Sodium channels are
integral membrane proteinAn Integral Membrane Protein is a protein molecule that is permanently attached to the biological membrane...
s that form ion channels, conducting sodium ions (
Na+Sodium is a metallic element with a symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals within "group 1"...
) through a
cell'sThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
plasma membrane. They are classified according to the trigger that opens the channel for such ions, i.e. either a voltage-change (voltage-gated sodium channels) or binding of a substance (a
ligandIn chemistry, a ligand is either an atom, ion, or molecule that binds to a central metal-atom to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The metal-ligand bonding ranges from covalent...
) to the channel (ligand-gated sodium channels).
In excitable cells such as
neuronA neuron is an excitable cell in the nervous system that processes and transmits information by electrochemical signaling. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves...
s,
myocytesMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
, and certain types of glia, sodium channels are responsible for the rising phase of action potentials.
Structure
Sodium channels consist of a large α subunit which associates with other proteins, such as β subunits. An α subunit forms the core of the channel and is functional on its own. When the α subunit protein is expressed by a cell, it is able to form channels which conduct Na
+ in a voltage-gated way, even if β subunits or other known modulating proteins are not expressed. When accessory proteins assemble with α subunits the resulting complex can display altered voltage dependence and cellular localization.
The α-subunit has four repeat domains, labeled I through IV, each containing six membrane-spanning regions, labeled S1 through S6. The highly
conservedConservation may refer to:* Conservation genetics - "an interdisciplinary science that aims to apply genetic methods to the conservation and restoration of biodiversity."...
S4 region acts as the channel's voltage sensor. The voltage sensitivity of this channel is due to positive amino acids located at every third position. When stimulated by a change in transmembrane voltage, this region moves toward the extracellular side of the cell membrane, allowing the channel to become permeable to ions. The ions are conducted through a pore, which can be broken into two regions. The more external (i.e., more extracellular) portion of the pore is formed by the "P-loops" (the region between S5 and S6) of the four domains. This region is the most narrow part of the pore and is responsible for its ion selectivity. The inner portion (i.e., more cytoplasmic) of the pore is formed by the combined S5 and S6 regions of the four domains. The region linking domains III and IV is also important for channel function. This region plugs the channel after prolonged activation, inactivating it.
Gating
Voltage-gated sodium channels have three types of states: deactivated (closed), activated (open), and inactivated (closed). Channels in the deactivated state are thought to be blocked on their intracellular side by an "activation gate", which is removed in response to stimulation that opens the channel. The ability to inactivate is thought to be due to a tethered plug (formed by domains III and IV of the alpha subunit), called an inactivation gate, that blocks the inside of the channel shortly after it has been activated. During an action potential the channel remains inactivated for a few milliseconds after depolarization. The inactivation is removed when the membrane potential of the cell repolarizes following the falling phase of the action potential. This allows the channels to be activated again during the next action potential. Genetic diseases that alter sodium channel inactivation can cause muscle stiffness or epileptic seizures because of the introduction of a so-called window current, during which sodium channels are tonically active, causing muscle and/or nerve cells to become over-excited.
The temporal behaviour of sodium channels can be modeled by a
MarkovianA hidden Markov model is a statistical model in which the system being modeled is assumed to be a Markov process with unobserved state. An HMM can be considered as the simplest dynamic Bayesian network....
scheme or by the
Hodgkin-HuxleyThe Hodgkin–Huxley model is a scientific model that describes how action potentials in neurons are initiated and propagated.It is a set of nonlinear ordinary differential equations that approximates the electrical characteristics of excitable cells such as neurons and cardiac myocytes.Alan Lloyd...
-type formalism. In the former scheme, each channel occupies a distinct
stateIn physics,the term state is used in several related senses,each expressing something about the way a physical system is.#In the sense state of matter, state describes the organization of matter in a phase....
with
differential equationA differential equation is a mathematical equation for an unknown function of one or several variables that relates the values of the function itself and its derivatives of various orders...
s describing transitions between states; in the latter, the channels are treated as a
populationIn statistics, a statistical population is a set of entities concerning which statistical inferences are to be drawn, often based on a random sample taken from the population. For example, if we were interested in generalizations about crows, then we would describe the set of crows that is of...
that are affected by three independent gating variables. Each of these variables can attain a value between 1 (fully permeant to ions) and 0 (fully non-permeant), the product of these variables yielding the percentage of conducting channels.
