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 f rom Beta vulgaris L. When the vacuoles contained 50 mM phosphate, the addition of phosphate to the bath shifted the apparent reversal potent ial for whole vacuole currents to more positive values, suggesting an outward rectifying current due to movement of phosphate ions out of th e vacuole. However absolute values for reversal potentials obtained fr om the current-voltage curves and tail currents for whole vacuoles sug gested that the membranes were relatively impermeable to phosphate. Si ngle-channel data showed that the vacuole preparations contained more than one species of ion channel and therefore the whole-vacuole data w ill not give definitive information about individual species of ion ch annels. One of the channels had a single channel reversal potential th at indicated a permeability of H2PO4- ions relative to Cl- of 7. The p robability of this outwardly rectifying channel being open had a marke d dependence on voltage and, in these experiments, it was effectively closed for potentials negative of +20 mV. The single-channel conductan ce was 19.4 +/- 3.1 pS with 50 mM KH2PO4 in the vacuole and 10 mM tota l phosphate concentration in the bath. A channel with these characteri stics has not been reported previously. In addition to the data identi fying 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 t he bath.