Download 1 Introduction to Cardiac Arrhythmias. Electrophysiology of Heart

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Introduction to Cardiac Arrhythmias. Electrophysiology of Heart,
Action Potential and Membrane Currents
Norbert Jost, PhD
Department of Pharmacology & Pharmacotherapy, Faculty of Medicine,
University of Szeged, Hungary
Correspondence:
Norbert Jost, PhD
Department of Pharmacology & Pharmacotherapy, Faculty of Medicine,
University of Szeged
Dóm tér 12, P.O. Box 427, H-6701 Szeged, Hungary
Tel.: (36-62) 546885, Fax: (36-62) 545680
E-mail: [email protected]
@All rights reserved to Dr. Norbert Jost, associate professor
Department of Pharmacology & Pharmacotherapy, Faculty of
Medicine, University of Szeged, Hungary
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Keywords:
cardiac membrane potential,
electrophysiology, transmembrane ion channels,
action
potential,
List of abbreviations:
AF = atrial fibrillation
APD = action potential duration
AVN = atrioventricular node
cAMP= cyclic adenosine monophosphate
EK, ENa = equilibrium potential for K+ or Na+, respectively
EM= resting membrane potential
HVA and LVA = high and low voltage-activated calcium channels,
respectively
ICaL= L-type calcium current
IK(ACh) = acetylcholine sensitive potassium current
IKATP = ATP sensitive potassium current
IK = delayed rectifier potassium current;
IKr = rapid component of the delayed rectifier potassium current;
IKs = slow component of the delayed rectifier potassium current;
IK1= inward rectifier potassium current
IKur = ultra-rapid component of the delayed rectifier potassium current;
INCX = Na+/Ca2+ exchanger current
Ito= transient outward potassium current;
INa= fast sodium current
INaL= late sodium current
KV = voltage gated K+ channels
LQT3 = long QT3 syndrome
mRNA= messenger RNA
NaV = voltage gated Na+ channels
Na/K pump= sodium-potassium pump
NCX= sodium-calcium exchanger current
SAN = sinoatrial node
Vm = membrane potential
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Table of contents
Abstract
1. Introduction
2. Cell membrane potentials. Electrical activity in the heart: conduction
system and cardiac action potential
2.1. Equilibrium potentials
2.2. Cardiac conduction system
2.3. The cardiac action potential
3. Transmembrane ion channels in the heart
3.1. Voltage-gated Na+ (NaV) currents
3.2. Voltage-gated Ca2+ (CaV) currents
3.3. Voltage gated K+ (KV) channels
3.3.1. Transient outward KV currents (Ito)
3.3.2. Delayed rectifier KV currents (IK)
3.3.3. Inward rectifier potassium currents (IK1)
3.3.3. ATP sensitive potassium currents (IKATP)
3.3.5. Background potassium channels
3.4. Summary
4. Concluding remarks
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Abstract
Myocytes represent the functional unit of cardiac muscle;
nonetheless, the heart behaves more or less like an electrical syncytium,
whose global activity depends on low resistance coupling between the
myocytes. The term “more or less” is used here intentionally to imply
that, while the activity intrinsic to individual myocytes is affected by
coupling, its features remain recognizable within the context of the whole
heart and are important to determine its function. Electrical changes
within the myocytes plays an important role to initiate the cardiac
contraction. This chapter addresses (a) the electrical activity of individual
myocytes, namely the resting membrane potentials and action potentials;
(b) the way action potentials are conducted throughout the heart to initiate
coordinated contraction of the entire heart; and (c) the transmembrane
ionic currents underlying cardiac action potential.
1. Introduction
The heartbeat arises from organized flow of ionic currents through ionspecific channels in the cell membrane, through the myoplasm and gap
junctions that connect cells, and through the extracellular space. The
action potential formation results from the opening and closing (gating
mechanism) of several inward and outward ion channels, which are
largely expressed within the sarcolemma of cardiomyocytes. Ion channels
possess distinct genetic, molecular, pharmacologic, and gating properties,
while exhibit heterogeneous expression levels within different cardiac
regions. By gating, ion channels permit ion currents across the
sarcolemma, thereby creating the different repetitive phases of the action
potential, which will be discussed later in detail (e.g., resting phase 
depolarization  repolarization circles). The importance of ion channels
in maintaining normal heart rhythm is reflected by the increased
incidence of arrhythmias in inherited diseases that are associated to
several mutations in genes encoding cardiac ion channels or
pumps/exchangers or their accessory proteins and in acquired diseases
that are associated with changes in ion channel expression levels or gating
properties (e.g., different forms of electrical, structural or contractile
remodelling linked to congestive heart failure, dilated cardiomyopathy,
permanent forms of atrial fibrillation, etc.). To understand the functioning
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of the transmembrane ion currents and their contribution to the cardiac
action potential, it is important to understand the biophysics of the
biological cell membranes, including the ion transports, membrane and
Nernst potentials as well.
2. Cell membrane potentials. Electrical activity in the heart:
conduction system and cardiac action potential
Cardiac cells, similar with the majority of the living cells from the
body, have an electrical potential across the cell membrane. This potential
can be investigated by inserting a microelectrode into the cell and to
determine the electrical potential in millivolts (mV) inside the cell relative
to the outside of the cell. By convention, the outside of the cell is
considered 0 mV. If measurements are taken with a resting ventricular
myocyte, a membrane potential of about –90 mV will be recorded. This
resting membrane potential (EM) is determined by the concentrations of
positively and negatively charged ions across the cell membrane, the
relative permeability of the cell membrane to these ions, and the ionic
pumps that transport ions across the cell membrane.
2.1. Equilibrium potentials
Of several others ions present inside and outside of cells, the
concentrations of Na+, K+, Cl-, and Ca2+ are most important in
determining the membrane potential across the cell membrane. Table 1
shows typical concentrations of these ions. Among these ions, K+ is the
most important in determining the resting membrane potential. In a
cardiac cell, the concentration of K+ is relatively high inside (about 140150 mM), while it is significantly lower outside (4-4.5 mM) the cell.
Table 1. Ion concentrations inside and outside of resting myocytes
ION
Na+
K+
Ca2+
Cl-
INSIDE (mM)
20
150
0.0001
25
Outside (mM)
145
4
2.5
140
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Therefore, a strong chemical gradient (concentration difference) exists for
K+ that facilitates the ion diffusion out of the cell. The opposite situation
is found for Na+; its chemical gradient favours an inward diffusion. The
concentration differences across the cell membrane for these and other
ions are determined by the activity of energy-dependent ionic pumps and
the presence of impermeable, negatively charged proteins within the cell
that affect the passive distribution of cations and anions. These
concentrations are approximations and are used to illustrate the concepts
of chemical gradients and membrane resting potential.
To understand how concentration gradients of ions across a cell
membrane affect membrane potential, consider a cell in which K+ is the
only ion other than the large negatively charged proteins inside of the cell.
In this cell, K+ diffuses down its chemical gradient out of the cell because
its concentration is much higher inside than outside the cell. As K+
diffuses out of the cell, it leaves behind negatively charged proteins,
thereby creating a separation of charge and a potential difference across
the membrane (leaving it negative inside the cell).
The membrane potential that is necessary to oppose the movement
+
of K down its concentration gradient is termed the equilibrium potential
for K+ (EK), and is expressed by the Nernst potential. The Nernst potential
for K+ at 37°C is as follows:
E K  61 log
[ K  ]i
 96mV
[ K  ]o
where the potassium concentration inside [K+]i = 150 mM and the
potassium concentration outside [K+]o = 4 mM. The –61 is derived from
RT/zF, in which R is the universal gas constant, z is the number of ion
charges (z=1 for K+; z=2 for divalent ions such as Ca2+), F is Faraday’s
constant, and T is absolute temperature (°K). The equilibrium potential is
the potential difference across the membrane required to maintain the
concentration gradient across the membrane. In other words, the
equilibrium potential for K+ represents the electrical potential necessary
to keep K+ from diffusing down its chemical gradient and out of the cell.
An increase in the outside K+ concentration will reduce the chemical
gradient for diffusion out of the cell, i.e. the membrane potential required
to maintain electrochemical equilibrium would be less negative according
to the Nernst equation. The EM for a ventricular myocyte is about –90
mV, which is very close to the equilibrium potential for K+. Because the
equilibrium potential for K+ is –96 mV and the resting membrane
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potential is –90 mV, a net driving force (net electrochemical force) acts
on the K+, causing it to diffuse out of the cell. In the case of K+, this net
electrochemical driving force is the EM (–90 mV) minus the EK (–96 mV),
resulting in +6 mV. Because the resting cell has a finite permeability to
K+ and a small net outward driving force is acting on K+, K+ slowly leaks
outward from the cell.
