Potassium permeability in rat myelinated nerve fibres

Potential clamp experiments were performed at 21-23 degrees C on single myelinated rat nerve fibres isolated from the sciatic nerve. A small delayed K-permeability (PK) was regularly identified in the normal rat node amounting to 0.067 cm3 s-1 X 10(-9) at large positive potentials. Measurements in isotonic KCl showed that about two-thirds of PK was already activated at resting potential. Substitution of Cs for axoplasmic K demonstrated that the rat node had large leak currents (33 nS). The leak current provided the main part of the repolarizing outward current. 10 mM tetraethylammonium chloride (TEA) slowly (tau approximately equal to 2 min) and incompletely (50%) blocked both PK activated at resting potential and the delayed increase in PK. I mM 4-aminopyridine (4-AP) had less effect on PK.