Reduced plasma concentration of 1,25-dihydroxyvitamin D in children with moderate renal insufficiency

Disordered FGF23 and mineral metabolism in children with CKD

BACKGROUND AND OBJECTIVES

In children with CKD, information is limited regarding the prevalence and determinants of fibroblast growth factor 23 excess and 1,25-dihyroxyvitamin D deficiency across the spectrum of predialysis CKD. This study characterized circulating concentrations of fibroblast growth factor 23 and 1,25-dihyroxyvitamin D, and investigated their interrelationships and associations with GFR and secondary hyperparathyroidism in children with CKD who were enrolled in the Chronic Kidney Disease in Children observational cohort study.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Plasma fibroblast growth factor 23 concentrations and determinants of mineral metabolism were measured in 464 children ages 1-16 years with predialysis CKD. GFR was measured by plasma disappearance of iohexol in 70% of participants and estimated by the Chronic Kidney Disease in Children estimating equation using serum creatinine and cystatin C concentrations in the remainder of the participants. Participants were grouped according to CKD stage and by 10-ml/min categories of GFR.

RESULTS

Median GFR for the cohort was 45 ml/min per 1.73 m(2) (interquartile range=33-57; range=15-109). Plasma fibroblast growth factor 23 concentration was above the normal range in 67% of participants (with higher levels observed among participants with lower GFR) before higher levels of serum parathyroid hormone and phosphorus were observed. Plasma fibroblast growth factor 23 levels were 34% higher in participants with glomerular disease than in participants with nonglomerular disease, despite similar GFR. Serum phosphorus levels, adjusted for age, were significantly lower at GFR of 60-69 ml/min per 1.73 m(2) than higher GFR, but thereafter they became higher in parallel with fibroblast growth factor 23 as GFR declined. Serum 1,25-dihyroxyvitamin D concentrations were lower in those participants with low GFR values, high fibroblast growth factor 23 levels, 25-hydroxyvitamin D deficiency, and proteinuria. Secondary hyperparathyroidism was present in 55% of participants with GFR<50 ml/min per 1.73 m(2).

CONCLUSION

In children with predialysis CKD, high plasma fibroblast growth factor 23 is the earliest detectable abnormality in mineral metabolism, and levels are highest in glomerular diseases.

Paricalcitol versus ergocalciferol for secondary hyperparathyroidism in CKD stages 3 and 4: a randomized controlled trial

BACKGROUND

The efficacy of 25-hydroxyvitamin D (25[OH]D) supplementation versus vitamin D receptor activators for the treatment of secondary hyperparathyroidism (SHPT) in patients with chronic kidney disease (CKD) stages 3 or 4 and vitamin D deficiency is unclear.

STUDY DESIGN

Randomized controlled trial.

SETTING & PARTICIPANTS

80 patients with CKD stages 3 or 4, 25(OH)D level <30 ng/mL, and SHPT in a single medical center.

INTERVENTION

Ergocalciferol, 50,000 units, titrated to achieve serum levels ≥30 ng/mL versus paricalcitol, 1 or 2 μg/d, for 16 weeks.

OUTCOMES

The occurrence of 2 consecutive parathyroid hormone (PTH) levels decreased by at least 30% from baseline. All analyses were intention to treat.

RESULTS

Baseline characteristics in the 2 groups were similar. 21 patients (53%) on paricalcitol and 7 patients (18%) on ergocalciferol treatment achieved the primary outcome measure (P = 0.002). After 16 weeks, PTH levels did not decrease significantly in patients receiving ergocalciferol, but were decreased significantly in those treated with paricalcitol (mean estimate of between-group difference over 16 weeks of therapy, 43.9 pg/mL; 95% CI, 11.2-76.6; P = 0.009). Serum 25(OH)D levels increased significantly after 16 weeks in only the ergocalciferol group, but not the paricalcitol group (mean estimate of between-group difference over 16 weeks of therapy, 7.08 ng/mL; 95% CI, 4.32-9.85; P < 0.001). Episodes of hyperphosphatemia and hypercalcemia were not significantly different between the 2 groups.

LIMITATIONS

Lack of blinding and use of surrogate end points.

CONCLUSIONS

Paricalcitol is more effective than ergocalciferol at decreasing PTH levels in patients with CKD stages 3 or 4 with vitamin D deficiency and SHPT.

Vitamin D deficiency and toxicity in chronic kidney disease: in search of the therapeutic window

Both vitamin D deficiency and vitamin D toxicity are associated with cardiovascular complications in chronic kidney disease (CKD). Clinical and experiment data indicate that the association of vitamin D levels with cardiovascular disease is best illustrated as a biphasic, or U-shaped, curve. Children and adolescents with CKD need vitamin D due to the demands of a growing skeleton, to prevent renal rickets. However, this therapy carries the risk of severe side effects and chronic toxicity. Observational studies show that vitamin D deficiency and toxicity are frequently present in patients with CKD. In view of the importance of cardiovascular complications for the long-term survival of young patients, these findings demand a judicious use of vitamin D preparations. In clinical practice, the therapeutic window is rather small, presenting a therapeutic challenge to avoid both vitamin D deficiency and toxicity.

Obesity is associated with increased parathyroid hormone levels independent of glomerular filtration rate in chronic kidney disease

The objective of the study was to examine the relationship of obesity and parathyroid hormone (PTH) levels among persons with chronic kidney disease (CKD). This was a cross-sectional analysis of 4551 participants in the National Kidney Foundation-Kidney Early Evaluation Program found to have CKD (estimated glomerular filtration rate <60 mL/[min 1.73 m(2)]) examining the relationship of body mass index (BMI) and PTH levels. In unadjusted analysis, PTH levels increased with increasing BMI quartiles. After adjustment for age, race, sex, diabetes, calcium, phosphorus, estimated glomerular filtration rate, and presence of microalbuminuria, PTH levels were 7.3% (P = .008), 11.9% (P < .0001), and 18.1% (P < .0001) higher in the second, third, and fourth BMI quartiles, respectively, as compared with the first quartile. In a companion analysis, higher BMI was associated with increased odds of having an elevated PTH measurement (>70 pg/mL). Compared with the first quartile, odds ratios for elevated PTH were 1.26 (95% confidence interval, 1.06-1.50; P = .01), 1.38 (1.15-1.65, P = .0005), and 1.66 (1.37-2.00, P < .0001) for the second, third, and fourth quartiles, respectively. We found no effect modification by race, diabetes, or presence of microalbuminuria. Therefore, in a large community-dwelling population with CKD, the presence of obesity and of increasing BMI is associated with higher PTH levels independent of measured confounders and may be an additional target in the management of secondary hyperparathyroidism in CKD.

