Characterization of an anion-permeable channel from sugar beet vacuoles: effect of inhibitors.

The vacuole occupies 25-95% of the plant cell volume and plays an essential role in maintaining cytoplasmic homeostasis of nutrients and ions. Recent patch-clamp studies identified ion channels and electrogenic pumps as pathways for the movement of ions and metabolites across the vacuolar membrane (tonoplast). At high cytoplasmic Ca (>10 M) and negative potentials (inside the vacuole) non-selective channels of the ;slow-vacuolar (SV)-type' were activated resulting in anion release or cation influx. In the present study these vacuolar channels were characterized pharmacologically by ion channel inhibitors. The cation-transport inhibitors Ba, TEA and amiloride caused only partial and reversible block of the ;SV-type'channels, whereas anion-transport inhibitors strongly affected the vacuolar channels. Pyridoxalphosphate and the dimethylaminecarboxylate derivates anthracene-9-carboxylic acid and C 144 reversibly blocked the channels up to 70% and Zncl(2) up to 95%. DIDS and SITS inhibited this channel irreversibly up to 95%. The block developed under a variety of experimental conditions using solutions containing combinations of permanent cations and anions. The DIDS binding site is located on the cytoplasmic surface of the tonoplast, as intravacuolar DIDS did not block the channels. DIDS concentrations in the micromolar range, efficient in blocking 70-80% of the ;SV-type' channels did not significantly affect ATP-induced or pyrophosphate-induced proton-pumps. Stilbene derivatives may therefore be useful tools for studies on the substrate binding site on this vacuolar channel and for channel isolation.