Cardiovascular Research

Cardiovascular Research Advance Access first published online on September 13, 2007
This version [Corrected Proof] published online on October 15, 2007
Cardiovascular Research, doi:10.1093/cvr/cvm009

This Article Abstract FREE Full Text (PDF) All Versions of this Article: 77/2/285    most recent cvm009v2 cvm009v1 E-letters: Submit a response Alert me when this article is cited Alert me when E-letters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Request Permissions Disclaimer Google Scholar Articles by Venetucci, L. A. Articles by Eisner, D. A. Search for Related Content PubMed PubMed Citation Articles by Venetucci, L. A. Articles by Eisner, D. A. Social Bookmarking          What's this?

The sarcoplasmic reticulum and arrhythmogenic calcium release

Luigi A. Venetucci, Andrew W. Trafford, Stephen C. O'Neill and David A. Eisner^*

Unit of Cardiac Physiology, University of Manchester, Core Technology Facility, 46 Grafton St, Manchester M13 9NT, UK

^* Corresponding author. Tel +44 161 275 2702; fax: +44 161 275 2703. E-mail address: eisner{at}man.ac.uk

Time for primary review: 26 days

	Abstract

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
There is much evidence showing that some lethal ventricular arrhythmias arise from waves of Ca²⁺ release from the sarcoplasmic reticulum (SR) that propagate along cardiac cells. The purpose of this review is to discuss the mechanism of production of these waves and how they depend on the properties of the SR Ca²⁺ release channel or ryanodine receptor (RyR). The best-known method of producing Ca²⁺ waves is by increasing the Ca²⁺ content of the cell by either increasing Ca²⁺ influx or decreasing efflux. Once SR Ca²⁺ content reaches a threshold level a Ca²⁺ wave is produced. Altering the properties of the RyR affects the threshold level of Ca²⁺ required to produce a wave. Patients with a mutation in the RyR suffer from catecholaminergic polymorphic ventricular tachycardia, and this may be due to a decrease in the SR Ca²⁺ threshold for wave production. Heart failure has also been suggested to result in Ca²⁺ waves due to a leak of Ca²⁺ through the RyR. We review the finding that these changes in RyR function will only result in Ca²⁺ waves in the steady state if some other mechanism maintains the SR Ca²⁺ content. The review concludes with a description of potential mechanisms for treating arrhythmias produced by Ca²⁺ waves.

KEYWORDS Calcium; Wave; Delayed afterdepolarization; Arrhythmia

	1. Introduction

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
Much of our current understanding of the role of the sarcoplasmic reticulum (SR) in arrhythmias comes from work done in the early 1970s on the arrhythmogenic effects of intoxication with digitalis glycosides. Studies on isolated Purkinje fibres found that digitalis-like glycosides produced a delayed after depolarization (DAD) that could reach threshold and result in an action potential.^1–3 Rosen et al.⁴ performed a cross-perfusion experiment in which they infused a dog with ouabain and took blood from this animal to superfuse an isolated Purkinje fibre. When the point was reached at which the dog developed ventricular ectopic beats, DADs were seen in the isolated Purkinje fibre thereby showing the link between DADs and ventricular arrhythmias.

Subsequent work used the voltage clamp technique to investigate the membrane currents responsible for the DAD. Pioneering work by Lederer and Tsien⁵ revealed a ‘transient inward current’ activated on repolarization in digitalis-intoxicated preparations. Subsequent studies showed that the transient inward current occurred at about the same time as a secondary tension development (aftercontraction) thereby linking it to a rise of [Ca²⁺]_i.^6,7 After a large amount of work from many groups, it was shown that the transient inward current was carried by inward current on Na-Ca exchange (NCX) as a result of its stoichiometry (3 Na⁺ entering the cell in exchange for 1 Ca²⁺ leaving).^8,9 Ca²⁺ release from the SR activates NCX leading to a decrease of [Ca²⁺]_i and an inward (depolarizing) current.

The underlying diastolic increase of Ca²⁺ propagates as a wave along the cell. Evidence for such Ca²⁺ waves was first inferred from tension changes.^6,10–12 Subsequent work on multicellular tissues showed that the aftercontraction was accompanied by a phasic rise of [Ca²⁺]_i^13–17 and direct cellular studies have measured the Ca²⁺ wave itself in single cells¹⁸ (Figure 1A). As an aid to discussion, Figure 1B shows a flow diagram of the events that are involved in the generation of calcium waves. Specifically, an increase of Ca²⁺ influx and/or a decrease of Ca²⁺ efflux across the cell membrane leads to an increase of SR Ca²⁺ content which in turn produces Ca²⁺ waves. Some of the Ca²⁺ in the wave is pumped out of the cell on NCX resulting in a DAD which, if large enough, produces an action potential.

View larger version (20K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 1 Ca2+ waves and flow diagram for the events involved in generation of Ca2+ waves and Ca2+ sparks and possible therapeutic strategies. (A) Confocal images of Ca2+ waves in a rat ventricular myocyte. The temporal sequence is indicated by the numbers. (B) Overall scheme. An increase of Ca2+ influx or decrease of efflux results in an increase of sarcoplasmic reticulum Ca2+ content, Ca2+ waves, and (via Na-Ca exchange) a delayed after depolarization which produces an action potential. The steps at which Ca2+ antagonists, ryanodine receptor inhibitors, Na-Ca exchange blockers, and local anaesthetics may act to interrupt this mechanism are indicated below. (C) A more detailed description of the events included in the dashed box in B.

 

	2. What is calcium overload?

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
The term ‘Ca²⁺ overload’ is applied to conditions in which Ca²⁺ waves and their consequences (DADs and aftercontractions) are observed. While it may be a useful description, it is not clear what exactly it corresponds to mechanistically. What is it that determines whether or not a Ca²⁺ wave is produced?

2.1 A sarcoplasmic reticulum Ca²⁺ content threshold for Ca²⁺ waves
The simplest way to study this question is to take a quiescent cell and increase Ca²⁺ influx by increasing external Ca²⁺ concentration. There is initially a measurable increase in SR Ca²⁺ content but no Ca²⁺ waves. A point is then reached at which Ca²⁺ waves begin to develop¹⁹. Further increase of Ca²⁺ influx results in increased frequency of Ca²⁺ waves but no further increase of SR Ca²⁺.¹⁹ Importantly, the amplitude of the Ca²⁺ waves is also unaffected (Figure 2A). These results were interpreted suggesting that Ca²⁺ waves occur when the SR content reaches a critical SR threshold. A low Ca²⁺ influx can be balanced by efflux on NCX at a sufficiently low [Ca²⁺]_i such that SR Ca²⁺ is maintained below the threshold level. At higher Ca²⁺ influx, there is an increase of cell Ca²⁺ content and thence of SR Ca²⁺ until the threshold is reached. The resulting wave then results in the loss of Ca²⁺ from the cell and this Ca²⁺ must be regained by the cell (and the SR) until the threshold is again reached. As the rate of Ca²⁺ influx into the cell is increased, by increasing the external Ca²⁺ concentration, the SR threshold will be reached more quickly and the frequency of waves will increase. Subsequent work measured the SR Ca²⁺ content at the end of a wave and showed that the decrease of SR content was equal to that amount of Ca²⁺ removed from the cell during the wave.²⁰ The idea of an SR Ca²⁺ threshold for waves has recently received further experimental support and indeed the term ‘SOICR’ for store-overload-induced Ca²⁺ release has been coined.²¹