Impermeability to other ions
The
porePore may refer to:In animal biology and microbiology:* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat...
of sodium channels contains a selectivity filter made of negatively charged
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
residues, which attract the positive Na
+ ion and keep out negatively charged ions such as
chlorideThe chloride ion is formed when the element chlorine picks up one electron to form an anion Cl
−...
. The cations flow into a more constricted part of the pore that is 0.3 by 0.5 nm wide, which is just large enough to allow a single Na
+ ion with a
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
moleculeA molecule is defined as an electrically neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds. Molecules are distinguished from polyatomic ions in this strict sense...
associated to pass through. The larger K
+ ion cannot fit through this area. Differently sized ions also cannot interact as well with the negatively charged glutamic acid residues that line the pore.
Diversity
Voltage-gated sodium channels normally consist of an alpha subunit which forms the ion conduction pore and one to two beta subunits which have several functions including modulation of channel gating. Expression of the alpha subunit alone is sufficient to produce a functional channel.
Alpha subunits
The family of sodium channels has nine known members, with amino acid identity >50% in the transmembrane and extracellular loop regions. A standardized nomenclature for sodium channels is currently used and is maintained by the IUPHAR.
The proteins of these channels are named Na
v1.1 through Na
v1.9. The gene names are referred to as SCN1A through SCN11A (the SCN6/7A gene is part of the Na
x sub-family and has uncertain function). The likely evolutionary relationship between these channels, based on the similarity of their amino acid sequences, is shown in figure 1. The individual sodium channels are distinguished not only by differences in their sequence but also by their kinetics and expression profiles. Some of this data is summarized in table 1, below.
Table 1. Nomenclature and some functions of voltage-gated sodium channel alpha subunits
| Protein name | Gene | Expression profile | Associated human channelopathies Channelopathies are diseases caused by disturbed function of ion channel subunits or the proteins that regulate them. These diseases may be either congenital or acquired .There are a large number of distinct dysfunctions known to be caused by ion channel...
|
|---|
| Nav1.1 Nav1.1, also known as the sodium channel, voltage-gated, type I, alpha subunit , is a protein which in humans is encoded by the SCN1A gene.-Function:...
|
| Central neurons The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains...
, [Peripheral Neurons] and cardiac myocytes |
Inherited febrile epilepsyEpilepsy is a common chronic neurological disorder characterized by recurrent unprovoked seizures...
, GEFS+Generalized epilepsy with febrile seizures plus is a syndromic autosomal dominant disorder where afflicted individuals can exhibit numerous epilepsy phenotypes. This disorder is differentiated from generalized epilepsy with febrile seizures as GEFS+ can persist beyond early childhood...
and myoclonic epilepsyMyoclonic epilepsy refers to a family of epilepsies which present with myoclonus.They are divided into two main families:* progressive myoclonic epilepsy* juvenile myoclonic epilepsy...
|
| Nav1.2 Navα1.2, also known as the sodium channel, voltage-gated, type II, alpha subunit is a human protein encoded by the SCN2A gene. Functional sodium channels contain an ion conductive alpha subunit and one or more regulatory beta subunits...
|
| Central neurons, peripheral neurons |
inherited febrile seizure A febrile seizure, also known as a fever fit or febrile convulsion, is a convulsion triggered by a rise in body temperature. They most commonly occur in children between the ages of 3 months and 5 years and are twice as common in boys than girls...
s and epilepsyEpilepsy is a common chronic neurological disorder characterized by recurrent unprovoked seizures...
|
| Nav1.3 Sodium channel, voltage-gated, type III, alpha subunit is a human gene encoding the Nav1.3 protein,.- External links :...
|
| Central neurons, peripheral neurons and cardiac myocytes |
none known |
| Nav1.4 |
| Skeletal muscle Skeletal muscle is a form of striated muscle tissue existing under control of the somatic nervous system. It is one of three major muscle types, the others being cardiac and smooth muscle. As its name suggests, skeletal muscle is linked to bone by bundles of collagen fibers known as...
|
hyperkalemic periodic paralysis Hyperkalemic periodic paralysis , also known as Impressive Syndrome, is an inherited autosomal dominant disorder which affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood...
, paramyotonia congenitaParamyotonia Congenita , also known as Paramyotonia congenita of von Eulenburg or Eulenburg disease, is a rare congenital autosomal dominant neuromuscular disorder characterized by “paradoxical” myotonia...
