What is a Ca2+ wave? Is it like an Electrical Wave?

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Abstract

Arrhythmia subcellular mechanisms are constantly being explored. Recent knowledge has shown that travelling Ca2+ waves in cardiac cells are critical for delayed afterdepolarisations and in some cases, early afterdepolarisations. In this review, we comment on the properties of cardiac Ca2+ waves and abnormal Ca2+ releases in terms of properties used to describe electrical waves; propagation, excitability and refractoriness.

Disclosure
The authors have no conflicts of interest to declare
Correspondence
Dr Penelope Boyden, Columbia University, 630 W 168th St NY NY 10032 US. E: Pab4@cumc.columbia.edu
Received date
12 January 2015
Accepted date
25 February 2015
DOI
http://dx.doi.org/10.15420/aer.2015.4.1.35

Abnormalities in electrical rhythm were studied by Einthoven at the start of the 20th century. In the 1940s, studies by Bozler et al.1 described contractile signals that appeared to be ‘triggered’ heart beats. Today we use the term delayed afterdepolarisations (DADs) to refer to oscillations in voltage that follow a driven action potential.

In the mid-1970s, progress was made when Lederer and Tsien developed a method to study the underlying electrical mechanism of DADs2 (see Figure 1). In a voltage clamped, multicellular canine Purkinje fibre, the transient depolarisation of the resting potential of the fibre was found to be due to a transient inward current (Iti) (see Figure 1A). Many initially challenged this idea but these authors went on to show that Iti was not an artifact and that the Iti they recorded in Purkinje fibres was Ca2+ dependent (see Figure 1B).2,3 This was a relatively new concept for cardiac electrophysiology; that is, the idea that Ca2+ inside the cell could feed back and affect the electrics of the cell’s membrane. In a recent review this was referred to as reverse mode excitation–contraction (EC) coupling.4

Here we will discuss the Ca2+ wave and address the question: ‘Is it like the electrical wave with which we are all familiar?’

Functional Anatomy

A propagating electrical wave utilises the energy of the chemical gradients set up by the cardiac sarcolemma.5 Electrical waves rely on activation of a series of ion channels (eg. Na channel proteins) for forward propagation of the wave.

Propagation of a Ca2+ wave also depends on the energy stored in the myocyte. But in this case the energy comes from the presence of Ca2+ stored in the sarcoplasmic reticulum (SR). The SR is a specialised intracellular membrane structure that in a myocyte stores Ca2+ that has been pumped into it by a SR membrane pump, SERCA2. In the absence of Ca2+ influx through the plasma membrane or mischievous Ca2+ wandering the cytosol, the Ca2+ in SR stays in the SR. This is because the SR ligand-operated Ca2+ channel, the ryanodine receptor channel (RyR), which guards this SR Ca2+ store, has a low probability of opening.

Interestingly, just as surface membrane ion channels (eg. Na channels) are positioned in a specific array6 to provide for smooth electrical wave propagation, RyR channel proteins in myocytes, Purkinje and atrial cells are clustered and aligned in a specific micro-anatomic pattern (see Figure 2).7,8,19 Presumably, and particularly in the tubulated structures of ventricular myocytes, this specific patterning is to allow for uniform Ca2+ release from SR during the action potential (forward mode EC coupling). The orderly pattern of RyRs on the SR sets up a series of potential release sites of Ca2+ in the cell.

Ca2+-induced Ca2+ Release

Fabiato’s work on the properties of the cardiac SR provided a potential explanation for spontaneous Ca2+ release in mechanically skinned cells in which the SR and RyR were intact and excessive Ca2+ loading of the SR caused spontaneous Ca2+ release.9,10,11 The mechanism for increased probability of opening of RyR when the SR is heavily loaded with Ca2+ is still uncertain, but suggests that the RyR channel is sensitive to both cytosolic and luminal [Ca2+] of the SR. Hence, the oscillatory character of a triggered arrhythmia in myocardium with a high cellular Ca2+ load may be due to further increase of Ca2+ entry into the cells during driven action potentials, which causes even more Ca2+ loading of the SR. So as soon as the release process has recovered after the electrically evoked Ca2+ release, the overloaded SR again releases a fraction of its Ca2+ into the cytosol. The requirement that the Ca2+ release mechanism must recover first (refractoriness) would explain the presence of a delay between aftercontractions and afterdepolarisations and the preceding beat.

