Download Repolarization Reserve in Cardiac Cells

Journal of Medical and Biological Engineering, 26(3): 97-105
97
Repolarization Reserve in Cardiac Cells
Edward Carmeliet*
Laboratory of Physiology, University Leuven, Leuven, B-3000 Belgium
Received 14 June 2006; Accepted 8 July 2006
Abstract
Repolarization reserve is a term used to indicate the existence of redundancy of repolarizing currents in cardiac
cells. It provides safety and prevents cardiac arrhythmias due to excessive prolongation of the action potential duration
(APD) and generation of early afterdepolarizations. The existence of repolarization reserve in cardiac cells and its
modulation is the main topic of this short review. Under physiological conditions repolarization reserve is modulated
by increase of frequency, sympathetic stimulation and the accompanying rise in [Ca2+]i, [Na+]i and [K+]e. The analysis
of the changes by frequency and activation of the sympathetic system reveals an increase in repolarization reserve. At
elevated rates APD shortening is caused by enhancement of outward currents (IKr, IKs and INa,K), while inward currents
are decreased (INa, late and ICaL due to faster inactivation; peak ICaL may be increased by acceleration of recovery from
inactivation). Under sympathetic stimulation not only sinus rate increases but specific effects occur at the ventricular
level by activation of α- and β-receptors. Major effects occur via activation of β-receptors which result in an increase
of ICaL, IKr, IKs and INa,K, elevation of the plateau and further shortening of the APD. Pathological reduction of
repolarization reserve with increased risk of deadly arrhythmias can be of hereditary (congenital LQTsyndromes) or of
an acquired nature. One of the more frequent and dangerous forms of acquired LQT is caused by the use of medications.
The drugs responsible belong to different groups but have the common effect of blocking K+ currents. Since not all
patients using these drugs show complications of arrhythmias attention should be given to the underlying risk factors,
such as hypokalemia, hypomagnesemia, female sex, predisposing DNA polymorphisms, congestive heart failure, left
ventricular hypertrophy. It is the aim of future research, especially in the case of hypertrophy and failure to unravel the
remodeling mechanisms responsible for increased risk. In this way it will become possible to prevent excessive
reduction of repolarization reserve and to preserve a normal repolarization.
Keywords: Repolarization reserve, Frequency, Sympathetic stimulation, Ion concentrations, LQT, Remodeling
Introduction
Redundancy is a characteristic property of biological
systems. It provides safety and prevents pathological
disturbances. In cardiac electrophysiology the repolarization
process of the action potential is guaranteed not by a single but
by multiple currents. The contribution of these currents varies
with cell type. Blocking of one current does not result in
failure of repolarization. Compensation by other outward
currents prevents dangerous arrhythmias and has been called
repolarization reserve [1].
The action potential in nerve, skeletal muscle and cardiac
cells is characterized by a rapid depolarization phase or
upstroke which is caused by activation of the fast Na +
conductance. Repolarization in cardiac cells is very different
from that in nerve and skeletal muscle. In these latter cells
repolarization immediately follows the depolarization phase
and total membrane conductance is high. Repolarization in
cardiac cells is characterized by the existence of a long plateau
during which the rate of voltage change is very slow. The
* Corresponding author: Edward Carmeliet
Tel: +32-16-402823;
E-mail: [email protected]
return to the resting potential is delayed. The membrane
conductance during the long plateau is about four times lower
than during diastole [2]. It is caused by a dramatic fall in the
K + conductance of the I K1 channel which is normally
responsible for the negative membrane resting potential (Fig 1;
see later inward rectification). The fall in outward K+ current
has the great advantage that K+ loss during the long plateau is
minimal. Energy waste via active transport is thus held at a
safe lower limit. However, since net current as well as total
current are small, any further important decrease of outward
current may lead to excessive prolongation of the action
potential duration (APD) and of the QT interval in the ECG.
