Lecture 4: Ion Channels and Transporters Flashcards
What are the 4 properties ion channels and the currents flowing through them should have?
- Large ion currents indicate that the channels have to be capable of allowing ions to move across the membranes at high rates.
- Channels have to make use of the electrochemical gradients of various ions.
- There are channels selective only for Na+ or only K+.
- Because the conductances are voltage-dependent, channels have to be able to sense changes in membrane potential
What does the patch clamp method measure?
the current flowing through a single ion channel
What are the 4 different configurations of the patch clamp method?
1.Cell-attached recording.
2. Whole-cell recording.
3. Inside-out recording.
4. Outside-out recording.
Describe cell-attached recording
there’s tight contact between the recording electrode/pipette and the membrane, the electrode is small enough to record from a single ion channel
Describe whole-cell recording
creates a tight seal between the pipette and membrane, then with a strong pulse of suction it creates a small break in the suction which makes the cytoplasm continuous with the pipette, now you can measure the current in the cell and can add anything intracellularly via the electrode
Describe inside-out recording
electrode attaches to a small region of the membrane and creates a break where you separate the membrane from the channels and now the cytoplasmic domain is accessible
Describe outside-out recording
use the electrode, and pull it out which creates a break at 2 points where the ends of the membrane anneal and makes the extracellular domain accessible
What is the probability of sodium channels opening w.r.t. to membrane potential?
The probability increases as you increase depolarization
What is the probability of potassium channels opening w.r.t. membrane potential?
The probability increases as you increase depolarization
true of false: channels for both sodium and potassium are voltage-gated
true
true or false: sodium and potassium channels open at the same time during depolarization
false
true or false: sodium and potassium channels close during hyperpolarization
true
What do sodium and potassium channels have that detect the potential across the membrane?
a voltage sensor
What channel opens first?
sodium
What channels open second?
potassium
What inactivates sodium channels?
depolarization but it leads to a nonconducting state, indicating an additional mechanism for inactivation
Describe the functional states of voltage-gated sodium and potassium channels
Initially, when the membrane depolarized, sodium channels go from the closed state to an open state and sodium can enter the neuron, potassium channels remain closed and as time goes on the sodium channels get inactivated and potassium channels open so potassium can flow out of the cell
When MP is brought back to rest, both channels close
What happens to a neuron treated with alpha scorpion toxin?
increases sodium current leading to the prolonged current of an action potential
What happens to a neuron treated with beta scorpion toxin?
changes the activation of sodium channels to a more hyperpolarized potential so sodium channels can be activated at higher hyperpolarized potentials than they could, and it shifts the conductance curve of sodium to the left
What are the 4 types of ion channels?
- Voltage-gated ion channels:
- Na+ channels.
- K+ channels.
- Ca2+ channels.
- Cl- channels. - Ligand-gated ion channels:
- Neurotransmitter-gated.
Cyclic nucleotide-gated. - Transient receptor potential family.
- Thermosensitive channel. - Mechanosensitive channel.
Describe voltage-gated calcium channels
Start at rest, hold membrane potential at a higher potential and leads to calcium current
Describe neurotransmitter-gated channel
AMPA receptors bind the excitatory neurotransmitter, glutamate, a neurotransmitter binds to the channel, they open and current can flow through the channel, activate synapses by binding to glutamate, get excitatory postsynaptic current
Describe cyclic nt-gated channel
the cyclic nts are acting on channels on the intracellular side, activated by light
Describe thermosensitive channels
open in response to temperature change, different temperatures activate - some open in response to higher or lower temperatures
Describe mechanosensitive channels
Open in response to mechanical stimuli, get a displacement, which leads to current flowing through the channels
How do ions permeate through channels?
via ion-selective pores
Describe the ion channel pore
The pore is composed of the arrangement of loops and helices of the subunits that make up the channel.
The pore is large enough for only the ion for which it is meant to pass through and therefore, makes the membrane permeable to that ion.
What is the selectivity filter?
Each ion channel has a specific selectivity filter that ushers in a particular ion, while excluding others, including those of the same valence
What organism was early work on ion channel pores done on?
bacteria potassium channels
What makes up the ion channel pore?
The transmembrane domains with a selectivity filter and the inner/outer helices
Why are molecular specializations important?
They permit gating of ion channels by different types of stimuli
Why are voltage-gated channels larger than other types of channels?
Each channel subunit has additional transmembrane structures that form the voltage sensors for that channel
What do voltage sensors contain?
positively charged amino acids
How do voltage sensors move?
depolarizations push the senors outward and hyperpolarizations pull the sensors inward then sensors pull on the helical linker connecting the sensors to the pore then pull it open or push it closed
Describe the structure of mammalian voltage-gated potassium channels
when the membrane is hyperpolarized, gets deconfiguration (channel closed)
when the membrane depolarized, leads to a conformational change that opens the pore, so voltage sensors detect changes in membrane potential and change conformation allowing the channel to open – other voltage-gated channels operate in a similar manner
What are some neurological diseases caused by altered ion channels?
Epilepsy
Ataxia
Migraine headaches
Pain
Deafness
Blindness
Describe how altered ion channels can cause pain
a mutation known as IEM (inherited erythromelalgia) in sodium channels affects sodium channel function, sodium current is larger and lasts longer in IEM, associated with burning pain and if you measured action potentials between wildtype and neurons with this mutation, higher frequency of AP in this mutation, get an increase in sodium current and it stays activated longer so greater amplitude and results in increased AP frequency
Describe how altered ion channels can cause deafness
a mutation known as SANDD (sinoatrial node dysfunction and deficit) is found in a gene for calcium channels, when you depolarize the neuron, the mutation prevents calcium current, and the affected calcium channel is specifically involved in hearing, so this mutation causes a loss of hearing
