Lecture 4 - Transporters - Channels Flashcards
What did the voltage clamp experiments by Hodgkin and Huxley predict?
1) Separate Na and K channels 2) Voltage sensors in channels 3) high conductance of channels to specific ions
What helped form Hodgkin and Huxley’s predictions?
voltage clamp experiment indicating how Na+ and K+ currents change with increasing
What happens once the patch clamp voltage reaches +52 mV?
that the early inward Na+ current is missing
In the voltage clamp experiment, what happens at +65 mV?
it reverses to an outward flow.
What happens as the voltage becomes more and more positive?
the later outward K+ current increases in magnitude
What does the path-clamp technique allow for?
It allows for characterization of single channels
Patch-clamp is
a refinement of the voltage-clamp technique where voltage change activates channel openings.
Who developed the patch clamp?
developed by Sackman and Neher (Nobel Prize winners).
What is the patch-clamp technique?
Glass pipette is pressed against a cell membrane – slight suction is applied to generate a ‘gigaseal’ (low noise).
What is the purpose of the gigaseal?
All current flows through electrode and does not leak through the seal.
Describe the current in the patch clamp technique?
1) Macroscopic currents ~10-100 picoAmps (pAs) due to current flow through many channels 2) Microscopic current amplitude ~fraction of pA to several pAs due to current flow through one channel (lower right panel).
What is the macroscopic current flow due to?
Current flow through many channels
What is the microscopic current flow due to?
Current flow through one channel
Patch clamp recordings
it detects current flowing through single membrane channels due to depolarization
Describe the channels in the patch clamp experiment.
1) channels open and close in an all or none fashion 2) there is fast switch between open and close states 3) channels open and close in stochastic (random) manner
In the patch clamp experiment, what does gating refer to?
1) Gating is the transition between open and closed states 2) gating involves a temporary conformational change in the channels structure
In the patch clamp, what happens in response to the depolarizing effect from the pipette?
single channels open and close in an all or none fashion. Random or stochastic in nature
In patch clamp, what does the probability of opening depend on?
The stimulus; 1) voltage change or 2) ligand binding
What does the patch clamp measurements of ionic currents through single Na channels reveal?
1) voltage gated Na channels 2) depolarization increases the probability of a channel being open and hyperpolarizing decreases it
Depolarizing stimulus
increases the probability that the Na+ channel is opened.
The greater the depolarization
the higher the probability of channel opening.
For patch clamp looking at Na channels what happens to K+ channels?
they were blocked in this experiment to look at Na channels. Therapeutic drugs that act on ion channels are now being tested using this technique.
What is the patch clamp measurements of inward ionic currents through single Na channels vs the cell?
Macroscopic current arises from the aggregate effects of 1000s of microscopic currents (individual channels)
Stimulus (membrane potential depolarization of patch)
changes the probability that channel is open or closed.
Comparing the time course of the macroscopic current and the sum of many trials of the single ion channel show what?
close correlations of time courses of the macroscopic and microscopic currents
Is channel opening controlled in the patch clamp experiment?
Random or stochastic opening of channels
Probability of opening
increases with depolarization
Microscopic current
single channel
Macroscopic current
summed activity of 1000s of Na+ channels (K+ channels blocked).
Compare the Na and K channel data from the patch clamp experiment.
opposite current direction, longer latency for activation and long duration of activation for the K+ channel vs the Na+ channel properties.
The sum of many microscopic trials approximates what?
the time course of the macroscopic currents from the whole cell.
Sustained response (patch clamp)
on average the K+ channels tend to be an open state while the membrane is depolarized.
K+ channels diversity.
Multiple types of voltage gated K+ channels exist that have different properties and influence neuron firing.
Microscopic and macroscopic currents
Properties of microscopic currents (patch clamp) are the same as those of macroscopic currents
Na channels
1) opening is voltage dependant 2) opening near beginning of depolarization pulse 3) inactivate 4) current reverses at Na equilibrium potential 5) TTX blocks
K channels
1) opening is voltage-dependant 2) opens later 3) many do not inactivate, they just close 4) TEA or (Cs) blocks it
K channels in the CNS
most CNS neurons have multiple Potassium channels with different characteristics
K channel diversity as it pertains to voltage
voltage dependence of activation (low voltage versus high voltage activation)
K channel diversity as it pertains to rate?
Diversity in the rate of activation (How fast the population reaches maximum conductance)
K channel diversity as it pertains to inactivation
inactivation properties, some inactivate quickly, some inactivate slowly, some don’t inactivate, this produces a diversity of spike waveforms and spike patterns for different cells
Functional roles of the after hyperpolarization
1) Fast AHP 2) Medium AHP 3) Slow AHP
Fast AHP
1) (2-5 ms) shortens the AP by quickly repolarizing the membrane. 2) Only affects early spike frequency at very high frequencies 3) BK K channels, activation by Ca and depolarization and then rapid inactivation
Medium AHP
1) (10-100 ms) controls early interspike interval 2) contributes to early spike frequency adaptation, slowly activating by Ca entry 3) controls late spike frequency adaptation (IK and SK, K channels, non inactivating)
Slow AHP
(100ms – 3000ms) Limits firing frequency by an unknown channel
For the functional roles of AHP, which are rapid inactivation and which are non-inactivating?
