Week 2 (K+ channels/disease) physiology

Question 1

Ion channel significance across organisms

Answer 1

human: hormone release (ACTH, corticotroph), muscle contraction (vascular SMC)
bacteria: ion channels can affect behaviour
virus: attack cells controlling ion channel activity

Question 2

Ohm’s law in the context of ion channels (voltage/current clamps)

Answer 2

V: voltage (membrane potential)
R: How many ion channels are open - allowing ions to go through
I: current

Question 3

voltage clamp

Answer 3

A raised constant voltage (-80mV to 0mV): measure current
this will open/close ion channels (changing resistance R)

TTX (showing only K+): 0mV is more positive than K+ equilibrium
pushes K+ out of the cell, outward positive current
Positive Membrane Current

TEA (showing only Na+): 0mV is more negative than Na+ equilibrium
attracts Na+ inwards, inward positive current
Negative Membrane Current

Question 4

equilibrium potential for sodium and potassium

Answer 4

+60mV for Na+, -90mV for K+

Question 5

patch clamp

Answer 5

a laboratory technique in electrophysiology used to study ionic currents in individual isolated living cells, tissue sections, or patches of cell membrane.

Question 6

current clamp (sub threshold stimulus)

Answer 6

• When a current stimulus is applied that is below the threshold, it causes a proportional change in the membrane potential according to Ohm’s Law.
• However, if this change in voltage is not sufficient to trigger sodium channels to open, there will be no action potential.

Question 7

current clamp (over threshold stimulus)

Answer 7

If the membrane potential reaches the threshold, it opens Na+ channels.

Na+ influx causes depolarization. This change in membrane potential is not directly proportional to the applied current anymore because the opening of sodium channels creates a positive feedback loop – more Na⁺ influx causes further depolarization, which opens even more sodium channels.

Question 8

current clamp (at action potential peak)

Answer 8

At the peak, the K+ channels will open, K+ efflux will hyperpolarise the membrane potential.

Question 9

Molecular structure of Na+ channels:

Answer 9

1. Has 4 repeated domains (6 transmembrane helices each)
2. S4 (4th transmembrane helix - voltage sensor) +ve amino acids
3. S4-S6 - ion channel pore
4. Between domains III and IV, an inactivation h-gate (turns off Na+ at AP peak)

Question 10

Molecular structure of K+ channels:

Answer 10

4 individual polypeptides (subunits) assemble together - a tetramer: to form the pore.
S1-S4 voltage sensor, S5-S6 pore domain: similar to Na+ channels
Evolutionarily: K+ channels have evolved first, so Na+ and Ca2+ only have to fuse the 4 subunits to a whole polypeptide.

Question 11

voltage sensor mechanism - ion channel

Answer 11

S4 is a positively charged transmembrane helix.
It is normally attracted inwards (cell have -ve resting membrane p.)
During an action potential, Na+ influx will increase m.p.
This will push S4 outwards.

Question 12

voltage sensor and S6 (pore) - ion channel

Answer 12

During an action potential, S4’s movement outwards will pull S6 (part of the pore) and open the ion channel.

Question 13

Na+ and K+ hydration energy

Answer 13

K+ requires less energy to remove the coat of water than Na+ (even though it is bigger in ionic radius)
- reason for K+ channel selectivity (much more selective than Na+/Ca2+ channels)

Question 14

K+ channel pore selectivity - selectivity filter

Answer 14

selectivity filter is dependent on the ion’s hydration energy, K+ has a lower hydration energy than Na+.
GYG sequence: Only K+ is fitted just right, but pore is too big for Na+. (Cannot strip water molecules off)
During the strip, tyrosine will act as surrogate water molecules for after-strip K+.

Question 15

Na channel inactivation

Answer 15

current restores from negative to normal due to:
the ‘hinged lid’ inactivation part of the Na channel.

can be removed site directed mutagenesis: current stays low reaches equilibrium potential.

