Activation by Ca2+ and block by divalent ions of the K+ channel in the membrane of cytoplasmic drops from Chara australis.
Patch-clamp studies of cytoplasmic drops from the charophyte Chara 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+ greater than Rb+ greater than NH4+ much greater than 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+ greater than Sr2+ greater than Ca2+ greater than 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 Ca(2+)-activated K+ channel in the tonoplast of Chara is discussed.
Laver
D R
DR
School of Biological Sciences, University of Sydney, Australia.
Walker
N A
NA
D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology; D000460:Chlorophyta / Q000378:metabolism*; D003593:Cytoplasm / Q000378:metabolism; D007700:Kinetics; D008566:Membranes / Q000378:metabolism; D010539:Permeability; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*
D002413:Cations, Divalent; D015221:Potassium Channels; D002118:Calcium