Microstructure, Cell-to-Cell Coupling, and Ion Currents as Determinants of Electrical Propagation and Arrhythmogenesis

HomeCirculation: Arrhythmia and ElectrophysiologyVol. 10, No. 9Microstructure, Cell-to-Cell Coupling, and Ion Currents as Determinants of Electrical Propagation and Arrhythmogenesis Free AccessReview ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessReview ArticlePDF/EPUBMicrostructure, Cell-to-Cell Coupling, and Ion Currents as Determinants of Electrical Propagation and Arrhythmogenesis Jan P. Kucera, MD, Stephan Rohr, MD and Andre G. Kleber, MD Jan P. KuceraJan P. Kucera From the Department of Physiology, University of Bern, Switzerland (J.P.K., S.R.); and the Department of Pathology, Harvard Medical School, Boston, MA (A.G.K.). , Stephan RohrStephan Rohr From the Department of Physiology, University of Bern, Switzerland (J.P.K., S.R.); and the Department of Pathology, Harvard Medical School, Boston, MA (A.G.K.). and Andre G. KleberAndre G. Kleber From the Department of Physiology, University of Bern, Switzerland (J.P.K., S.R.); and the Department of Pathology, Harvard Medical School, Boston, MA (A.G.K.). Originally published14 Sep 2017https://doi.org/10.1161/CIRCEP.117.004665Circulation: Arrhythmia and Electrophysiology. 2017;10:e004665Rapid electric impulse spread from the SA node to the atria, the AV node, and the ventricles is prerequisite for coordinated cardiac contraction. In cardiac arrhythmias, disturbed impulse spread can lead to ventricular or atrial tachycardia, fibrillation, and sudden cardiac death.A change in electric impulse propagation had already been proposed to underlie reentrant arrhythmias at the beginning of the 20th century.1 Slowing of propagation and the formation of unidirectional propagation block represent the 2 most important changes leading to circulating and reentrant propagation. Unidirectional propagation block is the prerequisite for the wavefront to reenter nonrefractory tissue of the original propagation path. Knowledge of the mechanisms governing abnormal impulse propagation and formation of propagation block at the level of cellular networks is important for our understanding of arrhythmogenesis and the principles underlying electric and drug therapy. This article reviews experimental and modeling studies performed to understand the basic mechanisms of cardiac impulse propagation, with specific emphasis laid on the relation between propagation and cardiac microstructure. Normally, layers and strands of electrically coupled myocytes separated by connective tissue create a mostly anisotropic compartmentation in atrial and ventricular myocardium. However, compartmentalization can assume pathological forms when fibrosis is enhanced with increasing age and during reparative fibrosis in pathological settings (myocardial infarction, volume/pressure overload, some forms of hereditary diseases). Although structural heterogeneities favor the formation of reentrant circuits, spiral waves occur in tissues with homogeneous electric and structural properties on the basis of time-dependent local changes in refractoriness, which set up the scenario for unidirectional block formation.2The selected references cited in this short review cannot cover the large body of published literature. Moreover, this article will cover neither the topic of spiral waves nor early and delayed afterdepolarizations (disturbed Ca2+ cycling), which play a major role in the initiation of cardiac arrhythmias.Continuous Electric Propagation in Cardiac Cell Strands: Effect of Depolarizing Ion Currents and Cell-to-Cell CouplingBasic Mechanism of Impulse Propagation and the Concept of Propagation SafetyHistorically, a simple model of cardiac impulse propagation was derived by drawing an analogy between a cardiac muscular trabecula, Purkinje fibers, and nonmyelinated nerves as cylindrically shaped excitable structures. In these models, the extracellular space is separated from the intracellular space by the excitable surface membrane of the cylinder.3 The simplified intracellular space lumps the cytoplasm of the cells and the cell-to-cell junctions into a single resistive compartment. With the advent of computer simulations and high-resolution optical mapping technology, more detailed models of cardiac tissue were established,.4,5 It became possible to define the role of gap junction channels in electric cardiac propagation experimentally and to compute the contribution of ion and gap junction channels to propagation.6,7 The model depicted in Figure 1A shows a simple chain of cardiomyocytes interconnected by electric resistors representing gap junction channels. Impulse propagation in this model is driven by upstream excitation of cardiac cells producing depolarizing current flowing axially into the downstream cells. This current, provided it is large enough, excites the downstream cells, which in turn deliver axial current to further downstream excitable elements. Accordingly, the wavefront separating excited (upstream) from not-excited (downstream) tissue propagates along the cell chain. As mentioned above, slow conduction and conduction block are major determinants for the initiation of reentrant excitation. Therefore, besides