Effect of parathyroid hormone and dietary phosphate on phosphate transport in renal outer cortical and outer medullary brush-border membrane vesicles

Abstract

Two distinctive sodium-dependent phosphate transport systems have been identified in early and late proximal tubules; a high-capacity process located only in outer cortical tissue, and a high affinity present in both outer cortical and outer medullary brush-border membranes ( K m 0.1-0.25 mM). A third, sodium-independent, pH gradient-stimulated system ( V max 4.7 ± 0.3 nmol · mg −1 · min −1, K m 0.15 ± 0.002 mM) is present in the outer medulla, but absent in outer cortex. Brush-border vesicles were prepared from outer cortical and outer medullary tissue of pigs maintained on low (< 0.05%), normal (0.4%), or high (4%) phosphate diets. Sodium-dependent phosphate uptake of the high-capacity system decreased ( V max, 9.4 to 2.2 nmol · mg −1 · min −1) from low to high phosphate diet, whereas uptake rates decreased about 50% in the high-affinity system. There were no changes in the respective K m values. The pH gradient-stimulated uptake also decreased ( V max, 6.9 to 3.0 nmol · mg −1 · min −1) with no change in mean K m value (0.15 ± 0.001 mM) with dietary manipulation. Administration of 1 U parathyroid hormone prior to study resulted in a decrease in sodium-dependent uptake by 40–50% and in pH-dependent uptake (36%) with no change in the respective K m values. In conclusion, the antecedent dietary phosphate intake and parathyroid hormone administration appropriately alters phosphate uptake across the brush-border membrane of all three systems, sodium-dependent and pH gradient-stimulated phosphate transport.