Reduced absorption of45calcium from isolated duodenal segmentsin vitro in juvenile but not adult X-linked hypophosphatemic mice

Abstract

In juvenile X-linked hypophosphatemic (Hyp) mice, whole body calcium balances are significantly lower than in genetically normal mice. This is associated with low duodenal vitamin D-dependent calcium-binding protein and a failure of skeletal mineralization. To seek more specific evidence of an intestinal defect in these mice, absorption of 45Ca was measured in isolated duodenal segments in vivo in mice from 2-13 weeks of age. The duodenum was isolated by sutures and 45Ca was injected into the lumen in 150 mM NaCl and 2 mM CaCl2 at pH = 7.2. Absorption was measured by the amount of isotope remaining in the lumen and by the plasma isotope level. Hemizygous Hyp male and heterozygous Hyp female mice absorbed significantly less 45Ca at 4 and 7 weeks of age than genetically normal mice while Hyp mice at 2, 10, and 13 weeks of age were not significantly affected. At 4 and 7 weeks of age, the Hyp mice also had significantly reduced plasma radioactivity midway through the collection period as well as at the end of the period. To explore a possible mechanism for this malabsorption, 1,25(OH)2-vitamin D receptors were measured in cytosol prepared from 4-week-old normal and Hyp duodenum. There were non-significant differences between the normal and Hyp mice in both binding affinity, Kd, and the number of receptors, nmax. In conclusion, juvenile Hyp mice at 4 and 7 weeks of ages malabsorbed calcium from their duodenum. Hyp mice younger than this period were not affected nor were adult Hyp mice.(ABSTRACT TRUNCATED AT 250 WORDS)

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

“…Hyp mice harbour a large deletion in the 3'-region of the Phex gene [3] and, like Npt2 -/-mice, exhibit renal P i wasting [9,10], secondary to a 50% reduction in renal Npt2 gene expression, and associated hypophosphataemia [31]. However, in contrast to Npt2 -/-mice, duodenal Ca absorption is significantly decreased in Hyp mice relative to normal littermates, in agreement with previous findings [22,23]. Moreover, we showed that the decrease in Ca absorption in Hyp mice is associated with a corresponding decrease in the duodenal abundance of ECaC1 and ECaC2 mRNAs.…”

“…3A) and 45 Ca appearing in the plasma at 2 and 4 min (Fig. 3B) were decreased significantly in young mutant Hyp mice compared with normal littermates, as previously reported [22,23]. Despite significant differences in duodenal Ca absorption in Npt2 -/-mice and Hyp mice, relative to their respective normal littermates (Figs.…”

“…Hyp/Y mice exhibit significant hypophosphataemia, renal P i wasting and inappropriate renal 1,25-(OH) 2 D synthesis in response to endogenous hypophosphataemia [21,30,37]. In addition, juvenile Hyp mice exhibit decreased intestinal Ca absorption, secondary to inappropriate renal production of 1,25-(OH) 2 D [22,23]. All animal studies were conducted in accordance with the guidelines of the Canadian Council on Animal Care and with the approval of the local Institutional Animal Care and Use Committees.…”

“…The duodenal absorption of Ca was measured in isolated intestinal segments in vivo according to [22]. Briefly, mice were anaesthetized with pentobarbitonal sodium.…”

“…In addition, we examined the duodenal expression of ECaC1 and ECaC2 mRNA in mice with X-linked hypophosphataemia (Hyp), which exhibit decreased duodenal Ca absorption, secondary to inappropriate renal production of 1,25-(OH) 2 D [22,23], relative to their respective wild-type counterparts. Our data suggest that ECaC1 and ECaC2 mediate the changes in duodenal Ca absorption in both mutant mouse models.…”