NESC 2570 Lecture Notes - Lecture 4: Calcium Channel
27 Jun 2018
School
Department
Course
Professor
Mathematical Reconstruction of the Action Potential
Voltage dependent sodium and potassium conductance
○
Scientists began to realize that there must be channels in order for the conductance to occur
○
In response to an excitatory stimulus (manual or automatic) depolarization trajectory to around 0mV - the membrane
potential then repolarizes
This involved inactivating sodium conductance by a somewhat slower potassium increase
○
The sodium conductance stops and the potassium conductance stops by very different mechanisms
○
Potassium conductance deactivates while the sodium conductance inactivates (membrane potentials)
○
Intrinsic mechanisms
○
○
Pharmacological Separation of Na+ and K+ Currents Into Components
By blocking channels with drugs, scientists were able to determine the fact that there were indeed channels
○
Voltage clamping was an involved technique
When held at -75mV then triggering the membrane potential to depolarize to 0mV
○
○
Outward current - positive current coming out of the cell which hyperpolarizes the inside of the cell
○
Inward current - positive current comes into the cell which causes it to depolarize
○
Adding tetrodoxtoxin will block the sodium current
○
Adding TEA (tetraethyl-ammonium) will block the outward current and you're only left with the inward sodium
current
○
This proves that the conductances could be separated
○
The Patch Clamp Method (Part 1)
The tip of this electrode is one micrometer in diameter and the electrode sticks on the membrane and captures the
events in the membrane in a patch of membrane that is caught into the electrode
○
Glass forms a bond with the lipids of the membrane and creates an intensely tight seal - it has a very high resistance
so current doesn't leak through
Nothing is lost
○
○
When this was first applied to cells, right away, single channel conductance's were observed
○
Cell Attached Clamping
Single cell activity
○
○
The Patch Clamp Method (Part 2)
Suction was applied to the electrode, however if too much suction was applied, the seal would break
○
This would leave a hole in the membrane
○
Giving you whole cell recording, no longer would you be able to record single channels
○
You would be recording thousands of channels because you have access to the whole cell
○
The Patch Clamp Method (Part 3)
Parts of the patch of membrane can break off and reseal
○
Now the pipette had a piece of membrane suck to it with a high resistance seal
○
In order to get this configuration, you would be doing whole cell recording
○
The Patch Clamp Method (Part 4)
Starts with cell attached
○
Membrane is yanked off and the cell reseals over
○
The ion channel can be recorded, but the cytoplasmic side is exposed to the "bath"
○
Patch Clamp Measurements of Ionic Currents Flowing Through Single Na+ Channels
Single channel patch produces Image B and the membrane potential is set, you are able to change it by inserting an
electrode
Can still hold the membrane under voltage clamp
○
○
Normal channel gating is statistical and probabilistic
You can see on average how long it will take to open and how long it will stay open
○
It is the same channel undergoing a conformational change, however kinetics are different
○
○
Ensemble average (Image C) - this gives off a small curve that goes down and comes right back up again
Similar to D
○
○
Each channel is different and has statistical properties. This can be described, each action potential takes
approximately 1ms
○
Single channel openings last around half a milisecond
○
B Graph 4th row - an example where a sodium channel doesn't know how to operate correctly. Drugs can lead to this.