During recent years congenital as well as acquired forms of
LQTsyndromes (LQTs) have been described (for review
see[3]). When repolarization rate falls below 0.1V/s the
probability for early afterdepolarizations (EADs) and
extrasystoles rises[4]. Evolution to full-sized polymorphic
tachycardia or torsade de pointes arrhythmia (TdP) is favoured
by the existence of dispersion in the effective refractory period
(ERP), which is caused by non-uniform prolongation of the
APD over the wall of the ventricle. Fortunately this doom
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98
Figure1. Schematic representation of a cardiac action potential (top)
and accompanying change in IK1 current (bottom). The
IK1 current shows a dramatic fall during the plateau phase,
due to block by intracellular Mg2+ ions and positively
charged polyamines. Unblock occurs during the final
repolarization phase. Tracings are not actual recordings
but drawings based on information provided by reference
[4].
Table 1. Three phases of repolarization in cardiac cells
Phase 1 or initial fast repolarization
inactivation of INa, activation of Ito
Phase 2 or plateau
voltage-dependent currents
inward currents: ICaL
outward currents: IKr , IKs
transporters: INCX (in and out), INaK(outward)
Phase 3 or terminal repolarization :
IK1 inward rectifier
scenario does not occur frequently because of the existence of
redundancy in the repolarization process. The existence of
repolarization reserve in cardiac ventricular cells and its
modulation is the main topic of this contribution.
The normal repolarization process
The repolarization in cardiac ventricular cells is generally
subdivided in three phases, during which a number of inward
and outward currents participate (for references see [5-6]
(Table 1).
Phase 1 or initial fast repolarization follows the upstroke of
the action potential
It is caused by inactivation of the fast inward Na+ current
(INa) and simultaneous activation of a transient K+ outward
current. The transient outward current is mainly carried by a
voltage-activated current, I to (also called I to1 ) with fast
activation followed by fast inactivation, but a Ca2+-activated
Cl- current, ICa.Cl (Ito2), which rises in parallel with intracellular
Figure 2. Schematic representation of the time course of the two
delayed K+ currents, IKr and IKs (bottom) during the
cardiac action potential (top). Note the late peaking of IKr.
Based on information in reference [4].
Ca2+ concentration, can also contribute to the outward current.
Phase 1 is variably expressed in different cardiac cells. It is
very pronounced in Purkinje cells and M cells, large in
subepicardial but small in subendocardial cells. It also varies
with species: in guinea pig it is much less present than in
human. The rate of voltage change is in the order of 10 V/s.
Phase 2 or plateau follows phase 1
During the plateau the maximum repolarization rate is a
thousand times smaller than during depolarization: 0.1 to 0.3
V/s compared to 200V/s to 500 V/s. The net current (0.1-0.2
pA/pF) as well as the total conductance of the membrane are
very small. The major cause for the small total conductance is
the dramatic fall in IK1 conductance, the K+ channel that is
normally responsible for the resting potential. Different
outward and inward currents contribute to the plateau
repolarization.
Outward currents
Two voltage-dependent K+ currents are activated during
the plateau, IKr and IKs, the rapid and slow delayed K+ currents
respectively (Fig 2). Contrary to expectation for a rapidly
activated current IKr does not carry any large current early
during the plateau but shows a peak only late at the end of
phase 2 [7]. This is not due to the activation process as such:
IKr channels are activated early for depolarizations
corresponding to the peak of the action potential (AP) (time
constant of about 40 ms). Current through the IKr channel
however is small or absent as long as the membrane potential
is positive to the zero level. The reason is inactivation, which
in this channel is actually a faster process than activation [8].
Channels remain closed and only open when the membrane
potential is brought back to around zero mV. In the
current-voltage (I-V) relation such a behaviour is seen as fall
of current (inward rectification) at positive membrane
potentials. Following recovery from inactivation at more
negative potentials the channels open and slowly deactivate.
This succession of events is responsible for the transient nature
of the IKr current.
Repolarization Reserve in Cardiac Cells
The second outward current is IKs or slow delayed K+
current. In voltage clamp experiments using the action
potential clamp method, the current has been shown to carry a
slowly rising current over the whole plateau duration [9-11].
The total charge carried by IKs is variable with species and
conditions and is usually less than that of IKr. In the absence of
sympathetic stimulation, block of IKs produces little APD
prolongation in isolated rabbit, dog and human cardiac
myocytes. The distribution of IKs differs between cells: its
expression in M cells is smaller than in other myocardial layers;
in both subepicardial and subendocardial cells the contribution
of IKs is prominent. In this way IKs determines to a large extent
transmural dispersion of APD, the longest AP being observed
in M cells [12]. The heterogeneity of IKs expression in different
cells also explains why block of the other delayed K+ current,
IKr, affects M cells preferentially.
