Ion%20Channels

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Ion Channels
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Active Transporters The proteins that created
and maintain ion gradients Ion channels give
rise to selective ion permeability changes
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ION CHANNELS
Ion channels are transmembrane proteins that
contain a specialized structure, THE PORE that
that allow particulars ions to cross the
membrane. Some ion channels contain voltage
sensor ( voltage gated channels) that open or
close the channel in response to changes in
voltage. Other gated channels are regulated by
extracellular chemical signals such as
neurotransmitter or by intracellular signals as a
second messengers.
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ACTIVE TRANSPORTERS
Membrane proteins that produce and maintain ion
concentration gradients. For example the Na
pump which utilizes ATP to regulate internal
concentration of Na and K.
Transporters create the ionic gradient that drive
ions through open channels, thus generating
electric signals
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What is the mechanism for ion movement across the
membrane?

• K and Na currents were distinct, suggesting
  distinct mechanisms
• Mechanism is voltage dependent (must sense
  voltage)
• Voltage clamp recordings showed that ions move
  across membrane at high rates ( 600,000 /s)
  inconsistent with an ion pump mechanism
• Ion selectivity of Na and K currents size
  dependent permeability suggests pore of certain
  diameter.
• Armstrong (1965-6) TEA block could be overcome
  by adding excess K to the extracellular fluid
  and stepping to hyperpolarized potentials (K
  comes into cell) suggesting that K ions dislodge
  TEA from pore

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Ion channels share several characteristics

• The flux of ions through the channel is passive
  .
• The kinetic properties of ion permeation are best
  described by the channel conductance (g) that is
  determinate by measuring the current flux (I)
  that flows through the channel in repose to a
  given electrochemical driving force.
  (Electrochemical driving force is determinate by
  difference in electric potential across membrane
  and gradient of concentration of ions) .

At the single channel level, the gating
transitions are stochastic. They can be predicted
only in terms of probability.
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Ion channels share several characteristics

• In some channels the current flow varies
  linearly with the driving force ( channels behave
  as resistors)
• In other channels, current flow is a non-linear
  function of driving force ( Rectifiers)

High conductance (?)
I (pA)
V (mV)
Low conductance (?)
Ohmic channel
Rectifying Channel
( IVm/R)
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Ion channels share several characteristics

• The rate of ion flux (current) depends on the
  concentration of the ions in solution ( At low
  concentrations the current increases linearly
  with the concentration, at higher concentrations
  the current reach a saturation point ) .
• The ionic concentration at which current flow
  reaches half its maximum defines the dissociation
  constant for ion binding.
• Some ion channels are susceptible to occlusion by
  free ions or molecules

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The Opening and closing of channels involve
conformational changes

• In all channel so far studied, the channel
  protein has two or more conformational states
  that are relatively stable. Each stable
  conformation represents a different functional
  state.. Each channel has an open state and one
  or two closed states. The transition between
  states is calling gating.

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The Opening and closing of channels involve
conformational changes

• Three major regulatory mechanisms have evolved to
  control the amount of time that a channel remains
  open and active.

• Under the influence of these regulators
  ,channels enter one of three functional states
  closed and activable (resting), open (active) or
  closed and nonactivable ( refractory).
• The signal that gate the channel also controls
  the rate of transition between states.

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The Opening and closing of channels involve
conformational changes

• Ligand -gated and voltage gated channels enter
  refractory states through different process.
  Ligand-gated channels can enter refractory state
  when the exposure to ligand sis prolonged
  (desensitization)
• Voltage-gate channels enter a refractory state
  after activation. The process is called
  inactivation.

Activation is the rapid process that opens Na
channels during a depolarization. Inactivation
is a process that closes Na channels during
depolarization. The membrane needs to be
hyperpolarized for many milliseconds to remove
inactivation.
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The Opening and closing of channels involve
conformational changes

• Exogenous factors such as drugs and toxins can
  affect the gating control sites.

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Structure of Ion Channels

• Ion channels are composed of several subunits.
  They can be constructed as heterooligomers from
  distinct subunits, as homooligomers from a single
  type of subunit o from a single polypeptide chain
  organized into repeated motifs.
• In addition to one or more pore forming unties,
  which comprise a central core, some channels
  contain auxiliary subunits which modulate the
  characteristics of the central core

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Structure of voltage gated ion channels
Repeated series of 6 TM a helices S4 helix is
voltage sensor Loop between S5 S6 composes
selectivity filter
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(No Transcript)
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Gating currents

• Movement of charges in S4 segment produces
  small outward current that precedes ion flux
  through channel

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Role of auxiliary subunits
Auxiliary (non pore) subunits affect Surface
expression Gating properties
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Voltage gated sodium channels
A large alpha subunit that forms the core of the
channel and its functional on its own. It can
associate with beta subunits
Blocked by TTX, STX, cain local anesthetics
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Persistent (non-inactivating) Na currents are
produced by an alternative channel gating mode
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Functions of voltage-gated Na channel alpha
subunits
Protein name Gene Expression profile Associated channelopathies
NaV 1.1 SCN1A Central and peripheral neurons and cardiac myocites Febrile epilepsy, severe myclonic epilepsy of infancy, infantile spasms, intractable childhood epilepsy, familial autisms
Nav1.2 SCN2A Central and peripheral neurons Febrile seizures and epilepsy
Nav1.4 SCN4A Skeletal muscle Periodic paralysis, potassium agravated myotonia
Nav1.5 SCN5A Cardiac myocites, skeletal muscle, central neurons Idiopathic ventricular fibrillation
Nav1.7 SCN9A Dorsal root ganglia, peripheral neurons. Heart, glia Insensitivity to pain.
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Voltage gated Ca2 channels

