Anion modulation of the slowly activating vacuolar channel.

Abstract

This study addresses the permeability and modulation by anions of the slowly-activating vacuolar (SV) channel in the tonoplast of Mesembryanthemum crystallinum. Although we conclude that the M. crystallinum SV channel is impermeable to anions, the anion species at the cytosolic side of the membrane clearly affects the channel. We discuss two types of modulation, both consistent with a single binding site for anions at the cytosolic face of the channel. The first effect is related to the conducting properties of SV. When the site is occupied, the bound anion exerts an effect on the Ca2+ and Na+ fluxes through the channel. Because the Stokes-diameter of both ion species is different, both fluxes are not influenced to the same extent. This differential effect on the permeation of both ion species is reflected in a permeability ratio Ca2+ over Na+ (P(Ca)/P(Na)) that depends on the identity of the anion species. The second type of modulation deals with the activation of the channel by anions. Previous studies revealed that a low level of cytosolic Cl- inactivates SV (Pantoja, Dainty & Blumwald, 1992a). We critically evaluated reported effects of anions on the whole-vacuole SV current. Given that cytosolic Ca2+ and Cl- both modulate SV, we conclude that both levels of regulation show some kind of interaction. High cytosolic Ca2+ can overrule the inactivation caused by low cytosolic Cl-.