5- Molecular Aspects Of Na And K Channels Flashcards
Ion channels can be grouped according to ____, ____ and ____
Ion channels can be grouped according to properties, pharmacology and sequence homology
Voltage-gated Na+ channels are highly selective for __________
Activate rapidly upon ___________
Inactivate rapidly at __________
- Voltage-gated Na+ channels are highly selective for sodium ions
- Activate rapidly upon depolarization (AP upstroke)
- positive feedback process - Inactivate rapidly at positive membrane potentials
Recovery from inactivation requires __________
Recovery from inactivation requires repolarization of the membrane potential (back to resting MP)
What fish/amphibian toxin(s) target voltage gated Na+ channels?
- Tetrodotoxin, TTX (puffer fish)
- highly selective for neuronal and skeletal muscle Na+ channels
-** inhibitor** - Batrachotoxin (arrow poison frogs)
- activator
Why would Na+ channel inhibitors be used clinically?
Local anesthetics and anti-arrhythmics
eg: Lidocaine
- inhibit rapid firing of Na+ in sensory neurons (frequency dependent)
_____ from anemonia sulcata is a voltage-gated Na+ channel activator that opens neuronal and muscular Nav channels
ATX II from anemonia sulcata is a voltage-gated Na+ channel activator that opens neuronal and muscular Nav channels
How can both Na channel inhibitors and activators lead to rapid paralysis?
Correct neuronal and muscle conduction/contraction relies on proper AP firing
Toxins can either stop AP from firing (inhibit) or prevent it from firing again (activators - na+ needs to recover)
- Na channel inhibitors prevent channels from opening = no AP (eg TTX)
- Na channel activators prevent channels from inactivating = no repolarization = No AP’s (eg ATX II)
What is the structure of Sodium channels?
- Alpha subunit
- encoded by SCNxA gene
- One polypeptide with 4 similar domains each with 6 membrane spanning regions
- Selectivity filter between 5 and 6 of each domain
- “Hinged lid” inactivation gate between 6 of third domain and 1 of 4th domain
- DIII-DIV interface (IFM)
What toxins act on site 1 of the sodium channel?
TTX, STX, u-conotoxins
Site 1 = on selectivity filter of first domain
What toxins act on Site 5 of Na channels?
Brevetoxins
Ciguatoxins
Site 5 is on 6th membrane spanning region of domain I
What toxins act on site 4 of Na channel?
Site four between 3rd and 4th membrane spanning region of domain II
Scorpion-beta-toxins
Spider beta-toxins
What toxins act on Site 2 of Na Channels?
Site 2 = on 6th membrane spanning region of Domain III
Batrachotoxins
Grayanotoxins
What toxins act on site 3 of Na channels?
Scorpion alpha-toxins
Sea anemone toxins
Site three = in btween 3rd and 4th membrane spanning regions of Domain IV
What are the three major types of potassium channel?
- Voltage gated
- Calcium activated
- Inward rectifier
Which potassium channels contribute to slow recovery following AP?
Voltage-gated K channels
Which potassium channels are active after depolarization?
Calcium-activated K channels
Which potassium channels are in charge of maintaining the resting membrane potential?
Inward rectifier k+ channels
Two types of voltage gated K channels:
- Delayed rectifiers
- Kv1.x to 6.x - Calcium activators
- BK, IK, SK
Three types of inward rectifying potassium channels:
- IRK1
- Kir1.x, 2.x, 4.x and 5.x
- GIRK
- Kir3.x and CIR
- KATP
- Kir6.x plus SURx
Why are there so many types of K channels when compared to Na Channels?
The duration of the AP and the RMP are really important in controlling cellular excitability
K channels control both
Lots of K channels to “fine tune” excitability
In contrast, the role of Na channels is mostly limited to initiating the upstroke of the AP only
How does potassium channel activation/inactivation compare to Na channels?
K+ channels activate and inactivate more slowly than Na channels upon depolarization
Three blockers of K channels:
- Tetra-ethyl-ammonium (TEA)
- Dendrotoxin
- Agitoxin
Big/Intermediate Calcium activated K+ channels:
Called: _____ or _____
- respond to ____
- Sensitive to ____
- Blocked by: ____ , ____, ____
Calcium activated K+ channels:
Called: BK/IK or *KCa *
- respond to voltage (ie voltage-dependent)
- Sensitive to micromolar internal calcium
- Blocked by: TEA, ChTX and IbTX (scorpion venom, buthus sp.)
What are the physiological roles of BK/IK or KCa calcium-activated K+ channels?
Repolarization of excitable cells
eg smooth muscle and beta-cells (insulin secretory response)
What are SK channels?
Sensitive to?
Physiological role?
- Small conductance calcium-activated K+ channels
- Sensitive to apamin (bee venom)
- Controls after potential and AP duration
- “shape of AP”
What modifies K+ channel gating (Voltage dependent (Kv) K+ channels)
beta-subunit modifies K+ channel gating
eg Kvbeta or mink
There are 11+ Kv channel families; each contains _______
Subunits co-assemble as ____ to form ______
There are 11+ Kv channel families; each contains several homologues (eg Kv1.1-Kv1.7)
Subunits co-assemble as* tetramers* to form functional channels
- only subunits from the same family can co-assemble
Compare Kv channel assembly to Nav channels
Kv channel assembly: 4 polypeptide subunits co-assemble to form the major alpha subunit tetramer with the major properties of the Kv channel
Only 1 polypeptide subunit is required for Nav channels
What effect do Kv channel inhibitors have?
