Electrical Characteristics of
Channelopathies Involving
Skeletal Muscle
Bob Ruff, M.D., Ph.D.
Chief, SCI Service
Louis Stokes Cleveland VAMC
Barbara E. Shapiro, M.D., Ph.D.
Case Western Reserve Univ.
Jacob Levitt, M.D.
Albert Einstein College of Medicine
Objectives
• To understand factors regulating membrane
excitability in skeletal muscle
• To understand how impaired Na+ channel
inactivation can produce myotonia
• To appreciate how persistent depolarization
produces paralysis (Myotonia vs HyperKPP)
• To learn different ways to produce a persistent
depolarization (HyperKPP vs HypoKPP)
Roles of Na+, K+ and Cl- Channels
in Membrane Excitability
•
•
•
•
Kir sets resting membrane potential
Kv (delayed rectifier) repolarizes after AP
Cl- channel stabilizes membrane potential
INa drives AP
Potassium Sets Membrane
Resting Potential
• K+ conductance 20% of membrane
conductance - Inward or anomalous rectifier
K+ channel (KIR)
• AP Termination - Delay Rectifier K+ Channel
Inward (Anomolous) Rectifier
Sodium channel gating
properties
• Depolarization
activates Na channels
- changes from a
closed to an open
state
• The declining portion
of INa - transition of
open channels to a
non-conducting fast
inactivated state
Two Types of Skeletal Muscle
Sodium Channel Inactivation
• Fast inactivation – msec, Slow inactivation - seconds
• Fast inactivation helps to terminate the AP
• Slow inactivation operates at more negative potentials regulates the number of excitable sodium channels as a
function of the membrane potential
Action Potential
Factors Determining Action
Potential Threshold
• Number of excitable Na+ channels (# of
channels and fraction that are excitable)
• Voltage dependence of Na+ channel opening
• Amount of Cl- conductance
• Inward rectifier K+ conductance with
depolarization
Periodic Paralysis
• Results from persistent membrane
depolarization  inactivation of normal Na+
channels  membrane inexcitability
• HyperKPP – Na+ channelopathy –
depolarization due to abnormal persistent INa
• HypoKPP –
– Type I - Indirect Ca2+ Channelopathy
– Type 2 - Na+ channelopathy – loss of function
Hyperkalemic Periodic
Paralysis (HyperPP) - AD
• episodic attacks of flaccid weakness
• myotonia is often present (vs HypoK-PP)
• paralysis caused by membrane
depolarization  Na+ channel inactivation
• Overlap: Na+ Ch myotonias, paramyotonia
Lehmann-Horn, Rudel, Ricker
Impaired fast inactivation can
produce myotonia
1 msec
Note: Loss of inactivation in a small % of
channels → myotonia
Myotonia stopped in part due to
accumulated slow inactivation
Key to Paralysis vs Myotonia
is Persistent Depolarization
Impairment of Slow Inactivation
will facilitate persistent opening
of mutant channels
Hypokalemic Periodic
Paralysis (HypoKPP) - AD
•
•
•
•
Episodic attacks of flaccid paralysis
Myotonia never present (vs HyperKPP)
Insulin  paralytic attack without  K+
Membrane excitability impaired – low
conduction velocity: Drs. Haenen,
Links, Oosterhuis, Stegeman, van der
Hoevan, van Weerden & Zwarts
Depolarization not blocked by TTX
Insulin Enhances Depolarization
Lehmann-Horn, Rudel, Ricker
Paralysis parallels drop in K+
In HypoKPP
Weakness
Parallels
Depolarization
& Reduction
in EMG
Amplitude
Skeletal Muscle Membrane Excitability
Is Impaired in HypoKPP (Type1)
• Muscle fibers very susceptible to
depolarization-induced inexcitable
• Small depolarizations (10 mV) make
HypoKPP fibers unexcitable
• Slow conduction velocity (Zwarts’ lab)
suggests impaired Na+ channel function in
HypoKPP
Two Genotypes - Similar Phenotype
• Type 1 HypoKPP is linked to 1Q31-32
• Defective gene (CACNL1A3) encodes a
skeletal muscle dihydropyridine (DHP)
sensitive or L-type calcium channel
• Mutations - segment 4 of domain 2
(R528H) and segment 4 of domain 4
(R1239H, R1239G) of the -subunit of
the skeletal muscle L-type Ca+2 channel
Two Genotypes - Similar Phenotype
• Type 2 HypoK-PP has a similar
phenotype to type 1 HypoK-PP
• Associated with point mutations in the
Na+ channel gene (SCN4A)
• Surface membrane INa is reduced to
about 50% of normal (reduced
expression and increased resting
inactivation)
Type 1 HypoKPP – Altered
Inward (Anomolous) Rectifier
Insulin  outward current
component of KIR in HypoKPP
Circle – no insulin
Square - insulin
Unfilled – HypoKPP
Filled – Control
+
K
Insulin Reduces
Conductance
Even When [K+]o is High
Circle – no insulin
Square - insulin
Unfilled – HypoKPP
Filled – Control
Summary of Alterations of Inward
Rectifier K+ Channel in HypoKPP
• Baseline Inward Rectifier Conductance
Including KATP Channels is Reduced
• Insulin selectively reduces the K+ conductance
for outward currents
• Lowering [K+]o causes depolarization due to
TTX- and DHP-insensitive depolarizing current
(low Kir conductance for outward current
facilitates depolarization)
• Note: Andersen-Tawil Syndrome due to Kir
mutation
Why do Type I and Type II HypoKPP
have similar phenotypes?
