The document discusses ion channelopathies and provides details on the cardiac action potential and the ion channels involved in each phase. It describes the major ion channels that generate the cardiac action potential, including sodium channels, transient outward potassium current, ultra-rapidly activating delayed outward rectifying current, and rapidly activating delayed outward rectifying current. It also discusses channelopathies associated with these ion channels like Brugada syndrome and long QT syndrome.

1. ION CHANNELOPATHY
15-09-2014

2. • The analysis of the molecular basis of the inherited cardiac
arrhythmias has been the driving force behind the
identification of the ion channels that generate the action
potential.
• The genes encoding all the major ion channels have cloned
and sequenced.
• The studies have revealed greater complexity than heretofore
imagined.
• Many ion channels function as part of macromolecular
complexes in which many components are assembled at
specific sites within the membrane.

3. The Cardiac Action Potential
• The normal sequence and synchronous contraction of the atria and
ventricles require the rapid activation of groups of cardiac cells.
• An activation mechanism must enable rapid changes in heart rate
and also respond to the changes in autonomic tone.
• The propagating cardiac action potential fulfils these roles.
– Phase 4, or the resting potential, is stable at 90 mV in normal working
myocardial cells.
– Phase 0 is the phase of rapid depolarization. The membrane potential
shifts into positive voltage range. This phase is central to rapid
propagation of the cardiac impulse (conduction velocity, 1 m/s).
– Phase 1 is a phase of rapid repolarization. This phase sets the potential for
the next phase of the action potential.
– Phase 2, a plateau phase, is the longest phase. It is unique among
excitable cells and marks the phase of calcium entry into the cell.
– Phase 3 is the phase of rapid repolarization that restores the membrane
potential to its resting value

4. • In general, the resting potential of atrial and ventricular myocytes
during AP phase 4 (resting phase) is stable and negative
(approximately 85 mV) due to the high conductance for K+ of the IK1
channels.
• Upon excitation by electric impulses from adjacent cells, Na
channels activate (open) and permit an inward Na+ current (INa),
which gives rise to phase 0 depolarization (initial upstroke).
• Phase 0 is followed by phase 1 (early repolarization), accomplished
by the transient outward K current (Ito).
• Phase 2 (plateau) represents a balance between the depolarizing L-
type inward Ca2+ current (ICa,L) and the repolarizing ultra-rapidly
(IKur), rapidly (IKr), and slowly (IKs) activating delayed outward
rectifying currents.

5. • Phase 3 (repolarization) reflects the predominance of the delayed
outward rectifying currents after inactivation (closing) of the L-type
Ca2+ channels.
• Final repolarization during phase 3 is due to K+ efflux through the IK1
channels.
• In contrast to atrial and ventricular myocytes, SAN and AVN
myocytes demonstrate slow depolarization of the resting potential
during phase 4.
• This is mainly enabled by the absence of IK1, which allows inward
currents (e.g., pacemaker current [If]) to depolarize the membrane
potential.
• Slow depolarization during phase 4 inactivates most Na+ channels
and decreases their availability for phase 0.
• Consequently, in SAN and AVN myocytes, AP depolarization is
mainly achieved by ICa,L and the T-type Ca2+ current (ICa,T)

7. • The action potentials of pacemaker cells in the sinoatrial (SA)
and atrioventricular (AV) nodes are significantly different from
those in working myocardium.
• The membrane potential at the onset of phase 4 is more
depolarized (50 to 65 mV), undergoes slow diastolic
depolarization, and gradually merges into phase 0.
• The rate of depolarization in phase 0 is much slower than that
in the working myocardial cells and results in slow
propagation of the cardiac impulse in the nodal regions (0.1 to
0.2 m/s).
• Cells in the His-Purkinje system may also show phase 4
depolarization under special circumstances.