understanding what factors determine conduction velocity, an important question is how safe action potential propagation is under normal and pathological conditions. Shaw and Rudy6 provided a comprehensive approach to the computation of this so-called safety factor (SF), as illustrated in Figure 1B and as formulated in Equation 1:Download figureDownload PowerPointFigure 1. Model of a cardiac cell chain used to model cardiac propagation.A, Single cell chain; cells separated by resistors representing gap junction channels (top), and excitable elements (equivalent circuit) formed by a capacitor representing the lipid bilayer and a current generator (I) representing the lumped ion channels and exchangers in the membrane (bottom). B, With the wavefront approaching the cellular element, part of the axial current (blue) flows into the element and depolarizes the membrane to threshold for activation of Na+ and/or Ca2+ channels (left). Once activated, depolarizing inward current (red) initiates the action potential by delivering charge to the membrane capacitance and delivers axial current for excitation of downstream cells (right).Download figure ((1)) The denominator of this equation corresponds to the electric charge flowing from upstream into a given cell during excitation (Figure 1B, blue). The first term of the numerator refers to the capacitive charge forming the upstroke of the action potential (red), and the second term to the charge flowing downstream out of the cell (green) to excite the downstream cells and propagate the action potential. The integration range A is defined as the time window during which the membrane is being depolarized.6 Intuitively, this definition is straightforward: propagation is safe (SF>1), if the denominator is smaller than the numerator, that is, if the axial (or so-called electrotonic) current required to excite a given downstream cell is smaller than the ion current generated by this same cell. Over the years, several algorithms for describing the margin of safety of propagation have been proposed.6,8–10 Although the definition given by Equation 1 is very useful for understanding the general concept of propagation safety, algorithms describing propagation safety in multidimensional or discontinuous tissues are necessarily more complex.9,10Effect of Depolarizing Ion Currents and Cell-to-Cell Uncoupling on Impulse PropagationIn pathological settings, ion channel inhibition and cell-to-cell uncoupling are the main causes underlying cardiac propagation slowing.7The effects of reducing the Na+ current versus electric cell-to-cell coupling on SF and propagation velocity are illustrated in Figure 2 taken from a computational study of propagation in a chain of single myocytes.6 There is a striking difference between the 2 effects, which is best explained by looking at the change in SF. Reducing Na+ current leads to a monotonic decrease in SF with propagation block occurring at ≈85% reduction of INa. Propagation velocity at which propagation block occurs is still relatively high, ≈15 cm/s. Reducing cell-to-cell coupling increases SF markedly (up to ≈2-fold). As a consequence, propagation becomes safer (more stable) and, therefore, can decrease to very low values in the order of a centimeter per second before the SF drops abruptly and propagation block occurs.Download figureDownload PowerPointFigure 2. Effect of Na+ channel inhibition and cell-to-cell uncoupling on the safety factor (SF) and velocity of electric propagation (computer model of a cell chain).A, Decrease of cell-to-cell coupling conductance (gj) and inhibition of Na+ channel conductance (gNa) have opposite effects on SF Reduction of gNa causes a monotonic decrease of SF with conduction block occurring at approximately 85% inhibition. Cell-to-cell uncoupling renders propagation safer with propagation block occurring only beyond an >100-fold decrease of gj. B, Na+ channel inhibition causes a continuous decrease in propagation velocity and abrupt block at a relatively high velocity (15 cm/s at 85% inhibition). By contrast, propagation block develops only beyond a 100-fold decrease of gj, at velocities in the order of 1 cm/s. Note the logarithmic scale of the abscissa for gj. Reprinted from Shaw and Rudy6 with permission of the publisher. Copyright © 1997, American Heart Association, Inc.In accordance with theoretical work, marked differences between the effects of Na+ channel inhibition and decrease in cell-to-cell coupling on propagation have been reported in experimental studies. Figure 3A illustrates propagation in engineered strands of neonatal rat cardiomyocytes in which tetrodotoxin has been added to inhibit inward Na+ current (INa).11 This inhibition led to propagation carried by the L-type slow inward Ca2+ current (ICa,L) at a velocity of 13 cm/s. In contrast to hearts of small rodents, inhibition of INa in large mammalian hearts by perfusion with elevated extracellular K+ (partial Na+ channel inactivation because of the elevation of resting membrane potential) produces propagation block in longitudinal direction of muscular fibers at 40 and 20 cm/s in transverse direction.12 Slow propagation carried by ICa,L at 12 cm/s is observed in these hearts after addition of