Regulation of the fast vacuolar channel by cytosolic and vacuolar potassium

At resting cytosolic Ca2+, passive K+ conductance of a higher plant tonoplast is likely dominated by fast vacuolar (FV) channels. This patch-clamp study describes K+-sensing behavior of FV channels in Beta vulgaris taproot vacuoles. Variation of K+ between 10 and 400 mM had little effect on the FV channel conductance, but a pronounced one on the open probability. Shift of the voltage dependence by cytosolic K+ could be explained by screening of the negative surface charge with a density sigma = 0.25 e(-)/nm(2). Vacuolar K+ had a specific effect on the FV channel gating at negative potentials without significant effect on closed-open transitions at positive ones. Due to K+ effects at either membrane side, the potential at which the FV channel has minimal activity was always situated at similar to50 mV below the potassium equilibrium potential, EK+. At tonoplast potentials below or equal to EK+, the FV channel open probability was almost independent on the cytosolic K+ but varied in a proportion to the vacuolar K+. Therefore, the release of K+ from the vacuole via FV channels could be controlled by the vacuolar K+ in a feedback manner; the more K+ is lost the lower will be the transport rate.