>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
excitable.
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.
