Renal adaptation to phosphate deprivation in the Hyp mouse with X-linked hypophosphatemia

The mechanism of renal adaptation to variation in dietary inorganic phosphate (Pi) was investigated in intact Hyp/+ (heterozygous) mice and +/+ (normal homozygous) female siblings. Hyp/+ mice were selected for expression of the X-linked Hyp allele, when fed the control diet (0.6% P), by evidence of persistent postweaning hypophosphatemia (1.78 ± 0.08 mM, mean ± SE, versus 2.68 ± 0.19 mM in +/+ siblings (p < 0.01)). Hyp/+ mice had an elevated fractional excretion index for Pi (FEIPi) (0.570 ± 0.024, mean ± SE) on this diet versus +/+ siblings (0.352 ± 0.025, p < 0.001). Renal cortex content of Pi (~46 nmol/mg protein) and net radioisotopic uptake of Pi by slices were similar in Hyp/+ and +/+ mice. Purified brush border membrane vesicles (BBMV) from Hyp/+ kidney cortex transported labelled Pi (100 μM) by a Na+-dependent mode at about one-half the rate (p < 0.001) observed in +/+ mice. Hyp/+ and +/+ mice fed a low P diet (0.03%) maintained their phenotypic differences in vivo and in vitro. Both adapted to chronic (> 2 week) Pi deprivation with a striking reduction (40- to 50-fold) of FEIPi (p < 0.001) and a fall in plasma [Pi] (p < 0.001). Neither the renal Pi content nor uptake of Pi by slices changed in deprived Hyp/+ and +/+ mice. On the other hand, BBMV uptake by Na+-dependent cotransport increased 200% (p < 0.001) during Pi deprivation in both the Hyp/+ and +/+ mouse. D-glucose transport did not increase. We conclude that renal adaptation to phosphate deprivation is achieved by modulation of a component of Na+-dependent cotransport in brush border membrane that is not controlled by the X-linked gene.

Rickets with alopecia: an inborn error of vitamin D metabolism

Rickets with alopecia, an inborn error of vitamin D metabolism, is described in two sisters. The rachitic disorder began during the first year of life and was refractory to 50,000 IU of vitamin D2/day. Surprisingly, both children had marked elevations in serum concentrations of 1,25-(OH)2D. Although the molecular basis for this disorder is not evident to date, intestinal end-organ unresponsiveness to exceedingly high levels of 1,25-(OH)2D was present, in addition to hyporesponsiveness of bone to these high levels of the hormone, since normocalcemia was maintained despite elevated serum levels of PTH. Therapy with oral 1,25-(OH)2D3 failed to reverse the disorder, but oral phosphorus supplements resulted in significant radiographic and clinical improvement.

Calcium, phosphate, and vitamin D

It is evident that there has been enormous progress within the last decade in our understanding of vitamin D metabolism and its interaction with calcium, phosphate, parathyroid hormone, and a number of other factors. As a result largely of therapeutic trials with various metabolites of vitamin D, we now know something of their probable involvement in the development of rickets and osteomalacia. We do not yet have a clear comprehension of exactly how these metabolites influence bone mineralization other than through their indirect effect on serum calcium and phosphate. Nonetheless it seems likely that at least one, and perhaps more, of the metabolites do exert a more direct effect. One must conclude that a deficiency of the 1alpha-hydroxylase is probably not the only defect in renal insufficiency, although it currently that there must well known. It is premature to go so far as to suggest that there must be another metabolite not being synthesized in the diseased or absent kidney.

[X-linked familial hypophosphatemic rickets report of six cases (author's transl)]

X-linked familial hypophosphatemic rickets (X.L.F.H.R.) is one of the D resistant rickets. The inheritance pattern is related to the X chromosome. Most constant feature is hypophosphatemia. Pathogenesis is still a subject of controversy. There are three main theories: a) An abnormal vitamin D metabolism. b) Secondary hyperparathyroidism developping as a result of the diminished calcium absorption by gut. c) A primary deffect of phosphate transport al various levels. Authors study and comment six cases of X.L.F.H.R., three of which belong to the same family. Clinical, radiological and higtological findings correspond to those of severe rickets. It is a chronic disease which affects children during growth period, giving rise to deforming bones invalidism. Treatment consists on continuous administration of oral phosphate and vitamin D.