Preview of channelopathy
○
Sodium channels open with depolarization meaning that the probability of finding an open sodium channels increases
when the cell in depolarizing (positive feedback)
○
E graph - S shaped once the sodium channel opens, it stays open for a certain amount of time and then automatically
closes
○
Openings are short so it is difficult to do a patch clamping
○
Patch Clamp Measurements of Ionic Currents Flowing Through Single K+ Channels
Potassium channels are easier to record because they are slower
○
At -100mV there is no potassium cell activity
○
Outward potassium current - the gates stay open for quite a long time and it also takes the gates a long time to open as
well
About 40 ms
○
○
Single channels with statistical probability gives rise to the smooth curves that are fit with single exponentials raised
to a certain power by Hodgekin and Huxley
○
In patch clamping, the experimenter controls the voltage unlike in real life where the cell controls it
That way, we can study the kinetics of cell inactivation and deactivation
○
○
Functional States of Voltage Gated Na+ and K+
Voltage clamp
○
-100mV extremely low probability that sodium channel is open
At +50mV after a slight depolarization, the sodium channels open
○
Automatically inactivates and it stays closed, the probablility of getting out of the closed state is extremely low
○
Only when the membrane potential is brought back to something extremely low does the channel undergo a
change that allows the gate to open again
○
It must be strongly hyperpolarized in order for the channel to open again
○
The absolute refractory period, there are not enough channels that are able to open
○
○
K+ Channel
Opens with a delay called "delayed conductance"
○
Positive charges in the membrane move more slowly and when the channels open, allowing potassium ions to
go out, it will stay out
○
The only thing that closes the gates is going back to -100mV
○
○
Sodium channel gating scheme:
C to O to I (general)
Closed to Open to Inactivated state
○
Even for a sodium channel, you can go from the closed state to the open state if the depolarization is fast
and strong enough
○
Rate constant will tell you the speed
○
Automatic rate constant that is not regulated by voltage
○
It is able to go the opposite way as well (going from open back to closed)
○
○
C to C to C to O to I (Hodgkin- Huxley)
Closed state to another closed state to another closed state to an open state to an inactive state
○
○
○
Types of Voltage Gated Ion Channels (Part 1)
Many voltage gated channels exist
Sodium channels
○
Calcium channels
Not as many ( 6 different calcium channel genes)
○
○
Potassium channels
100 different genes
○
○
Chloride channels
10 different genes
○
○
○
Types of Voltage Gated Ion Channels (Part 2)
GABA - the most inhibitory neurotransmitter
○
Ligand - chemical
○
Glutamate is a ligand and a neurotransmitter
90% of the neurons in your brain excite other neurons with glutamate
○
A chemical that binds to the ion channel pore and opens it
○
Ligand gated channels don' t discriminate between sodium and potassium but they do discriminate between
cations and anions
○
Glutamate gated channels will always let sodium and potassium in
○
It will cause the cell to depolarize because the cell can't tell the difference between sodium and potassium it will
depolarize it towards zero if it can
○
○
Calcium activated potassium channel
Opening is aided by the presence of calcium on the inside
○
Calcium will bind to the protein and help it on the inside
○
Calcium activated voltage gated channel
○
○
Cyclic nucleotide
Intracellular ligand (cAMP or cGMP)
○
Calcium ions come in to these channels as well
○
Both calcium gated and voltage dependent gate
○
○
Ligand gated channels (lots of different types) versus voltage dependent
○
Diverse Properties of K+ Channels ( Part 1)
Voltage clamp paradigm - same for all experiments
○
-60mV will hyperpolarize to -120mV which is a huge hyperpolarization
First a depolarization but then a very strong hyperpolarization acts on it
○
○
Diverse Properties of K+ Channels (Part 2)
KV2.1
When the membrane is repolarized to -60mV a current doesn't just drop flat but has a slight trajectory called "tail
current"
○
This is the closure of the potassium channels
○
○
KV4.1
Potassium channel, voltage gated
○
Depolarizing membrane causes maximum conductance but then immediately deactivates
○
Voltage dependence of activation is the same
○
○
They give neurons different firing properties
○
-60mV to -120mV nothing happened because voltage dependence is 0
○
Diverse Properties of K+ Channels (Part 3)
HERG (Human Ether of Gogo Related Gene Channel)
Ether is a gas that causes shaking in fruit flies because they had mutation in that channel
○
Remember that going from a closed to an open state can easily go backwards the other way (closed back to open),
however when the cell is hyperpolarized it is able to shift the gates from and inactivated state to a closed state
○
Open state is so short lived that it jumps into an inactivated state from a closed state almost instantaneously at +50mV
○
Everything binds to HERG so it is a very important channel
○
○
Inward Rectifier
This is a potassium channel
○
Nothing happens at +50mV but there is a huge inward potassium current at -120mV
○
Potassium ions go in and try to depolarize the cell, the activation increases with hyperpolarization
○
Gated by a blocking mechanism and open with hyperpolarization
○
If anything tries to set the neuron negative, they will open and let potassium depolarize it
○
○
October 5th 2015
Diverse Properties of Potassium Channels (Part 4)
Voltage gated channel, the activation goes up with depolarization - if you take a snapshot and step from -60mV
to the depolarization,
+50 depolarizing step elicited outward potassium current
○
The gating of this potassium current is enhanced with intracellular calcium
○
Intracellular calcium is the first stop in regards to intracellular messangers
○
Usually intracellular calcium levels have a very low concentration
○
Elevating intracellular calcium makes it easier for calcium gates to open
○
Experimenters in image 2 are altering the intracellular calcium concentration
The same voltage dependent channels occur at lower potentials
○
Shift the activation curve to a lower threshold
○
○
Image 1 has a typo, it is milimolar not micromolar
○
○
○
Calcium Activated Potassium Current ( Increased due to voltage gated Calcium channel mediated Calcium + influx)
TEST ALERT!!!!