View larger version (9K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 2 The effects of changing Ca2+ influx on Ca2+ waves. (A) Data taken from an experiment in which external Ca2+ concentration was altered (as shown on abscissa). The graphs show the effects on (from top to bottom): Ca2+ efflux on each wave; frequency of waves; sarcoplasmic reticulum Ca2+ content. (B) Dependence of wave frequency on sarcoplasmic reticulum Ca2+ content. Experiments on rat ventricular myocytes taken from Díaz et al.19

 
The apparent SR threshold for Ca²⁺ waves depends on the properties of the RyR. Thus, potentiating the opening of the RyR with caffeine decreases the threshold for Ca²⁺ release²² whereas decreasing it with tetracaine increases the threshold²³ (Figure 3). An obvious question concerns the mechanisms responsible for this apparent threshold SR Ca²⁺ content required for release. It is well known that the probability of the RyR opening (p_o) is increased by elevating cytoplasmic Ca²⁺ concentration.²⁴ In addition, an increase of SR Ca²⁺ content also increases p_o—the so-called ‘luminal’ effect.^25,26 The increased opening of clusters of RyRs is seen by the increased frequency of Ca²⁺ ‘sparks’. Measurements of Ca²⁺ sparks show that Ca²⁺ waves are initiated by an increase of spark frequency and sparks can often be seen at the initiation of a wave.²⁷ On this model, an increase of [Ca²⁺]_i increases the frequency of sparks and this would be augmented by an increase of SR Ca²⁺ content leading to a situation where wave propagation is due to Ca²⁺ released by sparks at one site diffusing to other sites and activating further sparks (Figure 1C).

View larger version (10K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 3 The dependence of the probability of a Ca2+ wave occurring on the sarcoplasmic reticulum Ca2+ content. Note the threshold dependence on sarcoplasmic reticulum content and the shift of this threshold to the left by caffeine and to the right by tetracaine. Experiments on rat ventricular myocytes: caffeine data taken from Venetucci et al.39 and tetracaine from Overend et al.73

 
A different interpretation of the propagation of Ca²⁺ waves has been proposed recently.²⁸ This is based on the observation that rapid inhibition of SERCA results in a slowing of the propagation velocity of Ca²⁺ waves. This result was interpreted with a hypothesis in which Ca²⁺ released from the SR in the wave is taken up into the next region of SR thereby increasing SR Ca²⁺ content and initiating further Ca²⁺ release (see Section 3.3 for further discussion).

2.2 Calcium waves during stimulation
The above review has focused on the Ca²⁺ waves observed in unstimulated conditions. However, the diastolic Ca²⁺ release that activates the arrhythmogenic transient inward current and therefore the DAD occurs following the end of an action potential. The fact that this diastolic calcium release and the spontaneous waves are seen under similar conditions has led to the assumption that they have similar origins but it is important to consider this. There is reasonable evidence that the calcium release accompanying the DAD propagates as a wave.^29,30 The next question then is what is responsible for initiating the DAD-related wave? There are two possibilities: (i) changes of SR content and (ii) some other factor. As regards the role of SR Ca²⁺ content, there is a lack of direct evidence relating to what happens to SR Ca²⁺ content when both diastolic and systolic Ca²⁺ release are seen. Recent papers have developed a technique to measure free intra-SR Ca²⁺ content using low affinity Ca²⁺ indicators,^31,32 but we are unaware of this technique being used to specifically address the question of whether the SR Ca²⁺ content is elevated above a threshold level at the start of the diastolic Ca²⁺ release. We have estimated the changes of cellular SR Ca²⁺ content by integrating the Ca²⁺ influx into the cell on the L-type Ca²⁺ current and the efflux (largely on the NCX). It should be noted that these calculations ignore the small entry of Ca²⁺ on ‘reverse mode’ and efflux on ‘forward mode’ NCX that that may occur during depolarization. Under control conditions influx and efflux are in balance (Figure 4A). When Ca²⁺ waves are produced by ß-adrenergic stimulation, then the increased Ca²⁺ influx (due to the augmented L-type Ca²⁺ current) is now balanced by the sum of the effluxes during (i) the systolic Ca²⁺ transient and (ii) the Ca²⁺ release accompanying the DAD (Figure 4B).³³ This means that at the end of the systolic Ca²⁺ transient, the Ca²⁺ content of the cell (and therefore presumably of the SR) is greater than that before systole. It is therefore possible that it is this increase of SR Ca²⁺ content that is responsible for the initiation of the Ca²⁺ wave. However, in addition to an increase of SR Ca²⁺ content, several other factors could be involved. (i) If the membrane potential is depolarized to very positive membrane potentials, there may be little Ca²⁺ influx into the cell due to the small electrochemical driving force for Ca²⁺ entry. On repolarization, there will be an immediate increase of driving force-promoting Ca²⁺ which will then terminate as the Ca²⁺ channels close due to voltage-dependent deactivation. If repolarization is rapid enough (as for example following a square voltage clamp pulse) then the Ca²⁺ influx is sufficient to activate Ca²⁺ release from the SR.^34,35 This may be less important following an action potential since the slower repolarization may allow channels to close before the driving force becomes large enough to allow significant Ca²⁺ entry. This hypothesis has been investigated experimentally by Egdell et al.³⁶ who examined the effects of adding cadmium to block the L-type channel before the diastolic release occurred. They found that despite the presumed inhibition of the L-type channels, the Ca²⁺ release persisted suggesting that this release was not triggered by Ca²⁺ entering the cell on repolarization.

View larger version (12K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 4 The effect of induction of Ca2+ waves on the Ca2+ flux balance. In each panel traces show: top, [Ca2+]i; middle membrane current (note the two gains used); bottom, calculated Ca2+ balance in the cell. This is calculated from the entry into the cell on the L-type Ca2+ current minus the efflux (largely on Na-Ca exchange). Three components of flux are identified: I, Ca2+ influx; SE, systolic efflux; DE, diastolic efflux. The three panels show: (A) control; (B) isoproterenol (1 µM); (C) isoprotereonol + tetracaine (50 µM). Note that in control systolic efflux balances the influx whereas in isoproterenol the systolic efflux is less than the influx predicting an increase of cellular Ca2+ during systole. Experiments on rat ventricular myocytes from Venetucci;33 used with permission.

