Transport of Phosphate by Plasma Membranes of the Jejunum and Kidney of the Mouse Model of Hypophosphatemic Vitamin D-Resistant Rickets

Abstract

The hypophosphatemic mouse is a useful model for the study of hypophosphatemic vitamin D-resistant rickets in humans. Hypophosphatemia and hyperphosphaturia are the main biochemical findings in the patients and in mice with the disorder. The exact membrane localization of the site of the defect in phosphate transport in humans is not known. We utilized a well-validated technique of brush border and basolateral membrane vesicles to investigate phosphate transport across the enterocyte and the renal tubule cells of the hypophosphatemic (Hyp) mice model. Phosphate uptake by brush border membranes of jejunal enterocytes revealed similar initial rates (slopes were 0.007 and 0.006 for Hyp and control mice, respectively). Kinetics of jejunal Na⁺-dependent phosphate uptake showed a V_max of 0.21 ± 0.03 and 0.19 ± 0.02 nmol/mg protein/15 sec, and <K_m of 0.12 ± 0.07 and 0.09 ± 0.02 mM in the Hyp and control mice, respectively. Kinetics of basolateral uptake of phosphate were also similar (V_max of 0.05 ± 0.01 and 0.06 ± 0.02 nmol/mg protein/10 sec and <K_m of 0.013 ± 0.004 and 0.028 ± 0.022 mM, respectively). On the other hand, kinetics of Na⁺-dependent phosphate uptake by renal brush border membrane vesicles (BBMV) were markedly decreased (V_max of 0.42 ± 0.03 and 1.09 ± 0.06 nmol/mg protein/15 sec, < P < 0.01, and <K_m of 0.01 ± 0.003 and 0.05 ± 0.02 <mM, P < 0.02, in the Hyp and control mice, respectively). Kinetics of Na⁺-dependent phosphate uptake by renal basolateral membrane were not decreased (V_max of 0.19 ± 0.02 and 0.21 ± 0.02 nmol/mg protein/10 sec and <K_m of 0.012 ± 0.003 and 0.012 ± 0.004 mM for Hyp and control mice, respectively). To determine whether the decrease in renal BBMV is secondary to alteration in the Na⁺-dependent phosphate transporter or due to changes in the Na⁺ gradient, two studies were conducted: first, a tracer exchange study in renal BBMV which showed a decrease in phosphate uptake in Hyp BBMV compared with controls, confirming the kinetic studies; and second, an Na⁺ permeability study in renal BBMV of Hyp and control mice which showed no differences in Na⁺ permeability across the renal BBMV.