Lecture 6 - Channels and Transporters

Question 1

True or False?:

Conductance is a combined measure of permeability and number of ions. Permeability is a property of the cell membrane (function of the number of open channels). Conductance is the ability of current (ions) to pass across the membrane.

Answer 1

True

Question 2

What shape do microscopic currents take?

Answer 2

Square Wave

Question 3

What determines the slope of the current-voltage (I-V) plot? Why?

Answer 3

The slope is equal to conductance (g) because I/V=g (as per Ohm’s law).

Question 4

How can you use an I-V plot to determine what type of ion channel you are studying?

Answer 4

You can experimentally produce an I-V plot, find its x-intercept, and see which ion channel’s E_x is equal to that.

Question 5

What is a activation curve (in terms of gating of ion channels)?

Answer 5

An activation curve is a plot of P_o vs V^_m. At negative potentials, the channels are likely to be closed and vice versa for positive. It has the shape of a hill function.

Question 6

What is gating?

Answer 6

Gating is the transition of channels between open and closed states.

Question 7

True or False?:

The Na⁺/K⁺ pump is electrogenic. It extrudes 3 cations for every 2 it imports. As such, it generates a very small, slow, outward, hyperpolarizing current.

Answer 7

True

Question 8

What does the macroscopic I-V plot for non-voltage gated (leak) channels look like? What does the macroscopic I-V plot for voltage-gated channels look like?

Answer 8

Question 9

What is the name of the seal made between the membrane and the pipette during patch-clamp recordings?

Answer 9

Giga Ohm Seal

Question 10

True or False?:

There are about 100 different K⁺ channel genes.

Answer 10

True

Question 11

What Nobel prize-winning method is used to conduct single channel recordings?

Answer 11

Patch-Clamp

Question 12

Consider that you are performing an outside-out patch-clamp recording with a K⁺ “leak” channel. There is no concentration gradient. In what direction will the current be (if any) at 0 mV.

Answer 12

No Current

Question 13

Describe what is happening in the attached figure showing microscopic currents of sodium ions.

Answer 13

A command voltage (a) is applied to a cell. Since the voltage increases, the probability of sodium channels opening increases. The individual channels open to allow current through (b). The individual microscopic currents can be summed (c) to produce a macroscopic current that displays the net current for the cell.

Question 14

How do ion exchangers work?

Answer 14

Ion exchangers harness energy from concentration gradient(s) in order to pump ions.

Question 15

What is the name of the segment that forms the selectivity pore in ion channels?

Answer 15

P-Segment

Question 16

True or False?:

In sodium channels, the S4 domain acts as the voltage sensor. It functions by moving up and down across the membrane as the membrane potential changes.

Answer 16

True

Question 17

How does the equilibrium potential for a channel affects its I-V plot?

Answer 17

It shifts it so that the x-intercept is at (E_x, 0).

Question 18

What two types of ions transporters are there?

Answer 18

ATPases and Ion Exchangers

Question 19

How does the selectivity filter in ion channels work?

Answer 19

The selectively filter works by replacing the hydration shell with residues along the channel.

Question 20

Describe the structure of a Na⁺ channel.

Answer 20

Na⁺ channels have 4 repeat motifs of 6 transmembrane spanning regions. They also can incorporate β-subunit adaptors.

Question 21

Consider that you are performing an outside-out patch-clamp recording with a K+ “leak” channel. There is no concentration gradient. In what direction will the current be (if any) at -20 mV. How will the current at -40 mV compare to this?

Answer 21

Since the inside of the pipette is negative, current flows in. At -40 mV, the current is twice as large as at -20 mV due to Ohm’s law.

Question 22

How does one deduce the structure of ion channels?

Answer 22

The oldest way to do this is to divide pools of mRNAs until you find single channels. You express the mRNA in Xenopus oocytes. You can also use X-ray crystallography or electron microscopy imaging.

Question 23

Consider that you are performing an outside-out patch-clamp recording with a K+ “leak” channel. There is no concentration gradient. In what direction will the current be (if any) at 20 mV.

Answer 23

Since the inside of the pipette is more positive, current flows out.

Question 24

Probability of open channels (P_o) is dynamic. What regulates it?

Answer 24

Modulation and Gating

Question 25

What are rectifiying channels?

Answer 25

Recitifying channels are channels that exhibit non-ohmic I-V relationships. In essence, current passes in one direction better than it does in another. They can be inward rectifiers (current flows better inward) or outward rectifiers (current flows better outward).

Question 26

How can you predict the reversal potential for a K⁺ channel when there is a concentration gradient and a membrane potential?

Answer 26

You can use the Nernst equation.

Question 27

The Na⁺/K⁺ pump and Ca²⁺ pump are what type of ion transporter?

Answer 27

ATPases

Question 28

True or False?:

Individual channel currents are called microscopic currents. They summate to produce the macroscopic current.

Answer 28

True

Question 29

What shape does the probability of Na⁺ channel opening vs. membrane potential graph take?

Answer 29

It resembles the Hill function.

Question 30

What blocks the Na⁺/K⁺ ATPase?

Answer 30

Ouabain

Question 31

What is the relationship between macroscopic current (I), single channel current (micro) (i), # of channels (n), and probability of open channels (P_o)?

Answer 31

I = i n P_o

Question 32

True or False?:

There is only 1 human Na⁺ channel gene and 1 human voltage-gated Ca²⁺ channel gene.

Answer 32

False

There are 9 or 10 human Na⁺ channel genes and about 16 human voltage-gated Ca²⁺ channel genes.

Question 33

When producing an I-V plot from single-channel recordings, if you increase the number of ions but keep the relative ratio (inside:outside), what happens? If you alter the relative concentration gradient, what happens?

Answer 33

If you increase the ions but keep the same ratio (inside:outside), the slope changes. If you alter the relative concentration gradient, the reveral potential changes (and the graph shifts) because the Nernst equation changes..

Question 34

What are the different types of gating?

Answer 34

• Voltage
• Ligand
• Temperature
• Stretch
• Light

Question 35

True or False?:

There are 20 human Cl^- channel genes. The Cl^- channel has 10-12 transmembrane regions.

Answer 35

True