Although IKs block has normally only a small effect on
APD, the effect of IKs block becomes prominent when the AP
is already prolonged. This is i) because the net outward current
in a prolonged AP is smaller and any further decrease in
outward current will have a larger proportional effect and ii)
because more IKs can be activated during a long AP (activation
is time-dependent). The proportion of IKs also enhances under
sympathetic stimulation and block of the channels will become
more efficient [10] (see section on sympathetic stimulation).
Na+, K+-pump current (INa,K). Active Na+, K+-transport is
electrogenic. In the range of membrane potentials
corresponding to the action potential the pump current is
outward. The amplitude is sensitive to Em, [Na+]i and [K+]e ,
with [Na+]i as the more important modulator. This explains
why pump rate and thus current is so sensitive to frequency of
stimulation. The density of pump sites is larger in
subepicardial than subendocardial cells, a characteristic
distribution similar to that of IKs and of interest for our
understanding of the transmural potential gradient [13].
Inward currents
Ca2+ current (ICaL). ICaL is activated early during the AP
and carries a substantial charge into the cell: its mean density
is about 5 times the net current during the repolarization
process [4]. The current undergoes a voltage-dependent
inactivation which is rather slow. A much faster inactivation is
caused by the rise in free Ca2+ ion concentration following
influx of Ca2+ ions from the extracellular medium and release
from the sarcoplasmic reticulum (SR). Ca2+-dependent
inactivation occurs via binding to calmodulin, which is
prebound to the C-terminus of the channel. Part of the
C-terminus then moves in the direction of the inner pore and
blocks the channel [14-15]. When repolarization is very slow
in the range of membrane potentials where activation and
inactivation overlap, Ca2+ channels can become reactivated:
this extra inward current generates secondary depolarizations
or EADs.
Plateau Na+ current. Most of the fast Na+ current is
inactivated early during phase 1 of the repolarization. During
the plateau the Na+ current however does not drop to zero. In a
limited range of potentials where activation and inactivation
overlap a small steady-state Na+ current or window current can
99
be recorded. Also a small component of slowly inactivating
Na+ current can be measured over a broad range of potentials
in many types of cardiac cells of different species [16],
including expressed human cardiac Na+ channels [17]. The
non-inactivating or slowly inactivating current also called late
or persistent current is enhanced in the congenital LQT3
syndrome [18], in chronic ischaemia [19], hypertrophy and
failure [20-21] (for review see [22]).
Na+, Ca2+ exchanger current (INCX). Transport of Ca2+
ions through the NCX is responsible for maintaining
intracellular Ca2+ at a physiological low level (for review
[23-24]). Since three Na+ ions and one Ca2+ ion are transported
per cycle the transport is electrogenic. At rest the reversal
potential for the exchanger is positive to the resting membrane
potential: Ca2+ is moved out of the cell, Na+ is moved in and
the current is inward. During the initial part of the AP the
situation reverses: Ca2+ moves in and the current becomes
outward. Later during the plateau, when the cytoplasmic Ca2+
concentration reaches its peak the reversal potential shifts
positive to the membrane potential, causing Ca2+ again to be
moved out and producing inward current [25]. The INCX thus
will mainly carry inward current. In situations of Ca2+ overload
INCX may be responsible for critically reducing the rate of
repolarization late during the plateau, allowing Ca2+ channels
to be reactivated and generate EADs [26].
Phase 3 or final repolarization: IK1.
In the Introduction we mentioned that a fall in IK1 was the
main phenomenon responsible for the long APD. The rate of
repolarization during the plateau is dependent on the interplay
between outward currents through IKr, IKs, INa,K and to a minor
extent through INCX, and inward current through ICaL and INCX.
