Ion Channels Block A

Question 1

What types of ion channels mediate excitation?

Answer 1

There are two channels with the important role in excitation of a cell. These are voltage gated sodium channels and voltage gated calcium channels. It should be noted that to actually activate these channels a rise in membrane potential must first occur. The channels that mediate this rise are nicotinic Ach receptors and muscarinic Ach receptors. This causes an increase in Na+ or Ca+ that increases membrane potential typically to -50mv which is the threshold for voltage gated sodium channels. At this voltage they are stimulated and open to allow a rush of sodium/calcium within the cell that causes excitation to occur.

Question 1

What is the function of the S4 segment?

Answer 1

S4 is the voltage sensor within the channel. It will go in the opposite direction to like charges. This is how an excitation is able to occur. In Na+ example when you reach -50mV the voltage sensor will change and the sensor will open. Causing the channel to open. It should be noted there are four domains for the S4 sensor and only when all four sensors are activated will the channel open. Why is there four S4 sensors? Basically the Na+ and Ca are made up of four homologous domains. Each domain contains a S1-S6 alpha helixes.

Question 1

What type of K+ channels is opened by hyperpolarisation? What is the gating mechanism of these channels?

Answer 1

The Potassium inward rectifiers are open in hyperpolarisation. As the charge decreases greatly within the cell the Mg2+ that is associated with the channel is attracted inwardly toward the cytoplasm and therefore the channel is open. When the membrane potential reaches approx. -80mV the Mg2+ and or polyamines will block the channel as the resting membrane potential has been reached.

Question 2

What are the equilibrium potentials for K+, Na+, Cl+ and Ca2+ in a neuronal cell?

Answer 2

Ek = -90mV

ENa = +63mV

ECa = +123 mV

ECl = -70mV

Equilibrium potentials can be determined using the Nernst equation, the equation is as follows:

Ex = - 58/z log ([X]in/[X]out)

Therefore when we know all the concentrations inside and outside for K, Na, Cl and Ca we can determine the Ex.

K conc, in: 140mM, out: 4mM
Na conc, in: 12mM, out: 145mM
Cl conc, in: 3-40mM, out: 145mM
Ca conc, in: 0.0001mM, out: 1.8mM

Now we can plug in these values into the eq to establish Ex.
Ek = -58log (140/4)
Ek = -90mV

Question 3

List the main types of channel gating mechanisms.

Answer 3

Three major gating mechanisms include: voltage changes, ligand binding, physical sensory stimuli.

Question 3

Why stimulation of GABAA receptors causes inhibitory response in CNS?

Answer 3

The pore is permeable to Cl- current through GABAa receptors drives the membrane potential towards hyperpolarisation and prevents membrane depolarisation.

Question 4

How membrane current is related to membrane conductance? What is the driving force?

Answer 4

Not 100% sure however I think: When you have more open channels in a membrane you therefore have a larger conductance. Since the ohms law for membrane currents is: Ix = Gx (Em – Ex) if you increase the conductance in this equation you therefore increase the overall membrane current. Basically when you have more open the cell is able to increase the permeability of the ion and therefore the current will increase because there is more charge entering (or exiting the cell)

Question 4

What is the difference between closed and inactivated states of the voltage gated Na+ channel?

Answer 4

Inactivation is the ball and chain and gives the refractory period in which time is taken hyperpolarise and the ball and chain fall off and un inactivate. While the closed state is when the neuron is resting and remains closed due to the S4 segments.

Question 4

Give examples of temperature sensitive TRP channels.

Answer 4

TRPA1 52C, TRPV1>42C

Question 6

If the membrane of a neuronal cell was equally permeable to Na+ and K+ and impermeable to Cl- and Ca2+, what membrane potential would be? How would membrane potential change if membrane was made permeable to Cl?

Question 7

What types of ion channels are blocked by TTX and STX? What is the natural source of TTX?

Answer 7

Voltage Gated Sodium channels. Puffer fish, blue ringed octopus.

Question 9

What types of ion channels maintain resting state?

Answer 9

The main ion channel involved in maintaining resting state is the leakage channels. Primarily potassium although chloride leakage channels are important. The potassium inward rectifier channel is extremely important in contributing to the resting membrane potential.

Question 10

How single channel open probability is determined? What equation describes voltage dependence of channel open probability?

Answer 10

Single channel open probability gives the chances of a single channel being opened at that specific membrane voltage.
Po = to/ (to + tc)
to – open time
tc – closed time

Question 11

What happens to action potentials in the axon in the presence of TTX? Why?