8. • The characteristics of the action potential change across the
myocardial wall from endocardium, midmyocardium, to
epicardium.
• Epicardial cells have a prominent phase 1 and the shortest
action potential.
• The action potential duration is longest in the midmyocardial
region
• The average duration of the ventricular action potential
duration is reflected in the QT interval on the ECG.
• Factors that prolong the action potential duration (eg, a
decrease in outward K currents or an increase in inward late
Na current) prolong the action potential duration and the QT
interval on the ECG.

9. • The generation of the action potential and the regional differences
that are observed throughout the heart are the result of the
selective permeability of ion channels distributed on the cell
membrane.
• The ion channels reduce the activation energy required for ion
movement across the lipophilic cell membrane.
• During the action potential, the permeability of ion channels
changes and each ion, eg, X, moves passively down its electro-
chemical gradients (ΔV=[Vm-Vx,] where Vm is the membrane
potential and Vx the reversal potential of ion X) to change the
membrane potential of the cell.
• The electrochemical gradient determines whether an ion moves
into the cell (depolarizing current for cations) or out of the cell
(repolarizing current for cations).
• Homeostasis of the intracellular ion concentrations is maintained by
active and coupled transport processes that are linked directly or
indirectly to ATP hydrolysis.

10. • Ion channels do not function as simple fluid-filled pores, but
provide multiple binding sites for ions as they traverse the
membrane.
• Ions become dehydrated as they cross the membrane as ion-
binding site interaction is favored over ion–water interaction.
• Like an enzyme–substrate interaction, the binding of the
permeating ion is saturable.
• Most ion channels are singly occupied during permeation; certain K
channels may be multiply occupied.
• The equivalent circuit model of an ion channel is that of a resistor.
• The electrochemical potential, V is the driving force for ion
movement across the cell membrane.

11. • Simple resistors have a linear relationship between V and
current I (Ohm’s Law, I=ΔV/R=ΔVg, where g is the channel
conductance).
• Most ion channels have a nonlinear current-voltage
relationship.
• For the same absolute value of V, the magnitude of the
current depends on the direction of ion movement into or
out of the cells.
• This property is termed rectification and is an important
property of K+ channels, they pass little outward current at
positive (depolarized) potentials.
• The molecular mechanism of rectification varies with ion
channel type.
• Block by internal Mg+ and polyvalent cations is the
mechanism of the strong inward rectification demonstrated
by many K+ channels.

12. • Ion channels have 2 fundamental properties, ion permeation
and gating.
• Ion permeation describes the movement through the open
channel.
• The selective permeability of ion channels to specific ions is a
basis of classification of ion channels (eg, Na, K, and Ca2
channels).
• Size, valency, and hydration energy are important
determinants of selectivity.
• The selectivity ratio of the biologically important alkali cations
is high. For example, the Na:K selectivity of sodium channels is
10:1.

13. • Gating is the mechanism of opening and closing of ion channels and
is their second major property.
• Ion channels are also subclassified by their mechanism of gating:
voltagedependent, ligand-dependent, and mechano-sensitive
gating.
• Voltage-gated ion channels change their conductance in response
to variations in membrane potential.
• Voltagedependent gating is the commonest mechanism of gating
observed in ion channels.
• A majority of ion channels open in response to depolarization.
• The pacemaker current channel (If channel) opens in response to
membrane hyperpolarization.
• The steepness of the voltage dependence of opening or activation
varies between channels.

14. • Ion channels have 2 mechanism of closure.
• Certain channels like the Na+ and Ca2+ channels enters a closed
inactivated state during maintained depolarization.
• To regain their ability to open, the channel must undergo a recovery
process at hyperpolarized potentials.
• The inactivated state may also be accessed from the closed state.
• Inactivation is the basis for refractoriness in cardiac muscle and is
fundamental for the prevention of premature re-excitation.
• If the membrane potential is abruptly returned to its hyperpolarized
(resting) value while the channel is open, it closes by deactivation, a
reversal of the normal activation process.

