Phosphate uptake by renal membrane vesicles of rabbits adapted to high and low phosphorus diets

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Abstract
Renal adaptation to changes in phosphate intake was studied by comparing phosphate uptake by proximal tubule brush border membrane vesicles from rabbits on a relatively high or low P diet. The low P diet increased Na+ gradient-dependent phosphate uptake. Uptake in the absence of Na+ and in the presence of Na+, but no gradient, was not significantly affected. The P diet did not alter Na+ gradient-dependent D-glucose and L-proline uptake. The low P diet increased Vmax; affinity for phosphate was not appreciably changed. At all concentrations of extravesicular Na+, phosphate uptake was higher in membrane vesicles from animals fed the low P diet; the kinetics of the phosphate uptake system, with respect to Na+, was also altered by the change in dietary phosphate. Adaptation involves an alteration in the rate of translocation of the Na+-phosphate carrier when energized by a Na+ gradient driving force rather than a change in the number of Na+-phosphate carrier sites. With membrane vesicles from rabbits fed a low P diet, phosphate uptake increased several-fold when the pH of the uptake medium was raised, whereas with membrane vesicles from animals fed a high P diet the enhancement of uptake with alkalinization was relatively small. Irrespective of the diet, divalent phosphate was the probable preferred species for transport. Dietary adaptation was associated with an alteration in the pH dependency of the transport system per se. The adaptation of the kidney phosphate transport system to dietary phosphate load involves an intrinsic change in the Na+-phosphate carrier.