The sodium ions play a major role in determining the membrane
potential. Because the Na concentration is higher outside the cell, this ion
would diffuse down its chemical gradient into the cell. To prevent this
inward flux of Na, a large positive charge is needed inside the cell
(relative to the outside) to balance out the chemical diffusion forces. This
potential is called the equilibrium potential for Na+ (ENa) and is calculated
using the Nernst equation, as follows:
[ Na  ]i
ENa  61 log
 52mV
[ Na  ]o
where the sodium concentration inside [Na+]i =20 mM and the sodium
concentration outside [Na+]o =145 mM. The calculated equilibrium
potential for sodium indicates that to balance the inward diffusion of Na+
at these intracellular and extracellular concentrations, the cell interior has
to be + 52 mV to prevent Na+ from diffusing into the cell.
2.2. The cardiac conduction system
The conducting system of the heart consists of group of several cardiac
muscle cells and conducting fibres, which are specialized for initiating
impulses and conducting them rapidly through the heart (Figure 1). They
initiate the normal cardiac cycle and coordinate the contractions of
cardiac chambers, first contract both atria, then the ventricles. The
conducting system provides the heart its automatic rhythmic beat. For the
heart to pump efficiently and the systemic and pulmonary circulations to
operate in synchrony, the events in the cardiac cycle must be coordinated.
The cardiac impulse originates in the sinoatrial node (SA node), located
in the right atrium, which is activated first followed by the left atrium.
The general direction of the atrial activation is inferiorly, to the left, and
posteriorly. This causes the atria to contract and pump blood from the
atria to the ventricles; it is recorded on an EKG as a P wave (Figure 1).
The atrial impulse is delayed in the atrioventricular node (AV node) to
allow the ventricular chambers to fill, and is then conducted rapidly
through the ventricles (the bundle of His, the right and left bundles, and
the Purkinje fibres). This causes the ventricles to pump blood out of the
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heart and to the body; it is recorded on an EKG as a QRS complex.
Recovery following the cardiac cycle, or repolarization, follows.
Figure 1. Electrical activity in the myocardium. Schematic
representation of a human heart with illustration of typical action
potential (AP) waveforms recorded in different regions, and their
contribution to surface electrocardiogram.
This is recorded as a T wave on an EKG. On the microscopic level, the
wave of depolarization propagates to adjacent cells via gap junctions
located on the intercalated disk. The heart is a functional syncytium (not
to be confused with a true "syncytium" in which all the cells are fused
together, sharing the same plasma membrane as in skeletal muscle). In a
functional syncytium, electrical impulses propagate freely between cells
in every direction, so that the myocardium functions as a single
contractile unit. This property allows rapid, synchronous depolarization of
the myocardium. While advantageous under normal circumstances, this
property can be detrimental, as it has potential to allow the propagation of
incorrect electrical signals. These gap junctions can close to isolate
damaged or dying tissue, as in a myocardial infarction.
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2.3. The cardiac action potential
The normal mechanical (pump) function of the mammalian heart
depends on proper electrical function [1,2], as reflected in the successive
activation of cells in specialized, "pacemaker" regions of the heart and the
propagation of activity through the ventricles. Myocardial electrical
activity is attributed to the generation of action potentials (AP) in
individual cardiac cells, and the normal coordinated electrical functioning
of the whole heart is readily detected in surface electrocardiograms
(Figure 1). Propagation of the electrical activity and coordination of the
electromechanical functioning of the ventricles strongly depend on
cellular electrical coupling mediated by gap junctions [3]. The generation
of myocardial action potentials reflects the consecutive activation and
inactivation of ion channels that conduct depolarizing, inward (Na+ and
Ca2+), and repolarizing, outward (K+), currents. The waveforms of action
potentials in different regions of the heart are different reflecting to
differences in the expression and/or the properties of the underlying ion
channels. These differences contribute to the normal unidirectional
propagation of excitation through the myocardium and to the generation
of normal cardiac rhythms [4,5,6].
The cardiac electrical cycle has been divided in to five “phases”,
four of them describing the AP contour and one the diastolic interval
(Figures 1 and 2). Phase 0 refers to the autoregenerative depolarization,
which occurs when the excitation threshold is exceeded. Phase 0 is
supported by activation of two inward (depolarizing) currents, INa and
ICaL. Membrane depolarization will quickly activate these channels and,
with a delay of several milliseconds for I Na and of tens of milliseconds for
ICaL, inactivates them. Thus, membrane depolarization provides both the
triggering and breaking mechanism for the autoregenerative
depolarization. Although short-lived, INa is large and provides most of the
charge influx required for AP propagation (see below). ICaL has a small
component with fast activation/inactivation (ICaT) and a larger one with
slower kinetics (ICaL). ICaL mediates most of Ca2+ influx required to trigger
myocyte contraction and may support propagation under conditions in
which INa is not expressed or functional (e.g. in the SA node). Phase 0
depolarization also activates K+ currents, which contribute to termination
of this phase and to subsequent repolarization. Among these, the transient
outward current (Ito) is fast enough to limit the depolarization rate during
phase 0.
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Figure 2. Phases of a typical atrial and ventricular APs and underlying
currents. The numbers refer to the five phases of the action potential. In
each current profile, the horizontal line represents the zero current level;
inward currents are below the line and outward currents are pointing
upward.
Phase 1 is the initial phase of repolarization, mainly supported by
Ito,f (the Ca independent Ito fast component –see section 3.3.1 for details),
a potassium current that, similarly to INa, is activated and quickly
inactivated by depolarization. Thus, Ito,f supports fast repolarization.
Phase 2, also named “plateau”, is the slow repolarization phase,
which accounts for the peculiar configuration of the cardiac AP. The net
transmembrane current flowing during phase 2 is small and it results from
the algebraic summation of inward and outward components. The
outward one (promoting repolarization) mainly consists of depolarizationactivated K+ currents collectively named “delayed rectifiers” (IK). IK is
actually the sum of rapid (IKr) and slow (IKs) components (and ultra-rapid
in atria, IKur), carried by separate channels with different properties and
pharmacology [7]. The inward phase 2 currents (opposing repolarization)
are mostly carried by “window” components of INa and especially of ICaL,
which flow when membrane potential (Vm) is such that the activated state
of these channels is not yet completely offset by the inactivation process.
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This inward calcium movement is through long-lasting (L-type) calcium
channels that open when the membrane potential depolarizes to about –40
mV. L-type calcium channels are the major calcium channels in cardiac
and vascular smooth muscle. They are opened by membrane
depolarization (they are voltage-operated) and remain open for a
relatively long duration consequently causing the relative long plateau
phase of AP (corresponding to the long ST segment of the ECG). The
current generated by operation of the Na+/Ca2+ exchanger (INCX) can
variably contribute to phase 2, according to the magnitude and course of
the cytosolic Ca2+ transient and to the subsarcolemmal Na+ levels.
Phase 3 is the terminal phase of repolarization, which differs from
phase 2 for its faster repolarization rate. Phase 3 is dominated by I Kr and
IK1, both characterized by a kinetic property, named “inward rectification”
[8,9], suitable to support autoregenerative processes [10]. Initiation of
phase 3 is probably supported by IKr, with a threshold determined by the
balance between its onset and waning of phase 2 inward currents. I K1
takes over during the final part of phase 3 and effectively “clamps”
membrane potential back to its diastolic value [11].
Phase 4 describes membrane potential during diastole. In
myocytes expressing a robust IK1 (e.g. atrial and ventricular myocytes),
Vm is stabilized at a value close to the current reversal and a significant
current source is required to re-excite the cell. Under such conditions
even small inward currents may cause progressive depolarization,
eventually leading to re-excitation (automatic behaviour) [12]. Besides
the time-dependent currents, specific for each AP phase, timeindependent (or “background”) currents may also contribute to the whole
AP course. These mainly include the Na+/K+ pump current (INaK) and the
Na+/Ca2+ exchanger current (INCX). Direction and magnitude of these
currents during the various AP phases will be determined essentially by
their electromotive force.
3. Transmembrane ion channels in the heart
3.1. Voltage-gated Na+ (NaV) currents
Voltage-gated cardiac Na+ (NaV) channels open extremely rapidly
(within 3 ms) on membrane depolarization and in principle determine
alone the rapidly rising phases of the action potentials recorded in
mammalian ventricular and atrial myocytes and in cardiac Purkinje fibres
[13, 14]. Although the properties of the NaV channels expressed in
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different cardiac cells are alike, the biophysical and pharmacological
properties of these channels are quite different from NaV channels
expressed in other excitable cells (neurons or skeletal muscle). Cardiac
NaV channels, for example, are remarkably insensitive to the NaV
channel toxin tetrodotoxin (TTX). On membrane depolarization, cardiac
NaV channels activate and inactivate very rapidly [15]. Figure 3 show
typical representative recordings from a single cell of I Na in control and
during superfusion with 10 and 30 μmol/L quinidine, a well-known
sodium channel blocker Class IA antiarrhythmic drug [16].