• The effects of the Na+ channel mutations in
Type II HypoKPP are to reduce membrane
channel density and to increase the amount of
resting inactivation - both lead to  INa
• Susceptibility of Type I HypoKPP fibers to
depolarization-induced inactivation and lower
AP conduction velocities suggest reduced INa
in HypoKPP (Zwarts’ lab)
Small Depolarizations Produce
Paralysis in HypoKPP
Comparison of Na+ Channel Properties and Action
Potential (AP) Thresholds in Fast Twitch, Type IIb,
Skeletal Muscle Fibers from Five Patients with
HypoKPP and Seven Controls.
Controls
HypoKPP
Na+ Channel Properties
Max INa (mA/cm2)
23.7
±1.3
15.4
±1.9 (p<0.001)
Action Potential (AP) Thresholds
AP Threshold (mV)
-58.7
±1.5
-53.4
±1.1 (p<0.001)
Which Membrane Change
Correlates Best with Paralytic
Attacks in Type 1 HypoKPP?
•INa correlated inversely with frequency of
paralytic attacks (Pearson’s correlation
coefficient, r = -0.996)
•AP threshold correlated with the frequency of
paralytic attacks (r=-0.921)
•Peak outward K+ conductance of the inward
rectifier K+ channel correlated weakly with the
frequency of paralytic attacks (r = -0.121).
 Na+ current correlated with the frequency of
paralytic attacks
 K+ current did not have a strong correlation
Patients
Peak INa
Max INa,max
(mA/cm2)
1
11.9
±1.8
2
3
12.2
±2.0
16.9
±1.8
17.7
±1.7
18.2
±1.9
-54.9
±1.7
-55.1
± 1.8
-55.4
±1.8
Action Potential (AP) Thresholds
AP Thresh
-50.6
-51.0
(mV)
±1.9
±1.7
4
Peak Outward IK in 80 mM K+ with 12mU/ml Insulin
Conductance
260
271
279
268
(µS/cm2)
±30
±29
±39
±42
5
251
±36
Number of Paralytic Attacks (lasting >1 hour) in one year
15
13
3
2
1
How Can Ca2+ Channel Mutations Alter
Na+ & K+ Channel Properties?
• The Ca2+ channel mutations may disturb
intracellular [Ca2+]
• Intracellular Ca2+ is known to regulate
Na+ channel expression and can alter the
expression and properties of other
channels
Intracellular [Ca2+] is increased in
HypoKPP Fibers
Intracellular [Ca2+] Determined with a Calcium
Sensitive Electrode in Type I, IIa and IIb Control and
HypoKPP Human Intercostal Muscle Fibers
Intracellular [Ca2+](µM) According to Fiber Type
Type I
Type IIa
Type IIb
Controls 0.113±0.005 0.094±0.005 0.081±0.003
n=27
n=22
n=58
HypoPP 0.129±0.009 0.112±0.008
n=11
n=12
p<0.05
p<0.05
0.100±0.006
n=16
p<0.01
Indirect Channelopathy -Intracellular
[Ca2+] may Down Regulate Na+ and KIR
(incl. KATP) Channels
Ca2+ mutations in HypoKPP may reduce
Na+ channel density (and perhaps alter
Inward Rectifier K+ Channel Function) by
elevating intracellular [Ca2+], which
reduces the level of the Na+ channel subunit mRNA (and perhaps reduces
expression of KATP Channels)
Thyrotoxic Periodic Paralysis:
the brother of Hypokalemic
Periodic Paralysis
Bob Ruff, M.D., Ph.D.
Chief, SCI Service
Louis Stokes Cleveland VAMC
Director Rehabilitation Research & Development
Department of Veterans Affairs.