○
Calcium activated potassium current that shows a bunch of depolarizing steps from a real cell
○
The real cell has voltage gated calcium channels as well as calcium activated potassium channels
As one steps more positive, the calcium influx will go down
○
Typically goes down starting around 10-20mV, as less calcium comes in, less calcium activated potassium
channels will be activated
○
○
In this current voltage relation for a calcium activated potassium current, why does the calcium concentration get
smaller?
Combination of voltage dependent gating
○
Calcium comes in from calcium channels and then gets smaller
○
○
○
Ion Channels and Patch Clamping
September(28,(2015
2:34(PM
Mathematical Reconstruction of the Action Potential
Voltage dependent sodium and potassium conductance
○
Scientists began to realize that there must be channels in order for the conductance to occur
○
In response to an excitatory stimulus (manual or automatic) depolarization trajectory to around 0mV - the membrane
potential then repolarizes
This involved inactivating sodium conductance by a somewhat slower potassium increase
○
The sodium conductance stops and the potassium conductance stops by very different mechanisms
○
Potassium conductance deactivates while the sodium conductance inactivates (membrane potentials)
○
Intrinsic mechanisms
○
○
Pharmacological Separation of Na+ and K+ Currents Into Components
By blocking channels with drugs, scientists were able to determine the fact that there were indeed channels
○
Voltage clamping was an involved technique
When held at -75mV then triggering the membrane potential to depolarize to 0mV
○
○
Outward current - positive current coming out of the cell which hyperpolarizes the inside of the cell
○
Inward current - positive current comes into the cell which causes it to depolarize
○
Adding tetrodoxtoxin will block the sodium current
○
Adding TEA (tetraethyl-ammonium) will block the outward current and you're only left with the inward sodium
current
○
This proves that the conductances could be separated
○
The Patch Clamp Method (Part 1)
The tip of this electrode is one micrometer in diameter and the electrode sticks on the membrane and captures the
events in the membrane in a patch of membrane that is caught into the electrode
○
Glass forms a bond with the lipids of the membrane and creates an intensely tight seal - it has a very high resistance
so current doesn't leak through
Nothing is lost
○
○
When this was first applied to cells, right away, single channel conductance's were observed
○
Cell Attached Clamping
Single cell activity
○
○
The Patch Clamp Method (Part 2)
Suction was applied to the electrode, however if too much suction was applied, the seal would break
○
This would leave a hole in the membrane
○
Giving you whole cell recording, no longer would you be able to record single channels
○
You would be recording thousands of channels because you have access to the whole cell
○
The Patch Clamp Method (Part 3)
Parts of the patch of membrane can break off and reseal
○
Now the pipette had a piece of membrane suck to it with a high resistance seal
○
In order to get this configuration, you would be doing whole cell recording
○
The Patch Clamp Method (Part 4)
Starts with cell attached
○
Membrane is yanked off and the cell reseals over
○
The ion channel can be recorded, but the cytoplasmic side is exposed to the "bath"
○
Patch Clamp Measurements of Ionic Currents Flowing Through Single Na+ Channels
Single channel patch produces Image B and the membrane potential is set, you are able to change it by inserting an
electrode
Can still hold the membrane under voltage clamp
○
○
Normal channel gating is statistical and probabilistic
You can see on average how long it will take to open and how long it will stay open
○
It is the same channel undergoing a conformational change, however kinetics are different
○
○
Ensemble average (Image C) - this gives off a small curve that goes down and comes right back up again
Similar to D
○
○
Each channel is different and has statistical properties. This can be described, each action potential takes
approximately 1ms
○
Single channel openings last around half a milisecond
○
B Graph 4th row - an example where a sodium channel doesn't know how to operate correctly. Drugs can lead to this.