Bone and mineral disorders in pre-dialysis CKD

Disorders in calcium, phosphorus, and parathyroid hormone (PTH) are common in chronic kidney disease (CKD) and may be associated with poor outcomes including a higher rate of CKD progression and increased death risk. Although these abnormalities have been examined extensively in patients with CKD stage 5 who are receiving chronic maintenance dialysis, they have not been studied to the same extent at earlier stages of CKD, in spite of the much larger numbers of patients in the early CKD population. We summarize the available literature on outcomes associated with bone and mineral disorders in patients with CKD not yet receiving maintenance dialysis. We have reviewed novel data linking fibroblast growth factor 23 (FGF-23) to phosphorus and vitamin D homeostasis. More rapid CKD progression is linked to hyperphosphatemia and its associated hyperparathyroidism and vitamin D deficiency. Hence, hyperphosphatemia may play a central role in the diverse disorders characterizing CKD. We provide a brief overview of the available treatment recommendations for bone and mineral disorders, with an emphasis on areas needing further research.

Mineral metabolism and bone abnormalities in children with chronic renal failure

Abnormalities in mineral metabolism and changes in skeletal histology may contribute to growth impairment in children with chronic renal failure. Hyperphosphatemia, hypocalcemia, metabolic acidosis, alterations in vitamin D and IGF synthesis and parathyroid gland dysfunction play significant roles in the development of secondary hyperparathyroidism and subsequently, bone disease in renal failure. The recent KDIGO conference has made recommendations to consider this as a systemic disorder (chronic kidney disease-mineral bone disorder) and to standardize bone histomorphometry to include bone turnover, mineralization and volume (TMV). The use of DXA to assess bone mass is controversial in children with chronic renal failure. Questions arise regarding the accuracy of bone measurements and difficulty in data interpretation especially in children with renal failure who are not only growth retarded but often have pubertal delay and osteosclerosis. The validity and feasibility of new modalities of skeletal imaging which can detect changes in both trabecular and cortical bone are currently being investigated in children. The management of mineral abnormalities and bone disease in chronic renal failure is multifactorial. To manage hyperphosphatemia, dietary phosphate restriction accompanied by intake of calcium-free and metal-free phosphate binding agents are widely utilized. Vitamin D analogs remain the primary therapy for secondary hyperparathyroidism, although the use of the less hypercalcemic agents is preferred due to concerns of calciphylaxis and vascular calcification. Future clinical studies are needed to evaluate the long-term effects of calcimimetic agents and bisphosphonate therapy in children with chronic renal failure.

Vitamin D receptor activation and survival in chronic kidney disease

Replacement of activated vitamin D has been the cornerstone of therapy for secondary hyperparathyroidism (SHPT). Recent findings from several large observational studies have suggested that the benefits of vitamin D receptor activators (VDRA) may extend beyond the traditional parathyroid hormone (PTH)-lowering effect, and could result in direct cardiovascular and metabolic benefits. The advent of several new analogs of the activated vitamin D molecule has widened our therapeutic armamentarium, but has also made therapeutic decisions more complicated. Treatment of SHPT has become even more complex with the arrival of the first calcium-sensing receptor (CSR) agonist (cinacalcet hydrochloride) and with the uncovering of novel mechanisms responsible for SHPT. We provide a brief overview of the physiology and pathophysiology of SHPT, with a focus on vitamin D metabolism, and discuss various practical aspects of VDRA therapy and its reported association with survival in recent observational studies. A detailed discussion of the available agents is aimed at providing the practicing physician with a clear understanding of the advantages or disadvantages of the individual medications. A number of open questions are also analyzed, including the present and future roles of CSR agonists and 25(OH) vitamin D replacement.

Obesity is associated with secondary hyperparathyroidism in men with moderate and severe chronic kidney disease

BACKGROUND AND OBJECTIVES

Obesity is associated with secondary hyperparathyroidism in the general population. The objective of this study is to explore whether the same association is present in patients with chronic kidney disease.

DESIGN, SETTING, PARTICIPANTS & MEASUREMENTS

Linear regression models were used to examine the association between intact parathyroid hormone level and body mass index in 496 male US veterans (age 69.4 +/- 10.2 yr, 22.8% black) who had chronic kidney disease stages 2 to 5 and were not yet on dialysis (estimated GFR 31.8 +/- 11.2 ml/min per 1.73 m2).

RESULTS

Higher intact parathyroid hormone was associated with higher body mass index after adjustment for age, race, diabetes, and serum calcium and phosphorus levels. This association was independent of age, race, diabetes status, and serum calcium and phosphorus but was limited to patient groups with lower albumin (P = 0.005 for the interaction term) or higher white blood cell count (P = 0.026 for the interaction term).

CONCLUSIONS

Higher body mass index is associated with secondary hyperparathyroidism in patients who have chronic kidney disease and are not yet on dialysis, especially in patients with evidence of malnutrition and inflammation. Confirmation of these findings in other patient groups with chronic kidney disease and better characterization of the underlying mechanisms of action will be necessary before advocating weight loss as a means to treat secondary hyperparathyroidism in chronic kidney disease.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Secondary hyperparathyroidism in children with chronic renal failure: pathogenesis and treatment

Despite advances in the management of patients with chronic renal failure, histologic features associated with secondary hyperparathyroidism remain the predominant skeletal findings; however, over the last decade the prevalence of adynamic bone has increased in both adult and pediatric patients with chronic renal failure. The management of children with secondary hyperparathyroidism and mild to moderate chronic renal failure should be started early, and should include correction of hypocalcemia and metabolic acidosis, maintenance of age-appropriate serum phosphorus levels, and institution of vitamin D therapy when serum intact parathyroid hormone (PTH) measurements are elevated to maintain the blood levels within normal limits; however, in children undergoing chronic dialysis therapy, the current recommendation is to maintain the serum intact PTH levels at least 2-4 times the upper limits of normal to prevent the development of low bone turnover disease. Serum calcium, phosphorus, alkaline phosphatase, and PTH levels should be monitored frequently, especially in infants and very young children. Discontinuation or reduction of vitamin D should be considered when there is a rapid decline in PTH levels, persistent elevation in serum calcium and serum phosphorus levels, and a significant diminution in alkaline phosphatase levels. In addition, a reduction in the calcium concentration of the dialysis fluid, and judicious use of calcium-containing salts as phosphate binding agents should also be performed in these patients. Although not yet extensively used in pediatric patients with secondary hyperparathyroidism, several therapeutic alternatives, such as the less calcemic vitamin D analogs, including paricalcitol [19-nor-1,25-(OH)(2)D(2)] and doxercalciferol [1-alpha-(OH)(2)D(2)], calcimimetics, and the availability of a calcium-free, aluminum-free phosphate binder such as sevelamer hydrochloride and lanthanum carbonate, may play significant roles in the future management of children with secondary hyperparathyroidism to promote linear growth, prevent parathyroid gland hyperplasia, avoid calciphylaxis and, in the long run, avert vascular calcifications.