 
It has been suggested that the SR Ca²⁺ content may not be the only (or indeed the major) factor determining when arrhythmogenic Ca²⁺ release occurs. As the frequency of stimulation of guinea-pig ventricular myocytes was increased, aftercontractions were eventually observed. However, the SR Ca²⁺ content was not different when aftercontractions were observed compared to when the stimulation rate was just subthreshold for production of aftercontractions.³⁷ Similar results were obtained when aftercontractions were elicited by exposure to catecholamines.³⁸ It was found that when aftercontractions occur, the cytoplasmic Ca²⁺ concentration is elevated and these authors therefore suggested that it is the increase of cytoplasmic as well as SR Ca²⁺ that is important. It should, however, be noted that the steep dependence of Ca²⁺ release on SR content¹⁹ may have made it difficult to observe a small increase of SR Ca²⁺ content accompanying the appearance of aftercontractions. The SR Ca²⁺ content hypothesis can also account for the fact that when the RyR is potentiated with caffeine, aftercontractions³⁹ and Ca²⁺ waves²² occur at a lower SR Ca²⁺ content. However, a consensus hypothesis could be that both SR and cytoplasmic Ca²⁺ play important roles. It is certainly not easy to separate the two factors since a change of cytoplasmic Ca²⁺ will alter SR Ca²⁺.

	3. Causes of calcium waves

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
3.1 Increased sarcoplasmic reticulum Ca²⁺ content
Either increasing Ca²⁺ influx or decreasing efflux across the cell membrane results in Ca²⁺ waves. Experimentally, the influx can be increased by elevating external Ca²⁺ or by ß-adrenergic stimulation.^33,38,40,41 Manoeuvres that decrease the ability of NCX to pump Ca²⁺ out of the cell such as elevating intracellular Na concentration (by inhibiting the Na-K pump with cardiac glycosides⁶ or potassium depleted solutions)^12,42,43 also produce waves.

3.2 The effects on Ca²⁺ waves of modifying the ryanodine receptor
3.2.1 Experimental modulation
As mentioned earlier, changing the properties of the RyR affects the occurrence of calcium waves. Addition of caffeine increases the frequency but decreases the amplitude of these waves (or the effects on contraction and membrane current fluctuations).^10,22,44 Conversely, tetracaine (which decreases RyR opening) decreases the frequency but increases the amplitude of waves.^23,44 These effects can be explained as follows. Addition of caffeine will potentiate the opening of the RyRs. Therefore, at a given SR Ca²⁺ content, the open probability of the RyR will be increased. It follows that if a critical open probability is required to initiate a wave, then caffeine will decrease the content at which the wave occurs (Figure 3). The fact that the threshold content is reduced has two implications. (i) The amount of Ca²⁺ in the SR when the wave begins will be less and therefore the amplitude of the wave will be decreased. (ii) As a consequence of the smaller Ca²⁺ release, less Ca²⁺ will be pumped out of the cell and therefore less Ca²⁺ need enter the cell to refill the SR back to threshold. Therefore (assuming that Ca²⁺ influx into the cell is unaffected), the frequency of waves will be increased. Formally, the frequency of waves will be increased by the same fraction by which the efflux per wave is decreased so that the time averaged Ca²⁺ efflux from the cell is constant. This is required as in the steady-state Ca²⁺ efflux must equal influx. The effects of tetracaine can be explained by the fact that the threshold is increased resulting in a larger Ca²⁺ release requiring a longer interval to refill the SR thereby decreasing wave frequency.

3.2.2 Pathological modulation of the ryanodine receptor
Two forms of modulation of the RyR have been suggested to be involved in Ca²⁺-dependent arrhythmias: mutation of the RyR and phosphorylation in heart failure. We will consider these in turn.

3.2.2.1 Mutation of the ryanodine receptor and other proteins
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a severe inherited arrhythmic syndrome caused by the occurrence of bidirectional or polymorphic VT following catecholamine stimulation (during exercise and other stress).⁴⁵ It has been shown to be caused by a mutation in the RyR.^46,47 The mutation causes increased opening of the RyR in the presence of PKA-dependent phosphorylation.⁴⁸ A mouse created to express the same mutation in the RyR shows a predisposition to exhibit DADs.^49,50 Subsequent work has found a large number of other mutations in the RyR and some have also been shown to be associated with arrhythmias. (See⁵¹ for a review of this area). As well as mutations of the RyR, those of the SR Ca²⁺-binding protein calsequestrin (CSQ) have also been implicated in CPVT.^52,53

3.2.2.2 Hyperphosphorylation of the ryanodine receptor in heart failure
It has been reported that the RyR becomes excessively phosphorylated (hyperphosphorylated) in heart failure.⁵⁴ This phosphorylation has been suggested to cause dissociation of the regulatory protein FKBP12.6 resulting in increased leak of Ca²⁺ from the RyR. This mechanism has the attraction that a single hypothesis can account for the fact that in heart failure, there is both (i) a decrease of SR Ca²⁺ content and consequent decreased Ca²⁺ transient and (ii) initiation of arrhythmogenic Ca²⁺ waves and thence DADs and arrhythmias.^48,55,56 It is, however, worth noting that many aspects of this scheme are still controversial.^50,51,57 A recent review has also highlighted the controversies in understanding the effects of ß-adrenergic stimulation on the RyR.⁵⁸

3.2.3 Effects of ryanodine receptor modulators in the absence of Ca²⁺ waves
The above considerations deal with the effects of RyR modifiers on pre-existing Ca²⁺ waves. The next question, however, is what happens if a RyR modifier is applied to a cell that does not show waves? We have investigated this issue in recent work. Figure 5A shows the effects of adding a low concentration of caffeine (to increase the open probability of the RyR) to a cell that was not showing Ca²⁺ waves. Ca²⁺ waves are produced following the first three or so stimuli. However, subsequent stimuli do not produce waves. The explanation for this is provided by the calculated SR Ca²⁺ content (lower panel). The decrease in SR Ca²⁺ is due to the extra efflux produced by the initial Ca²⁺ waves. In the steady state, the decrease of SR Ca²⁺ content will compensate for the direct effect of caffeine on the RyR and no waves will be seen. Figure 5B shows what happens when the experiment is repeated in the presence of ß-adrenergic stimulation with isoprenaline. Now, the addition of caffeine evokes diastolic Ca²⁺ waves in the majority of cells. The explanation for this is that the ß-adrenergic stimulation has increased SR Ca²⁺ content to a sufficiently high level such that even when caffeine is added the level of SR Ca²⁺ is still high enough to be above the threshold SR content.³⁹ Figure 5C shows the SR Ca²⁺ content as a function of caffeine concentration. Caffeine decreases SR content in both control and isoprenaline. However, at a given caffeine concentration, the SR Ca²⁺ content is always increased by isoprenaline.

