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[QUESTION] How determine or define single channel conductances ?

Richard Norman rsnorman at mw.mediaone.net
Sat Apr 24 09:13:34 EST 1999


Gunnar Wrobel wrote in message <3720B95A.E43A3E4A at bbb.caltech.edu>...
>In my opinion the ionic conductance of a channel is defined by the
following
>equation:
>I=g(E-V)   [with I being the current flowing through the channel, g
as the
>conductance, E as equilibrium for the ion(s) conducted by the channel
and V for
>the voltage across the membrane]

>This would incorporate any effects caused by changes in the ion
concentration in
>the value of E.
>Might this be the answer to your question?
>
>Gunnar


You have it partly correct.  The equation you describe is the
definition of
conductance.  It is measured by measuring the current flow through
open
channels and dividing by the net "driving force", E-V.

You are wrong in assuming that the effect of ion concentration is
contained purely in the E term, the equilibrium potential.  Yes, that
does depend on concentration, but so does the conductance.

Think of the conductance as the conductance of a thin cylinder of salt
solution, whose length is the thickness of the membrane and whose
diameter is the width of the ion channel.  (Biophysically, things are
a little more complex, but this is close enough).  The conductance
depends on the geometry of the channel (length and width).  But it
also depends on the bulk resistivity (conductivity) of the salt
solution
which, in turn, depends on concentration.

The measurement of conductance is a tool to learn something about
the properties of the ion channel -- the length is constant, so it is
the
width (the "open-ness") that is of interest.  However, that
measurement
is confounded by the ion concentration.  If you measure conductance
under one concentration condition and I measure it under another,
our measurements will differ, even though the channel is the same.






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