Question 16

ion channel mutations

Answer 16

1. biophysical mutations - activity (open probability, gating kinetics, conductance, regulation, selectivity, time of activation/deactivation)
2. number of channels - synthesis, expression, degradation.

Question 17

ion channel mutation examples - epilepsy

Answer 17

Hyperexcitability:
Increased Na current (SCN1B): increased spike freq.
or
Decreased K current (KCNQ2/3): increased freq

Question 18

resting membrane potential of a cell (mechanism)

Answer 18

often

Question 19

RMP (resting m.p.) of excitable cells - neurons and muscle cells

Answer 19

Closer to potassium’s equilibrium potential (-70,-80mV)
These type of cells are more permeable to potassium, therefore closer to K+’s EP.

Question 20

evolution of K+ channels

Answer 20

voltage gated channels part of 6TM
Even voltage gated channels have different branches that lead to slightly different functions.

Question 21

2,4,6,8 TM

Answer 21

2 TM(P) & 4 TM(2P): only have pore forming regions - leak channels
6TM(P) & 8 TM(2P): have voltage sensors - voltage gated channels

Question 22

cardiac muscle cell action potential

Answer 22

1. Na+ influx
2. Na+ close and Ca2+ open (Ca2+ influx)
3. Ca2+ close and slow K+ open (slow repolarisation)
4. return to resting state.

Question 23

slow K+ channels

Question 24

QT interval

Answer 24

interval from Na+ to K+ conduct, ECG measured on skin
from ventricular depolarisation to ventricular repolarisation

Q wave: from endocardium to epicardium (inner to outer heart wall)
T wave: negative charge returns and resets the heart (a small positive bump)

Question 25

QT interval significance to action potential

Answer 25

QT interval is equivalent to how long the action potential is

Question 26

KvLQT1(KCNQ1 gene)

Answer 26

Protein that forms the voltage-gated potassium channel.

Question 27

minK gene

Answer 27

potassium channel accessory subunit - single transmembrane protein

Question 28

KCNQ1 + minK

Answer 28

Compared to classic ion channel (KCNQ1 only)
slower activation - allows delayed repolarisation
bigger current towards an increasing MP: allows more channels on the membrane

Question 29

long QT syndrome (1)

Answer 29

mutations in KCNQ1 or minK:
reduction in K+ current – longer QT interval

Heart collapse when HR demand is high - exercise or shock
arrythmias

Question 30

rectifier - ion channel

Answer 30

selectively allows ions to pass to a specific direction

Question 31

ATP sensitive - 2TM(P) - insulin secretion

Answer 31

sets RMP — K channels are typically open at rest. (low blood glucose)
Increasing glucose — elevated ATP level will close K channel
K channel closure — cell depolarisation -> Ca2+ influx
Ca2+ influx — insulin secretion

Question 32

Inward rectifier 2TM(P)

Answer 32

Fine tuning:
Slight depolarisation: a slight K+ efflux to return to RMP
When MP is very negative: K+ influx to return to RMP

However: When depolarisation is significant, this channel is blocked by Mg+ or polyamine (inward rectifying) to stop K+ efflux. This wouldn’t happen with non-rectifiers.

Question 33

K (ir - inward rec.) 6.2 – ATP sensitivity

Answer 33

ATP concentration is proportional to inhibition of the channel’s conductance

Question 34

SUR1 (sulphonylurea receptor 1)

Answer 34

SUR1 combines with Kir6.2 (insulin secretion)
confer sensitivity to ADP and sulphonylurea - a drug used to treat T2D.

S. binds to the SUR1 subunit and close the K_ATP channels, inducing insulin release even at lower glucose concentrations.

Question 35

Kir6.2 or SUR mutations

Answer 35

PHHI: unregulated insulin secretion
class 1 mut. -rare- : complete function loss (12th amino acid became stop codon - Y12 stop - Kir6.2)
class 2 mut. -more common -: impaired activity. ADP cannot activate K+ channel to hyperpolarise. HOWEVER, ATP inhibition is not affected. Leads to hyperinsulinaemia.