15. • Ligand-dependent gating is the second major gating mechanism of cardiac
ion channels.
• The most thoroughly studied channel of this class is the acetylcholine
(Ach)-activated K channel.
• Acetylcholine binds to the M-2 muscarinic receptor and activates a G
protein–signaling pathway, culminating in the release of the subunits Gαi
and Gβγ.
• The Gβγ subunit activates an inward-rectifying K channel, IKAch that
abbreviates the action potential and decreases the slope of diastolic
depolarization in pacemaker cells.
• IKAch channels are most abundant in the atria and the SA and
atrioventricular nodes.
• IKAch activation is a part of the mechanism of the vagal control of the heart.
• The ATP-sensitive K+ channel, also termed the ADP-activated K+ channel, is
a ligand-gated channel distributed abundantly in all regions of the heart.

16. • The open probability of this channel is proportional to the
[ADP]/[ATP] ratio.
• This channel couples the shape of the action potential to the
metabolic state of the cell.
• Energy depletion during ischemia increases the [ADP]/[ATP]
ratio, activates IK ATP, and abbreviates the action potential.
• The abbreviated action potential results in less force
generation and may be cardioprotective.
• This channel also plays a central role in ischemic
preconditioning.

17. • The mechanosensitive or stretch-activated channels are the least
studied.
• They belong to a class of ion channels that can transduce a physical
input such as stretch into an electric signal through a change in
channel conductance.
• Acute cardiac dilatation is a well-recognized cause of cardiac
arrhythmias.
• Stretch-activated channel are central to the mechanism of these
arrhythmias.
• Blunt chest wall impact at appropriately timed portions of the
cardiac cycle may also result in PVCs or ventricular fibrillation (the
VF of commotio cordis).
• The channels that transduce the impact into an electric event are
unknown.

18. Sodium Channels
• Sodium channels are the arch-type of voltage-gated ion channels.
• By enabling phase 0 depolarization in atrial, ventricular, and
Purkinje APs, INa+ determines cardiac excitability and electrical
conduction velocity.
• The -subunit of cardiac Na channels (Nav1.5, encoded by SCN5A)
encompasses four serially linked homologous domains (DI–DIV),
which fold around an ion-conducting pore.
• Each domain contains six transmembrane segments (S1–S6).
• S4 segments are held responsible for voltage-dependent activation.
• At the end of phase 0, most channels are inactivated and can be
reactivated only after recovery from inactivation during phase 4.
• Some channels remain open or reopen during phases 2 and 3, and
they carry a small late Na current (INaL).

20. • Na channel dysfunction is linked to several inherited
arrhythmia syndromes, emphasizing the important role of this
channel in cardiac electrical activity.
• In LQTS type 3 (LQT3), mutations in SCN5A delay
repolarization, mostly by enhancing INaL.
• Accordingly, drugs that block INaL (e.g., ranolazine, mexiletine)
may effectively shorten repolarization in LQT3 patients.

21. Brugada Syndrome
• BrS is characterized by ST-segment elevation with “coved”
morphology in the right precordial leads and complete or
incomplete right bundle-branch block.
• This ECG pattern is intermittent and may be unmasked by
pharmacological challenge with sodium channel blockers such
as procainamide, flecainide, ajmaline, or pilsicainide.
• The onset of ventricular arrhythmias causes the occurrence of
syncope and may lead to sudden death, usually at rest.
• Known triggers for arrhythmic events are fever and the
consumption of large meals; the latter has been related to
glucose-induced insulin secretion that might enhance ST-
segment elevation.

22. • Brugada syndrome is characterized by prolonged conduction
intervals, right precordial ST-segment elevation, and increased risk
for ventricular tachyarrhythmia.
• Prolonged conduction intervals are attributed to conduction
slowing due to INa reduction.
• ST-segment elevation is hypothesized to be due to preferential
conduction slowing in the right ventricle and/or aggravation of
transmural voltage gradients.
• Ten different genes causally linked to BrS have been reported.
• Loss-of-function mutations of SCN5A (BrS1), the gene encoding for
the cardiac sodium channel, were the first to be identified, and this
gene is currently the only BrS key gene.
• Reduced sodium current and a BrS phenotype can be also due to
mutations in SCN5A-regulating genes: GPD1-L, SCN1B, and SCN3B.

