AJP - Renal Physiology, Vol 248, Issue 5 705-F710, Copyright © 1985 by American Physiological Society
Effect of pH on phosphate transport in rat renal brush border membrane vesicles
M. Amstutz, M. Mohrmann, P. Gmaj and H. Murer
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 sodium gradient-dependent phosphate uptake about 20-fold.
Sodium-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 sodium increased from about 100 to 200 mM
when pH was decreased from 7.4 to 6.4. The Hill coefficient for sodium
interaction was close to 2 and was unaffected by pH. Increasing external
sodium 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 sodium concentrations, small effects
(20-30%) of pH on the apparent Vmax of the transport system were found;
measured at saturating sodium 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 data indicate that the major effect of
pH is to modify the interaction of the transport system with sodium. At
nonsaturating sodium concentrations, this resulted indirectly in a
reduction in the affinity for phosphate related to a different occupancy of
the sodium binding site. The differences of transport rate at low phosphate
and high sodium concentrations could be explained by preferential transport
of divalent phosphate as well as by pH effects on other carrier properties.
