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

1. The central portion of single frog internodes under voltage-clamp conditions was acutely demyelinated with lysolecithin to resolve whether potassium channels are normally present only in the paranode (Chiu & Ritchie, 1980, 1981) or are in fact present throughout the entire internode of a myelinated nerve fibre. 2. The experiments were performed in the absence of nodes of Ranvier to ensure that all currents observed originated from the internode. 3. 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 and caesium and exhibited a potassium-dependent reversal potential, seems to be a potassium current similar to that normally present in the frog node and in squid nerve. 4. The present method of acute demyelination revealed no detectable sodium currents in the internodal segments. 5. These results suggest that potassium channels, but virtually no sodium channels, are normally present throughout a frog internode covered by the myelin.