By – SARVESH MAURYADR.L.H.HIRANANDANI COLLEGE OF PHARMACY.

MOLECULAR ARCHITECTURE OF ION CHANNELS

• Ion channels are large and elaborate molecules. All consist of several (often four) domains, which are similar or identical to each other, organised either as an oligomeric array of separate subunits, or as one large protein.

• Each subunit or domain contains a bundle of two to six membrane-spanning helices. Most ligand-gated channels have the basic structure, comprising a pentameric array of non-identical subunits, each consisting of four transmembrane helices.

•The large extracellular N-terminal region contains the ligand-binding region

Types Of Na+ Channels

•Voltage gated –Changes in membrane polarity open the channel

•Ligand gated (nicotinic acetylcholine receptor) – Ligand binding alters channel/receptor conformation and opens the pore

•Mechanically gated (stretch receptor) –Physical torsion or deformation opens the channel pore

Voltage-gated sodium channels:

Introduction

• Voltage-gated sodium channels are responsible for action potential initiation and propagation in excitable cells, including nerve, muscle, and neuroendocrine cell types.

• They are also expressed at low levels in nonexcitable cells, where their physiological role is unclear. Sodium channels are the founding members of the ion channel superfamily in terms of their discovery as a protein and determination of their amino acid sequence.

Sodium Channel Subunits

• Sodium channels consist of a highly processed α subunit, which is approximately 260 kDa, associated with auxiliary β subunits Sodium channels in the adult central nervous system and heart contain β1 through β4 subunits, whereas sodium channels in adult skeletal muscle have only the β1 subunit .

• 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, and these auxiliary subunits are involved in channel localization and interaction with cell adhesion molecules, extracellular matrix, and intracellular cytoskeleton.

•The α subunits are organized in four homologous domains (I-IV), each of which contain six transmembrane α helices (S1-S6) and an additional pore loop located between the S5 and S6 segments (Fig. 1). The pore loops line the outer, narrow entry to the pore, whereas the S5 and S6 segments line the inner, wider exit from the pore. The S4 segments in each domain contain positively charged amino acid residues at every third position.

•These residues serve as gating charges and move across the membrane to initiate channel activation in response to depolarization of the membrane. The short intracellular loop connecting homologous domains III and IV serves as the inactivation gate, folding into the channel structure and blocking the pore from the inside during sustained depolarization of the membrane.

FIG. 1. Transmembrane organization of sodium channel subunits.

•The primary structures of the subunits of the voltage-gated ion channels are illustrated astransmembrane-folding diagrams. Cylinders represent probable α-helical segments. Bold linesrepresent the polypeptide chains of each subunit, with length approximately proportional to thenumber of amino acid residues in the brain sodium channel subtypes.

•The extracellular domains of the β1 and β2 subunits are shown as immunoglobulin-like folds.ψ, sites of probable N-linked glycosylation; P, sites of demonstrated protein phosphorylation byprotein kinase A (circles) and protein kinase C (diamonds); shaded, pore-lining S5-P-S6segments; white circles, the outer (EEDD) and inner (DEKA) rings of amino residues that formthe ion selectivity filter and tetrodotoxin binding site; ++, S4 voltage sensors; h in shadedcircle, inactivation particle in the inactivation gate loop; open shaded circles, sites implicated informing the inactivation gate receptor.

•Sites of binding of α- and β-scorpion toxins and a site of interaction between α and β1subunits are also shown.

Sodium Channel Classification and Nomenclature

In this nomenclature system, the name of an individual channel consists of the chemicalsymbol of the principal permeating ion (Na) with the principal physiological regulator (voltage)indicated as a subscript (NaV).

The number following the subscript indicates the gene subfamily (currently only NaV1), andthe number following the full point identifies the specific channel isoform (e.g., NaV1.1). Thislast number has been assigned according to the approximate order in which each gene wasidentified. Splice variants of each family member are identified by lowercase letters followingthe numbers (e.g., NaV1.1a).

Genetic mutations in sodium channel genes give rise to channelopathies and

clinical disease

Channel

nomenclatureGene

Chromosomal

location

(human)

Tetrodotoxin

sensitivity

Major tissue

expression

Effect of

mutation

Nav1.1 SCN1A 2q24 CNS, PNS Epilepsy

Nav1.2 SCN2A 2q23–24 CNS, PNS Epilepsy

Nav1.3 SCN3A 2q24 CNS, PNS None reported

Nav1.4 SCN4A 17q23–25 Skeletal muscle

Myotonia,

periodic

paralysis

Nav1.5 SCN5A 3p21 Heart

Long QT,

Brugada

syndrome,

progressive

familial heart

block

Nav1.6 SCN8A 12q13 CNS, PNSCerebellar

atrophy

Nav1.7 SCN9A 2q24PNS (SNS and

PAs)

Increased and

decreased pain

sensitivity

Voltage-gated sodium channel nomenclature, chromosomal location and tissue distribution

A restingstate

• when it can respond to a depolarizing voltage changes

Activated• when it allows flow of Na+ ions through the

Inactivated

• when subjected to a “suprathreshold” potential, the channel will not open

Sodium Channels - Function

Can be in different functional states (3)

• Tetrodotoxin (TTX)(blocks voltage gated sodium channel)

• Amioderone (blocks cardiac channel)

• Lidocaine (blocks voltage sodium channel in neuronal cell membrane)

• Procainamide

• Mexilitine

• Ketamine

Na + Channel Blockers/Pharmacological Agents

Tetrodoxin

Saxitoxin, a natural product

from dinoflagellates, acts in a similar way and is also a potent nerve poison."

•Tetrodotoxin is the poison that is produced by the puffer fish and a number of other animals. It is a virulent poison, the LD50 for the mouse is 10 nanograms.• It acts by blocking the conduction of nerve impulses along nerve fibers and axons. The victim eventually dies from respiratory paralysis.

If tetrodotoxin is such a powerful toxin why does it not poison the host?

This is a common question in virtually all cases where a toxin is present.

The sodium ion channel in the host must be different than that of the victim.

It must not be susceptible to the toxin.

It has been demonstrated for one of the pufferfish that the protein of the sodium ion channel has undergone a mutation that changes the amino acid sequence making the channel insensitive to tetrodotoxin.

The spontaneous mutation that caused this structural change is beneficial to the pufferfish because it allowed it to incorporate the symbiotic bacteria and utilize the toxin it produces to its best advantage, A single point mutation in the amino acid sequence of the sodium-ion channel in this species renders it immune from being bound and blockaded by TTX.

Dizziness

Cough

trembling or shaking of the hands

Side effects occur while taking Amiodarone

Common side

efects

lightheadedness, or fainting, Dizziness

unusual and uncontrolled

movements of the body

shortness of

breath

Cough, fever

(slight)

numbness or tingling in the fingers or toes

Less common side effects

Blue-gray coloring of the skin on the face,

neck, and arms

blurred vision or blue-green halos seen around

objects

fast or irregular heartbeat

dry eyes

dry, puffy skin

•Adapted from the original article in Pharmacological Reviews

•REFERENCES

•William A. Catterall, Alan L. Goldin, Stephen G. Waxman.Voltage-gated sodium channels, introductory chapter.