Past paper revision Part 1 Paper 1

Question 1

Using the evidence from part 1, and the additional data presented: Provide a diagnosis of the condition the patient has and explain why you think this. Can you identify any other symptoms the patient may have? Discuss how the Nav1.4 mutation described leads to the symptoms observed in the patient.

Answer 1

Symptoms listed in part 1:
- Skeletal muscle stiffness
- Cold appears to impact the Nav1.4 currents > figure 1 mutant has persistent currents > this can be seen in the 20 degrees experiment compared to the 30 degrees experiment. The 30 degrees shows no change in voltage-dependent activation btwn WT and mut unchanged, whereas there is a negative shift with the 20 degrees experiment
> Positive shift in inactivation

PARAMYOTONIA CONGENITA

Question 2

Calculate the Nernst potentials for Na+ and K+ with the two bath solutions:

Answer 2

140 Na+ Ringer in bath (140NaCl, 5KCl) and 130K+ ringer (130KCl, 6NaCl) in the pippette
At room temperature

EC: 140 Na, 5 K
IC: 6 Na, 130 K

*Do values individually:
ENa = 58.2 x log 140/6 = +79.6mV
EK = 58.2 x log 5/130 = -82.4mV

One at 135 K+ ringer bath ( 10 mM NaCl, 135mM KCl ) with the same 130K+ ringer pipette

EC: 10 Na, 135 K
IC: 6 Na, 130 K

ENa = 58.2 x log 10/ 6 = +12.9mV
EK = 58.2 x log 135/130 = +1.0mV
*Always check to what decimal place

Question 3

How would you choose the correct value as the Vrev when plotting the currents recorded from the Na+ Ringer?

Answer 3

• Vrev is where there is no net movement of the ions, leaving the current at 0.
• Graphically, it is seen where the line crosses the X acxis on the voltage-current curve plotted
• This should be similar to the ion’s Nernst potential

Question 4

Knowing that the Vrev the K+ Ringer was 14mV, calculate the relative selectivity of Na+ to K+ to 2 decimal places.

Answer 4

Vrev Solution B - Vrev Solution A = Shift in Vrev

*Write out Goldmann of each solution
Example of workings out:
Vrev Sol B - Vrev Sol A = Shift in Vrev, therefore:
Shift in Vrev = Goldman B - Goldman A
VREV B (KCl) = 58.2 x log ((0+ 100)/(0.1 + 100))
VREV A (NaCl) = 58.2 x log ((97 + 3)/(0.1+100))
therefore:
-70mV = 58.2 x log ((97 + 3)/(0.1+100))
-4 = 58.2 x log ((0+ 100)/(0.1 + 100))
* This forms a RATIO when rearranged

-4 - -70 = +66mV
Rearranging:
+66 = [58.2 x log ((0+ 100)/(0.1 + 100))]-[58.2 x log ((97 + 3)/(0.1+100))]
*as it is using logirithms, that minus can become a divide
+66 = [58.2 x log ((0+ 100)/(0.1 + 100))]/[58.2 x log ((97 + 3)/(0.1+100))]
+66 = [58.2 x log ((0+ 100)/(0.1 + 100))]x[58.2 x log (((0.1+100/97 + 3)))] (when dividing fractions, keep switch flip
Cancel out top of fraction:
+66 = 58.2 x log 100/3+97
+66/58.2 = log 100/3+97
1.13= log 100/3+97 > to remove log from ratio, take inverse log of 1.13:
13.49 = 100/3+97
13.49(3+97) = 100
40.27PK + 1308.53PNa = 100PK > move over PK so that theyre together
1308.53PNa = 59.53PK
PNa:PK = 59.53/1308.53 = 0.04549379…
Ratio is 0.05 (to 2 decimal places)

*This is not the answer for this question, just an example of presenting workings

Question 5

Looking carefully at Figure 1 (shows typical currents recorded from WT and mutant Nav1.4 channels expressed in HEK cells at 30OC and 20 OC.), describe the impact of temperature on the WT and the mutant

Answer 5

The 30 degrees shows no change in voltage-dependent activation btwn WT and mut unchanged, whereas there is a negative shift with the 20 degrees experiment
> Positive shift in inactivation

• Graph F shows a larger amount of inactivation, shift to the right compared to WT at 20 degrees

Question 6

WT and a mix of WT and mutant KCNQ1 channels were expressed in HEK cells. Tail currents were recorded under physiological conditions. The conductance at +60mV for WT was 2.31 nS, while that for the mutant mix was 1.14nS. Exposure of cells to forskolin and IBMX (to increase IC cAMP levels) increased WT current magnitude, but not the magnitude of the WT/ mutant channel mix. What condition might the patient have, explain their likely symptoms and explain why the symtoms occur.

Answer 6

Long QT syndrome
- Impact on a cellular level > describe cardiac AP and role of Q1 in this > repolarisation > K+ channels
- Smaller currents > ventricular repolarisation delayed so long QT observed on ECG recording
- Typical QT = 0.36s
LQT = around 0.54s

Symptoms: syncope, arrhythmia risk, ventricular tachycardia, sudden death
- Triggered activity of organs and re-entrant excitation (secondary aps fired)

Response to excercise in LQT:
- lack of response to cAMP, PKA and SNS
- Normal SNS activation and shortening of QT interval
- Mutants reduced response to PKA = reduced changes in rate of repolarisation, ventricular myoctytes cannot keep up with signals from SAN/atria.
- Increased risk arrhythmias on exercise = more likely to be a mis-match between atria and ventricles during exercise

Question 7

Describe the physiological role of WT Nav1.4 channels in skeletal muscle

Answer 7

• Initiation and propogation of action potentials in Nav1.4 channels > influx of Na+ into a cell increases current in cell and changes membrane potential leading to depolarisation and therefore initation of ap. This then propagates along the muscle fibers, to lead to muscle contraction.