When the membrane potential approaches the level of –50 mV
repolarization speeds up reaching values of 0.5 to 5 V/s
(variable with type of cell and species). The main mechanism
responsible for this faster repolarization is outward current
through IK1[27]. The inward rectifier current is indeed a major
determinant of the final repolarization and carries a substantial
charge (about 4 times that of IKr at the end of the plateau). This
increase in outward current is not due to a voltage-dependent
activation but to an unblocking of the channel. At voltages
positive to –50 mV the channel is blocked by Mg2+ and
polyamines which enter the channel from the intracellular side
in a potential-dependent way [28]. At more negative levels the
channel is unblocked and outward current increases. This
greater outward current at more negative potentials generates a
negative resistance and the I-V relation is characterized by
inward rectification [29]. The system forms a positive
feedback (Fig 3): the greater the hyperpolarization, the less
block by Mg2+ or polyamines and thus the greater the outward
current which in turn causes more hyperpolarization.
Repolarization is thus a regenerative phenomenon. During
phase 3 the regenerative process is graded and, once started,
progresses to completion; the instantaneous I-V relation
contains a region of negative slope but is outward over the
whole voltage region. Earlier during the plateau regenerative
repolarization can be induced by hyperpolarizing impulses and
shows a threshold similarly to the Na+ conductance system
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Figure 3. Steady-state I-V relation of the IK1 current in cardiac cells
(top). Note the strong inward rectification, with a region
of negative resistance between –65 mV and –20 mV.
During phase 3 of the cardiac action potential the
existence of negative resistance in the
I-V relation
generates a positive feedback (bottom) and is responsible
for the regenerative nature of the final repolarization.
Schematic drawing based on reference [27].
during the depolarization [30-31]. The existence of a threshold
means not only that there is a negative resistance but that the
instantaneous I-V crosses the voltage axis with outward
current negative and inward current positive to the threshold
voltage.
Physiological modulation of repolarization reserve
Repolarization reserve is variable: net current and total
conductance of the membrane during the repolarization
process are not fixed. Under physiological conditions
repolarization reserve is modulated by increase in frequency of
stimulation, sympathetic stimulation and the accompanying
increase in [Ca2+]i, [Na+]i and [K+]e. All these processes are
interrelated as shown schematically in Fig 4.
From an electrophysiological point of view sympathetic
stimulation has two important effects on the heart: i) it
increases substantially the spontaneous sinus node rate, and ii)
at the level of atrial and ventricular cells it modulates a number
of ionic currents and transporters. The relative amplitude of
both effects depends on a dynamic equilibrium between
activity of the right and left stellate ganglion. The sinus rate is
selectively innervated by postganglionic fibers of the right
stellate ganglion, while most of the left ventricular
myocardium is governed by the left stellate ganglion. We will
first discuss the effect of rate as such.
Effect of rate.
The higher the frequency, the greater the shortening of the
APD in steady state (for review [22]). Initial changes for a
sudden increase in rate differ between species and type of
preparation. In human ventricle an important initial shortening
is followed by a slow and gradual shortening of APD, which
develops over a time course of tens of seconds to a few
minutes. The underlying processes responsible for the APD
shortening demonstrate an increase of repolarization reserve.
Inward currents decrease whereas outward currents increase: at
higher rates the safety factor for repolarization is enhanced.
Figure 4. Interplay between sympathetic stimulation, heart rate,
change of [Na+]i , [K+]e and [Ca2+]i in modulating
repolarization reserve. Most of the currents and
transports indicated in this scheme are enhanced;
negative changes are explicitly mentioned. Sinus rate is
augmented via β-receptor stimulation and activation of a
number of ionic currents. Increase in sinus rate decreases
INa,late and enhances IKs and IKr because of the specific
kinetics of these currents. INa,late shows slow recovery
from inactivation. For IKs activation is accelerated ; at
high rates in some species IKs tails may also show
summation. For IKr the increase is the consequence of the
change in AP shape and the accompanying changes in
activation-inactivation. Slower changes in ionic currents
with increase in sinus rate are secondary to increase of
[Na+]i, [Ca2+]i and [K+]e. The increase in sinus node rate
is normally caused by activation of the right stellate
ganglion of the sympathetic system. Stimulation of the
left stellate ganglion causes release of norepinephrine
at the level of left ventricular cells with activation of αand β-receptors, followed by activation of PKC and PKA
and subsequent phosphorylation of a number of ionic
channels and transporters (SERCA or sarcoplasmic
reticulum Ca2+-ATP-ase and INa,K or Na+,K+-ATP-ase).