Figure 3. The voltage gated fast sodium channel (INa). Panel A. Original
INa trace recorded in rabbit ventricular myocytes (adapted from [16] with
permission). The original recording show representative recordings of
peak INa from a single cell in the absence (control) and the presence of 10
and 30 μM/L quinidine (a typical Class IA, i.e., a known sodium channel
blocker, antiarrhythmic drug). Inset shows current–voltage curves in the
absence (open circles) and the presence (solid circles) of 30 μM/L
quinidine. Panel B. The schematic contribution of INa to the phase 0
(depolarization) on the action potential shape. Panel C. The biophysical
double gating model of the sodium channel. Sodium channel has two
gates, one activating gate (m) and one inactivating gate (h). In the resting
(closed) state, the m-gates (activation gates) are closed, although the hgates (inactivation gates) are open in order to wait activating pulse to
open the channel. Rapid depolarization to threshold opens the m-gates
(voltage activated), thereby opening the channel and enabling sodium to
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enter the cell. Shortly thereafter, as the cell begins to repolarize, the hgates close and the channel becomes inactivated. Toward the end of
repolarization, the m-gates again close and the h-gates open. This brings
the channel back to its resting state. Summarizing, the channel can be
activated only after reaching again the resting phase (i.e., the activating
gate must first close and the inactivated gate must reopen). This
phenomenon represents the subcellular basis of the refractoriness of the
heart, i.e., the heart cannot be tetanised.
The threshold for NaV channel activation is negative (approximately –55
mV), and the activation of these channels is strongly voltage dependent.
Importantly, the inactivation is also voltage dependent, and so fast that
cardiac NaV channels can undergo voltage-dependent inactivation
without ever opening. At values corresponding to the action potential
plateau in ventricular myocytes, present estimates are that 99% of the
NaV channels are already in an inactivated, nonconducting state [17].
There is, therefore, a finite, albeit small (1%), probability of NaV
channels being opened at potentials corresponding to the action potential
plateau [17]. This slow component of NaV channel inactivation called
late Na current (INaL) has indeed been described in normal human
ventricular myocytes, [18].
The probability of NaV channel opening at depolarized potentials
(i.e., during phase 2) is determined by the overlap of the curves describing
the voltage dependences of channel activation and inactivation.
Consistent with these predictions, electrophysiological studies reveal the
presence of a sustained component of inward NaV current, i.e., a
"persistent" NaV current, during prolonged membrane depolarizations,
which was named “window” current. Although the "window" current is
relatively small in comparison with the magnitude of the INa current, this
current may contribute to action potential durations [19]. It has been
reported that the expression level of the persistent NaV current
component is different in various regions of the ventricles, differences
that could contribute to the observed regional heterogeneities in
ventricular action potential durations [20]. The impact of alterations in the
"persistent" NaV channel window current on cardiac rhythms has now
been unequivocally demonstrated with the identification of mutations in
the gene, SCN5A, encoding the TTX-insensitive cardiac NaV channels in
patients with an inherited form of long QT syndrome, LQT3 [21]. Several
SCN5A mutations in different affected individuals/families have been
identified and linked to Brugada syndrome and to conduction defects, in
addition to LQT3 [22,23].
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3.2. Voltage-gated Ca2+ (CaV) currents
In contrast to skeletal muscle, it has long been recognized that
Ca2+ entry from the extracellular space is required for excitationcontraction coupling in the mammalian myocardium [2,24]. Several
studies reported late that a "slow inward" (in comparison with NaV
channel) current is carried by Ca2+ through a membrane conductance
distinct from the Na+ movement [25]. Further studies characterized the
time- and voltage-dependent properties of voltage-gated cardiac Ca2+
(CaV) currents in multicellular preparations, and later in isolated single
cardiac cells as well [26,27].
Based on differences in the thresholds for channel activation two
different types of CaV currents/channels were reported to be present in
chick and rat sensory neurons [28,29]. These channels were termed high
voltage-activated (HVA) and low voltage-activated (LVA) CaV channels,
respectively. In the heart HVA and LVA CaV channels were first
described in isolated canine atrial cells [30]. LVA CaV channels, also
referred to as T-type Ca2+ channels [31], activate at relatively
hyperpolarized membrane potentials, to about –50 mV, and these
channels activate and inactivate rapidly [28,29]. On the contrary, HVA
CaV channels (L-type), open on depolarization to more positive
membrane potentials (approximately –20 mV), and inactivate over a
longer time scale (several tens of milliseconds to seconds). Under
physiological conditions, with Ca2+ as the charge carrier, HVA channels
in most cells inactivate in <100 ms at depolarized voltages [30]. Figure 4
shows typical representative recordings from a dog ventricular single cell
of ICaL in control and during superfusion with 10 μM/L dronedarone, an
antiarrhythmic drug known to possess strong ICaL channel blocker effect
[32].
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Figure 4. The voltage gated L-type calcium channel (ICaL). Panel A.
Original ICaL trace recorded in dog ventricular myocytes (modified from
[32] with permission). The original recording show representative
recordings of ICaL from a single cell in the absence (control) and the
presence of 10 μM/L dronedarone (known as iodine free “amiodarone” a
typical multichannel antiarrhythmic drug, which
is similar to
amiodarone and a strong ICaL channel blocker). Panel B. The schematic
contribution of ICaL to the phase 2 (depolarization) on the action potential
shape. Lower Panels C. The biophysical double gating model of the
calcium channel. Calcium channels, like sodium channel, have two gates,
one activating gate (m) and one inactivating gate (h). In the resting phase
(left panel) m gate is closed, while h gate is open waiting the activating
pulse to open the channel. When membrane reaches the activating
voltage threshold, m gate opens facilitating the entrance of the Ca2+ ion
from the extracellular space into the cell (2nd panel). The resulting
inward current is extremely fast activating (within 1-3 s, see panel A,
left). The inactivating gate (h) fast closes (3rd panel) causing the relative
fast inactivation (to about 50-150 ms) of the ICaL current. This medium
fast inactivating calcium current is responsible for the relative long
plateau phase of the AP shape. The double gated cardiac calcium channel
has the intrinsic properties that it cannot be reactivated again from the
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inactive state to open the channel. The channel can be activated only after
reaching the resting phase (4th right panel) (i.e., the activating m gate
must first close and h the inactivated gate must reopen).
In the mammalian heart, L-type CaV channels currents appear to
be ubiquitously expressed. Apparently there are no interspecies and
transregional differences in the properties and the densities of L-type CaV
channel currents occur, suggesting similar molecular compositions of the
underlying channel. Since the opening of cardiac L-type CaV channels in
response to membrane depolarization is delayed relative to the NaV
channels, ICaL contribute little to phase 0 depolarization in Purkinje, atrial
and ventricular cells (Figures 1 and 2). The opening of L-type CaV
channels and the Ca2+ entry through these channels is responsible for the
action potential plateau (phase 2), which is particularly prominent in
ventricular and Purkinje cells.
In addition, Ca2+ influx through the L-type CaV channels triggers
2+
Ca release from intracellular Ca2+stores and underlies excitationcontraction coupling in the working (ventricular) myocardium [1,2,33]. Ltype CaV channels are also expressed in other heart regions as SAN and
AVN cells, where they play an important role in action potential
generation and automaticity regulation too [34]. Cardiac L-type CaV
channels undergo rapid voltage- and Ca2+-dependent inactivation [35],
processes that will also influence the action potential waveforms (Figure
2) by interfering the duration of the plateau (phase 2) and the time course
of action potential repolarization.
3.3. Voltage gated K+ channels
Voltage-gated K+ (KV) channels are the primary determinants of
action potential repolarization in the mammalian myocardium. There is
considerable electrophysiological and functional cardiac KV channel
diversity as compared with cardiac NaV and CaV channels, there is
considerable electrophysiological and functional cardiac KV channel
diversity [36]. Based primarily on differences in time- and voltagedependent properties and pharmacological sensitivities [6,36], two large
classes of repolarizing cardiac KV currents have been distinguished:
transient outward K+ currents (Ito) and delayed, outwardly rectifying K+
currents (IK). Ito currents activate and inactivate rapidly on membrane
depolarizations to potentials positive to approximately –30 mV and
underlie the early phase (phase 1) of repolarization (notch) in ventricular
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and atrial cells (Figure 2). The cardiac delayed rectifiers (IK) activate at
relative similar membrane potentials, however having variable kinetics. IK
currents determine the latter phase (phase 3) of repolarization back to the
diastolic potential. Multiple types of myocardial Ito and IK channels were
identified in various species. The distinct time- and voltage-dependent
properties and differences in the densities and the biophysical properties
of these channels contribute to variations in the waveforms of action
potentials recorded in different cardiac cell types [4,36].
