NaV Channels

Question 1

Gating and pore selectivity of the NaV channel

Answer 1

• pore is closed at -65mV
• pore is opened at -40mV
• partially hydrated Na+ ions fit through sodium channel selectivity filter, K+ ions do not

Question 2

Describe the function of channels during the refractory period

Answer 2

channels are inactivated

Question 3

How can channels be opened

Answer 3

• depolarisation
• open with little delay
• stay open for 1msec
• cannot be opened again by depolarisation

Question 4

Function of the activation gate

Answer 4

• closes the channel at resting membrane potential
• Na+ enters the cell while activation gate is open
• reset to original position during repolarisation (K+ ions leaving cell)

Question 5

Function of inactivation gate

Answer 5

• closes and stops Na+ entry
• delayed response to depolarisation

Question 6

Explain what is meant by “activation is voltage dependent”

Answer 6

• the amount of Na+ current is dependent on the magnitude of the depolarisaiton
• as you get closer to equilibrium potential of sodium, drive for sodium to enter cell becomes smaller

Question 7

The process of inactivation determines…

Answer 7

the number of Na+ channels available to open at any given membrane potential

Question 8

Role of the Na+ channel auxillary subunits

Answer 8

• modulate channel gating allowing rapid activation and inactivation
• mutation in B1 associated with epileptic seizures
• immunoglobulin domains bind extracellular proteins and are important determinants of channel localisation in cells

Question 9

How can changes in expression of TTX-resistant channels cause chronic inflammatory pain

Answer 9

• Na v1.8 and 1.9 are TTx-resistant isoforms found in peripheral NS
• found in sensory dorsal root ganglion neurons, including C-fibres (transmit pain)
• sustain repetitive firing of depolarised nerves

Question 10

Role of Na v 1.7 in pain

Answer 10

• TTx sensitive
• expressed selectively in dorsal root ganglion nerves
• Nav1.7 mice have increased mechanical and thermal pain thresholds
• GoF mutation = cause of primary erythomelalgia (burning sensation and redness in response to mild thermal stimuli)

Question 11

Mutations in Nav1.7 in humans

Answer 11

loss of Nav1.7 function, unable to feel pain

Question 12

What happens to Na currents in grasshopper mouse in presence of venom

Answer 12

substantially diminished due to inactivation of Nav1.8 by venom

Question 13

How local anesthetics work to block Na+ channels

Answer 13

• prevent action potential propagation
• cross the nerve sheath and cell membrane to reach site of action
• blocking is dependent on Na channel being open as it only blocks from the inside (use-dependent block)
• block is also voltage-dependent (enhanced by depolarisation)
• enhance Na+ inactivation process by stabilising it

Question 14

Structure of local anaesthetics

Answer 14

aromatic group linked with amide or ester bond to basic side chain, mostly charged at physiological pH

Question 15

What is Tetrodotoxin (TTX)

Answer 15

• naturally occurring
• virulent poison
• blocks nerve conduction
• causes death by respiratory paralysis

Question 16

Where is TTX found

Answer 16

internal organs of Pacific puffer fish (ovaries, liver)

Question 17

Mechanism of action of TTX

Answer 17

• blocks voltage-gated Na channels of neurons (nerves) and skeletal muscle
• blocks Na channels from outside of the channel (no effect from inside)
• binds to amino acid residues associated with outer mouth of channel

Question 18

Effect of glutamate (-) to glutamine (neutral) mutation on P-loop

Answer 18

mM sensitivity is lost and TTX does not block

Question 19

Effect of cysteine to tyrosine mutation in cardiac Na channel P-loop

Answer 19

affinity to TTX is increased

Question 20

Where is the P-loop on the Na channel domain

Answer 20

between 5 and 6

Question 21

What is saxitoxin (STX)

Answer 21

• has properties similar to TTX and blocks Na channels

Question 22

Which toxins increase duration of Na channel opening

Answer 22

• batrachotoxin
• pyrethins
• b-scorpion toxins
• sea anemone and a-scorpion toxins

Question 23

What are batrachotoxins

Answer 23

• secreteed by skin of Columbian poison frogs
• inhibits inactivation and shifts activation voltage to more negative potentials
• enters cell and acts internally

Question 24

What are pyrethrins

Answer 24

• insecticides produced by plants
• non-toxic for mammals but rapid effect on insect Na channels
• inhibit inactivation

Question 25

What are b-scorpion toxins

Answer 25

- bind to outer side of IIS4 voltage sensor - toxin binding alone = no effect on activation - when channel is activated, toxin enhances activation by negatively shifting voltage dependence

Question 26

What are sea anemone and a-scorpion toxins

Answer 26

- uncouple activation from inactivation - bind to receptor site at extracellular end of IVS4 segment - prevent normal gating movement - upward movement of IVS4 initiates fast inactivation

Question 27

What are some inherited disorders of the Na channels

Answer 27

- myotonias and periodic paralysis - ventricular arrhythmia - inherited epilepsy syndromes

Question 28

Myotonias and periodic paralysis

Answer 28

- mutations of NaV1.4 - may affect fast inactivation gate -> channels have slower inactivation kinetics and faster recovery - aggravated by potassium-rich foods - missense mutations of positive charged residues lead to hypoexcitability and inability to depolarise muscle

Question 29

Ventricular arrhythmia

Answer 29

- prolonged ventricular AP duration - persistent inward Na+ current causing abnormal repolarisation - various mutations of Nav1.5