[Frontiers in Bioscience E5, 893-899, June 1, 2013]

Can triggered arrhythmias arise from propagation of Ca2+ waves between cardiac cells?

Amanda F. Nahhas1, Manvinder S. Kumar1, Matthew J. O'Toole1, Gary L. Aistrup1, J. Andrew Wasserstrom1

1Department of Medicine (Cardiology) and the Feinberg Cardiovascular Research Institute, Northwestern University Feinberg School of Medicine, Chicago, IL 60611

TABLE OF CONTENTS

1. Abstract
2. Introduction
3. Intracellular Ca2+ cycling and arrhythmias
4. Connexins, gap junctions and cell-cell communication
5. Ca2+ spread between cardiac myocytes in culture and cell pairs
6. Ca2+ spread between myocytes in intact tissues and whole heart
7. Mechanisms and regulation of intercellular Ca2+ wave propagation
8. Intercellular Ca2+ wave propagation and triggered arrhythmias
9. References

1. ABSTRACT

Intracellular Ca2+ overload can induce regenerative Ca2+ waves that activate inward current in cardiac myocytes, allowing the cell membrane to achieve threshold. The result is a triggered extrasystole that can, under the right conditions, lead to sustained triggered arrhythmias. In this review, we consider the issue of whether or not Ca2+ waves can travel between neighboring myocytes and if this intercellular Ca2+ diffusion can involve enough cells over a short enough period of time to actually induce triggered activity in the heart. This review is not intended to serve as an exhaustive review of the literature summarizing Ca2+ flux through cardiac gap junctions or of how Ca2+ waves move from cell to cell. Rather, it summarizes many of the pertinent experimental studies and considers their results in the theoretical context of whether or not the intercellular propagation of Ca2+ overload can contribute to triggered beats and arrhythmias in the intact heart.

2. INTRODUCTION

Abnormal intracellular Ca2+ cycling is responsible for triggered arrhythmias, particularly in the setting of intracellular Ca2+ overload that results from ischemia, cell damage, pharmacological or neurohormonal alterations in intracellular Ca2+ concentration (e.g. digitalis toxicity and b-adrenergic stimulation), genetic mutations in Ca2+ cycling proteins (e.g. mutations of the cardiac ryanodine receptor, RyR2, or calsequestrin in catecholaminergic polymorphic ventricular tachycardia), and other interventions that alter Ca2+ homeostasis. It is well known how altered Ca2+ cycling in the form of spontaneous Ca2+ waves can induce cellular depolarization and how this cellular depolarization can achieve the voltage threshold for activation of inward Na+ current, producing an extrasystole. However, it is essential for this aberrant excitation to spread among a sufficient number of cells across a sufficiently large mass of ventricular tissue (the liminal mass) in order to induce a triggered beat that conducts throughout the myocardium.

One of the ways that has been proposed to spread enough depolarizing current to achieve threshold in intact heart is that Ca2+ waves might spread from a focal region of intracellular Ca2+ overload to neighboring myocytes through gap junctions, thus spreading depolarization by involving enough adjoining tissue to bring it to threshold to produce a conducted beat. It is important to note, however, that the spread of Ca2+ between myocytes is carefully regulated by the myocyte through regulation of gap junctions which are responsible for physiologically essential functions of maintaining small substrate, metabolic and ionic balance among cells. In addition, regulation of intercellular communication also insures that pathophysiological processes (including Ca2+ overload) do not spread easily between myocytes, thus limiting the spread of tissue damage.

The purpose of this work is to review what is known about the diffusion of Ca2+ between cardiac myocytes and to consider how Ca2+ spread from cell to cell might be involved in the generation of triggered arrhythmias.

3. INTRACELLULAR Ca2+ CYCLING AND ARRHYTHMIAS

Excitation-contraction coupling (ECC) is a process that allows the heart to pump blood efficiently throughout the body (1,2). ECC begins with activation of the ventricular action potential, which depolarizes the cell and triggers calcium-induced calcium release (CICR), resulting in intracellular Ca2+ cycling. Once the action potential has spread throughout the cell, Ca2+ enters through L-type Ca2+ channels found primarily on the plasma membrane in the t-tubules that spread activation to the cell core. This entering Ca2+ binds to and activates ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR), inducing the release of more Ca2+ from this Ca2+ storage organelle into the cytosol, in a local event known as a Ca2+ spark. The summation of thousands of Ca2+ sparks throughout the cell produces a homogeneous rise in Ca2+ throughout the cell, resulting in the formation of Ca2+ transients. This rise in cytosolic Ca2+ causes the interaction between actin and myosin filaments, resulting in cardiac contraction. Once contraction is completed, most Ca2+ is pumped back into the SR while some exits the cell via the sodium-calcium exchanger (NCX), thus restoring Ca2+ gradients in time for the next cardiac cycle.

