Cellular Arrhythmogenic Effects of Congenital and Acquired Long-QT Syndrome in the Heterogeneous Myocardium
Background—Certain alterations by mutations or drugs of the potassium currents IKs and IKr and the sodium current INa give rise to several types of the long-QT syndrome. IKs is heterogeneously distributed across the ventricular wall.
Methods and Results—We investigated the effects of reducing IKs or IKr or enhancing late INa (to simulate the 3 forms of long-QT syndrome) on action potential duration (APD) in the context of IKs heterogeneity. We introduced IKs heterogeneity in the Luo-Rudy dynamic cell model to simulate epicardial, endocardial, and midmyocardial (M) cells. Results demonstrated higher susceptibility of M cells to the development of arrhythmogenic early afterdepolarizations (EADs) in isolated cells and poorly coupled tissue. An important observation is that IKr block or late INa acts to increase APD differences between the cell types, whereas IKs block minimizes such differences. Also, for normal transverse coupling, EADs develop in the endocardial region rather than in the M region as the result of strong electrotonic interaction.
Conclusions—IKs heterogeneity and intercellular coupling strongly influence EAD development during interventions or disorders that prolong APD. M cells in isolation or in poorly coupled tissue are more susceptible to EAD development than epicardial or endocardial cells. In well-coupled myocardium, EAD formation in the subendocardium can be the source of focal arrhythmias or provide the trigger for reentrant excitation. The efficacy of IKs block in minimizing APD dispersion could have important implications for antiarrhythmic therapy.
Asubpopulation of cells (midmyocardial [M] cells) has been described1 2 in the ventricular wall. These cells display a longer action potential duration (APD) and a steeper dependence of APD on rate than epicardial or endocardial cells. M cells display greater responsiveness to interventions that prolong APD (eg, agents with class III antiarrhythmic action) and a higher susceptibility to the development of arrhythmogenic early afterdepolarizations (EADs).3 These properties suggest that M cells play an important role in arrhythmias associated with abnormal repolarization such as the congenital or acquired long-QT syndrome (LQTS). We recently showed that differences in IKs:IKr density ratios result in heterogeneity of the repolarization properties of cells.4 A smaller density of IKs results in longer APD and its greater prolongation with slowing of rate, an experimentally observed behavior typical of M cells.5
The density of functional channels can be altered by disease, as occurs in the congenital LQTS, in which mutations in the genes that encode IKs (KvLQT1 or MinK) or IKr (HERG) act through a dominant negative mechanism to cause a loss of function of IKs (LQT1) or IKr (LQT2), thereby slowing action potential repolarization. LQT3 involves mutations in the SCN5A gene that cause incomplete inactivation of INa and a persistent inward current that prolongs APD. Prolonged repolarization also can be acquired. Methanesulfonanilides (E-4031) block IKr and chromanol 293B blocks IKs, whereas the neurotoxin anthopleurin A slows INa inactivation, resulting in a sustained inward current during the action potential (AP) plateau. All these interventions prolong APD.
The different IKs:IKr density ratios in different cell types and the fact that these currents are affected in the LQTS and by antiarrhythmic agents necessitates characterization of the cellular responses to such pathologies and interventions. In this study we used theoretical models of isolated cells and multicellular fibers to investigate the effects of heterogeneities of IKs on steady-state APD and EAD formation during prolonged repolarization caused by the different LQTS types or antiarrhythmic agents that block IKr or IKs.
The theoretical Luo-Rudy model of a mammalian ventricular action potential provides the basis for the simulations in this study (Figure 1⇓). The action potential is numerically reconstructed from ionic processes that are formulated on the basis of experimental data obtained mostly from the guinea pig. The model also accounts for processes that regulate dynamic concentration changes of Na+, K+, and Ca2+. References 6 and 76 7 provide a detailed description of the cell model and a list of equations governing its behavior. The 3 different cell types (epicardial, midmyocardial, and endocardial) are formulated by altering IKs density while keeping IKr density constant.4 Heterogeneity of IKs density is introduced by altering its maximum conductance, Ḡks.
