4 - Channels & Transporters

Question 0

Patch Clamp Experiment

Answer 0

• Whole cell recording measures macroscopic currents due to flow of ions through many channels (of all ions)
• Inside-out recording measures microscopic currents due to flow through as ingle channel by a single ion
  
  • Channels open and close in an all-or-none fashion
  • Fast switch between open and close, does so randomly
  • Gating involves temporary conformational change

Question 1

Voltage Clamp Experiment

Answer 1

Indicated how Na+ and K+ currents change with depolarization

At 52mV, early inward Na+ current disappeared, and at 65mV it reverses direction to an outward flow

Outward K+ current increases magnitude with increased depolarization

Question 2

Voltage-gated Na+ Channels

Answer 2

• Depolarization increases probability of a channel being open
• Summed microscopic currents prove that only one type of Na+ channel is required for the Na+ component of an action potential

Question 3

Voltage-gated K+ Channels

Answer 3

• Depolarization increases the probability of a channel being open
• Multiple types with different properties
• Have a longer latency for activation and longer duration of activation compared to that of Na+ channels (Sustained Response)

Question 4

After Hyperpolarization (AHP)

Answer 4

• Fast AHP shortens the AP by quickly repolarizing the membrane
  
  • only affects early spike frequency at very high frequencies
  • ex. Bk K+ channels activate by Ca2+ depolarization and then rapidly inactivate
• Medium AHP controls interspike interval, contributes to early spike frequency adaptation: slowly activating by Ca2+ entry
  
  • ex. Ik and Sk K+ channels control late spike-frequency adaptation, they are non-inactivating

-Slow AHP limits firing frequency by an unknown channel

Question 5

K+ channel properties (Kv2.1 channels vs Kv4.1 channels, etc)

Answer 5

Kv2.1 channels have little inactivation, are involved with repolarization

Kv4.1 channels inactivate rapidly to depolarization

Inward rectifiers allow more current flow during hyperpolarization than during depolarization

The Ca2+ activated K+ channel opens in response to increased intracellular Ca2+ and sometimes to membrane depolarization

Question 6

Channelopathies (Ca2+, Na+, and K+)

Answer 6

1. Voltage-gated Ca2+ channels
   
   • Congenital Stationary Night Blindness
   • Familial Hemiplegic Migraine
   • Episodic Ataxia type 2
2. Na+ channel defects
   
   • Epilepsy w/febrile seizures
3. K+ channel mutations
   
   • Benign Familial Neonatal Convulsion

Question 7

Toxins that block Ion channels

Answer 7

Na+
Tetrodotoxin (TTX) & saxitoxin block Na+ channels (pufferfish)
α-Toxin prolongs duration of Na+ currents (scorpion)
β-Toxin shifts voltage activation of Na+ currents (scorpion)
Batrachotoxin inactivates Na+ channels (poison dart frogs)
K+
Dendrotoxin (wasp) & Apamin (bee) blocks K+ channels
TEA blocks K+ channels
Ca2+
ω-conotoxin (cone snail) blocks N-type Ca2+ channels
ω-agatoxin (spider) blocks P/Q-type Ca2+ channels