Cytoplasmic magnesium regulates the fast activating vacuolar cation channel

Fast activating vacuolar (FV) channels, which are permeable for small monov alent cations, dominate the ion conductance of the vacuolar membrane at phy siological Ca2+ concentrations. Here the effect of Mg2+ on FV channels was studied. Patch-clamp measurements were performed on whole barley (Hordeum v ulgare) mesophyll vacuoles and on excised tonoplast patches. Free Mg2+ conc entrations in the millimolar range inhibited FV channels from the cytosolic and the vacuolar side. Increasing cytosolic free Mg2+ decreased the open p robability of FV channels without affecting single channel current amplitud es. The Mg2+ effect showed a bell-shaped voltage-dependence and was most pr onounced at voltages between -40 and -60 mV. The dose dependence of the FV channel inhibition by cytosolic Mg2+ could be described by a simple Michael is-Menten type of binding with K-d values of 10 and 35 mu M at -60 mV and 100 mV, respectively.