Evidence for the presence of potassium channels in the internode of frog myelinated nerve fibres.

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Abstract
The central portion of single frog internodes under voltage-clamp conditions were acutely demyelinated with lysolecithin to resolve whether K+ channels are normally present only in the paranode or are in fact present throughout the entire internode of a myelinated nerve fiber. The experiments were performed in the absence of nodes of Ranvier to ensure that all currents observed originated from the internode. The demyelinating internodal segment showed a steady increase in the capacity and leakage for the first 40-50 min, after which a delayed outward current was revealed. This delayed current, which was blocked by TEA [tetraethylammonium] and Cs and exhibited a K+-dependent reversal potential, seems to be a K+ current similar to that normally present in the frog node and in squid nerve. The method of acute demyelination revealed no detectable Na+ currents in the internodal segments. K+ channels, but virtually no Na+ channels, are normally present throughout a frog internode covered by the myelin.