Agnès Bourdeau

Long-term nocturnal calcium infusions can cure rickets and promote normal mineralization in hereditary resistance to 1,25-dihydroxyvitamin D.

We report the beneficial effects of calcium infusions in a child with hereditary resistance to 1,25(OH)2D and alopecia. This patient after transient responsiveness to vitamin D derivatives became unresponsive to all therapy despite serum 1,25(OH)2D concentrations maintained at levels approximately 100-fold normal. A 7-mo trial with calcium infusions led to correction of biochemical abnormalities and healing of rickets. Bone biopsies (n = 3) showed a normal mineralization and the disappearance of the osteomalacia. Cultures of bone-derived cells demonstrated a lack of activation of 25-hydroxyvitamin D 24-hydroxylase and osteocalcin synthesis by 1,25(OH)2D3 (10(-9) and 10(-6) M). These results demonstrate that even in the absence of a normal 1,25(OH)2D3 receptor-effector system in bone cells, normal mineralization can be achieved in humans if adequate serum calcium and phosphorus concentrations are maintained; and calcium infusions may be an efficient alternative for the management of patients with this condition who are unresponsive to large doses of vitamin D derivatives.

Human parathyroid cell proliferation in response to calcium, NPS R-467, calcitriol and phosphate.

It remains uncertain how calcium, phosphate and calcitriol regulate parathyroid cell growth. The present study was aimed at examining possible direct effects of these modulators and of the calcimimetic NPS R‐467 on parathyroid cell growth in vitro.

Suppressive Effects of 24,25-Dihydroxycholecalciferol on Bone Resorption Induced by Acute Bilateral Nephrectomy in Rats

The possible suppressive effects of 24,25-dihydroxycholecalciferol on secondary hyperparathyroidism and increased bone resorption were investigated in adult rats raised on a diet normal in calcium, phosphorus, and vitamin D, and subjected to acute bilateral nephrectomy. The animals had received subcutaneous radiocalcium 4 wk before the experiment. 5 h after nephrectomy an increase in serum total calcium, (45)Ca-specific activity, citrate, phosphorus, and magnesium concentrations were observed. Serum immunoreactive parathyroid hormone increased, while serum calcitonin decreased. The osteoclast count in the tibial metaphyses was augmented. The biochemical and histological changes observed were partly parathyroid hormone and calcitonin independent, as they also occurred in parathyroidectomized hypocalcemic rats. Pretreatment with 650 pmol of 24,25-dihydroxycholecalciferol 16 h before nephrectomy prevented bone calcium mobilization and diminished the rise in serum total calcium and citrate both in parathyroid-intact and in parathyroidectomized animals. In parathyroid-intact rats, serum immunoreactive parathyroid hormone and calcitonin remained normal in spite of the fall in serum-ionized calcium, and the number of osteoclasts did not increase. In parathyroidectomized rats, 24,25-dihydroxycholecalciferol did not prevent the postnephrectomy rise in the osteoclast count. This latter observation suggests that this metabolite exerts its effect on bone either by acting on cells other than osteoclasts, i.e., the osteocytes, or by inhibiting cell activity. At equimolar dosage 1,25-dihydroxycholecalciferol had a potent stimulatory effect on bone resorption. This effect of 1,25-dihydroxycholecalciferol was partly blocked by the simultaneous administration of 24,25-dihydroxycholecalciferol. The potential clinical significance of these observations remains to be determined.

Persistence of Ca2+-Sensing Receptor Expression in Functionally Active, Long-Term Human Parathyroid Cell Cultures

An original human parathyroid cell culture model from uremic patients with II° hyperparathyroidism has been developed, with its main feature being long‐term functionally active viability up to 5 months, as assessed by persistent responsiveness to changes of extracellular Ca2+ concentrations ([Ca2+]e). In addition to the inhibitory effect of increasing [Ca2+]e, increasing extracellular phosphate exerted a biphasic effect on parathyroid hormone (PTH) secretion. The presence of the Ca2+‐sensing receptor (CaR), on which depends the response to [Ca2+]e and its persistence, has been demonstrated in our culture system both by direct detection and by inhibition of its activity. CaR protein was detected by Western blot analysis with a specific anti‐CaR antibody. CaR gene transcripts have been identified by reverse transcription‐polymerase chain reaction analysis. mRNA (by in situ hybridization) and protein (by immunocytochemistry) expression were detected for both CaR and PTH. Adding a specific anti‐CaR antibody to the medium induced a marked reduction of low [Ca2+]e‐stimulated PTH release, which decreased to levels equivalent to those obtained in high [Ca2+]e medium. The described long‐term functionality could be due to several factors, including the clustered cell type of culture yielded by our preparation procedure, the growth characteristics of hyperplastic uremic tissue, and the use of a phosphate‐rich medium. The present model, because of its long‐term functionality, is a unique tool for the exploration of PTH synthesis and secretion and for studies of parathyroid cell growth in vitro.

