Characterization of the defect in the Na⁺-Phosphate transporter in vitamin D-resistant hypophosphatemic mice

Hypophosphatemic vitamin D-resistant rickets is the most common form of vitamin D-resistant rickets in man. The hypophosphatemic mouse model (Hyp) is phenotypically and biochemically similar to the human disease. Biochemically, hypophosphatemia is the hallmark of this disorder. The cause of the hypophosphatemia is thought to be secondary to a defect in the renal and/or intestinal Na⁺-phosphate transporter. The current studies were designed to investigate and characterize the localization of the defect in the Na⁺-phosphate transporter in this disorder. Phosphate uptake by renal brush border membrane vesicles (BBMV) showed a significant decrease in the slope of the initial rate of phosphate uptake in (Hyp) compared with control mice (0.009 versus 0.013, respectively). The slopes representing initial rates of phosphate uptake by jejunal BBMV were similar in (Hyp) and control mice (0.004 and 0.004, respectively). Kinetics of jejunal Na⁺-dependent phosphate uptake showed a V_max of 0.63 ± 0.12 and 0.64 ± 0.12 nmol/mg protein/15 s in (Hyp) and control mice, respectively, whereas K_m values were 0.12 ± 0.08 and 0.2 ± 0.11 mM, respectively. Similar kinetic analysis in the kidney showed a V_max of 0.32 ± 0.06 and 1.6 ± 0.1 (p < 0.01) and K_m of 0.07 ± 0.06 and 0.39 ± 0.05 (p ̈ 0.02(Hyp) and control mice, respectively. Na⁺-dependent D-glucose uptake by BBMVs of intestine and kidney showed typical over-shoot phenomena in (Hyp) and control mice. In order to explore these findings further, Na⁺-phosphate transporter expression from intestine and kidney was accomplished by microinjection of 50 ng of poly(A)⁺ RNA into Xenopus laevis oocytes. Na⁺-dependent phosphate uptake was expressed 6 days after the microinjection of intestinal and kidney poly(A)⁺ RNA from control mice. However, expression of the transporter from (Hyp) mice occurred only from the intestine, and not from the kidney. The decrease in the expression of the Na⁺-dependent phosphate transporter was not secondary to accelerated efflux of phosphate or decreased metabolism in oocytes injected with poly(A)⁺ RNA from (Hyp) mice. α-Methyl-D-glucose was expressed equally from the kidneys of (Hyp) and control mice. The combined results of brush border membrane and X. laevis oocyte studies suggest that the cause of hypophosphatemia in (Hyp) mice is secondary to a defect in coding for the phosphate transporter, resulting in diminished activity and/or function of the Na⁺-dependent phosphate transporter in the kidney, but not in the intestine. Alternatively, the diminished expression could be secondary to a decrease in mRNA concentration for the Na⁺-phosphate co-transporter.