Na+-activated K+ channels localized in the nodal region of myelinated axons of Xenopus Journal Article

Author(s): Koh, Duk S; Jonas, Peter; Vogel, Werner
Article Title: Na+-activated K+ channels localized in the nodal region of myelinated axons of Xenopus
Abstract: 1. A potassium channel activated by internal Na+ ions (K+Na channel) was identified in peripheral myelinated axons of Xenopus laevis using the cell-attached and excised configurations of the patch clamp technique. 2. The single-channel conductance for the main open state was 88 pS with [K+]o = 105 mM and pS with [K+]o = 2.5 mM ([K+]i = 105 mM). The channel was selectively permeable to K+ over Na+ ions. A characteristic feature of the K+Na channel was the frequent occurrence of subconductance states. 3. The open probability of the channel was strongly dependent on the concentration of Na+ ions at the inner side of the membrane. The half-maximal activating Na+ concentration and the Hill coefficient were 33 mM and 2.9, respectively. The open probability of the channel showed only weak potential dependence. 4. The K+Na channel was relatively insensitive to external tetraethylammonium (TEA+) in comparison with voltage-dependent axonal K+ channels; the half-maximal inhibitory concentration (IC50) was 21.3 mM (at -90 mV). In contrast, the channel was blocked by low concentrations of external Ba2+ and Cs+ ions, with IC50 values of 0.7 and 1.1 mM, respectively (at -90 mV). The block by Ba2+ and Cs+ was more pronounced at negative than at positive membrane potentials. 5. A comparison of the number of K+Na channels in nodal and paranodal patches from the same axon revealed that the channel density was about 10-fold higher at the node of Ranvier than at the paranode. Moreover, a correlation between the number of K+Na channels and voltage-dependent Na+ channels in the same patches was found, suggesting co-localization of both channel types. 6. As weakly potential-dependent ('leakage') channels, axonal K+Na channels may be involved in setting the resting potential of vertebrate axons. Simulations of Na+ ion diffusion suggest two possible mechanisms of activation of K+Na channels: the local increase of Na+ concentration in a cluster of Na+ channels during a single action potential or the accumulation in the intracellular axonal compartment during a train of action potentials.
Keywords: Animals; Sodium; Immunohistochemistry; Patch-Clamp Techniques; Sodium/metabolism; Sodium Channels/metabolism; Microscopy, Electron; Potassium Channels; Axons/metabolism; Xenopus laevis; Cell Membrane Permeability; Membrane Potentials; Ranvier's Nodes/metabolism; Sodium Channels; Axons/ultrastructure; Potassium Channels/drug effects; Axons/drug effects; Barium/pharmacology; Cesium/pharmacology; Demyelinating Diseases/chemically induced; Demyelinating Diseases/metabolism; Myelin Sheath/drug effects; Myelin Sheath/metabolism; Myelin Sheath/ultrastructure; Potassium Channel Blockers; Potassium Channels/metabolism; Ranvier's Nodes/drug effects; Ranvier's Nodes/ultrastructure; Tetraethylammonium; Tetraethylammonium Compounds/pharmacology; Tetraethylammonium Compounds; Barium; Cesium
Journal Title: Journal of Physiology
Volume: 479
ISSN: 1469-7793
Publisher: Wiley-Blackwell  
Date Published: 1994-01-01
Start Page: 183
End Page: 197
DOI: 10.1113/jphysiol.1994.sp020287
Open access: yes (repository)