Answer 11

Action potentials cease to exist. This is because for an action potential to excite the cell a depolarisation must occur. Because this is primarily due to sodium channels when the TTX poison blocks the sodium channels the cell can’t generate a sufficient rise in positive charge. Even if there was a rise in positive charge the depolarisation wouldn’t occur because sodium channels wouldn’t open and fully excite the cell.

Question 12

What determines membrane potential?

Answer 12

The difference in electrical potential between the interior and exterior of a biological cell. Results from the concentration gradients of ions across the cell.

Question 13

Why chilli peppers give sensation of heat?

Answer 13

Chilli peppers act as a chemical stimuli on the TRPV1 receptors. As such, when these channels open they tell the body that the body temperature is >42C and we feel hot. People often start to sweat and go red when eating chilli peppers but it isn’t because they are really hot. It’s because they are incorrectly activating certain temperature sensitive channels.

Question 14

What is the main difference between action potential in an axon and in a cardiac myocyte?

Answer 14

Action potentials in axons use sodium channels in order to cause the depolarisation. These channels have a very fast inactivation time and therefore the action potential is very fast. In the cardiac myocyte calcium channels are primarily used in order to create the depolarisation. However the inactivation of calcium channels takes a lot longer to occur and therefore the action potential is prolonged. While the action potential excitation takes approximately the same time it is the inactivation that varies.

Question 15

Describe the main properties and the functions of nicotinic ACh receptor.

Answer 15

One of the main receptors in neurotransmission. Composed of five sub-units - two alpha, one beta, one gamma and one delta sub-unit. The pore of the channels is formed by all subunits. Activated by binding of two ACh molecules. The pore is permeable to both Na+ and K+ - current through ACh receptors drives the membrane potential towards 0mV. Located at neuromuscular junctions (N1 receptors), and CNS, autonomic ganglia and adrenal medulla (N2 receptors). Activation of ACh receptors at neuromuscular junction creates end plate potential (EPP) that initiates AP in the muscle and muscle contraction.

Question 16

How activation of mechanosensitive channels changes membrane potential in neuronal cells?

Answer 16

Mechanically activated current is typically a non-selective cation current which depolarises membrane potential. i.e. depolarisation (more positive)

Question 17

22. How many genes in human genome encode alpha subunit of voltage gated Ca2+ channels?

Answer 17

10 genes

Question 18

What physiological functions depend on mechanosensitive channels?

Answer 18

Important in touch, hearing, cardiovascular regulation, sensing of gravity and osmotic stress.

Question 19

Give at least 3 examples of intracellular ligand gated channels.

Answer 19

Ca2+ activated K+ channels
Ca2+ activated Cl- channels
ATP-regulated K+ channels

Question 20

List the main techniques used to investigate ion channel function. What is the difference between voltage clamp and current clamp?

Answer 20

Electrophysiology, blockers, mutagenesis, substituted cysteine accessibility method, fluorescent probes, simulation, crystallography.
Current clam – Used to measure membrane potential
Voltage clamp – Used to measure membrane current

Question 21

What is the general structure of a voltage gated K+ channel?

Answer 21

Made from four protein subunits (alpha subunit) assembled around a central pore. Each alpha subunit consist of six transmembrane segments (S1-S6). With S4 voltage sensor and a P loop that contains the pore.

Question 23

What are the main functions of voltage gated Ca2+ channels?

Answer 23

Cause excitatory response. Contributes to repolarisation phase through activation of calcium dependent potassium channels. Provide transient increases in intracellular calcium. Required for muscle contraction, neurotransmitter release, hormone secretion. Inactivated by intracellular calcium.

Question 24

What are the major classes of ligand-gated channels? Give some examples.

Answer 24

The two classes are typical extracellular ligand gated channels and typical intracellular ligand gated channels. The extracellular ligand gated channel are ionotropic neurotransmitter receptors - Ach, glutamate, serotonin, GABAa and glycine. The intracellular ligand gated channels are gated by second messengers - calcium, cyclic nucleotides, ATP, G-proteins, lipids.

Question 25

What stimuli activate TRPM8 channels?

Answer 25

The channel is also stimulated by the chemical stimulus menthol (mint).

Question 27

What is equilibrium potential, and how is it determined?

Answer 27

Membrane potential at which net movement of ions across the membrane is zero.