Later epinephrine, set free from the adrenal medulla, will
also reach the heart. In general, currents are enhanced ,
except in the case of INa,late. INCX is modulated by changes
in [Na+]i, [Ca2+]i and PKC-dependent phosphorylation;
depending on the reversal potential the current can be
inward or outward. For further information see text and
[3, 61, 22].
Inward currents during the plateau are decreased. At
higher rate of stimulation, ICaL declines faster. The mechanism
is amplified Ca2+-dependent inactivation consequent to the
enhanced release of Ca2+ ions from the SR. The peak current
however, may be increased by Ca2+-dependent facilitation.
Recently Ca2+-dependent activation of endogenous
Calmodulin kinase II has been shown to play an important role
in this facilitating mechanism by accelerating recovery from
inactivation [32].
The late Na+ current or slowly inactivating Na+ current is
also reduced. Two mechanisms play a role. i) Recovery from
inactivation of the late Na+ current, in contrast to that of the
fast INa is characterized by a slow time course [33]. At higher
rates of stimulation with shorter diastole, recovery from
inactivation thus remains incomplete and causes a fall in
Repolarization Reserve in Cardiac Cells
current. ii) A second reason for a decline in current is a
reduction in maximum conductance caused by PKC-dependent
phosphorylation of the channel as a consequence of increase in
intracellular free Ca2+ concentration at higher rates [34].
In contrast to inward currents, outward currents are
increased at higher stimulation rates. Summation of tail
currents at high rates has been described for IKs in guinea pig
ventricle, but this does not seem a general mechanism. In
many species deactivation is too fast to allow important
summation at short diastolic intervals [7, 35-36]. The kinetics
of IKs activation during the AP on the other hand are
accelerated and outward current reaches higher levels when
stimulation rate increases [7]. The change in kinetics and the
increase in conductance have been explained in the following
way. Opening of the channels is not a single reaction from
closed to open but requires passage through a number of
intermediate non-conducting states. At the end of an AP not all
channels have reached the open state but a number have
moved near to the open state. During diastole the channels
follow the reverse reaction but not all return to the rested state.
Some remain at an intermediate state from which activation
during the next AP is more readily turned on (for a modeling
study see [37]). This creates an available reserve of channels
that are ready to open ‘on demand’. A repolarization reserve is
generated in the channel itself.
A second mechanism for a rise in amplitude of IKs with
rate of stimulation is increase in conductance following
phosphorylation of the channel by PKC. This enzyme in turn
is activated by the rise in [Ca2+]i at elevated rates of stimulation
[38]. Phosphorylation however is not the only mechanism by
which [Ca2+]i modulates IKs. A phosphorylation-independent
but calmodulin-dependent stimulation has been suggested by
Nitta et al.[39]. Recently two groups [40-41] have
demonstrated the existence a direct binding of Ca2+ to the
channel complex via calmodulin. This binding facilitates
channel assembly, prevents inactivation, shifts the activation to
more negative potentials and thus mediates a considerable
Ca2+-sensitive increase of the IKs current.
Modulation of IKr seems less developed. The existence of
a direct effect of rate, i.e. summation and/or faster activation
kinetics, is more controversial. No effect has been found in the
guinea pig [35] or in the dog [36]. According to Gintant [36]
this is due to the rapid activation of IKr, which is complete
within 150 ms or the duration of a single AP. In the guinea pig
Rochetti et al [7] explained the increase in IKr level at higher
rates of stimulation by taking into account the accompanying
change in shape of the AP. The dependency of IKr on AP shape,
according to the authors, can be justified by its known kinetic
properties of activation and inactivation. This is a remarkable
result because the system forms a positive feedback system:
the higher the rate of repolarization the greater the activation
level of IKr.
As for IKs, IKr has been shown to be phosphorylated by a
Ca2+-dependent PKC. The result is a marked decrease in
inward rectification which means enhancement of current at
positive potentials [42].
As a final comment one should stress the fact that not
101
only [Ca2+]i but also [Na+]i and [K+]e rise at higher frequency
(Fig 4). The rise in [Na+]i will stimulate the Na+,K+-pump
and generate extra outward current during the whole plateau
favouring repolarization (review see [22]). [K+]e increase also
stimulates the pump, and exerts moreover an enhanced effect
on IKr [43] and IK1 [44].