• Gene Product Cav1.1-1.4 Cav2.1 Cav2.2 Cav2.3 Cav3.
  1-3.3
• Tsien Type L P/Q N R T
• Characteristics High voltage Mod voltage High
  voltage Mod voltage Low voltage
• activated, activated, activated, activated acti
  vated
• slow inactivation moderate moderate fast fast
• (Ca2 dependent) inactivation
  inactivation inactivation inactivation
• Blocked by dihydropiridines Agatoxin Conotoxin
  SNX 482 Mibefridil
• (nimodipine) IVA GVIA High Ni2

Form by different subunitsa1, a2d,ß and ?. The
a1 subunit forms the pore, the other subunits
modulate gating.
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Ca2 dependent Ca2 channel inactivation
Ca2
Ca2
Ca2
Ca2
Ca2
Ca2
Ca2
Ca2
Ca2
Ca2 channel
CaM
-
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Potassium Channels
Inwardly rectifying
Voltage gated
2 pore (leak)
Ca2 activated
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Inwardly-rectifying and leak K channels
Inwardly-rectifying
2 pore leak
Inwardly-rectifying channels ? subunits Kir
1.X - 7.X Rectifying character due to internal
block by Mg2 and polyamines Roles
Constitutively active resting K conductance (eg.
Kir1, Kir2) G-protein activated (Kir3) ATP
sensitive (Kir6) 2 pore leak channels many
different ? subunits, nomenclature still argued
Outwardly rectifying due to unequal K across
the membrane Roles Constitutively active
resting K conductance pH sensing Mechanosen
sitive Thermosensitive Second messenger
sensitive (cAMP, PKC, arachadonic acid)
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Voltage gated K channels

• Gene Product Kv1.X (1-8) Kv2.X (1-2) Kv3.X
  (1-4) Kv4.X (1-4) Kv7.X (1-5)
• D type Delayed Delayed A type M
  current
• rectifier rectifier
• Characteristics Low voltage High voltage High
  voltage Low voltage Low voltage
• activated (50 mV), activated (0 mV), activated
  (-10 mV), activated (-60 mV) activated (-60 mV)
• fast activation mod activation fast
  activation fast activation slow activation
• (lt 10 ms) (gt20 ms) (10-20 ms) (10-20
  ms) (gt100 ms)
• slow inactivation very slow very slow fast no
• inactivation inactivation inactivation inactiva
  tion
• fast deactivation
• Blocked by 4-AP (100 µM) TEA (5-10 mM) TEA
  (0.1-0.5 mM) 4-AP (5 mM) XE991
• dendrotoxin 4AP (1-5 mM) 4AP (0.5-1 mM)
• BDS (50 nM)

Kv4 (A type)
Kv1 (D type)
Kv2 (DR type)
Kv3 (DR type)
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Ca2 activated K channels - role in
repolarization following APs
Voltage response
currents mediating AHP
Spike frequency accommodation
Role of IKCa in burst duration
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Ca2 activated K channels

• Channel Type BK SK sAHP
• maxi K, IC fAHP mAHP sAHP
• Gene product slo 1-3 SK1-3 ????
• Voltage dep? Yes No No
• Ca2 to activate 1-10 µM 0.1-1 µM 0.1-1 µM
• Ca2 binding direct to ??subunit calmodulin hippoc
  alcin?
• Single channel 100-400 pS 5-20 pS 5-10 pS
• Conductance
• Blocked by charybdotoxin apamin TEA (gt 20 mM)
• TEA (lt 1 mM) TEA (gt 20 mM)

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Many drugs and toxins act on voltage gated ion
channels
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Effect of drugs and toxins

• Many toxins block ion channels directly either
  from the outer (TTX) or inner (lidocaine) surface
  of the channel
• Other toxins change the properties of the channel
  without blocking it
• Delaying inactivation
• Shifting voltage dependence

FUGU
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Modulation of Ion Channels

• Example, enhancement of Ca2 channels in cardiac
  myocytes by NE

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Dendritic ion channels participate in synaptic
amplification and integration
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Channelopathies
Condition Channel type
Paramyotonia congenita Vgated Na channel
Hemiplegia of childhood Na/K ATPase
Congenital hyperinsulinism IR K channel
Cystic fibrosis Cl- Channel
Episodic ataxia Vgated K channel
Erythromegalia Vgated K channel
Generalized epilepsy with febrile seisures Vgated Na channel
Hyperkalemic periodic paralysis Vgated Na channel
Malignant hyperthermia L gated Ca2 channel
Myasthenia Gravis Lgated Na channel
Neuromyotonia Vgated K channel
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Recommended Readings
Kandel. Principles of Neural Science, 4 th
Edition chapter 6 Hille. Ion Channels of
Excitable Membranes. 3 ed. Edition.