Lack of effect of vitamin D and its metabolites on intestinal phosphate transport in familial hypophosphatemia of mice

Low levels of intestinal calbindin-D28K in X-linked hypophosphatemic mice

Using our radioimmunoassay for chick intestinal calbindin-D28K, a protein homologous to that in the chick intestine on the basis of immuno-crossreactivity, molecular weight and charge properties was identified in mouse intestinal mucosa. No such protein, however, was found in the kidney, cerebellum, cerebrum, liver or myocardium of the mice. In X-linked hypophosphatemic mice (Hyp mice) maintained on a standard diet containing vitamin D, the basal level of intestinal calbindin-D28K was much lower (average 65.13 +/- 7.39%) than that in breeding pairs of normal mice maintained under the same conditions. Although the mean intestinal level of calbindin-D28K was as low as 118.23 +/- 20.08 (SD) ng/mg protein in vitamin D-deficient mice (-D), the level of this protein was markedly increased to the control level (198.68 +/- 9.98 vs. 198.49 +/- 14.29 ng/mg protein of control) (P less than 0.001) in response to the administration of vitamin D3 (1000 I.U. s.c. for 10 days) (+D). These results indicate that calbindin-D28K, biochemically indistinguishable from the chick intestinal calbindin-D28K, is present in the mouse intestine, and that the lower amount of this protein in the Hyp mouse intestine may, at least in part, be responsible for the resistance to the effects of vitamin D.

Evidence that low plasma 1,25-dihydroxyvitamin D causes intestinal malabsorption of calcium and phosphate in juvenile X-linked hypophosphatemic mice

X-linked hypophosphatemic (Hyp) mice are a model for human sex-linked vitamin D-resistant rickets. We have reported intestinal malabsorption of calcium in young Hyp mice, and in this report we have explored the mechanism for it. To test for resistance of the intestine to 1,25(OH)2 vitamin D3, this hormone was continually infused via osmotic minipumps into 4-week-old normal and Hyp mice at 0, 17, 50 or 150 ng/kg/day. After 3 days, 45Ca and inorganic 32P were administered by gavage, and the mice were sacrificed on the fifth day. The Hyp mice showed responses to the hormone equivalent to the normal mice in terms of increased intestinal absorption of both 45Ca and 32P, increased plasma isotope levels, increased femoral isotope content, and increased duodenal and renal 9 kD vitamin D-dependent calcium-binding protein (calbindin-D9K; CaBP). Plasma 1,25(OH)2D was measured in these mice. There were significant correlations of plasma 1,25(OH)2D to the intestinal absorption of 45Ca and 32P and to duodenal and renal CaBP. Plasma 1,25(OH)2D was also measured in stock normal and Hyp mice and was found to be lower in 4-week-old Hyp mice than in 4-week-old normal mice (113 +/- 10 pM (n = 18) vs. 67 +/- 10 (n = 20), normal vs. Hyp, p less than .01), but unchanged at 13 weeks of age (77 +/- 13 (n = 13) vs. 70 +/- 15 (n = 15), NS). This observed difference in plasma 1,25(OH)2D between normal and Hyp mice at 4 weeks of age was sufficient to explain the observed normal-to-Hyp differences in intestinal absorption of 45Ca and duodenal and renal CaBP. It also explained 72 +/- 18% of the observed difference in 32P absorption. We conclude that Hyp mouse intestine is not resistant to 1,25(OH)2D and that the lower plasma 1,25(OH)2D of 4-week-old Hyp mice causes intestinal malabsorption of calcium and phosphate.

Impaired phosphorus conservation and 1,25 dihydroxyvitamin D generation during phosphorus deprivation in familial hypophosphatemic rickets

The pathogenesis of familial hypophosphatemic rickets (FHR) is incompletely understood. We therefore examined the effects of acute dietary phosphorus deprivation to see whether renal phosphate conservation and increased 1,25 dihydroxyvitamin D [1,25(OH)2D] plasma levels, which normally follow restriction of phosphorus intake, could be induced in patients with FHR. Six healthy male volunteers (age 26 +/- 3 yr) and seven male patients with FHR (age 24 +/- 3 yr) were placed on a low phosphorus diet supplemented with aluminum hydroxide and studied over a 4-d period. The patients with FHR excreted more than five times as much phosphorus per day at the conclusion of the study than did the controls (176 +/- 61 mg/24 h vs. 33 +/- 11 mg/h). In the normal subjects, maximum tubular reabsorptive capacity for phosphorus/glomerular filtration rate (TmP/GFR) rose progressively during phosphorus deprivation, and the rise from base line was more than two times greater than that seen in patients with FHR. Immunoreactive parathyroid hormone levels and nephrogenous cyclic AMP were initially normal in both groups and no change was seen in either group with phosphorus deprivation. In the normal subjects, 1,25(OH)2D levels rose progressively over the 96 h of the study (49 +/- 3 to 63 +/- 6 pg/ml, P less than 0.05), while mean circulating 1,25(OH)2D in the patients with FHR did not change (34 +/- 3 to 29 +/- 3 pg/ml). The changes in individual plasma 1,25(OH)2D levels correlated strongly with the change in individual nephrogenous cyclic AMP measurements in the patients with FHR (r = +0.93), while no such correlation was observed in the normal subjects. These data demonstrate a defective renal response to phosphorus deprivation in patients with FHR including a qualitatively abnormal response in 1,25(OH)2D generation.

