Remodeling in cells from different regions of the reentrant circuit during ventricular tachycardia

Abstract

Background: Anisotropic reentrant excitation occurs in the remodeled substrate of the epicardial border zone (EBZ) of the 5-day infarcted canine heart. Reentry is stabilized because of the formation of functional lines of block. We hypothesized that regional differences of ionic currents in cells of the EBZ form these lines of block. Therefore, we first mapped reentrant circuits of sustained tachycardias, then dispersed cells (infarct zone cells, IZs) from the central common pathway of the circuit (IZc) as well as from the other side of the line of block (outer pathway, IZo) for study.

Methods and results: We mapped reentrant circuits in the EBZ of infarcted hearts during sustained ventricular tachycardias (>30 seconds, n=17 episodes, cycle lengths=218+/-7.9 ms). INa density was reduced in both IZc and IZo, and the kinetic properties of IZc INa were markedly altered versus IZo. Structural remodeling of the sodium channel protein Nav1.5 occurred in IZs, with cell surface localization differing from normal cells. Both IZc and IZo have similar but reduced ICaL, whereas IZc showed changes in Ca2+ current kinetics with an acceleration of current decay. Computer simulations of the 2D EBZ showed that incorporating only differences between INa in IZc and IZo prevented stability of the reentrant circuit. Incorporating only differences between ICaL in the IZc and IZo cells also prevented stability of the circuit. However, incorporating both INa and ICaL current differences stabilized the simulated reentrant circuit, and lines of block formed between the 2 distinct regions.

Conclusions: Despite differences in INa and ICaL properties in cells of the center and outer pathways of a reentrant circuit, the resulting changes in effective refractory periods tend to stabilize reentry in this remodeled substrate.

Figure 1

A, Activation map of 1 reentrant beat in the 5-day EBZ during a sustained VT (top). Isochrones (10-ms intervals) and arrows show path of excitatory wave fronts. Activation during the time window shown begins in the central common pathway between the 2 thick parallel lines of functional conduction block and moves upward toward the base of the heart (10 to 60 ms). Two wave fronts then move around the outer aspects of the lines of block, to the left and to the right (60 to 120 ms, arrows) and reenter the central common pathway at the 120-ms isochrone. Cells were made from 2 different regions of this reentrant path, the central and the outer pathways (noted by dotted boxes). ECG scale bar is 100 ms. B, Electrograms at indicated sites during sustained VT of A. Arrow indicates pattern of excitation.

Figure 2

A, Average peak I_Na in each of the 3 cell groups, NZ normal cells from noninfarcted epicardium, n=45, N=35), IZc (center pathway cells, n = 16, N=12), IZo (outer pathway cells, n=14, N=10). B, Family of Na current tracings from a cell in each group. Clamp protocol shown below. C, Average IZs for all cell groups. IZc (squares), IZo (circles), and NZs (triangles). At each test voltage (VT), mean±SEM is plotted. All data were collected at similar times after whole-cell membrane rupture (NZs, 22.3±1.0 minutes; IZc, 25.4±3.0 minutes; IZo, 24.4±1.8 minutes).

Figure 3

A, Cell staining for the α-subunit of the cardiac sodium channel, Na_v1.5. Note that in the NZ, the optical plane through the surface shows SL staining (pseudocolor is reddish orange) that has a particular pattern. Notably, the SL shows robust, nearly uniform staining. An enlargement of SL pattern (see inset) shows that cell surface is stained except for every 1.9 µm. This is consistent with T-tubular invaginations in NZs. The subsurface optical plane shows robust SL staining but no staining in the cell core (blue), consistent with lack of staining of T-tubular membranes. Calibration bar is 25 µm. In the IZ, SL staining is present but nonuniform in both planes. Gap junction staining remains in this IZc. B, The heights of the bars depict the frequency of NZ (n=26), IZc (n=29), and IZo (n=28) that showed either uniform SL and gap pattern, nonuniform SL and gap pattern, or no staining at all (none). All cells were prepared and viewed in a similar manner. For each cell, optical planes were taken at several layers of the cell.

Figure 4

A, Ca²⁺ currents in NZ, IZo, and IZc under conditions of these experiments: 5 mmol/L Ca²⁺; 10 mmol/L EGTA; holding voltage, −70 mV to various test voltage as shown. B, Average peak I_Ca density-voltage relations in NZ (filled circles) (n=25, N=11), IZo (unfilled circles) (n=18, N=13), and IZc (filled triangles) (n=22, N=16). Data were collected at similar times after whole-cell membrane rupture (NZ, 18±1.3; IZo, 16±0.5; IZc, 18±0.3 minutes; P>0.05). *P<0.05 vs NZ. C, Average time constants (tau1, τ₁) of decay of I_CaL at the test pulse after various conditioning prepulse voltage (V_cs) in NZ, IZo, and IZc. Average τ₁ values are plotted as a function of conditioning prepulse Vc. The protocol is shown in the inset. *P<0.05 vs NZ, #P<0.05 vs IZo.

Figure 5

A, Simulated I_Na density-voltage curves for IZc (solid line) and IZo (dashed line). B, Activation and steady-state inactivation curves. Same voltage-clamp protocols and sodium concentrations as used experimentally. Rate constants were estimated from values measured at room temperature to simulate currents at a physiological temperature of 37°C. C, Single cell AP for IZc (solid line) and IZo (dashed line) with I_Na differences incorporated. Temperature 37°C. [Na]_o=140 mmol/L. [Na]_i=10 mmol/L. D, Table of values obtained with simulations. E, Single-cell AP computer model of IZc (solid line) and IZo (dashed line) with both I_Na and I_CaL current changes incorporated (CL=300 ms). F is table of values obtained from 1D simulations (CL=300 ms).

Figure 6

Simulated reentrant wave initiated at the boundary between the center and outer regions. A, Consecutive isochronal maps of the last 3 beats of the nonstationary reentrant wave rotating. The vertical dashed line indicates the separation between the center (left) and outer (right) regions. Isochrones are drawn every 20 ms. Asterisks indicate the location of the first isochrone for each beat. Right, Trajectory of each of the spiral tips during the 6 beats of the reentering wave. B, APs calculated at sites a–h indicated in the isochronal maps in A. The scale bar is 100 ms. Numbers in parentheses next to each AP indicate the beat number in A maps. C, Reentrant wave initiated at the boundary between the center and outer regions in the computer model. Consecutive isochronal maps of the last 5 beats of the stationary reentrant wave. D, APs calculated at sites a–h indicated by the black dots in the isochronal maps in C. The scale bar is 200 ms. Numbers in parentheses next to each AP indicate the beat number in maps in C.

Figure 7

A, Dynamics of action potentials during simulated VT when remodeled I_CaL and I_Na are considered. B depicts the changes in I_Na during these beats. C depicts the dynamics of I_CaL during these beats. In all panels, dotted lines are data from IZo and solid lines are from IZc.

Figure 8

Summary of CL changes during simulated beats when only I_Na differences were used, when only I_CaL differences were incorporated, and for simulations including both inward current differences in the center and outer pathway. These data have been obtained from movies shown in Data Supplement.