Bone disease in long-term adult kidney transplant patients with normal renal function

BACKGROUND

In successful renal transplantation, the degree of renal function recovery is usually incomplete and information is scarce about the abnormalities of mineral metabolism in long-term adult renal recipients with normal renal function. This study was designed to investigate bone mineral metabolism in patients with a long-term normal functioning kidney.

METHODS

Twenty-nine adult asymptomatic renal transplant (RT) recipients with stable graft function for more than 10 years and serum creatinine <2 mg/dL were studied. They were classified into two groups according to glomerular filtration rate: Group A (N = 12; nine men, three women)>70 mL/min (x: 126 +/- 55 mL/min) and Group B (N = 17; nine men, eight women) <70 mL/min (x: 56 +/- 11 mL/min). Circulating biochemical markers of bone remodelling, bone histomorphometry, and densitometry (lumbar spine and hip) were obtained to investigate bone disease in these patients.

RESULTS

Serum PTH was slightly elevated in 10 patients (83%) in group A. Serum PTH levels were positively related to serum calcium, osteocalcin, BAP, telopeptide, OH-proline, and creatinine. There was no histologic data to support overactivity on bone in this group of patients, with only one showing high bone turnover. Mineralization was prolonged in 34% of patients. Twenty-two patients (75%) exhibited normal bone turnover. In the group with GFR>70 mL/min the prevalence of mineralization defect in the presence of normal serum levels of calcitriol suggested vitamin D resistance. Lumbar and femoral neck osteoporosis was present in 25% and 33% of patients in group A, and 23% and 53% in group B, respectively. T-score at lumbar spine was negatively correlated with months since transplantation. Patients under treatment with cyclosporine (CsA) showed increased concentrations of osteocalcin and D-pyr and higher lumbar bone mineral density (BMD), but bone histomorphometry was not influenced by CsA.

CONCLUSION

Patients with long-term renal transplantation with normal renal function frequently present with slight increases in PTH, but without an effect on bone histology. CsA did not induce changes in bone histology and delayed mineralization was frequently observed.

The severity of secondary hyperparathyroidism in chronic renal insufficiency is GFR-dependent, race-dependent, and associated with cardiovascular disease

Secondary hyperparathyroidism (SHPT) is an important complication of end-stage renal disease. However, SHPT begins during earlier stages of chronic renal insufficiency (CRI), and little is known about risk factors for SHPT in this population. This study evaluated 218 patients in an ethnically diverse ambulatory nephrology practice at the University of California San Francisco during calendar years 1999 and 2000. Demographic data, comorbid diseases, medications, and laboratory parameters were collected, and independent correlates of intact parathyroid hormone (PTH) were identified by using multiple linear regression. The mean estimated GFR was 34 ml/min per 1.73 m(2) (10%-90% range, 13 to 61 ml/min per 1.73 m(2)); PTH was inversely related to GFR (P < 0.0001). The adjusted mean PTH was higher among African Americans and lower among Asian/Pacific Islanders compared with white patients (233 versus 95 versus 139 pg/ml; P < 0.0001). Moreover, among the 196 patients with GFR <60 ml/min per 1.73 m(2), the slope of GFR versus PTH was significantly steeper among African Americans than among white patients (10.6 versus 3.9 pg/ml per ml per min per 1.73 m(2); P = 0.01). After adjusting for age and diabetes, PTH was associated with a history of myocardial infarction (OR, 1.6; 95% CI, 1.1 to 2.3 per unit natural log PTH) and congestive heart failure (OR, 2.0; 95% CI, 1.3 to 2.9 per unit natural log PTH) and not associated with other co-morbid conditions. These factors should be considered when screening and managing SHPT in CRI.

Different effects of 22-oxacalcitriol and calcitriol on the course of experimental chronic renal failure

Calcitriol, 1,25(OH)(2)D(3), has been reported to have a beneficial effect on bone histology in patients with predialysis chronic renal failure; however, such treatment involves a risk of hypercalcemia. To investigate the effects of 1,25(OH)(2)D(3) and 22-oxacalcitriol (OCT) on the progression of histologic deterioration, we administered intraperitoneal 1,25(OH)(2)D(3) or OCT, three times a week, to rats with adriamycin-induced progressive renal failure, from the 10th week after the induction of ADR-induced nephropathy. The rats were divided into the following groups: (1) high-dose 1,25(OH)(2)D(3), 0.2 microg/kg (group D(3)-0.2); (2) low-dose 1,25(OH)(2)D(3), 0.04 microg/kg (group D(3)-0.04); (3) high-dose OCT, 0.2 microg/kg (group OCT-0.2); (4) low-dose OCT, 0.04 microg/kg (group OCT-0.04); and (5) ADR-induced nephropathy (group ADR). The death rate at week 20 in group D(3)-0.2 was 50%, significantly higher than the death rates in the other groups, except for group D(3)-0.04 (P <.05). The serum creatinine and blood urea nitrogen levels were the highest in group D(3)-0.2, although the difference was not significant. In contrast, in groups OCT-0.2 and OCT-0.04, tubular changes and interstitial volume were smaller than in groups D(3)-0.2 and D(3)-0.04 (P <.05). Although calcium deposits increased in group D(3)-0.2, the difference was not significant. Glomerular expression of transforming growth factor-beta1 (TGF-beta1) expression was less in groups OCT-0.2 and OCT-0.04 than in groups D(3)-0.2 and D(3)-0.04 (P <.05). Glomerular fibronectin expression was less in group OCT-0.2 than in groups D(3)-0.2 and ADR (P <.05). Tubulointerstitial expression of TGF-beta1 was greater in group D(3)-0.2 than in group ADR and greater in group D(3)-0.04 than in group OCT-0.04 (P <.05). We conclude that a high dose of 1,25(OH)(2)D(3) accelerated the progressive renal deterioration of ADR-induced nephropathy, and, as a result, OCT was able to attenuate renal histologic lesions.