Sympathetic stimulation.
The effect of rate has been discussed in the previous
section: an increase in rate shortens the APD, it enhances
repolarization reserve. We will now concentrate on changes in
currents and transporters by sympathetic stimulation which are
due to interaction of norepinephrine and epinephrine with
specific receptors in ventricular cells. Norepinephrine, released
from the nerve endings, and epinephrine, set free from the
adrenal medulla, bind to α- and β-receptors in the target cell.
In heart the major effects occur via β-receptor stimulation.
Three pathways are involved: direct activation of G-proteins,
binding of cAMP and, the most important, cAMP-mediated
activation of PKA and phosphorylation of the target.
β-receptor activation causes an increase in ICaL, IKs and INa,K
and results in elevation and shortening of the AP plateau.
Ca2+ current. ICaL is enlarged by β-receptor stimulation
via PKA-dependent phosphorylation. The effect is an increase
in conductance (recruitment of more channels) and
acceleration of activation [45]. The increase in ICaL would
normally prolong the APD in ventricular cells and this
effectively occurs at low concentrations of norepinephrine
where stimulation of ICaL prevails [46]. A larger ICaL at higher
concentrations of norepinephrine however, also means a
more positive plateau. This larger depolarization allows for
greater activation of IKr and IKs. Since outward currents at
these concentrations are positively modulated by direct
β-receptor stimulation (IKs see below) and rise of [Ca2+]i (IKs
and IKr) the final effect under normal physiological conditions
is a higher plateau but shorter duration.
Delayed K+ currents. As noted in the previous discussion
on rate effects, both IKr and IKs are enhanced by
Ca2+-dependent PKC stimulation. In the presence of
sympathetic stimulation the rise in [Ca2+]i will be amplified by
an enhanced Ca2+ influx through ICaL and larger storage in the
SR by a stimulated Ca2+-ATPase The IKs channel furthermore,
is specifically stimulated by Ca-calmodulin-dependent and
PKA-dependent phosphorylation. It has become evident that
the IKs channel, composed of KCNQ1 and KCNE1 proteins,
forms a macromolecular complex with calmodulin and with
PKA and β-receptor signaling molecules [47][40-41].
Modulation of IKs by Ca-calmodulin has been discussed in the
section on rate. Phosphorylation by PKA moreover, shifts
kinetics and increases conductance and causes a faster and
greater outward current during the AP [48]. The changes in
both delayed K+ currents are primordial in causing shortening
of the APD under sympathetic stimulation. The absence of one
of the delayed K+ currents in congenital LQT syndromes
explains why catecholamines may induce QT prolongation
under those conditions [49]. The effect on ICaL then prevails.
α-receptor increases the activity of the Na+,K+ pumpATPase and of the Na+, Ca exchanger via activation of lipases
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102
Table 2. LQT syndromes
and secondarily of PKC [50]. At the channel level α-receptor
stimulation inhibits Ito, IK1 and late INa Some of these actions
result in shortening others in prolongation of APD.
Under normal physiological conditions the effect of
β-receptor activation on heart is prevalent over the effect of
α-receptor activation. It may thus be concluded that
repolarization reserve under activation of the sympathetic
system under normal physiological conditions is substantially
enhanced.
Pathological changes in repolarization reserve
Congenital LQT syndromes (LQTs) (for review see [3, 51])
A great number of mutations of channel or transporter
genes resulting in dramatic reduction of repolarization reserve
have been described during recent years. Most of these
mutations cause reduction of repolarizing currents ( IKr, IKs, IK1,
and INa,K), only one type concerns a gain of function and
causes an increase of the late inward Na+ current (see table 2).
Loss of function is due to a deficient trafficking of channel
molecules to the cell membrane, to changes in kinetics,
conductance or selectivity of the channel.
Gain of function in the case of INa, is caused by slowing
of inactivation. They all result in congenital LQTs
characterized by life-threatening arrhythmias and often sudden
death. Fortunately these mutations are relatively rare.