Preview of channelopathy
○
Sodium channels open with depolarization meaning that the probability of finding an open sodium channels increases
when the cell in depolarizing (positive feedback)
○
E graph - S shaped once the sodium channel opens, it stays open for a certain amount of time and then automatically
closes
○
Openings are short so it is difficult to do a patch clamping
○
Patch Clamp Measurements of Ionic Currents Flowing Through Single K+ Channels
Potassium channels are easier to record because they are slower
○
At -100mV there is no potassium cell activity
○
Outward potassium current - the gates stay open for quite a long time and it also takes the gates a long time to open as
well
About 40 ms
○
○
Single channels with statistical probability gives rise to the smooth curves that are fit with single exponentials raised
to a certain power by Hodgekin and Huxley
○
In patch clamping, the experimenter controls the voltage unlike in real life where the cell controls it
That way, we can study the kinetics of cell inactivation and deactivation
○
○
Functional States of Voltage Gated Na+ and K+
Voltage clamp
○
-100mV extremely low probability that sodium channel is open
At +50mV after a slight depolarization, the sodium channels open
○
Automatically inactivates and it stays closed, the probablility of getting out of the closed state is extremely low
○
Only when the membrane potential is brought back to something extremely low does the channel undergo a
change that allows the gate to open again
○
It must be strongly hyperpolarized in order for the channel to open again
○
The absolute refractory period, there are not enough channels that are able to open
○
○
K+ Channel
Opens with a delay called "delayed conductance"
○
Positive charges in the membrane move more slowly and when the channels open, allowing potassium ions to
go out, it will stay out
○
The only thing that closes the gates is going back to -100mV
○
○
Sodium channel gating scheme:
C to O to I (general)
Closed to Open to Inactivated state
○
Even for a sodium channel, you can go from the closed state to the open state if the depolarization is fast
and strong enough
○
Rate constant will tell you the speed
○
Automatic rate constant that is not regulated by voltage
○
It is able to go the opposite way as well (going from open back to closed)
○
○
C to C to C to O to I (Hodgkin- Huxley)
Closed state to another closed state to another closed state to an open state to an inactive state
○
○
○
Types of Voltage Gated Ion Channels (Part 1)
Many voltage gated channels exist
Sodium channels
○
Calcium channels
Not as many ( 6 different calcium channel genes)
○
○
Potassium channels
100 different genes
○
○
Chloride channels
10 different genes
○
○
○
Types of Voltage Gated Ion Channels (Part 2)
GABA - the most inhibitory neurotransmitter
○
Ligand - chemical
○
Glutamate is a ligand and a neurotransmitter
90% of the neurons in your brain excite other neurons with glutamate
○
A chemical that binds to the ion channel pore and opens it
○
Ligand gated channels don' t discriminate between sodium and potassium but they do discriminate between
cations and anions
○
Glutamate gated channels will always let sodium and potassium in
○
It will cause the cell to depolarize because the cell can't tell the difference between sodium and potassium it will
depolarize it towards zero if it can
○
○
Calcium activated potassium channel
Opening is aided by the presence of calcium on the inside
○
Calcium will bind to the protein and help it on the inside
○
Calcium activated voltage gated channel
○
○
Cyclic nucleotide
Intracellular ligand (cAMP or cGMP)
○
Calcium ions come in to these channels as well
○
Both calcium gated and voltage dependent gate
○
○
Ligand gated channels (lots of different types) versus voltage dependent
○
Diverse Properties of K+ Channels ( Part 1)
Voltage clamp paradigm - same for all experiments
○
-60mV will hyperpolarize to -120mV which is a huge hyperpolarization
First a depolarization but then a very strong hyperpolarization acts on it
○
○
Diverse Properties of K+ Channels (Part 2)
KV2.1
When the membrane is repolarized to -60mV a current doesn't just drop flat but has a slight trajectory called "tail
current"
○
This is the closure of the potassium channels
○
○
KV4.1
Potassium channel, voltage gated
○
Depolarizing membrane causes maximum conductance but then immediately deactivates
○
Voltage dependence of activation is the same
○
○
They give neurons different firing properties
○
-60mV to -120mV nothing happened because voltage dependence is 0
○
Diverse Properties of K+ Channels (Part 3)
HERG (Human Ether of Gogo Related Gene Channel)
Ether is a gas that causes shaking in fruit flies because they had mutation in that channel
○
Remember that going from a closed to an open state can easily go backwards the other way (closed back to open),
however when the cell is hyperpolarized it is able to shift the gates from and inactivated state to a closed state
○
Open state is so short lived that it jumps into an inactivated state from a closed state almost instantaneously at +50mV
○
Everything binds to HERG so it is a very important channel
○
○
Inward Rectifier
This is a potassium channel
○
Nothing happens at +50mV but there is a huge inward potassium current at -120mV
○
Potassium ions go in and try to depolarize the cell, the activation increases with hyperpolarization
○