View larger version (16K): [in this window] [in a new window] [Download PowerPoint slide]   Figure 5 The effects of increasing ryanodine receptor opening by application of caffeine on the probability of Ca2+ waves. (A) The effects of caffeine (0.5 mmol/L) in the absence of pre-existing Ca2+ waves. Traces show: top [Ca2+]i; bottom, calculated change of sarcoplasmic reticulum content. Caffeine (0.5 mmol/L) was applied for the period shown. (B) The effects of caffeine (0.5 mmol/L) in the presence of pre-existing Ca2+ waves produced by the application of isoproterenol (1 µmol/L). Note that only in the presence of isoproterenol does caffeine application result in Ca2+ waves in the steady state. (C) The effects of caffeine on sarcoplasmic reticulum Ca2+ content in the presence and absence of isoproterenol. Experiments on rat ventricular myocytes taken from Venetucci et al.;39 used with permission.

 
Why is it that waves only persist in the steady state in the presence of ß-adrenergic stimulation? One simple explanation is that in the steady state the calcium influx into the cell must equal the amount that leaves the cell during the systolic Ca²⁺ transient plus any that leaves on a wave. The efflux associated with a Ca²⁺ wave is typically 8 µmol/L.³³ This means that the Ca²⁺ influx must be larger than this value and as the amplitude of the systolic Ca²⁺ transient increases and therefore the systolic efflux increases then the influx must be even larger. On this basis it will be the increase of the L-type Ca²⁺ current produced by ß-adrenergic stimulation that provides sufficient Ca²⁺ influx to allow Ca²⁺ waves to occur A further consideration might suggest that ß-adrenergic stimulation should be even more arrhythmogenic than is inotropy via other mechanisms since the Ca²⁺ transient decays more quickly (due to stimulation of SERCA by phosphorylation of phospholamban). Therefore the time available for Ca²⁺ to leave the cell on NCX during systole will be decreased and, for a given sized Ca²⁺ transient there will be less efflux of Ca²⁺ from the cell and therefore a greater requirement for Ca²⁺ to leave the cell on waves. These considerations are, of course, consistent with the clinical observation that patients with mutations in the RyR are most likely to develop arrhythmias in the presence of ß-adrenergic stimulation. It should, however, be noted that (see section 3.3) stimulation of SERCA may make it harder for waves to propagate. Furthermore ß-adrenergic stimulation is normally associated with an increase in heart rate and abbreviation of the diastolic period which will give less time for diastolic release to occur. The net result of these factors is therefore difficult to predict.

3.3 The effect of modulation of SERCA on Ca²⁺ waves and arrhythmias
When SERCA is greatly inhibited with TBQ (2,5-di-tert-butyl-1,2,benzohydroquinone) in unstimulated cells, there is a marked decrease in the frequency of calcium waves.⁵⁹ This is accompanied by an increase in the duration of each wave (as a result of decreased reuptake of Ca²⁺ into the SR) resulting in more Ca²⁺ efflux per wave (on NCX). The combination of a lower frequency of waves with greater efflux per wave results in no change of time-averaged Ca²⁺ efflux from the cell.

It was also noted that the apparent threshold SR Ca²⁺ content for waves was decreased.⁵⁹ This was suggested to result from an effect of SERCA interfering with wave propagation. In other words, Ca²⁺ released from one region in the cell has to diffuse through the cytoplasm (past the SERCA-containing longitudinal SR) to the next SR Ca²⁺ release site. Some of the Ca²⁺ will be taken back into the SR by SERCA thus decreasing the efficiency of wave propagation. This effect will be decreased by inhibition of SERCA. However, it has also been suggested that SERCA may be required for wave propagation. As mentioned in Section 2.1, rapid inhibition of SERCA using a photolabile inhibitor found a decrease of propagation velocity of the wave,²⁸ an effect that was interpreted as showing that Ca²⁺ uptake into the SR ahead of the propagating wave is required to trigger release. This result might suggest that stimulation of SERCA would promote the occurrence of Ca²⁺ waves. However, it is worth noting that overexpression of SERCA has been shown to decrease the occurrence of both Ca²⁺ dependent aftercontractions⁶⁰ and reperfusion arrhythmias⁶¹ suggesting that SERCA may be antiarrhythmogenic. In other words, increased SERCA activity may have two opposite effects on the probability of Ca²⁺ waves occurring (i) an increase due to increased SR content and (ii) a decrease due to making it more difficult for waves to propagate.

	4. Ca2+ waves—friend or foe?

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
Much attention has been paid to the fact that Ca²⁺ waves result in arrhythmogenic inward currents. However, they do provide a way for a cardiac cell to come into Ca²⁺ flux balance in the presence of increased Ca²⁺ influx. If it was not for the waves then diastolic Ca²⁺ would have to increase to a sufficient level to increase Ca²⁺ efflux. Abolishing spontaneous waves with caffeine results in this predicted increase of diastolic Ca²⁺.¹⁹ Sufficient increase of diastolic Ca²⁺ might interfere with diastolic mechanical relaxation and it is not obvious whether this effect is more serious than the effects of the Ca²⁺ waves.

	5. What determines whether or not Ca2+ waves produce arrhythmias?

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
At a single cell level, this can be answered easily. All that is required is that the DAD resulting from the transient inward current is large enough to reach the threshold for action potential generation. The situation is more complicated for the intact heart. This is because the cells are electrically coupled and a given transient inward current will therefore produce a smaller DAD than if all the current was only injected into a single cell. It would therefore require many cells to have DADs for an ectopic beat to emerge. Simulations using an atrial model comprising an array of 512 by 512 cells suggested that a DAD would have to be produced in about 1000 cells for an ectopic beat to result.⁶² The exact number of cells depends on factors such as geometry and the intercellular coupling. Presumably as the degree of cell Ca²⁺ overload increases, then the probability that sufficient cells have waves will increase. Of course, if the cells in a given region have similar properties then it is likely that many of them will develop Ca²⁺ waves at the same time. Another important consideration is whether or not Ca²⁺ waves can propagate between cells⁶³ since, if this occurs, it will be easier to have waves in this critical number of cells. It should also be remembered that although most experimental studies on Ca²⁺ waves are carried out on ventricular muscle, there is considerable evidence that arrhythmias may arise in the Purkinje fibres (see Janse and Wit⁶⁴ and ter Keurs and Boyden⁶⁵ for reviews). Indeed some of the first work on DADs showed that these are elicited more easily in Purkinje fibres than ventricular muscle.¹

	6. How might Ca2+-wave-dependent arrhythmias be treated?

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
As shown in Figure 1B, in principle there are three ways to treat arrhythmias that originate from Ca²⁺ waves; (i) abolish the Ca²⁺ waves; (ii) stop the Ca²⁺ waves producing a DAD, and (iii) stop the DAD producing an ectopic beat. We will discuss the first strategy below. The second approach would require either increasing the background K conductance of the membrane or using an NCX inhibitor to prevent the Ca²⁺ wave activating the NCX current but such inhibitors might have untoward effects on Ca²⁺ handling (see Sipido et al.⁶⁶ for a review). As far as the third possibility is concerned, this is the mechanism by which the local anaesthetic group of drugs operate (by decreasing the excitability of the cell membrane). In this context, it is worth noting that a trial (on heart failure patients) has shown that such local anaesthetic antiarrhythmic drugs can be dangerous.⁶⁷

6.1 Abolition of Ca²⁺ waves as a therapeutic strategy
We will now consider how Ca²⁺ waves can be removed. There are two ways to do this; (i) abolish the causal Ca²⁺ overload and (ii) in the maintained presence of Ca²⁺ overload remove the Ca²⁺ waves.