Loss of IKs channel function is seen in LQT1 and LQT5
due to mutations in KCNQ1 and KCNE1 gene, loss of IKr is
characteristic for LQT2 and LQT6 due to mutations in
KCNH2 and KCNE2, loss of IK1 in LQT7 is due to mutation in
KCNJ2. A gain of function in the late Na+ channels is
characteristic for LQT3, which is caused by mutations in
SCN5A. The LQT4 is caused by deficient functioning of an
anchoring protein, ankyrin-B, normally responsible for
membrane targeting of the Na+,K+pump-ATPase and NCX
protein. The result is aberrant intracellular Ca2+ regulation. In
all these mutations the QT interval or cellular APD is increased
to a large extent such that EADs may be generated.
The majority of serious arrhythmias in LQT1, LQT5 and
LQT2 and LQT6 syndromes occur during exercise or
emotional stress, when an increase in sympathetic activity is
expected [52]. This may seem strange since sympathetic
stimulation has been shown to enhance repolarization reserve
in normal physiological conditions via increase in outward and
decrease in inward currents. The reason for this negative effect
is that excessive stimulation of the sympathetic system on the
background of reduced outward currents may increase ICaL to
such extent that EADs are generated.
The reduction of outward current caused by the mutation,
can be due to deficient channels or in the case of IKs to
uncoupling of the channel from the PKA system [47]. As
mentioned previously the IKs channel forms a macromolecular
complex together with β-signaling molecules. In some
mutations linked to LQT1 and LQT5 sympathetic stimulation
is disrupted in such a way that IKs phophorylation is deficient.
IKs is uncoupled from PKA modulation, whereas ICaL
modulation is still normal. The enhanced ICaL may then lead to
excessive prolongation of APD and EADs.
Experimentally, β-receptor stimulation has been shown to
prolong QT in LQT1 and LQT2 patients and to increase
transmural dispersion in LQT1 and LQT2 animal models [53,
49, 54]. Excessive prolongation of the AP and existence of a
high grade of dispersion are pivotal mechanisms in the genesis
of arrhythmias. The increase in dispersion is caused by a
selective increase of APD in M cells while APD is shortened in
subepicardial and subendocardial cells. Due to the mutations,
IKs and IKr are less expressed especially in M cells, and the
APD under β-receptor stimulation will be prolonged. In
subepicardial and subendocardial cells on the other hand,
shortening of APD is caused by enhancement of IKs and IKr
which are sufficiently expressed and represent a substantial
repolarization reserve against ICaL.
In LQT3, arrhythmias are generated
preferentially
during rest or sleep, when cardiac frequency is very low.
Excessive APD prolongation at long cycle lengths has been
confirmed in the transgenic ∆KPQ mouse model [18]. EADs
may develop under those circumstances and are responsible for
the initiation of arrhythmia. At higher rates, summated
inactivation of the late current [33] together with larger and
faster activation of IKs will act in the opposite way and tend to
normalize the APD as a consequence of enhanced
repolarization reserve. The antiarrhythmic effect of
isoproterenol can be explained by the net increase of
repolarizing current (K+ currents and Na+,K+ pump current) by
β-receptor stimulation and possibly by a reduction of late INa
as a result of PKC-dependent phosphorylation induced by
increase of intracellular Ca2+.
β-blockers are the mainstay therapy for LQT1 and LQT2
patients. This is reasonable since most of the arrhythmias in
these patients occur in the presence of sympathetic overdrive.
For LQT3 patients however the usefulness of β-receptor
therapy has been questioned [55]. During rest or sleep when
most cardiac events occur, β-receptor antagonists may further
slow heart rate and promote arrhythmias. At elevated rates on
the other hand, β-receptor agonism (isoproterenol) exerts
antiarrhythmic activity in the ∆KPQ transgenic mouse model
[18]. In LQT3 patients and cellular models, β-receptor
stimulation antagonizes LQT and causes shortening of the
APD independent of the rate. The use of β-blockers thus
remains questionable.
Repolarization Reserve in Cardiac Cells
Acquired LQT syndrome.