Gated by a blocking mechanism and open with hyperpolarization
○
If anything tries to set the neuron negative, they will open and let potassium depolarize it
○
○
October 5th 2015
Diverse Properties of Potassium Channels (Part 4)
Voltage gated channel, the activation goes up with depolarization - if you take a snapshot and step from -60mV
to the depolarization,
+50 depolarizing step elicited outward potassium current
○
The gating of this potassium current is enhanced with intracellular calcium
○
Intracellular calcium is the first stop in regards to intracellular messangers
○
Usually intracellular calcium levels have a very low concentration
○
Elevating intracellular calcium makes it easier for calcium gates to open
○
Experimenters in image 2 are altering the intracellular calcium concentration
The same voltage dependent channels occur at lower potentials
○
Shift the activation curve to a lower threshold
○
○
Image 1 has a typo, it is milimolar not micromolar
○
○
○
Calcium Activated Potassium Current ( Increased due to voltage gated Calcium channel mediated Calcium + influx)
TEST ALERT!!!!
○
Calcium activated potassium current that shows a bunch of depolarizing steps from a real cell
○
The real cell has voltage gated calcium channels as well as calcium activated potassium channels
As one steps more positive, the calcium influx will go down
○
Typically goes down starting around 10-20mV, as less calcium comes in, less calcium activated potassium
channels will be activated
○
○
In this current voltage relation for a calcium activated potassium current, why does the calcium concentration get
smaller?
Combination of voltage dependent gating
○
Calcium comes in from calcium channels and then gets smaller
○
○
○
Ion Channels and Patch Clamping
September(28,(2015
2:34(PM
Mathematical Reconstruction of the Action Potential
Voltage dependent sodium and potassium conductance
○
Scientists began to realize that there must be channels in order for the conductance to occur
○
In response to an excitatory stimulus (manual or automatic) depolarization trajectory to around 0mV - the membrane
potential then repolarizes
This involved inactivating sodium conductance by a somewhat slower potassium increase
○
The sodium conductance stops and the potassium conductance stops by very different mechanisms
○
Potassium conductance deactivates while the sodium conductance inactivates (membrane potentials)
○
Intrinsic mechanisms
○
○
Pharmacological Separation of Na+ and K+ Currents Into Components
By blocking channels with drugs, scientists were able to determine the fact that there were indeed channels
○
Voltage clamping was an involved technique
When held at -75mV then triggering the membrane potential to depolarize to 0mV
○
○
Outward current - positive current coming out of the cell which hyperpolarizes the inside of the cell
○
Inward current - positive current comes into the cell which causes it to depolarize
○
Adding tetrodoxtoxin will block the sodium current
○
Adding TEA (tetraethyl-ammonium) will block the outward current and you're only left with the inward sodium
current
○
This proves that the conductances could be separated
○
The Patch Clamp Method (Part 1)
The tip of this electrode is one micrometer in diameter and the electrode sticks on the membrane and captures the
events in the membrane in a patch of membrane that is caught into the electrode
○
Glass forms a bond with the lipids of the membrane and creates an intensely tight seal - it has a very high resistance
so current doesn't leak through
Nothing is lost
○
○
When this was first applied to cells, right away, single channel conductance's were observed
○
Cell Attached Clamping
Single cell activity
○
○
The Patch Clamp Method (Part 2)
Suction was applied to the electrode, however if too much suction was applied, the seal would break
○
This would leave a hole in the membrane
○
Giving you whole cell recording, no longer would you be able to record single channels
○
You would be recording thousands of channels because you have access to the whole cell
○
The Patch Clamp Method (Part 3)
Parts of the patch of membrane can break off and reseal
○
Now the pipette had a piece of membrane suck to it with a high resistance seal
○
In order to get this configuration, you would be doing whole cell recording
○
The Patch Clamp Method (Part 4)
Starts with cell attached
○
Membrane is yanked off and the cell reseals over
○
The ion channel can be recorded, but the cytoplasmic side is exposed to the "bath"
○
Patch Clamp Measurements of Ionic Currents Flowing Through Single Na+ Channels
Single channel patch produces Image B and the membrane potential is set, you are able to change it by inserting an
electrode
Can still hold the membrane under voltage clamp
○
○
Normal channel gating is statistical and probabilistic
You can see on average how long it will take to open and how long it will stay open
○
It is the same channel undergoing a conformational change, however kinetics are different
○
○
Ensemble average (Image C) - this gives off a small curve that goes down and comes right back up again
Similar to D
○
○
Each channel is different and has statistical properties. This can be described, each action potential takes
approximately 1ms
○
Single channel openings last around half a milisecond
○
B Graph 4th row - an example where a sodium channel doesn't know how to operate correctly. Drugs can lead to this.
