1. Essential Animal Cell Biology Department of Biomedical Sciences
Cell membranes 4
Ion channels
Dr Gordon McEwan
Department of Biomedical Sciences

2. Ion channels
Channel proteins form transmembrane aqueous pores which allow passive movement of small water-soluble molecules into or out of the cell or organelle
Most channel proteins in plasma membrane form narrow pores which are only permeable to inorganic ions - ion channels
Ion channels are highly selective for specific ions (eg Na+, K+, Ca2+, Cl-)
Ion selectivity depends on:
diameter - narrow channels can’t pass large ions
shape - only single ions of correct species can access
charge - distribution of charged amino acids in pore
Passage of ions through narrowest portion of channel rate limiting  saturable transport

3. Ion channel gating Ion channels not continuously open
Can switch between open and closed state by changing conformation
Conformation change regulated by conditions inside and outside cell
acetylcholine
binding site
lipid
bilayer
gate
cytosol
Na+
overall structure
closed
open
Adapted from ECB Fig 12-18

4. Ion channel recording
Ion movements across membrane can be detected by electrical measurements
Technical advances now permit measurement of electrical current through single channel molecule - patch clamp recording
glass microelectrode
conducting fluid
tight seal
ion channel
cell membrane
cell-attached
detached patch
metal wire
constant
voltage source
trace of ion channel currents
metal
electrode
Adapted from ECB Fig 12-20

5. Ion channel recording (contd)
Patch clamp technique permits recording from ion channels in large variety of cell types
By changing ionic composition of bathing solution - can establish which ions go through channels
Can “clamp” membrane potential at different voltages and establish effects on channel activity
If patch is sufficiently small, only one channel molecule present; Can measure ion flow through single channel amps = picoamp (pA)
Current randomly switches between two levels due to channel being either open or closed
Regulation  channel being in open (or closed) state for a greater proportion of time (but still opening and closing at random)

6. Ion channel recording (contd)
Current (pA)
Time (ms)
state of
channel:
closed
open
Adapted from ECB Fig 12-21
Na+ channel activated by acetylcholine   open state probability (Po)
When acetylcholine absent channel spends most of time in closed state

7. Classes of ion channel Two distinct properties of ion channels:
(1) ion selectivity - type of ions which can pass
(2) gating - conditions which influence opening and closing
closed
open
out in
voltage-
gated
ligand-gated
extracellular
intracellular
stress-
activated
 membrane potential
+++
--- + -
molecule binds to channel
mechanical
Adapted from ECB Fig 12-22

8. Stress-activated channel
eg auditory hair cells in ear
basilar
membrane
auditory
nerve fibres
stereo-cilia
tectorial membrane
hair cells
supporting
cell
channel
closed
linking
filament
entry of positively charged ions
open
Bundle not
tilted
Bundle
Adapted from ECB Fig 12-23
Sound vibrations cause basilar membrane to move up and down  stereocilia of hair cells tilt  stretching of linking filaments  opening of channels

9. Voltage-gated channels
Voltage-gated channels play major role in propagating electrical signals in nerve and muscle cells
Channels have special charged protein domains - voltage sensors - which are extremely sensitive to changes in membrane potential
When potential changes beyond threshold voltage, channel switches from closed to open configuration
Open state probability increases at threshold potential
Q What controls membrane potential?
A Opening and closing of ion channels
 control loop:
ion channels  membrane potential  ion channels
Fundamental principal of electrical signaling in cells

10. Basis of membrane potential
cation channel mV mV
charges balanced
cations move through channels  non-zero membrane potential
Adapted from ECB Fig 12-25

11. Resting membrane potential
Negative charge on intracellular organic anions balanced by K+
High intracellular [K+] generated by Na+-K+ ATPase
Large K+ concentration gradient ([K+]i:[K+]o  30)
Plasma membrane contains spontaneously active K+ channels  K+ move freely out of cell
As K+ moves out of cell, leaves negative charge build up  opposes further K+ exit
At equilibrium, electrical force balances concentration gradient and electrochemical gradient for K+ is zero (even though there is still a very substantial K+ concentration gradient)
Resting membrane potential = flow of positive/negative ions across plasma membrane precisely balanced

12. Resting membrane potential
Membrane potential measured as voltage difference across membrane
For animal cells, resting membrane potential varies between -20 and -200 mV
Negative value due to negativity of intracellular compartment compared to extracellular fluid
Because K+ channels predominate in resting plasma membrane, resting membrane potential mainly due to K+ concentration gradient
Nernst equation permits calculation of membrane potential (V):
V = 62log10(Cout/Cin)
where Cout and Cin are extracellular and intracellular ion concentrations of monovalent cation at 37°C

13. Changing the membrane potential
Resting membrane potential determined by K+ permeability
If Na+ channels open, because [Na+]out>>[Na+]in, Na+ will move into cell  membrane potential less negative
New equilibrium potential established - compromise between negative K+ potential and positive Na+ potential
Membrane potential determined by state of ion channels and transmembrane ion concentrations
Because electrical changes much more rapid than ion concentration changes - ion channel activity most important in controlling membrane potential
Voltage-gated ion channels control electrical signalling in nerve cells