Under abnormal conditions, such as during myocardial damage, excessive phosphorylation, excessive Ca2+ influx and Ca2+ mismanagement of Ca2+ release by genetically or pharmacologically altered protein function, a state known as Ca2+ overload is induced. Under these conditions, the SR becomes overloaded with Ca2+ and cannot sequester enough to restore the normal Ca2+ balances. Because of the high SR Ca2+ content, RyRs open spontaneously (in the absence of L-type Ca2+ influx and CICR) causing Ca2+ release that spreads from release unit to release unit, in a regenerative manner as a propagating Ca2+ wave (2). As Ca2+ overload progresses, the high Ca2+ sensitivity of RyRs and SR Ca2+ overload cause the continuous generation of abnormal Ca2+ waves (2,3). Most importantly, Ca2+ waves activate the sarcolemmal Na+-Ca2+ exchanger (NCX), an equilibrium exchanger that is also electrogenic. The result is that removal of the excess Ca2+ released spontaneously by the SR during a wave also produces inward depolarizing current which can be of sufficient magnitude and timing to cause a cell depolarization above the voltage threshold, generating a triggered action potential (4,5). Under the right conditions, triggered arrhythmias may result as the final outcome at the whole heart level.

4. CONNEXINS, GAP JUCTIONS AND CELL-CELL COMMUNICATION

In most living tissue, cells communicate by regulating conductances of ions, second messengers and small metabolites through gap junctional proteins called connexins, allowing rapid cell-to-cell communication (6-8). Connexin isoforms identified in the heart include Cx-40, Cx-43 and Cx-45 depending on tissue location (9-11). Although homomeric channels may form the bulk of active gap junctional channels, heteromeric forms of the channel exist and are functional (12,13). It is also known that disease changes the balance of connexin expression patterns, with a decrease in Cx-43 expression (14,15) and compensatory increase in Cx-45 expression (16). Moreover, there is a close link between changes in Cx-43 expression levels and electrical conduction disturbances in disease (17-19), demonstrating the importance of normal intercellular communication to normal electrical function. In addition, Cx-43 expression is reduced at cell ends and increases at lateral cell connections so there are important consequences of cardiac disease on connexin isoform expression levels, phosphorylation and location (18-22).

Numerous studies have demonstrated gap junction formation and conductances in cell pairs (23-25) so it is quite clear how intercellular communication is accomplished between cardiac cells. The question then arises: how can an inherently intracellular event (spontaneous Ca2+ release from the SR) cause the spread of excitation across the intact heart?

5. Ca2+ SPREAD BETWEEN CARDIAC MYOCYTES IN CULTURE AND CELL PAIRS

Early experiments using myocyte pairs demonstrated low resistance junctions to the passage of electrical current (26). Gap junctions are regulated by hydrogen ions, cyclic 3',5'-adenosine monophosphate (probably through channel phosphorylation) and Ca2+, among other ligands (27), making them vitally important to the maintenance of cell viability during both acute and chronic disease states.

Aside from its ability to regulate gap junctional conductance, there have been numerous studies reporting the ability of Ca2+ itself to move between cardiac cells via gap junctions. Suadicani et al. compared neonatal cardiac myocytes from wild type (WT) and Cx-43 knock-out mice (KO) and found that the intercellular movement of Ca2+ also occurs primarily via gap junctions (28). Mechanical induction of Ca2+ waves caused slow conduction of Ca2+ waves (14m/sec) across 80% of myocytes across the entire WT culture, with a loss of 87% of transmitted Ca2+ waves between cultured neonatal KO myocytes. Fewer myocytes showed Ca2+ waves in KO than WT when type 2 purinergic receptors were blocked, suggesting the involvement of intracellular ATP release in regulation of gap junctional conductances and a mechanism for reduced intercellular communication in ischemia. The authors concluded that the slow propagation velocity occurred because of poor SR organization in neonatal myocytes (29,30) as well as poor organization of connexins in the intercalated disk (31).

Cell pairs from adult heart have also served as one of the major models for the study of intercellular Ca2+ flow. One of the first applications of laser scanning confocal microscopy demonstrated that Ca2+ waves propagated with little delay between guinea pig ventricular myocyte pairs (32). In contrast, cell pairs taken from rabbits (7) showed no indication of intercellular Ca2+ flow under physiological conditions despite intracellular Ca2+ wave propagation under conditions of SR Ca2+ overload.

6. Ca2+ SPREAD BETWEEN CARDIAC MYOCYTES IN INTACT TISSUES AND WHOLE HEART

In intact myocardium, Minamikawa et al. were the first to demonstrate that Ca2+ waves occur in perfused whole rat hearts in the subepicardium (33). They periodically saw Ca2+ waves during physiological extracellular Ca2+ perfusion in a paced and arrested heart, which occurred more frequently when extracellular Ca2+ levels were increased to the point where Ca2+ overload was induced. They noted that Ca2+ waves traversed cell borders in all three dimensions and crossed to nearby cells; however, in damaged cells, intercellular Ca2+ propagation was absent, even though spontaneous Ca2+ waves did occur frequently. This observation is consistent with the fact that high intracellular Ca2+ can close connexin channels which is presumably a protective device that prevents Ca2+ dysregulation of cell functions and hypercontracture from spreading to neighboring cells (27,34).