Preview of channelopathy
○
Sodium channels open with depolarization meaning that the probability of finding an open sodium channels increases
when the cell in depolarizing (positive feedback)
○
E graph - S shaped once the sodium channel opens, it stays open for a certain amount of time and then automatically
closes
○
Openings are short so it is difficult to do a patch clamping
○
Patch Clamp Measurements of Ionic Currents Flowing Through Single K+ Channels
Potassium channels are easier to record because they are slower
○
At -100mV there is no potassium cell activity
○
Outward potassium current - the gates stay open for quite a long time and it also takes the gates a long time to open as
well
About 40 ms
○
○
Single channels with statistical probability gives rise to the smooth curves that are fit with single exponentials raised
to a certain power by Hodgekin and Huxley
○
In patch clamping, the experimenter controls the voltage unlike in real life where the cell controls it
That way, we can study the kinetics of cell inactivation and deactivation
○
○
Functional States of Voltage Gated Na+ and K+
Voltage clamp
○
-100mV extremely low probability that sodium channel is open
At +50mV after a slight depolarization, the sodium channels open
○
Automatically inactivates and it stays closed, the probablility of getting out of the closed state is extremely low
○
Only when the membrane potential is brought back to something extremely low does the channel undergo a
change that allows the gate to open again
○
It must be strongly hyperpolarized in order for the channel to open again
○
The absolute refractory period, there are not enough channels that are able to open
○
○
K+ Channel
Opens with a delay called "delayed conductance"
○
Positive charges in the membrane move more slowly and when the channels open, allowing potassium ions to
go out, it will stay out
○
The only thing that closes the gates is going back to -100mV
○
○
Sodium channel gating scheme:
C to O to I (general)
Closed to Open to Inactivated state
○
Even for a sodium channel, you can go from the closed state to the open state if the depolarization is fast
and strong enough
○
Rate constant will tell you the speed
○
Automatic rate constant that is not regulated by voltage
○
It is able to go the opposite way as well (going from open back to closed)
○
○
C to C to C to O to I (Hodgkin- Huxley)
Closed state to another closed state to another closed state to an open state to an inactive state
○
○
○
Types of Voltage Gated Ion Channels (Part 1)
Many voltage gated channels exist
Sodium channels
○
Calcium channels
Not as many ( 6 different calcium channel genes)
○
○
Potassium channels
100 different genes
○
○
Chloride channels
10 different genes
○
○
○
Types of Voltage Gated Ion Channels (Part 2)
GABA - the most inhibitory neurotransmitter
○
Ligand - chemical
○
Glutamate is a ligand and a neurotransmitter
90% of the neurons in your brain excite other neurons with glutamate
○
A chemical that binds to the ion channel pore and opens it
○
Ligand gated channels don' t discriminate between sodium and potassium but they do discriminate between
cations and anions
○
Glutamate gated channels will always let sodium and potassium in
○
It will cause the cell to depolarize because the cell can't tell the difference between sodium and potassium it will
depolarize it towards zero if it can
○
○
Calcium activated potassium channel
Opening is aided by the presence of calcium on the inside
○
Calcium will bind to the protein and help it on the inside
○
Calcium activated voltage gated channel
○
○
Cyclic nucleotide
Intracellular ligand (cAMP or cGMP)
○
Calcium ions come in to these channels as well
○
Both calcium gated and voltage dependent gate
○
○
Ligand gated channels (lots of different types) versus voltage dependent
○
Diverse Properties of K+ Channels ( Part 1)
Voltage clamp paradigm - same for all experiments
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-60mV will hyperpolarize to -120mV which is a huge hyperpolarization
First a depolarization but then a very strong hyperpolarization acts on it
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Diverse Properties of K+ Channels (Part 2)
KV2.1
When the membrane is repolarized to -60mV a current doesn't just drop flat but has a slight trajectory called "tail
current"
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This is the closure of the potassium channels
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KV4.1
Potassium channel, voltage gated
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Depolarizing membrane causes maximum conductance but then immediately deactivates
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Voltage dependence of activation is the same
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They give neurons different firing properties
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-60mV to -120mV nothing happened because voltage dependence is 0