3.3.1. Transient outward KV currents
Although cardiac transient outward currents were first described in
sheep Purkinje fibres and thought to reflect Cl– conductance [37],
subsequent work demonstrated the presence of two transient outward
currents with distinct properties and they were referred to as Ito1 and Ito2
[38]. Pharmacological studies revealed that the K+ selective Ito1 is blocked
by high concentration of 4-aminopyridine (3-5 mM 4-AP) and is not
affected by changes in extracellular Ca2+, whereas Ito2 cannot be blocked
by 4-AP and is Ca2+ dependent [38].
Figure 5. The transient outward K+ channel (Itof). Panel A. Original Itof
trace recorded in dog ventricular myocytes (This panel is modified from
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[42] with permission). Ito current was activated by 300 ms long
depolarizing voltage pulses (inset top panel shows applied voltage
pulse), while bottom panel shows the magnified Ito current recorded
under the first 20 s long depolarizing pulse (corresponding with the
magnification of Ito marked by the dashed line rectangle on the top
panel). Panel B. The schematic contribution of Ito,f to the phase 1 (early
repolarization, the notch phase of the AP) on the action potential shape.
Panels C. The biophysical double gating model of the transient outward
channel. Like fast sodium (see Fig 3) and inward calcium (see Fig 4)
channels, Ito channels, have two gates, one activating gate (m) and one
inactivating gate (h). In the resting phase (left panel) m gate is closed,
while h gate is open waiting the activating pulse to open the channel.
When membrane potential reaches the activating voltage threshold, m
gate opens facilitating the exit of the K+ ion from the intracellular space
to the extracellular space (2nd panel). The resulting outward current is
extremely fast activating (within 1-3 s, see panel A, left). The
inactivating gate (h) fast closes (3rd panel) causing the relative fast
inactivation (to about 50-150 ms) of the Ito current. The double gated
cardiac Ito channel has the intrinsic properties that it cannot be reactivated
again from the inactive state. The channel can be activated only after
reaching again the resting phase (4th panel) (i.e., the activating gate must
first close and the inactivated gate must reopen).
In further studies, it was shown that the Ca2+-dependent Ito2 in Purkinje
fibres and ventricular cells is a Cl– channel (Ca2+ activated chloride
channel) [39]. The transient outward K+ currents, referred to variably by
different laboratories as Ito, or Ito1 [40], have now been described in many
cardiac cell types and in most species [41]. Comparison of the detailed
biophysical properties of the transient outward K+ currents described in
various cell types and species, however, suggested there are two types of
transient outward K+ currents (Figure 5) [42]. This hypothesis is
supported by several electrophysiological and pharmacological studies. In
adult mouse and rat ventricular myocytes, for example, two transient K+
currents, termed Ito,fast (Ito,f) and Ito,slow (Ito,s), have been distinguished [43].
On membrane depolarization, mouse ventricular Ito,f channels activate and
inactivate rapidly, and on membrane repolarization, these (Ito,f) channels
recover rapidly from steady-state inactivation. Similar to Ito,f, mouse
ventricular Ito,s channels activate and inactivate rapidly. Although the
properties of the transient K+ currents in different cell types and species
are similar and are amenable to classification as either I to,f or Ito,s, there are
large cell type and inter-species variations in the detailed biophysical
19
properties of the (Ito,f and Ito,s) currents [41]. These observations suggest
that there may well be subtle, albeit potentially important, cell type and/or
species dependent molecular heterogeneity among Ito,f and Ito,s channels in
different cell types and/or in different species.
Although originally identified in Purkinje fibres, Ito,f is a
prominent repolarizing current in atrial and ventricular myocytes in the
other species including humans [44,45,46]. In humans and other larger
mammals (as dog), Ito,f underlies the early phase (phase 1, notch) of
repolarization in ventricular and atrial cells (Figure 5) and likely also
contributes to determining the plateau (phase 2) [32,41,47,48]. Recent
studies reported that the transient outward potassium current Ito,f, may
influence not only indirectly by modulating the plateau duration, but also
directly the phase 3 of the ventricular repolarisation in dog and human
hearts [32,49]. However, there are exceptions. In guinea pig ventricular
cells, for example, Ito,f has not been detected except when extracellular
Ca2+ is removed [50]. These channels are absent in rabbit atrial and
ventricular cells [46]. Nevertheless, there are transient KV currents in
rabbit myocytes (typically referred to as transient inward current It),
which inactivate slowly and recover from (steady-state) inactivation very
slowly, whose properties more closely resemble mouse ventricular I to,s
than Ito,f and reflect possible different molecular structure background.
Transient KV currents classified as Ito,f have also been shown to be
expressed in (rabbit) SAN cells, although, similar to NaV currents, Ito,f
densities vary markedly among SAN cells. In addition, when expressed,
Ito,f appears to play a role in shaping action potential waveforms and in
regulating automaticity in SAN cells [51]. Cells isolated from the (rabbit)
AVN also express Ito,f, and detailed kinetic analysis of the currents reveals
the presence of two components with distinct rates of inactivation and
recovery [52]. It is unclear whether these findings reflect differences in
the kinetic properties of a single type of Ito,f channel or if two distinct
types of Ito channels are expressed in (rabbit) AVN cells.
3.3.2. Delayed rectifier KV currents
The delayed rectifier potassium current (IK) is a major outward
current responsible for ventricular muscle action potential repolarisation
[53]. This current was first described in 1969 by Noble and Tsien using
the two-microelectrode voltage-clamp technique in sheep cardiac Purkinje
fibre strands [54]. Since its discovery it has been examined in single
isolated myocytes obtained from various regions of the heart in several
mammalian species [7,55,56]. In most species, I K can be separated into
20
rapid (IKr) and slow (IKs) components (Figure 6) that differ from one
another in terms of their sensitivity to drugs, rectification characteristics,
and kinetic properties [55,57,58,59].
Figure 6. The fast and the slow components of the delayed rectifier K+
channel (IKr and IKs). Upper panels. Original I Kr (panel A) and IKs (panel
B) traces recorded in human ventricular myocytes (modified from [58]
and [59] with permission. IKr and IKs currents were activated using test
pulses of 1000 ms (IKr) or 5000 ms (IKs) in duration to 20 mV from the
holding potential of -40 mV. The decaying tail current at -40 mV after the
test pulse were assessed as IKr or IKs tail currents. Panels C. The
biophysical single gating model of the delayed rectifier potassium
channels. IK channels unlike the previously presented INa, ICa and Ito
channels (see Fig 3, 4, 5, respectively) have only one channel gate, which
in resting state is closed (left panel). When cells are depolarized, the
channel activates (2nd panel). In I Kr channels this activation is fast, while
in IKs channels is relatively slow. When cells are repolarized the channels
is closed (3rd and 4th panels). IKs channels are not inactivating, thereby the
closing mechanism is called deactivation. Previously it was thought that
IKr behaves a similar way that IKs having one gate based activating and
deactivating kinetics, until Spector et al [60] reported that IKr channels
have in fact double gating mechanism like Ito channels. IKr channels
21
inactivate having extremely fast inactivation kinetics, however this
inactivation display a voltage dependent behaviour. At positive membrane
potential, the inactivation is blocked, while when membrane potential
repolarizes and become more negative the inactivation blocking effect
disappears and channels deactivate with a relative slow kinetics.
Although I Kr activates rapidly, inactivates very rapidly and thereby
displays marked inward rectification, no inward rectification is evident
for the slowly activating I Ks [60]. In cardiac ventricular myocytes isolated
from healthy human donor hearts, IKr activated fast and deactivated
slowly and biexponentially (600 ms and 6700 ms) [61], while I Ks
exhibited slow and voltage independent activation at more positive than 0
mV (900 ms) and had fast and monoexponential deactivation kinetics
[58,59]. This deactivation was voltage dependent, being fast at more
negative (at -50 mV,  90 ms), and slow at more positive voltages (at 0
mV, 350 ms) [59]. In dog cardiac ventricular cells IKr activated fast and
deactivated slowly and biexponentially, (360 ms and 3300 ms),
while IKs activated slowly (800 ms) at voltage more positive than 0 mV,
and deactivated rapidly and monoexponentially (150 ms) [55,62]. In
the guinea pig, where it was first described, I Ks activated very slowly, not
saturating even after 5-7 s at +50 mV, and deactivated slowly, within 5001000 ms [63]. Initially, it was not observed in the rabbit, but later studies
revealed a large and consistent IKs in the rabbit ventricle [56; 64].
