Week 2 (K+ channels/disease) physiology Flashcards
Ion channel significance across organisms
human: hormone release (ACTH, corticotroph), muscle contraction (vascular SMC)
bacteria: ion channels can affect behaviour
virus: attack cells controlling ion channel activity
Ohm’s law in the context of ion channels (voltage/current clamps)
V: voltage (membrane potential)
R: How many ion channels are open - allowing ions to go through
I: current
voltage clamp
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
equilibrium potential for sodium and potassium
+60mV for Na+, -90mV for K+
patch clamp
a laboratory technique in electrophysiology used to study ionic currents in individual isolated living cells, tissue sections, or patches of cell membrane.
current clamp (sub threshold stimulus)
- 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.
current clamp (over threshold stimulus)
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.
current clamp (at action potential peak)
At the peak, the K+ channels will open, K+ efflux will hyperpolarise the membrane potential.
Molecular structure of Na+ channels:
- Has 4 repeated domains (6 transmembrane helices each)
- S4 (4th transmembrane helix - voltage sensor) +ve amino acids
- S4-S6 - ion channel pore
- Between domains III and IV, an inactivation h-gate (turns off Na+ at AP peak)
Molecular structure of K+ channels:
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.
voltage sensor mechanism - ion channel
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.
voltage sensor and S6 (pore) - ion channel
During an action potential, S4’s movement outwards will pull S6 (part of the pore) and open the ion channel.
Na+ and K+ hydration energy
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)
K+ channel pore selectivity - selectivity filter
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+.
Na channel inactivation
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.
ion channel mutations
- biophysical mutations - activity (open probability, gating kinetics, conductance, regulation, selectivity, time of activation/deactivation)
- number of channels - synthesis, expression, degradation.
ion channel mutation examples - epilepsy
Hyperexcitability:
Increased Na current (SCN1B): increased spike freq.
or
Decreased K current (KCNQ2/3): increased freq
resting membrane potential of a cell (mechanism)
often
RMP (resting m.p.) of excitable cells - neurons and muscle cells
Closer to potassium’s equilibrium potential (-70,-80mV)
These type of cells are more permeable to potassium, therefore closer to K+’s EP.
evolution of K+ channels
voltage gated channels part of 6TM
Even voltage gated channels have different branches that lead to slightly different functions.
2,4,6,8 TM
2 TM(P) & 4 TM(2P): only have pore forming regions - leak channels
6TM(P) & 8 TM(2P): have voltage sensors - voltage gated channels
cardiac muscle cell action potential
- Na+ influx
- Na+ close and Ca2+ open (Ca2+ influx)
- Ca2+ close and slow K+ open (slow repolarisation)
- return to resting state.
slow K+ channels
QT interval
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)
QT interval significance to action potential
QT interval is equivalent to how long the action potential is
KvLQT1(KCNQ1 gene)
Protein that forms the voltage-gated potassium channel.
minK gene
potassium channel accessory subunit - single transmembrane protein
KCNQ1 + minK
Compared to classic ion channel (KCNQ1 only)
slower activation - allows delayed repolarisation
bigger current towards an increasing MP: allows more channels on the membrane
long QT syndrome (1)
mutations in KCNQ1 or minK:
reduction in K+ current – longer QT interval
Heart collapse when HR demand is high - exercise or shock
arrythmias
rectifier - ion channel
selectively allows ions to pass to a specific direction
ATP sensitive - 2TM(P) - insulin secretion
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
Inward rectifier 2TM(P)
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.
K (ir - inward rec.) 6.2 – ATP sensitivity
ATP concentration is proportional to inhibition of the channel’s conductance
SUR1 (sulphonylurea receptor 1)
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.
Kir6.2 or SUR mutations
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.
