Electrical Characteristics of Channelopathies Involving Skeletal Muscle

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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

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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)

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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

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Potassium Sets Membrane Resting Potential

• K conductance 20 of membrane conductance -
  Inward or anomalous rectifier K channel (KIR)
• AP Termination - Delay Rectifier K Channel

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Inward (Anomolous) Rectifier
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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

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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

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Action Potential
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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

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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

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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
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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
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Key to Paralysis vs Myotonia is Persistent
Depolarization
Impairment of Slow Inactivation will facilitate
persistent opening of mutant channels
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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

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Depolarization not blocked by TTXInsulin
Enhances Depolarization
Lehmann-Horn, Rudel, Ricker
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Paralysis parallels drop in K
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In HypoKPP Weakness Parallels Depolarization
Reduction in EMG Amplitude
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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

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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 Ca2
  channel

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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)

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Type 1 HypoKPP Altered Inward (Anomolous)
Rectifier
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Insulin ? outward current component of KIR in
HypoKPP
Circle no insulin Square - insulin Unfilled
HypoKPP Filled Control
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Insulin Reduces K Conductance Even When Ko is
High
Circle no insulin Square - insulin Unfilled
HypoKPP Filled Control
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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 Ko 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

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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)

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Small Depolarizations Produce Paralysis in HypoKPP
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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
15.4 1.3 1.9
(plt0.001) Action Potential (AP) Thresholds AP
Threshold (mV) -58.7 -53.4
1.5 1.1 (plt0.001)
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Which Membrane Change Correlates Best with
Paralytic Attacks in Type 1 HypoKPP?

• INa correlated inversely with frequency of
  paralytic attacks (Pearsons 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).

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? Na current correlated with the frequency of
paralytic attacks ? K current did not have a
strong correlation Patients 1
2 3 4
5 Peak INa Max INa,max 11.9 12.2 16.9
17.7 18.2 (mA/cm2) 1.8 2.0 1.8
1.7 1.9 Action Potential (AP) Thresholds AP
Thresh -50.6 -51.0 -54.9 -55.1
-55.4 (mV) 1.9 1.7 1.7 1.8
1.8 Peak Outward IK in 80 mM K with 12mU/ml
Insulin Conductance 260 271 279
268 251 (µS/cm2) 30 29 39 42
36 Number of Paralytic Attacks (lasting gt1
hour) in one year
15 13 3 2 1
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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

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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 0.1130.005
0.0940.005 0.0810.003 n27
n22 n58 HypoPP 0.1290.009
0.1120.008 0.1000.006 n11 n12
n16 plt0.05 plt0.05 plt0.01
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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 a-subunit mRNA (and perhaps
  reduces expression of KATP Channels)

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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.

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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

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Clinical TPP vs HypoKPP
TPP HypoKPP
Predominance Asian Non-Asian
Age of Onset 3rd 4th decades 1st 2nd decades
Genetics Sporadic, expression linked to thyroid state MgtgtgtF AD, specific mutations MgtF
Rx Beta-blocker Acetazolamide may worsen K replacement Acetazolamide Prevents
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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

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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

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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

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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).

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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

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KIR in TPP (nA/mm2)
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Max INa (mA/cm2)
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AP Threshold (mV)
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Intracellular Ca2 (nM) in TPP HypoKPP
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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

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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

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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

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Supported by the Clinical Research and
Development Service, Office of Research and
Development, Department of Veterans Affairs
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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

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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

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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

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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.

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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 K-sparing diuretic such a
  triamterene or spironolactone may help.
• 3) Supplemental oral K alone or combined with a
  carbonic anhydrase inhibitor may prevent
  paralytic attacks

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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.

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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 25-50 mg two to three times a
  day. Monitor cardiac function.

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Rx of Anderson-Tawil Syndrome

• TREATMENT OF ARRHYTHMIAS - Arrhythmias may
  respond poorly to anti-arrhythmic agents.
  Imipramine may be useful. Manage with a
  cardiologist.

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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.

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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.

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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.