Inorganic phosphate homeostasis in sodium-dependent phosphate cotransporter Npt2b^+/−mice

Abstract

An inorganic phosphate (Pi)-restricted diet is important for patients with chronic kidney disease and patients on hemodialysis. Phosphate binders are essential for preventing hyperphosphatemia and ectopic calcification. The sodium-dependent Pi (Na/Pi) transport system is involved in intestinal Pi absorption and is regulated by several factors. The type II sodium-dependent Pi transporter Npt2b is expressed in the brush-border membrane in intestinal epithelial cells and transports Pi. In the present study, we analyzed the phenotype of Npt2b−/− and hetero+/− mice. Npt2b−/− mice died in utero soon after implantation, indicating that Npt2b is essential for early embryonic development. At 4 wk of age, Npt2b+/− mice showed hypophosphatemia and low urinary Pi excretion. Plasma fibroblast growth factor 23 levels were significantly decreased and 1,25(OH)2D3 levels were significantly increased in Npt2b+/− mice compared with Npt2b+/+ mice. Npt2b mRNA levels were reduced to 50% that in Npt2b+/+ mice. In contrast, renal Npt2a and Npt2c transporter protein levels were significantly increased in Npt2b+/− mice. At 20 wk of age, Npt2b+/− mice showed hypophosphaturia and reduced Na/Pi cotransport activity in the distal intestine. Npt2b+/+ mice with adenine-induced renal failure had hyperphosphatemia and high plasma creatinine levels. Npt2b+/− mice treated with adenine had significantly reduced plasma Pi levels compared with Npt2b+/+ mice. Intestinal Npt2b protein and Na+/Pi transport activity levels were significantly lower in Npt2b+/− mice than in the Npt2b+/+ mice. The findings of the present studies suggest that Npt2b is an important target for the prevention of hyperphosphatemia.

Npt2b Deletion Attenuates Hyperphosphatemia Associated with CKD

“…6,15 Multiple studies provide indirect evidence that Npt2b may also participate in CKD-associated hyperphosphatemia. 6,11,13,[16][17][18] To directly assess the therapeutic potential of targeting Npt2b, we evaluated the long-term effect of Npt2b deletion in a chronic CKD model.…”

“…12,14 A recent study in uremic Npt2b +/2 mice provides initial evidence that Npt2b reduction specifically decreases hyperphosphatemia. 15 However, because this study was performed using developmental heterozygous mice with ARF, it is unknown whether the effects of Npt2b deletion can be sustained over the course of CKD progression due to upregulation of Npt2a. In addition, bone disease, a known contributor of systemic phosphate burden, was likely not present in this acute model.…”

Effect of dietary components on renal inorganic phosphate (Pi) excretion induced by a Pi-depleted diet

“…Egg white was used as the protein source instead of casein (16). An AIN93G mineral mixture without Pi and Ca sources was used to adjust the concentration of Ca and Pi for each test diet.…”

“…Concentrations of plasma or urinary Pi, and Ca were determined using commercial kits (Wako, Osaka, Japan) (16). Concentrations of plasma PTH were determined using the PTH ELISA kit (Immunotopics Inc., San Clemente, CA) (16).…”

“…Concentrations of plasma PTH were determined using the PTH ELISA kit (Immunotopics Inc., San Clemente, CA) (16). Tail vein blood collection was performed using a 1-mL syringe with a 25-gauge needle at 11 : 00 AM for all studies.…”