Potassium permeability in thin amphibian myelinated fibres.

Potential clamp analysis of thin (10-12 micrometers) myelinated fibres from amphibia (Xenopus laevis and Rana temporaria) showed that they had a large delayed K-permeability (PK) and a PK/PNa-ratio of 0.32, which is similar to the ratio in large (25-30 micrometers) frog fibres. These results agree with earlier evidence of a large delayed PK in 10-15 micrometers Rana fibres; a recent description of a size-dependent difference in PK was thus not confirmed. The large PK (0.32 cm3s-1 X 10(-9) in 10-12 micrometers amphibian fibres contrasted with the small PK (0.05 cm3 s-1 X 10(-9) in 12 micrometers rat fibres. Frog and toad fibres also had a larger nodal capacitance (4 pF), but smaller leak conductance (18 nS) than rat fibres (1.4 pF and 29 nS). These differences may be related to the morphological evidence of a more open node with a larger surface area in the amphibian as compared to the rat node.