Figure 1: Evidence of Iti in Multicellular Canine Purkinje Fibres

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Figure 2: Architecture of Ca2+ Release Channels in Purkinje (A) and Atrial (B) Cells

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Ca2+ waves occurring in cardiac cells depend on the regenerative production of a diffusible molecule that triggers Ca2+ release from adjacent SR stores. Cytosolic Ca2+ is one such ion and thus the process is called Ca2+ induced (intracellular) Ca2+ release (CICR) (see Figure 3B).12 This schematic shows Ca2+ wave propagation from one RyR cluster to another. Calsequestrin (CASQ), a Ca2+ binding protein, is found in the SR lumen and aids wave propagation inside cells (see Figure 3C). The released Ca2+ constitutes a leak from the SR and tends to reduce the overload. This phenomenon has been observed in different forms, all of which fall under the general definition of Ca2+ leak: increased probability of opening of RyR in lipid bilayer experiments,13 a biochemically detectable loss of Ca2+ from the SR;14 Ca2+ sparks in isolated cells and muscle;15,16 micro Ca2+ waves in isolated cells and muscle13,14 and Purkinje cells after infarction;7,19 and multicellular cellular Ca2+ waves.17–19 The threshold for Ca2+ leak is reduced in some arrhythmogenic mutations of the RyR,13 CASQ20 and in acquired dysfunction of the RyR such as in congestive heart failure and post MI.7,21–23

Intracellular Ca2+ waves can be seen in normal canine atrial and Purkinje myocytes during forward mode EC coupling (see Figure 4).7 Here, a line of Ca2+ release is seen peripherally just after the plateau of the AP and this Ca2+ then via CICR, sets up a Ca2+ transient that moves to the core of the Purkinje cell (see Figure 4).

Figure 3: Simple Schematic Showing the Important Components of Ca2+ Wave Propagation

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Figure 4: AP-evoked Global Ca2+ Transients in Purkinje Cells are Robust

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Figure 5A shows that spontaneous Ca2+ waves occur often during diastolic intervals in Purkinje cells dispersed from the infarcted heart.7
In some cases the waves formed varied oscillatory changes in voltage as

Figure 5: Ca2+ Waves Lead to Spontaneous Depolarisations

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Ca2+ of wave is pumped out of cell via the sodium-calcium exchanger (Iti) (see Figure 5B). However, in the same cell the oscillatory voltage change is large enough to reach threshold, triggering a nondriven AP (see Figure 6). In these cells the small voltage signals and triggered activity are sensitive to an agent that blocks Ca2+ release of the RyR protein, ryanodine.

Propagation Between Cardiac Cells

Intercellular electrical transmission occurs via a set of ion channel proteins and specialised membrane structures called gap junctions.24 Each channel is formed by close apposition of two hemichannels each of which is in an opposing cell.25 Gap junctions can provide passage of many molecules (cAMP, Ca2+, IP3, ATP).26–28 In cardiac cells gap junctional conductance can be regulated acutely by pH, Ca2+, cAMP and cGMP.29 Therefore Ca2+ ions can flow through gap junctions as well as inhibiting gap junctional conductance.

Since Ca2+ waves propagate along the cell, it is important to know whether they propagate between cells via gap junctions. Many have observed Ca2+ waves passing between two cardiac cells30 and have assumed a role for gap junctions. Ca2+ ions released upon RyR activation can travel as a wave across cells31 and propagate to adjoining cells via gap junctions.19 In cells transfected with both

Figure 6: Large Extensive Ca2+ Waves Lead to Sufficient Depolarisation to Elicit Nondriven APs

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connexin 43 (Cx43) and RyR receptors the propagation of Ca2+ waves between cells was sensitive to octanol.32 Furthermore, in this experimental cell model, both Ca2+ wave propagation and gap junctional conductance between paired cardiac cells are related to the state of tyrosine phosphorylation of Cx43.33 How the Ca2+ wave crosses
the gap junction is unknown but extracellular disulphide bonds of
the Cx43 proteins between the adjoining cells appear critical for
wave propagation.32

Arguably the occurrence of Ca2+ wave propagation from one cell to another is not a frequent event, but when one does happen, it appears to be due to a CICR mechanism (see Figure 7). In adult rat cells, Li et al.30 assert that Ca2+ wave propagation between cells mostly occurs at side-to-side junctions and the ultrastructure of the connections between the gap and SR release units is critical. Propagation failure occurs when distance between the disc membrane and neighbouring SR release unit is too large, such as occurs at end-to-end junctions.