The general requirement for Ca2+ overload in intercellular transmission of Ca2+ is similar to that in intact tissues where Ca2+ overload, often in the setting of cell damage, promotes wave propagation between myocytes of healthy and infarcted hearts. In fact, Ca2+ waves in myocytes of intact rat ventricular trabeculae caused different types of responses in downstream myocytes, including subthreshold Ca2+ "spritzes" as well as full-blown Ca2+ waves that propagated to both cell ends, but only during Ca2+ overload induced by elevated (Ca2+) in the external solution (8). The likelihood of wave transmission increased with b-adrenergic stimulation and decreased when cells were uncoupled with heptanol. However, propagation increased during Ca2+ overload induced by tissue damage when more waves occurred. These investigators also reported that occasional transverse wave propagation also occurred in these conditions, although most wave propagation occurred from cell ends, which is consistent with the idea that Ca2+ accumulation occurs normally in the intercalated disks, increasing the probability of spontaneous Ca2+ release from that site (7). These cellular regions have a particularly high density of gap junctions which might contribute to a higher likelihood of both Ca2+ accumulation and spontaneous Ca2+ release from these sites. Finally, although Ca2+ waves did propagate between cells in this study, propagation was slow, requiring hundreds of milliseconds to fully evoke a wave in a neighboring cell.

Among the most extensive studies demonstrating Ca2+ wave transmission are those performed in damaged ventricular trabeculae. Studies in rat trabeculae focused on the generation of Ca2+ waves (35,36) induced by simulation of either local mechanical or ischemic myocardial damage or by using regional application of caffeine, low Ca2+ concentration, or 2,3-butanedione monoxime. Each condition showed that non-uniform ECC can produce Ca2+ waves that underlie triggered propagated contractions, presumably as a result of passive stretch in damaged or weak myocardial regions causing release of Ca2+ from myofilaments which then causes a rise in cytoplasmic (Ca2+) that is late enough to induce triggered SR Ca2+ release in the form of Ca2+ waves. These authors proposed that the early release of Ca2+ from damaged cells causes Ca2+ waves that propagate across the myocardium that could produce triggered beats in the setting of non-uniform contraction in heterogeneously damaged myocardium (35-38). This proposition is particularly interesting because they documented a wide range of propagation velocities for large and small waves (ranging from 0.5-1.5mm/sec) and distances of propagation up to 450 microns (35), making this behavior in damaged myocardium more likely to recruit enough tissue to produce a triggered beat than in nearly all other experimental models reported to date.

We have also observed the propagation of Ca2+ waves between myocytes of intact rat heart (39) although these events are extremely rare and usually involve only a single neighboring myocyte. Only rarely have we observed Ca2+ spread across multiple cells and this can take up to hundreds of milliseconds to occur. Figure 1A shows an example of Ca2+ wave propagation between adjoining myocytes in an intact heart from a spontaneously hypertensive rat following rapid pacing. A Ca2+ wave signals spontaneous Ca2+ release in a cell near the center of the field of myocytes (6th myocyte from the top, cells separated by white horizontal lines). There is a slow cascade of Ca2+ waves toward the cells at the top of the image, with successive activation spreading from cell-to-cell. Figure 1B shows an expanded image (taken from the red bar to the right of the image in Figure 1A) in which Ca2+ "spritzes" (subthreshold Ca2+ transmission to downstream cells) can be seen to activate waves in adjoining cells (red arrows). We have seen this behavior very rarely in intact rat hearts (and never in guinea pig hearts) and it is consistent with other reports of this behavior in diseased myocardium, where it has been reported to occur only occasionally.

Experiments by Kaneko et al. (3) also used rat whole heart subepicardial myocardium to monitor the incidence of Ca2+ waves. They simulated Ca2+ overload conditions through the induction of mechanical damage by microelectrodes to observe how this damage influences Ca2+ wave propagation and found that intercellular wave propagation involved primarily transverse (side-to-side) Ca2+ spread to as many as 4-7 adjacent cells. In addition, they found that only about 20% of Ca2+-overloaded myocytes had waves that crossed cell borders. More recently, similar experiments in rat heart confirmed intercellular propagation of Ca2+ waves but found that Ca2+ waves spread to at most 2-3 cells (40).

Thus, there are some reports of the spread of Ca2+ waves between cells of intact tissue under Ca2+ overload conditions, although spread involves only a few layers of neighboring myocytes and takes a relatively long time to occur. There seems to be a somewhat different situation in non-uniformly damaged tissue where the spread of Ca2+ waves may be sufficient to induce triggered beats (4,41), but this appears to be a special situation. In contrast, it appears that, under most normal and pathophysiological conditions, it is difficult for intact myocardium to meet the spatial and temporal conditions required for activation of triggered activity (discussed below).

