Download Intracellular Ca waves, afterdepolarizations, and

Cardiovascular Research (2012) 95, 265–268
doi:10.1093/cvr/cvs155
VIEWPOINT EDITORIAL
Intracellular Ca21 waves, afterdepolarizations,
and triggered arrhythmias
Yohannes Shiferaw1, Gary L. Aistrup 2, and J. Andrew Wasserstrom 2*
1
Department of Physics and Astronomy, California State University (Northridge), Northridge, CA, USA; and 2Department of Medicine (Cardiology) and the Feinberg Cardiovascular
Research Institute, Northwestern University Feinberg School of Medicine, 310 E. Superior St., Morton 7-607, Chicago, IL 60611, USA
1. Introduction
Clinical studies have shown that sudden death is initiated by an illtimed propagated ectopic beat that leads to fibrillation.1 – 4
However, the mechanism underlying these focal excitations is not
completely understood. Experimental studies have demonstrated
that abnormal calcium (Ca2+) cycling is a critical factor in the development of focal excitations.5 – 9 These excitations can be caused by
spontaneous Ca2+ release (SCR) in the form of intracellular Ca2+
waves. These waves are initiated when Ca2+ release from a few
Ca2+ release units (CRUs) on the sarcoplasmic reticulum (SR)
causes regenerative release in adjoining units via Ca2+-induced Ca2+
release (CICR), causing Ca2+ wave propagation. The resulting depolarizing inward current through the electrogenic Na+ – Ca2+ exchanger (NCX) depolarizes the cell membrane to threshold,
producing a triggered beat.10 – 14
The relationship between subcellular Ca2+ waves and focal excitations in cardiac tissue is, however, still not completely understood.
The basic unanswered question is how Ca2+ release within a population of cardiac cells can induce sufficient inward current to overcome the electrotonic load of the neighbouring cells. In this
paper, we will discuss some key ideas that are essential in answering
this question, focusing on the probabilistic nature of SCR and the
importance of measuring the timing distribution of Ca2+ waves in
multicellular populations in tissue. We will also discuss our recent
results showing that the likelihood of a triggered beat is determined
by the variance of the timing distribution, which is dictated by the
time course of SR reloading.15,16 In addition, we will discuss our
observations of a form of Ca2+ wave that is distinct from SCR.
These Ca2+ waves occur only during rapid pacing and occur with
a latency that is significantly shorter than the spontaneous waves
observed following cessation of pacing. We argue that these Ca2+
waves are ‘triggered’ as opposed to ‘spontaneous’, since they must
be initiated by L-type Ca2+ channel (LCC) openings rather than
random ryanodine receptor (RyR) openings. Finally, we discuss the
complex dynamics and timing of these waves and why they may
be at least as—and possibly more—arrhythmogenic than spontaneous waves.
2. How do spontaneous Ca21 waves
cause triggered arrhythmias?
Spontaneous Ca2+ waves are typically observed under conditions of
SR Ca2+ overload and are known to cause depolarization of the
cardiac cell membrane that, under the right conditions, can achieve
the threshold for activation for spontaneous electrical activity.17,18
This process occurs because the Ca2+ released during Ca2+ waves
induces sufficient inward current via the activation of Ca2+-sensitive
currents, particularly forward-mode NCX current but possibly
other Ca2+-sensitive inward currents. The resulting depolarization,
known as a delayed afterdepolarization (DAD), is regulated by the
timing and the amount of Ca2+ released by the Ca2+ wave so that
if the DAD is early enough and large enough, it can activate the
rapid Na+ inward current, producing a triggered beat.12,19,20
The bi-directional coupling between subcellular Ca2+ and voltage
is thought to be the mechanism underlying a variety of triggered
arrhythmias. However, a fundamental question regarding the arrhythmogenic potential for Ca2+ waves is how Ca2+ release in single cells
becomes sufficiently synchronized in both time and space to produce
enough depolarization to bring a critical mass of tissue to voltage
threshold. To address this question, recent studies have applied confocal imaging of subcellular Ca2+ in individual cardiac myocytes of the
whole rat heart.15,21 These studies show that the timing of SCR is
determined by localized Ca2+ release events which propagate as
Ca2+ waves. The precise origin of these localized release events is
not known, but it is likely that they are due to localized, SCR
events of sufficient magnitude to propagate as a fire-diffuse-fire
wave. Based on this physical picture, the probability distribution of
the time to the first propagating SCR event (first latency distribution)
dictates how Ca2+ waves are coordinated among cells in tissue. To
quantify this probability distribution, we measured the average and
variance of the first latency distribution.15 These quantities both
decreased with increasing pacing rate and external Ca2+ concentration, thus explaining the reduced latency and increased synchronization of SCR across large cell populations.