Effect of oral glucose ingestion on renal phosphate reabsorption and clearance in vitamin D-resistant rickets

The effect or oral glucose ingestion on renal phosphate reabsorption was studied in 13 patients with the inherited form of vitamin D-resistant rickets (VDRR) and 5 normal subjects. In contrast to the normal subjects, glycosuria developed in six VDRR patients after glucose ingestion and resulted in a further 43% decrease in renal phosphate reabsorption. This was accompanied by a 33% increase in phosphate clearance. This was not attended by differences in fasting glucose or phosphorus levels between groups, or in their respective values 1 h after glucose ingestion. Baseline renal phosphate reabsorption was less and baseline phosphate clearance was greater in those VDRR subjects who developed glycosuria. The accumulated data suggest that excessive glucose ingestion by some patients with VDRR may add an additional insult to the phosphaturia characteristic of this disorder. This, in turn, would further compromise the response of circulating phosphate to therapeutic attempts at oral phosphate supplementation, thereby reducing the efficacy of oral phosphate therapy on skeletal growth and development in this disorder.

The defect in transcellular transport of phosphate in the nephron is located in brush-border membranes in X-linked hypophosphatemia (Hyp mouse model)

We purified renal cortex brush-border membranes from mutant hemizygous hypophosphatemic (Hyp/Y) mice and male control (+/Y) littermates. Tenfold purification of mutant and wild-type membranes was obtained. Phosphate enters +/Y brush-border membrane vesicles by a saturable Na+-dependent arsenate-inhibited component and also by a diffusional component observed in the presence of a potassium gradient. Phosphate is not bound or incorporated significantly by mouse brush-border membrane vesicles. Parallel studies with rat renal cortex brush-border membrane vesicles revealed that phosphate and D-glucose transport in rat and mouse vesicles are similar and have the characteristics reported by other workers. Brush-border membrane vesicles prepared from Hyp/Y renal cortex have significant (p less than 0.001) partial loss of phosphate transport on the Na+-dependent arsenate-inhibited component. D-Glucose transport is not affected. Our previous studies reveal that other components of transcellular phosphate flux in kidney are normal. Therefore, we conclude that the mutant gene product in the Hyp mouse is confined to the brush-border membrane. Stability of the X-chromosome in mammalian evolution implied that the same gene product is involved in the classic human disease, familial 'vitamin D 'resistant' X-linked hypophosphatemia.

Vitamin-D-dependent rickets type II. Resistance of target organs to 1,25-dihydroxyvitamin D

Studies were done to determine the cause for hypocalcemia, secondary hyperparathyroidism, osteomalacia and osteitis fibrosa cystica in a 22-year-old black woman. The patient had normal serum 25-hydroxyvitamin D (14 ng per milliliter) and markedly elevated serum 1,25-dihydroxyvitamin D (137 pg per milliliter). Vitamin D3, 4000 units per day for four weeks, increased the serum 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D to as high as 29 and 297 pg per milliliter, respectively, and corrected the hypocalcemia and secondary hyperparathyroidism. The results suggest that the disorder results from impaired end-organ response to 1,25-dihydroxyvitamin D. We propose that the entity be called vitamin-D-dependent rickets Type II.

Hypophosphatemic vitamin D-resistant rickets: metabolic balance studies in a child receiving 1,25 dihydroxyvitamin D3, phosphate, and ascorbic acid

A child with hypophosphatemic vitamin D-resistant rickets was treated for three years with the conventional vitamin D-inorganic phosphate supplementation followed by a new therapeutic regimen consisting of 1,25 dihydroxyvitamin D3 (1,25 (OH)2D3) and half of the previous phosphate supplementation. The effectiveness of the two treatment regimens was compared by calcium, phosphate, and magnesium balance techniques and by serial radiological examinations as well as careful height measurements. In addition, the lowering of the urinary pH with ascorbic acid supplementation seems to be associated with improvement in the renal tubular reabsorption of phosphate, but its distinct effect, separate from the rest of the treatment modalities, was not tested in this study. The conventional treatment did not correct the hypophosphatemia and alkaline phosphatase elevation, whereas the 1,25 (OH)2 D3-inorganic phosphate regimen is well tolerated and effective in achieving a sustained normalization of these variables. In addition, the improved growth and healing of rickets further attest to the efficacy of the new treatment.

Osteomalacia and disorders of vitamin D metabolism

A rapidly growing understanding of the biochemical and physiological processes that underlie the metabolism of vitamin D has provided new insights into the pathogenesis of oestomalacia. Many of the vitamin D--resistant osteomalacia syndromes can now be explained on the basis of defects in the metabolic conversion of vitamin D to the biologically active dihydroxylated metabolite 1,25(OH)2D and perhaps, in some instances, to impairement of the actions of 1,25(OH)2D on target tissues. The availability of this new information has made possible the synthesis of 1-hydroxylated forms of the vitamin for therapeutic use in states of vitamin D resistance. Although many questions regarding the pathogenesis and most effective approaches in the management of osteomalacia remain unanswered, considerable progress has been made in this direction as a result of continued research on the subject.