Acquired and Congenital Long-QT Syndrome
Evidence suggests that congenital long-QT syndromes (LQTS) LQT1 and LQT2 involve reduction in the density of functional IKs or IKr channels, respectively, through a dominant negative mechanism. Acquired LQTS results from drugs that prolong action potential duration (APD) by blocking IKs or IKr. In our studies we reduced the maximum conductance of IKs or IKr to represent the effects of the congenital or acquired LQTS. We simulated LQT3 by altering the steady-state inactivation of the h (fast) and j (slow) inactivation gates of INa.8 This results in a persistent late current that is 0.2% of peak INa during a voltage clamp protocol (step from −120 to −30 mV). The overall behavior of the current is in good agreement with whole cell recordings of mutant INa channels.9 10 11
Cells were paced for 5 minutes at each cycle length (CL) to reach steady state. The effects of drugs were simulated by reducing IKs or IKr at steady state. Results are reported for CL=300 ms (rapid pacing) and CL=2000 ms (slow pacing). These CLs are in the range of clinically observed heart rates. APD was measured from stimulus onset to 90% repolarization (APD90). Simulations were performed for 37°C.
The theoretical fiber12 (Figure 1B⇑) is composed of 190 Luo-Rudy cells. It contains an endocardial region (cells 1 to 80), M-cell region (81 to 110), and epicardial region (111 to 190). Gap-junction conductance (gj) is homogeneous throughout the fiber and for different simulations varies from 2.5 μS (normal longitudinal coupling) to 0.025 μS (poor coupling). A stimulus is applied to cell 1 to simulate normal endocardial to epicardial activation. Pacing studies were conducted by stimulating cell 1 at CL=300 ms (3.3 Hz) or 2000 ms (0.5 Hz) for 90 seconds.
Reduction of IKr or Presence of Late INa Enhances APD Differences Between Isolated Cells of the Different Types. Reduction of IKs Eliminates Such Differences
Figure 2⇓ shows the effects of IKs or IKr block, simulating the effects of congenital or acquired LQT1 and LQT2, respectively, on APD during rapid pacing. IKr block prolongs M-cell APD more than that of epicardial or endocardial cells. In contrast, IKs block has a smaller effect on the M cell than on the other cell types. At a cycle length (CL)=300 ms, 100% IKr block prolongs APD by 20%, 36%, and 25% in epicardial cells, M cells, and endocardial cells, respectively, whereas 100% IKs block prolongs the respective APDs by 43%, 26%, and 37%. Figure 3⇓ shows the effect of persistent INa on APD of the 3 cell types at CL=300 ms. Late INa prolongs APD of the M cell much more than that of the other cell types. APD prolongation is 17%, 35%, and 22% in epicardial cells, M cells, and endocardial cells, respectively. An important observation from Figures 2⇓ and 3⇓ is that IKr block and late INa act to increase APD differences between the cell types. In contrast, IKs block acts to reduce such APD differences. Figure 4⇓ shows APDs from the 3 cell types during different interventions (complete IKs block, complete IKr block, and in presence of late INa) at CL=300 ms. IKs block decreases ΔAPD (APDM−APDEpi), whereas IKr block or presence of late INa increases ΔAPD. In the absence of intervention, ΔAPD=27 ms. IKr block or late INa increases ΔAPD to 53 or 55 ms, respectively; IKs block reduces ΔAPD to 17 ms.
Figure 5⇓ shows the effect of 50% IKr or IKs block and Figure 6⇓ shows the effect of late INa on APD during slow pacing (CL=2000 ms). A 50% IKr block causes EAD development in the M cell only, whereas 50% IKs block fails to produce EADs even in these cells. A higher degree of IKs block results in EAD development in all 3 cell types (data not shown). The degree of IKs block required to develop EADs in endocardial cells, M cells, and epicardial cells is 85%, 64%, and 90%, respectively. Late INa also results in EADs in the M cell but not in other cell types. These results demonstrate the higher susceptibility of M cells to the development of arrhythmogenic EADs as a consequence of processes (congenital or interventional) that reduce IKr or IKs or enhance late INa.
Effect of Gap-Junction Coupling on APD and Development of EADs
In the previous section we focused on the effects of IKs and IKr reduction and of late INa on APD and the development of EADs in isolated cells. In the intact heart, cells are coupled through gap junctions and are subject to electrotonic interactions. In this section, we study the effects of intercellular coupling on the development of EADs during simulated LQTS.