In order to understand the mechanisms responsible for reduced
timing variability, we applied numerical simulations of subcellular
The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.
* Corresponding author. Tel: +1 312 503 1384; fax: +1 312 503 3891, Email: [email protected]
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2012. For permissions please email: [email protected].
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Online publish-ahead-of-print 27 April 2012
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Ca2+ release within cells and groups of cells.15,16 These simulations
revealed that the decrease in variance of the first latency distribution
can be explained by the faster recovery of the SR Ca2+ load. This
occurs because the uptake of Ca2+ back into the SR is up-regulated
due to increased diastolic Ca2+ concentrations during rapid pacing.
Thus, the faster recovery kinetics of SR Ca2+ content are directly
responsible for both reduced latency and decreased timing variability.
Therefore, even though the timing of Ca2+ waves is statistically
independent between cells, these events tend to occur at the same
time because all cells in the population experience a faster SR Ca2+
recovery. Thus, the apparent synchronization of Ca2+ waves among
cells can be explained by the intrinsic properties of Ca2+ cycling,
without invoking Ca2+ diffusion between cells.
In order to initiate a Ca2+ wave, Ca2+ release must first occur within
a single CRU and then diffuse to initiate Ca2+ release at the neighbouring CRUs via CICR. Hence, it is important to understand the
mechanisms that initiate the first Ca2+ release event. There are two
distinct possibilities: (i) an LCC transitions to the open state and
allows a flux of Ca2+ to enter, diffuse, and then trigger Ca2+
release from the local RyR cluster; (ii) an RyR channel in the cluster
randomly transitions to the open state and allows Ca2+ to flow
from the SR into the CRU, which induces Ca2+ release via CICR.
Ca2+ waves induced by the above mechanisms will be referred to
as ‘triggered Ca2+ waves’ and ‘spontaneous Ca2+ waves’, respectively.
Most experimental studies typically measure Ca2+ waves following
Figure 1 Triggered waves in congestive heart failure. (A) Longitudinal line-scan recording of a single cell on the epicardial surface of a
failing intact SHR heart paced at BCL ¼ 400 ms. Arrows indicate
triggered waves. The top trace is the ECG and the bottom trace is
the mean intensity profile. (B) Transverse recording of two neighbouring myocytes during pacing at BCL ¼ 700 and 400 ms. Reproduced from Wasserstrom et al. 22 with permission.
the cessation of pacing. In this case, the first Ca2+ wave typically
occurs during the resting membrane potential, when the open probability of LCCs is low and they are therefore more likely to be spontaneous rather than triggered. However, triggered Ca2+ waves can
be observed under the appropriate conditions, such as in heart
failure.22,23 Figure 1A shows a longitudinal recording of a line-scan confocal image of a single myocyte from the epicardial surface of an intact
failing spontaneously hypertensive rat (SHR) heart. Despite small
Ca2+ transients evoked during stimulation at a basic cycle length
(BCL) of 400 ms, a series of large Ca2+ waves (indicated by
arrows) occurred during stimulation but were absent following the
cessation of pacing. The incidence of triggered waves is non-uniform
in the failing heart as shown in the transverse recording (Figure 1B) of
two neighbouring myocytes in which normal excitation–contraction
(E–C) coupling is present in Cell 1 while triggered waves (arrows)
occur only during pacing in Cell 2. Supplementary material online,
Video S1, shows a site where there are small Ca2+ transients in all
cells in the visual field, even though many of the cells are also
showing propagated triggered Ca2+ waves which cease immediately
when pacing stops, after which several spontaneous waves develop
in some cells before pacing is re-initiated. Hence, we can conclude
that these waves are not spontaneous and are most likely triggered
by LCC openings during the action potential (AP).