norepinephrine.12Download figureDownload PowerPointFigure 3. Effect of Na+ channel inhibition and cell-to-cell uncoupling on safety factor and velocity of electric propagation (experimental study).A, Engineered strand of neonatal rat ventricular myocytes. Action potential upstrokes are measured by a voltage-sensitive dye. Left, Rapid action potential upstrokes during normal propagation at a velocity of 43 cm/s. Right, INa inhibition (tetrodotoxin): upstrokes carried by the L-type Ca2+ currents are slow, propagation velocity, Θ, has decreased to 13 cm/s. B, Left, Column plots depicting the intercellular conductance, gj, in pairs of fetal murine ventricular myocytes. Genetic ablation of Cx43 produces a >90% decrease of gj. Right, Strand of fetal murine ventricular myocytes engineered from Cx43−/Cx43− cells. The cell from which the cluster of action potential upstrokes represented in blue were recorded was excited 1026 μs after the previous upstream cell (red upstrokes). This saltatory type of propagation produces a very slow propagation velocity of 2.1 cm/s. Reprinted from Rohr et al11 and Beauchamp et al13 with permission of the publisher. Copyright © 1998, 2004, American Heart Association, Inc.Propagation slowing because of cell-to-cell uncoupling is shown in Figure 3B. Experimentally, uncoupling was achieved by either adding an uncoupling agent (palmitoleic acid11) or genetically ablating connexin43 (Cx43) in engineered fetal murine myocytes.13 As illustrated in Figure 3B, genetic ablation of Cx43 leads to a reduction of intercellular electric conductance (gj), by >90% (the small remaining conductance being because of the presence of Cx4513,14) and to a decrease in propagation velocity to very slow values (2.1 cm/s).The increase of the safety of propagation with cell-to-cell uncoupling is explained by a changed relation between the upstream source of axial current and the downstream sink: the increase in cell-to-cell resistance will, on the one hand, slow the charging of the membrane capacitance by axial current flow and cause slow propagation (Figures 1B and 2). On the other hand, it will decrease the downstream current sink (ie, increase its impedance) and constrain the axial current to a smaller region downstream. This protective effect of increased downstream impedance has been recognized early on and was used to explain, for instance, why a small SA node can excite a large atrium.15,16Effect of Structural Discontinuities on Propagation: Interaction Between Microstructure, Ion Currents, and Cell-to-Cell CouplingThe mechanisms explained above can now be used to discuss the effect of the cardiac microstructure on propagation. This discussion is important because, as will be shown, the discontinuous structure is, besides intercellular coupling and depolarizing current flow, a third important determinant of cardiac propagation.Both normal atrial and ventricular myocardium develop as discontinuous structures. Atria show large areas of trabeculation; ventricular myocardium shows a highly organized laminar structure with tissue layers that are 4 to 6 cells thick being wrapped around the left ventricular cavity.17–19 These layers are tilted with respect to one another and connected by small muscular bridges (Figure 4A). Moreover, a network of muscular trabeculae outlines parts of the ventricular and atrial cavities. An increase of the amount of fibrous tissue separating muscle layers at a smaller scale is observed with increasing age,20 in myocardial infarction, and other diseases.21Download figureDownload PowerPointFigure 4. Continuous versus discontinuous propagation.A, Transmural section across the left ventricular wall of a pig heart showing laminar architecture. Adapted from Hooks et al19 with permission of the publisher. Copyright © 2007, American Heart Association, Inc. B, Propagation in canine atrial trabecula with continuous structure (Ba, young dog) vs discontinuous structure (Bb, aged dog). The structural differences are visualized in the histological sections (right) showing the presence of fine fibrous septa between the longitudinally oriented cardiac fibers. The extracellular electrograms in Ba are of a smooth shape with a single instrinsic deflection, typical for a homogeneous wavefront. The electrograms in Bb are fractionated, indicating heterogeneous transverse impulse spread. Slightly adapted from Spach et al22 with permission of the publisher. Copyright © 1982, American Heart Association, Inc. Authorization for these adaptations have been obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.Basic Rules Governing Discontinuous Cardiac PropagationExperimental and theoretical studies on discontinuous electric propagation in the heart were pioneered by Spach et al.22–26 A result from an early experiment by is depicted in Figure 4B.22 It shows that increasing fibrosis leads to heterogeneous impulse spread in atrial trabeculae, a change that the same group later associated with microreentry in human atria.26With the introduction of voltage-sensitive dyes to monitor action potentials from multiple sites at high resolution, it became possible to follow electric excitation as it propagates across regions of discontinuous tissue geometries and to