At the tissue level, the subcellular Ca2+ dynamics combine with cell coupling and tissue architecture to generate multicellular Ca2+ wave dynamics. We understand focal electrical excitations due to triggered activity but are the dynamics of Ca waves similar? Spontaneous Ca2+ releases that triggered Ca2+ waves have been mapped in both normal and failing heart tissues34 (see Figure 8).35 Notably each

Figure 7: Intercellular Ca2+ Wave Propagation

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Figure 8: Multicellular Ca2+ Wave Propagation

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spontaneous event occurred in a region of myocardium comprised of many cells (~3000 cells).34 In failing myocardium it is the rate of rise of the Ca2+ releases (waves) rather than their amplitude that is associated with triggered beats. In atria from CASQ-/- mice, the runs of APs during nondriven electrical activity were always preceded by rises in Ca2+, however total time of atrial activation increases with number of beats.36

Refractoriness

Electrical refractoriness is clearly related to time course of action potential repolarisation as well as the status of the sodium channel. In forward mode EC coupling, after the cellular action potential evoked Ca2+ transient due SR Ca2+ release, time is needed before a second Ca2+ release occurs of similar amplitude. Thus there is also a recovery process of Ca2+ release in cardiac cells that is independent of membrane voltage. In the electrically stimulated cell, the recovery of the SR Ca2+ release process or refractoriness is determined by recovery of the L-type calcium channel influx as well as the time course of the SR refilling. The latter can be examined by assessing the interval between spontaneous Ca2+ sparks occurring at the same release site (see Figure 9). Ca2+ spark termination is due to local depletion of Ca2+ within junctional SR. The time between one spark and another is related to the ryanodine sensitivity or threshold for Ca2+ release. How fast junctional SR refills after depletion is important for the recovery of both spark amplitude and CICR or Ca2+ wave formation.37 Shortened refractoriness of this process has been seen in both acquired (post MI)38 and genetic disease.39 For example, loss of calsequestrin (CASQ) in SR of cells in some CVPT patients produces fast SR refilling and greater likelihood of a trigger for re-release40 and Ca2+ waves.

Figure 9: Refractoriness of the Ca2+ Release Events

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Conclusion

While cellular electrical events and Ca2+ waves can occur independently of each other, it is when they interact and feed back on each other that complicated arrhythmogenic behaviour can occur (eg. alternans).4 Each physiological process has its own mechanisms of initiation, propagation and refractoriness and thus would be expected to have its own possible targets for effective therapeutic agents. For example, CamKII,41 sodium-calcium exchanger protein42 and EHD343 proteins have all emerged as possible targets for Ca2+-dependent arrhythmias. n

