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 mu M) concentrations of Zn2+ produced a substantial (similar to threefold) 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 mu M 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.