Potassium Channels

Question 1

What is the meaning of delayed rectifier?

Answer 1

the inward Na+ current activates much more rapidly than the K+ current

Question 2

What are the 2 broad divisions of K+ channels and their properties?

Answer 2

1.) ‘Leak’ channels
- always open
concerned with setting resting membrane potentials
- inward rectifiers (Kir, KcsA)
- tandem - pore, outward rectifiers (K2P)

2.) ‘Gated’ Channels
- opened and closed
concerned with repolarising APs and controlling excitability
- ligand - gated (cyclic nucleotide, G-proteins, Ca2+)
- voltage-gated (Kv - delayed rectifiers)

Question 3

What are the 7 main roles of potassium channels?

Answer 3

1. ) Regulators of excitability
2. ) Foundation upon which all other forms of neuronal excitation is built
3. ) Mediates the action potential firing frequency
4. ) Mediates the action potential repolarisation
5. ) Mediates the action potential threshold
6. ) Mediates the resting membrane potential
7. ) Mediates the after-hyperpolarisation

Question 4

What are the 8 major K+ channel families and what types of channels are they?

Answer 4

1. ) KCNA-D ==> voltage gated
2. ) KCNE-G ==> voltage-gated
3. ) KCNH ==> voltage gated
4. ) KCNJ ==> leak -non-gated
5. ) KCNK ==> leak - non-gated
6. ) KCNM ==> ligand & voltage gated
7. ) KCNN ==> ligand & voltage gated
8. ) KCNQ ==> ligand & voltage gated

Question 5

What is the structure of potassium channels?

Answer 5

• Tetrameric transmembrane proteins
• K+ permeable pore with a “GYG” motif
• Voltage dependent activation (S4)
• voltage dependent inactivation (N and C intracellular domains)
• 6 domains to make 1 subunit
• 4 subunits must combine to form a functional channel
• Subunit assembly T1 domain ==> stabilises the structure
• beta subunits

Question 6

What are the 4 mechanisms of ion selectivity?

Answer 6

1. ) Plenty of water
2. ) Helix dipoles - separation of charge. Opposite charges attract, like charges repel
3. ) Customised oxygen cages
4. ) Multiple ion occupancy

Question 7

Why don’t Na+ ions pass through the channel?

Answer 7

Na+ ions are smaller so done line up with the oxygen ions so don’t pass through the channel

Question 8

What are the two ways for potassium channels to close and describe them?

Answer 8

1. ) Deactivation - voltage dependent

2. ) Inactivation - when the channels become blocked even though they are activated

Question 9

What are the 4 major families of voltage gated K+ channels?

Answer 9

1. ) Kv1 (gene 7) - LVA
2. ) Kv2 (gene 2) - LVA/HVA
3. ) Kv3 (gene 4) - HVA
4. ) Kv4 (gene 3) - LVA

Question 10

What are the 2 antagonists of voltage gated K+ channels and which channels do they block?

Answer 10

.1.) Tetraethylammonium (TEA) - low concentrations particularly block BK and KV3 (Kv1.1)

2.) Dendrotoxin (DTx) - blocks Kv1.1, Kv1.2 and Kv1.6 ==> all in the brain

Question 11

What are the 4 methods to identify and characterise K+ currents in neurones

Answer 11

1. ) Electrophysiology: a neuron is voltage-clamped and other voltage-gated currents blocked
   - Na+ channels blocked with TTx
   - Ca2+ channels blocked with Dihydropyridine
2. ) Pharmacological agents
3. ) Immunohistochemistry: uses antibodies to identify the presence of Kv1 subunits protein in the membrane
4. ) PCR: measures specific mRNA of K+ channels

Question 12

Give the main example of a native Kv function and its explanation

Answer 12

Kv in the brainstem auditory pathway

• MNTB neurons give inhibitory projections to the MSO, LSO, the SPN (Superior Paraolivary Nucleus) and the NLL (Nuclei of the Lateral Lemniscus)
• all 4 target nuclei receive direct excitatory inputs from the VCN (Ventral Cochlear Nucleus)
• This pathway converts excitation on one side of the ear and inhibition to the other side

Question 13

What channels are contained in MNTB neurons?

