Phosphate transport in kidneys: effect of transmembrane electrical potential

The effect of a transmembrane electrical potential on phosphate transport by kidney brush-border membrane vesicles was studied. The initial rate of Na(+)-dependent phosphate influx was twice as high as that of efflux. Generation of a negative transmembrane potential had a stimulatory effect on the rate of influx but had no effect on efflux. The Na+ saturation curve for phosphate influx was sigmoidal, and the Hill coefficients were similar, in the presence and absence of a transmembrane potential. The membrane potential increased both the affinity for phosphate and the maximal velocity (Vmax) of the transporter. In the absence of a Na+ gradient, the stimulation by the potential was 1.78-fold. When a proton gradient (in greater than out) was the driving force, the electrical potential stimulated phosphate transport 1.71-fold. Internal Na+ (trans) inhibited phosphate influx whether a potential was present or not. Internal phosphate (trans) stimulated phosphate influx in the absence of a potential but not in its presence. These results indicate that the electrical potential is an important driving force for the Na(+)-phosphate carrier and that the translocation of the carrier is a potential-dependent step.