The LQT syndrome is not necessarily of hereditary origin
but can also be of an acquired nature [1]. One of the more
frequent and dangerous forms of acquired LQT is caused by
the use of medications. The drugs responsible belong to
different groups but have the common effect of blocking K+
currents. Among the antiarrhythmics, drugs of class I and III
should be mentioned: procainamide, quinidine, disopyramide,
sotalol, dofetilide, ibutilide. Others belong to the
antihistamines e.g. terfenadine, astemizole; antimicrobials e.g.
erythromycine, ketoconazole; gastrointestinal drugs e.g.
cisapride, and psychotropic drugs e.g. haloperidol. Chemically
they all have at least one aromatic ring, they block IKr and are
trapped when the channel closes [56].
During recent years it has become evident that IKs under
conditions of IKr block can compensate for the loss of IKr [11].
This is due to the longer and more positive plateau, allowing
more channels to open and more channels to move close to the
open state such that they readily open during the next AP.
Not all patients using these drugs show complications of
TdP arrhythmias. It is thus important to delineate the risk
factors that amplify the danger of blocking certain repolarizing
current or reducing repolarization reserve. Among these factors
one should mention hypokalemia, hypomagnesemia,
congestive heart failure, left ventricular hypertrophy, female
sex [57] and predisposing DNA polymorphisms [1]. The
hypertrophic and failing heart undergoes a number of
remodeling modifications in the myocytes which are
predisposing to arrhythmias [58]. Ventricular ectopy and
arrhythmias are a frequent complication which may result in
sudden death. The mechanisms involved are enhanced
tendency to triggered activity on one hand and favourable
conditions for reentry on the other hand. The fall in different
K+ currents and of INa,K shift the background in the inward
direction and slow repolarization. The occurrence of EADs is
favoured by the prolongation of the APD especially at low
frequencies of stimulation. Increased INCX again plays a
conditioning role and facilitates reactivation of ICaL [59]. It is
the aim of future research to unravel the underlying
remodeling mechanisms. In this way it will become possible to
prevent or reverse remodeling and avoid LQT arrhythmias.
2.
3.
Redundancy in repolarizing currents, called repolarization
reserve, guarantees a normal repolarization process in cardiac
cells. Increase in rate and sympathetic stimulation under
physiological conditions enhances repolarization reserve.
Pathological reduction of repolarization reserve which
increases the risk of deadly arrhythmias can be of hereditary
(congenital LQT syndromes) or acquired nature. By way of
conclusion, the following aspects should be stressed.
1. The effects of sympathetic stimulation at the level of
ionic channels are multiple and interrelated. Changes in
ionic currents are caused via modulation of kinetics and
conductance, trafficking and assembly of the channel,
PKA- and PKC-dependent phosphorylation. An important
component is the increase in [Ca2+]i which modulates a
number of currents via PKC-dependent phosphorylation,
stimulates IKs via Ca2+-CaM-binding, and activates INCX
acting as a substrate. The rise in [Ca2+]i feeds back on ICaL
itself: in a negative way by accelerating inactivation
(Ca2+-CaM binding) and in a positive way by accelerating
recovery from inactivation (Ca2+-CaMkinase II). Under
physiological conditions the total result is elevation of the
plateau level and shortening of the APD.
Under conditions of pathological prolongation of the APD
(LQT syndrome), activation of the sympathetic system
causes dual effects. When delayed K+ currents are
deficient, the amplified ICaL will act as an arrhythmogenic
by further prolonging the APD and favouring EADs. It
explains why the majority of arrhythmias occur during
exercise or stress in LQT1, LQT2, LQT4 and LQT6.
However, when APD prolongation is caused by a gain in
INa,late, i.e. in LQT3, arrhythmias are generated
preferentially during rest of sleep, when frequency is low.
In this case, β-receptor stimulation has been shown to be
antiarrhythmic, because of the fall in INa,late and the
concomitant increase in IKs and INa,K currents.
Excessive increase in repolarization reserve has not been
discussed in the present review. It may occur in atria and
favour the transition from acute to chronic atrial
fibrillation. Remodeling as a consequence of high rate
stimulation causes a drastic reduction in the expression of
Ca2+ channels and induces a stable shortening of the APD
with enhanced probability of reentry arrhythmias [60].
Excessive shortening of the APD and eventual
inexcitability may also occur in the ventricle during acute
ischaemia following activation of ATP-dependent K+
channels [5]. It may favour reentry arrhythmias, but is
also assumed to play a role in cell survival by arresting
depletion of ATP.
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