Bone response to phosphate and vitamin D metabolites in the hypophosphatemic male mouse

The hypophosphatemic male mouse (Hyp/y), the proposed model for human vitamin D-resistant rickets (VDRR), is characterized by chronic hypophosphatemia, dwarfism, and rachitic and osteomalacic bone lesions. We have reported that treatment of Hyp/y mice with phosphate salts (Pi) heals rickets but does not correct the defective endosteal bone mineralization. In an attempt to cure osteomalacia, mutant male animals were treated with Pi combined with 25-hydroxyvitamin D3 (25OHD3, 1 microgram/kg/day), 24,25-dihydroxyvitamin D3 [24,25(OH)2D3, 0.5 microgram/kg/day], or 1,25-dihydroxyvitamin D3 [1,25(OH)2D3, 0.05--0.25 microgram/kg/day] infused constantly for 3 weeks. The biochemical and skeletal effects of treatment were assessed by analytical methods and bone histomorphometry. The results show that only 1,25(OH)2D3 produced a dose-dependent elevation of serum calcium and phosphorus, and greatly improved bone mineralization at doses high enough to increase serum calcium and phosphorus concentrations within or above the normal range. Better improvement of bone mineralization was obtained when Pi was combined to 1,25(OH)2D3. In conjunction with the correction of hypocalcemia, Pi + 1,25(OH)2D3 suppressed the stimulation of bone turnover induced by Pi supplementation. The results show that, as in VDRR children, 1,25(OH)2D3 produces beneficial effects on bone lesions in Hyp/y mice, mainly through enhancement of mineral availability. However, the persistence of osteomalacia despite correction of serum mineral concentrations suggests that there is a specific bone cell resistance to mineral and/or hormonal influences in Hyp/y mice.

Altered vitamin D, cyclic nucleotide and trace mineral metabolism in the X-linked hypophosphatemic mouse

Hyp mice have a defective control system for the synthesis of 1,25-(OH)2-vitamin D that does not respond to a low phosphate stimulus. While the plasma levels of 25-OH-vitamin D are reduced somewhat, this seems to be not a serious defect since plasma 1,25-(OH)2-vitamin D levels are normal. Hyp kidneys synthesize and excrete elevated levels of cyclic AMP and cyclic GMP. The elevated tissue levels of trace minerals suggests increased food intake with normal intestinal absorption of the minerals. In addition there is the reduced renal tubular reabsorption of phosphate caused by a change in the brush border transport of phosphate. This leads to hypophosphatemia, osteomalacic bone disease and altered Mg metabolism. There is intestinal resistance to 1,25-(OH)2-vitamin D stimulation. Many of these defects seem unrelated to the reduced renal tubular reabsorption of phosphate. Yet all are derived from one mutation in the Hyp gene. The underlying explanation must account for all abnormalities by a single mutation in a single gene product. The near normal phosphate levels in soft tissues and the multiple defects argue against a genetic change in the phosphate pump as an ultimate explanation. Still to be explored are possible changes in a phosphate recognition site on a cell membrane or a change in an as yet unknown phosphate regulating system.

1,25-Dihydroxyvitamin D3 increases the activity of the intestinal phosphatidylcholine deacylation-reacylation cycle

The activity of the intestinal phosphatidylcholine deacylation-reacylation cycle has been found to be stimulated by 1,25-dihydroxy-vitamin D3. The stimulation of this cycle thus provides a possible mechanism for the reported retailoring of the fatty acid composition of phosphatidylcholine in intestinal cell membranes by 1,25-dihydroxy-vitamin D3 and its analogue, 1alpha-hydroxyvitamin D3.

Serum 1,25-dihydroxyvitamin D levels in normal subjects and in patients with hereditary rickets or bone disease

The serum concentration of 1,25-dihydroxylvitamin D (1,25-[OH]2D) in normal children and in children with inherited diseases of bone was compared by use of a competitive binding assay. Observed values were: in 12 normal children and adolescents, 37.1 +/- 1.9 pg per milliliter (mean +/- S.D.); in 14 patients with X-linked hypophosphatemic rickets treated with vitamin D2 and phosphate supplements, 15.6 +/- 7.8 (P less than 0.01 versus control); in six patients with autosomal recessive vitamin D dependency treated with vitamin D2, 9.5 +/- 2.9 (P less than 0.01 versus control); and in four untreated patients with autosomal dominant hypophosphatemic (non-rachitic) bone disease, 30.2 +/- 6.3 (not significantly different from the controls). The difference in bone disease between X-linked hypophosphatemia (severe) and hypophosphatemic bone disease (mild) at comparable low serum levels of phosphate implies that 1,25-(OH)2D and phosphate may have independent roles in the pathogenesis of defective bone mineralization.