Considering the kinetic properties, the human cardiac slow delayed
rectifier potassium current best resembles those measured in the dog (55;
62) and rabbit [56; 64] ventricle, but significantly differs from those
found in the guinea pig heart [57]. IKs was augmented with an increase in
sympathetic tone as a result of elevation in intracellular cAMP levels
[58,65].
In canine [66] and human [67] atrial myocytes, a novel, very
rapidly activating, and largely non-inactivating, outward KV current, now
typically referred to as IKur [66], has been described. In most species, IKur
is not detected in ventricular cells. It is likely that the expression and the
properties of IKur, together with Ito,f, contribute to the more rapid
repolarization evident in atrial, as compared to ventricular, myocytes
(Figures 1 and 2). This suggests that IKur channels might represent a
therapeutic target for the treatment of atrial arrhythmias without
undesirable effects on impulse propagation, ventricular functioning, or
cardiac output [68]. However, Wettwer et al. [69] reported that IKur
22
blockade may either prolong or shorten atrial APD depending on the
disease status of the atria, which questioned the selective IKur blockade to
atrial specific antiarrhythmic potential [69].
3.3.3. Inward rectifier potassium currents
In addition to the depolarization-activated KV currents, inwardly
rectifying K+ (Kir) currents, through IK1 channels, also contribute to
myocardial action potential repolarization, particularly in ventricular cells
[71]. As the "inward rectifier" terminology implies, Kir channels carry
inward K+ currents rather than outward K+ currents [71]. Nevertheless, it
is the outward K+ currents through these channels that are important
physiologically because myocardial membrane potentials never reach
values more negative than the K+ reversal potential (approximately –90
mV).
At the macroscopic level, I K1 channels have been characterized in
human guinea pig and rabbit atrial and ventricular myocytes and in rabbit
SAN cells [11,71,72]. The properties of the IK1 channels in each of these
preparations are similar in that all are K+ selective, blocked by
extracellular Ba2+ and intracellular Cs+ and strongly inwardly rectifying
[72]. The strong inward rectification evident in cardiac I K1 channels is
attributed to block by intracellular Mg2+, Ca2+ and polyamines [9,73,74].
The expression of IK1 is clearly reflected in the negative slope region
(between approximately –50 and –10 mV) of the (total steady-state)
myocyte conductance-voltage relation, which is prominent in ventricular
myocytes, and smaller in atrial cells [72]. The fact that the strongly
inwardly rectifying IK1 channels conduct at negative membrane potentials
suggests that these channels play a primarily role in establishing the
resting membrane potentials of Purkinje fibres, as well as of atrial and
ventricular myocytes. Direct experimental support for this hypothesis was
provided with the demonstration that ventricular membrane potentials are
depolarized in the presence of large concentration of Ba2+ [70], which
blocks IK1 channels. In addition, action potentials are prolonged, and
phase 3 repolarization is slowed in the presence of extracellular Ba2+
(Figure 7), suggesting that IK1 channels also contribute to repolarization
[75] and especially of repolarization reserve and arrhythmogenesis
[58,77], particularly in the ventricular myocardium. Similar to the KV
channels, IK1 densities and the detailed biophysical properties of the
currents vary in different myocardial cell types. In the human heart, for
example, I K1 density is more than two-fold higher in ventricular, than in
23
atrial, cells [77]. A recent study reported that ventricular IK1 current
density in human is significantly lower than in dog; therefore, selective
blockade of I K1 lengthens ventricular APD less in human than in dog [76].
However, this behaviour indirectly cause a more less pronounced I Kr
blockade induced APD lengthening in the dog than in human ventricle,
therefore any I Kr block drug effect tested in dogs can be underestimated
when it is extrapolated to humans [76].
Figure 7. The inward rectifier potassium channel (IK1). Panel A. Original
IK1 recording in a dog ventricular myocyte in the absence (control) and the
presence of 10 μM/L BaCl2 (a typical I K1 channel inhibitor). Panel B. The
effect of selective I K1 channel blockade by BaCl2 on the action potential
recorded in dog right ventricular preparation (both two panels are adapted
from [75] with permission). Panels C. The biophysical single gating
model of the inward rectifier potassium channel. I K1 channels like IKs
channels have only one channel gate, which is closed in the resting state.
When cells are depolarized, the channel is activated, and later when the
membrane is repolarized, the extremely slow deactivation of the channel
occurs.
Another important and special ligand gated cardiac Kir channel
type is the acetylcholine sensitive potassium channel IK(ACh), which is
24
gated through a G protein-coupled mechanism mediated by muscarinic
acetylcholine receptor activation (78). Physiologically, I K(ACh) channels
are activated by the binding of G protein-subunits in response to the
acetylcholine released on vagal stimulation (68). Although IK(ACh)
channels are expressed in AVN, SAN, atrial, and Purkinje cells, and are
activated by acetylcholine released on vagal stimulation, these channels
are not thought to contribute appreciably to action potential repolarization
under normal physiological conditions. Consistent with this hypothesis,
targeted deletion of one of the Kir encoding IK(ACh) channels, Kir3.4, does
not measurably affect resting heart rates. Interestingly, atrial fibrillation
(AF) is not evident in Kir3.4 null mice exposed to the acetylcholine
receptor agonist carbachol, suggesting that activation of I K(ACh) channels
is involved in the cholinergic induction of atrial fibrillation (79). Similar
to IKur, IK(ACh) is also an atrial specific channel. Thus, it is interesting
whether selective I K(ACh) blockade cause repolarization lengthening effect,
and be a basis for developing new antiarrhythmic drugs for treating
chronic AF (cAF). In the study of Dobrev et al [80], IK(ACh) channels in
cAF are constitutively active without any direct ligand stimulation. This
observation suggests that selective I K(ACh) blockade may be a target for
developing new antiarrhythmic drugs for treating cAF. Several
pharmacological studies have supported this hypothesis [81].
3.3.4. ATP sensitive potassium currents
The existence of K+ channels which could be blocked by internal
ATP in cardiac muscle was suggested a long time ago [82], but direct
demonstration of the presence was possible only later with the aid of
voltage-clamp techniques. The ATP sensitive potassium current (IKATP)
channels, are inhibited by intracellular ATP, which are activated by
nucleotide diphosphates; thus providing a link between cellular
metabolism and membrane potential [83]. The current is time
independent, and is activated when intracellular ATP concentration
decreases below 1 mM. The reversal potential of I KATP (reported to be at 85 mV) is close to the K+ equilibrium potential [84], suggesting K+ as the
major ion carrier. The current can be activated by several vasodilators
such as cromakalim (Figure 8A), pinacidil (Figure 8B) and nicorandil,
and is blocked by Ba2+, Cs+, tetraethylammonium, tedisamil (Figure 8A
and B) [41,85,86]. However, under normal conditions the current is
completely blocked by physiological intracellular ATP level. In the
ventricular myocardium, the opening of I KATP channels is thought to be
important under conditions of metabolic stress, as occurs during ischemia
25
or hypoxia, and to result in shortening action potential durations and
minimizing K+ efflux [83].
Figure 8. The ATP sensitive potassium channel (IKATP). Panel A. Original
IKATP recordings in a rabbit ventricular myocyte under control conditions,
after opening of the IKATP channels with 50 µM cromakalim and after
application of 1 µM tedisamil (an investigational antiarrhythmic drug
having multichannel blocker profile) in the continuous presence of
cromakalim. Panel B. The effect of 1 µM tedisamil on the pinacidil (10
µM) induced APD shortening in dog ventricular muscle fibres (all panels
modified from [41] with permission).
Activation of I KATP contributes to the repolarization and shortening
of refractoriness in hypoxic/ischaemic conditions, which may cause life
threatening cardiac arrhythmias. Drugs that inhibit I KATP can prevent or
lessen the hypoxia/ischaemia induced shortening of repolarization and
refractoriness and may protect from dangerous arrhythmias. On the other
hand, however, growing evidence suggests that shortening of the APD
can protect the myocardium from calcium overload, thus providing
significant cardioprotection during acute ischaemia [87]. Although I KATP
channels appear to be distributed uniformly in the RV and LV and
through the thickness of the ventricular wall, these channels are expressed
at much higher density than other sarcolemmal K+ channels, suggesting
that action potentials could be shortened markedly when only very small
numbers of I KATP channels are activated.
Table 2 summarizes the description of the ionic currents operating
in cardiac muscle.
26
Table 2. Summary of the ionic currents operating in cardiac muscle, their
physiological roles and pharmacologic modulation.
