This topic covers voltage gated type of ion channel, general structure and functioning of ion channels and involvement of different ion channel types in the pathogenesis as wella as a target for the development of various diseases.

1. Ion Channels
FARAZA JAVED
MPHIL PHARMACOLOGY

2. Ion Channels
Ion channels are pore-forming membrane proteins
whose function is establishing a resting membrane
potential, shaping action potentials and other
electrical signals by gating the flow of ions across
the cell membrane, controlling the flow of ions
across membranes, and regulating cell volume.

3. They are often described as narrow, water-filled
tunnels that accept only specific type of ions. This
characteristic is called selective permeability.
Ion channels are integral membrane proteins,
formed as assemblies of several proteins. Such
"multi-subunit" assemblies usually make a circular
arrangement of identical or homologous proteins
closely packed around a water-filled pore through
the plane of the lipid bilayer membrane.

4. Ion channels are different from other transporter
proteins:
The rate of ion transport through the channel is
very high (often 106 ions per second or above).
Ions pass through channels down their
electrochemical gradient, which is a function of ion
concentration and membrane potential,
"downhill", without the input of metabolic energy
(e.g. Adenosine triphosphate, active transport
mechanisms, co-transport mechanisms).

5. History
By the late 1800s, the chemical mechanism
underlying nerve and muscle tissue messaging was
a mystery.
Ludimar Hermann was able to conclude that nerve
and muscle cells were capable of exhibiting a "self-propagating
wave of negative charge which
advances in steps along the tissue ".

6. Julius Bernstein made the first real theoretical
contribution, for he postulated the ionic theory,
the Nernst equation, and the assumption of a semi-permeable
membrane surrounding nerve and
muscle cells could all help explain the mounting
electrophysiological evidence of the previous
statement.

7. Sidney Ringer used a solution of water and ran it
through the vessels of an isolated heart from a frog
in the 1880s and discovered that in order for the
heart to continue beating salts needed to be
present in the water.
Specifically, sodium, calcium, and potassium salts
were needed and they had to be in special
concentrations relative to each other.

8. In 1937, John Z. Young
use squid neuron to study
ion current.
He made important experiments possible for the
first time, including the first intracellular recordings
of the nerve cell action potential.

9. The next improvement in instrumentation took place
in the late 1940s by Kenneth Cole. This involved
placing a glass electrode inside the cell in order to
"voltage clamp" the interior of the cell.
 Voltage clamping made it possible to distinguish the
voltage effects caused by influx of sodium or efflux of
potassium.

10.  Later in the 1960s and 70s, many other small
molecules and peptides would be discovered to
help gate these channels, including glutamate,
GABA, glycine, serotonin, dopamine, and
norepinephrine.

11. In 1970s, the existence of ion channels was
confirmed by the invention of ‘patch clamp’
technique by Erwin Neher and Bert Sakmann who
won a Nobel Prize for it.
In 2003, the Nobel Prize was awarded to American
scientists, Roderick MacKinon and Peter Agre for
their x-ray crystallographic structure studies on
ion channels.

12. Since the discovery of ion channels, the etiology of
many diseases has been traced back to
channelopathies. Many toxins produced by snakes,
fish, spiders and other insects paralyze the ion
channels. Many physiological mechanisms have
been studied at molecular level and have
confirmed the involvement of ion channels. The
ion channels are the recent target sites for
pharmaceutical biosynthesis of new drugs.

13. Types of Ion Channels
2 major types:
Voltage gated ion channels
Ligand gated ion channels

14. Voltage Gated Ion Channels

15. Structure and Function
Voltage-dependent channels are
made of three basic parts:
Voltage sensor
The pore or conducting
pathway and
Selectivity filter

16. Voltage gated ion channels consist of a highly
processed
 α subunit, associated with
 auxiliary β subunits.
The pore-forming α subunit is sufficient for functional
expression, but the kinetics and voltage dependence
of channel gating are modified by the β subunits.

17. The α subunits are organized in four homologous
domains (I-IV) each with six transmembrane
segments (S1-S6) - 24 transmembrane segments in
total. The pore forming segments are formed by S5
and S6.

18. Each of these segments
coils is called a
transmembrane
domain, and within
a transmembrane domain
the side chains necessarily
face outward where they
readily interact with the
lipids of the membrane are known as Polypeptide
chain.

19. For clarity in the figure, the alpha-helices are shown
spread out and in a row.
In an actual membrane, the
alpha-helices are
not in a line, but clustered.
This is shown in top view in
the figure.
At the center of the four
domains is the channel
through which the ions movement take place.

20. Voltage
Gated
Channels
Domains TM
Segment
s
Sub-
Units
Pore
Forming
Regions
Voltage
Sensor
V.G Na+ 4 6 1 S5-S6 S4
V.G Ca2+ 4 6 4 S5-S6 S4
V.G Cl- 4 6 4 S5-S6 S4
V.G K+ 4 6 1 S5-S6 S4
Channel Structure

21. In this diagram, a single transmembrane domain is
shown as the voltage sensor that operates the gate.
The S4 segment of voltage gated
channel is the voltage sensor
that is responsible for changing
conformation as the voltage
changes. All voltage gated
channels have this S4 segment.

