Gap junction heterogeneity as mechanism for electrophysiologically distinct properties across the ventricular wall

Abstract

Gap junctions are critical to maintaining synchronized impulse propagation and repolarization. Heterogeneous expression of the principal ventricular gap junction protein connexin43 (Cx43) is associated with action potential duration (APD) dispersion across the anterior ventricular wall. Little is known about Cx43 expression patterns and their disparate impact on regional electrophysiology throughout the heart. We aimed to determine whether the anterior and posterior regions of the heart are electrophysiologically distinct. Multisegment, high-resolution optical mapping was performed in canine wedge preparations harvested separately from the anterior left ventricle (aLV; n = 8) and posterior left ventricle (pLV; n = 8). Transmural APD dispersion was significantly greater on the aLV than the pLV (45 +/- 13 vs. 26 +/- 8.0 ms; P < 0.05). Conduction velocity dispersion was also significantly higher (P < 0.05) across the aLV (39 +/- 7%) than the pLV (16 +/- 3%). Carbenoxolone perfusion significantly enhanced APD and conduction velocity dispersion on the aLV (by 1.53-fold and 1.36-fold, respectively), but not the pLV (by 1.27-fold and 1.2-fold, respectively), and produced a 4.2-fold increase in susceptibility to inducible arrhythmias in the aLV. Confocal immunofluorescence microscopy revealed significantly (P < 0.05) greater transmural dispersion of Cx43 expression on the aLV (44 +/- 10%) compared with the pLV wall (8.3 +/- 0.7%), suggesting that regional expression of Cx43 expression patterns may account for regional electrophysiological differences. Computer simulations affirmed that localized uncoupling at the epicardial-midmyocardial interface is sufficient to produce APD gradients observed on the aLV. These data demonstrate that the aLV and pLV differ importantly with respect to their electrophysiological properties and Cx43 expression patterns. Furthermore, local underexpression of Cx43 is closely associated with transmural electrophysiological heterogeneity on the aLV. Therefore, regional and transmural heterogeneous Cx43 expression patterns may be an important mechanism underlying arrhythmia susceptibility, particularly in disease states where gap junction expression is altered.

Fig. 1.

Action potential duration (APD) dispersion is greater in the anterior left ventricular (aLV) than the posterior left ventricular (pLV) wall. Representative transmural APD profiles are plotted for the aLV (A) and pLV (C) with associated APD gradients (B and D) at baseline (squares) and carbenoxolone (circles) conditions. Summary data in E indicated that APD dispersion is higher across the transmural wall of the aLV compared with the pLV and is further enhanced upon the administration of the gap junction uncoupling agent carbenoxolone. Summary data of APD gradient in F illustrate that carbenoxolone amplifies the difference in APD gradient between the aLV and pLV.

Fig. 2.

Conduction velocity is heterogeneous in aLV but not pLV wall. Representative transmural conduction velocity is plotted for the aLV (A) and pLV (B) at baseline (squares) and carbenoxolone (circles) conditions. Conduction velocity is significantly slower in the epicardial (Epi) compared with the midmyocardial (Mid) layers in the aLV (A). Across the pLV, conduction velocity is uniform (B). Summary data in C indicate that conduction velocity dispersion is higher in the aLV compared with pLV.

Fig. 3.

Arrhythmia inducibility is greater in the aLV than the pLV wall. A representative example of a ventricular tachycardia is shown in A. Arrhythmias were typically self-terminating, lasting for 4 s or longer, and polymorphic in appearance. Under conditions of reduced coupling by the administration of carbenoxolone, the aLV exhibited significantly greater susceptibility to inducible ventricular tachycardia (B). NS, not significant.

Fig. 4.

Connexin43 (Cx43) is heterogeneously expressed across the aLV but not pLV wall. Representative immunofluorescence images from the Epi and Mid layers (A) and quantified Cx43 (B) reveal that across the aLV, there is less overall Cx43 signal in the Epi as compared with deeper muscle layers. The transmural Cx43 gradient as measured by the difference in Cx43 expression between the Epi and Mid layer in B, left inset, illustrates a greater heterogeneity of Cx43 expression across the transmural wall of the aLV compared with the pLV. Interestingly, total Cx43 expression levels in the aLV and pLV were similar (B, right inset). AU, arbitrary units.

Fig. 5.

Localized uncoupling across the Epi-Mid interface increases APD dispersion. The effect of homogenous (solid lines) and heterogeneous (dashed lines) coupling across the theoretical fiber (bottom) on the transmural APD profile is illustrated (top). Unlike uncoupling within the Epi (dashed blue line), uncoupling across the Epi-Mid interface (dashed black line) produces significant alterations in the transmural APD profile that are similar to the case of homogeneous uncoupling (red line). The effect of uncoupling position on APD dispersion is illustrated in the inset, which summarizes that uncoupling must cross the Epi-Mid interface to unmask APD differences inherent to cellular layers.

Fig. 6.

Coupling profiles of the aLV and pLV yield higher APD gradients across the aLV but not pLV wall. The effect of aLV and pLV Cx43 expression patterns on theoretical transmural APD gradient is illustrated for baseline and carbenoxolone conditions. The aLV has a larger APD gradient than the pLV, and this difference is further augmented when coupling is reduced to simulate carbenoxolone infusion.