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

The renal Na+/phosphate (Pi) cotransporter Npt2 is expressed in the brush b order membrane (BBM) of proximal tubular cells. We examined the effect of N pt2 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-ty pe mice (Npt2(+/+)), but not in mice homozygous for the disrupted Npt2 alle le (Npt2(-/-)). At all ages, Na+/Pi cotransport in Npt2-/- mice is approxim ately 15% of that in Npt2(+/+) Littermates. Only Npt1 mRNA abundance increa ses with age in Npt2(+/+) mice, whereas Npt1, Glvr-1, and Ram-1 mRNAs show an age-dependent increase in Npt2(-/-) mice. Pi deprivation significantly i ncreases Na+/Pi cotransport, Npt2 protein, and mRNA in Npt2(+/+) mice. In c ontrast, Pi-deprived Npt2-/- mice fail to show the adaptive increase in tra nsport 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 increa se in Npt1, Glvr-1, and Ram-1 mRNAs in Npt2(-/-) mice is insufficient to co mpensate for loss of Npt2; and (c) Npt2 is essential for the adaptive BBM N a+/Pi cotransport response to Pi deprivation.