References
  1. Bozler E. The initiation of impulses in cardiac muscle. Amer J Physiol 1943;138:273–82.
    Crossref
  2. Lederer WJ, Tsien RW. Transient inward current underlying arrhythmogenic effects of cardiotonic steriods in Purkinje fibers. J Physiol 1976;263:73–100.
    Crossref | Pubmed
  3. Kass RS, Lederer WJ, Tsien RW, Weingart R. Role of calcium ions in transient inward currents and aftercontractions induced by strophanthidin in cardiac Purkinje fibres. J Physiol (Lond) 1978;281:187–208.
    Crossref | Pubmed
  4. Ter Keurs HEDJ, Boyden PA. Calcium and arrhythmogenesis. Physiol Rev 2007;87:457–506.
    Crossref | Pubmed
  5. Kleber AG, Rudy Y. Basic mechanisms of cardiac impulse propagation and associated arrhythmias. Physiol Rev 2004;84:431–88.
    Crossref | Pubmed
  6. Dun W, Lowe JS, Wright PA, et al. Ankgrin-G participates in INa remodeling in myocytes from the broder zone of infarcted canine hearts. PloS ONE 2013;e78087.
    Crossref | Pubmed
  7. Boyden PA, Barbhaiya C, Lee T, Ter Keurs HEDJ. Nonuniform Ca2+ transients in arrhythmogenic purkinje cells that survive in the infarcted canine heart. Cardiovasc Res 2003;57:681–93.
    Crossref | Pubmed
  8. Thul R, Coombes S, Roderick HL, Bootman MD. Subcellular calcium dynamics in a whole-cell model of an atrial myocyte. Proc Natl Acad Sci 2012;109:2150–5.
    Crossref | Pubmed
  9. Fabiato A. Time and calcium dependence of activation and inactivation of calcium induced release of calcium from the sarcoplasmic reticulum of a skinned cardiac Purkinje cell.
    J Gen Physiol 1985;85:247–90.
    Crossref | Pubmed
  10. Fabiato A. Simulated calcium current can both cause loading
    in and trigger calcium release from the sarcoplasmic reticulum of a skinned canine cardiac Purkinje cell. J Gen Physiol
    1985;85:291–320.
    Crossref | Pubmed
  11. Fabiato A. Spontaneous versus triggered contractions of calcium tolerant cardiac cells from the adult rat ventricle. Basic Res Cardiol 1985;80(Suppl 2):83–8.
    Pubmed
  12. Swietach P, Spitzer KW, Vaughan-Jones RD. Modeling calcium waves in cardiac myocytes: importance of calcium diffusion. Front Biosci (Landmark Ed) 2010;15:661–80.
    Crossref | Pubmed
  13. Jiang D, Xiao B, Yang D, et al. RyR2 mutations linked to ventricular tachycardia and sudden death reduce the threshold for store-overload-induced Ca2+ release (SOICR). Proc Natl Acad Sci 2004;101:13062–7.
    Crossref | Pubmed
  14. Yano M, Ikeda Y, Matsuzaki M. Altered intracellular Ca2+ handling in heart failure. J Clin Invest 2005;115:556–64.
    Crossref | Pubmed
  15. Wier WG, ter Keurs HE, Marban E, et al. Ca2+ ‘sparks’ and waves in intact ventricular muscle resolved by confocal imaging. Circ Res 1997;81:462–9.
    Crossref | Pubmed
  16. Shannon TR, Pogwizd SM, Bers DM. Elevated sarcoplasmic reticulum Ca2+ leak in intact ventricular myocytes from rabbits in heart failure. Circ Res 2003;93:592–4.
    Crossref | Pubmed
  17. Miura M, Boyden PA, terKeurs HEDJ. Ca2+ waves during triggered propagated contractions in intact trabeculae: determinants of the velocity of propagation. Circ Res 1999;84:1459–68.
    Crossref | Pubmed
  18. Lamont C, Luther PW, Wier WG. Intercellular Ca2+ waves in
    rat heart muscle. J Physiol 1998;512:669–76.
    Crossref | Pubmed
  19. Stuyvers BD, Dun W, Matkovich SJ, et al. Ca2+ sparks and
    Ca2+ waves in Purkinje cells: a triple layered system of activation. Circ Res 2005;97:35–43.
    Crossref | Pubmed
  20. di Barletta MR, Viatchenko-Karpinski S, Nori A, et al. Clinical phenotype and functional characterization of CASQ2 mutations associated with catecholaminergic polymorphic ventricular tachycardia. Circ 2006;114;1012–9.
    Crossref | Pubmed
  21. Wehrens XHT, Lehnart SE, Reiken S, et al. Enhancing
    calstabin binding to ryanodine receptors improves cardiac and skeletal muscle function in heart failure. PNAS 2005;102:9607–12.
    Crossref | Pubmed
  22. Yano M, Ono K, Ohkusa T, et al. Altered stoichiometry of FKBP12.6 versus ryanodine receptor as a cause of abnormal Ca2+ leak through ryanodine receptor in heart failure. Circ 2000;102:2131–6.
