Lectrues 1 - 10

Question 1

How are ion channels classified ?

Answer 1

Selectivity - K+,Na+, Cl+ etc)
Gating - voltage/ligand dependent
Regulation - what regulates when an ion channel opens and how long it stays open (ATP, G-proteins, Ca2+ etc.)

Question 2

Describe structural and functional differences between molecular ion channel families.

Question 3

Explain the parameters that contribute to the current (I) carried by a population of channels.

Answer 3

I = N.Po.g.(Vm-Ei)

I = total current carried by population channels in the cell membrane
N = no of channels
Po = open probability
g = single channel conductance
Vm = membrane potential
Ei = Nernst (equilibrium) potential ion I -

Question 4

What are the different numbers of subunits in potassium voltage ion channels ?

Answer 4

Each gene that codes for a Kv produes a subunit with this structure, 4th one is voltage sensor, 6 transmembrane spanning domains. Set by membrane potential, mediated by specific aa in the 4th transmembrane domain.
1 ion channel needs 4 of the 6 transmembrane subunits to make one.

Question 5

What is open probability ?

Answer 5

How often channels are open 0=closed all the time, 1= open all the time, 0.5=open half of the time.

Question 6

What is single channel conductance ?

Answer 6

Measure of how many ions move through the pore in a given time

Question 7

What is Nernst potential ?

Answer 7

Eion = RT X Ln [ion]out
zF [ion]in

Question 8

Explain the patch clamp technique

Answer 8

It involves a glass micropipette forming a tight gigaohm seal with the cell membrane.

Glass pipette gently touches the membrane. Highly resistant seal, so you can observe the individual ion channels that sit inside the pipette.

Question 9

How can the patch, clamp technique be used to identify ion channel populations in cells ?

Answer 9

Identify what regulates the channel. Can evaluate directly the physiological function of the ion channels in the cells of the tissues we are interested in.

Question 10

Discuss an example of how specific ion channel mutations impact on electrophysiological parameters.

Answer 10

The mutation of KCNK-4 causes FHEIG.

Neurons expressing WT KCNK-4 and some do not

Neurons in the brain that are mutant means K+ is lost into the interstitial space. Increased K+. Depolarized Ek. Neighboring cells are depolarized. more positive membrane potential, so neurons are firing AP’s when they shouldn’t be

When channel gets mutated K+ moves into interstitial space, lowering the depolarization threshold

Question 11

What is FHEIG and what does it cause ?

Answer 11

FACIAL DYSMORPHISM, HYPERTRICHOSIS, EPILEPSY, INTELLECTUAL/DEVELOPMENTAL DELAY, AND GINGIVAL OVERGROWTH SYNDROME.
Causes:
Overgrown top lip and small upper lip.
Bi-temporal narrowing, hypertrichosis (hair), thin upper lip, bushy/long eyebrows

Delayed development of intellectual ability and motor skills
Seizures/EEG anomalies

Question 12

What is ‘g’ in I = N.Po.g.(Vm-Ei)

Answer 12

g = single channel conductance – measure of how many ions move through the pore in a given time

Question 13

What is Vm ?

Answer 13

The membrane potential

Question 14

What is Ei in

Answer 14

It is the Nearest potential, helps set membrane potential

Question 15

In a patch clamp study, current mediated by WT potassium channels was 500 pA at a potential of +20 mV. In contrast, the current from mutant channels at the same potential was 200 pA. From what we have covered in the introduction lecture, identify 1 possible mechanism that may underlie this finding

Answer 15

Reduced number of channels
Reduced open probability
Driving force is same as expected.

Question 16

What experimental approach could be used to determine which channels are present and open?

Answer 16

1.Whole cell patch clamp technique
2.Clamp potential to different potentials and measure the current
3.Add specific blockers of the channels.
4.If the currents are reduced with the blocker, then the channels are contributing to the total whole cell conductance.
5.It is likely that there are more than 1 type of channel expressed and functioning in a cell.

Question 17

Under physiological conditions the resting potential of a cell was measured as -20 mV. Which ion channel (or channels) might be playing a role in setting this value ?

Answer 17

Potassium
Sodium
Chloride
Calcium

Question 18

Assuming a starting situation of physiological concentrations for sodium, what would happen to ENa if the EC concentration decreased (while the IC concentration remained the same).

Answer 18

1.Under physiological conditions we have high EC and low IC Na+. The Ena is typically around +60mV.
2.If the EC conc falls, then it is moving closer to the IC conc.
3.When EC and IC concs are equal, then Ena – 0 mV
4.Therefore the Ena must shift in a negative direction, i.e towards 0.

Question 19

Differences in the composition of the EC and IC fluids is critical for normal physiological function of cells. Taking this into consideration, discuss the impact of a 3 mM rise in plasma K+ on neuronal function.

Answer 19

1.Explain key differences, normal values for Na+ and K+
2.Explain how these set the Nernst potentials for the ions, calculate typical values these differences.
3.Explain what channels are open at rest (K+), and how this sets the resting potential.
4.Evaluate the impact of a change of 3mM – new Nernst calculation.
5.Discuss the impact this has on the resting potential. Link to activation of the Na+ channels, more likely to fire an AP.
6.Finish with a summary sentence – e.g regulation of the composition of the fluids important for normal neuronal cell function.