Effect of pH on phosphate transport in rat renal brush border membrane vesicles

Abstract

The initial linear rate of phosphate uptake was analyzed in rat renal brush border membrane vesicles. An increase in medium pH from 6.0 to 8.0 increased the Na gradient-dependent phosphate uptake about 20-fold. Na-independent phosphate uptake about 20-fold. Na-independent phosphate uptake was not altered in this pH range. At pH 7.4 an intravesicular acid pH stimulated the initial linear uptake rate (20-25%). The apparent Km for Na increased from about 100 to 200 mM when pH was decreased from 7.4-6.4. The Hill coefficient for Na interaction was close to 2 and was unaffected by pH. Increasing external Na reduced the apparent Km of the transport system for phosphate independent of pH. Variations of phosphate concentration had no influence on the apparent Km for sodium. At high Na concentrations, small effects (20-30%) of pH on the apparent Vmax of the transport system were found; measured at saturating Na concentrations, the apparent Km values calculated on the basis of total phosphate were increased (50-60%) when pH was decreased from 7.4 to 6.4. The major effect of pH is to modify the interaction of the transport system with Na. At nonsaturating Na concentrations, this resulted indirectly in a reduction in the affinity for phosphate related to a different occupancy of the Na binding site. The differences of transport rate at low phosphate and high-Na concentrations could be explained by preferential transport of divalent phosphate as well as by pH effects on other carrier properties.