7. MECHANISMS AND REGULATION OF INTERCELLULAR Ca2+ WAVE PROPAGATION

If intercellular flow of Ca2+ is to occur between myocytes, Ca2+ will propagate through gap junctions either longitudinally between cell ends at the intercalated disks or laterally between cells connected side-to-side. Gap junctions are more heavily concentrated at cell ends than along cells connected laterally in normal tissue (31,42). In fact, cell-end gap junction proteins, in particular Cx-43, account for approximately 80% of all gap junction proteins within cardiac cells, as seen in sections of canine myocardium. It is thought that gap junctions are less likely to be concentrated laterally because they would have difficulty enduring the shear movements produced by cell contractions (31).

Certain factors are known to influence the likelihood and occurrence of intercellular movement of Ca2+. The most notable ones include gap junction conductivity, the amount of tissue damage caused by myocardial infarction or hypertrophy, and the ability of Ca2+ waves to become repetitive within a given myocyte (43,44). In regard to gap junction conductivity, experimental evidence suggests that Ca2+-overloaded regions tend to be less conductive and that phosphorylation of Cx-43 is critical to Ca2+ wave propagation (2,3).

It is well known that intercellular Ca2+ waves may be responsible for the depolarizations that cause triggered arrhythmias (4,35,45). Many common arrhythmias occur as a result of structural heart disease, such as left ventricular dysfunction and/or the presence of myocardial infarction and scar formation (46). However, it is also known that Ca2+ overload-induced waves promote cell depolarization in non-uniform heart tissue, which can then lead to both cellular contraction and triggered arrhythmias (3). To achieve an effective increase in the Ca2+ load, action potential initiation from a delayed afterdepolarization in the whole heart requires the generation of spontaneous Ca2+ waves in multiple, closely located cells or that a single Ca2+ wave quickly propagate across cell borders (47). Given that the results described in most studies found both a low incidence and a low propagation distance of Ca2+ waves, it is unlikely that Ca2+ waves alone may serve as a frequent cause of triggered cardiac arrhythmias.

It is important to note, however, that hyperphosphorylation of RyR receptors, such as occurs in heart failure (HF), causes Ca2+ leak from the SR. Therefore, there is a decreased Ca2+ load of the SR, which is commonly observed in heart failure (48,49). This increased SR leak is responsible both for reduced contractile force and potentially the increased risk of spontaneous diastolic Ca2+ release which could cause triggered arrhythmias (48,49). This occurs partly as the result of the fact that the electrogenic Na+ -Ca2+ exchanger is upregulated in many models of heart failure which promotes membrane depolarization and resulting triggered activity. In fact, the incidence of Ca2+ waves is increased in both overt HF and during the development of HF (39,50). Despite reported changes in connexin distribution and many additional changes in ion channels expression and distribution, t-tubule disruption, cell/tissue hypertrophy and many additional changes in cell structure and function, however, there have been no systematic studies of Ca2+ wave propagation in HF models. It may be that this and other pathophysiological conditions might influence intercellular Ca2+ diffusion and promote the activation of Ca2+ waves from cell-to-cell through diseased tissue.

8. INTERCELLULAR Ca2+ WAVE PROPAGATION AND TRIGGERED ARRHYTHMIAS

In consideration of previous studies on intercellular Ca2+ movement in myocytes, it is clear that intercellular Ca2+ diffusion in ventricular tissues does indeed occur in some species and under a variety of experimental conditions, although results are not consistent even in similar tissues. However, despite evidence that this phenomenon can and does indeed occur, the fact that Ca2+ diffusion and subsequent activation of Ca2+ waves only affects a very few adjoining cell layers makes it very unlikely that the spread of Ca2+ waves can involve enough tissue to induce a triggered beat. It has been difficult to obtain accurate estimates based on experimental results of the liminal tissue mass required but estimates range from tens of thousands to nearly a millions cells (51,52) depending on whether the tissue is 2- or 3-dimensional, far exceeding the number of cells observed experimentally to be involved thus far. In contrast, the estimate of the liminal length in Purkinje fibers is far less (about 0.2mm, 53), probably involving tens to hundreds of cells) because of the 2-dimensional architecture of these cable-like structures, bringing the experimental observation closer to the theoretical requirement.

