Effects of reducing transient Na⁺ current (I_Na,T) to 70% of its control value in the absence (i) and presence (ii) of I_Na,L of 6% magnitude on SAN activity using a peripheral SAN cell model. A–F: corresponding effects on APs (A), CLs (B), APDs (C), peak amplitudes (D), (dV/dt)_max values (E), and MDPs (F).

Fig. 7.

Effects of varying I_Na,L to levels corresponding to 0% (A), 3% (B), 6% (C), and 18% (D) of a constant, control (100%) level of I_Na,T on AP conduction in a two-dimensional (2-D) tissue model of the SAN coupled to the atrial septum (upward direction) and atrial muscle (downward direction). E and F: calculated conduction times (E) from the SAN center to the atrial septum or atrial muscle and the CLs (F) with the corresponding I_Na,L values.

Fig. 8.

Effects of I_Na,T reduced to 35% of its control value and different values of I_Na,L corresponding to 0% (A), 3% (B), 6% (C), and 18% (D) of the control I_Na,T on AP conduction in a 2-D tissue model of the SAN coupled to the atrial septum (upward direction) and atrial muscle (downward direction). E and F: calculated conduction times (E) from the SAN center to the atrial septum or atrial muscle and the CLs (F) with the corresponding I_Na,L values.

Fig. 9.

Effects of a contribution from I_Na,L of magnitude 6% of the control I_NaT in combination with varying I_Na,T of 100% (A), 80% (B), 70% (C), and 35% (D) of the control I_Na,T on AP conduction in a 2-D tissue model of the SAN coupled to the atrial septum (upward direction) and atrial muscle (downward direction). E and F: calculated conduction times (E) from the SAN center to the atrial septum or atrial muscle and the CLs (F) with the corresponding I_Na,L values.