Pharmacology hubio543 at u.washington.edu
Sun Oct 1 23:02:41 EST 1995

One could also ask if the Ca pump is disabled during the action 
potential, because Ca ions also enter cells during the AP.  But these are 
somewhat ridiculous questions because the ion channels Na and Ca, 
respectively, are separate and distinct from the ion transport ATPases 
for Na/K and Ca respectively.  

Think of the AP like the starter motor on an automobile.  It draws a huge 
current for a short time.  In the background the alternator recharges the 
battery (resting membrane potential).  That is what the pumps do.  They 
are not inactivated during the AP, but in the long haul repay the 
thermodynamic debt of letting Na, K and Ca move down their 
electrochemical gradients.  

A more common misconception, often held by medical students, is that the 
Na/K pump causes repolarization of the membrane (phase 3 of the AP).  
This is, of course, erroneous but it is difficult to convince students 
that the AP is an entirely passive process once the membrane is charged 
(by the pumps) and once threshold is achieved.  Ouabain and other potent 
and specific inhibitors of the Na/K pump do not prevent the AP (although 
they will, by virtue of preventing recharging of the membrane potential 
cause the system to fail eventually).  Unfortunately, I am unaware of any 
potent and specific inhibitor of the plasma membrane calcium pump.  I 
anybody out there has any suggestions I would appreciate hearing them.

Frank F. Vincenzi
University of Washington
vincenz at u.washington  

On 2 Oct 1995, Dr. Alex wrote:

> In article <009971A7.615D10BF.108 at hg.uleth.ca>, melvnr at hg.uleth.ca wrote:
> >When an action potential is firing, is the Na/K pump disabled?
> I don't think so. The AP is a lot faster than the Na/K transport transfer 
> rate, and the charge transfer with the AP, across the membrane, is a lot 
> larger.
> Alex.

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