Aside from the spatial requirements, however, the temporal requirements are also critical. It is not sufficient to simply have enough cells produce Ca2+ waves. They must do so nearly simultaneously in order to provide enough inward depolarizing current to achieve threshold (54). Very few of the studies summarized here demonstrated rapid enough intercellular Ca2+ movements to provide sufficient current density over a short enough period of time to overcome the passive membrane properties that maintain the resting potential. It is the integrated depolarizing influence among many cells over a short period of time that determines the efficacy of the stimulus, requiring nearly simultaneous depolarization to produce the most effective influence in bringing the tissue to threshold, at least in normal tissue. Furthermore, the fact that there is evidence for preferential wave propagation in the longitudinal direction compared to the transverse direction would mean that wave spread could occur preferentially in only one dimension with wave propagation left to occur much more slowly in the other two dimensions, creating an enormous spatiotemporal mismatch that is likely to discourage uniform waves and depolarization in a local mass of tissue. We have recently reported that the alignment of spontaneous release in time occurs as a natural consequence of accelerated SR Ca2+ uptake during Ca2+ overload (54). This explains the temporal organization of spontaneous Ca2+ release in intact myocardium, at least in normal hearts. It is important to recognize that these conditions may vary in different disease states, such as heart failure, where both passive and active membrane properties of the tissue could make it easier to meet the temporal and spatial requirements for triggered activity. These changes include increasing cell size during hypertrophy, inducing levels and location of gap junctions (lateral localization rather than at cell ends), altered ion channel expression, loss of t-tubule organization and so forth. Furthermore, it is not yet known if these and other changes occur in Purkinje cells in diseased hearts where the properties of a 2-dimensional cable already probably favor triggered beat formation. The outcome of tissue remodeling in disease may thus serve to close the gap between physiological behavior and theoretical requirements for triggered activity. Future research should focus on the direction of intercellular Ca2+ movement (transverse vs. longitudinal), wave velocity, the distribution of gap junctions, the distance Ca2+ waves can travel and the conditions that cause Ca2+ waves to traverse cell borders, including comparisons between normal and diseased tissues and between conducting tissue and working myocardium. Clarifying the mechanisms and causes of successful intercellular Ca2+ flow could be instrumental in developing new therapeutic strategies in the treatment of triggered arrhythmia.

9. REFERENCES

1. Donald Bers: Cardiac excitation-contraction coupling. Nature 415, 198-205 (2002)
http://dx.doi.org/10.1038/415198a
PMid:11805843

 

2. Hendrik ter Keurs and Penelope Boyden: Calcium and arrhythmogenesis. Physiol Rev 87, 457-506 (2007)
http://dx.doi.org/10.1152/physrev.00011.2006
PMid:17429038

 
 

3. Tomoyuki Kaneko, Hideo Tanaka, Masahito Oyamada, Satoshi Kawata, and Tetsuro Takamatsu: Three distinct types of Ca(2+) waves in Langendorff-perfused rat heart revealed by real-time confocal microscopy. Circ Res 86, 1093-1099 (2000)
http://dx.doi.org/10.1161/01.RES.86.10.1093
PMid:10827140

 
 

4. Joshua Berlin, Mark Cannell, and W. Jonathan Lederer: Cellular origins of the transient inward current in cardiac myocytes. Role of fluctuations and waves of elevated intracellular calcium. Circ Res 65, 115-126 (1989)
http://dx.doi.org/10.1161/01.RES.65.1.115
PMid:2736729

 
 

5. Penelope Boyden, Jielin Pu, Judith Pinto, and Hendrik ter Keurs: Ca(2+) transients and Ca(2+) waves in purkinje cells : role in action potential initiation. Circ Res 86, 448-455 (2000)
http://dx.doi.org/10.1161/01.RES.86.4.448
PMid:10700450

 
 

6. Walmor De Mello: Intercellular communication in cardiac muscle. Circ Res 51, 1-9 (1982)
http://dx.doi.org/10.1161/01.RES.51.1.1
PMid:6282496

 
 

7. Norbert Klauke, Godfrey Smith, and Jonathan Cooper: Microfluidic systems to examine intercellular coupling of pairs of cardiac myocytes. Lab Chip 7, 731-739 (2007)
http://dx.doi.org/10.1039/b706175g
PMid:17538715

 
 

8. Christine Lamont, Paul Luther, C. William Balke, and W. Gil Wier: Intercellular Ca2+ waves in rat heart muscle. J Physiol 512 (Pt 3), 669-676 (1998) 13 Thomas Desplantez, Deborah Halliday, Emmanuel Dupont, and Robert Weingart: Cardiac connexins Cx43 and Cx45: formation of diverse gap junction channels with diverse electrical properties. Pflugers Arch 448, 363-375 (2004)

 
 

9. Lloyd Davis, H. Lee Kanter, Eric Beyer, and Jeffrey Saffitz: Distinct gap junction protein phenotypes in cardiac tissues with disparate conduction properties. J Am Coll Cardiol 24, 1124-1132 (1994)
http://dx.doi.org/10.1016/0735-1097(94)90879-6

 
 

10. Lee Kanter, Jeffrey Saffitz, and Eric Beyer: Cardiac myocytes express multiple gap junction proteins. Circ Res 70, 438-444 (1992)
http://dx.doi.org/10.1161/01.RES.70.2.438
PMid:1310450

 
 

11. Toon van Veen, A.A., Harold van Rijen, and Tobias Opthof: Cardiac gap junction channels: modulation of expression and channel properties. Cardiovasc Res 51, 217-229 (2001)
http://dx.doi.org/10.1016/S0008-6363(01)00324-8

