Slowly activating vacuolar channels can not mediate Ca²⁺‐induced Ca²⁺ release

Abstract

In order to test the hypothesis that slowly activating vacuolar (SV) channels mediate Ca2+‐induced Ca2+ release the voltage‐ and Ca2+‐dependence of these K+ and Ca2+‐ permeable channels were studied in a quantitative manner. The patch‐clamp technique was applied to barley (Hordeum vulgare L.) mesophyll vacuoles in the whole vacuole and vacuolar‐free patch configuration. Under symmetrical ionic conditions the current‐voltage relationship of the open SV channel was characterized by a pronounced inward rectification. The single channel current amplitude was not affected by changes in cytosolic Ca2+ whereas an increase in vacuolar Ca2+ decreased the unitary current in a voltage‐dependent manner. The SV channel open‐probability increased with positive potentials and elevated cytosolic Ca2+, but not with elevated cytosolic Mg2+. An increase of cytosolic Ca2+ shifted the half‐activation potential to more negative voltages, whereas an increase of vacuolar Ca2+ shifted the half‐activation potential to more positive voltages. At physiological vacuolar Ca2+ activities (50 μM to 2 mM) changes in cytosolic Ca2+ (5 μM to 2 mM) revealed an exponential dependence of the SV channel open‐probability on the electrochemical potential gradient for Ca2+ (ΔμCa). At the Ca2+ equilibrium potential (ΔμCa = 0) the open‐probability was as low as 0.4%. Higher open‐probabilities required net Ca2+ motive forces which would drive Ca2+ influx into the vacuole. Under conditions favouring Ca2+ release from the vacuole, however, the open‐probability further decreased. Based on quantitative analysis, it was concluded that the SV channel is not suited for Ca2+‐induced Ca2+ release from the vacuole.

Signal transduction and ion channels in guard cells

“…It was suggested that the SV channel would also carry e¥ux of Ca 2+ from the vacuole, in a process of Ca 2+ -induced Ca 2+ release, but recent more detailed study of the voltage conditions for activation of the SV channel in barley mesophyll vacuoles argues against this. Pottosin et al (1997) found that the open probability of the SV channel depended on the electrochemical gradient for Ca 2+ across the tonoplast. At the equilibrium potential for Ca 2+ the open probability was as low as 0.4%, and declined further in conditions favouring Ca 2+ release from the vacuole; essentially the channel opened signi¢cantly only when the gradient was such as to drive Ca 2+ into the vacuole.…”

Reduced Tonoplast Fast-Activating and Slow-Activating Channel Activity Is Essential for Conferring Salinity Tolerance in a Facultative Halophyte, Quinoa

“…The SV channel is permeable to both monovalent and divalent cations and is activated by cytosolic Ca 2+ and positive vacuolar voltage (Hedrich and Neher, 1987;Ward and Schroeder, 1994;Pottosin et al, 1997Pottosin et al, , 2001. The FV channel is permeable for monovalent cations only, is activated by large voltages of either sign, and is inhibited by divalent cations from either side of the membrane Brüggemann et al, 1999aBrüggemann et al, , 1999b.…”

The Clickable Guard Cell, Version II: Interactive Model of Guard Cell Signal Transduction Mechanisms and Pathways

“…Absence of Ca 2+ -activated SV currents was confirmed in an independent study (Ranf et al, 2008). SV channels are downregulated by vacuolar Ca 2+ (Pottosin et al, 1997;Pei et al, 1999), but can allow Ca 2+ transport from the vacuole to the cytoplasm (Bewell et al, 1999), which is for example illustrated by "tail" currents recorded in pure CaCl 2 gradients (Ward et al, 1995). SV channels are ubiquitous and have been found in all plant vacuoles, but in spite of their broad distribution in plant vacuoles, their physiological functions are not yet understood.…”