1 α - Hydroxylase gene ablation and Pi supplementation inhibit renal calcification in mice homozygous for the disrupted

Disruption of the major renal Na/phosphate (Pi) cotransporter gene, Npt2a, in mice leads to a substantial decrease in renal brush border membrane Na/Pi cotransport, hypophosphatemia and appropriate adaptive increases in renal 25-hydroxyvitamin D 3-1α-hydroxylase (1αOHase) activity and the serum concentration of 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D). The latter is associated with increased intestinal Ca absorption, hypercalcemia, hypercalciuria and renal calcification in Npt2-/-mice. To determine the contribution of elevated serum 1,25(OH) 2 D levels to the development of hypercalciuria and nephrocalcinosis in Npt2-/-mice, we examined the effects of 1αOHase gene ablation and long-term Pi supplementation on urine Ca excretion and renal calcification by micro-computed tomography. We show that the urine Ca/creatinine ratio is significantly decreased in Npt2-/-/1αOHase-/-mice when compared to Npt2-/-mice. In addition, renal calcification, determined by estimating the calcified volume to total renal volume (CT/TV), is reduced by ~80% in Npt2-/-/1αOHase-/-mice when compared to that in Npt2-/-mice. In Npt2-/-mice derived from dams fed a 1% Pi diet and maintained on the same diet, we observed a significant decrease in urine Ca/creatinine that was also associated with ~80% reduction in CV/TV when compared with counterparts fed a 0.6% diet. Taken together, the present data demonstrate that both 1αOHase gene ablation and Pi supplementation inhibit renal calcification in Npt2-/-mice and that 1,25(OH) 2 D is essential for the development of hypercalciuria and nephrocalcinosis in the mutant strain.