Cesium for K, but what for Na channels?

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gio
gio's picture
Cesium for K, but what for Na channels?

Hi everyone,
a stupid question on electrophysiology: why do we use Cs to block K channels and therefore isolate Na currents and do not do the same when studying K channels? I mean, why don't we use something to block Na channels and isolate therefore K currents?
thanks
g

The FFM
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NMDG is the non-permeant ion

NMDG is the non-permeant ion that you use to sustitute for Na+ when you want to reduce sodium currents.

gio
gio's picture
ok thanks,

ok thanks,
but still...my question remains: why are we not usually that bothered to block Na currents when measuring K currents as we are to block K when recording Na? I'm doing hERG channels, and using NaCl in the external with no concern. Is it because Na is usually negligible compared to hERG, or what?
thanks again

g

The FFM
The FFM's picture
If you are heterologously

If you are heterologously expressing hERG in a mammalian cell line like HEK or CHO, the chances are that the cell has no appreciable native voltage-dependent Na currents and so the point of blocking Na current is moot (although there is some recent evidence for low level expression of NaV1.7 in HEKs).  Furthermore, if there is appreciable expression of Na channels, the necessity to pharmocologically block them (with a toxin like TTX) when recording hERG is small because they  will have rapidly inactivated during the preceding depolarization step and they do not reopen during the hyperpolarizing step used to fully open the hERGs from their inactivated state to yield their characteristic tail current (Na+ and other types of K+ channels can move from an inactivated to a closed state without passing through the open state and thus the tail currents are pure hERG in cell expressing the hERG protein).  
If you are recording native 'IKr' from cardiac myocytes then you will need to apply a blocker like HMR1556 (100 nM), an inhibitor of the slow delayed rectifier potassium current 'IKs ', and ~300 μM CdCl2 to inhibit L-type Ca2+ current, to the recording solutions.  When you record the IKr tail currents you will not have to be concerned by Na+ contamination because like in the experiments with cell lines the Na+ channels will have rapidly inactivated during the preceding depoalization step and they do not reopen during the hyperpolarizing step used to open the hERGs that gate the IKr .  An alternative method to isolate IKr in myocytes is to study the Cs+ permeation.

In HEK cells there is some endogenous K+ current, but this only contaminates the depolarization step portion of the recording protocol.  To get a pure hERG current trace for the full protocol some people will block the hERG portion of the current with E4031 after they have performed their main experiment and subtract the remaining current from their original traces.  Also 10mM TEA generally blocks most K+ currents except the hERG current, and thus can be applied to isolate hERG currents for the complete protocol from a mixed K+ current background.

gio
gio's picture
crystal clear, thank a lot,

crystal clear, thank a lot, really.
G