Figure 7⇓ shows APs (aligned for comparison) from the middle of each region of the multicellular fiber for CL=300 ms and gap-junctional conductance gj=0.43 μS (corresponding to velocity of 30 cm/s, typical of transmural propagation transverse to fibers). During control conditions, M-cell APD is longest (panel A), and the difference in APD between M and epi cells (ΔAPD=APDM−APDEpi) is ≈20 ms. ΔAPD at CL=2000 ms is 30 ms (not shown). This suggests that baseline level of APD differences exists in the normal myocardium. On block of IKs (panel B), APD differences are greatly reduced. For 100% IKs block, ΔAPD decreases to 6 ms. In contrast, complete IKr block (panel C) or late INa (panel D) increases ΔAPD from 20 to 32 or 33 ms, respectively. This behavior is similar to that observed in isolated cells (Figure 6⇑) in which IKr block and late INa enhanced the differences of APD between cell types, whereas IKs block eliminated it.
Figure 8⇓ shows APD along the multicellular fiber for longitudinal coupling (panel A, gj=2.5 μS, conduction velocity of 56 cm/s) and for transverse coupling as in Figure 7⇑ (panel B, gj=0.43 μS). A 100% IKr block or late INa increases APD in the M region preferentially, thereby increasing APD differences between M and epicardial or endocardial cells (compared with control). A 100% IKs block, on the other hand, decreases APD differences between M cells and other cell types.
The above studies were conducted at a short CL of 300 ms. At this rate, APDs were significantly shorter than during slower pacing. At this fast rate, EADs did not develop during complete block of either IKr or IKs even during extreme gap-junctional uncoupling. Figure 9⇓ shows APs of the middle cell from each region of the fiber for CL=2000 ms with gap-junctional coupling of 0.43 μS (panel A) or 0.025 μS (corresponding to a slow velocity of 3.5 cm/s, observed during pathological conditions such as infarction). For the normal transverse coupling (gj=0.43 μS), 85% reduction of IKs results in the appearance of an EAD in the endocardial cell and not in the M cell, in contrast to the single cell behavior in which EADs developed in the M cell (Figure 5⇑ and 6⇑ and corresponding text). However, a similar degree of IKs block on the background of pathologically reduced intercellular coupling resulted in the development of EADs in the M region (Figure 9B⇓) similar to the isolated cell behavior. We also conducted simulations in which the pacing stimulus was applied epicardially or in the M region (not shown). In all cases, EADs develop in the endocardial region for 85% IKs block when intercellular coupling is normal. This phenomenon highlights the importance of electrotonic influences in modulating the behavior of different cell types. During normal coupling, the epicardial region acts as a sink for axial current from the M region because of its shorter APD and earlier repolarization than that of the other regions. Note the existence of a large potential gradient between cells in the M and epicardial regions during the plateau and phase 3 of the AP (Figure 9A⇓). This causes shortening of M-cell APD as axial current is lost to the epicardial region. Shortening of the APD prevents EAD development in the M region.
We have demonstrated4 that changes in IKs:IKr density ratios can explain heterogeneity of the repolarization properties of cells in the ventricular myocardium. Consistent with the experimentally observed behavior,1 2 simulated M cells could be distinguished from epicardial and endocardial cells by their longer APD and greater APD prolongation with slowing of stimulation rate. In this study, we have investigated the rate-dependent effects of reduced IKr or IKs or the presence of late INa to mimic the 3 forms of the LQTS and to simulate the effects of drugs with class III antiarrhythmic action. Effects on APD and the development of EADs were considered in isolated cells and in a multicellular fiber with varying degrees of gap-junctional coupling. Important findings of this study include the following: (1) At fast pacing, IKr block and late INa prolong APD of isolated M cells more than that of isolated epicardial or endocardial cells. In contrast, IKs block prolongs APD of M cells less than that of the other cell types. (2) Greater prolongation of M-cell APD by IKr block or late INa augments APD differences between the different cell types. In contrast, smaller prolongation of M-cell APD by IKs block acts to minimize such APD differences. (3) At slow pacing, M cells are highly susceptible to the development of plateau EADs upon IKr block or presence of late INa. A higher degree of IKs block is necessary for EAD development in the M cells. (4) When cells are electrotonically coupled through gap junctions, IKr block or late INa enhances APD dispersion along the fiber during rapid pacing. In contrast, IKs block reduces APD dispersion, similar to the behavior observed in isolated cells. (5) At slow pacing and when cells are well coupled, IKs block results in the appearance of EADs in the endocardial region. However, in the presence of reduced coupling, EADs develop in the M region.