6.1.1 Removal of Ca²⁺ overload
The Ca²⁺ overload can be removed by abolishing its cause. For the case of Ca²⁺ overload caused by an increase of [Na⁺]_i then manoeuvres which decrease [Na⁺]_i may be efficacious. One interesting example concerns the effects of the local anaesthetic group of drugs such as lidocaine and orally effective equivalents like mexiletine that inhibit the TTX-sensitive sodium channel. These also decrease the intracellular Na concentration⁶⁸ and (via increased NCX-mediated Ca²⁺ efflux) will thereby decrease the degree of Ca²⁺ overloading.

6.1.2 Removal of Ca²⁺ waves without affecting Ca²⁺ overload
The alternative to removing the calcium overload is to prevent the occurrence of Ca²⁺ waves in the presence of this overload. One way to do this would be to decrease the open probability of the RyR and thereby (Figure 3) increase the SR Ca²⁺ threshold at which Ca²⁺ waves occur. The problem with this strategy is to target diastolic Ca²⁺ waves as opposed to the normal systolic Ca²⁺ release, a distinction which is made more difficult by the fact that both occur through the RyR. Two approaches have been tried. (i) As mentioned in Section 3.2.2.2, it has been suggested that hyperphosphorylation of the RyR makes the accessory protein FKBP12.6 dissociate thereby making the RyR leaky during diastole. The agent JTV519 increases the binding of FKBP12.6 to the RyR and when given to a heterozygous FKBP12.6 knockout mouse decreased the occurrence of ventricular arrhythmias.⁴⁸ It was also found that mutant RyRs had a lower affinity for FKBP12.6 and suggested that this decreased binding would result in increased dissociation of FKBP12.6 from the RyR during ß-adrenergic stimulation. However, a study on mice expressing a mutant RyR showed no effect on the binding neither of FKBP12.6 nor of JTV519 on arrhythmias.⁵⁰ It should also be noted that recent work has found that JTV519 can decrease RyR opening by mechanisms that do not involve FKBP12.6.^69,70 An alternative strategy is based on the observation that the local anaesthetic tetracaine decreases the open probability of the RyR⁷¹ and increases the threshold SR Ca²⁺ content at which waves are observed.²³ Tetracaine has recently been shown to abolish diastolic Ca²⁺ waves in cell in which a state of Ca²⁺ overload had been produced by excessive ß-adrenergic stimulation. Importantly, this was accompanied by an increase of the amplitude of the systolic Ca²⁺ transient (Venetucci et al.³³ and Figure 4C). The increased systolic Ca²⁺ transient may be due to the fact that Ca²⁺ waves decrease the size of a subsequent systolic response.⁷² Tetracaine itself would be of no use as a therapeutic agent against cardiac arrhythmias since it also blocks sodium channels and would abolish nerve and muscle action potentials. However, the development of an analog of tetracaine with specificity for the RyR might be a promising approach.

	7. Conclusions

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 
As discussed in this review, in the steady state, there is an absolute requirement for Ca²⁺ influx and efflux to be in balance. Ca²⁺ waves can result when the Ca²⁺ influx into the cell is increased. Modulation of the RyR can also promote Ca²⁺ waves but only in the presence of a sufficiently high SR Ca²⁺ content. Finally, there are a number of potential sites against which therapeutic strategies can be targeted.

	Acknowledgements

 
Work from the authors laboratory was supported by The British Heart Foundation.

Conflict of interest: none declared.

	References

Top Abstract 1. Introduction 2. What is calcium... 3. Causes of calcium... 4. Ca2+ waves--friend or... 5. What determines whether... 6. How might Ca2+-wave-dependent... 7. Conclusions References

 