Prevention of renal osteodystrophy in predialysis patients

Impaired calcitriol synthesis is one of the major factors contributing to the development of secondary hyperparathyroidism in patients with chronic renal failure. Vitamin D therapy, particularly 1alpha-hydroxyvitamin D3, even in low doses, has been shown to be effective in the treatment of secondary hyperparathyroidism in patients with mild-to-moderate chronic renal failure. Complications associated with calcitriol and alfacalcidol therapy, which include hypercalcemia and progressive deterioration of renal function, have been reported in some patients. The majority of the studies reviewed, however, demonstrated that daily calcitriol and alfacalcidol doses below 0.25 microg are rarely associated with hypercalcemia, hyperphosphatemia, or progressive decline in renal function. In addition, these complications usually resolve with the reduction in dose or discontinuation of the medication. Thus, vitamin D therapy may be valuable in the treatment of patients with mild-to-moderate chronic renal failure who may be at high risk of developing secondary hyperparathyroidism.

Constitutive expression of 25-hydroxyvitamin D3-1alpha-hydroxylase in a transformed human proximal tubule cell line: evidence for direct regulation of vitamin D metabolism by calcium

Circulating levels of the active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) are dependent on activity of the renal mitochondrial cytochrome P450 enzyme, 25-hydroxyvitamin D3-1alpha-hydroxylase (1alpha-hydroxylase). Production of 1,25-(OH)2D3 occurs predominantly in the renal proximal tubule, with 1alpha-hydroxylase activity being impaired in renal insufficiency and renal disease. The expression and activity of 1alpha-hydroxylase are tightly regulated in response to serum levels of PTH, calcium, phosphate, and 1,25-(OH)2D3 itself. As a consequence of this, the characterization of 1alpha-hydroxylase in human renal tissue has proved difficult. In this study we have characterized constitutive 1alpha-hydroxylase expression in a simian virus 40-transformed human proximal tubule cell line, HKC-8. Initial analyses of [3H]25-hydroxyvitamin D3 (25OHD3) metabolism in these cells using straight and reverse phase HPLC revealed product peaks that coincided with authentic 1,25-(OH)2D3 as well as 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3). Enzyme kinetic studies indicated that the Km for synthesis of 1,25-(OH)2D3 in HKC-8 cells was 120 nmol/liter 25OHD3, with a maximum velocity of 21 pmol/h/mg protein. This activity was inhibited by treatment with ketoconazole, but not diphenyl phenylenediamine. RT-PCR analysis of RNA from HKC-8 cells revealed a transcript similar in size to that observed in keratinocytes and primary cultures of human proximal tubule cells, and protein was detected by Western blot analysis. Synthesis of 1,25-(OH)2D3 was up regulated by treatment with forskolin (10 micromol/liter, 24 h) and was down-regulated by 1,25-(OH)2D3 (10 nmol/liter, 24 h). 1Alpha-hydroxylase activity in HKC-8 cells was also sensitive to the concentration of calcium. Cells grown in low calcium (0.5 mmol/liter) showed a 4.8-fold induction of 1alpha-hydroxylase, whereas treatment with medium containing high levels of calcium (2 mmol/liter) significantly inhibited 1,25-(OH)2D3 production. These data suggest that direct effects of calcium on proximal tubule cells may be an important feature of the regulation of renal 1,25-(OH)2D3 production.

The importance of dietary calcium and phosphorous in the secondary hyperparathyroidism of patients with early renal failure

Secondary hyperparathyroidism (HPT) was evaluated in 157 patients with chronic renal failure (CRF). It was noted that HPT developed early in CRF at a time when plasma calcium and phosphorous were within normal limits. As creatinine clearance decreased below 80 mL/m, there was a significant decrease in plasma calcitriol and a slow and progressive significant increment in plasma parathyroid hormone (PTH). The effect of dietary intake of calcium and phosphorous was evaluated in these patients with early renal failure (ERF). They were divided into two groups. Group A was placed on a protein- and phosphorous-restricted diet (10 days) followed by a daily phosphorous-load diet (10 days). Group B had similar sequential diets plus a calcium supplement throughout the study. Dietary protein and phosphorous restriction resulted in an amelioration of the HPT only in the group of patients receiving calcium supplementation. The phosphate-load diet resulted in worsening of HPT in both groups. In summary, a deficit of calcitriol occurs early in CRF, which in turn leads to a significant increase in PTH. Phosphorous restriction, together with calcium supplementation, ameliorated the HPT of patients with ERF.

Levels of serum 1,25(OH)2D in patients with pre-dialysis chronic renal failure

One hundred and ninety-five outpatients in pre-dialysis period served as subjects. The mean age of subjects was 58.0 +/- 11.2 (range: 29-82) years. The subjects were divided into 8 groups according to their serum creatinine (Cr) levels (Cr < or = 1.0, 1.0 < Cr < or = 2.0, 2.0 < Cr < or = 3.0, 3.0 < Cr < or = 4.0, 4.0 < Cr < or = 5.0, 5.0 < Cr < or = 6.0, 6.0 < Cr < or = 8.0, and Cr > 8.0 mg/100 ml). The level of 1,25-dihydroxyvitamin D (1,25(OH)2D) decreased in accordance with the progression of chronic renal failure (CRF). Even in subjects with 1.0 < Cr < or = 2.0 mg/100 ml, the levels of 1,25(OH)2D were significantly lower than those in subjects with Cr < or = 1.0 mg/100 ml. The levels of calcium adjusted by serum albumin levels (adjusted Ca) were relatively maintained within the normal range until Cr > 8.0 mg/100 ml. The levels of inorganic phosphate (IP) were significantly lower in subjects with 1.0 < Cr < or = 2.0 mg/100 ml, but significantly higher in subjects with Cr > 4.0 mg/100 ml than those in subjects with Cr < or = 1.0 mg/100 ml. The levels of immunoreactive high-sensitive parathyroid hormone (HS-PTH) were greatly increased and the levels of intact PTH were significantly increased even in subjects with 1.0 < Cr < or = 2.0 mg/100 ml, in association with a decrease in levels of 1,25(OH)2D suggesting that a decline in 1,25(OH)2D production due to a decrease in renal mass contributes to the acceleration of secondary hyperparathyroidism. When the levels of adjusted Ca, intact PTH and 1,25(OH)2D of subjects with hypophosphatemia (IP < 2.8 mg/100 ml), normophosphatemia and hyperphosphatemia (IP > 4.4 mg/100 ml) were compared, there were not any significant differences in the levels of adjusted Ca among these subjects in each group. But, the levels of 1,25(OH)2D in subjects with hypophosphatemia were significantly higher than those with normophosphatemia in groups with Cr < or = 2.0 mg/100 ml, and those with hyperphosphatemia were significantly lower than those with normophosphatemia in groups with 3.0 < Cr < or = 4.0, 5.0 < Cr < or = 6.0 and Cr > 8.0 mg/100 ml. These results suggest that increased secretion of PTH might compensate the decreased production of 1,25(OH)2D3 by lowering phosphate in the early phase of CRF, and phosphate retention inhibits the activity of 1 alpha-hydroxylase and contributes to the decrease in 1,25(OH)2D3 in the advanced stage of CRF. Monitoring of 1,25(OH)2D is considered to be vitally important for diagnosing the 1,25(OH)2D3 deficiency in CRF.