Current
Region
Activated by
Blocked by
Fast
inward
sodium
current
(INa)
A, V,
AVN,
Purkinje
Depolarization
beyond -60 mV
TTX,
local
anaesthetics
Main
charge
carrier
Na+
Effect
Rapid
depolarization
(phase 0), and
maintenance of
plateau (phase
2)
Inward calcium currents
L type
ICa
A,V,
SAN,
AVN,
Purkinje
Depolarization
beyond -30 mV
Cd2+,
Mn2+,
organic
Ca
channel
blockers
Ca2+
T type
ICa
A, SAN,
AVN
Depolarization
beyond -60 mV
mibefradil,
amiloride,
flunarizine
Ca2+
Depolarization,
maintenance of
plateau (phase
2),
calcium
induced calcium
release
Depolarization,
automaticity
Transient outward currents
Ito,f
A, V,
SAN,
AVN,
Purkinje
Depolarization
beyond -20 mV
Ito,s
A, V
Depolarization
and
intracellular Ca2+
4
aminopyridine
(4-AP),
heteropoda
toxin
Intracellular
EGTA, anion
channel
blockers
(SITS, DIDS)
K+
Rapid
repolarization
(phase 1)
Cl-
Early
repolarization
(notch, phase 1)
K+
Termination of
repolarization
(phase 3)
K+
Termination of
repolarization
(phase 3)
Delayed rectifier potassium currents
IKr
A,V,
SAN,
AVN,
Purkinje
Depolarization
beyond -20 mV
and
IKr
activators
(NS3623,
NS1643)
IKs
A,V,
SAN,
AVN,
Depolarization
beyond 0 mV
and
Class IA and
III
antiarrhythmic
drugs
(quinidine,
dofetilide, E4031,
dsotalol)
L-735,821,
HMR-1556,
Chromanol
27
Purkinje
IKur
A
sympathetic
activation
Depolarization
beyond -30 mV
293B
Low
concentration
(50 µM) 4-AP,
AVE0118
K+
Modulation of
plateau (phase
2)
and
termination of
repolarization
(phase 3)
Reduction
of
membrane
conductance
during plateau
(phase
2),
termination of
repolarization
(phase 3) and
restoring of the
membrane
resting potential
(phase 4)
Shortening of
repolarization,
decreases
resting
membrane
potential (phase
4)
Decreases
membrane
potential
Inward rectifier potassium currents
IK1
A,V,
AVN,
Purkinje
Depolarization
Ba2+,
indapamide
K+
IKACh
A, SAN,
AVN
Depolarization
and stimulation
of muscarinic
receptor
(carbachol)
Tertiapine,
Ba2+
K+
unknown
K+
glibenclamide
K+
IKp
A,
V, depolarization
undefined
in other
regions
ATP sensitive potassium current
IKATP
A,V
Depolarization
and decrease of
intracellular
ATP,
vasodilator
agents
(cromakalim,
pinacidil,
nicorandil)
Shortening of
repolarization
during hypoxia
Abbreviations: A - atria; V – ventricle; SAN – sinoatrial node; AVN – atrioventricular
node; SITS (4-acetamido-4-isothiocyanatostilbene-2,2-disulfonic acid); DIDS (4,4'Diisothiocyanato-2,2'-stilbenedisulfonic acid disodium salt); E-4031 (IUPAC name: (1-
28
[2-(6-methyl-2-pyridyl)ethyl]-4-(4-methylsulfonyl-aminobenzoyl)piperidine); L-735821
(IUPAC name: (E)-3-(2,4-dichlorophenyl)-N-[(3R)-1-methyl-2-oxo-5-phenyl-3H-1,4benzodiazepin-3-yl]prop-2-enamide); HMR-1556 (IUPAC name: N-[(3R,4S)-3-hydroxy2,2-dimethyl-6-(4,4,4-trifluorobutoxy)chroman-4-yl]-N-methylethanesulfonamide);
chromanol-293B (IUPAC name: trans-N-[6-Cyano-3,4-dihydro-3-hydroxy-2,2-dimethyl2H-1-benzopyran-4-yl]-N-methyl-ethanesulfonamide); AVE0118 (IUPAC name: 2’-{[2(4-methoxy-phenyl)-acetylamino]-methyl}-biphenyl-2carboxylic acid (2-pyridin-3-ylethyl)-amide
3.3.5. Background potassium channels
A small and time-independent potassium conductance has been
described in guinea-pig cardiac myocytes [88]. None of the cloned 6Tm–
1P or 2Tm–1P channels encode such current, but several recently cloned
tandem pore subunits (4Tm–2P) encode currents with this ‘leak’ current
behaviour, e.g. the fairly ubiquitous TWIK subunit (“Tandem of Pdomains in a Weakly Inward rectifying K+ channel”) [89]. Presence of
TWIK mRNA was found in large amount in human atrial and ventricular
preparations [90]; however, the properties of the current formed by these
channels are still poorly understood. A related subunit TASK (“TWIKrelated acid sensitive K+ channel”) is also highly expressed in the heart
[91]. The channel is sensitive to pH variations in the physiological range,
and contains a C-terminal PDZ binding motif. This relatively new family
is rapidly expanding and subunits responsible for the cardiac background
potassium channel (IKp) component yet to be established.
4. Concluding remarks
This chapter has focused on ion channels that contribute to AP
formation in normal hearts in physiological conditions. However,
several ion channels play only a role in pathophysiological conditions
related to different cardiac diseases or during early development.
Several studies have indicated that ion channels function properly only
in the presence of various exo- and/or endogenous regulatory
molecules; moreover, next to several identified gene mutations and
polymorphisms, ion channel expression are strongly influenced by
molecular variants (genes and/or small noncoding RNAs and
microRNAs). The investigations of these effects on channel expression
29
near electrophysiological properties of the transmembrane ionic
currents are of importance, because: i) exploring their function may
provide novel mechanistic insights into the pathophysiology of several
specific cardiac diseases; ii) the investigation of the transmembrane
ion current at organ, cellular and subcellular levels may identify new
and more specific targets to treat arrhythmias, which will provide a
more potent effect without proarrhythmic side effects that are present
with the currently used antiarrhythmic drugs.
Acknowledgements
This work was supported by a grant of the Ministry of National
Education, CNCS – UEFISCDI, project number PN-II-ID-PCE-2012-40512 and grants by the Hungarian Scientific Research Fund (OTKA
NN109904 and ANN113273), by the Hungarian Academy of Sciences
and by TÁMOP 4.2.4. A/2-11-1-2012-0001 „National Excellence
Program – Elaborating and operating an inland student and researcher
personal support system” key project to Norbert Jost. The project was
subsidized by the European Union and co-financed by the European
Social Fund.
REFERENCES
1. Bers DM. (2001). Excitation-Contraction Coupling and Contractile
Force (2nd ed.). Kluwer, The Netherlands.
2. Fozzard HA (1991). Excitation-contraction coupling in the heart. Adv
Exp Med Biol, 308, 135–142.
3. Sáez JC, Berthoud VM, Brañes MC, Martinez AD, and Beyer EC
(2003). Plasma membrane channels formed by connexins: their
regulation and functions. Physiol Rev, 83, 1359–1400.
4. Antzelevitch C and Dumaine R (2002). Electrical heterogeneity in the
heart: Physiological, pharmacological and clinical implications. In: Page
E, Fozzard HA, Solaro RJ, eds. Handbook of Physiology: The Heart.
New York: Oxford University Press; pp 654–692
5. Kléber AG, Rudy Y (2004). Basic mechanisms of cardiac impulse
propagation and associated arrhythmias. Physiol Rev, 84, 431–488.
30
6. Nerbonne JM, Guo W (2002). Heterogeneous expression of voltagegated potassium channels in the heart: roles in normal excitation and
arrhythmias. J Cardiovasc Electrophysiol, 13, 406–409.
7. Sanguinetti MC, Jurkiewicz NK (1990). Two components of cardiac
delayed rectifier K+ current. Differential sensitivity to block by class III
antiarrhythmic agents. J Gen Physiol, 96, 195-215.
8. Ishihara K, Mitsuiye T, Noma A, Takano M.T (1989). The Mg2+ block
and intrinsic gating underlying inward rectification of the K+ current in
guinea-pig cardiac myocytes. J Physiol, 419, 297-320
9. Lopatin AN, Makhina EN, Nichols CG (1994). Potassium channel block
by cytoplasmic polyamines as the mechanism of intrinsic rectification.
Nature, 372, 366–369.
10. Rocchetti M, Besana A, Gurrola GB, Possani LD, Zaza A (2001). Rate
dependency of delayed rectifier currents during the guinea-pig
ventricular action potential. J Physiol, 534, 721-732.
11. Shimoni Y, Clark RB, Giles WR (1992). Role of an inwardly rectifying
potassium current in rabbit ventricular action potential. J Physiol, 448,
709-727.
12. Miake J, Marbán E, Nuss HB (2002). Biological pacemaker created by
gene transfer. Nature, 419, 132-133.
13. Brown AM, Lee KS, Powell T (1981). Sodium current in single rat heart
muscle cells. J Physiol, 318, 479-500.