4. Mechanism for triggered waves
Ca2+ waves propagate in cardiac cells due to a fire-diffuse-fire mechanism that allows local Ca2+ release events to spread throughout
the cell. A crucial requirement for this to occur is that there must
be a large enough population of CRUs in the cell that have not
already been activated. If this condition is not met, then Ca2+
waves will abort since they will quickly encounter regions of the
cell that have recently released Ca2+ and are refractory. Therefore,
an essential condition for the formation of triggered waves is the presence of a large population of available CRUs shortly after the AP
upstroke. Under normal pacing conditions, the number of available
CRUs following the AP upstroke is low since most CRUs have
been triggered by LCC openings. However, under conditions of
Ca2+ overload or rapid pacing, it is likely that the number of available
CRUs is much larger than normal. There are two distinct mechanisms
for this to occur. First, the open probability of LCCs during the AP
plateau may be much smaller than normal so that only a few LCCs
open to trigger Ca2+ sparks, because high diastolic Ca2+ levels
during rapid pacing will induce Ca2+-induced inactivation of LCCs
and reduce their open probability. Furthermore, elevated diastolic
Ca2+ increases the likelihood of Ca2+ wave propagation, since RyR
open probability increases with Ca2+ concentration. Secondly, the
response of CRUs to LCC openings is a sharp threshold function of
the SR load24 so that the availability of CRUs depends on the time
course of the SR reloading. Consequently, triggered waves can
occur if the SR load is below some threshold immediately following
an AP upstroke but then crosses that threshold during the ensuing
AP. In this case, the timing of the triggered waves is dependent on
the time course of the SR load immediately following the AP
upstroke.
The mechanisms described above can explain the presence of triggered waves during heart failure. In these cells, E–C coupling is disrupted due to an abnormal t-tubule pattern, which leaves a large
number of RyRs that can no longer be triggered by LCC channel
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3. Initiation of Ca21 waves:
triggered vs. spontaneous
Editorial
267
Editorial
openings.25,26 Thus, LCC openings in a large number of CRUs are
insufficient to induce Ca2+ release. This leaves a large population of
CRUs that can support a Ca2+ wave, thus promoting triggered
waves. Furthermore, elevated diastolic Ca2+ in heart failure ensures
that the number of LCC openings is reduced due to Ca2+-induced
inactivation, while RyR channels are more prone to induce and
support Ca2+ waves.
5. Triggered waves as a novel
mechanism of early
afterdepolarizations
6. Triggered waves and
arrhythmias: role of ectopic foci
How might triggered waves contribute to cardiac arrhythmias? In
order for an ectopic beat to occur due to triggered Ca2+ waves, it
will be necessary that these events occur in large cell populations.
For this to occur, a triggered wave should occur within a critical fraction of cells in tissue, at roughly the same time, in order to summate
to overcome the electrotonic current drain to the surrounding cells. It
is possible for this to occur if the frequency of triggered waves is high.