The renal bone diseases in children treated with dialysis

Renal osteodystrophy represents a spectrum from high- to low-turnover bone lesions. The specific pattern, however, may change during selected therapeutic interventions. As in the past, osteitis fibrosa remains the most frequent histologic lesion in pediatric patients on dialysis, although recently the prevalence of low-turnover bone lesions without aluminum toxicity has been increasing in the pediatric population. This may be a consequence of aggressive calcitriol and calcium therapy. The different factors involved in the development of secondary hyperparathyroidism include hyperphosphatemia, hypocalcemia, altered vitamin D synthesis, impairments in parathyroid hormone (PTH) secretion and metabolism, and, recently, possible downregulation of renal PTH/PTH-rP messenger RNA receptor. New developments in molecular biology have demonstrated the relationship between vitamin D and PTH. The use of high-dose pulse intravenous, intraperitoneal, and oral calcitriol therapy has significantly decreased serum PTH levels and retarded the progression of osteitis fibrosa. These therapeutic interventions, however, may have led to the development of adynamic bone lesions. The impact of adynamic bone lesions in the young and growing skeleton remains to be determined.

Bone and mineral metabolism in childhood end-stage renal disease

Renal osteodystrophy represents a spectrum from high to low turn-over lesions of bone, and the specific pattern may change during the evolution of chronic renal failure and as a consequence of specific therapeutic interventions. Although secondary hyperparathyroidism remains the predominant histologic lesion in patients undergoing maintenance dialysis, recent evidence indicates higher frequency of adynamic lesion not associated with aluminum intoxication. The different factors involved in the development of each of the histologic subtypes have been described together with the clinical manifestations of renal bone disease in childhood. Avoidance of aluminum-containing medications and the intermittent administration of calcitriol are effective approaches for the management of the renal bone diseases. The long-term consequences of the adynamic lesion remain to be determined.

Indications for parathyroidectomy and extent of treatment for patients with secondary hyperparathyroidism

Some manifestations of secondary hyperparathyroidism affect most if not all patients with chronic renal failure and can affect many different organ systems. Proper medical treatment is essential and should be attempted before considering surgical intervention. The symptoms that most often resolve after parathyroidectomy include bone pain and intractable pruritus. Other useful indications for operation include a marked elevation of the parathyroid hormone level and the elevation of the calcium x phosphate product over 70. Both subtotal parathyroidectomy and total parathyroidectomy with autotransplantation have been advocated as the best operative approach. Each of these procedures has its own advantages and disadvantages which should be considered for each individual case. Localizing procedures should be reserved for patients with persistent or recurrent hyperparathyroidism, as diffuse parathyroid hyperplasia is the most common operative finding in secondary hyperparathyroidism.

Secondary hyperparathyroidism in renal failure: the trade-off hypothesis revisited

Our understanding of the mechanism responsible for secondary hyperparathyroidism (HPTH) has advanced significantly since the "trade-off" hypothesis was formulated. It appears that in early renal failure a deficit of calcitriol synthesis is an important factor. However, additional factors, such as a defect of the vitamin D receptor or the newly cloned calcium sensor receptor (BoPCaR1), may be present in the parathyroid cells. As renal failure progresses, the lack of calcitriol becomes more pronounced, inducing HPTH. With advanced chronic renal failure, hyperphosphatemia is an additional important factor in worsening HPTH. In addition, resistance of the parathyroids to calcitriol due to a reduced density of calcitriol receptors also may contribute to HPTH. Finally, uremia per se not only may cause a receptor abnormality in the parathyroid but at the level of the bone it may aggravate the impaired calcemic response to PTH. In conclusion, after reviewing the "trade-off" hypothesis, although some of the original concepts may have been simplistic, most of the factors postulated 30 years ago are still operative in the pathogenesis of secondary HPTH in renal failure.

A prospective, double-blind study of growth failure in children with chronic renal insufficiency and the effectiveness of treatment with calcitriol versus dihydrotachysterol. The Growth Failure in Children with Renal Diseases Investigators

Because controlled trials in adults have shown accelerated deterioration of renal function in a small number of patients receiving calcitriol for renal osteodystrophy, we initiated a prospective, randomized, double-blind study of the use of calcitriol versus dihydrotachysterol in children with chronic renal insufficiency. We studied children aged 1 1/2 through 10 years, with a calculated glomerular filtration rate between 20 and 75 ml/min per 1.73 m2, and with elevated serum parathyroid hormone concentrations. Ninety-four patients completed a mean of 8.0 months of control observations and were randomly assigned to a treatment period; 82 completed the treatment period of at least 6 months while receiving a calcitriol dosage (mean +/- SD) of 17.1 +/- 5.9 ng/kg per day or a dihydrotachysterol dosage of 13.8 +/- 3.3 micrograms/kg per day. With treatment the height z scores for both calcitriol- and dihydrotachysterol-treated groups showed no differences between the two groups. In relation to cumulative dose, there was a significant decrease in glomerular filtration rate for both calcitriol and dihydrotachysterol; for calcitriol the rate of decline was significantly steeper (p = 0.0026). The treatment groups did not differ significantly with respect to the incidence of hypercalcemia (serum calcium concentration > 2.7 mmol/L (> 11 mg/dl)). We conclude that careful follow-up of renal function is mandatory during the use of either calcitriol or dihydrotachysterol because both agents were associated with significant declines in renal function. There was no significant difference between calcitriol and dihydrotachysterol in promoting linear growth or causing hypercalcemia in children with chronic renal insufficiency. Dihydrotachysterol, the less costly agent, can be used with equal efficacy.