Heterogeneity of APD and the resulting dispersion of repolarization provide a substrate for unidirectional block and reentry. This study demonstrates that at rapid rates, IKr block or late INa enhances APD differences between different cell types not only in isolated cells (Figure 6⇑) but also in the multicellular tissue (Figures 7⇑ and 8⇑). This is due to the greater effect of these interventions in prolonging the APD of M cells than epicardial or endocardial cells. IKr block or late INa prolongs APD of the M cell most because of its smaller total repolarizing current (caused by a smaller IKs). Both these interventions have a similar effect on M cells because these currents are uniformly distributed in all cell types. In contrast, IKs block prolongs the APD of M cells less than that of epicardial or endocardial cells because of the smaller density of IKs in the M cells. This theoretical observation is consistent with experimental observations by Shimizu and Antzelevitch,13 who showed that the percentage of APD prolongation in epicardium or endocardium is greater than in the M cell upon application of chromanol 293B, a selective IKs blocker. This tends to minimize APD differences between the 3 cell types. It is therefore observed that IKr block or late INa increases APD differences and dispersion, thereby creating a substrate for the development of unidirectional block and reentry, whereas IKs block tends to minimize APD differences and eliminate dispersion.
During slow pacing, IKr or IKs block and late INa result in preferential EAD development in isolated M cells similar to experimental observations. A 50% IKr block was sufficient to cause EADs in the M cell, whereas EADs did not develop in epicardial or endocardial cells, even for complete IKr block. A higher percentage of IKs block (64%) was necessary to cause EADs in the M cell. In a multicellular fiber, EADs developed in the M region under conditions of reduced gap-junctional coupling. For example, when gj=0.025 μS, EADs developed in the M region, which in turn prolonged APD and greatly enhanced APD dispersion. The higher likelihood of EADs in the M region suggests that M cells could become a source of triggered activity and focal arrhythmias3 when gap-junctional coupling is reduced. On the background of enhanced APD dispersion caused by greater APD prolongation in the M region, such EADs can provide the triggering event for the development of reentry.
Surprisingly, when cells are well coupled (gj=0.43 μS), endocardial cells are more susceptible to APD prolongation and EAD development than M cells, which suggests their involvement in EAD-related arrhythmias. This behavior is consistent with experimental findings demonstrating that focal activity (possibly caused by EADs) in the subendocardium generates the initial beat of polymorphic tachycardias in the LQTS14 or in ventricular arrhythmias in patients with idiopathic dilated cardiomyopathy.15 Purkinje fibers are also known to be highly susceptible to interventions that prolong APD. It is possible that EADs generated in Purkinje fibers play an important role in these arrhythmias. The results of this study demonstrate the possibility that EADs in endocardial cells can also provide the trigger for ventricular arrhythmias associated with abnormal repolarization.
An important mechanistic observation is that in all cases (single cell or multicellular fiber), EAD depolarization is due to recovery from inactivation and reactivation of the L-type calcium channel current, ICa(L), as we have shown previously.16 Prolongation of the AP to provide sufficient time for ICa(L) recovery is achieved in a nonspecific manner. In LQT1 and LQT2 it is achieved by reduction of outward repolarizing currents IKs or IKr, respectively. In LQT3 it is achieved by an increase of an inward depolarizing current, late INa. In a related study8 we showed that pause-induced EADs in the 3 types of LQTS also involve recovery and reactivation of ICa(L) during a prolonged AP plateau. These observations suggest a universal mechanism for EAD formation at plateau potentials, namely ICa(L) reactivation, whereas prolongation of the plateau that is necessary for ICa(L) recovery does not involve a specific mechanism, and its underlying ionic current is different in the different types of LQTS.