Question 28

What types of ion channels are blocked by conotoxins?

Answer 28

Depends on the conotoxin. It can block Na(v), Ca(v), K(v), Ach receptors. It is a neurotoxic peptide from the venom of the marine cone snail.

Question 29

Draw a typical I-V plot of a voltage dependent Na+ current. Mark the threshold of activation and the equilibrium potential for Na+.

Answer 29

Question 30

What is the role of KATP channels in pancreatic beta cells?

Answer 30

Under normal conditions the KATP channels in pancreatic beta cells are spontaneously active, allowing potassium ions to flow out of the cell. In the presence of higher glucose metabolism and consequently increased levels of ATP, the KATP channels close, causing the membrane potential of the cell to depolarize thus promoting insulin release. The change from one state to the other happens quickly and synchronously due to the c terminus multimerization among proximate KATP channel molecules. In short, it controls insulin secretion.

Question 32

What are the main functions of ion channels?

Answer 32

Ion channels primarily function is to allow passage of ions into and out of a cell down an electrochemical gradient. They function to mediate fast electrical signalling in the NS. Regulate osmotic balance and fluid movement across cell membrane. Regulate a large number of physiological functions through calcium signalling – fertillsation, release of neurotransmitters and hormones, muscle contraction, vascular tone, gene expression and cell death.

Question 33

Draw a typical I-V plot of a voltage dependent Ca2+ current. Mark the threshold of activation and the equilibrium potential for Ca2+.

Answer 33

Question 34

Draw a typical I-V plot of a voltage dependent K+ current. Mark the equilibrium potential for K+ at normal conditions.

Answer 34

Using Nersnt equation will give you the answer. Potassium will be -90mV.

Question 35

If you need to record Na+ current in a neuronal cell using patch clamp, what voltage protocol, bath and pipette solutions will you use? How would you know that you are recording Na+ current?

Answer 35

You would need to know the threshold for sodium and equilibrium potential. Therefore the range of voltage for sodium would be measured between -55 to + 50mV. Although threshold is at -50 an extra -5 is given for other conditions. Bath solution would be that of the inside of the cell. The pipette would mimic the components outside the cell. If we wanted to get purely sodium we would replace others with components that suit. For example calcium could be replaced by copper to ensure that calcium is taken out of the equation.

Question 36

What are the advantages and disadvantages of the whole cell patch clamp, compared to two-electrode voltage clamp?

Answer 36

Patch clamps can be used to record currents through single channels. Patch clamps allow a specific drug or substance to be added and the effects on specific ion channels can be observed. The two electrode voltage clamp can’t use a high voltage as the cell can burst.

Question 37

How many alpha subunits form the pore of the voltage gated Na+ channels?

Answer 37

Only one, it’s trying to be tricky. But remember K+ consists of four alpha subunits, but both Ca and Na consist of only one with many 4 homologous domains. But all four homologous subunits form the pore. It’s a weird question.

Question 38

Why an outward current causes membrane hyperpolarisation?

Answer 38

Because K+ are going out of the cell, the loss of + means that the cell will hyperpolarise.

Question 39

What are the main differences between ion channels and transporters?

Answer 39

Channels are in charge of a passive flow of ions. As such, passive flow can only be down an ions electrochemical gradient. Exploit ion gradients do the work. Species of permeating ions depends on selectivity and abundance. High conductance. But wtf does conductance mean? Ions move across the cell membrane through specific ion channels. When these channels open, the permeability and electrical conductance to their respective ion increases, which leads to a change in the membrane potential.

Question 40

Describe the sequence of events during action potential in an axon.

Answer 40

First off there is neurotransmitter released into the synapse of two axons. Ach is released into the terminal and interacts with the other cell. The Ach travels the synaptic cleft and attaches to Muscarinic nicotinic acetylcholine receptors or nicotinic Ach receptors. This causes a ligand binding transformation in which Na+ is released into the cell. This causes a slight change in depolarisation and the membrane potential changes slightly towards positive. At the -50mV the sodium channels are opened due to threshold reached and open to allow Na+ to enter the cell. This causes an opening of all sodium channels and makes the membrane potential rise in order to depolarise the cell.

Question 41

What is Na,K-ATPase, and what its main function?

Answer 41

Transport sodium and potassium. It transports 2 potassium in for 3 sodium transported out. Run on ATP. It balances the membrane potential through the ions lost in the leakage channels. Contributes approximately -5mV to the resting membrane potential.