Fast‐activating cation channel in barley mesophyll vacuoles. Inhibition by calcium

Abstract

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+ overs C− 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≈ 6 µM. 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.