Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts

Abstract

Loss of connexin43 (Cx43) gap junction channels in the heart results in a marked increase in the incidence of spontaneous and inducible polymorphic ventricular tachyarrhythmias (PVTs). The mechanisms resulting in this phenotype remain unclear. We hypothesized that uncoupling promotes regional ion channel remodeling, thereby increasing electrical heterogeneity and facilitating the development of PVT. In isolated-perfused control hearts, programmed electrical stimulation elicited infrequent monomorphic ventricular tachyarrhythmias (MVT), and dominant frequencies (DFs) during MVT were similar in the right ventricle (RV) and left ventricle (LV). Moreover, conduction properties, action potential durations (APDs), and repolarizing current densities were similar in RV and LV myocytes. In contrast, PVT was common in Cx43 conditional knockout (OCKO) hearts, and arrhythmias were characterized by significantly higher DFs in the RV compared to the LV. APDs in OCKO myocytes were significantly shorter than those from chamber-matched controls, with RV OCKO myocytes being most affected. APD shortening was associated with higher levels of sustained current in myocytes from both chambers as well as higher levels of the inward rectifier current only in RV myocytes. Thus, alterations in cell-cell coupling lead to regional changes in potassium current expression, which in this case facilitates the development of reentrant arrhythmias. We propose a new mechanistic link between electrical uncoupling and ion channel remodeling. These findings may be relevant not only in cardiac tissue but also to other organ systems where gap junction remodeling is known to occur.

Figure 1

MVT in CTR mice. A) Activation map recorded during MVT from the anterior view. B) Pseudo-ECG shows stable repetitive activity. C) FFT of the pseudo-ECG shows the power spectra and a dominant frequency of 28 Hz.

Figure 2

Regional electrophysiological characteristics of control mice. CV measurements from the LV and RV of CTL hearts are shown. A) Anisotropic ratio measured in the RV and LV. B) No significant differences with respect to CV or anisotropic ratios between the RV and LV were found. C) APs recorded from right (dashed line) and left (solid line) ventricular myocytes isolated from CTL. D) Comparison of APDs measured from RV and LV myocytes of CTL myocytes.

Figure 3

Potassium current levels recorded from CTL RV and LV myocytes. A, B) Representative current traces from RV (A) and LV myocytes (B). Insets show voltage clamp protocols. C) Peak I_to recorded from the RV and LV. Peak outward I_to densities in CTL myocytes are higher in the RV compared to the LV. D) I_sus levels recorded from RV and LV myocytes are similar. E, F) Representative IK1 current traces from RV (E) and LV myocytes (F). G) I_K1 levels measured in the RV and LV of CTL hearts.

Figure 4

MTC in OCKO mice. A–D) Activation maps of an MVT recorded from the RV (A), anterior (B), LV (C), and posterior views (D). The reentrant source can be seen on the anterior surface, with the center of rotation positioned near the junction of the RV and LV (*). E) Pseudo-ECG obtained from the movie shown in A, showing stable monomorphic activity. F) Pseudo-ECG obtained from the movie shown in C. G) Power spectrum of the movie shown in A shows a highly periodic signal with a dominant frequency at 27 Hz. H) Power spectrum obtained from the movie shown in C. Color scale indicates 0–35 ms for A and C; 0–22 ms for B and D.

Figure 5

Dominant frequency maps of the RV and LV during polymorphic VT in OCKO mice. A, B) DF map obtained during PVT from the RV (A) and LV (B). C, D) Pseudo-ECG recordings obtained from the RV (C) and LV (D). E, F) Power spectra of the pseudo-ECG for the RV (E) and LV (F).

Figure 6

RV- and LV-dominant frequencies recorded during monomorphic (A) and polymorphic (B) arrhythmias. During MVT, the average dominant frequency was not significantly different between the RV and LV (RV DF=27.3±1.7; LV DF=26.8±1.3; n=6; P=0.84). Similar dominant frequencies were recorded from the LV during PVT; however, significantly higher dominant frequencies were recorded in the RV (RV DF=33±2.5; LV DF=26±2.5; n=5; P=0.016).

Figure 7

RV remodeling in OCKO hearts. A) LV and RV CV measurements in OCKO hearts. CV was reduced by ~50% in both the RV and LV of OCKO mice compared to CTLs (Fig. 2). B) Anisotropic ratios in the LV and RV of OCKO hearts. However, no significant differences with respect to CV or anisotropic ratios between the RV and LV were found. C) Examples of Aps recorded from right (dashed line) and left (solid line) ventricular myocytes isolated from OCKO hearts. D) Comparison of APDs measured from right and left ventricular myocytes of OCKO hearts. Significant differences in APDs measured at 50, 70, and 90% repolarization were found in the OCKO myocytes.

Figure 8

Remodeling of potassium currents in the OCKO RV myocytes. A, B) Representative current traces from RV (A) and LV myocytes (B). Insets show voltage clamp protocols. C) I_to levels in the RV and LV. D) I_sus levels recorded from RV and LV myocytes. E, F) Representative I_K1 current traces from RV (E) and LV myocytes (F). G) I_K1 levels recorded from RV and LV myocytes.