1) Fast AHP = rapid inactivation 2) Medium AHP = non-inactivating
In some types of neurons what is the role of voltage gated Ca channels?
They result in bursts of APs that may last 100ms or longer
Channel timings
1) Na channels open 2) Na channels inactivate, Ka+ and Kdr+ channels open 3) Kbk+ channels open 4) Ca channels open 5) other known and unknown K+ channels open
What results in neurons with diverse electrical properties?
Larger number of ion channel genes
Voltage gated channels typically
allow only a single type of ion to pass through the channel although there are exceptions.
Ligand gated ion channels often
allow two or more types of ions to pass through the channel.
Ionic channels are organized based on
sequence homology
Voltage dependant ion channels differ in
their cellular expression and subcellular localization impacting their relative contribution to brain function
Kv4.1
these channels play a positive role in tumorigenic human mammary cells.
What does double immunofluorescence staining for Kv1.4 and Kv2.1 in the adult hippocampus show?
1) Staining for Kv1.4 is red and are axons 2) Staining for Kv2.1 is green and are soma proximal dendrites
In adult hippocampus, Kv1.4 staining?
In terminal fields of the medial perforant path in the middle molecular layer of the dentate gyrus and mossy fiber axons and terminal s. lucidum of CA-3
In adult hippocampus, Kv2.1 staining?
It is most prominent in the pyramidal cell CA-1 layer
Why are there so many genes encoding K+ channels?
So the genes can differ in: 1) activation 2) gating 3) inactivation
What does the diversity of K channels allow?
Influence the duration of AP and resting membrane potential
The Kv2.1 channels
show little inactivation and are related to channels involved in repolarization.
The Kv4.1 channels
inactivate rapidly to depolarization.
The inward rectifier channels
allow more current flow during hyperpolarization than during depolarization.
The Ca++ activated K+ channel
opens in response to increased intracellular Ca++ and sometimes to membrane depolarization.
Ion channels encoded by
large and diverse families of homologous genes
Ion channel differences
they differ widely in cellular expression and subcellular localization
Voltage gated channel differences
different voltage gated channels differ in functional properties (activation, inactivation and gating)
Ion channels contribute to
rich electrical responses
Ion channel diversity
is key to developing new therapeutics for central nervous system disorders
Channelopathies
they are genetic diseases resulting from mutations in channel genes
Channelopathies, voltage gated Ca channels
1) congenital stationary night blindness 2) familial hemiplegic membrane 3) episodic ataxia type 2
Channelopathies, Na channel defect
generalized epilepsy with febrile seizures
Channelopathies, K channel mutations
benign familial neonatal convulsion
Toxins target what sites on ion channels
extracellular domains and pore regions
Tetrodotoxin
(puffer fish) block Na channels
Saxitoxin
(red tide) a homologue of TTX
Alpha toxins
(scorpion) prolong duration of Na currents
Beta toxins
(scorpion) shift voltage activation of Na channels
Batrachotoxin
(frogs) inactivation of Na channels (used by South American indians)
Dentrotoxin
(wasps) K+ channel blockers
Amapin
(bees) ?
Omega – conotoxins
(cone snails) – N-type Ca channels
Omega – agatoxin
(spiders) P/Q – type Ca channels
Active ion transporters are
membrane proteins that create and maintain ion gradient
Active ion transporters that translocate what?
Translocate ions against their electrochemical gradient (consume energy)
Active ion transporters form what?
Form complex with ion they transport
Active transport; binding and unbinding
is slow (ms)
Active transport versus channels
ion translocation is slower in transporters than in channels (1000/sec)
ATPase pumps (Na/K, Ca)
acquire energy from hydrolysis of ATP
Ion exchangers and co-transporters
depend on the electrochemical gradient of other ions as other sources
Ion exchangers
trade an intracellular ion for an extracellular ion, e.g., Na/Ca, Na/H and do not use ATP as an energy source.
Ion co-transporters
transport two or more ions/molecules in the same direction across the membrane.
Ion channels regulate
the flow of ions across the membrane, influencing cell activities
Channels differ in
ion selectivity and in factors that control their gating
Ion selectivity
is achieved through interactions between the ion and the amino acids that line the walls of the channel pore (selectivity filter)
Patch clamp technique
measure current flow through single open channels
Gene cloning
determine the sequence of genes that encode channels
X-ray crystallography
provided detailed 3D structure of the bacterial K channel
Channels are targets of
blockers, toxins, and various diseases resulting from genetic mutations
Active ion transporters are
membrane proteins that create and maintain ion gradients using ATP as the energy source
Ion exchangers
use the electrochemical gradients of co-transported ions as an energy source to exchange ions