24. • Loss of function mutations in the CACNA1c and CACNB2 genes
encoding for the α and β subunits of the cardiac calcium
channel can also cause BrS and may represent the second
most frequent cause of the disease.
• Management of BrS is based on the use of ICD in selected
high-risk individuals.
• No drug has been definitely proven effective in reducing the
cardiac arrest burden.

25. • Cardiac conduction disease is manifested by progressive
conduction defects at the atrial, atrioventricular, and/or
ventricular level and is commonly associated with SCN5A
mutations that are also linked to Brugada syndrome.
• How a single mutation may cause different phenotypes or
combinations thereof is often not known.
• Dilated cardiomyopathy–linked SCN5A mutations cause
divergent changes in gating, but how such changes evoke
contractile dysfunction and arrhythmia is not understood.
• Finally, mutations in SCN5A have occasionally been linked to
sick sinus syndrome, which includes sinus bradycardia, sinus
arrest, and/or sinoatrial block. SCN5A mutations may impair
sinus node function by slowing AP depolarization or
prolonging AP duration in SAN cells.

26. Transient outward K current (Ito)
• Ito supports early repolarization during phase 1.
• The transient nature of Ito is secondary to its fast activation
and inactivation upon depolarization.
• Ito displays two phenotypes. Ito, fast recovers rapidly from
inactivation, and its α-subunit (Kv4.3) is encoded by KCND3.
Ito, slow recovers slowly from inactivation; its -subunit (Kv1.4)
is encoded by KCNA4.
• Kv4.3 is abundantly expressed in the epicardium and is
responsible for shorter AP duration there compared to
endocardium, where Kv1.4 is expressed to a much lesser
extent.

27. Ultra-rapidly activating delayed outward
rectifying current (IKur)
• KCNA5 encodes the -subunit (Kv1.5) of the channel carrying
Ikur.
• Kv1.5 is mainly expressed in the atria, and IKur is detected only
in atrial myocytes.
• Thus, IKur plays a role in atrial repolarization. It activates
rapidly upon depolarization but displays very slow
inactivation.
• Inactivation accelerates when Kv1.5 is co-expressed with its β
–subunits.
• Genetic studies identified KCNA5 mutations in individuals with
familial AF.

28. Rapidly activating delayed outward rectifying
current (IKr)
• KCNH2, also called the human-ether-a-go-go-related gene (hERG),
encodes the -subunit (Kv11.1) of the channel carrying IKr.
• Belying its name, IKr activation upon depolarization is not rapid, but
inactivation thereafter is very fast, resulting in a small outward K
current near the end of the AP upstroke.
• However, during early repolarization, the channel rapidly recovers
from inactivation to produce large IKr amplitudes during AP phases 2
and 3.
• Next, the channel deactivates (closes) slowly (in contrast to
inactivation, deactivation is a voltage-independent process).
• Under normal conditions, IKr is largely responsible for repolarization
of most cardiac cells.

29. • LQTS type 2, the second most prevalent type of LQTS, is caused by
KCNH2 loss-of-function mutations .
• This translates into AP and QT interval prolongation and may
generate EADs to trigger torsades de pointes.
• KCNH2 mutations reduce IKr, mostly by impairing the trafficking of
Kv11.1 proteins to the sarcolemma.
• Moreover, mutations in KCNE2 also reduce IKr and cause the less
prevalent LQTS type 6.
• Short QT syndrome is a rare disease associated with short QT
intervals and increased risk for atrial and ventricular fibrillation.
• A gain-of-function mutation in KCNH2 is linked to short QT
syndrome type 1.
• The resulting Ikr increase achieved by altered gating hastens
repolarization, thereby shortening AP duration and facilitating
reentrant excitation waves to induce atrial and/or ventricular
arrhythmia.

