L8 Ion Channels

Question 1

What can lead to an increase in membrane capictance of sodium (g_Na⁺)

Answer 1

Increasing intracellular sodium woud increase g_Na⁺

Question 2

What can cause a focal depolarization of the plasma
membrane?

Answer 2

Inward Ionic Current (+ charge moves in)

Question 3

What is voltage clamping used for?

Answer 3

1. A method for measuring the “whole cell response” for a given class of ion channels.
2. A method of compensating for membrane capacitance so that membrane resistance can be directly measured in
   current - voltage plots (V=IR).

Question 4

How is Voltage Clamping done?

Answer 4

1. Apply a constant command
   voltage (V_com) across the plasma
   membrane.
2. Record the transmembrane current
   (Itm) required to sustain the constant
   command voltage

Question 5

What are the results of voltage clamping?

Answer 5

A current - voltage plot that
describes the channels under
investigation.

Slope of the curve = G_x (whole cell conductance of ion x)

Question 6

What do the downward and upward slopes of a current response plot represent in volatge clamping of fast Na+ channels.

Answer 6

The downward slope is due to the opening of Na⁺ channels and cell depolarization (inward Ionic current), An upward slope is the result of outward current.

Question 7

What does voltage clamping an area with no voltage gated channels demonstrate?

Answer 7

Leakage Channels

Question 8

What is the relationship between whole cell
and single channel conductance?

Answer 8

G_x = N_xo * Y_x

G_x = conductance of all channel in a whole cell

N_xo = # open channels for a given ion

Y_x = conductance of 1 open channel for that ion

Question 9

What is single channel recording?

What are alpha and beta in terms of an ion channel’s state?

How is the magnitude of a single channel current determined?

Answer 9

Sectioning off a segment of membrane “membrane patch” to measure magnitude if current and the amount of time channels are open and closed.

alpha = rate of a channel closing if open
beta = rate of a channel opening if closed

The magnitude of the
single channel current (i)
depends on the value of
the single channel
conductance (gamma) and the
electrochemical driving
force (Vm - Ex):
i = gamma (Vm - Ex)

Question 10

What are 5 functional characterizations of ion channel?

Answer 10

1. Leakage
2. Voltage-gated
3. Ligand-gated
4. Second messenger gated
5. Mechanically gated

Question 11

What is the proposed model for “Fast Na⁺ channels”
opening and closing?

Answer 11

4 Motifs each containing 6 transmembrane subunits. The 4th subunit is the Activation Gate
Deactivation gate is a cytoplasmic loop between motifs 3 and 4

Question 12

What are the 5 “moleulcar faimilies” of ion channels and an example of each?

Answer 12

1. Gap Junction Channels.
2. Depolarization - activated ion channels.
3. Inward Rectifying K+ channels.
4. Pentameric Ligand - gated channels.
5. Ionotropic Glutamate Receptors.

Question 13

Describe gap junction ion channels

Answer 13

6 Connexins make a connexon, connexons of adjacet cells unite to form a gap juntion.

Very High Conductance

Question 14

What’s an example of a ligand gated channel (the one most focused on in calss)?

How does it function?

What are some other ligand gated channels?

Answer 14

Acetylcholine receptor (AChR)

AChR has 5 subunits and 2 binding sites for acetyl choline. If no ACh is bound, channel is closed. If two ACh bind, channel is open, allowing ions to flow in/out.

Other ligand gated channels include ATP receptors, GABA receptors, glycine receptors and glutamate receptors (he said don’t worry about the glutamate receptors)