Objectives
• To understand distinguishing features of
Thyrotoxic Periodic Paralysis (TPP)
• To compare channel defects in TPP with
HypoKPP
• To consider how thyrotoxicosis contributes to
the pathogenesis of TPP
Clinical: TPP vs HypoKPP
TPP
Predominance Asian
HypoKPP
Non-Asian
Age of Onset
3rd & 4th decades
1st & 2nd decades
Genetics
Sporadic, expression AD, specific
linked to thyroid state mutations
M>>>F
M>F
Rx
Beta-blocker
Acetazolamide may
worsen
K replacement
Acetazolamide
Prevents
Periodic Paralysis
• Results from persistent membrane depolarization 
inactivation of normal Na+ channels  membrane
inexcitability
• HyperKPP – Na+ channelopathy – depolarization due
to abnormal persistent INa
• HypoKPP –
– Type I - Indirect Ca2+ Channelopathy
– Type 2 - Na+ channelopathy – loss of function
• TPP – Not Associated with HypoKPP channel defects
Common Features of TPP &
HypoKPP
•
•
•
•
Episodic attacks of flaccid paralysis
Myotonia never present (vs HyperKPP)
Insulin  paralytic attack without  K+
Membrane excitability impaired – low
conduction velocity, low CMAP
amplitude, CMAP reduction with
exercise
Genetics of TPP
•
•
•
•
Familial cases increasingly recognized
HypoKPP Na channel mutations not found
HypoKPP Ca channel mutations not found
Reports of selective single nucleotide
polymorphisms (SNP) in regulatory region
of Ca channel gene – region of thyroid
hormone binding sites
Methods - Patient with TPP
• 32 yo man with TPP in the T-toxic state and 4
months later when euthyroid & asymptomatic
• Measured INa with a loose patch voltage clamp,
inward rectifier IK with a 3-electrode voltage
clamp, action potential (AP) threshold with a 2
electrode clamp and intracellular [Ca2+] using
Ca2+-sensitive electrodes
• Intercostal type IIb muscle fibers from patient
with TPP, 5 patients with Type I HypoKPP
(R528H mutation) and 7 controls(C).
Summary of Alterations of Inward
Rectifier K+ Channel in HypoKPP
• Baseline Inward Rectifier Conductance
Including KATP Channels is Reduced
• Insulin selectively reduces the K+ conductance
for outward currents
KIR in TPP (nA/mm2)
120
100
80
no Insulin
+ Insulin
60
40
20
0
Control
HypoKPP
T-toxic
Euthyroid
Max INa (mA/cm2)
25
20
15
10
5
0
Control
HypoKPP
T-toxic
Euthyroid
AP Threshold (mV)
-49
-50
-51
-52
-53
-54
-55
-56
-57
-58
-59
Control
HypoKPP
T-toxic
Euthyroid
Intracellular [Ca2+] (nM) in TPP & HypoKPP
100
90
80
70
60
50
40
30
20
10
0
IIb
Control HypoKPP
T-toxic
Euthyroid
TPP & HypoKPP- Indirect
Channelopathies -[Ca2+] may Down
Regulate Na+ and KIR Channels
• Ca2+ mutations in HypoKPP may reduce Na+
channel density and alter KIR function by
elevating intracellular [Ca2+]
• In TPP - SNPs at the thyroid hormone
responsive element may affect the binding
affinity of the thyroid hormone responsive
element and modulate the stimulation of thyroid
hormone on the Ca(v)1.1 gene
Summary – HyperKPP
• Paralysis produced by prolonged membrane
depolarization
• Difference between mutations that produce
myotonia vs paralysis is probably that
paralysis is associated with prolonged
pathological INa
• Impairment of slow inactivation will facilitate
prolonged pathological INa
• Mutations that impair slow inactivation
associated with paralysis
Summary – HypoKPP
• INa is reduced in both types of HypoKPP
• Inward Rectifier K+ conductance is altered
in Type I HypoKPP and Andersen-Tawil
Syndrome
• Type I HypoKPP - Frequency of paralytic
attacks correlates with decrease of INa
• Type I HypoKPP – indirect Channelopathy
- alteration of Na+ and K+ channel function
may be mediated by  intracellular [Ca2+]
Supported by the Clinical
Research and Development
Service, Office of Research
and Development,
Department of Veterans
Affairs
Rx of HyperKPP
• REDUCE PARALYTIC ATTACK
FREQUENCY
– 1) Eat regular meals high in
carbohydrates and low in K
– 2) Avoid strenuous exercise followed by
rest, emotional stress and cold
Rx of HyperKPP
• ABORT PARALYTIC ATTACKS –
– 1) Ingest high carbohydrate food such as
candy bar
– 2) use beta-adrenergic agonist inhaler.