Phosphate homeostasis, 1,25-dihydroxyvitamm D(3), and hyperparathyroidism in early chronic renal failure

In early chronic renal failure, low plasma levels of calcitriol (1,25[OH](2)D(3)) do not seem to be merely the consequence of a reduced mass of functional nephrons. Indeed, this alteration can be considered as a compensatory mechanism, as analyzed according to a new concept of inorganic phosphate (P(i)) homeostasis that integrates both 1,25(OH)(2)D(3) production and renal P(i) reabsorption as essential regulating elements. Accordingly, the observed reduction in the renal production of 1,25(OH)(2)D(3) that occurs concomitantly with a decrease in tubular P(i) reabsorptive capacity (TmP(i)/GFR) may well represent a secondary adaptive response to a primary alteration in P(i) homeostasis. This crucial alteration in P(i) homeostasis would consist of an overload of a putative regulated intracellular P(i) pool, the localization of which remains to be determined. The observed hypophosphatemia, hypocalcemia, and PTH hypersecretion would represent alterations secondary to a low TmP(i)/GFR and to reduced 1,25(OH)(2)D(3) production. According to this pathophysiologic sequence, 1,25(OH)(2)D(3), rather than PTH as proposed in a former theory, would be "traded off" to preserve P(i) homeostasis in early chronic renal failure. Both theories predict that dietary P(i) restriction represents a logical preventive therapy at least until the nature of the primary defect in P(i) homeostasis is understood. However, assuming that low 1,25(OH)(2)D(3) levels in early chronic renal failure represent a compensatory phenomenon, this new theory suggests that calcitriol should be only administered at a later stage of the disease, when the production of 1,25(OH)(2)D(3) becomes inappropriately low to maintain mineral homeostasis.

Clinical performance of parathyroid hormone immunometric assays

Three immunometric assays of parathyroid hormone (PTH)--a commercial immunoradiometric assay, an in-house immunoradiometric assay, and an immunochemiluminometric assay--were evaluated in 50 patients with surgically proven primary hyperparathyroidism. Of these patients, 43 had increased values with the commercial assay (sensitivity, 86%), whereas 45 patients had increased concentrations with both the in-house immunoradiometric and the in-house immunochemiluminometric assays (sensitivities, 90%). Because of the results of this comparison study, we confidently chose the immunochemiluminometric assay as our routine assay; this assay was evaluated retrospectively in 361 patients with surgically proven primary hyperparathyroidism. In 45 patients, PTH values were below the upper limit of normal (sensitivity, 88%). The results indicate that the sensitivities of current immunometric assays are approximately 90%. Twenty patients who had hypercalcemia associated with malignant involvement were assessed with the immunochemiluminometric assay. Of these 20 patients, 19 had subnormal PTH values, and 1 had a value within the normal range. In contrast, in the past, PTH values determined with radioimmunoassays have often been in the normal range for such patients. Thus, an immunometric PTH assay is superior to a radioimmunoassay in the differential diagnosis of hypercalcemia associated with malignant disease.

Calcemic response to parathyroid hormone in renal failure: role of phosphorus and its effect on calcitriol

The calcemic response to parathyroid hormone (PTH) is decreased in renal failure. The reduction of hyperphosphatemia improves the calcemic response to PTH in animals with advanced renal failure. However, since low calcitriol levels in renal failure may also contribute to the decreased calcemic response to PTH, the improved calcemic response observed during the reduction of serum phosphorus may be partially mediated by an increase in serum calcitriol levels. The present study evaluated the calcemic response to PTH in rats with moderate and advanced renal failure and how this response was modified by a high and a low phosphorus diet. In addition, the effect of a change in dietary phosphorus on calcitriol levels was also evaluated. A 48-hour continuous infusion of 1-34 rat PTH increased the serum calcium level to 18.2 +/- 0.4 mg/dl in normal rats, versus 13.7 +/- 0.9 and 12.1 +/- 0.2 mg/dl in rats with moderate and advanced renal failure, respectively. During the PTH infusion, a high phosphorus diet increased the serum phosphorus and resulted in a reduced calcemic response to PTH at each level of renal function; respective serum calcium levels were 13.8 +/- 0.6 mg/dl in normals, 11.2 +/- 0.2 mg/dl in moderate renal failure and 9.6 +/- 0.5 mg/dl in advanced renal failure. In normal rats and in rats with moderate renal failure, dietary phosphorus restriction during the PTH infusion increased serum calcitriol levels. In rats with advanced renal failure, serum calcitriol levels were lower than in the other two groups and were not affected by changes in dietary phosphorus.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of vitamin D on growth cartilage cell proliferation in vitro

Disturbed calcification of the growth plate and stunting is a frequent finding in vitamin D-deficiency rickets, vitamin D-dependency rickets and renal osteodystrophy, illustrating that chondrocytes are a target for vitamin D. This observation prompted an investigation of Ic, 25-dihydroxy vitamin D3 [1,25(OH)2D3] receptor expression and action of vitamin D metabolites on chondrocyte proliferation. In tibial growth plates and in primary cultures of tibial growth cartilage of male Sprague-Dawley rats (80 g) specific binding of [3H]-1,25(OH)2D3 was noted. Scatchard analysis revealed the presence of a single class of non-interacting binding sites. Kd was 10(-11) M irrespective of growth phase. The binding macromolecule had a sedimentation coefficient of 3.5 S. Interaction with DNA was demonstrated by DNA-cellulose affinity chromatography. By immunohistology, growth cartilage cells (rabbit tibia) were shown to express nuclear 1,25(OH)2D3 receptors, most prominently in the proliferative and early hypertrophic zone. This corresponds to binding data which showed highest binding of 1,25(OH)2D3 in the logarithmic growth phase (12,780 molecules/cell versus 4,538 molecules/cell in confluent cells) in primary cultures of growth plate chondrocytes. In the presence of delipidated fetal calf serum 1,25(OH)2D3 had a biphasic effect on cell proliferation and density, i.e. stimulation at 10(-12) M and dose-dependent inhibition at 10(-10) M and below. Inhibition was specific and not seen with 24 (R), 25-dihydroxy-vitamin D3 or dexamethasone. Growth phase-dependent 1,25(OH)2D3 receptor expression and specific effects of 1,25(OH)2D3 on chondrocyte proliferation in vitro point to a role for vitamin D in the homeostasis of growth cartilage of the rat.

