Enhanced transmural fiber rotation and connexin 43 heterogeneity are associated with an increased upper limit of vulnerability in a transgenic rabbit model of human hypertrophic cardiomyopathy

Abstract

Human hypertrophic cardiomyopathy, characterized by cardiac hypertrophy and myocyte disarray, is the most common cause of sudden cardiac death in the young. Hypertrophic cardiomyopathy is often caused by mutations in sarcomeric genes. We sought to determine arrhythmia propensity and underlying mechanisms contributing to arrhythmia in a transgenic (TG) rabbit model (beta-myosin heavy chain-Q403) of human hypertrophic cardiomyopathy. Langendorff-perfused hearts from TG (n=6) and wild-type (WT) rabbits (n=6) were optically mapped. The upper and lower limits of vulnerability, action potential duration (APD) restitution, and conduction velocity were measured. The transmural fiber angle shift was determined using diffusion tensor MRI. The transmural distribution of connexin 43 was quantified with immunohistochemistry. The upper limit of vulnerability was significantly increased in TG versus WT hearts (13.3+/-2.1 versus 7.4+/-2.3 V/cm; P=3.2e(-5)), whereas the lower limits of vulnerability were similar. APD restitution, conduction velocities, and anisotropy were also similar. Left ventricular transmural fiber rotation was significantly higher in TG versus WT hearts (95.6+/-10.9 degrees versus 79.2+/-7.8 degrees; P=0.039). The connexin 43 density was significantly increased in the mid-myocardium of TG hearts compared with WT (5.46+/-2.44% versus 2.68+/-0.77%; P=0.024), and similar densities were observed in the endo- and epicardium. Because a nearly 2-fold increase in upper limit of vulnerability was observed in the TG hearts without significant changes in APD restitution, conduction velocity, or the anisotropy ratio, we conclude that structural remodeling may underlie the elevated upper limit of vulnerability in human hypertrophic cardiomyopathy.

Figure 1

Sample vulnerability grids from a WT and TG heart showing the outcome at each shock strength and CI combination.

Figure 2

Optical imaging data during shock application. A, Photograph of a Langendorff-perfused WT heart with the optical field of view indicated with a red square. B, Map of VEP at shock end (8 V/cm, 79% APD). Red and blue regions represent depolarized and hyperpolarized tissue, respectively. C, Activation map showing the shock-induced spread of reentrant activation. Isochrones are 4 ms apart. D, Optical action potentials recorded from the location are indicated with a black square in B. Shock-induced deexcitation is observed at this site, and the tissue is immediately reexcited by reentrant activation. The arrhythmia self-terminates after several beats. E through H, Similar images as A through D for a TG heart. In this case, the shock (8 V/cm, 86% APD) produced a sustained arrhythmia.

Figure 3

APD restitution curves. A, APD vs diastolic interval (DI) for each photodiode pixel before normalizing. The different colors represent different S2 pacing intervals. The points within each color represent all photodiode pixels at each S2 interval. B, APD restitution curve after normalizing and fitting with a biexponential function (see the online data supplement for details). C, The steepest portion of the restitution curve shown in the red box in B, along with a linear fit. Comparisons between TG and WT animals revealed no significant differences for either fitting method.

Figure 4

Epicardial conduction velocities. A, Sample activation map and conduction velocity vectors near the pacing site (field of view similar to those shown in Figure 2). B, Magnitude and direction of each conduction velocity vector and sinusoidal fit (see text and the online data supplement for details).

Figure 5

Diffusion tensor MRI. A, Mid-ventricle short-axis slices from a WT and TG heart showing the helix angle (α). sep indicates septal; post, posterior; lat, lateral; ant, anterior. B, Average helix angle from all hearts at 10 steps across the LV wall from endocardium to epicardium. The TG hearts were found to have a significantly larger helix angle compared with WT hearts (95.6 ± 10.9° vs 79.2 ± 7.8°; P = 0.039).

Figure 6

Transmural expression of Cx43. A, Sample confocal images from a WT and TG heart taken from endo-, mid-, and epicardial locations. B and C, Individual Cx43 densities obtained from each WT and TG heart studied. D, Average Cx43 densities at each tissue location. All Cx43 densities are listed in supplemental Table IV.