30. Slowly activating delayed outward rectifying
current (IKs)
• Kv7.1, encoded by KCNQ1, is the -subunit of the channel
responsible for IKs.
• IKs is markedly enhanced by –adrenergic stimulation through
channel phosphorylation by protein kinase A and protein
kinase C.
• This implies that IKs contributes to repolarization, especially
when –adrenergic stimulation is present.
• Accordingly, selective blocking of IKs by chromanol-293B
prolongs AP duration minimally under baseline conditions but
markedly under –adrenergic stimulation.
• Interestingly, KCNQ1 and KCNE1 are also expressed in the
inner ear, where they enable endolymph secretion.

31. • The most common type of LQTS, type 1 (LQT1), is caused by loss-of-
function mutations in KCNQ1.
• The resulting IKs reduction is responsible for prolonged AP
durations and QT intervals.
• Individuals with the less prevalent LQTS type 5 carry loss-of-
function mutations in KCNE1 and display a similar phenotype as
LQT1 patients.
• Loss-of-function mutations in both alleles of KCNQ1 or KCNE1 cause
the very rare Jervell and Lange-Nielsen syndrome (JLNS) type 1 or 2,
respectively.
• KCNQ1 gain-of-function mutations are anecdotally linked to short
QT syndrome (type 2).
• Moreover, an KCNQ1 gainof- function mutation is reported to cause
familial AF by shortening atrial AP duration and facilitating reentry.

32. Inward rectifying current (IK1)
• IK1 stabilizes the resting membrane potential of atrial and
ventricular myocytes during phase 4 and contributes to the
terminal portion of phase 3 repolarization.
• IK1 channels are closed during AP phases 1 and 2. Voltage
dependent block by intracellular Mg2+ underlies channel closing,
while unblocking enables channel opening.
• IK1 is almost absent in SAN and AVN myocytes.
• Its α–subunit (Kir2.1) is encoded by KCNJ2 and consists of one
domain with two transmembrane segments.
• Loss-of-function mutations in KCNJ2 are linked to Andersen- Tawil
syndrome, a rare disease characterized by skeletal developmental
abnormalities, periodic paralysis, and usually nonsustained
ventricular arrhythmia, often associated with prominent U waves
and mild QT interval prolongation (LQTS type 7).
• To date, one KCNJ2 gain-of-function mutation, found in one small
family, is linked to short QT syndrome type 3.

34. Cardiac Ca2+ current (ICa) and intracellular
Ca2+ transients
• The L-type (long-lasting) inward Ca2+ current (ICa,L) is largely
responsible for the AP plateau.
• Ca2+ influx by ICa,L activates Ca2+ release channels (ryanodine
receptor [RyR2]), located in the sarcoplasmic reticulum
membrane.
• Sarcoplasmic reticulum Ca2+ release (Ca2+ transients) via RyR2
channels couples excitation to contraction in myocytes.
• CACNA1C encodes the α-subunit (Cav1.2) of L-type channels
• Beside ICa,L, T-type (tiny) Ca2+ current (ICa,T) is identified in SAN
and AVN myocytes.
• ICa,T is believed to contribute to AP formation in pacemaker
cells.

35. • CACNA1C mutations are linked to Timothy syndrome, a rare multisystem
disease with QT interval prolongation (LQTS type 8), ventricular
tachyarrhythmia, and structural heart disease.
• In Timothy syndrome, CACNA1C mutations delay repolarization by
increasing ICa,L.
• RyR2 mutations cause catecholaminergic polymorphic ventricular
tachycardia, a disease associated with exerciseand emotion-induced
arrhythmia.
• Mutant RyR2 channels permit Ca2+ leakage from the sarcoplasmic
reticulum into the cytoplasm.
• Ca2+ leakage induces extrusion of Ca2+ to the extracellular matrix by the
Na+/Ca2+ exchanger, which exchanges one Ca2+ ion for three Na+ ions.
• By doing so, the Na+/Ca2+ exchanger generates an inward Na+ current,
which underlies delayed afterdepolarization (abnormal depolarization
during phase 4 due to activation of Na+ channels).
• Delayed afterdepolarizations are believed to cause ventricular
tachyarrhythmia.

