NaV channels Flashcards
at what mV is the NaV closed
-65
at what mV is the NaV open
-40
how is it selective towards Na
only allows Na+ ions to pass
ex - K+ ions are too large
how does the channel open
by depolarisation
how long do they stay open for, and what happens once they close again
~1msec, and then they become inactive and cannot be opened again
what happens in response to a membrane depolarisation
they open rapidly from a resting state
what happens when depolarisation is maintained
the Na+ channels exit open state and enter inactivated state
define whole cell current
individual channels open at different times, so this is ever changing
R =
resting (closed) state (favoured by hyperpolarisation)
O =
open state (transiently favoured by depolarisatoon)
I =
inactivated state (favoured by maintained depolarisation)
how many gates do Na+ channels have within axons
2
what are the 2 gates within axons called
activation gate and inactivation gate
describe the events of an action potential in a cell at the Na+ channels
- at the resting membrane potential, the activation gate closes the channel
- depolarising stimulus arrives at the channel and opens the channel
- with activation gate open, Na+ enters the cell
- inactivation gate closes and Na+ entry stops due to hyperpolarisation
- during repolarization caused by K+ leaving the cell, the 2 gates reset to their original positions
what is activation of NaV channels dependant on
voltage
more depolarisation =
more sodium influx
the amount of Na+ current (i.e. proportional to number of open channels) is dependant on
the magnitude of the depolarisation
what is inactivation of NaV channels dependant on
voltage
the process of inactivation determines …
the number of Na+ channels avaliable to open at any given membrane potential
how many beta subunits are there
4 (beta1-beta4)
what is the difference between all the beta subunits
they have similar structures but are seperate proteins
what to beta protein subunits modulate
channel gating, allowing rapid activation and inactivation
what is a mutation in the beta1 subunit associated with
epileptic seizures
what do immunoglobulin domains thought do do
bind extracellular proteins and be important determinants of channel localisation in cells
what does the extracellular domain of both beta proteins possess
an immunoglobulin-like fold
how many sodium channel alpha subunit genes are there in the human genome
9 (NaV 1.1-1.9)
what are the subunits that predominate in the CNS
Nav1.1, 1.2 and 1.3
what are NaV 1.1-1.3 sensitive to
nM TTX
what are the TTX-resistant isoforms and where are they found
NaV 1.8 and 1.9 - found in the peripheral NS
what are the TTX-resistant isoforms expressed as
small-diameter dorsal root ganglion neurons (including C-fibres which transmit nociceptive pain)
what are the TTX-resistant isoforms thought to sustain
repetitive firing of depolarised nerves
what are TTX-resistant channels involved in
the pathophysiology of chronic inflammatory pain
where are NaV 1.8 and 1.9 increased
in nerve fibres, proximal to site of injury
what happens when expression of NaV1.8 is reduced
attenuated neuropathic pain and analgesic to noxious mechanical stimuli
which alpha subunit is TTX sensitive
NaV1.7
where are NaV1.7’s expressed
selectively in dorsal root ganglion neurons, particularly nociceptive cells
what sets gain for pain in nociceptors (sets pain threshold)
NaV1.7
what happens when mutations occur in NaV1.7 in mice
cause of primary erythromelagia an autosomal dominant, inherited disorderm severe burning sensation and redness in extremities in response to mild thermal stimuli
what happens with mutations/loss of NaV1.7 in humans
unable to feel pain
how do local anaesthetics prevent AP propagation of nerve axons
by blocking NaV channels
what are LAs
small lipid soluble molecules and as such cross the nerve sheath and cell membrane to reach site of action
what do LAs consist of
an aromatic group linked by an amide or ester bind to a basic side chain, and at physiolgical pH’s are usually charged (+)
list some clinally useful LAs
procaine, lignocaine and bupivacaine
why are permenantly charged derivitives of LAs ineffective
they are unable to penetrate nerve cell membranes
what is the blocking action of LAs dependant on
the NaV channel being OPEN
what else do LAs enhance
the NaV inactivation process (they stabilise it)
what is tetrodotoxin (TTX)
a naturally occuring, virulent poison that blocks nerve conduction and causes death by respiratory paralysis
where is TTX found
in internal organs of the pacific puffer fish
what does TTX do
blocks NaV channels of nerves and skeletal muscle in nanomolar range - but cardiac NaV channels are much less sensitive (micromolar range)
how does TTX work
blocks NaV channels from the outside of the cell
it binds to the AA residue on the outer mouth of the channel
what does saxitoxin do (STX)
similar to TTX - blocks NaV channels from AA site on outside of cell
where is STX found
produced by dinoflagellates (a unicellular organisms in marine plankton)
what can STX cause if ingested
paralytic shellfish poisoning (which is often fatal)
what are u-conotoxins
constituents of venom of the group of predatory molluscs - cone shells
they are positively charged peptides
how do u-conotoxins work
they are injected into pray via a disposable tooth - they prey are then paralysed and die
what do u-conotoxin GIIIA do
block skeletal NaV channels (little effect of neuronal NaV channels)
what is batrachotoxin and how does it work
secreted by the skin of columbian poison frogs
it inhibits and shifts the activation voltage to more negative potentials so channels can stay open longer - then it enters the cell and acts internally
what is pyrethrins and how do they work
a natural insecticide produced by plants (non-toxic on mammals)
rapid effect on insect Na channels - prolong activation and inhibit inactivation
how do b-scorpion toxins work
bind to the outer side of the IIs4 voltage sensor - toxin binding alone has no effect but when the channel is activated by depolarization, the bound toxin enhances activation by negatively shifting the voltage dependance
what do sea anemone and a-scorpion toxins do
uncouple activation from inactivation by binding to a receptor site at the extracellular end of the IVs4 segment and preventing its normal gating movement - as upward movement of IVS4 is thought to initiate fast inactivation, the channel remains in an activates “gated” state
what do mutations of NaV 1.4 do
cause epidsodic and transient weakness or paralysis or relaxation defect
may affect fast inactivation gate causing channels to have slower inactivation kinetics and faster recovery from inactivation resulting in delayed muscle relaxation
what do missense mutations of positive charged residues in the voltage sensor lead to
hypoexcitability or inexcitability and inability to depolarize muscle
sodium channels may also open after a delay and cause late depolarization and firing
explain ventricular arrhythmia
conegnital long QT syndrome (10% of cases) and idiopathic ventricular fibrillation. observe a prolonged ventricular action potential duration. this is due to a persistent inward Na+ current causing abnormal repolarisation - various mutations of alpha subunit of NaV1.5 observed
explain inherited epilepsy syndromes
many mutations (>100) found in alpha subunits (1.1 and 1.2) and some associated with beta1 subunit - variable outcomes - many induce persistent sodium currents or alter voltage dependence of activation/inactivation - causing enhanced excitability
