, Volume 120, Issue 2, pp 131-139

Activation by Ca2+ and block by divalent ions of the K+ channel in the membrane of cytoplasmic drops fromChara australis

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Patch-clamp studies of cytoplasmic drops from the charophyteChara australis have previously revealed K+ channels combining high conductance (170 pS) with high selectivity for K+, which are voltage activated. The cation-selectivity sequence of the channel is shown here to be: K+>Rb+>NH 4 + ≫Na+ and Cl. Divalent cytosolic ions reduce the K+ conductance of this channel and alter its K+ gating in a voltage-dependent manner. The order of blocking potency is Ba2+>Sr2+>Ca2+>Mg2+. The channel is activated by micromolar cytosolic Ca2+, an activation that is found to be only weakly voltage dependent. However, the concentration dependence of calcium activation is quite pronounced, having a Hill coefficient of three, equivalent to three bound Ca2+ needed to open the channel. The possible role of the Ca2+-activated K+ channel in the tonoplast ofChara is discussed.