title: Mechanism of Ba2+ block of M-like K channels of rod photoreceptors of tiger salamanders.
abstract: IKx is a voltage-dependent K+ current in the inner segment of rod photoreceptors that shows many similarities to M-current. The depression of IKx by external Ba2+ was studied with whole-cell voltage clamp. Ba2+ reduced the conductance and voltage sensitivity of IKx tail currents and shifted the voltage range over which they appeared to more positive potentials. These effects showed different sensitivities to Ba2+: conductance was the least sensitive (K0.5 = 7.6 mM), voltage dependence intermediate (K0.5 = 2.4 mM) and voltage sensitivity the most sensitive (K0.5 = 0.2 mM). Ca2+, Co2+, Mn2+, Sr2+, and Zn2+ did not have actions comparable to Ba2+ on the voltage dependence or the voltage sensitivity of IKx tail currents. In high K+ (100 mM), the voltage range of activation of IKx was shifted 20 mV negative, as was the tau-voltage relation. High K+ did not prevent the effect of Ba2+ on conductance, but abolished its ability to affect voltage dependence and voltage sensitivity. Ba2+ also altered the apparent time-course of activation and deactivation of IKx. Low Ba2+ (0.2 mM) slowed both deactivation and activation, with most effect on deactivation; at higher concentrations (1-25 mM), deactivation and activation time courses were equally affected, and at the highest concentrations, 5 and 25 mM Ba2+, the time course became faster than control. Rapid application of 5 mM Ba2+ suggested that the time dependent currents in Ba2+ reflect in part the slow voltage-dependent block and unblock of IKx channels by Ba2+. This blocking action of Ba2+ was steeply voltage-dependent with an apparent electrical distance of 1.07. Ba2+ appears to interact with IKx channels at multiple sites. A model which assumes that Ba2+ has a voltage-independent and a voltage-dependent blocking action on open or closed IKx channels reproduced many aspects of the data; the voltage-dependent component could account for both the Ba(2+)-induced shift in voltage dependence and reduction in voltage sensitivity of IKx tail currents.
authors.affiliation: Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle 98195.
authors.forename: L P
authors.initials: LP
authors.lastname: Wollmuth
mesh_terms: D000557:Ambystoma; D000818:Animals; D001464:Barium / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D004594:Electrophysiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects; D007700:Kinetics; D008954:Models, Biological; D010786:Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*
chemical_list: D015221:Potassium Channels; D001464:Barium; D011188:Potassium