, and potassium-aggravated myotonia Potassium-aggravated myotonia is a rare genetic disorder that affects skeletal muscle. Beginning in childhood or adolescence, people with this condition experience bouts of sustained muscle tensing that prevent muscles from relaxing normally...
|
| Nav1.5 |
| Cardiac myocytes, uninnervated skeletal muscle, central neurons |
Long QT syndrome The long QT syndrome is a rare congenital heart condition with delayed repolarization following depolarization of the heart, associated with syncope due to ventricular arrhythmias, possibly of type torsade de pointes, which can deteriorate into ventricular fibrillation and ultimately sudden death...
, Brugada syndromeThe Brugada syndrome is a genetic disease that is characterised by abnormal electrocardiogram findings and an increased risk of sudden cardiac death...
, and idiopathic ventricular fibrillationVentricular fibrillation is a condition in which there is uncoordinated contraction of the cardiac muscle of the ventricles in the heart, making them tremble rather than contract properly...
|
| Nav1.6 Sodium channel, voltage gated, type VIII, alpha subunit also known as SCN8A or Nav1.6 is a protein which in humans is encoded by the SCN8A gene.-Further reading:...
|
| Central neurons, dorsal root ganglia, peripheral neuronsThe peripheral nervous system resides or extends outside the central nervous system , which consists of the brain and spinal cord. The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain...
, heart, glia cells |
none known |
| Nav1.7 Nav1.7 is a sodium ion channel which in humans is encoded by the gene. It is usually expressed at high levels in two types of neurons, the nociceptive dorsal root ganglion neurons and sympathetic ganglion neurons, which are part of the autonomic nervous system.- Function...
|
| Dorsal root ganglia, sympathetic neurons, Schwann cells, and neuroendocrine cells |
erythromelalgia Erythromelalgia, also known as acromelalgia, Mitchell's disease , red neuralgia, or erythermalgia, is a rare neurovascular peripheral nerve disorder in which blood vessels, usually in the lower extremities, are episodically blocked and inflamed...
, PEPDParoxysmal extreme pain disorder , originally named familial rectal pain syndrome, is a rare disorder whose most notable features are pain in the mandibular, ocular and rectal areas as well as flushing. PEPD often first manifests at the beginning of life, perhaps even in utero, with symptoms...
and channelopathy-associated insensitivity to painCongenital insensitivity to pain , also known as congenital analgia, congenital analgesia and congenital pain insensitivity, is one or more rare conditions where a person cannot feel physical pain...
|
| Nav1.8 Sodium channel, voltage gated, type X, alpha subunit also known as SCN10A or Nav1.8 is a protein which in humans is encoded by the SCN10A gene.Nav1.8 is a sodium channel subunit.-Function:...
|
| Dorsal root ganglia |
none known |
| Nav1.9 Sodium channel, voltage-gated, type XI, alpha subunit also known as SCN11A or Nav1.9 is a voltage-gated sodium ion channel protein which in humans is encoded by the SCN11A gene....
|
| Dorsal root ganglia |
none known |
| Nax Sodium channel protein type 7 subunit alpha is a protein that in humans is encoded by the SCN7A gene.-See also:* Sodium channelScn7a is the name of the gene that encodes to a membrane protein, in particular a Sodium Channel Nax It belongs to a family of Sodium Channel known as...
|
| heart, uterus, skeletal muscle, astrocytes, dorsal root ganglion cells |
none known |
Beta subunits
Sodium channel beta subunits are type 1 transmembrane glycoproteins with an extracellular N-terminus and a cytoplasmic C-terminus. As a member of the Ig superfamily, beta subunits contain a prototypic V-set Ig loop in their extracellular domain. Interestingly, beta subunits share no homology with their counterparts of calcium and potassium channels. Instead, they are homologous to neural cell adhesion molecules (CAMs) and the large family of L1 CAMs. There are four distinct betas named in order of discovery: SCN1B, SCN2B, SCN3B, SCN4B (table 2). Beta 1 and beta 3 interact with the alpha subunit non-covalently while beta 2 and beta 4 associate with alpha via disulfide bond..
Role of Beta Subunits as Cell Adhesion Molecules
In addition to regulating channel gating, sodium channel beta subunits also modulate channel expression and form links to the
intracelluarNot to be confused with intercellular, meaning "between cells".In cell biology, molecular biology and related fields, the word intracellular means "inside the cell".It is used in contrast to extracellular...
cytoskeletonThe cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm. The cytoskeleton is present in all cells; it was once thought this structure was unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
via
ankyrinAnkyrins are a family of adaptor proteins that mediate the attachment of integral membrane proteins to the spectrin-actin based membrane skeleton. Ankyrins have binding sites for the beta subunit of spectrin and at least 12 families of integral membrane proteins...
and
spectrinSpectrin is a cytoskeletal protein that lines the intracellular side of the plasma membrane of many cell types in pentagonal or hexagonal arrangements, forming a scaffolding and playing an important role in maintenance of plasma membrane integrity and cytoskeletal structure...
.
Voltage-gated sodium channels also assemble with a variety of other proteins, such as FHF proteins (
Fibroblast grwoth factor
Homologous
Factor), calmodulin, cytoskeleton or regulatory kinases which form a complex with sodium channels, influencing its expression and/or function. Several beta subunits interact with one or more
extracellular matrixIn biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
(ECM) molecules. Contactin, also known as F3 or F11, associates with beta 1 as shown via co-immunoprecipitation.
FibronectinFibronectin is a high-molecular weight extracellular matrix glycoprotein that binds to membrane-spanning receptor proteins called integrins. In addition to integrins, fibronectin also binds extracellular matrix components such as collagen, fibrin and heparan sulfate proteoglycans Fibronectin is a...
-like (FN-like) repeats of
TenascinTenascins are extracellular matrix glycoproteins. They are abundant in the extracellular matrix of developing vertebrate embryos and they reappear around healing wounds and in the stroma of some tumors.-Types:...
-C and
TenascinTenascins are extracellular matrix glycoproteins. They are abundant in the extracellular matrix of developing vertebrate embryos and they reappear around healing wounds and in the stroma of some tumors.-Types:...
-R bind with beta 2 in contrast to the Epidermal
Growth FactorA growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....
-like (EGF-like) repeats that repel beta2. A disintegrin and metalloproteinase (ADAM) 10 sheds beta 2's
ectodomainAn ectodomain is the part of a membrane protein that extends into the extracellular space . Ectodomains are usually the part of a protein that initiate contact with surface which leads to signal transduction. In SARS-CoV the ectodomain of the spike protein is responsible for attachment to and entry...
possibly inducing neurite outgrowth. Beta 3 and beta 1 bind to neurofascin at Nodes of Ranvier in developing neurons.
Table 2. Nomenclature and some functions of voltage-gated sodium channel beta subunits
| Protein name | Gene link | Assembles with | Expression profile | channelopathies Channelopathies are diseases caused by disturbed function of ion channel subunits or the proteins that regulate them. These diseases may be either congenital or acquired .There are a large number of distinct dysfunctions known to be caused by ion channel...
>|-
| Navβ1Sodium channel subunit beta-1 is a protein that in humans is encoded by the SCN1B gene....
|
|---|
| Nav1.1 to Nav1.7 |
Central Neurons, Peripheral Neurons, skeletal muscle, heart, glia |
|
>-
| Navβ2Sodium channel subunit beta-2 is a protein that in humans is encoded by the SCN2B gene....
| Nav1.1, Nav1.2, Nav1.5 to Nav1.7 |
Central Neurons, peripheral neurons, heart, glia |
|
>-
| Navβ3Sodium channel subunit beta-3 is a protein that in humans is encoded by the SCN3B gene....
| Nav1.1 to Nav1.3, Nav1.5 |
central neurons, adrenal gland, kidney, peripheral neurons |
|
>-
| Navβ4Sodium channel, also known as SCN4B is a protein which in humans is encoded by the SCN4B gene.SCN4B is a sodium channel associated with long QT syndrome....
| Nav1.1, Nav1.2, Nav1.5 |
heart, skeletal muscle, central and peripheral neurons |
Ligand-gated
Ligand-gated sodium channels are activated by binding of a
ligandIn chemistry, a ligand is either an atom, ion, or molecule that binds to a central metal-atom to produce a coordination complex. The bonding between the metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The metal-ligand bonding ranges from covalent...
instead of a change in membrane potential.
They are found e.g. in the
neuromuscular junctionA neuromuscular junction is the synapse or junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
as nicotinic receptors, where the ligands are
acetylcholineThe chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans. Acetylcholine is one of many neurotransmitters in the autonomic nervous system and the only neurotransmitter used in the motor division...
molecules. Most channels of this type are permeable to potassium to some degree as well as to sodium.
Role in action potential
- See main article: Action potential
An action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
Voltage-gated sodium channels play an important role in
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
s. If enough channels open when there is a change in the cell's
membrane potentialMembrane potential is the voltage difference between the interior and exterior of a cell...