 
 

12. G. Trevor Cottrell, and Janis Burt: Functional consequences of heterogeneous gap junction channel formation and its influence in health and disease. Biochim Biophys Acta 1711, 126-141 (2005)
http://dx.doi.org/10.1016/j.bbamem.2004.11.013
PMid:15955298

 
 

14. Xun Ai and Steven Pogwizd: Connexin 43 downregulation and dephosphorylation in nonischemic heart failure is associated with enhanced colocalized protein phosphatase type 2A. Circ Res 96, 54-63 (2005)
http://dx.doi.org/10.1161/01.RES.0000152325.07495.5a
PMid:15576650

 
 

15. Emmaneul Dupont, Tsutomu Matsushita, Riyaz Kaba, Cristina Vozzi, Steven Coppen, Natasha Khan, Raffi Kaprielian, Magdi Yacoub, and Nicholas Severs: Altered connexin expression in human congestive heart failure. J Mol Cell Cardiol 33, 359-371 (2001)
http://dx.doi.org/10.1006/jmcc.2000.1308
PMid:11162139

 
 

16. Kathryn Yamada, Joseph Rogers, Rune Sundset, Thomas Steinberg, and Jeffrey Saffitz: Up-regulation of connexin45 in heart failure. J Cardiovasc Electrophysiol 14, 1205-1212 (2003)
http://dx.doi.org/10.1046/j.1540-8167.2003.03276.x
PMid:14678136

 
 

17. Nicholas Peters, James Coromilas, Nicholas Severs, and Andrew Wit: Disturbed connexin43 gap junction distribution correlates with the location of reentrant circuits in the epicardial border zone of healing canine infarcts that cause ventricular tachycardia. Circulation 95, 988-996 (1997)
http://dx.doi.org/10.1161/01.CIR.95.4.988
PMid:9054762

 
 

18. Nicholas Severs, Alexandra Bruce, Emmanuel Dupont, and Stephen Rothery: Remodelling of gap junctions and connexin expression in diseased myocardium. Cardiovasc Res 80, 9-19 (2008)
http://dx.doi.org/10.1093/cvr/cvn133
PMid:18519446 PMCid:2533424

 
 

19. Mahmud Uzzaman, Haruo Honjo, Yoshiko Takagishi, Luni Emdad, Anthony Magee, Nocholas Severs, and Itsuo Kodama: Remodeling of gap junctional coupling in hypertrophied right ventricles of rats with monocrotaline-induced pulmonary hypertension. Circ Res 86, 871-878 (2000)
http://dx.doi.org/10.1161/01.RES.86.8.871
PMid:10785509

 
 

20. Fadi Akar, David Spragg, Richard Tunin, David Kass and Gordon Tomaselli: Mechanisms underlying conduction slowing and arrhythmogenesis in nonischemic dilated cardiomyopathy. Circ Res 95, 717-725 (2004)
http://dx.doi.org/10.1161/01.RES.0000144125.61927.1c
PMid:15345654

 
 

21. Fadi Akar, Robert Nass, Samuel Hahn, Eugenio Cingolani. Manish Shah, Geoffrey Hesketh, Deborah DiSilvestre, Richard Tunin, David Kass, and Gordon Tomaselli: Dynamic changes in conduction velocity and gap junction properties during development of pacing-induced heart failure. Am J Physiol Heart Circ Physiol 293, H1223-1230 (2007)
http://dx.doi.org/10.1152/ajpheart.00079.2007
PMid:17434978

 
 

22. Geoffrey Hesketh, Manish Shah, Victoria Halperin, Carol Cooke, Fadi Akar, Timothy Yen, David Kass, Carolyn Machamer, Jeffinfer Van Eyk, and Gordon Tomaselli: Ultrastructure and regulation of lateralized connexin43 in the failing heart. Circ Res 106, 1153-116 (2010)
http://dx.doi.org/10.1161/CIRCRESAHA.108.182147
PMid:20167932 PMCid:2896878

 
 

23. Virginijus Valiunas, Feliksas Bukauskas, and Robert Weingart: Conductances and selective permeability of connexin43 gap junction channels examined in neonatal rat heart cells. Circ Res 80, 708-719 (1997)
http://dx.doi.org/10.1161/01.RES.80.5.708
PMid:9130452

 
 

24. Richard Veenstra and Robert DeHaan: Cardiac gap junction channel activity in embryonic chick ventricle cells. Am J Physiol Heart Circ Physiol 254, H170-180 (1988)

 
 

25. Richard Veenstra, Hong-ZhanWang, Eric Beyer, and Peter Brink: Selective dye and ionic permeability of gap junction channels formed by connexin45. Circ Res 75, 483-490 (1994)
http://dx.doi.org/10.1161/01.RES.75.3.483
PMid:7520372

 
 

