4 - K+ Channels Flashcards
Most voltage-dependent ion channels have a __________ structure.
Most voltage-dependent ion channels have a tetrameric structure.
- 4- fold (4-domains) symmetric structure each with 6 TMD’s linked together via a “linker”
- Domains have a fixed composition and cannot ‘mix’
What is the structure of a K+ channel?
K+ channel genes usually encode only one 6TM domain, so four proteins assemble to form the ion channel
- results in diverse channel functions depending on which subunits assemble
What are three sources of variability of K+ channel function?
- Subunits may come from different genes (heteromeric channels)
- Alternative splice
- different proteins arising from the same genes
- Assembly with modifying subunits (‘accessory subunits’ ; ‘auxiliary’ subunits ; beta subunits)
What are 3 ‘Single’ Action Potential Effects of K+ channels?
- Set resting membrane potential
- influences which stimulus is needed for neuron to fire (sensitivity)
- Influence AP shape
- broad AP controlled by K+ channel
- Repolarize/hyperpolarize membrane
What is the subunit composition of voltage-gated K+ channels? (how many subunits, what arrangement?)
Four-fold (ie four domain) symmetric structure each with 6 membrane spanning regions
K+ channel genes usually encode only one 6TMD so four proteins assemble to form the ion channel = heteromeric channel = great diversity
Modifying subunits include: accessory subunits, auxiliary subunits, beta subunits
Differences between voltage-gated K+ channels vs Voltage-gated Na+ or Ca++ channels (selectivity, subunit arrangement)
Voltage-gated Na+ and Ca++ channels have a tetrameric structure with 4 homologous domains (I, II, III, IV) each with 6 membrane spanning regions. The domains are linked together with a “linker”
These domains have a fixed composition
What is the definition of a delayed rectifier? (why delayed? Why rectifier?)
- Delayed: channels have a delayed sigmoidal opening
- because these channels have many ‘pre-open’ states to go through before opening (all subunits must be activated for pore to open)
- Rectifier: K+ channels preferentially conduct ions OUT of the cell
- 2 reasons:
-
intrinsic voltage dependence Their activity depends on voltage -
- at neg voltages = mostly closed = low conductance
-
concentration gradient
- K+ ions have a much higher concentration inside the cell relative to outside so at all physiological voltages, K+ ions tend to move out of the cell
-
intrinsic voltage dependence Their activity depends on voltage -
- 2 reasons:
- Rectifier: K+ channels preferentially conduct ions OUT of the cell
- 2 reasons:
- Rectifier: K+ channels preferentially conduct ions OUT of the cell
- 2 reasons:
- Their activity depends on voltage -
- at neg voltages = mostly closed = low conductance
- Concentration gradient: K+ ions have a much higher concentration inside the cell relative to outside so at all physiological voltages, K+ ions tend to move out of the cell
- Their activity depends on voltage -
- 2 reasons:
What is a conductance-voltage relationship? What does it describe?
describes the conductance of a channel dependent of various voltages
What controls activity of M- channels?
closed by?
activated by?
M-channels are closed by the muscarinic action of acetylcholine (ACh)
M-channels are activated (slowly) by depolarization
What does closure of M-channel cause?
depolarization and suppression of the mechanism which limits repetitive discharge
- ACh is strongly excitatory in the CNS
- depolarization
- Enhanced repetitive discharge
- Increased excitability
M-channels require _____ to active
M-channels require PIP2 to active – cleaving pip2 shuts down the channel
ACh binding to M-channel cleaves PIP2
_______ is a first-in-class potassium channel opener
Non-selective activator of neuronal ______ channels
Retigabine is a first-in-class potassium channel opener
Non-selective activator of neuronal KCNQ(2-5) (encode M-channels in NS) channels
What happens to RMP when M-channels are inhibited by ACh? How does this effect AP?
RMP is higher because M-channels typically contribute to RMP
Neuron is hyperexcitable = bursting pattern (repetitive firing)
What controls activity of BK channels? What underlies Ca++ sensitivity?
BK channels depend on both depolarization and internal [Ca++]
Ca++ sensing mechanism is intrinsic to the channel (encoded by channel gene) ie intracellular Ca++ sensing domain
What controls the activity of SK channels? What underlies Ca++ sensitivity?
SK channels are Ca++ sensitive K+ channels
- Weakly voltage dependent
- Strongly Ca++ dependent
- Calcium sensitivity due to association with calmodulin (not encoded by the channel gene)
Ca++ sensitivity of SK channels is due to:
Intracellular binding of Calmodulin - calmodulin senses Ca++
SK channels are important for which phase of the action potential?
After hyperpolarization phase
AHP - effects further AP firing
In absence of Ca++ what happens to the AHP?
After hyperpolarization phase
Absence of Ca++ = loss of AHP
IAHP (SK) channels generate _______ action potentials
IAHP (SK) channels generate slow afterhyperpolarization of action potentials
= reduces excitability and eventually prevents firing
IAHP (SK) channels are blocked by ________
IAHP (SK) channels are blocked by apamin
What is spike frequency adaptation?
Activation of specific Ca++ activated potassium channels (SK (IAHP)) results in termination of a burst of action potentials
How is K,Ca channel (Ca++ activated K+ channel eg IAHP (SK)) altered during repeated AP’s?
Influx of Ca++ leads to activation of K+ conductance that reduces excitability and eventually prevents firing = Spike frequency adaptation
Activation of Ca++ activated K+ channels results in termination of a burst of AP’s
IAHP can be responsible for ________ (terminate bursts) - generate rhythmic activity in neurons
IAHP can be responsible for bursting pacemakers (terminate bursts) - generate rhythmic activity in neurons
At negative voltages an excitatory current is activated leading to subsequent burst