Divalent Cations are Antagonists of the Sodium-Dependent Potassium Channel Slo2.2

Abstract

We studied the effect of different divalent cations on the Na+-dependent potassium channel Slo2.2 (Slack) using inside-out patches from Xenopus oocytes and a Slack-HEK cell line heterologously expressing Slack channels. In the presence of intracellular sodium, the addition of divalent cations reduces significantly Slack currents recorded in macropatches. Among the divalent cations studied, the most effective in reducing channel activity was Cd++ followed by Ni++, Ca++ and Mg++. Several results suggest that this effect is not caused by blocking the pore of the channel. First, the decrease in currents is not voltage dependent as expected for a cation blocking the channel pore. Second, single channel recordings show a decrease in open probability but not a significant reduction in single channel conductance. Third, some of these cations activate rather than block the Ca++-dependent channel Slo1, which is likely to have a pore structure similar to Slo2.2. Outside-out patches have been used to show that divalent cations do not have an inhibitory effect when applied to the extracellular side of the channel. We propose that the divalent cations may be competing with Na+ for the Na+ binding site. To test this hypothesis, we will examine the effect of divalent cations on several channel mutants and chimeras. These experiments also may help to reveal the structure and localization of the Na+ binding site.