26. Masaki Kameyama: Electrical coupling between ventricular paired cells isolated from guinea-pig heart. J Physiol 336, 345-357 (1983)
PMid:6875911 PMCid:1198973

 
 

27. David Spray, Roy White, Francoise Mazet, and M. Bennett: Regulation of gap junctional conductance. Am J Physiol Heart Circ Physiol 248, H753-764 (1985)

 
 

28. Sylvia Suadicani, Monique Vink, and David Spray: Slow intercellular Ca(2+) signaling in wild-type and Cx43-null neonatal mouse cardiac myocytes. Am J Physiol Heart Circ Physiol 279, H3076-3088 (2000)
PMid:11087266

 
 

29. B Husse and Manfred Wussling: Developmental changes of calcium transients and contractility during the cultivation of rat neonatal cardiomyocytes. Mol Cell Biochem 163-164, 13-21 (1996)
http://dx.doi.org/10.1007/BF00408636
PMid:8974035

 
 

30. Masashi Seguchi, Jennifer Harding, and Jay Jarmakani: Developmental change in the function of sarcoplasmic reticulum. J Mol Cell Cardiol 18, 189-195 (1986)
http://dx.doi.org/10.1016/S0022-2828(86)80471-0

 
 

31. Robert Hoyt, Mark Cohen, and Jeffrey Saffitz: Distribution and three-dimensional structure of intercellular junctions in canine myocardium. Circ Res 64, 563-574 (1989)
http://dx.doi.org/10.1161/01.RES.64.3.563
PMid:2645060

 
 

32. Tetsuro Takamatsu: Arrhythmogenic substrates in myocardial infarct. Pathol Int 58, 533-543 (2008)
http://dx.doi.org/10.1111/j.1440-1827.2008.02269.x
PMid:18801068

 
 

33. Tetsuhiro Minamikawa, Stephen Cody, and David Williams: In situ visualization of spontaneous calcium waves within perfused whole rat heart by confocal imaging. Am J Physiol Heart Circ Physiol 272, H236-243 (1997)

 
 

34. Julian Thimm, Adam Mechler, Hai Lin, Seung Rhee, and Ratnesh Lal: Calcium-dependent open/closed conformations and interfacial energy maps of reconstituted hemichannels. J Biol Chem 280, 10646-10654 (2005)
http://dx.doi.org/10.1074/jbc.M412749200
PMid:15615707

 
 

35. Hendrik ter Keurs, Yuji Wakayama, Masahito Miura, Tsuyoshi Shinozaki, Bruno Stuyvers, Penelope Boyden, and Amir Landesberg: Arrhythmogenic Ca(2+) release from cardiac myofilaments. Prog Biophys Mol Biol 90, 151-171 (2006)
http://dx.doi.org/10.1016/j.pbiomolbio.2005.07.002
PMid:16120452

 
 

36. Yuji Wakayama, Masahito Miura, Bruno Stuyvers, Penelope Boyden, and Hendrik ter Keurs: Spatial nonuniformity of excitation-contraction coupling causes arrhythmogenic Ca2+ waves in rat cardiac muscle. Circ Res 96, 1266-1273 (2005)
http://dx.doi.org/10.1161/01.RES.0000172544.56818.54
PMid:15933267

 
 

37. Masahito Miura, Penelope Boyden, and Hendrik ter Keurs: Ca2+ waves during triggered propagated contractions in intact trabeculae. Determinants of the velocity of propagation. Circ Res 84, 1459-1468 (1999)
http://dx.doi.org/10.1161/01.RES.84.12.1459
PMid:10381899

 
 

38. Hendrik ter Keurs, Yinhui Zhang, Amy Davidoff, Penelope Boyden, Yuji Wakayama, and Masahito Miura: Damage induced arrhythmias: mechanisms and implications. Can J Physiol Pharmacol 79, 73-81 (2001)
http://dx.doi.org/10.1139/y00-114
PMid:11201504

 
 

39. Sunil Kapur, Gary Aistrup, Rohan Sharma, James Kelly, Rishi Arora, Jiabo Zheng, Mitra Veramasuneni, Alan Kadish, C. William Balke, and J. Andrew Wasserstrom: Early development of intracellular calcium cycling defects in intact hearts of spontaneously hypertensive rats. Am J Physiol Heart Circ Physiol 299, H1843-1853 (2010)
http://dx.doi.org/10.1152/ajpheart.00623.2010
PMid:20889840 PMCid:3006292

 

40. Andreas Baader, Lorenz Buchler, Lilly Bircher-Lehmann, and Andre Kleber: Real time, confocal imaging of Ca(2+) waves in arterially perfused rat hearts. Cardiovasc Res 53, 105-115 (2002)
http://dx.doi.org/10.1016/S0008-6363(01)00423-0

 

