Modulation of potassium channel gating by external divalent cations.
We have examined the actions of Zn2+ ions on Shaker K channels. We found that low (100 microM) concentrations of Zn2+ produced a substantial (approximately three-fold) slowing of the kinetics of macroscopic activation and inactivation. Channel deactivation was much less affected. These results were obtained in the presence of 5 mM Mg2+ and 4 mM Ca2+ in the external solution and so are unlikely to be due to modification of membrane surface charges. Furthermore, the action of 100 microM Zn2+ on activation was equivalent to a 70-mV reduction of a negative surface potential whereas the effects on deactivation would require a 15-mV increase in surface potential. External H+ ions reduced the Zn-induced slowing of macroscopic activation with an apparent pK of 7.3. Treatment of Shaker K channels with the amino group reagent, trinitrobenzene sulfonic acid (TNBS), substantially reduced the effects of Zn2+. All these results are qualitatively similar to the actions of Zn2+ on squid K channels, indicating that the binding site may be a common motif in potassium channels. Studies of single Shaker channel properties showed that Zn2+ ions had little or no effect on the open channel current level or on the open channel lifetime. Rather, Zn2+ substantially delayed the time to first channel opening. Thus, K channels appear to contain a site to which divalent cations bind and in so doing act to slow one or more of the rate constants controlling transitions among closed conformational states of the channel.
Spires
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Department of Physiology, University of Rochester Medical Center, New York 14642-8642.
Begenisich
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D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology*; D002460:Cell Line; D004330:Drosophila; D006863:Hydrogen-Ion Concentration; D015640:Ion Channel Gating / Q000187:drug effects*; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D014302:Trinitrobenzenesulfonic Acid / Q000494:pharmacology; D015032:Zinc / Q000378:metabolism / Q000494:pharmacology*
D015221:Potassium Channels; D014302:Trinitrobenzenesulfonic Acid; D008274:Magnesium; D015032:Zinc; D002118:Calcium