Conclusions and Implications for Antiarrhythmic Therapy
Electrophysiological heterogeneity is an important property of the myocardium that must be considered in the determination of arrhythmogenic mechanisms and the administration of antiarrhythmic drug therapy. Results from the multicellular fiber suggest that APD dispersion in the myocardium is greatly influenced by the degree of gap-junctional coupling and the type of intervention that prolongs APD. Substantial APD differences can exist in the myocardium when intercellular coupling is reduced, even in the absence of ion channel modification by drugs or disease. Conversely, the higher susceptibility of M cells to pathologies and interventions that prolong APD suggests that large APD dispersions can arise even in the normally coupled myocardium. A combination of reduced cellular coupling and enhanced cellular heterogeneity can result in extreme levels of APD dispersion, setting the stage for unidirectional block and reentry. Such a situation can arise when an antiarrhythmic drug that prolongs APD is administered in an aging or infarcted heart in which intercellular coupling is reduced. Studies (for review see Reference 1717 ) have shown that antiarrhythmic drugs could become proarrhythmic when the substrate is altered by ischemia or infarction. Similar changes can occur during electrophysiological remodeling caused by various pathologies (eg, hypertrophic or dilated cardiomyopathy) or as a result of the fast rates associated with the arrhythmia itself (as can occur during atrial flutter and fibrillation).18 Our results show that drugs that prolong APD by blocking IKr aggravate APD dispersion in the presence of reduced cellular coupling. Recent studies19 20 showed changes in the distribution and concentration of the gap-junctional protein connexin 43 during acute ischemia and infarction. It is likely that heterogeneity of gap junction distribution acts synergistically with ion channel heterogeneity to aggravate APD dispersion and EAD formation during administration of drugs that prolong APD or in the presence of disease that prolongs repolarization (eg, LQTS). A striking result of our simulations is that IKs block minimized APD dispersion even in the presence of reduced cellular coupling. This is a very important observation because it could be a beneficial antiarrhythmic property that is not shared by drugs that block IKr.
Limitations of the Study
The role of gap-junctional coupling in APD heterogeneity and EAD formation during prolonged repolarization was studied in a 1D fiber. This is a simplified model that allowed us to investigate this phenomenon at the cellular and ionic channel levels. Moreover, propagation that can be simulated in a 1-D model occurs frequently in the 3D heart when broad-plane waves are generated. Importantly, this is the situation during normal sinus rhythm in which planar waves created by the Purkinje network propagate from endocardium to epicardium. A similar situation occurs in the wedge preparation used extensively21 to study myocardial heterogeneity. It should be emphasized that local dispersion on a cellular scale is involved in the generation of unidirectional block and reentry. Even in the presence of complex 3D global wave fronts, local excitation can be 1D in nature. This is particularly true in the presence of pathological structural complexities such as narrow fibers connecting islands of surviving tissue in an infarct.
Clinically, LQT3 is usually associated with arrhythmias occurring at slow heart rates (during sleep), whereas arrhythmias in LQT1 often occur during exercise and/or excitement.22 It is likely that these different scenarios reflect different levels of sympathetic activity. A possible explanation for this behavior is that in LQT3, prolongation of APD is greater at slow rates, when the mutant late INa is opposed by a smaller repolarizing current as the result of greater deactivation of IKs between APs. In the case of LQT1, the mutation results in reduced IKs. Sympathetic activity augments both IKs and ICa(L), in addition to other effects on calcium cycling, INa and the Na/K pump. The net effect on APD depends on the balance between IKs reduction by the mutation and the enhancement of IKs and ICa(L) by β-adrenergic stimulation. This effect is likely to be nonuniform because of the heterogeneous distribution of IKs in the myocardium, thereby increasing dispersion of repolarization and enhancing arrhythmogenesis. The multifactorial cellular effects of β-adrenergic stimulation can be simulated in the Luo-Rudy model.16 Their interactions with the different types of LQTS are complex and require extensive simulation protocols that are outside the scope of this study.
This study was supported by National Institutes of Health–National Heart, Lung, and Blood Institute grants R01-HL-49054 and R37-HL-33343 (to Dr Rudy) and by a Development Award from the Whitaker Foundation.
- Received June 14, 1999.
- Revision received September 2, 1999.
- Accepted September 23, 1999.
- Copyright © 2000 by American Heart Association
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