Channel kinetics.....

Gerry S. Oxford gsox at med.unc.edu
Tue Jul 5 16:23:27 EST 1994

In article <m0qLFPh-000BzUC at bootes.cus.cam.ac.uk> mat10 at CUS.CAM.AC.UK (M.A. Tester) writes:
 To our surprise, we cannot find any paper in which
>people have tried measuring single channel currents with Ca activities
>below about 1 mM Ca. Surely we have missed something. Do any
>of you know of such measurements; or why such measurements
>have not been made?   (We are aware of measurements of Ca 
>inhibition of Li fluxes [e.g. Kuo & Hess, 1993, J.Physiol. 466, 629]
>which show a high affinity binding of Ca to the channel, but are
>uncertain of the connection between this type of measurement 
>of binding and our own measurement of conductance.

You _have_ actually struck on one of the important appropriate references.
The common view of mammalian calcium channel permeation is that a channel
ion pathway (pore) is capable of being occupied by more than one permeant
ion at a time.  Such "multiple occupancy" results in predictions that
ion passage will be blocked or restricted by the lengthy occupancy of a
pore site by any ion.  It has been demonstrated that divalents, in
particular calcium, ions spend more time in the channel (i.e. have a
higher affinity for binding sites) than some monovalent cations.  Hence,
calcium ions can impede not only the flow of monovalents but also their
own passage.  In the absence of divalents, monovalent passage can be
very large, owing (in theory) to the absence of divalent block.

I suggest you seek some of the references that are in the Kuo and Hess
article.  One should be Hess, Lansman, & Tsien (1986, J. General
Physiology 88:293-319).

BTW, when you say "only divalent cations except for H+", I am puzzled.  How
do you maintain osmotic strength?  Are there monovalent cations around or
only sucrose, glucose, or another molecule?  Messing around with ionic
strength has its _own_ set of problems as far as voltage-gated ion channels
are concerned.

Gerry Oxford, Ph.D.
UNC Physiology Dept.

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