External barium influences the gating charge movement of Shaker potassium channels.
External Ba2+ speeds the OFF gating currents (IgOFF) of Shaker K+ channels but only upon repolarization from potentials that are expected to open the channel pore. To study this effect we used a nonconducting and noninactivating mutant of the Shaker K+ channel, ShH4-IR (W434F). External Ba2+ slightly decreases the quantity of ON gating charge (QON) upon depolarization to potentials near -30 mV but has little effect on the quantity of charge upon stepping to more hyperpolarized or depolarized potentials. More strikingly, Ba2+ significantly increases the decay rate of IgOFF upon repolarization to -90 mV from potentials positive to approximately -55 mV. For Ba2+ to have this effect, the depolarizing command must be maintained for a duration that is dependent on the depolarizing potential (> 4 ms at -30 mV and > 1 ms at 0 mV). The actions of Ba2+ on the gating current are dose-dependent (EC50 approximately 0.2 mM) and are not produced by either Ca2+ or Mg2+ (2 mM). The results suggest that Ba2+ binds to a specific site on the Shaker K+ channel that destabilizes the open conformation and thus facilitates the return of gating charge upon repolarization.
Hurst
R S
RS
Department of Anesthesiology, University of California at Los Angeles 90095, USA. raymond@cvmail.anes.ucla.edu
Roux
M J
MJ
Toro
L
L
Stefani
E
E
D000818:Animals; D001464:Barium / Q000494:pharmacology*; D005260:Female; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D008954:Models, Biological; D009865:Oocytes / Q000502:physiology; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D011994:Recombinant Proteins / Q000187:drug effects / Q000378:metabolism; D017384:Sequence Deletion; D051637:Shaker Superfamily of Potassium Channels; D014982:Xenopus laevis
D015221:Potassium Channels; D011994:Recombinant Proteins; D051637:Shaker Superfamily of Potassium Channels; D001464:Barium