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.