14. Makielski JC, Sheets MF, Hanck DA, January CT, Fozzard HA (1987).
Sodium current in voltage clamped internally perfused canine cardiac
Purkinje cells. Biophys J, 52, 1-11.
15. Antoni H, Bocker D, Eichorn R (1988). Sodium current kinetics in intact
rat papillary muscle: measurements with the loose-patch-clamp
technique. J Physiol, 406, 199-213.
16. Wu L, Guo D, Li H, Hackett J, Yan GX, Jiao Z, Antzelevitch C,
Shryock JC, Belardinelli L (2008). Role of late sodium current in
modulating the proarrhythmic and antiarrhythmic effects of quinidine.
Heart Rhythm, 5, 1726-1734.
17. Wang DW, Yazawa K, George ALJ, Bennett PB (1996).
Characterization of human cardiac Na+ channel mutations in the
congenital long QT syndrome. Proc Natl Acad Sci USA, 93, 13200–
13205.
18. Maltsev VA, Sabbah HN, Higgins RS, Silverman N, Lesh M,
Undrovinas A (1998). Novel, ultraslow inactivating sodium current in
human ventricular cardiomyocytes. Circulation 98, 2546–2552
19. Attwell D, Cohen I, Eisner D, Ohba M, Ojeda C (1979). The steady state
TTX-sensitive ("window") sodium current in cardiac Purkinje fibres.
Pflügers Arch, 379, 137–142.
31
20. Sakmann BF, Spindler AJ, Bryant SM, Linz KW, Noble D (2000).
Distribution of a persistent sodium current across the ventricular wall in
guinea pigs. Circ Res, 87, 910–914.
21. Bennett PB, Yazawa K, Makita N, George AL (1995). Molecular
mechanism for an inherited cardiac arrhythmia. Nature, 376, 683–685.
22. Antzelevitch C (2003). Molecular genetics of arrhythmias and
cardiovascular conditions associated with arrhythmias. J Cardiovasc
Electrophysiol, 14, 1259–1272.
23. Remme CA, Wilde AA, Bezzina CR (2008). Cardiac sodium channel
overlap syndromes: different faces of SCN5A mutations. Trends
Cardiovasc Med, 18,78-87.
24. Noble D (1979). The initiation of the heart beat. 2nd Edition. University
Press, New York, Oxford
25. Mascher D and Peper K (1969). Two components of inward current in
myocardial muscle fibers. Pflügers Arch, 307, 190–203
26. Isenberg G, Klöckner U (1982) Calcium currents of isolated bovine
ventricular myocytes are fast and of large amplitude. Pflügers Archiv,
395, 30-41.
27. Reuter H (1968). Slow inactivation of currents in cardiac Purkinje
fibres. J Physiol, 197, 233–253.
28. Carbone E, Lux HD (1987). Kinetics and selectivity of a low-voltageactivated calcium current in chick and rat sensory neurones. J Physiol,
386, 547–570.
29. Carbone E, Lux HD (1987). Single low-voltage-activated calcium
channels in chick and rat sensory neurones. J Physiol, 386, 571–601.
30. Bean BP (1985). Two kinds of calcium channels in canine atrial cells.
Differences in kinetics, selectivity, and pharmacology. J Gen Physiol,
86, 1–30.
31. Perez-Reyes E (2003). Molecular physiology of low-voltage-activated ttype calcium channels. Physiol Rev, 83: 117–161.
32. Varró A, Takács J, Németh M, Hála O, Virág L, Iost N, Baláti B,
Ágoston M, Vereckei A, Pastor G, Delbruyère M, Gautier P, Nisato D,
Papp JG (2001). Electrophysiological effects of dronedarone (SR
33589), a noniodinated amiodarone derivative in the canine heart:
comparison with amiodarone. Br J Pharmacol, 133, 625–634.
33. Bers DM and Perez-Reyes E (1999). Ca channels in cardiac myocytes:
structure and function in Ca influx and intracellular Ca release.
Cardiovasc Res, 42, 339–360.
34. Bénitah JP, Gómez AM, Fauconnier J, Kerfant BG, Perrier E, Vassort
G, Richard S (2002). Voltage-gated Ca2+ currents in the human
pathophysiologic heart: a review. Basic Res Cardiol, 97/Suppl 1, 11–18.
35. Findlay I (2004). Physiological modulation of inactivation in L-type
Ca2+ channels: one switch. J Physiol, 554, 275–283
32
36. Snyders DJ (1999). Structure and function of cardiac potassium
channels. Cardiovasc Res, 42, 377-390.
37. Dudel J, Peper K, Rudel R, Trautwein W (1967). The dynamic chloride
component of membrane current in Purkinje fibers. Pflügers Arch, 295,
197–212.
38. Coraboeuf E and Carmeliet E (1982). Existence of two transient outward
currents in sheep cardiac Purkinje fibers. Pflügers Arch, 392, 352–359.
39. Zygmunt AC and Gibbons WR (1992). Properties of the calciumactivated chloride current in heart. J Gen Physiol, 99, 391–414.
40. Campbell DL, Rasmusson RL, Qu Y, Strauss HC (1993). The calciumindependent transient outward potassium current in isolated ferret right
ventricular myocytes. I. Basic characterization and kinetic analysis. J
Gen Physiol, 101, 571–601.
41. Jost N, Virág L, Hála O, Varró A, Thormählen D, Papp JG (2004).
Effect of the antifibrillatory compound tedisamil (KC-8857) on
transmembrane currents in mammalian ventricular myocytes. Curr Med
Chem, 11, 3219-3228.
42. Virág L, Jost N, Papp R, Koncz I, Kristóf A, Kohajda Zs, Harmati G,
Carbonell-Pascual B, Ferrero (Jr) JM, Papp JGy, Nánási PP, Varró A
(2011). Analysis of the contribution of Ito to repolarization in canine
ventricular myocardium. British Journal of Pharmacology, 164, 93–105.
43. Xu H, Guo W, and Nerbonne JM. (1999). Four kinetically distinct
depolarization-activated K+ currents in adult mouse ventricular
myocytes. J Gen Physiol, 113, 661–678.
44. Bertaso F, Sharpe CC, Hendry BM, James AF (2002). Expression of
voltage-gated K+ channels in human atrium. Basic Res Cardiol, 97, 424–
433.
45. Liu DW, Gintant GA, and Antzelevitch C (1993). Ionic bases for
electrophysiological distinctions among epicardial, midmyocardial, and
endocardial myocytes from the free wall of the canine left ventricle. Circ
Res, 72, 671–687.
46. Wang Z, Feng J, Shi H, Pond A, Nerbonne JM, Nattel S (1999).
Potential molecular basis of different physiological properties of the
transient outward K+ current in rabbit and human atrial myocytes. Circ
Res, 84, 551–561.
47. Akar FG, Wu RC, Deschenes I, Armoundas AA, Piacentino V 3rd,
Houser SR, Tomaselli GF (2004). Phenotypic differences in transient
outward K+ current of human and canine ventricular myocytes: insights
into molecular composition of ventricular Ito. Am J Physiol Heart Circ
Physiol, 286, H602-H609.
48. Magyar J, Iost N, Körtvély Á, Bányász T, Virág L, Szigligeti P, Varró
A, Opincariu M, Szécsi J, Papp JG, Nánási PP (2000). Effects of
endothelin-1 on calcium and potassium currents in undiseased human
ventricular myocytes, Pflug Arch Eur J Phys, 441, 144-149.
33
49. Varró A Virág L, Acsai K, Jost N, Lengyel Cs, Papp JGy (2006). Role
of the slowly inactivating transient outward potassium current in cardiac
repolarisation. Heart Rhythm, 3, S302.
50. Inoue M and Imanaga I (1993). Masking of A-type K+ channel in guinea
pig cardiac cells by extracellular Ca2+. Am J Physiol Cell Physiol, 264,
C1434–C1438.
51. Lei M, Honjo H, Kodama I, Boyett MR (2000). Characterisation of the
transient outward K+ current in rabbit sinoatrial node cells. Cardiovasc
Res, 46, 433–441.
52. Mitcheson JS, Hancox JC (1999). Characteristics of a transient outward
current (sensitive to 4-aminopyridine) in Ca2+-tolerant myocytes isolated
from the rabbit atrioventricular node. Pflügers Arch, 438, 68–78.
53. Sanguinetti MC, Keating MT (1997). Role of delayed rectifier
potassium channels in cardiac repolarization and arrhythmias. News in
Physiological Sciences, 12, 152-158.
54. Noble D, Tsien RW (1969). Outward membrane currents activated in
the plateau range of potentials in cardiac Purkinje fibres. J Physiol, 200,
205-231.
55. Gintant GA (1996). Regional differences in IK density in canine left
ventricle: role of IK,s in electrical heterogeneity. Am J Physiol, 268,
H604-H613.