1. Ferrier GR, Saunders JH, Mendez C. A cellular mechanism for the generation of ventricular arrhythmias by acetylstrophanthidin. Circ Res (1973) 32:600–609.[Abstract/Free Full Text]
2. Ferrier GR. The effects of tension on acetylstrophanthidin-induced transient depolarizations and after-contractions in canine myocardial and Purkinje tissues. Circ Res (1976) 38:156–162.[Abstract/Free Full Text]
3. Rosen MR, Gelband H, Merker C, Hoffman BF. Mechanisms of digitalis toxicity: Effects of ouabain on phase four of canine Purkinje fiber transmembrane potentials. Circulation (1973) 47:681–689.[Abstract/Free Full Text]
4. Rosen MR, Gelband H, Hoffman BF. Correlation between effects of ouabain on the canine electrocardiogram and transmembrane potentials of isolated Purkinje fibers. Circulation (1973) 47:65–72.[Abstract/Free Full Text]
5. Lederer WJ, Tsien RW. Transient inward current underlying arrhythmogenic effects of cardiotonic steroids in Purkinje fibers. J Physiol (Lond) (1976) 263:73–100.[Abstract/Free Full Text]
6. 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.[Abstract/Free Full Text]
7. Kass RS, Tsien RW, Weingart R. Ionic basis of transient inward current induced by strophanthidin in cardiac Purkinje fibres. J Physiol (Lond) (1978) 281:209–226.[Abstract/Free Full Text]
8. Mechmann S, Pott L. Identification of Na-Ca exchange current in single cardiac myocytes. Nature (1986) 319:597–599.[CrossRef][Medline]
9. Fedida D, Noble D, Shimoni Y, Spindler AJ. Inward current related to contraction in guinea-pig ventricular myocytes. J Physiol (Lond) (1987) 385:565–589.[Abstract/Free Full Text]
10. Glitsch HG, Pott L. Spontaneous tension oscillations in guinea-pig atrial trabeculae. Pflügers Arch (1975) 358:11–25.[CrossRef][Web of Science][Medline]
11. Lakatta EG, Lappé DL. Diastolic scattered light fluctuation, resting force and twitch force in mammalian cardiac muscle. J Physiol (Lond) (1981) 315:369–394.[Abstract/Free Full Text]
12. Eisner DA, Lederer WJ. The role of the sodium pump in the effects of potassium-depleted solutions on mammalian cardiac muscle. J Physiol (Lond) (1979) 294:279–301.[Abstract/Free Full Text]
13. Allen DG, Kurihara S. Calcium transients in mammalian ventricular muscle. Eur Heart J (1980) 1:5–15.[Free Full Text]
14. Wier WG, Hess P. Excitation-contraction coupling in cardiac Purkinje fibers. Effects of cardiotonic steroids on the intracellular (Ca²⁺) transient, membrane potential, and contraction. J Gen Physiol (1984) 83:395–415.[Abstract/Free Full Text]
15. Orchard CH, Eisner DA, Allen DG. Oscillations of intracellular Ca²⁺ in mammalian cardiac muscle. Nature (1983) 304:735–738.[CrossRef][Medline]
16. Wier WG, Kort AA, Stern MD, Lakatta EG, Marban E. Cellular calcium fluctuations in mammalian heart: direct evidence from noise analysis of aequorin signals in Purkinje fibers. Proc Natl Acad Sci USA (1983) 80:7367–7371.[Abstract/Free Full Text]
17. Eisner DA, Valdeolmillos M. A study of intracellular calcium oscillations in sheep cardiac Purkinje fibres measured at the single cell level. J Physiol (Lond) (1986) 372:539–556.[Abstract/Free Full Text]
18. Wier WG, Cannell MB, Berlin JR, Marban E, Lederer WJ. Cellular and subcellular heterogeneity of [Ca²⁺]_i in single heart cells revealed by fura-2. Science (1987) 235:325–328.[Abstract/Free Full Text]
19. Díaz ME, Trafford AW, O'Neill SC, Eisner DA. Measurement of sarcoplasmic reticulum Ca²⁺ content and sarcolemmal Ca²⁺ fluxes in isolated rat ventricular myocytes during spontaneous Ca²⁺ release. J Physiol (Lond) (1997) 501:3–16.[Abstract/Free Full Text]
20. Díaz ME, Trafford AW, O'Neill SC, Eisner DA. A measurable reduction of s.r. Ca content follows spontaneous Ca release in rat ventricular myocytes. Pflügers Arch (1997) 434:852–854.[CrossRef][Web of Science][Medline]
21. Jiang D, Xiao B, Yang D, Wang R, Choi P, Zhang L, 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) 101:13062–13067.[Abstract/Free Full Text]
22. Trafford AW, Sibbring GC, Díaz ME, Eisner DA. The effects of low concentrations of caffeine on spontaneous Ca release in isolated rat ventricular myocytes. Cell Calcium (2000) 28:269–276.[CrossRef][Web of Science][Medline]
23. Overend CL, Eisner DA, O'Neill SC. The effect of tetracaine on spontaneous Ca release and sarcoplasmic reticulum calcium content in rat ventricular myocytes. J Physiol (Lond) (1997) 502:471–479.[Abstract/Free Full Text]
24. Rousseau E, Smith JS, Henderson JS, Meissner G. Single channel and ⁴⁵Ca²⁺ flux measurements of cardiac sarcoplasmic reticulum calcium channel. Biophys J (1986) 50:1009–1014.[Web of Science][Medline]
25. Sitsapesan R, Williams AJ. Regulation of current flow through ryanodine receptors by luminal Ca²⁺. J Memb Biol (1997) 159:179–185.[CrossRef][Web of Science][Medline]
26. Lukyanenko V, Viatchenko-Karpinski S, Smirnov A, Wiesner TF, Gyorke S. Dynamic regulation of sarcoplasmic reticulum Ca²⁺ content and release by luminal Ca²⁺-sensitive leak in rat ventricular myocytes. Biophys J (2001) 81:785–798.[Web of Science][Medline]
27. Cheng H, Lederer MR, Lederer WJ, Cannell MB. Calcium sparks and [Ca²⁺]_i waves in cardiac myocytes. Am J Physiol (1996) 270:C148–C159.[Web of Science][Medline]
28. Keller M, Kao JPY, Egger M, Niggli E. Calcium waves driven by ‘sensitization’ wave-fronts. Cardiovasc Res (2007) 74:39–45.[Abstract/Free Full Text]
29. Berlin JR, Cannell MB, Lederer WJ. Cellular origins of the transient inward current in cardiac myocytes: role of fluctuations and waves of elevated intracellular calcium. Circ Res (1989) 65:115–126.[Abstract/Free Full Text]
30. Miura M, Ishide N, Oda H, Sakurai M, Shinozaki T, Takishima T. Spatial features of calcium transients during early and delayed afterdepolarizations. Am J Physiol (1993) 265:H439–H444.[Web of Science][Medline]
31. Shannon TR, Guo T, Bers DM. Ca scraps: local depletions of free [Ca] in cardiac sarcoplasmic reticulum during contractions leave substantial ca reserve. Circ Res (2003) 93:40–45.[Abstract/Free Full Text]
32. Terentyev D, Nori A, Santoro M, Viatchenko-Karpinski S, Kubalova Z, Gyorke I, et al. Abnormal interactions of calsequestrin with the ryanodine receptor calcium release channel complex linked to exercise-induced sudden cardiac death. Circ Res (2006) 98:1151–1158.[Abstract/Free Full Text]
33. Venetucci L, Trafford AW, Diaz ME, O'Neill SC, Eisner DA. Reducing ryanodine receptor open probability as a means to abolish spontaneous Ca²⁺ release and increase Ca²⁺ transient amplitude in adult ventricular myocytes. Circ Res (2006) 98:1299–1305.[Abstract/Free Full Text]
34. Cannell MB, Berlin JR, Lederer WJ. Effect of membrane potential changes on the calcium transient in single rat cardiac muscle cells. Science (1987) 238:1419–1423.[Abstract/Free Full Text]
35. Barcenas-Ruiz L, Wier WG. Voltage dependence of intracellular [Ca²⁺]_i transients in guinea pig ventricular myocytes. Circ Res (1987) 61:148–154.[Abstract/Free Full Text]
36. Egdell RM, Milnes JT, MacLeod KT. The role of L-type calcium current in the generation of repolarization- induced contraction in cardiac myocytes. Cardiovasc Res (2000) 48:59–67.[Abstract/Free Full Text]
37. Edgell RM, De Souza AI, MacLeod KT. Relative importance of SR load and cytoplasmic calcium concentration in the genesis of aftercontractions in cardiac myocytes. Cardiovasc Res (2000) 47:769–777.[Abstract/Free Full Text]
38. Tweedie D, Harding SE, MacLeod KT. Sarcoplasmic reticulum Ca Content, sarcolemmal Ca influx and the genesis of arrhythmias in isolated guinea-pig cardiomyocytes. J Mol Cell Cardiol (2000) 32:261–272.[CrossRef][Web of Science][Medline]
