Phosphate and slow vacuolar channels in Beta vulgaris

Movement of phosphate through slow vacuolar (SV) ion channels and the effects of phosphate on SV currents were investigated using vacuoles from Beta vulgaris L. When the vacuoles contained 50 mM phosphate, the addition of phosphate to the bath shifted the apparent reversal potential for whole vacuole currents to more positive values, suggesting an outward rectifying current due to movement of phosphate ions out of the vacuole. However absolute values for reversal potentials obtained from the current-voltage curves and tail currents for whole vacuoles suggested that the membranes were relatively impermeable to phosphate. Single-channel data showed that the vacuole preparations contained more than one species of ion channel and therefore the whole-vacuole data will not give definitive information about individual species of ion channels. One of the channels had a single channel reversal potential that indicated a permeability of H2PO4- ions relative to Cl- of 7. The probability of this outwardly rectifying channel being open had a marked dependence on voltage and, in these experiments, it was effectively closed for potentials negative of +20 mV. The single-channel conductance was 19.4 +/- 3.1 pS with 50 mM KH2PO4 in the vacuole and 10 mM total phosphate concentration in the bath. A channel with these characteristics has not been reported previously. In addition to the data identifying a phosphate channel, it was found that the presence of phosphate in the bath solution slowed the rate of activation of the SV currents. This effect was partially reversed when phosphate was removed from the bath.