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Na pump or leakage K

Thor Eysteinsson thoreys at rhi.hi.is
Thu Jun 23 16:56:43 EST 1994

In <2ubis9$d6m at aragorn.unibe.ch> larkum at optolab.unibe.ch (Matthew Larkum) writes:

>In article hxie at vms2.macc.wisc.edu, hxie at VMS2.MACC.WISC.EDU ("huiwen xie") writes:
>>I have a question regarding the change in resting potential of motoneuron
>>during embryogenesis. What I found in my study was that the resting
>>potential of motoneuron increased as motoneurons matured.(The increase in
>>resting potential means resting potential is more negative.). Theoretically,
>>there are at least two factors that may be responsible for this change. One
>>is the increased expression of electrogenic Na-pump onto the membrane, while
>>the other will be the increased leakage K current. Although I don't know
>>what really happened during their maturation, I prefer the first
>>possibility. I welcome your comments. It will be appreciated if you can give
>>any relevant references on Na-pump and resting potential during development.
>>Steven Schie
>>Department of Physiology
>>University of Wisconsin-Madison

This discussion would be more to the point if you would provide more detail
of your experimental design.  What animals did you use, in vivo/in vitro? 
What recording method? Where you doing long term recordings of Vm (unlikely)
or impaling/patching cells at various stages of embryogenesis?  As for the
hypotheses (Na-pumps, potassium leakage), which seem reasonable, there are
various ways to examine these by physiological and pharmacological means.
You could block your Na-pumps with oubain (it«s a bit drastic!), and see
if the response to it changes with embryogenesis.  Does the I-V relations
ship change with embryogenesis? It all depends on your methods of course.

 It strikes me that a study of membrane potential would be hard
>because one would introduce a variable leak just by impaling a neuron
>and the average would always be skewed in the positive direction, never
>the more hyperpolarized direction. 

This I don«t think is a problem if the methods are consistent. My experience
with intracellular recordings of Vm (retinal neurons) with impalement is
that if you get consistently good electrodes, and the set up is in shape,
then the Vm is very consistent from cell to cell.  This also would be a
problem that should be independent of embryogenesis (or not?)

 (By the way, I find I consistently
>get a slightly better Vm by patching rather than impaling a neuron).

What do you mean by "better"? More stable? It should be pointed that 
if you are getting your cells for whole-cell clamping by enzymatic
dissociation, then there is the danger that that treatment may change
membrane properties (e.g. reveal "new" channels!).  So, for this 
project I would prefer as intact a preparation as possible and
of course then you would have to impale.  Recordings that are unstable
are suspect and should be discarded. A good impalement means you can
hold the cell and record a stable Vm for at least an hour.

>Also, if the dendrites are slightly depolarized (or hyperpolarized)
>relative to the soma due to constant synaptic activity, then the Vm at
>the soma might be dependent on the dendritic structure of the neuron
>which might also undergo changes during maturation (although, then
>I, at least, would have expected Vm to increase not decrease).

But according to the original post (above) it DID increase with
embryogenesis.  But these points are still important.

>Well, just a few thoughts.

>Department of Physiology
>University of Berne

To the original poster: this sounds very interesting. Have you published?
If you have, send me a reprint.  Good luck.

Thor Eysteinsson
Dept. of Physiology
University of Iceland
Vatnsmyrarvegi 16            ---I do NOT live in an Igloo---
101 Reykjavik                   

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