39. Venetucci L, Trafford AW, Eisner DA. Increasing ryanodine receptor open probability alone does not produce arrhythmogenic Ca waves: threshold Ca content is required. Circ Res (2007) 100:105–111.[Abstract/Free Full Text]
40. Kurihara S, Allen DG. Intracellular Ca⁺⁺ transients and relaxation in mammalian cardiac muscle. Jpn Circ J (1982) 46:39–43.[Medline]
41. Marban E, Robinson SW, Wier WG. Mechanisms of arrhythmogenic delayed and early afterdepolarizations in ferret ventricular muscle. J Clin Invest (1986) 78:1185–1192.[Web of Science][Medline]
42. Eisner DA, Lederer WJ. Inotropic and arrhythmogenic effects of potassium-depleted solutions on mammalian cardiac muscle. J Physiol (Lond) (1979) 294:255–277.[Abstract/Free Full Text]
43. Eisner DA, Lederer WJ. The relationship between sodium pump activity and twitch tension in cardiac Purkinje fibres. J Physiol (Lond) (1980) 303:475–494.[Web of Science][Medline]
44. Nieman CJ, Eisner DA. Effects of caffeine, tetracaine, and ryanodine on calcium-dependent oscillations in sheep cardiac Purkinje fibers. J Gen Physiol (1985) 86:877–889.[Abstract/Free Full Text]
45. Leenhardt A, Lucet V, Denjoy I, Grau F, Ngoc DD, Coumel P. Catecholaminergic polymorphic ventricular tachycardia in children: a 7-year follow-up of 21 patients. Circulation (1995) 91:1512–1519.[Abstract/Free Full Text]
46. Priori SG, Napolitano C, Tiso N, Memmi M, Vignati G, Bloise R, et al. Mutations in the cardiac ryanodine receptor gene (hRyR2) underlie catecholaminergic polymorphic ventricular tachycardia. Circulation (2001) 103:196–200.[Abstract/Free Full Text]
47. Priori SG, Napolitano C, Memmi M, Colombi B, Drago F, Gasparini M, et al. Clinical and molecular characterization of patients with catecholaminergic polymorphic ventricular tachycardia. Circulation (2002) 106:69–74.[Abstract/Free Full Text]
48. Wehrens XH, Lehnart SE, Huang F, Vest JA, Reiken SR, Mohler PJ, et al. FKBP12.6 deficiency and defective calcium release channel (ryanodine receptor) function linked to exercise-induced sudden cardiac death. Cell (2003) 113:829–840.[CrossRef][Web of Science][Medline]
49. Cerrone M, Colombi B, Santoro M, di Barletta MR, Scelsi M, Villani L, et al. Bidirectional ventricular tachycardia and fibrillation elicited in a knock-in mouse model carrier of a mutation in the cardiac ryanodine receptor. Circ Res (2005) 96:e77–e82.[Abstract/Free Full Text]
50. Liu N, Colombi B, Memmi M, Zissimopoulos S, Rizzi N, Negri S, et al. Arrhythmogenesis in catecholaminergic polymorphic ventricular tachycardia - Insights from a RyR2 R4496C knock-in mouse model. Circ Res (2006) 99:292–298.[Abstract/Free Full Text]
51. George CH, Jundi H, Thomas NL, Fry DL, Lai FA. Ryanodine receptors and ventricular arrhythmias: emerging trends in mutations, mechanisms and therapies. J Mol Cell Cardiol (2007) 42:34–50.[CrossRef][Web of Science][Medline]
52. Lahat H, Pras E, Olender T, Avidan N, Ben Asher E, Man O, et al. A missense mutation in a highly conserved region of CASQ2 is associated with autosomal recessive catecholamine-induced polymorphic ventricular tachycardia in Bedouin families from Israel. Am J Hum Gen (2001) 69:1378–1384.[CrossRef][Web of Science][Medline]
53. Viatchenko-Karpinski S, Terentyev D, Gyorke I, Terentyeva R, Volpe P, Priori SG, et al. Abnormal calcium signaling and sudden cardiac death associated with mutation of calsequestrin. Circ Res (2004) 94:471–477.[Abstract/Free Full Text]
54. Marx SO, Reiken S, Hisamatsu Y, Jayaraman T, Burkhoff D, Rosemblit N, et al. PKA phosphorylation dissociates FKBP12.6 from the calcium release channel (ryanodine receptor): defective regulation in failing hearts. Cell (2000) 101:365–376.[CrossRef][Web of Science][Medline]
55. Lehnart SE, Terrenoire C, Reiken S, Wehrens XH, Song LS, Tillman EJ, et al. Stabilization of cardiac ryanodine receptor prevents intracellular calcium leak and arrhythmias. Proc Natl Acad Sci USA (2006) 103:7906–7910.[Abstract/Free Full Text]
56. Díaz ME, Graham HK, Trafford AW. Enhanced sarcolemmal Ca²⁺ efflux reduces sarcoplasmic reticulum Ca²⁺ content and systolic Ca²⁺ in cardiac hypertrophy. Cardiovasc Res (2004) 62:538–547.[CrossRef][Web of Science][Medline]
57. Bers DM, Eisner DA, Valdivia HH. Sarcoplasmic Reticulum Ca²⁺ and Heart Failure: Roles of Diastolic Leak and Ca²⁺ Transport. Circ Res (2003) 93:487–490.[Free Full Text]
58. Sipido KR. CaM or cAMP: Linking ß-Adrenergic Stimulation to ‘Leaky’ RyRs. Circ Res (2007) 100:296–298.[Free Full Text]
59. O'Neill SC, Miller L, Hinch R, Eisner DA. Interplay between SERCA and sarcolemmal Ca efflux pathways controls spontaneous release of Ca from the sarcoplasmic reticulum in rat ventricular myocytes. J Physiol (2004) 559:121–128.[Abstract/Free Full Text]
60. Davia K, Bernobich E, Ranu HK, Del Monte F, Terracciano CM, MacLeod KT, et al. SERCA2A overexpression decreases the incidence of aftercontractions in adult rabbit ventricular myocytes. J Mol Cell Cardiol (2001) 33:1005–1015.[CrossRef][Web of Science][Medline]
61. Del Monte F, Lebeche D, Guerrero JL, Tsuji T, Doye AA, Gwathmey JK, et al. Abrogation of ventricular arrhythmias in a model of ischemia and reperfusion by targeting myocardial calcium cycling. Proc Natl Acad Sci USA (2004) 101:5622–5627.[Abstract/Free Full Text]
62. Winslow RL, Varghese A, Noble D, Adlakha C, Hoythya A. Generation and propagation of ectopic beats induced by spatially localized Na-K pump inhibition in atrial network models. Proc Biol Sci (1993) 254:55–61.[Abstract/Free Full Text]
63. Lamont C, Luther PW, Balke CW, Wier WG. Intercellular Ca²⁺ waves in rat heart muscle. J Physiol (Lond) (1998) 512:669–676.[Abstract/Free Full Text]
64. Janse MJ, Wit AL. Electrophysiological mechanisms of ventricular arrhythmias resulting from myocardial ischemia and infarction. Physiol Rev (1989) 69:1049–1169.[Free Full Text]
65. ter Keurs HEDJ, Boyden PA. Calcium and arrhythmogenesis. Physiol Rev (2007) 87:457–506.[Abstract/Free Full Text]
66. Sipido KR, Varro A, Eisner D. Sodium calcium exchange as a target for antiarrhythmic therapy. Handb Exp Pharmacol (2006) 171:159–199.[Medline]
67. The Cardiac Arrhythmia Suppression Trial (CAST) Investigators. Preliminary report: effect of encainide and flecainide on mortality in a randomized trial of arrhythmia suppression after myocardial infarction. The Cardiac Arrhythmia Suppression Trial (CAST) Investigators. N Engl J Med (1989) 321:406–412.[Abstract]
68. Eisner DA, Lederer WJ, Sheu SS. The role of intracellular sodium activity in the anti-arrhythmic action of local anaesthetics in sheep Purkinje fibres. J Physiol (Lond) (1983) 340:239–257.[Web of Science][Medline]
69. Hunt DJ, Jones PP, Wang R, Chen W, Bolstad J, Chen K, et al. K201 (JTV519) suppresses spontaneous Ca2+ release and [3H]ryanodine binding to RyR2 irrespective of FKBP12.6 association. Biochem J (2007) 404:431–438.[CrossRef][Web of Science][Medline]
70. Loughrey CM, Otani N, Seidler T, Craig MA, Matsuda R, Kaneko N, et al. K201 modulates excitation-contraction coupling and spontaneous Ca²⁺ release in normal adult rabbit ventricular cardiomyocytes. Cardiovasc Res (2007) 76:236–246.[Abstract/Free Full Text]
71. Xu L, Jones R, Meissner G. Effects of local anesthetics on single channel behavior of skeletal muscle release channel. J Gen Physiol (1993) 101:207–233.[Abstract/Free Full Text]
72. Allen DG, Eisner DA, Pirolo JS, Smith GL. The relationship between intracellular calcium and contraction in calcium-overloaded ferret papillary muscles. J Physiol (Lond) (1985) 364:169–182.[Abstract/Free Full Text]
73. Overend CL, O'Neill SC, Eisner DA. The effect of tetracaine on stimulated contractions, sarcoplasmic reticulum Ca²⁺ content and membrane current in isolated rat ventricular myocytes. J Physiol (Lond) (1998) 507:759–769.[Abstract/Free Full Text]