For severe attacks I.V. glucose and
insulin can be administered in a carefully
monitored environment
Rx of HyperKPP
• IF PARAMYOTONIA AND STIFFNESS
ARE PRESENT –
– 1) Mexiletine 150 mg twice a day
increasing to 300 mg three times a day to
reduce stiffness
– 2) Tocainide is a second line agent if
mexiletine fails; however blood counts
must be monitored due to the risk of bone
marrow suppression. The dose of
tocainide is 400-1200 mg per day
Rx of HypoKPP
• REDUCE PARALYTIC ATTACK
FREQUENCY –
– 1) Follow a low carbohydrate and sodium
restricted diet
– 2) Avoid precipitating factors such as
strenuous exercise followed by rest, high
carbohydrate meals or alcohol.
Rx of HypoKPP
• MEDICATION TO REDUCE ATTACK FREQUENCY
• 1) Initiate carbonic anhydrase inhibitor. Usual agent is
acetazolamide. Initial dose of 125 mg twice a day and
increasing as needed to final dose of 250 mg four times a
day (some will need a total daily dose of 1500mg). An
alternative carbonic anhydrate inhibitor is dichlorphenamide
starting at 25 mg twice a day and increasing to 25-50 mg
two to three times a day. Note that some HypoPP patients
worsen with carbonic anhydrase inhibitors.
• 2) If carbonic anhydrase inhibitors are not successful, a Ksparing diuretic such a triamterene or spironolactone may
help.
• 3) Supplemental oral K alone or combined with a carbonic
anhydrase inhibitor may prevent paralytic attacks
Rx of HypoKPP
• ABORT PARALYTIC ATTACKS –
• 1) Oral KCl 0.25 mEq/kg repeating every
half hour until the weakness improves.
Carefully monitor electrolytes and EKG in
an intensive care setting. Avoid intravenous
KCl unless KCl cannot be given orally.
Avoid giving glucose and insulin as this will
worsen paralysis.
Rx of Anderson-Tawil Syndrome
• MEDICATION TO REDUCE ATTACK
FREQUENCY – Initiate an oral carbonic
anhydrase inhibitor. The usual agent is
acetazolamide, with the initial dose of 125 mg
twice a day and increasing as needed to final dose
of 250 mg four times a day. An alternative
carbonic anhydrate inhibitor is dichlorphenamide
starting at 25 mg twice a day and increasing to 2550 mg two to three times a day. Monitor cardiac
function.
Rx of Anderson-Tawil Syndrome
• TREATMENT OF ARRHYTHMIAS Arrhythmias may respond poorly to antiarrhythmic agents. Imipramine may be
useful. Manage with a cardiologist.
Rx of ThyrotoxicPP
• PRIMARY TREATMENT IS TO
CORRECT HYPERTHYROIDISM. When
it is not possible to correct thyrotoxicosis,
treatment with propranolol may reduce the
frequency of paralytic attacks as may the
treatments used to reduce the frequency of
paralytic attacks in patients with HypoPP.
Carbonic anhydrase inhibitors are not
effective for treating TPP.
Rx of ThyrotoxicPP
• ABORT PARALYTIC ATTACKS - Administer
oral KCl 0.25 mEq/kg repeating every half hour
until the weakness improves. Carefully monitor
electrolytes and EKG in an intensive care setting.
Avoid intravenous KCl unless KCl cannot be
given orally. Avoid giving glucose and insulin as
this will worsen paralysis. Intravenous
propranolol, given with EKG monitoring may be
useful in treating acute paralytic attacks in TPP
when hyperthyroidism has not yet been addressed.
Rx of HyperKPP
• IF PARALYTIC ATTACKS REMAIN FREQUENT – 1)
Start oral HCTZ diuretic, with initial dose of 12.5
mg/day and increasing slowly in increments of 12.5 mg
to a final dose of 100-200mg/day
• 2) If HCTZ alone is not sufficient initiate an oral
carbonic anhydrase inhibitor. The most common agent is
acetazolamide, with the initial dose of 125 mg twice a
day and increasing as needed to final dose of 250 mg
four times a day (some will need a total daily dose of
1500mg). An alternative carbonic anhydrate inhibitor is
dichlorphenamide starting at 25 mg twice a day and
increasing to 25-50 mg two to three times a day. Note
that carbonic anhydrase inhibitors may precipitate
weakness in patients with HyperPP and paramyotonia.