Fast-activating cation channel in barley mesophyll vacuoles. Inhibition by calcium
1997
Tikhonova, L.I. | Pottosin, I.I. | Dietz, K.J. | Schonknecht, G.
In contrast to the vacuolar ion channels which are gated open by an increase of cytosolic Ca2+ the vacuolar ion currents at resting cytosolic Ca2+ are poorly explored. Therefore, this study was performed to investigate the properties of the so-called fast-activating vacuolar (FV) current which dominates the electrical characteristics of the tonoplast at physiological free Ca2+ concentrations. Patch-clamp measurements were performed on whole barley (Hordeum vulgare) mesophyll vacuoles and on excised tonoplast patches. Single ion channels were identified, which, based on their selectivity, activation kinetics, Ca2+-and voltage-dependence, carry the whole-vacuole FV current. Reversal potential determinations indicated a K+ over Cl- permeability ratio of about 30. Both inward and outward whole-vacuole currents as well as the activity of single FV channels were inhibited by an increase of cytosolic Ca2+, with a Kd approximately 6 micromolar. At physiological vacuolar Ca2+ activities, the FV channel is an outward-rectifying potassium channel. The FV channel was activated in less than a few milliseconds both by negative and positive potential steps, having a minimal activity that is 40 mV negative of the K+ equilibrium potential. It is proposed that transport of K+ through this cation channel controls the electrical potential difference across the tonoplast.
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