Re: resting potential and K+ [HAPP-L]

[Alan Magid <amagid@xxxxxxxxx>]

on 9/3/03 4:03 PM, Pat Bowne at Pat.Bowne@xxxxxxxxxxx wrote:

> I understood, and all my texts agree, that opening the K+ channels and
> allowing K+ to diffuse out of the cell would hyperpolarize the cell, lowering
> its resting potential.

(No! 'raising' it's resting potential, that is more negative but bigger! In
other words, avoid the terms like raise or lower. Be concrete: such as -80
to - 90 mV for example.)

> But my new text has a question in it about what will happen if you remove K+
> from the intracellular fluid. It explains that removing K+ will decrease the
> concentration gradient and therefore less K+ will diffuse out of the cell, so
> the cell will be HYPOpolarized. This makes sense mathematically, when I plug
> the values into the Nernst equation; a decrease in intracellular K+ has the
> same depolarizing effect as an increase in extracellular K+.
>
> BUT if that is the case, how could opening K+ channels hyperpolarize a cell?
> When K+ channels open and K+ diffuses out of the cell, internal K+ decreases
> AND external K+ increases, and you ought to have a doubled depolarizing
> effect!
>
> Help!

Pat-

The author is correct, if medically irrelevant. If the gradient is made
smaller, the value of E(sub)K, the potassium ion equilibrium (Nernst)
potential will become smaller and so will the membrane potential. That is,
it will tend to depolarize the cell. Medically, the more realistic problem
leading to a shallower gradient is a rise in extracellular K+, for example
from renal failure. This will tend to depolarize excitable cells such as the
heart bringing on the risk of arythmias or even asystole.

The reason why increasing the K+ permeability hyperpolarizes is that the
normal resting  membrane potential lies more positive than E(sub)K due to
the small but important sodium ion permeability (~5% of total ion
permeability). When the K channels open, the membrane potential becomes more
strongly influenced by the K 'battery', Nernst potential, and the cell
hyperpolarizes. This accounts for the 'after-hypolarization' phase of the
action potential.

In thinking about these matters, IGNORE any changes in the bulk
concentrations of the ions. They hardly change and so the reasoning in your
last paragraph is unrealistic. The ion movements are trivial, requiring only
the miniscule quantities of charge to change the charge on the membrane
capacitance.

Hope this helps. You may need to consult an advanced text or monograph to
get more of the physics here. The early papers by Goldman, Hodgkin, Huxley,
Katz mostly  in J. Physiol. develop this topic. I was obliged to wade thru
them in grad school as a biophysicist. Run-of-the-mill A&P texts rarely
treat these matters correctly.

-Alan




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