title: Single channel characterization of multiple types of potassium channels in demyelinated Xenopus axons.
abstract: Single channel currents in internodes of demyelinated Xenopus axons were measured with the gigaseal patch-clamp technique. Demyelination induced by injected lysolecithin allows the entire internodal axolemma to be accessible to a patch electrode. Four classes of K+ channels in this region were characterized. A Ca(2+)-activated K+ channel [K(Ca)] with a single channel conductance of 235 pS was found. In the presence of 10(-3), 10(-4), 10(-5), and 10(-6) M intracellular free [Ca2+]i, the half-activation voltages are -24.1, -20.8, 30.2, and 111 mV, and the voltage sensitivities are 18.3, 17.2, 23.7, and 21 mV per e-fold change in open probability, respectively. The half-activation Ca2+ concentration at 40 mV is 10(-5) M and the Hill coefficient of Ca2+ binding is 1.7. The K(Ca) channels were sometimes found in clusters, three to six channels in a patch. A 125 pS ATP-sensitive K+ channel was inhibited by the internal application of 2 mM ATP. Its activation was voltage independent. This channel may be important in the regulation of resting potential. A background K+ channel exhibited outwardly rectifying unitary current (176 pS) in symmetrical 115 mM KCl solutions but the ensemble-averaged I-V curve was ohmic. The voltage dependence is very weak, 220 mV per e-fold change in open probability. The nearly symmetrical macroscopic I-V curve of the background channel suggests a role in maintaining the axonal resting potential. A 28 pS delayed-rectifier K+ channel is found to be blocked internally by 2 mM 4-aminopyridine and by 10 mM tetraethylammonium. The half-activation voltage is -41 mV and the voltage sensitivity is 8 mV per e-fold change in open probability.
authors.affiliation: Department of Physiology, University of Rochester Medical Center, New York 14642-8642.
authors.forename: J V
authors.initials: JV
authors.lastname: Wu
authors.forename: C T
authors.initials: CT
authors.lastname: Rubinstein
authors.forename: P
authors.initials: P
authors.lastname: Shrager
mesh_terms: D015761:4-Aminopyridine / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001369:Axons / Q000502:physiology*; D002118:Calcium / Q000494:pharmacology; D004553:Electric Conductivity; D008564:Membrane Potentials / Q000187:drug effects; D009186:Myelin Sheath / Q000502:physiology*; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D014982:Xenopus laevis
chemical_list: D015221:Potassium Channels; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D000255:Adenosine Triphosphate; D015761:4-Aminopyridine; D002118:Calcium