Effects of Npt2 gene ablation and low-phosphate diet on renal Na+/phosphate cotransport and cotransporter gene expression

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Abstract
The renal Na+/phosphate (Pi) cotransporter Npt2 is expressed in the brush border membrane (BBM) of proximal tubular cells. We examined the effect of Npt2 gene knockout on age-dependent BBM Na+/Pi cotransport, expression of Na+/Pi cotransporter genes Npt1, Glvr-1, and Ram-1, and the adaptive response to chronic Pi deprivation. Na+/Pi cotransport declines with age in wild-type mice (Npt2+/+), but not in mice homozygous for the disrupted Npt2 allele (Npt2–/–). At all ages, Na+/Pi cotransport in Npt2–/– mice is approximately 15% of that in Npt2+/+ littermates. Only Npt1 mRNA abundance increases with age in Npt2+/+ mice, whereas Npt1, Glvr-1, and Ram-1 mRNAs show an age-dependent increase in Npt2–/– mice. Pi deprivation significantly increases Na+/Pi cotransport, Npt2 protein, and mRNA in Npt2+/+ mice. In contrast, Pi-deprived Npt2–/– mice fail to show the adaptive increase in transport despite exhibiting a fall in serum Pi. We conclude that (a) Npt2 is a major determinant of BBM Na+/Pi cotransport; (b) the age-dependent increase in Npt1, Glvr-1, and Ram-1 mRNAs in Npt2–/– mice is insufficient to compensate for loss of Npt2; and (c) Npt2 is essential for the adaptive BBM Na+/Pi cotransport response to Pi deprivation.