    Crossref | Pubmed
  23. Bers DM, Eisner DA, Valdivia HH. Sarcoplasmic reticulum Ca2+ and heart failure: roles of diastolic leak and Ca2+ transport. Circ Res 2003;93:487–90.
    Crossref | Pubmed
  24. Kleber AG, Rudy Y. Basic mechanisms of cardiac impulse propagation and associated arrhythmias. Physiol Rev 2004;84:431–88.
    Crossref | Pubmed
  25. Yeager M. Structure of cardiac gap junction membrane channels. In: Spooner PM, Joyner RW, Jalife J, eds. Discontinuous Conduction in the Heart. First ed. Armonk: Futura; 1997;161–84.
  26. Boitano S, Dirksen ER, Sanderson MJ. Intercellular propagation of calcium waves mediated by inositol triphosphate. Science 1992;258:292–5.
    Crossref | Pubmed
  27. Saez JC, Connor JA, Spray DC, Bennett MV. Hepatocyte gap junctions are permeable to the second messenger, inositiol 1,4,5-trisphosphate and to calcium ions. Proc Natl Acad Sci 1989;86:2708–12.
    Crossref | Pubmed
  28. Sandberg K, Ji H, Iida Y, Catt KJ. Intercellular communication between follicular angiotensin receptors and Xenopus laevis oocytes: mediation by an inositol 1,4,5-trisphosphate-dependent mechanism. J Cell Biol 1992;117:157–67.
    Crossref
  29. Rosen MR, Boyden PA. Is there a pharmacology of discontinuous conduction? In: Spooner PM, Joyner RW, Jalife J, eds. Discontinuous Conduction in the Heart. First ed. Armonk: Futura, 1997; 471–82.
  30. Li Y, Eisner DA, O’Neill SC. Do calcium waves propagate between cells and synchronize alternating calcium release in rat ventricular myocytes? J Physiol 2012;590:6353–61.
    Crossref | Pubmed
  31. Zhang Y, Miura M, Ter Keurs HEDJ. Triggered propagated contractions in rat cardiac trabeculae; inhibition by octanol and heptanol. Circ Res 1996;79:1077–85.
    Crossref | Pubmed
  32. Toyofukyu T, Yabuki M, Otsu K, et al. Intercellular calcium signaling via gap junction in connexin43 transfected cells.
    J Biol Chem 1998;273:1519–28.
    Crossref | Pubmed
  33. Toyofuku T, Yabuki M, Otsu K, et al. Functional role of c-Src in gap junctions of the cardiomyopathic heart. Circ Res 1999;85:672–81.
    Crossref | Pubmed
  34. Katra RP, Laurita KR. Cellular mechanism of calcium-mediated triggered activity in the heart. Circ Res 2005;96:535–42.
    Crossref | Pubmed
  35. Hoeker GS, Katra RP, Wilson LD, et al. Spontaneous calcium release in tissue from the failing canine heart. Am J Physiol Heart Circ Physiol 2009;297:H1235–42.
    Crossref | Pubmed
  36. Lou Q, Belevych AE, Liu B, et al. Alternating membrane potential/calcium interplay underlies repetitive focal activity in a genetic model of calcium dependent atrial arrhythmias.
    J Physiol 2015;593:1443–58.
    Crossref | Pubmed
  37. Ramay HR, Liu OZ, Sobie EA. Recovery of cardiac calcium release is controlled by sarcoplasmic reticulum refilling and ryanodine receptor sensitivity. Cardiovasc Res 2011;91:598–605.
    Crossref | Pubmed
  38. Belevych AE, Terentyev D, Terentyeva R, et al. Shortened Ca2+ signaling refractoriness underlies cellular arrhythmogenesis in a postinfarction model of sudden cardiac death. Circ Res 2012;110:569–77.
    Crossref | Pubmed
  39. Brunello L, Slabaugh JL, Ho HT, et al. Decreased RyR2 refractoriness determines myocardial synchronization of aberrant Ca2+ release in a genetic model of arrhythmia. Proc Natl Acad Sci 2013;110:10312–7.
    Crossref | Pubmed
  40. Liu N, Denegri M, Dun W, et al. Abnormal propagation of calcium waves and ultrastructural remodeling in recessive catecholaminergic polymorphic ventricular tachycardia. Circ Res 2013;113:142–52.
    Crossref | Pubmed
  41. Pellicena P, Schulman H. CaMKII inhibitors: from research tools to therapeutic agents. Front Pharmacol 2014;5:21.
    Crossref | Pubmed
  42. Nagy N, Kormos A, Kohajda Z, et al. Selective Na+/Ca2+ exchanger inhibition prevents Ca2+ overload-induced triggered arrhythmias. Br J Pharmacol 2014;171:5665–81.
    Crossref | Pubmed
  43. Curran J, Makara MA, Little SC, et al. EHD3-dependent endosome pathway regulates cardiac membrane excitability and physiology. Circ Res 2014;115:68–78.
    Crossref | Pubmed