Exaggerated phosphaturic response to circulating parathyroid hormone in patients with familial X-linked hypophosphatemic rickets

Generation of hypophosphatemia in rats by continuous oral administration of cadmium

To obtain further information on the negative calcium balance caused by cadmium (Cd), the factors associated with serum calcium and phosphorus homeostasis other than inhibition of intestinal calcium absorption were studied by using parathyroid hormone (PTH) and 1 alpha-hydroxycholecalciferol (1 alpha-OH-D3). In rats exposed to Cd for 30 or 90 days, the concentrations of serum calcium after treatment with PTH, parathyroidectomy (PTX) or 1 alpha-OH-D3 showed almost the same patterns as those of control animals. It was considered that the mechanism of regulation of calcium in Cd-exposed rats was normal. The continuous oral administration of Cd generated hypophosphatemia in rats. On the other hand, in 30- and 90-day-treated rats, the low concentration of serum phosphorus caused by Cd was further decreased by administration of PTH, whereas it was increased by PTX. The hypophosphatemia found in rats exposed to Cd for 30 days, but not for 90 days, was reversed by treatment with 1 alpha-OH-D3. From these results, it was concluded that the hypophosphatemia caused by long term oral administration of Cd resulted from secondary hyperparathyroidism, due to inhibited calcium absorption from the intestine, as was demonstrated previously.

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.

Evidence for an intrinsic renal tubular defect in mice with genetic hypophosphatemic rickets

To investigate the role of parathyroid hormone (PTH) and(or) an intrinsic renal tubular reabsorptive defect for phosphate in mice with hereditary hypophosphatemic rickets, we performed clearance and micropuncture studies in hypophosphatemic mutants and nonaffected littermate controls. Increased fractional excretion of phosphate in mutants (47.2+/-4 vs. 30.8+/-2% in controls) was associated with reduced fractional and absolute reabsorption in the proximal convoluted tubule and more distal sites. Acute thyropara-thyroidectomy (TPTX) increased phosphate reabsorption in both mutants and controls with a fall in fractional phosphate excretion to congruent with7.5% in both groups indicating that PTH modified the degree of phosphaturia in the intact mutants. Absolute reabsorption in the proximal tubule and beyond remained reduced in the mutants, however, possibly because of the reduced filtered load. Serum PTH levels were the same in intact mutants and normals as was renal cortical adenylate cyclase activity both before and after PTH stimulation. To evaluate the possibility that the phosphate wasting was caused by an intrinsic tubular defect that was masked by TPTX, glomerular fluid phosphate concentration was raised by phosphate infusion in TPTX mutants to levels approaching those of control mice. Phosphate excretion rose markedly and fractional reabsorption fell, but there was no change in absolute phosphate reabsorption in either the proximal tubule or beyond, indicating a persistent reabsorptive defect in the absence of PTH. We conclude that hereditary hypophosphatemia in the mouse is associated with a renal tubular defect in phosphate reabsorption, which is independent of PTH and therefore represents a specific intrinsic abnormality of phosphate transport.