36. Long QT syndrome
• LQTS is a group of genetically transmitted disorders marked by
QT prolongation and by episodes of syncope and sudden
death due to torsades de pointes.
• QT prolongation and susceptibility to torsades de pointes
results from ion channel dysfunction that prolongs cellular
repolarization.
• This is most often caused by decreased outward potassium
current IKs (LQT1, LQT5) or IKr (LQT2, LQT6), or by enhanced
activity of mutant inward sodium current (LQT3).
• LQTS usually is transmitted in an autosomal dominant pattern.

37. • Mutations of different genes lead to distinct clinical
syndromes.
• Patients with LQT1 classically have a broad-based T wave and
tend to have syncope or sudden death during physical
exercise.
• Patients with LQT2 tend to have a notched or low-amplitude T
wave, and they classically have syncope or sudden death with
sudden auditory stimuli or strong emotion.
• Patients with LQT3 have a long, flat ST segment, a tendency
toward abnormal bradycardia, and a higher incidence of
sudden death during sleep.

39. • In addition to avoiding QT-prolonging drugs and high-intensity
sports, standard treatment of LQTS involves the use of high-
dose beta-blockers.
• Beta-blocker therapy appears to be somewhat less effective in
LQT2 and perhaps useless in LQT3.
• Patients with a history of aborted sudden death, documented
torsades de pointes, or syncope despite beta-blocker therapy
generally should be treated with an implantable cardioverter-
defibrillator (ICD).

40. Short-QT Syndrome
• Short-QT syndrome is described as a disorder characterized by
abbreviated QT interval, ventricular and atrial arrhythmias,
and sudden cardiac death.
• There have been 70 short-QT syndrome cases reported
worldwide, with the mean QTc value in the entire population
of 310 milliseconds.
• Symptomatic (sudden death or cardiac arrest) individuals,
accounting for 25% of the total, tend to present with shorter
QTc (average, 300 milliseconds).
• Quinidine can normalize the QT interval, but its long-term
efficacy is not proven.
• Therefore, an ICD is the only way to prevent sudden death.

42. CPVT
• Catecholaminergic polymorphic ventricular tachycardia (CPVT) was
described in the 1970s.
• Patients present unremarkable resting ECG and a peculiar pattern of
ventricular arrhythmias (bidirectional or polymorphic ventricular
tachycardia) reproducibly triggered by exercise or acute emotion.
• CPVT is characterized by a high incidence of cardiac events among
untreated patients (79% in patients up to 40 years of age) and 30%
incidence of sudden cardiac death.
• Investigations directed to disclose the molecular basis of CPVT led
to the identification of mutations of 2 genes, the ryanodine
receptor RyR255 and the cardiac calsequestrin CASQ2, which are
associated with the autosomal dominant and recessive forms of
CPVT, respectively.
• Both genes are involved in the control of calcium release from the
sarcoplasmic reticulum (SR): RyR2 is the SR calcium-releasing
channel, and CASQ2 is a calcium buffering protein in the SR that
may also exert a regulatory function of RyR2.

43. • CPVT mutations lead to a loss of calcium release inhibition from the
SR during diastole.
• As a consequence, when SR calcium content augments adrenergic
activation, it drives a pathological increase in Ca2+ release (leak) that
leads to delayed afterdepolarization and triggered activity.
• Approximately 60% of CPVT individuals carry an RyR2 mutation.
• No prognostic value is linked to specific RyR2 mutations
• When a CPVT diagnosis is established, beta-blockers should be
administered.
• Although this approach affords protection in the majority of
patients, 30% experience at least 1 arrhythmic event while on
therapy.
• In these cases, an ICD may be indicated.

44. Conclusions
• Ion channel disease offers a paradigm for the understanding
of a molecular lesion in the patient and its translation to
phenotype and eventually management decisions.
• Nonetheless there are many gaps in our understanding,
particularly of incomplete penetrance, risk stratification and
the underlying pathophysiology of some of the conditions.
• Still, progress is rapid, and patients and their families will
continue to benefit as our knowledge and understanding
improves.