Answer 13

High (KV3) and Low (Kv1) voltage activated K+ currents

Question 14

Where are Kv1 channels found and what are their roles?

Answer 14

• found at the Nodes of Ranvier
  are under the myelin, so have little influence during AP’s
• Cause AP failure when exposed during demyelinating diseases such as multiple sclerosis
• limit excitability (maintains 1 output AP to each EPSP)
• suppress firing of multiple APs e.g. in the brainstem auditory pathway by raising the threshold at the AIS, juxtaparanodal at nodes of Ranvier and the synaptic terminal

Question 15

Where are Kv3 channels found and what are their roles?

Answer 15

• found on the soma, AIS and Heminode & Terminal
• keeps AP’s short and fast
• shorten AP’s by repolarisation

Question 16

How do Kv3.1b channels function in the auditory pathway?

Answer 16

• Kv3.1 channels activate at high voltages and accelerate AP replarisation
• Kv3.1 expressing cells have shorter APs and can sustain firing at higher rates
• Phosphorylation at ser503 by PKC causes reduced Kv3 current
• This allows Kv3 currents to be changed in response to auditory environment
• Kv3.1 is basally phosphorylated in the auditory brainstem
• sound stimulation induces Kv3.1b dephosphorylation via PP1/PP2A

Question 17

What are the 5 other form of potassium channels and briefly describe them

Answer 17

1. ) inward rectifiers (Kir)
   - No voltage sensor and no gate
   - Outward current blocked by internal [Mg2+]I  blocks the pore at potentials positive to EK  hence inward rectification
   - Are blocked by TEA, Cs+ and Ba2+
   - Kir1 – weak inward rectifiers
   - Kir2 – strong rectifiers (blocked by Spermine & Mg2+)
   - Kir3 – G-protein β/ƴ subunit gated and thereby regulated by neurotransmitters e.g. Muscarinic Ach (heart) and opioid receptors
   - Kir6 – associate with Sulphonylurea receptor (SUR) to form KATP channel (open as ATP declines)

2.) Two-pore (K2p) K+ channels - Leak Channels
- only 2 subunits needed to form 1 channel
- No voltage sensor or gate
- Outward rectifying normally, but linear I/V in symmetrical K+
Blocked by Cs+ and Ba2+
Many are activated by halothane and other volatile anaesthetics, suggesting K2P activation may underlie volatile anaesthetic mechanism of action

3. ) voltage gated potassium channels
   - Kv1 shaker  mutations associated with episodic ataxia (group of neurological disorders that affect balance, coordination, and speech) and epilepsy
   - Kv2 – shab  Expressed in heart and CNS
   - Kv3 – shaw  associated with fast spiking neurons. Accelerates AP repolarisation
   - Kv4 – shal  the ‘A-current’ which quickly inactivates. Is enriched on neuronal dendrites
   - Mutations of KCNQ1 – associated with Long QT1 (prolonged cardiac AP), familial neonatal epilepsy and hearing loss
   - Gated by voltage and intracellular ligands (Gβ/ƴ)

4.) Calcium dependent K+ channels
A.) BK – (KCNM)
- Gated by voltage and internal Ca2+ which binds to C-terminal domain
- Blocked by TEA or iberiotoxin
- Underlies slow After-hyperpolarisation
B.) SK – (KCNN)
- NOT voltage – dependent. They are gated by Ca2+ binding and activated mediated by calmodulin
- Blocked by apamin
- Underlie intermediate after-hyperpolarisation HC – expressed on hair cells

5.) Ether - a - go - go and cyclic nucleotide gated channels
A.) ERG
- Related to Kv10 and Kv11
- Not calcium sensitive
- Some inactivate faster than they activate  mutation underlie Long – QT2
- Blocked by Ergtoxin – 1 and E4031
B.) CNG
- Includes cGMP – gated non-specific cation channels -
(CNGA/B) in retinal rod cells, olfactory neurones and -
- HCN1-4 channels underlying IH in neurones
- Similar structure to BK/Kv subunits but possess cyclic nucleotide binding domains (rather than Ca2+ binding) with a pore permeable to K+ and Na+