22. Changes in the membrane potential modulate the
channel's opening or closing, as changing the
membrane potential changes the relative amounts of
positive and negative charges on the inside and
outside of the membrane. Like charges repel, so the
positively charged S4 segment
will be pushed away from
a positive intracellular fluid
towards the negative extra
cellular fluid, changing the
protein's conformation and opening the channel.

23. At a typical resting membrane potential (for example, -70
mV) the channel is closed. Then should any
factor depolarize the
membrane potential
sufficiently (for example,
to -50 mV), the voltage
sensor moves outward
and the gate opens.
The channel can also
close (deactivate) by negative voltages that restore the
down position of S4 and close the gate.

24. Types of V.G Ion Channels
4 major types:
V.G Sodium Channels
V.G Calcium Channels
V.G Potassium Channels
V.G Chloride Channels

25. Voltage Gated Sodium Channels
The founding member of the ion channel superfamily
in terms of its discovery as a protein is the voltage
gated sodium channel. These channels are
responsible for the rapid influx of sodium ions that
underlies the rising phase of the action potential in
nerve, muscle, and endocrine cells.
Sodium channel composed of one principal alpha
subunit and one or two auxiliary beta subunits.

26. The a subunits of sodium channels are composed of
four homologous domains that each contain six
transmembrane segments.
Different distinct neurotoxin binding sites have been
identified within the Na channel protein, with
different effects on ion permeation and gating
resulting in either inhibition or enhancement of Na
currents.

27. Channelopathies
Extensive research has been done and is continued on
ion channels. Ion channels are a favorite site for
invention of new drugs.
Moreover many genetic disorders are found to be
caused by defective channel proteins. In addition to
that, many toxins and venoms produced by spiders,
snakes, scorpion, bees, fish, snails and others act by
incapacitating ion channels.

28. Paramyotonia Congenita (PMC)
Paramyotonia Congenita (PMC) is one of the periodic
paralyses caused by mutations in (alpha-1 subunit)
the sodium channel. PMC causes muscle stiffness
(myotonia) which is made worse by chilling or
activity.
Mechanism: In Paramyotonia the sodium channels
fail to regulate the flow of ions properly. At first this
imbalance causes the muscle fiber to contract
uncontrollably, but as the imbalance worsens the
muscle stops responding to nerve signals and
becomes weak or paralyzed.

29. This can clearly be seen in an EMG measurement of
muscle from a PMC patient.
Treatment:
Tocainide is a new
antiarrhythmic agent
which seems to
reduce effectively
sodium conductance.

30. Brugada syndrome
The Brugada syndrome is a genetic disease that is
characterized by abnormal electrocardiogram (ECG)
findings and an increased risk of sudden cardiac
death. It is the major cause of sudden death in
young’s and termed as sudden unexplained/ adult
death syndrome (SUDS) or (SADS).

31. Genetics:
The cases of Brugada syndrome have been shown to be
associated with mutations (loss of function
mutation) in the gene (named SCN5A of alpha
subunit) that encodes for the sodium ion channel in
the cell membranes of the muscle cells of the heart
(the myocytes).

32. Treatment: The cause of death in Brugada syndrome
is ventricular fibrillation. These arrhythmias appear
with no warning. While there is no exact treatment
modality, treatment lies in termination of this
lethal arrhythmia before it causes death.
This is done via implantation of an implantable
cardioverter-defibrillator(ICD), which continuously
monitors the heart rhythm and will defibrillate an
individual if ventricular fibrillation is noted.

33. Voltage Gated Calcium Channels
Voltage-gated calcium channels mediate calcium
influx in response to membrane depolarization and
regulate intracellular processes such as contraction,
secretion, neurotransmission.
Like sodium channels, the α1 subunit of voltage gated
calcium channels is organized in four homologous
domains (I-IV), with six transmembrane segments
(S1-S6) in each.
An intracellular β subunit and a transmembrane,
disulfide-linked α2β subunit complex are
components of most types of calcium channels.

34. A γ subunit has also been found in skeletal muscle
calcium channels, and related subunits are expressed
in heart and brain.
There are several different kinds of high-voltage-gated
calcium channels (VGCCs).
N-type channel (Most often
found throughout the brain
and peripheral nervous
system).

35. P/Q-type channel (Purkinje neurons in the
cerebellum)
L-type (Skeletal muscle, smooth muscle, bone
(osteoblasts), ventricular myocytes (responsible for
prolonged action potential in cardiac cell), dendrites
and dendritic spines of cortical neurones).
L-type channels are responsible for excitation-contraction
coupling of skeletal, smooth, and cardiac
muscle and for hormone secretion in endocrine cells.

36. Malignant Hyperthermia
MH is a life-threatening clinical syndrome of
hypermetabolism involving the skeletal muscle. It is
triggered in susceptible individuals primarily by the
volatile inhalational anesthetic agents (Halothane)
and the muscle relaxant succinylcholine.

