R. Wilders

Gap Junctions in Cardiovascular Disease

Connexins, the protein molecules forming gap junction channels, are reduced in number or redistributed from intercalated disks to lateral cell borders in a variety of cardiac diseases. This gap junction remodeling is considered to be arrhythmogenic. Using a simple model of human ventricular myocardium, we found that quantitative remodeling data extracted from the literature gave rise to only small to moderate changes in conduction velocity and the anisotropy ratio. Especially for longitudinal conduction, cytoplasmic resistivity (and thus cellular geometry) is much more important than commonly realized. None of the remodeling data gave rise to slow conduction on the order of a few centimeters per second.

Effects of anisotropy on the development of cardiac arrhythmias associated with focal activity

Abstract. The anisotropy that normally exists in the myocardium may be either enhanced in peri-infarction zones by loss of lateral cell connections or reduced by redistribution of gap junctions. To test how the degree of anisotropy affects the development of ectopic focal activity, we carried out computer simulations in which a model of an ectopic focus is incorporated as the central element of a two-dimensional sheet of ventricular cells. At low values of intercellular coupling conductance (Gc), the focus region is spontaneously active, but the limited intercellular current flow inhibits propagation. At high Gc, automaticity is suppressed by the loading effects of the surrounding cells. At intermediate Gc, the ectopic activity may propagate into the sheet. In the case of isotropic coupling, the minimum size of the focus region for propagation to occur (in terms of number of collaborating cells within the focus) is as small as approximately ten cells, and this number decreases with increasing anisotropy. Thus, the presence of anisotropy facilitates the development of ectopic focal activity. We conclude that the remodeling that occurs in peri-infarction zones may create a substrate that either facilitates (enhanced anisotropy) or inhibits (reduced anisotropy) the development of cardiac arrhythmias associated with ectopic focal activity.

Quantitative analysis of dual whole-cell voltage-clamp determination of gap junctional conductance

Abstractu2002The dual whole-cell voltage-clamp technique is used widely for determination of kinetics and conductance of gap junctions. The use of this technique may, however, occasion to considerable errors. We have analysed the errors in steady state junctional conductance measurements under different experimental conditions. The errors in measured junctional conductance induced by series resistance alone, and by series resistance in combination with membrane resistance, were quantified both theoretically and experimentally, on equivalent resistive circuits with known resistance values in a dual voltage-clamp setup. We present and analyse a method that accounts for series resistance and membrane resistance in the determination of true junctional conductance. This method requires that series resistance is determined during the experiment, and involves some calculations to determine membrane resistance. We demonstrate that correction for both membrane and series resistance reduces the error in measured junctional conductance to near zero, even when membrane resistances on both sides of the gap junction are as low as 20xa0MΩ and the (true) junctional conductance is as high as 100xa0nS.

Action potential conduction between a ventricular cell model and an isolated ventricular cell

We used the Luo and Rudy (LR) mathematical model of the guinea pig ventricular cell coupled to experimentally recorded guinea pig ventricular cells to investigate the effects of geometrical asymmetry on action potential propagation. The overall correspondence of the LR cell model with the recorded real cell action potentials was quite good, and the strength-duration curves for the real cells and for the LR model cell were in general correspondence. The experimental protocol allowed us to modify the effective size of either the simulation model or the real cell. 1) When we normalized real cell size to LR model cell size, required conductance for propagation between model cell and real cell was greater than that found for conduction between two LR model cells (5.4 nS), with a greater disparity when we stimulated the LR model cell (8.3 +/- 0.6 nS) than when we stimulated the real cell (7.0 +/- 0.2 nS). 2) Electrical loading of the action potential waveform was greater for real cell than for LR model cell even when real cell size was normalized to be equal to that of LR model cell. 3) When the size of the follower cell was doubled, required conductance for propagation was dramatically increased; but this increase was greatest for conduction from real cell to LR model cell, less for conduction from LR model cell to real cell, and least for conduction from LR model cell to LR model cell. The introduction of this model clamp technique allows testing of proposed membrane models of cardiac cells in terms of their source-sink behavior under conditions of extreme coupling by examining the symmetry of conduction of a cell pair composed of a model cell and a real cardiac cell. We have focused our experimental work with this technique on situations of extreme uncoupling that can lead to conduction block. In addition, the analysis of the geometrical factors that determine success or failure of conduction is important in the understanding of the process of discontinuous conduction, which occurs in myocardial infarction.