41. Hendrik ter Keurs, Yuji Wakayama, Yoshinao Sugai, Guy Price, Yutaka Kagaya, Penelope Boyden, Masahito Miura, and Bruno Stuyvers: Role of sarcomere mechanics and Ca2+ overload in Ca2+ waves and arrhythmias in rat cardiac muscle. Ann N Y Acad Sci 1080, 248-267 (2006)
http://dx.doi.org/10.1196/annals.1380.020
PMid:17132788

 
 

42. Ernest Page and Janet Buecker: Development of dyadic junctional complexes between sarcoplasmic reticulum and plasmalemma in rabbit left ventricular myocardial cells. Morphometric analysis. Circ Res 48, 519-522 (1981)
http://dx.doi.org/10.1161/01.RES.48.4.519
PMid:7460221

 
 

43. Thomas Smith and Raymond Kelly. Therapeutic strategies for CHF in the 1990s. Hosp Pract (Off Ed) 26, 127-134, 139-140, 145-126 passim (1991)

       
         
 

44. Beatrice Wittenberg, Roy White, Rosemary Ginzberg, and David Spray: Effect of calcium on the dissociation of the mature rat heart into individual and paired myocytes: electrical properties of cell pairs. Circ Res 59, 143-150 (1986)
http://dx.doi.org/10.1161/01.RES.59.2.143
PMid:2427246

 
 

45. Bruno Stuyvers, Penelope Boyden, and Hendrik ter Keurs: Calcium waves: physiological relevance in cardiac function. Circ Res 86, 1016-1018 (2000)
http://dx.doi.org/10.1161/01.RES.86.10.1016
PMid:10827129

 
 

46. Manish Shah, Fadi Akar, and Gordon Tomaselli: Molecular basis of arrhythmias. Circulation 112, 2517-2529 (2005)
http://dx.doi.org/10.1161/CIRCULATIONAHA.104.494476
PMid:16230503

 
 

47. Michael Rubart and Douglas Zipes: Mechanisms of sudden cardiac death. J Clin Invest 115, 2305-2315 (2005)
http://dx.doi.org/10.1172/JCI26381
PMid:16138184 PMCid:1193893

 
 

48. Steven Marx, Steven Reiken, Yuji Hisamatsu, Thotalla Jayaraman, Daniel Burkhoff, Nora Rosemblit, and Andrew Marks: PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (ryanodine receptor): defective regulation in failing hearts. Cell 101, 365-376 (2000)
http://dx.doi.org/10.1016/S0092-8674(00)80847-8

 
 

49. Thomas Eschenhagen: Is ryanodine receptor phosphorylation key to the fight or flight response and heart failure? J Clin Invest 120, 4197-4203 (2010)
http://dx.doi.org/10.1172/JCI45251
PMid:21099119 PMCid:2994341

 
 

50. J. Andrew Wasserstrom, Rohan Sharma, Sunil Kapur, James Kelly, Alan Kadish, C. William Balke and Gary Aistrup: Multiple defects in intracellular calcium cycling in whole failing rat heart. Circ Heart Fail 2, 223-232 (2009)
http://dx.doi.org/10.1161/CIRCHEARTFAILURE.108.811539
PMid:19808344

 
 

51. Yuanfang Xie, Daisuke Sato, Alan Garfinkel, Zhilin Qu, and James Weiss: So little source, so much sink: requirements for afterdepolarizations to propagate in tissue. Biophys J 99, 1408-1415 (2010)
http://dx.doi.org/10.1016/j.bpj.2010.06.042
PMid:20816052 PMCid:2931729

 
 

52. James Weiss, Michael Nivala, Alan Garfinkel, and Zhilin Qu: Alternans and arrhythmias: from cell to heart. Circ Res 108, 98-112 (2011)
http://dx.doi.org/10.1161/CIRCRESAHA.110.223586
PMid:21212392 PMCid:3076605

 
 

53. Harry Fozzard and Mark Schoenberg: Strength-duration curves in cardiac Purkinje fibres: effects of liminal length and charge distribution. J Physiol 226, 593-618 (1972)
PMid:4637625 PMCid:1331166

 
 

54. J. Andrew Wasserstrom, Yohannes Shiferaw, Wei Chen, Satvik Ramakrishna, Heetabh Patel, James Kelly, Matthew O'Toole, Amanda Pappas, Nimi Chirayil, Nikhil Bassi, Lisa Akintilo, Megan Wu, Rishi Arora, and Gary Aistrup: Variability in timing of spontaneous calcium release in the intact rat heart is determined by the time course of sarcoplasmic reticulum calcium load. Circ Res 107, 1117-1126 (2010)
http://dx.doi.org/10.1161/CIRCRESAHA.110.229294
PMid:20829511 PMCid:2967435

 

Key Words: Calcium Waves, Arrhythmias, Delayed Afterdepolarizations, Intracellular Calcium Overload. Review

Send correspondence to: J. Andrew Wasserstrom, 310 E. Superior St., Tarry 12-723, Chicago, Il 60611, Tel: 312-503-1384, Fax: 312-503-3891, E-mail:ja-wasserstrom@northwestern.edu