Pharmacokinetics of calcitriol in continuous ambulatory and cycling peritoneal dialysis patients

Oral calcitriol is commonly used for the treatment of secondary hyperparathyroidism in patients undergoing long-term dialysis, but it has been suggested that intravenous (IV) or intraperitoneal (IP) administration enhances the therapeutic efficacy of the sterol. To examine potential mechanisms for this difference, the bioavailability of calcitriol was evaluated after single oral (PO), IV, and IP doses of 60 ng/kg in each of six adolescent patients with osteitis fibrosa undergoing continuous ambulatory peritoneal dialysis (CAPD) or continuous cycling peritoneal dialysis (CCPD). Serum calcitriol levels were 3.6 +/- 4.3, 8.2 +/- 7.5, and 2.5 +/- 3.0 pg/mL, respectively, before IV, PO, and IP doses of the sterol; these values increased to similar levels at 24 hours: 55.6 +/- 14.6 pg/mL after PO, 56.4 +/- 17.6 pg/mL after IV, and 53.8 +/- 20.1 pg/mL after IP. Serum calcitriol levels were higher 1, 3, and 6 hours after IV injections than after PO or IP doses; values thereafter did not differ among groups. The bioavailability of calcitriol, determined from the 24-hour area under the curve (AUC0-24) for the increase in serum calcitriol concentration above baseline values was 50% to 60% greater after IV, 2,340 +/- 523 pg.mL-1.h-1, than after PO, 1,442 +/- 467 pg.mL-1.h-1, or IP, 1,562 +/- 477 pg.mL-1.h1, dosages, P less than 0.05. These differences were due to higher values for AUC during the first 6 hours after calcitriol administration. Although IP calcitriol did not increase sterol bioavailability, radioisotope tracer studies indicated that 35% to 40% of the hormone adheres to plastic components of the peritoneal dialysate delivery system.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent advances in pediatric metabolic bone disease: the consequences of altered phosphate homeostasis in renal insufficiency and hypophosphatemic vitamin D-resistant rickets

Over the past decade our understanding of the pathogenesis of altered mineral homeostasis in chronic renal failure (CRF) and X-linked hypophosphatemic vitamin D-resistant rickets (XLH) has increased, and has provided a rational approach for the use of the 1 alpha-hydroxylated analogues of vitamin D in their therapy. Recent evidence suggests that intracellular phosphate (Pi) retention in CRF plays a major role in decreasing serum 1,25-dihydroxyvitamin D (1,25(OH)2D) levels, which are responsible for the progressive rise in serum parathyroid hormone (PTH) concentrations through the direct action of 1,25(OH)2D on the parathyroid gland. 1,25(OH)2D levels affect the number of intracellular 1,25(OH)2D receptors, preproPTH mRNA levels and the set point for calcium suppression of PTH release. Further in experimental CRF, the maintenance of normal 1,25(OH)2D levels prevents parathyroid gland hyperplasia. These studies indicate that depressed renal 1 alpha-hydroxylase activity due to Pi retention is a major factor in directly increasing PTH secretion, which in turn contributes significantly to the severity of renal osteodystrophy. Thus the aim of therapy in early CRF should be to maintain normal levels of 1,25(OH)2D which can be achieved by either dietary Pi restriction and oral Pi binders or by administering small doses of 1 alpha-hydroxylated metabolites. The long term consequences of these two different therapeutic regimens still need to be assessed. In XLH, evidence is rapidly accumulating that alterations in 1 alpha-hydroxylase activity secondary to impaired Pi handling by the proximal renal tubule, results in decreased serum 1,25(OH)2D levels, which might be responsible for a number of the associated abnormalities documented in both treated and untreated XLH patients. These abnormalities include decreased calcium and Pi absorption by the intestine and low normal serum calcium values. In vitamin D- and Pi-treated patients 1,25(OH)2D levels are further depressed, with a resultant increase in PTH values, and the development of tertiary hyperparathyroidism in a small number of patients. The use of 1 alpha-hydroxylated analogues rather than vitamin D together with Pi supplements decreases the severity of hyperparathyroidism, improves Pi absorption from the intestine and markedly ameliorates the degree of osteomalacia. Whether long-term therapy with these analogues will prevent the development of tertiary hyperparathyroidism in patients with XLH is unclear.

1,25(OH)2D3 administration in moderate renal failure: a prospective double-blind trial

This study represents the first randomized prospective, double-blind, placebo-controlled trial of the efficacy of 1,25(OH)2D3 on bone histology and serum biochemistry in patients with mild to moderate renal failure. Sixteen patients with chronic renal impairment (creatinine clearance 20 to 59 ml per min) received either 1,25(OH)2D3, at a dose of 0.25 to 0.5 microgram daily (eight patients), or placebo. Transiliac crest bone biopsies were performed before entrance into the study and after 12 months of experimental observation. None of the patients were symptomatic or had radiological evidence of bone disease. Of the thirteen patients who completed the study, initial serum 1,25(OH)2D levels were low in seven patients and parathyroid hormone levels were elevated in seven patients. Bone histology was abnormal in all patients. 1,25(OH)2D3 treatment was associated with a significant fall in serum phosphorus and alkaline phosphatase concentrations as well as with histological evidence of an amelioration of hyperparathyroid changes. In contrast to previous reports, no deterioration of renal function attributable to the treatment occurred, perhaps because a modest dose of 1,25(OH)2D3 was employed combined with meticulous monitoring. Further investigation is required to determine whether alternative therapeutic strategies (smaller doses or intermittent therapy) may avoid the potential for suppressing bone turnover to abnormally low levels in the long term.

Vitamin D metabolite profiles in moderate renal insufficiency of childhood

In a prior study, we reported vitamin D metabolite profiles in the plasma of healthy children. In view of these findings, we investigated these same profiles in children with moderate renal insufficiency. Specimens were obtained in both summer and winter in untreated patients, and before and after treatment for up to 1 year, with either 25(OH)D3 or 1,25(OH)2D3. Mean pretreatment 1,25(OH)2D levels were normal. Levels of 25(OH)D3 were also normal and continued to vary with season. Mean pretreatment 24,25(OH)2D3 levels were significantly lower in patients and, interestingly, did not show the normal seasonal variation. Treatment with 25(OH)D3 resulted in consistent and sustained rises in 25(OH)D3 and 24,25(OH)2D3 levels, but no increase in 1,25(OH)2D levels. After 1,25(OH)2D3 treatment, 25(OH)D3 levels were unchanged but still showed seasonal variation in some patients, suggesting a lack of feedback control by 1,25(OH)2D3. Levels of 24,25(OH)2D3 were not significantly different from baseline values. Levels of 1,25(OH)2D increased initially then dropped to pretreatment levels or lower. Normal 1,25(OH)2D levels and reduced 24,25(OH)2D3 levels with the loss of seasonal variation suggests in our patients that the kidney was able to maintain 1,25(OH)2D levels at the expense of 24,25(OH)2D3 levels, presumably to preserve calcium homeostasis.