compare experimental findings with theoretical results obtained from computer modeling.4,9,27 Impulse propagation in these basic geometric patterns share common features characterized by a mismatch between the source (excited tissue) and the sink (resting tissue), which may consist in a single mismatch or in recurrent mismatches at small spatial intervals.The main differences between continuous and discontinuous propagation relates to the fact that, in discontinuous propagation, the effects of depolarizing inward currents (INa and ICa,L) and cell-to-cell coupling become interdependent. The complex role of depolarizing ion currents in discontinuous cardiac structures has been recognized in experimental settings28 and in theoretical work.6,29Figure 5Aa illustrates propagation from a narrow strand of cultured neonatal rat myocytes into a large bulk of tissue.28 At the geometric transition, the source-to-sink mismatch produces a local delay in the sequence of the action potential upstrokes indicating locally delayed propagation. In Figure 5Ab, the width of the narrow cell strand was reduced, which resulted in a larger source-to-sink mismatch that produced a local delay >2 ms. As a consequence, excitation of the bulk tissue occurs at a time when the action potentials of the upstream strand have reached their peak. At this instant, a large part of Na+ channels upstream is inactivated and ICa,L flowing during the early action potential plateau becomes pivotal for eliciting action potentials downstream. Under these conditions, inhibitors of ICa,L blocked propagation at the site of the source-to-sink mismatch. When the source-to-sink mismatch was made large enough to induce unidirectional conduction block, enhancers of ICa,L rescued propagation (Figure 5Ac). The role of ICa,L in discontinuous propagation has further been demonstrated in theoretical and experimental studies describing action potential transfer between a cell pair, either an isolated myocyte and a virtual (simulated) myocyte connected by a resistor or 2 simulated myocytes.30,31 Common to all of these experiments is the demonstration that local propagation delays shift the role of ion currents in the upstream driver cell(s) from a current with fast kinetics (INa) to a current with slower kinetics (ICa,L). In addition to ICa,L, transient outward current, Ito, can modulate propagation at sites of significant propagation delay.32Download figureDownload PowerPointFigure 5. Cardiac tissue discontinuities: role of L-type Ca2+ current and cell-to-cell coupling.A, Left, a, Source-to-sink mismatch at a transition from a small bundle to a large bulk of tissue in a patterned culture of neonatal rat ventricular myocytes. Middle, b, Forward propagation from a narrow strand to the bulk tissue produces a local propagation delay >2 ms. Nifedipine, an inhibitor of ICa,L blocks propagation across at the transition. Right, c, Complete block at the transition because of source-to-sink mismatch under control conditions (strand <50 μm in width). Enhancement of ICa,L with Bay K 8644 restores anterograde propagation. B, Effect of partial cell-to-cell uncoupling at a site of source-to-sink mismatch. Ba, Source-to-sink mismatch produces block at the transition of a narrow cell strand (≤50 μm) to the bulk tissue (red: excited tissue; blue: nonexcited tissue). Bb, Partial uncoupling by local superfusion with palmitoleic acid restores propagation across the expansion. Bc, Full local uncoupling causes conduction block. Bd, Graph from computer simulations showing that restoring propagation depends on the increase of cell-to-cell resistance transverse to the main axis of the strand, whereas changes in the longitudinal resistance do not affect block formation (hc=width of the strand at which block occurs). Reprinted and adapted from Rohr and Kucera,28 Fast and Kléber,33 and Rohr et al34 with permission of the publishers. Copyright © 1997, Elsevier. Copyright © 1995, American Heart Association, Inc. Copyright © 1997, The American Association for the Advancement of Science. Authorization for these adaptations have been obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.Similarly, electric propagation at a site of source-to-sink mismatch is affected by the degree of local cell-to-cell coupling.33,34 This effect has been termed as paradoxical34 because, as shown in Figure 5B, partial uncoupling of tissue restores propagation at a site where a marked source-to-sink mismatch produced block during normal cell-to-cell coupling. This suggests that cell-to-cell coupling has complex effects on conduction in tissues with discontinuous structures.34 On the one hand, partial gap junctional uncoupling decreases propagation velocity; on the other hand, it stabilizes propagation and may restore transmission of electric impulses at the sites of block. These findings challenge the paradigm that drugs enhancing cell-to-cell coupling exert exclusively antiarrhythmic effects via an increase in propagation velocity as they may simultaneously precipitate unidirectional conduction blocks.As shown in Figure 6, propagation across tissues with repetitive branches (branching tissue, such as found in infarct scars and, possibly, in the AV node) can be