Atrial fibrillation-induced gap junctional remodeling

With great interest we have read the study by Polontchouk et al. [(1)][1]on changes in the distribution of gap junctions—clusters of intercellular channels that allow action potentials to propagate from one cardiomyocyte to another—during atrial fibrillation (AF). In atrial tissue from chronic

Action potential transfer at the Purkinje-ventricular junction: role of transitional cells

At the Purkinje (P) - ventricular (V) junction a zone of transitional (T) cells is found. In the present study we investigated the role of these T cells in P-to-V conduction. Using the model clamp technique, an experimentally recorded rabbit P cell was coupled to a phase-2 Luo and Rudy (LR) model cell, which in turn was coupled to a strand of phase-2 LR model cells. In our experiments, the single LR model represents the T cell, while the strand of LR models represents subendocardial V cells. This approach enabled us to change selectively coupling conductance (G/sub c/) between cells, presence or T cell, and relative size of cells. We demonstrated that: 1) a decrease of G/sub c/ between P-T and T-V increases the delay of V activation, 2) the delay of V activation is importantly due to conduction between T and V cells, 3) there is a critical Cc for successful conduction at the P-V junction, 4) the critical value of Cc for conduction at the P-V junction is lower in presence (11.0/spl plusmn/0.7 nS) than in absence (13.7/spl plusmn/0.8 nS) of the T cell, and 5) enlargement of the T zone size hampers successful P-to-V conduction.

Modulation of propagation from an ectopic focus by electrical load and by extracellular potassium concentration

Cardiac arrhythmias may be due to spontaneous automatic activity from an ectopic focus. We have developed a technique in which a computer model of an ectopic focus (represented by an SA Node cell model), running in real time, can be coupled by a variable conductance, G/sub c/, to a real ventricular cell. We used this technique to investigate the effects of G/sub c/, cell size, and elevated potassium on the ability of an ectopic focus to successfully drive the ventricular cell. For the hybrid cell pair there are three possible outcomes in the steady state: (1) pacing of the SAN model cell but not driving of the ventricular cell, (2) cessation of pacing, or (3) successful pacing of the SAN model cell and driving of the ventricular cell. Elevation of potassium concentration increases both the lower and upper bound of the values of G/sub c/ which define the successful pacing and driving range.

Modeling of the long-QT syndrome type 1 and 2

Types 1 and 2 of the long-QT syndrome (LQTS/sub 1/ and LQTS/sub 2/) are caused by mutations in genes encoding the ion channels carrying the slow and rapid delayed rectifier potassium currents I/sub Ks/ and I/sub Kr/, respectively. Electrocardiographic dispersion in repolarization varies between these subtypes of the long-QT syndrome. We studied transmural dispersion in repolarization in a computer model of LQTS/sub 1/ and LQTS/sub 2/. Transmural activation was simulated in a linear strand of 500 human ventricular cells (300 subendocardial, 150 M-cells, and 50 subepicardial cells) that were arranged transversally and coupled at low or high normal intercellular coupling conductance (2.5 and 7.0 /spl mu/S, respectively). Missense mutations in LQTS/sub 1/ and LQTS/sub 2/ were simulated by setting I/sub Ks/ and I/sub Kr/ to 6.25% of the original value, respectively. To simulate truncating LQTS2 mutations, I/sub Kr/ was set to 50%. Repolarization moment was obtained by summation of the local activation time and the local action potential duration. Both at low and high normal intercellular coupling, the latest repolarization moments and the largest dispersion are found in the LQTS/sub 2/-missense simulation, whereas the earliest repolarization moments and the least dispersion are found in LQTS/sub 1/. This complies with our clinical data obtained from a group of 87 LQTS patients.

Electrical interactions between cardiac cells studied with coupling clamp and model clamp

In the early nineties, Joyner et al. introduced the coupling clamp technique in which an isolated cardiac cell can be electrically coupled to either another isolated cardiac cell or to an analog model cell (RC circuit). In brief, an amplifier system does a continuous analog computation of the current that would be flowing between the two cells if there had been an intercellular coupling conductance G/sub c/, and then provides current inputs to the cells accordingly. Building on this concept, the authors developed a computer-controlled coupling clamp system, as well as a model clamp system, in which an isolated cardiac cell is dynamically coupled in real time to a comprehensive mathematical cell model (e.g., the phase-2 Luo-Rudy model). With this system the authors have the ability to vary coupling conductance, effective size of both model cell and real cell, and intrinsic cellular properties of the model cell. The authors used the digital coupling clamp to study the synchronization of two sinoatrial nodal cells, and found that the critical value of coupling conductance required for 1:1 frequency entrainment was <0.5 nS, whereas G/sub c//spl ges/10 nS was required for waveform entrainment. The authors used the model clamp to assess alterations in the critical value of coupling conductance required for action potential conduction from a real ventricular cell to the Luo-Rudy model ventricular cell when the real cell was exposed to a solution mimicking acute ischemia. They observed that conduction was inhibited in the standard ischemic solution, but facilitated when noradrenaline was added to the ischemic solution.

Propagation of the cardiac action potential from an ectopic focus into a two-dimensional sheet of ventricular cells

We have carried out computer simulations in which a model of an ectopic focus is incorporated as the central element of a two-dimensional sheet of ventricular cells in which the coupling conductances may be different in the X and Y directions and a specific region of lack of coupling conductance may serve as a resistive barrier. We determined the critical size of the central element for successful propagation of its action potential into the sheet and found that this critical size was decreased when anisotropy was present compared to the isotropic case and was further decreased when the central site of stimulation was close to the resistive barrier. We conclude that the normal existence of anisotropy and enhancement of the degree of anisotropy under pathophysiological conditions may play a facilitating role in the development of ectopic foci which may lead to cardiac arrhythmias.