Na/P i cotransporter ( Npt2 ) gene disruption increases duodenal calcium absorption and expression of epithelial calcium channels 1 and 2

Abstract

Mice homozygous for the disrupted type-II Na/P(i) cotransporter gene ( Npt2(-/-)) exhibit hypophosphataemia, increased serum concentration of 1,25-dihydroxyvitamin D (1,25-(OH)(2)D) and calcium (Ca) and elevated urinary Ca excretion. To determine whether the hypercalcaemia and hypercalciuria are secondary to 1,25-(OH)(2)D-stimulated intestinal Ca absorption, we examined the effect of Npt2 gene disruption on serum Ca and urinary Ca excretion after an overnight fast, and on duodenal Ca absorption. We also compared the duodenal expression of the epithelial Ca channels, ECaC1 and ECaC2, and calbindinD(9K) mRNAs, relative to that of beta-actin mRNA, in Npt2(+/+) and Npt2(-/-) mice. Both serum Ca and urine Ca/creatinine were significantly decreased in Npt2(-/-) mice after an overnight fast and were no longer different from that in wild-type mice. Absorption of (45)Ca from isolated duodenal segments in vivo and (45)Ca appearing in the plasma were significantly increased in Npt2(-/-) compared with Npt2(+/+) mice. In addition, the duodenal abundance of ECaC1, ECaC2 and calbindinD(9K) mRNAs was significantly elevated in mutant mice relative to that in wild-type mice. In contrast, both duodenal Ca absorption and ECaC1 and ECaC2 mRNA abundance were lower in mice with X-linked hypophosphataemia ( Hyp) than in normal littermates. In summary, we provide evidence for increased duodenal Ca absorption in Npt2(-/-) mice and suggest a role for ECaC1, ECaC2 and calbindinD(9K) in mediating this response.

Type IIc Sodium–Dependent Phosphate Transporter Regulates Calcium Metabolism

“…Hypercalcemia and hypercalciuria in Npt2a Ϫ/Ϫ mice were due to induction of intestinal Ca absorption via transcellular transport (TRPV6 and calbindinD 9k ). 10 In contrast, the level of intestinal TRPV6 and calbindinD 9k mRNA was not increased in Npt2c Ϫ/Ϫ mice. The mechanisms causing elevation of intestinal Ca absorption in Npt2c Ϫ/Ϫ mice may differ from those in Npt2a Ϫ/Ϫ mice.…”

“…10 In the kidney, the levels of calbindinD 28K mRNA were significantly increased in Npt2a Ϫ/Ϫ mice compared with Npt2a ϩ/ϩ mice, but there was no differences in renal TRPV5 mRNA between Npt2a Ϫ/Ϫ mice and Npt2a ϩ/ϩ mice ( Figure 6I, E and F). 10 TRPV6 mRNA levels were slightly but not significantly decreased in the proximal and distal intestine of Npt2c ϩ/Ϫ and Npt2c Ϫ/Ϫ mice when compared with Npt2c ϩ/ϩ mice ( Figure 6II, A and B).…”

“…10 To determine whether the hypercalciuria in Npt2c Ϫ/Ϫ mice is dependent on dietary Ca intake, we examined the effect of fasting on plasma Ca concentration and urinary Ca excretion ( Figure 5). Under control conditions, the levels of plasma Ca concentration and urinary Ca excretion were significantly increased in Npt2c Ϫ/Ϫ mice compared with Npt2c ϩ/ϩ mice at 4 wk of age ( Figures 2IIB and 5).…”

Molecular Mechanisms of Primary Hypercalciuria

“…These symptoms are similar to the phenotype of mice with constructed deletion of the gene for the kidney-specific Na-Pi cotransporter Npt2, although the bone phenotype is different; while HHRH patients display rickets and osteomalacia, the Npt2 knockout (KO) mice show a delay in bone mineralization at 21 d after birth and an increase in number of trabeculae and decrease in marrow space at 115 d (61). Npt2 KO mice display higher levels of duodenal mRNA for ECaC, CaT1, and calbindin D 9k , consistent with their known increase in intestinal calcium absorption (62). However, mapping of the Npt2 locus in HHRH kindreds revealed no mutations that cosegregated with the disease (63,64).…”

1α-Hydroxylase gene ablation and Pisupplementation inhibit renal calcification in mice homozygous for the disruptedNpt2agene

“…Ϫ/Ϫ mice is associated with significantly increased intestinal Ca absorption (21), hypercalcemia, decreased circulating levels of PTH, and hypercalciuria (2).…”