Kv channel inhibitors favor depolarization = increasing excitability and AP firing rate
What effect do Kv channel activators have?
Kv channel activators favor repolarization = decreasing excitability and AP firing rate
What is the effect of hypo-active neurons?
- Decreased frequency of AP firing
- CNS depression, neuronal-conduction disorders (eg multiple sclerosis) and cognition disorders
What is the effect of hyper-active neurons?
- Increased frequency of AP firing
- CNS hyperexcitability, seizures, pain, ADHD, anxiety, bipolar disorder and schizophrenia
What are the physiological roles of strong inward rectifier K channels?
- Kir2,4,5.x
- Clamp membrane potential near Ek
- Allow small efflux of K ions positive to Ek
- Stabilizes neuronal membrane potential
Properties of Strong inward rectifier K channels:
- Open probability compared to voltage?
- Current flow?
- Strong inward rectification caused by internal block by ____
Properties of Strong inward rectifier K channels:
- Open probability independent of voltage (no S4 segment)
- Current flow passively dependent on direction of K ion flow (ie Ek)
- Strong inward rectification caused by internal block by* polyamines and Mg++ ions*
Weak inward rectifier K channels are always ____
When open, they have what physiological role:
Weak inward rectifier K channels are always gated (usually closed and require activation)
When open, they:
- clamp membrane potential near Ek and affect the AP shape
- Allow larger efflux of K ions pos to Ek (when compared to strong IRKs)
Properties of weak inward rectifier K channels:
- Open probability vs voltage?
- Current flow?
- weak inward rectification caused by:
Properties of weak inward rectifier K channels:
- Open probability independent of voltage (no S4 segment)
- Current flow **passively dependent on direction of K ion flow **(ie Ek)
- weak inward rectification caused by a reduced internal block by polyamines and Mg++ ions (compared to strong IRKs)
Weak IRKs influence the ____ and the ____ (of the AP)
Weak IRKs influence the *membrane potential *and most of the AP
Strong IRKs influence the ____ and ____ (of the AP)
Strong IRKs ONLY influence the resting membrane potential and the “foot” of the AP
The IKACh channel is formed by a ____
Activated by ____ and ____
The IKACh channel is formed by a heterotetramer of Kir3.1 and 3.4 (CIR)
Activated by membrane delimited G-protein activation coupled to muscarinic receptors and acetylcholine and analogs
Provide an example of physiological role of IKACh channel?
IKACh is a weak inward rectifier K channel activated by ACh and analogs
- Found in high density in the SA node, AV node and atrium of the heart. VAGAL nerve stimulation slows heart rate by releasing ACh at the neuromuscular junction and activating the IKACh channels
What is KATP?
ATP sensitive K channel
- Found in many different tissues and primarily inhibited by ATP
- ⤓ATP (⤓glucose) ⇒ ⤒ K channel
The ATP-sensitive K channel has rich pharmacology because of the ____ receptor
The ATP-sensitive K channel has rich pharmacology because of the sulphonylurea receptor (SUR)
What are 7 physiological roles of ATP-sensitive K channels (KATP)?
- Repolarization of cardiac AP in hypoxia/ischemia
- Cardioprotective
- Control of glucose/insulin secretion
- Control of smooth muscle tone
- Myoprotection during skeletal muscle fatigue
- CNS protection during ischemia (eg stroke)
- Found in “energy-sensing” neurons in the hypothalamus
INHIBITED BY SULPHONYLREAS
What is the subunit assembly of strong inward rectifier K+ channels (IK1)
IK1 is a homotetramer
Subunit assembly of IKACh?
Heterotetramer
- passive gating
Subunit assembly of KATP inward rectifier K+ channels
Hetero-octomer
Sulphonylrea receptor
Nucleotide binding folds (where ATP binds)
What are the features of the KcsA channel? (voltage gated K+ channel solved to 3.2A resolution)
A. 4 identical subunits
B. Each subunit has two transmembrane helices
C. Selectivity filter
D. Water-filled cavity
E. Inverted “teepee”
D. Gating mechanism
What is the purpose of the water-filled cavity in KcsA?
Helps to overcome electrostatic destabilization (ions don’t like hydrophobic regions)
What are three features of the selectivity filter/ion conduction pore of KcsA?
Motif?
What fits?
What determines selectivity?
- Signature “GYG” motif
- Space for 3 K+ ions and a water molecule
- The 4 tyrosine residues of the filter determine selectivity
Antibodies bind to ____ of KcsA channel
Antibodies bind to extracellular loop of KcsA channel
What happens to water of hydration before ions enter selectivity filter of KcsA?
Water of hydration stripped before ions enter selectivity filter
How many conformational positions are there for K+ ions in the selectivity filter? (KcsA)
four
Oxygen cage defines selectivity for K+ ions
What feature of KcsA defines selectivity for K+ ions
Oxygen cage
What is present on valinomycin and nonactin (antibiotic and antimycotic compounds) that allows K+ to enter bacterial and fungal cell membranes?
Oxygen cages
Valinomycin and Nonactin have oxygen cages and act by allow K+ to enter bacterial and fungal cell membranes
What are four essential elements in the structure of K channels?
- Plenty of water
- Helix dipoles
- Customized oxygen cages
- Multiple ion occupancy
Which K channels are more positively charged?
BK are more positively charged than IK/SK
What is the importance of the helix dipoles in K channels?
Helix dipoles attract ions into the water filled cavity
High [K+] relative to the outside of the channel = energy in
