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[QUESTION] How determine or define single channel conductances ?

Gunnar Wrobel gwrobel at bbb.caltech.edu
Fri Apr 23 13:18:03 EST 1999


Hi!

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

sos wrote:

> Thank you for your kind answer !
> May I ask some further questions ?
>
> In almost all experimental report, authors usually specify the condition in
> which conductance was determined.
> However, as you know, often in "general textbook"(or some review articles),
> authors usually describe channel conductance without specifying the measured
> condition.
> Let me give you one example.
> In the famous textbook "From neuron to brain(third edition, 118p)", authors
> describe the K channel type, and they distinguish the Ca activated K
> channel(Kca) by single channel conductance, they say ...  the conductance of
> small conductance Kca is 10pS .. and that of large conductance Kca is 200pS
> ....
> In such case, they mentioned about the intracellular Ca concentration, but
> mentioned nothing about the concentration of the transported ions or ...
> something like that.
> Then how should we figure out the meaning of the conductance value ?
> I think symmetric(inside & outside) K concentration may be used. (even
> though that experimental setting is not physiological condition,.. since
> actually K concentration is not symmetric in physiological condition)
> If symmetric concentration was used, how much concentration ???
> So, what I'd like to know is, in many cases like that, how should we guess
> the measured condition, .. or figure out the meaning of the value of single
> channel conductance.
>
> One more question ,... You say 120mM KCl, 120mM NaCl... then symmetric
> Chloride concentration..OK ... but osmolality is 240mOsm/kgH2O ? ...
> physiological osmolality is 300mOsm/kgH2O , isn't it ? ... then, is there
> any reason for you to say 120mM ? ... I guess... 150mM would be more
> reasonable ?????  I'm not sure for that...
>
> Thank you in advance !!!
> Good luck !
>
> Matt Jones ÀÌ(°¡) <7f0aae$3bm$1 at fremont.ohsu.edu> ¸Þ½ÃÁö¿¡¼­
> ÀÛ¼ºÇÏ¿´½À´Ï´Ù...
> >In article <7eujme$5kk$1 at news2.kornet.net> sos, tsnam at yumc.yonsei.ac.kr
> >writes:
> >>Hello, everyone !
> >>
> >>As you know, ion channel conductance is dependent on the concentration of
> >>the ions that is transported through the channel (so called "normal"
> >>rectification). For example, when we increase the ion concentration, the
> >>channel conductance measured by single channel current will also be
> >>increased, even in the condition of same electrochemical potential
> gradient.
> >>However, in some paper, the value of the ion channel conductances is
> >>described without specifying the detailed ion concentration. In that
> >>ase( namely, not mentioned about ion concentration or measured condition
> >>such as symmetrical ? etc...), I wonder if there are any general rules(?)
> or
> >>conditions(?) for defining(or describing) single channel conductances ?
> >>
> >>Thank you in advance !!!
> >>Good luck
> >>
> >
> >
> >If a paper really doesn't give any information about the ion
> >concentrations used when the channel conductance was determined, you have
> >two options: 1) Ignore the paper completely, and disregard everything
> >that it says, because this is an incredibly sloppy way of reporting data;
> >or 2) assume that they used some relatively "standard" set of solutions,
> >designed to mimic the endogenous physiological environment. For example,
> >if someone reports that a ligand-gated channel has a 30 pS conductance,
> >you might be charitable and assume that this was measured in
> >approximately 120 mM NaCl outside and 120 mM KCl inside. But this is
> >dangerous in my opinion, because more often than not, people studying
> >single channels are using special solutions to maximize their signal to
> >noise ratio, or to isolate their channel of interest from other channels
> >in the same membrane.
> >
> >Matt Jones




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