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Diverse Properties of K+ Channels (Part 3)
HERG (Human Ether of Gogo Related Gene Channel)
Ether is a gas that causes shaking in fruit flies because they had mutation in that channel
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Remember that going from a closed to an open state can easily go backwards the other way (closed back to open),
however when the cell is hyperpolarized it is able to shift the gates from and inactivated state to a closed state
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Open state is so short lived that it jumps into an inactivated state from a closed state almost instantaneously at +50mV
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Everything binds to HERG so it is a very important channel
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Inward Rectifier
This is a potassium channel
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Nothing happens at +50mV but there is a huge inward potassium current at -120mV
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Potassium ions go in and try to depolarize the cell, the activation increases with hyperpolarization
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Gated by a blocking mechanism and open with hyperpolarization
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If anything tries to set the neuron negative, they will open and let potassium depolarize it
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October 5th 2015
Diverse Properties of Potassium Channels (Part 4)
Voltage gated channel, the activation goes up with depolarization - if you take a snapshot and step from -60mV
to the depolarization,
+50 depolarizing step elicited outward potassium current
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The gating of this potassium current is enhanced with intracellular calcium
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Intracellular calcium is the first stop in regards to intracellular messangers
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Usually intracellular calcium levels have a very low concentration
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Elevating intracellular calcium makes it easier for calcium gates to open
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Experimenters in image 2 are altering the intracellular calcium concentration
The same voltage dependent channels occur at lower potentials
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Shift the activation curve to a lower threshold
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Image 1 has a typo, it is milimolar not micromolar
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Calcium Activated Potassium Current ( Increased due to voltage gated Calcium channel mediated Calcium + influx)
TEST ALERT!!!!
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Calcium activated potassium current that shows a bunch of depolarizing steps from a real cell
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The real cell has voltage gated calcium channels as well as calcium activated potassium channels
As one steps more positive, the calcium influx will go down
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Typically goes down starting around 10-20mV, as less calcium comes in, less calcium activated potassium
channels will be activated
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In this current voltage relation for a calcium activated potassium current, why does the calcium concentration get
smaller?
Combination of voltage dependent gating
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Calcium comes in from calcium channels and then gets smaller
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Ion Channels and Patch Clamping
September(28,(2015
2:34(PM
Document Summary
Scientists began to realize that there must be channels in order for the conductance to occur. In response to an excitatory stimulus (manual or automatic) depolarization trajectory to around 0mv - the membrane potential then repolarizes. This involved inactivating sodium conductance by a somewhat slower potassium increase. The sodium conductance stops and the potassium conductance stops by very different mechanisms. Potassium conductance deactivates while the sodium conductance inactivates (membrane potentials) Pharmacological separation of na+ and k+ currents into components. By blocking channels with drugs, scientists were able to determine the fact that there were indeed channels. When held at -75mv then triggering the membrane potential to depolarize to 0mv. Outward current - positive current coming out of the cell which hyperpolarizes the inside of the cell. Inward current - positive current comes into the cell which causes it to depolarize. Adding tea (tetraethyl-ammonium) will block the outward current and you"re only left with the inward sodium current.