56. Salata JJ, Jurkiewicz NK, Jow B, Folander K, Guinosso PJ Jr, Raynor B,
Swanson R, Fermini B (1996). IK of rabbit ventricle is composed of two
currents: evidence for IKs. Am J Physiol, 271, H2477-H248.
57. Heath BM, Terrar DA (1996). The deactivation kinetics of the delayed
rectifier components IKr and IKs in guinea-pig isolated ventricular
myocytes. Exp Physiol, 81, 605-621.
58. Jost N, Virág L, Bitay M, Takács J, Lengyel Cs, Biliczki P, Nagy ZA,
Bogáts G, Lathrop DA, Papp JG, Varró A (2005). Restricting excessive
cardiac action potential and QT prolongation: a vital role for IKs in
human ventricular muscle. Circulation, 112, 1392-1399.
59. Virág L, Iost N, Opincariu M, Szolnoky J, Szécsi J,. Bogáts G.
Szenohradszky P. Varró A, Papp JG (2001). The slow component of the
delayed rectifier potassium current in undiseased human ventricular
myocytes. Cardiovasc Res, 49, 790-797.
60. Spector PS, Curran ME, Zou A, Keating MT, Sanguinetti MC (1996).
Fast inactivation causes rectification of the IKr channel. J Gen Physiol,
107, 611-619.
61. Iost N, Virag L, Opincariu M, Szecsi J, Varro A, Papp JG (1998).
Delayed rectifier potassium current in undiseased human ventricular
myocytes. Cardiovasc Res, 40, 508–515.
62. Varró A, Baláti B, Iost N, Takács J, Virág L, Lathrop DA, Lengyel C,
Tálosi L, Papp JG (2000). The role of the delayed rectifier component
34
63.
64.
65.
66.
67.
68.
69.
70.
71.
72.
73.
74.
75.
IKs in dog ventricular muscle and Purkinje fibre repolarization. J Physiol,
523, 67-81.
Jurkiewicz NK, Sanguinetti MC (1993). Rate-dependent prolongation of
cardiac action potentials by a methanesulfonanilide class III
antiarrhythmic agent. Specific block of rapidly activating delayed
rectifier K+ current by dofetilide. Circ Res, 71, 75-83.
Lengyel Cs, Iost N, Virág L, Varró A., Lathrop DA, Papp JG (2001).
Pharmacological block of the slow component of the outward delayed
rectifier current (IKs) fails to lengthen rabbit ventricular muscle QTc and
action potential duration. Br J Pharmacol, 132, 101-110.
Han W, Wang Z, Nattel S (2001). Slow delayed rectifier current and
repolarization in canine cardiac Purkinje cells. Am J Physiol, 280,
H1075-H1080.
Yue L, Feng J, Li GR, Nattel S (1996). Characterization of an ultrarapid
delayed rectifier potassium channel involved in canine atrial
repolarization. J Physiol, 496, 647–662.
Wang Z, Fermini B, Nattel S (1993). Sustained depolarization-induced
outward current in human atrial myocytes. Evidence for a novel delayed
rectifier K+ current similar to Kv1.5 cloned channel currents. Circ Res,
73, 1061–1076.
Dobrev D, Ravens U (2003). Remodeling of cardiomyocyte ion
channels in human atrial fibrillation.
Basic Res Cardiol, 98, 137-148.
Wettwer E, Hála O, Christ T, Heubach JF, Dobrev D, Knaut M, Varró
A, Ravens U (2004). Role of IKur in controlling action potential shape
and contractility in the human atrium: influence of chronic atrial
fibrillation. Circulation, 110, 2299-2306.
Lopatin AN, Nichols CG (2001). Inward rectifiers in the heart: an
update on I(K1). J Mol Cell Cardiol, 33, 625–638.
Heidbüchel H, Vereecke J, and Carmeliet E (1990). Three different
potassium channels in human atrium. Contribution to the basal
potassium conductance. Circ Res, 66, 1277–1286.
Giles WR, Imaizumi Y (1988). Comparison of potassium currents in
rabbit atrial and ventricular cells. J Physiol, , 405, 123–145.
Lopatin AN, Makhina EN, Nichols CG (1995). The mechanism of
inward rectification of potassium channels: "long-pore plugging" by
cytoplasmic polyamines. J Gen Physiol, 106, 923–955.
Yan DH, Nishimura K, Yoshida K, Nakahira K, Ehara T, Igarashi K,
Ishihara K (2005). Different intracellular polyamine concentrations
underlie the difference in the inward rectifier K+ currents in atria and
ventricles of the guinea-pig heart. J Physiol, 563, 713-724.
Biliczki P, Virág L, Iost N, Papp J G, Varró A (2002). Interaction of
different potassium channels in cardiac repolarization in dog ventricular
preparations: role of the repolarization reserve. Br J Pharmacol, 137,
361-368.
35
76. Jost N, Virág L, Comtois P, Ördög B, Szűts V, Seprényi Gy, Bitay M,
Kohajda Zs, Koncz I, Nagy N, Szél T, Magyar J, Kovács M, Puskás LG,
Lengyel Cs, Wettwer E, Ravens U, Nanasi PP, Papp JGy, Varró A,
Nattel S (2013). Ionic mechanisms limiting cardiac repolarizationreserve in humans compared to dogs. J Physiol, 591, 4189–4206.
77. Varró A, Nanasi PP, Lathrop DA (1993). Potassium currents in isolated
human atrial and ventricular cardiocytes. Acta Physiol Scand, 149, 133–
142.
78. Kurachi Y (1995). G protein regulation of cardiac muscarinic potassium
channel. Am J Physiol Cell Physiol, 269, C821–C830.
79. Kovoor P, Wickman K, Maguire CT, Pu W, Gehrmann J, Berul CI,
Clampham DE (2001). Evaluation of the role of I(KACh) in atrial
fibrillation using a mouse knockout model. J Am Coll Cardiol, 37,
2136–2143.
80. Dobrev D, Friedrich A, Voigt N, Jost N, Wettwer E, Christ T, Knaut M,
Ravens U (2005). The G-protein gated potassium current IK,ACh is
constitutively active in patients with chronic atrial fibrillation.
Circulation, 112, 3697-3706.
81. Nishida A, Reien Y, Ogura T, Uemura H, Tamagawa M, Yabana H,
Nakaya H (2007). Effects of azimilide on the muscarinic acetylcholine
receptor-operated K+ current and experimental atrial fibrillation in
guinea-pig hearts. J Pharmacol Sci, 105, 229-239.
82. Webb JL, Hollander PB (1956). Metabolic aspects of the relationship
between the contractility and membrane potentials of the rat atrium.
Circ Res, 4, 618-626.
83. Isomoto S and Kurachi Y (1997). Function, regulation, pharmacology,
and molecular structure of ATP-sensitive K+ channels in the
cardiovascular system. J Cardiovasc Electrophysiol, 8, 1431–1446.
84. Horie M. Irishawa H, Noma A (1987). Voltage-dependent magnesium
block of adenosine-triphosphate-sensitive potassium channel in guineapig ventricular cells. J Physiol, 387, 251-272.
85. Riccioppo Neto F, Mesquita Júnior O, Olivera GB (1997).
Antiarrhythmic and electrophysiological effects of the novel KATP
channel opener, rilmakalim, in rabbit cardiac cells. Gen Pharmacol. 29,
201-205.
86. Németh M, Varró A, Virág L, Hála O, Thormählen D, Papp JG 1997,
Frequency-dependent Cardiac Electrophysiologic Effects of Tedisamil:
Comparison With Quinidine and Sotalol. J Cardiovasc Pharmacol Ther.
2 (4), 273-284.
87. Cole WC, McPherson CD, Sontag D (1991). ATP-regulated K+ channels
protect the myocardium against ischemia/reperfusion damage. Circ Res,
69, 571-581.
36
88. Backx P.H, Marban E (1993). Background potassium current active
during the plateau of the action potential in guinea pig ventricular
myocytes. Circ Res, 72, 890–900.
89. Lesage F, Guillemare E, Fink M, Duprat F, Lazdunski M, Romey G,
Barhanin J (1996). TWIK-1, a ubiquitous human weakly inward
rectifying K+ channel with a novel structure. EMBO J, 15, 1004–1011.
90. Gaborit N, Le Bouter S, Szuts V, Varro A, Escande D, Nattel S,
Demolombe S (2007). Regional and tissue specific transcript signatures
of ion channel genes in the non-diseased human heart. J Physiol, 582,
675-693.
91. Duprat F, Lesage F, Fink M, Reyes R, Heurteaux C, Lazdunski M
(1997). TASK, a human background K+ channel to sense external pH
variations near physiological pH. EMBO J, 16, 5464–5471.