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				S. C.W. Stevens, D. Terentyev, A. Kalyanasundaram, M. Periasamy, and S. Györke Intra-sarcoplasmic reticulum Ca2+ oscillations are driven by dynamic regulation of ryanodine receptor function by luminal Ca2+ in cardiomyocytes J. Physiol., October 15, 2009; 587(20): 4863 - 4872. [Abstract] [Full Text] [PDF]

				M. Ruiz-Meana, A. Abellan, E. Miro-Casas, E. Agullo, and D. Garcia-Dorado Role of sarcoplasmic reticulum in mitochondrial permeability transition and cardiomyocyte death during reperfusion Am J Physiol Heart Circ Physiol, October 1, 2009; 297(4): H1281 - H1289. [Abstract] [Full Text] [PDF]

				L. M. Blayney, J.-L. Jones, J. Griffiths, and F. A. Lai A mechanism of ryanodine receptor modulation by FKBP12/12.6, protein kinase A, and K201 Cardiovasc Res, September 2, 2009; (2009) cvp273v2. [Abstract] [Full Text] [PDF]

				J. Inserte, J. A. Barrabes, V. Hernando, and D. Garcia-Dorado Orphan targets for reperfusion injury Cardiovasc Res, July 15, 2009; 83(2): 169 - 178. [Abstract] [Full Text] [PDF]

				D. H. MacLennan and S. R. W. Chen Store overload-induced Ca2+ release as a triggering mechanism for CPVT and MH episodes caused by mutations in RYR and CASQ genes J. Physiol., July 1, 2009; 587(13): 3113 - 3115. [Full Text] [PDF]

				C. Soeller, I. D. Jayasinghe, P. Li, A. V. Holden, and M. B. Cannell Three-dimensional high-resolution imaging of cardiac proteins to construct models of intracellular Ca2+ signalling in rat ventricular myocytes Exp Physiol, May 1, 2009; 94(5): 496 - 508. [Abstract] [Full Text] [PDF]

				D. A. Eisner and E. Cerbai Beating to time: calcium clocks, voltage clocks, and cardiac pacemaker activity Am J Physiol Heart Circ Physiol, March 1, 2009; 296(3): H561 - H562. [Full Text] [PDF]

				S. R. Houser Ca2+ Signaling Domains Responsible For Cardiac Hypertrophy and Arrhythmias Circ. Res., February 27, 2009; 104(4): 413 - 415. [Full Text] [PDF]

				G. Antoons and K. R. Sipido Targeting calcium handling in arrhythmias Europace, December 1, 2008; 10(12): 1364 - 1369. [Abstract] [Full Text] [PDF]

				D. R. Laver and B. N. Honen Luminal Mg2+, A Key Factor Controlling RYR2-mediated Ca2+ Release: Cytoplasmic and Luminal Regulation Modeled in a Tetrameric Channel J. Gen. Physiol., October 1, 2008; 132(4): 429 - 446. [Abstract] [Full Text] [PDF]

				L. A. Venetucci and D. A. Eisner Calsequestrin Mutations and Sudden Death: A Case of Too Little Sarcoplasmic Reticulum Calcium Buffering? Circ. Res., August 1, 2008; 103(3): 223 - 225. [Full Text] [PDF]

				D. Garcia-Dorado, H. M. Piper, and D. A. Eisner Sarcoplasmic reticulum and mitochondria in cardiac pathophysiology Cardiovasc Res, January 15, 2008; 77(2): 231 - 233. [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) All Versions of this Article: 77/2/285    most recent cvm009v2 cvm009v1 E-letters: Submit a response Alert me when this article is cited Alert me when E-letters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Request Permissions Disclaimer Google Scholar Articles by Venetucci, L. A. Articles by Eisner, D. A. Search for Related Content PubMed PubMed Citation Articles by Venetucci, L. A. Articles by Eisner, D. A. Social Bookmarking          What's this?

Online ISSN 1755-3245 - Print ISSN 0008-6363

Copyright © 2009 European Society of Cardiology

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics