Ion Channels 101 Flashcards
What is the membrane potential?
Voltage difference across cell membrane
Maintained by ion pumps and influenced by ion distribution
What does RMP stand for and what is its typical value in neurons?
Resting Membrane Potential, approximately -70 mV
It is influenced by the leak of potassium and fixed negative charges
Define hyperpolarization
Membrane potential more negative than RMP
Define depolarization
Membrane potential more positive than RMP
What is an electrochemical gradient?
Combination of chemical gradient (due to concentration differences) and electrical gradient (due to membrane potential)
I.e. Na+ ions are driven into a cell by both their high extracellular concentration (chemical gradient) and their negative charge inside the cell (electrical gradient)
Influences direction and rate of ion movement
What is current in the context of ion movement?
Flow of charged ions across the membrane driven by the electrochemical gradient
measured in amps (A) and represented by I
What does conductance refer to?
How easily ions can flow through an ion channel, reciprocal of resistance
Denoted using G, e.g., GNa
What are BK channels?
Big conductance potassium channels
What is rectification in ion channels?
Channel conducts better in one direction than the other depending on membrane potential
Inward rectification: better at more negative potentials; Outward rectification: better at more positive potentials
What is a carrier/transporter protein’s maximum transport rate?
Approximately 10,000 ions per second
What is the maximum transport rate of Na+/K+ ATPase?
Approximately 1,000,000 ions per second
Define selective permeability in ion channels
Ability to allow certain ions to pass while blocking others
Includes cations like K+, Na+, Ca2+ and anions like Cl-
What types of gating mechanisms exist for ion channels?
- Second messenger
- Ligand-gated
- Voltage-gated
- G protein-gated
What characterizes ion channels?
Transmembrane proteins that are selectively permeable and have controlled openings
How are ion channels named?
Characterized by gating and ion selectivity
For ligand-gated channels, named after natural ligand (e.g., GABAA receptor)
What are the typical ionic gradients of a mammalian cell?
Na+ high outside, K+ high inside, Cl- high outside, Ca2+ low inside
What structure do voltage-gated potassium channels have?
Tetramer of four equivalent subunits = alpha subunits
Can be homotetramers or heterotetramers
Has 6 TM domains with the 4th one as a voltage centre and a membrane dipping domain (forms channel lining) between the 5th and 6th
How many Kv channel genes are there in the human genome?
40 Kv channel genes
What is significant about the evolution of KV channels?
KV channels appear early in evolution and are present in prokaryotes
What is the function of voltage-gated sodium channels?
Depolarization phase of neuronal and skeletal muscle action potential
What is the role of voltage-gated potassium channels?
Repolarization phase of action potentials and inhibitory responses to neurotransmitters
What is the significance of voltage-gated calcium channels?
Depolarization phase of cardiac action potentials and vesicular release
True or False: Voltage-gated channels are always open.
False
Most channels are closed most of the time.
What therapeutic applications do calcium channel blockers have?
Used for hypertension and angina
List the functional subtypes of calcium channels.
- L type (CaV 1.1-1.4)
- P/Q type (CaV 2.1)
- N type (CaV 2.2)
- R type (CaV 2.3)
- T type (CaV 3.1-3.3)
What is the significance of the selectivity filter in ion channels?
Allows permeant ions to pass while blocking impermeant ions
Fill in the blank: The flow of sodium into the cell is an ______.
inward current
What percentage of human drug targets are ion channels?
11%
What are carrier/transporter proteins?
Function by binding a substrate on one side of the membrane then a transporter binds to the substrate and undergoes conformational change = flips the substrate to the other side & releases it
Can be passive or active transport
E.g. Na+/K+ ATPase
Describe how the ion channels have selective permeability
They have a selectivity filter which selects ions based on charge and size
Rings of charge around the selectivity filter which has the opposite charge to the type of ion channels it’s selective for & it repels ions of the wrong charge
Evolution of the CaV and NaV channels
- A single KV gene undergoes gene duplication
- Forms a 2 pore channel which undergoes further gene duplication = gets mutated over time so is not identical to the original K+ channel
- Results in 4 copies of the original gene strung together
Structure of CaV and NaV channels
- Instead of having 4 separate subunits that come together, the alpha subunit folds so that the 4 pseudo-subunits form the channel
- Pseudo-subunits relate back to the original voltage gated K+ channel
- Alpha subunit is the pore forming subunit and consists of 4 copies of a KV channel-like structure strung together in a single polypeptide
Translation and assembly of KV channels
- Potassium channel alpha subunit
- 6 TM domains
- 1 voltage sensing domain (4th TM domain)
- 1 pore forming domain (between the 5th and 6th TM domain)
4 copies of a potassium channel subunit is translated from mRNA and come together to form a channel
Translation and assembly of CaV and NaV channels
- Sodium/calcium channel alpha subunit is 4x bigger than that of the K+ channel
- 4 pseudo-subunits in a single polypeptide chain with each subunit similar to a KV channel subunit and containing a voltage sensor & pore forming domain
- A single sodium or calcium channel alpha subunit is translated from mRNA and the 4 pseudo-subunits fold to form a channel
- The 4 pseudo-subunits act in a similar way to the separate subunits of the K+ channel
Calcium channel subunits
10 different Ca2+ channel alpha subunits which are divided into 3 families
- α CaV 1.1-1.4
- α CaV 2.1-2.3
- α CaV 3.1-3.3
4β 4α2δ 8γ subunits
Native channel (natural state) = contains small accessory subunits that modify the function of the alpha subunit
- Possibly = 1α:1β:1α2δ ?1γ
Sodium channel subunits
9 different Na+ channel alpha subunits
- α NaV 1.1-1.9
4β subunits
Native channel possibly
1α:1/2β
α2δ subunits
2 separate proteins formed from a single polypeptide
Important as a drug target as it’s the site of action in some drugs used to treat epilepsy, anxiety and pain
- Gene produces a single polypeptide
- Disulfide bond formed in extracellular domain
- Extracellular domain is cleaved to yield 2 linked peptides
Calcium channel subtypes
Categorised into 5 main families
L-type
- Long lasting
- Alpha subunits CaV 1.1-1.4
- Located in the heart, smooth muscle, CNS, retina and cochlear hair cells
P/Q type
- Purkinje
- Alpha subunits CaV 2.1
- Located in neurons
N type
- Neuronal
- Alpha subunit CaV 2.2
- Located in neurons
R type
- Residual
- Alpha subunit CaV 2.3
- Located in neurons
T type
- Transient
- Alpha subunits CaV 3.1-3.3
- Located in neurons, heart and smooth muscle
What are the 3 main states of voltage gated ion channels?
- Resting (at -70 mV)
- Open
- Inactivated
Transition between the states is dependent on membrane potential
Describe the state of the channel at resting membrane potential
- High Na+ outside the cell
- Activation gate is closed to prevent Na+ crossing the membrane
- Inactivation gate = ball and chain like structure
Describe the state of the channel at threshold potential (-55 mV)
- Na+ channels start to open
- Gating mechanism = opening of channel is caused by movement of the voltage sensor
Describe the state of the channel when membrane potential is positive
- Channel inactivation
- As more Na+ enters, the membrane potential becomes more positive
- The inactivation gate swings up to block the inner mouth of the channel = Na+ can’t pass through
- Inactivation in other channels such as Ca2+ and K+ channels is more complex
Physiological significance of voltage gated ion channels
- NaV channels —> depolarisation phase of neuronal & skeletal muscle action potential
- KV channels —> repolarisation phase of action potentials (2nd phase) and inhibitory responses to neurotransmitters
- CaV channels —> depolarisation phase of cardiac action potentials (excitatory phase), vesicular release of neurotransmitters and excitation-contraction coupling for muscle contraction
Therapeutic importance of voltage gated ion channels
- Hypertension —> Calcium channel blockers = work by relaxing smooth muscle in blood vessels, increasing their diameter
- Pain —> Sodium channel blockers (e.g. in dental surgery) and calcium channel blockers = derived from snail toxin and is injected into the spinal cord to treat chronic pain
- Disorders of cardiac rhythm —> Calcium, sodium and potassium channel blockers
- Angina (O2 deprivation to heart muscle) —> Calcium channel blockers