, a small but significant number of Na
+ ions will move into the cell down their
electrochemical gradientAn electrochemical gradient is a spatial variation of both electrical potential and chemical concentration across a membrane. Both components are often due to ion gradients, particularly proton gradients, and the result can be a type of potential energy available for work in a cell...
, further
depolarizingIn biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
the cell. Thus, the more Na
+ channels localized in a region of a cell's membrane, the faster the action potential will propagate, and the more
excitable that area of the cell will be. This is an example of a positive feedback loop. The ability of these channels to assume a closed-inactivated state causes the refractory period and is critical for the propagation of action potentials down an
axonAn axon or nerve fiber is a long, slender projectionof a nerve cell, or neuron, that conducts electrical impulsesaway from the neuron's cell body or soma....
.
Na
+ channels both open and close more quickly than
K+ channelsIn the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
, producing an influx of positive charge (Na
+) toward the beginning of the
action potentialAn action potential is a transient alteration of the transmembrane voltage across an excitable membrane generated by the activity of voltage-gated ion channels embedded in the membrane. Action potentials play multiple roles in several types of excitable cells such as neurons, myocytes, and...
and an efflux (K
+) toward the end.
Ligand-gated sodium channels, on the other hand, creates the change in the membrane potential in the first place, in response to the binding of a ligand to it.
Activators
The following naturally produced substances persistently activate (open) sodium channels:
- Alkaloid
Alkaloids are naturally occurring chemical compounds containing basic nitrogen atoms. The name derives from the word alkaline and was used to describe any nitrogen-containing base. Alkaloids are produced by a large variety of organisms, including bacteria, fungi, plants, and animals and are part of...
based toxins
- aconitine
Aconitine is a highly poisonous alkaloid derived from various aconite species. It is a neurotoxin that opens TTX-sensitive Na
+ channels in the heart and other tissues, and is used for creating models of cardiac arrhythmia...
- batrachotoxin
Batrachotoxins are extremely potent cardiotoxic and neurotoxic steroidal alkaloids found in certain species of frogs , Melyridae beetles, and birds .- Chemistry :...
- brevetoxin
Brevetoxin , or brevetoxins, are a suite of cyclic polyether compounds produced naturally by a species of dinoflagellate known as Karenia brevis...
- ciguatoxin
thumb|300px|right|Chemical structure of the ciguatoxin CTX1BCiguatoxin is a fish poison which causes ciguatera.There are several types: - Ciguatoxin 1 - Ciguatoxin 2 - Ciguatoxin 3 - Ciguatoxin 4B-Toxic Effect:...
- delphinine
Delphinine is a toxic alkaloid found in plants from the Delphinium family. It is related in structure and has similar effects to aconitine, acting as an allosteric modulator of voltage gated sodium channels, and producing hypotension, bradycardia and cardiac arrythmia...
- grayanotoxin
Grayanotoxin is a toxin found in rhododendrons and other plants of the family Ericaceae. It can be found in honey made from their nectar and cause a very rare poisonous reaction called grayanotoxin poisoning, honey intoxication, or rhododendron poisoning...
- veratridine
Veratridine is a steroid-derived alkaloid from the family of Liliaceae that functions as a neurotoxin by activating sodium ion channels. It is a derivative of veracevine. It is primarily obtained from the herb hellebore and sabadilla seeds. It binds to intramembrane receptor site 2 and...
Gating modifiers
The following toxins modify the gating of sodium channels:
- Peptide
Peptides are short polymers formed from the linking, in a defined order, of α-amino acids. The link between one amino acid residue and the next is called an amide bond or a peptide bond....
based toxins
- μ-conotoxin
A conotoxin is one of a group of neurotoxic peptides isolated from the venom of the marine cone snail, genus Conus.Conotoxins, which are peptides consisting of 10 to 30 amino acid residues, typically have one or more disulfide bonds. Conotoxins have a variety of mechanisms of actions, most of...
- δ-atracotoxin
See also
- Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of all living cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
s
- Calcium channel
A Calcium channel is an ion channel which displays selective permeabiltiy to calcium ions. It is sometimes synonymous as voltage-dependent calcium channel, although there are also ligand-gated calcium channels.-Comparison tables:...
s
- Potassium channel
In the field of cell biology, potassium channels are the most widely distributed type of ion channel and are found in virtually all living organisms. They form potassium-selective pores that span cell membranes...
s
- Resting ion channels
- Epithelial sodium channel
The epithelial sodium channel is a membrane-bound ion-channel that is permeable for Li+-ions, protons and especially Na+-ions. It is a constitutively active ion-channel...