The effect of uninephrectomy on mineral metabolism in normal human kidney donors

A longitudinal prospective study of 17 normal human kidney donors before and after uninephrectomy revealed changes in mineral metabolism during compensation for loss of renal mass. Increases in carboxy terminal parathyroid hormone (PTH) and urinary cyclic adenosine monophosphate (AMP) occurred at 1 week and persisted for up to 3 years after surgery. 1,25(OH)2D levels fell from 26.5 +/- 2.0 to 18.6 +/- 1.7 pg/mL (P less than 0.05) at 1 week. Tubular reabsorption of phosphate (TRP) fell from 83.4% to 72.3% at 1 month and remained at this level throughout the study. At 6 months, several changes developed that were suggestive of increased PTH effect: hypocalciuria, a decrease in serum phosphate, and the return of 1,25(OH)2D levels to baseline or above. 1,25(OH)2D levels showed an inverse correlation with serum phosphate at 6 months (r = 0.75, P less than 0.005) and 1 year (r = 0.60, P less than 0.01). In addition, at 6 months, an increase in bone mineral content by forearm photon absorptiometry was indirect evidence for a period of positive calcium balance. The compensatory changes demonstrated in this study after loss of renal mass took place over the course of several months and persisted for up to 3 years.

Calcitriol deficiency with retained synthetic reserve in chronic renal failure

Serum calcitriol and the free calcitriol index together with factors considered to regulate calcitriol production were measured in eleven patients with moderate chronic renal failure (MCRF) and eleven age- and sex-matched normal subjects. Although the serum dialysable calcium levels were similar in the two groups, there was depression of calcitriol levels and an elevation of PTH and nephrogenous cyclic AMP (NcAMP) levels in the MCRF patients. Furthermore, plasma phosphate levels were higher and the renal phosphate threshold was depressed in this patient group. When all subjects were grouped together calcitriol was positively correlated with GFR. When calcitriol levels were factored for GFR, to permit an assessment of calcitriol production per unit functioning renal mass, there was no significant difference between normal and MCRF subjects. To determine whether reserve for calcitriol production existed, six of the MCRF patients and six of the age- and sex-matched normal subjects received a low calcium diet for one week supplemented by cellulose phosphate to bind calcium within the gut. In both groups there was a significant rise in calcitriol, although the absolute levels were much lower in the MCRF patients than the normal subjects. These results suggest that calcitriol deficiency is a major feature of MCRF despite marked hyperparathyroidism. The rise in calcitriol levels in MCRF suggests persistent reserve secretory capacity in this condition. Therefore, the low serum calcitriol concentration may be due not only to structural renal damage, but also to suppression of calcitriol formation perhaps due to altered renal phosphate handling.

Recent advances and controversies in childhood renal osteodystrophy

Renal osteodystrophy starts very early in chronic renal failure. Although vitamin D levels are normal in patients with 70-80% function, the levels are not appropriate to the prevailing biochemical milieu. Renal osteodystrophy may contribute to renal growth failure but a correlation between the degree of renal osteodystrophy and growth failure is not observed. Catch-up growth cannot be obtained over a longer period of time with vitamin D. The main reason for osteomalacia is Al intoxication. Aluminium osteopathy is more common in pediatric renal patients than anticipated. The mechanism whereby Al produces its effect on bone is uncertain. Guidelines for the diagnosis and therapy of renal osteopathy are presented. Prophylaxis of renal osteopathy can be attempted by phosphate restriction and/or vitamin D and by avoiding Al-containing drugs. All vitamin D compounds can be used for treatment and all have their advantages and disadvantages.

Review: Renal osteodystrophy--pathogenesis and treatment

Histologic bone changes of osteitis fibrosa and osteomalacia are commonly present in patients with end-stage renal disease. Although many patients are not symptomatic from these bone changes, some patients are severely disabled. Altered metabolism of vitamin D, calcium, phosphorus, and parathyroid hormone occurs in renal failure and contributes to the development of uremic bone disease. This article reviews the current theories of pathogenesis and treatment of renal osteodystrophy. In addition, the clinical presentation, pathogenesis, and treatment of the various aluminum-associated osteomalacic syndromes in uremia are discussed.

Oral intake of phosphorus can determine the serum concentration of 1,25-dihydroxyvitamin D by determining its production rate in humans

Changes in the oral intake of phosphorus could induce the reported changes in the serum concentration of 1,25-dihydroxyvitamin D (1,25-(OH)2D) by inducing changes in its production rate (PR) or metabolic clearance rate (MCR), or both. To investigate these possibilities, we employed the constant infusion equilibrium technique to measure the PR and MCR of 1,25-(OH)2D in six healthy men in whom the oral intake of phosphorus was initially maintained at 1,500 mg/70 kg body weight per d for 9 d, then restricted to 500 mg/d (coupled with oral administration of aluminum hydroxide) for 10 d, and then supplemented to 3,000 mg/d for 10 d. With phosphorus restriction, the serum concentration of 1,25-(OH)2D increased by 80% from a mean of 38 +/- 3 to 68 +/- 6 pg/ml, P less than 0.001; the PR increased from 1.8 +/- 0.2 to 3.8 +/- 0.6 micrograms/d, P less than 0.005; the MCR did not change significantly. The fasting serum concentration of phosphorus decreased from 3.5 +/- 0.2 to 2.6 +/- 0.2 mg/dl, P less than 0.01. With phosphorus supplementation, the serum concentration of 1,25-(OH)2D decreased abruptly, reaching a nadir within 2 to 4 d; after 10 d of supplementation, the mean concentration of 27 +/- 4 pg/ml was lower by 29%, P less than 0.01, than the value measured when phosphorus intake was normal. The PR decreased to 1.3 +/- 0.2 micrograms/d, P less than 0.05; the MCR did not change significantly. The fasting serum concentration of phosphorus increased significantly, but only initially. These data demonstrate that in healthy men, reductions and increases in the oral intake of phosphorus can induce rapidly occurring, large, inverse, and persisting changes in the serum concentration of 1,25-(OH)2D. Changes in the PR of 1,25-(OH)2D account entirely for the phosphorus-induced changes in serum concentration of this hormone.

The physiology and pathophysiology of vitamin D

The vitamin D endocrine system plays an important role in the maintenance of normal calcium homeostasis. Abnormalities of this system occur in many conditions, such as rickets, osteomalacia, hypoparathyroidism, and hyperparathyroidism. The diagnosis and treatment of these disorders will be facilitated if the clinician understands the general mechanisms by which defects in vitamin D metabolism and action occur. We review this information and discuss the use and limitations of vitamin D metabolite assays for diagnosis of clinical disorders of mineral metabolism.