Cytoplasmic magnesium regulates the fast activating vacuolar cation channel

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