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Joseph J. Pancrazio jjp2h at galen.med.Virginia.EDU
Wed Jun 29 15:09:25 EST 1994

>Nurses in a class I teach claim that cramping is frequent symptom of
>hypokalemia in their patients. According to the Nernst equation and
>everything I've read, hypokalemia should hyperpolarize membranes and
>decrease excitability. I seem to be missing something. I would greatly
>appreciate any information regarding this matter.
>Thanks, D. Fonner

Of course you are correct regarding the Nernst potential, however the
key is that the input conductance, largely set by the inwardly rectifying
K+ current (IK1), is decreased in low [K+]o.  IK1 is decreased by
diminshed [K+]o (single channel conductance is dependent on the [K+]o).
So, decreasing IK1 causes the cell to be less influenced by the 
equilibrium (Nernst) potential for K+ ions and, therefore, more 

A few years ago, we performed some classic microelectrode experiments
in guinea pig papillary muscle which showed that as the [K+]o fell, 
the membrane potential decreased but also, the incidence of 
spontaneous APs increased.

With regard to hypokalemia, I would be interested to know whether 
excitation (i.e. cramping) involves neural input, or rather, it 
occurs at the muscle level.  Many neurons do express K+ channels
which behave like the inwardly rectifying K+ channels in muscle.

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