perceived as a process where a small source excites a large sink, and once excited, the large sink functions as the large source to excite a smaller downstream sink. Accordingly, this structure produces slow and, at the same time, safe conduction. In the presence of elevated extracellular potassium, conduction velocities assume values in the range of a few centimeters per second thereby approaching values measured in uniform tissue structures during critical gap junctional uncoupling.35 The biophysical basis of the effect alternation of opposite mismatches as found in experiments has been defined in a theoretical study.9Download figureDownload PowerPointFigure 6. Propagation in Propagation in a strand 2 branches The structure is illustrated on with to the of used to show activation in as As excitation the propagation by the of activation This effect is enhanced in the presence of extracellular a that INa. B, slowing propagation. of propagation velocity in the main strand on the of the The current sink (or increases up to a of 1 Reprinted from Kucera et with permission of the publisher. Copyright © 1997, American Heart Association, of Propagation in Discontinuous of ion currents in the of the action potential has been for several In the of the major depolarizing ion currents for INa and ICa,L, from inactivation of the ion channels and for the excitation on the membrane potential from which excitation and on the resting membrane Although the of depolarizing inward current and propagation velocity on resting membrane potential is complex and such as the of membrane resistance on the K+ and of it is that to more than leads to marked in Na+ inward conduction and propagation source-to-sink as by heterogeneities in tissue the of Slowing of propagation across an or an is on the degree of structural and the of excitation. Figure shows that of a strand of patterned neonatal ventricular myocytes that into a bulk of cells produces increasing local propagation delays at the site of source-to-sink mismatch propagation blocked after a of This (Figure is markedly enhanced with increasing source-to-sink A further factor to of excitation is the of intracellular Na+ associated with increased changes in intracellular ion will that the action potential and the time This effect increases and restores propagation (Figure These theoretical results have been by experimental figureDownload PowerPointFigure Effect of heterogeneous tissue structure on of propagation.A, Propagation across a tissue in cultured neonatal rat ventricular myocytes. is the effect of a change in basic from a of to for With at a of conduction delays increase block occurs after the The time associated with block partial of the tissue in the and, the activation produces a relatively short conduction delays and block occurs after 6 This 40 when propagation is The shows the degree of block as a of the B, Graph taken from experiments in which a narrow was into an taken from of a Propagation blocked to a on the width of the At a of propagation is blocked second at an width and propagation is observed at in at leads to a of conduction on microstructure with of propagation only at an width >2 Reprinted from et and et with permission of the publishers. Copyright © 1997, American Heart Association, Inc. Copyright © 2007, American of propagation in tissues with heterogeneous structure is an important Once an is and is markedly local propagation delays and blocks are to to sites of source-to-sink mismatch in a on the specific microstructure. at these are to be out of producing complex excitation patterns that change from to in Cardiac With discontinuous conduction can at the cellular scale from the intercellular of important of tissue is heterogeneous gap junctional coupling. a may for because of intercellular differences in the to to gap junctional coupling can from of Cx43 leading to a genetic of cardiac tissue to be A contribution of heterogeneous cell-to-cell coupling to arrhythmogenesis has been in with heart Experimentally, the of a of gap junctional coupling has been in the ventricles of in which heterogeneous activation patterns were and in strands of and murine ventricular In the conduction velocity optical mapping decreased with the of cells and at a velocity of 2 to cm/s that was by experiments that the between velocity and cell is and that conduction in cell strands in which the 2 cellular are is to into the mechanisms of conduction slowing and block were obtained from computer simulations a detailed high-resolution model in which the cellular of the tissue and the of cell were taken into In tissue of a of and cells, conduction was highly discontinuous with of myocytes being very whereas activation was This heterogeneous was by conduction myocytes between and connected regions from the of the 2 cell This led to large and very mismatches that conduction and increased the of block. These results that conduction in heterogeneous tissue is from conduction in uniform and of Currents With Propagation above, structural and is associated with a of local current and a mismatch between the upstream source and the downstream sink leading to a change in the between excited and resting In contrast to a where excited tissue