Voltage gated Na+

Question 1

What is whole-cell and single cell current?

Answer 1

Different patch clamp techniques.

Question 2

What is the mechanism of anaesthetics? Example:lignocaine

Answer 2

The channel is in the open state; the drug ONLY BLOCKS FROM THE INSIDE.

The block is VOLTAGE-DEPENDANT (stimulated by cell depolarisation)

The block stabilises inactivation

Question 3

What is the mechanism of neurotoxin Tetrodotoxin (TTX)

Answer 3

TTX is a virulent poison that blocks nerve conduction and causes death by respiratory paralysis.

TTX blocks Na+ channels in the neurones, skeletal (and the cardiac) muscle from outside of the channel (no effect when applied from the inside) by binding to amino acids of the outer pore.

In particular the amino acid GLUTAMATE on the p-loop; other residues help too (Cystene in cardiac and Serine in neuronal cells)

Question 4

What are the functions of the beta subunit of Na+ channels?

Answer 4

4 beta subunits (b1-4; b1/3 at one end and b2/4at the other) that modulate channel gating allowing RAPID activation and inactivation.

Mutation in b1 subunit associated with epileptic seizures.

The immunoglobulin domains (extracellular NH2) bind extracellular proteins and determine channel LOCALIZATION in cells.

Question 5

What disorders arise from muations of this channel?

Answer 5

Many mutaions in all domains

1) Disturbances in skeletal muscle function ( in 1.4 isoform) such as brief weakness/paralysis/ relaxation defects.

This may be due to mutations that affect fast INACTIVATION gate causing channels to have SLOWER inactivation and faster recovery from inactivation resulting in DELAYED muscle relaxation.

2) Cardiac and neuronal sodium channelopathies such as
i) Ventricular arrhythmia (non stop Na influx, prolonged A.P)
ii) epilepsy syndromes (enhanced excitability)

Question 6

What is the basic structure of he Na+ pump?

Answer 6

4 transmembrane domains, around an ion selective pore; structural loops come together in this pore

Question 7

What are the voltages needed to open/close channel?

Answer 7

at rest- closed at -65mV

in response to cell depolarisation, a more positive cell the channel open at -40nV

Question 8

Describe the pattern of ionic current

Answer 8

The channels open rapidly from resting

If maintained will enter inactive state when there is no current

Question 9

Describe the equation I + N x Po x i

Answer 9

I =current
N= total number of sodium channels
Po= probality of opening
i= individual current

Question 10

What governs the two gates of the sodium channel; activation and inactivation gates.

Answer 10

entry based on a positive feedback loop which only stops if intervened.

1) channel closed
2) depolarisation opens activation channel as voltage of the cell has become POSITIVE enough (voltage dependant opening)

The size of depolarisation determines the size of the Na current; more positive, more conductance

3) Na enters
4) inactivation then closes. In this way the inactivation gates are DELAYED in response to depolarisation.

inactivation is also voltage dependent

5) Na influx stops
6) gates reset in repolarisation.

Question 11

What are LAs?

Answer 11

Local anaesthetics (LAs) are small lipid soluble molecules that cross membrane to reach site of action.

The PREVENT action potential propagation of nerve axons by blocking voltage-activated Na+ channels; cell cannot depolarise further.

They consist of an aromatic group linked by an amide or ester bond to a basic side chain, (mostly positively charged (+))

Question 12

What the problem with a LA deritve OX 314?

Answer 12

It is Permanently charged derivatives such as QX 314 and so ineffective as LAs because they cannot penetrate nerve cell membrane

Question 13

What is Saxitoxin (STX)

Answer 13

Toxin that blocks Na+ AT THE SAME SITE

In humans can be ingested in clams/shellfish

Question 14

What is micro-conotoxins

Answer 14

peptide parts of the cone shell (mollusc) venom causing paralysis.

µ-conotoxin GIIIA selectively blocks skeletal muscle Na+ channels ( but has little effect on neuronal Na+ channels

Question 15

List toxins that can modulate the channel to maintain its open state leading to REPETITIVE firing of neurons.

Answer 15

1) Batrachotoxin (frogs)
2) Pyrethrins (plant insecticide afecting only insects)
3) b-scorpion toxins (negatively shifts voltage dependance, activaes at more negative levels)
4) Sea anemone and a-scorpion toxins (prevents gating movement)

Question 16

Describe how channel isoforms can help with pain relief.

Answer 16

Isoform Na+ channels (1.8/1.9) are thought to withstand repetitive firing of depolarised nerves and are said to be TTX-resistant

The presence of these isoforms are involved with pathophysiology of chronic inflammatory pain as they sense pain signals.

Reduced expression of NaV1.8 weakens neuropathic pain .

No NaV1.8 shows complete pain relief effects.

Question 17

Describe how channel isoforms 1.7 effects the brain

Answer 17

Nav1.7 KO mice have increased mechanical and thermal pain thresholds + reduced inflammatory pain responses.

Without 1.7 at all, we cant feel pain.