3 - Ion Channels

Question 1

What types of channels are essentially always open?

Answer 1

Gap junctions and “background” channels such as K+ leak channels

Question 2

What are four additional regulators of channels (aside from voltage or ligands)

Answer 2

1. Phosphorylation
   
   • Ca++ Beta-adrenergic channel ;
   • L-type - increase phosphorylation = increase HR and contraction
2. Temperature
   
   1. TRP
3. Membrane Lipids
   
   1. K+ channels require phospholipids
4. Light

Question 3

What are the three types of patch clamp techniques?

Answer 3

1. Cell-Attached (Gigaohm seal)
2. Inside out patch
3. Outside out patch

Question 4

Which domain is accessible in an inside-out patch clamp recording?

Answer 4

The cytoplasmic domain

Question 5

What is a whole cell recording

Answer 5

When the cytoplasm is continuous with the pipette interior

Question 6

Which domain is accessible in an outside out patch clamp recording

Answer 6

extracellular domain

Question 7

What is ohms law?

Answer 7

V=IR

voltage = current x resistance

V=I/g

g - conductance

Question 8

the _________ (p) of a channel defines how easily a permeant ion can pass through the channel when it is open

Answer 8

the permeability (p) of a channel defines how easily a permeant ion can pass through the channel when it is open

Question 9

the ________ (gamma) of an open channel is dependent on the concentration of the permeant ion(s) as charge carrier(s)

Answer 9

the conductance(gamma) of an open channel is dependent on the concentration of the permeant ion(s) as charge carrier(s)

Question 10

What is rectification?

Answer 10

When the plot of current as a function of potential is NOT straight because ion concentration gradient is changing

Question 11

What two features could contribute to a non-linear I-V (current-voltage) relationship?

Answer 11

• Concentration gradient effects speed of ion flow
  
  • appears as lower conductance in one direction
• Rectification resulting from properties of the channel
  
  • Channel itself passes a permeant ion better in only one direction independently of the effect on concentration gradient
    
    • eg an outwardly rectifying K+ channel with 150mM K+ on both sides

Question 12

What is the nernst equation? What does it define?

Answer 12

Defines the electrical potential when the concentration gradient of an ion is balanced by electrical gradient (no NET current flow even when channel is open)

Also called reversal potential

E_k=(RT/zF) ln ([K]_o / [K]_i)

R - gas constant

F- faradays constant

T - degrees in Kelvin

z - valence of the ion (eg +1 for K)

Question 13

Which cation channel discussed in lecture has selectivity determined by the “waistline” of the pore?

Answer 13

The poorly selective cation channel of the nicotinic ACh receptor

Poorly selective = most inorganic monovalent cations can pass through

Cut off size is tetraethylammonium (TEA)

Low filter = reasonable conductance

Question 14

What feature of channels gives hem the ability to discriminate well between different cations (eg K+ Na+)?

Answer 14

The ability of the selective filter to partially dehydrate specific ions

Question 15

When is the GHK equation applicable?

Answer 15

For multiple species of ions that either:

• each has its own passive channel on the membrane
• or they all pass independently through the same channel

Question 16

What is the GHK equation?

Answer 16

Vm = 58mV x log ((p_k[K]_o + p_Na[Na]_o + p_Cl[Cl]_i) / (p_k[K]i + p_Na[Na]_i + p_Cl[Cl]_o))

where px refers to permeability of ion x

58mV is used because all ions are monovalent and the terms involving [Cl] are flipped (anion)