37. Genetics:
The defect is typically located on chromosome
19 (involving the ryanodine receptor) located in the
N-terminus of the protein, which interacts with L-type
calcium channels. This region is important for
allowing Ca2+ passage through the protein following
opening.

39. Treatment: During an episode of malignant
hyperthermia, wrapping the patient in a cooling
blanket can help reduce fever and the risk of serious
complications.
The current treatment of choice is the intravenous
administration of Dantrolene, the only known
antidote, and supportive therapy directed at
correcting hyperthermia.

40. Migraine
Calcium channel blockers are effective second-line
agents. They are a viable alternative in patients who
cannot tolerate β-blockers.
Mechanism: Results of recent studies suggest that
cerebral blood flow during the initial phase of
migraine is decreased and this decrease probably
leads to ischemia and hypoxia. Cellular hypoxia, in
turn, can cause an increase in the flow of calcium,
resulting in calcium overload and cellular
dysfunction.

41. Nimodipine, a calcium-channel blocker that exhibits
selective effects on cerebral vessels, seems to offer
protection against the cerebral ischemia and hypoxia
presumed to be operative during migraine attacks.

42. Analgesic Activity of CCB
Ziconotide (Prialt) is the synthetic form of the N-type
Ca2+ channel blocker in the final stages of
clinical development, a peptide toxin derived from
a marine cone snail.
In humans, spinal infusion of Prialt produces
significant pain relief in patients with intractable
pain associated with cancer, AIDS and in some
neuropathic pain conditions.

43. Voltage Gated Potassium Channels
Potassium channels are the most widely distributed
type of ion channel and are found in virtually all
living organisms.
The α1 subunit of voltage gated potassium channels is
organized in four homologous domains (I-IV), with
six transmembrane segments (S1-S6) in each.
The core of the channel consists
of helices 5 & 6 & the
intervening H5 segment of
each of the 4 copies of the protein.

44. Mutation studies showed that the H5 segment is
essential for K+ selectivity.
Potassium channels act to set or reset the resting
potential in many cells. In excitable cells, such
as neurons, the delayed counter flow of potassium
ions shapes the action potential.
They also regulate cellular processes such as the
secretion of hormones (e.g., insulin release
from beta-cells in the pancreas) so their malfunction
can lead to diseases.

45. Congenital Hyperinsulinism
Congenital hyperinsulinism is a condition that causes
individuals to have abnormally high levels of insulin.
People with this condition have frequent episodes of
low blood sugar (hypoglycemia). These conditions
are present at birth but milder forms may not be
detected until adult years.

46. Mechanism:
In approximately half of people with congenital
hyperinsulinism, the cause is unknown. Mutations in
genes that regulate the release (secretion) of insulin,
which is produced by beta cells in the pancreas is the
proposed cause.

47. Treatment:
Diazoxide and octreotide are the primary
medications used in long-term treatment of CHI.

48. Multiple sclerosis
Multiple sclerosis (MS) is an inflammatory disease of
CNS characterized by demyelination of axons.
Uregualtion of V.G Potassium channels increase the
autoimmune disease process of MS.
The idea that neurologic function might be improved if
conduction could be restored in CNS demyelinated
axons led to the testing of potassium channel
blockers as a symptomatic treatment.

49. To date, only 2 broad-spectrum K+ channel blockers,
4-aminopyridine (4-AP) and 3,4-diaminopyridine
(3,4-DAP), have been tested in MS patients.
Although both 4-AP and 3,4-DAP produce clear
neurologic benefits, their use has been limited due to
toxicity.

50. Epilepsy
 Instead of blocking excitatory ion channels, another
potentially antiexcitable strategy is to enhance the
activity of Kv channels. The past decade has seen
increased interest in Kv channel opening as
antiepileptic mechanism with focus on Kv channels.
The leading compound in the pipeline is
Retigabine which activates neuronal Kv channels.
Retigabine has successfully completed Phase III trial
and is presently awaiting approval as an
antiepileptic.

51. Voltage Gated Chloride Channels
Chloride channels are a superfamily of poorly
understood ion channels.
CLC is involved in setting and restoring the resting
membrane potential of skeletal muscle, while other
channels play important parts in solute
concentration mechanisms in the kidney.
A number of human disease-causing mutations have
been identified in the genes encoding CLCs. These
mutations have been demonstrated to reduce or
abolish CLC function.

52. Cystic Fibrosis
Cystic fibrosis transmembrane conductance
regulator (CFTR) is a membrane protein that is
encoded by the CFTR gene. Mutations of the CFTR
gene affecting chloride ion channel function lead to
dysregulation of epithelial fluid transport in the lung,
pancreas and other organs, resulting in cystic
fibrosis.

53. Treatment:
Respiratory therapy is any treatment that slows
down lung damage and improves breathing.
 FDA has just approved a new drug called Ivacaftor
that will almost certainly be a godsend for 4% of
cystic fibrosis (CF) sufferers.

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phenotypes of a common channelopathy.
Neurology.2009;72(3):208–9.
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