This is precisely what we would expect in Ca2+ overload and heart
failure where, as observed experimentally, the frequency of triggered
waves increased dramatically. A particularly important feature of triggered waves is that they occur during systole at rapid rates. Thus, the
timing variability of these events is naturally less than that of spontaneous Ca2+ waves, which can occur over a much longer time interval
during diastole. Thus, it may be possible that during rapid pacing, triggered waves can occur nearly simultaneously in many cells. Also, it is
important to consider that an ectopic excitation due to triggered
waves may be more dangerous than that due to spontaneous Ca2+
waves, because triggered waves can occur at short time intervals
after an AP upstroke. Therefore, the coupling interval between a triggered excitation and the previous paced beat can occur during or
7. Triggered waves and
arrhythmias: formation of a
dynamic substrate
Wave break occurs when ectopic excitation propagates in cardiac
tissue with a non-uniform AP duration (APD) distribution. In particular, if there are large gradients in APD, then a triggered excitation can
propagate into a region where a short APD occurred, whereas block
occurs in a refractory region following a long APD. Triggered waves
can lead to a heterogeneous distribution of APD, since they occur
during an AP and therefore significantly change the APD on a
beat-to-beat basis. Thus, if a region of tissue exhibits a larger than
average fraction of triggered waves on a certain beat, then the APD
in this region may be significantly larger than in neighbouring
regions. Another mechanism for the formation of a heterogeneous
APD substrate is that since Ca2+ and voltage are bi-directionally
coupled, triggered waves destabilize the periodic response of the
APD. Thus, a triggered Ca2+ wave between beats will typically
induce an increase in the APD. This effect will then feed back on
Ca2+ on the next beat, since a larger APD in one beat will tend to
lead to reduced LCC current in the next, owing to the shorter diastolic interval. Furthermore, a triggered Ca2+ wave will deplete the
SR and prevent Ca2+ release on the next beat. Thus, we expect
that a triggered wave will disrupt the dynamics of voltage and Ca2+
under rapid pacing conditions. In the tissue setting, this will lead to
spatial gradients in APD, since different cells will respond differently
to rapid pacing, preventing spatiotemporal synchronization of the
voltage and Ca2+ response. This mechanism is similar to spatially discordant APD alternans where one region of tissue alternates in a
long–short–long– short pattern, while a neighbouring region alternates out of phase (short –long –short–long). Here, we expect an
even more complex dynamic pattern, as triggered waves will disrupt
the non-linear response of both voltage and Ca2+ cycling.
8. Conclusions
Numerous studies have shown that various cardiac arrhythmias are
initiated by triggered excitations that are caused by SCR. However,
it is not well understood how Ca2+ release at the single-cell level
can lead to triggered activity in tissue. In this manuscript, we described
several ideas that are necessary to understand the precise connection
between subcellular Ca2+ and the triggers for arrhythmias in the
heart. In particular, the timing distribution of SCR within a large cell
population in tissue is the key factor that determines whether SCR activity can summate to form ectopic foci. Moreover, it is the time
course of SR calcium release that determines the variance of this distribution and that is the critical factor to determine the likelihood of
triggered activity. Insights gained from these studies suggest novel
gene-based or pharmacological treatments that seek to target the formation of dangerous ectopic beats. For example, our work has identified the rate of SR Ca2+ refilling as a potential therapeutic target
since it controls the relative synchrony of SCR in a population of
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An important and novel feature of triggered waves is that they occur
during the AP, whereas spontaneous waves occur only during diastole. Thus, the timing of spontaneous Ca2+ waves suggests their
involvement in DADs while triggered waves might also be involved
in the induction of early afterdepolarizations (EADs), depending on
the timing of the first triggered spark. This mechanism for EADs is
novel as it is generally believed that EADs are due to a voltagedependent mechanism involving Ca2+ and K+ currents, although
there is some evidence that SR Ca2+ release, and perhaps even
Ca2+ waves, may cause EADs.14 Certainly, the timing of triggered
waves is consistent with this intriguing notion since these Ca2+
release events are both large and late during systole, so that the
inward current generated late during the AP plateau could be sufficient in both timing and magnitude to provide enough inward
current to produce a late Phase-2 or -3 depolarization. However, if
it is in fact true that EADs might also occur as the result of Ca2+
cycling, possibly arising from triggered waves, then this behaviour
should respond quite readily to pharmacological interventions
which target intracellular Ca2+ cycling that is independent of traditional notions of SR Ca2+ overload, since this is not a requirement
for triggered waves as it is for spontaneous Ca2+ waves.
towards the end of the AP so that the ensuing wave front will
closely follow the wave back of the preceding beat and will have
a higher likelihood of propagation failure. Hence, we expect that
ectopic foci due to triggered waves would have a higher probability
of leading to wave break and fibrillation.
268
cells. Furthermore, we have described ‘triggered waves’ which may be
highly arrhythmogenic since they occur early in the AP and thus can
induce propagating excitations that are more likely to undergo wave
break. Also, owing to their early onset, triggered waves provide a putative mechanism for EADs that has not been described before. These
findings should stimulate further exploration of these complex spatiotemporal features of Ca2+ and their connection with cardiac
arrhythmias.
Supplementary material
Supplementary material is available at Cardiovascular Research online.
Funding
This work was supported by the National Institutes of Health (grant
#R01HL101196) and the American Heart Association Grant 0830298N.
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Conflict of interest: none declared.
Editorial