Disturbances in acid-base balance during hypophosphatemia and phosphate depletion

Phosphate depletion leads to bone resorption, resulting in the accession of alkali to the blood and urine. There may also be a release of lactic acid from liver, resulting from ATP depletion and intracellular alkalosis. In general, the net result of these tissue effects is an alkali load discharged into the extracellular fluid. In the kidneys, phosphate depletion causes impaired proximal tubular reabsorption of bicarbonate, and perhaps other substrates, owing in all likelihood to both intracellular alkalosis and depletion of ATP. This impaired proximal capacity to reclaim bicarbonate results in hyperchloremic acidosis. The distal nephron may contribute to acidosis by virtue of inability to produce titratable acid (phosphate-free urine) and perhaps impaired ammonia production. Severe phosphate depletion, therefore, tends to produce hyperchloremic acidosis as a result of a diminished capacity for bicarbonate reclamation in the proximal tubule and probably some impairment of bicarbonate regeneration in the distal nephron. These effects, when marked, ultimately outweigh the contribution of alkali coming from bone breakdown. In the liver, phosphate depletion may stimulate lactic acid production by reducing hepatic ATP stores (Pasteur effect) and producing intracellular alkalosis. ATP depletion may also limit hepatic uptake of lactate. Finally, in muscle cells ATP depletion, resulting from either profound hypophosphatemia or the metabolism of massive fructose loads, may eventuate in frank rhabdomyolysis.

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

Intestinal calcium and phosphate transport was studied in normal and hypophosphatemic mice fed a variety of dietary regimens with and without vitamin D. Regardless of dietary phosphorus levels, the genetic hypophosphatemic mice had drastically reduced levels of serum inorganic phosphate and intestinal phosphate transport while showing only slightly reduced serum calcium and intestinal calcium transport levels. The inclusion of vitamin D in the diet did not increase the low serum phosphorus levels and low rates of intestinal phosphate transport in the genetic hypophosphatemic mice, while it did increase serum calcium and intestinal calcium transport levels. The administration of 1,25-dihydroxyvitamin D3 to the hypophosphatemic mice stimulated intestinal calcium transport but had no effect on intestinal phosphate transport. In contrast, the 1,25-dihydroxyvitamin D3 stimulated both phosphate and calcium transport in the intestine of normal mice. The results obtained are consistent with the hypothesis that the primary metabolic disturbance in familial hypophosphatemia involves a defect in phosphate transport mechanisms.

A case of McCune-Albright syndrome with hyperthyroidism and vitamin D-resistant rickets

A girl aged 1 11/12 year with the unusual combination of McCune-Albright syndrome (polyostotic fibrous dysplasia, cutaneous pigmentation and precocious puberty), hyperthyroidism and vitamin D-resistant rickets is described. Urinary estrogens are increased, while serum LH and FSH responses to LH-RH are subnormal. Thyroid hormone levels in serum and 131I thyroidal uptake are increased, while TSH response to TRH is decreased. Serum phosphorus level is low and phosphate clearance high, while PTH is within normal range. Basal GH level and GH responses to various stimulations are high, but high basal GH is suppressed by oral glucose. The oral glucose tolerance test shows normal blood glucose with hypersecretion of insulin. These data suggest that the common pathogenesis of the various aspects of the syndrome is a hypersensitivity of the target organs, which include the pituitary, the thyroid, the gonads, the pancreatic islet cells and the proximal tubules of the kidney.

Effects of hypocalcemia and theophylline on glucose tolerance and insulin release in human beings

A study was carried out to investigate the role of serum calcium in the regulation of insulin secretion and glucose tolerance in response to an oral glucose load on human subjects in hypocalcemie and normocalcemic states. The subjects for this study comprised three groups. Group 1 consisted of 15 patients with hypoparathy-roidism. They were subjected to an oral glucose load, and blood samples were taken at 0, 30, 60, 90, 120, 150, and 180 minutes to determine the blood glucose and serum insulin levels. These patients were treated with vitamin D2; after normocalcemic states were achieved the same study was repeated. This group of patients showed an abnormal glucose tolerance test and a markedly reduced insulin release during their hypocalcemie state; these abnormalities returned to normal after treatment. Group 2 consisted of six patients with hypoparathyroidism and one patient with vitamin-D-resistant rickets. These patients were subjected to an oral glucose load and a theophylline infusion (1 gm. in three hours) when they were hypocalcemie. The theophylline infusion increased the insulin release to the level of control subjects' in spite of hypocalcemia. Ten healthy volunteers served as a control group (group 3).

Our results indicate that serum calcium has an important role in glucose utilization by insulin release.

This is the first experiment to be performed on human beings and shows that theophylline has a stimulatory effect on insulin secretion in the presence of hypocalcemia.