Many thanks, Matt. This is exactly the technical detail I was seeking.
There actually is a small component of excellent information in this
newsgroup, worth sifting through the noise.
Incidentally, if you hold the membrane hyperpolarized, wouldn't the
voltage-gated potassium channels close whereas the "leak" will persist?
And what about the numerous cell types (most of the body) that are not
electrically excitable. Do they still have voltage-gated channels? They do
have resting permeabilities and a resting potential primarily produced by
potassium.
Wouldn't these argue against the window effect as the source?
Matt Jones wrote in message <7vcv2j$lin$1 at fremont.ohsu.edu>...
>There are two sources of leak, which can be hard to sort out from each
>other because of the way leak is defined. In an experiment, one usually
>does an I-V curve, and then maybe does one again in the presence of a
>cocktail of toxins to block known voltage-gated channels. When you
>subtract these two I-Vs, you get something relatively Ohmic, which is
>defined as "leak". The problem is that an Ohmic conductance is what you
>would expect from just not having a very good seal with the membrane, so
>it's not obvious that there are in fact ANY channels at all underlying
>this Ohmic leak. However, if you plot resting potential versus various
>ion concentrations, you usually find that the membrane's resting
>potential is dominated by potassium conductance, with some smaller
>contributions from other selectivities.
>>There are potassium channels and chloride channels that have been
>identified and cloned and visualized in single channel studies, that are
>relatively Ohmic in their I-V responses, so these would be prime
>candidates for "leak" channels. Note that these channels are NOT always
>open (if they were, you probably couldn't identify them easily in single
>channel studies). However, another way the leak can arise through
>channels is through something known as a window current. Here's what that
>means:
>>Voltage-gated channels are both "activated" and "inactivated" by voltage.
>That is, if you step from a constant holding potential to various
>activating potentials, you can get a conductance vs voltage curve that
>starts at zero conductance and increases sigmoidally. If instead, you
>start from various "test potentials" and jump to a fixed activating
>voltage, you get an "inactivation" curve (or "h-infinity curve") that
>starts at a high level of availability and DECREASES sigmoidally. These
>two curves cross over each other at some potential, and the area under
>this intersection is a region where the channel would be open with some
>finite probability at steady-state. The window current region for the
>Hodgkin-Huxley k-channel is in fact near the resting potential for the
>squid axon, and if you take the HH equations and remove the "leak" term,
>you still wind up with a similar resting potential because of this window
>in the potassium channel activation/inactivation curves.
>>Hille's book and Johnston's book must both have some discussion of this
>window current effect.
>>Cheers,
>>Matt Jones
>>>>>>>>>>>>>>>>>>>>>>In article <4JXR3.2431$u3.148177 at typhoon1.rdc-detw.rr.com> Richard
>Norman, rsnorman at mediaone.net writes:
>>Austin So (Hae Jin) wrote in message <38175D80.96AC5698 at netinfo.ubc.ca>...
>>>Since no one but that guy ken collins replied...
>>>>Actually Ken Collin's reply, for once, was on topic and actually relevant!
>>>>>"leak" currents are due to things like transporters (in either direction,
>>and
>>>may or may not involve ATP) which generally co-transport ions through the
>>>membrane.
>>>>Since leak currents are responsible for electrotonic potentials, they are
>>directly proportional to the driving force. That is usually not the case
>>with transporters, especially the active ones involving ATP. Those
usually
>>show saturation kinetics. The pumps, in particular, are controlled by
other
>>factors and do not readily respond to changes in potential. And, besides,
>>the leaks continue in the absence of metabolic energy.
>>>>>And yes...ion channels do not "leak".
>>>>No, in the closed position they do not. But at resting potential some may
>>remain open and therefore be responsible for the "resting" or "leak"
>>permeability.
>>>>>Any good neurophysiology text should give you good information about
them.
>>>>The specific reason I ask is that no good neurophysiology text gives the
>>information
>>I requested. Of course, they all describe the leak and they all describe
in
>>detail
>>the molecular structure of the gated channels. And some have long lists
of
>>varieties of gated channels. But NO text I checked indentifies
specifically
>>what
>>ion channel is reponsible for the resting leak.
>>