Absence of response to human parathyroid hormone in athymic mice grafted with human parathyroid adenoma, hyperplasia or parathyroid cells maintained in culture

In athymic mice we have developed a model of long-term human PTH hypersecretion, using xenotransplantation of respectively parathyroid gland fragments obtained from patients with primary (1°) or secondary (2°) uremic hyperparathyroidism (HPT), and parathyroid cells maintained in culture from patients with 2° uremic HPT. Both grafted parathyroid tissue fragments and cultured cells induced prolonged and marked secretion of human intact PTH (iPTH) in nude mice. Despite extremely high plasma iPTH levels, hypercalcemia or hypophosphatemia was not observed. Moreover, PTH secretion was not significantly modified by low-calcium, high-phosphate diet for 3 weeks. Four mice which had a mean plasma human iPTH level of 237±152 pg/ml for more than 9 months and 4 age-matched, sham-grafted control mice with undetectable human iPTH levels underwent bone histomorphometry examination. No difference was found between the two groups with respect to active bone resorption surface or number of osteoclasts/ mm2. We hypothesize that the characteristic deficit of T cell function and of cytokine and growth factor production may protect nude mice with chronic hypersecretion of human PTH from hypercalcemia and bone lesions. We suggest that this strain of mice could be used for better understanding the relationship between cytokines and bone turnover.

Calcium metabolism in children during long-term total parenteral nutrition : the influence of calcium, phosphorus, and vitamin D intakes

Hypercalciuria and bone disease are frequently associated with total parenteral nutrition (TPN) in children and adults. The aim of this study was to assess the influence of calcium, phosphorus, and vitamin D intakes on hypercalciuria. We observed seven children aged 4–13 years receiving home cyclic TPN for 4 consecutive years. Calcium and phosphorus intakes, constant during the 1st year, were reduced during the last 3 years to 50 and 30% of the initial intakes, and vitamin D was stopped during the 3rd and the 4th years. All children had hypercalciuria and one of them had acute painful osteopenia and nephrocalcinosis at the beginning of the study. Hypercalciuria was corrected and painful bone disease did not occur during the three following years, with TPN daily intakes of calcium, 0.35 mmol/kg, and phosphorus, 0.70 mmol/kg. Cessation of vitamin D administration during 48 months led to no further decrease in calciuria nor to the occurrence of clinical or biological signs of vitamin D deficiency. However, we hypothesize that excessive vitamin D intake may have facilitated the occurrence of the TPN-related bone disease in one patient and should be avoided. The possible role of parenteral aluminum loading is also discussed.

Effect of Calcitriol in the Control of Plasma Calcium after Parathyroidectomy

Severe, prolonged hypocalcemia in observed in some, but not all, hemodialysis patients after parathyroidectomy performed because of uncontrolled hyperparathyroidism. The aim of the present study was to investigate whether calcitriol and calcium supplementation in the immediate period after parathyroidectomy (days 1-14) was of more help in the control of plasma calcium than calcium supplementation alone. Fourteen hemodialysis patients were enrolled in a prospective, randomized, double-blind and placebo-controlled study. From the day after parathyroidectomy, 7 patients received calcitriol and the remaining 7 a placebo using incremental doses adjusted to the degree of hypocalcemia (up to 4 micrograms/day for calcitriol). Plasma calcium, phosphorus, alkaline phosphatase and immunoreactive parathyroid hormone levels before parathyroidectomy were comparable in both patients groups, as was the lowest plasma calcium achieved after parathyroidectomy. The decrease in plasma calcium after parathyroidectomy was related to plasma alkaline phosphatase and to the number of osteoclasts and osteoblasts on bone biopsy surface before parathyroidectomy. The mean decrement of plasma calcium (days 3-9) as compared to that before parathyroidectomy was less pronounced in calcitriol-treated than in placebo-treated patients (0.25 +/- 0.06 versus 0.45 +/- 0.05 mM, mean +/- SEM, p less than 0.025). Treatment with placebo was interrupted before day 14 because of persistent severe hypocalcemia in 4 of 7 patients, whereas calcitriol treatment was continued in all 7 patients up to 14 days. Patients on calcitriol treatment required less mean calcium supplements (days 1-9) than patients receiving placebo (37.4 +/- 3.2 versus 49.4 +/- 3.7 g, p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic Hypophosphatemia in Kidney Transplanted Children and Young Adults

Different mechanisms have been suggested to account for the frequently observed hypophosphatemia after renal transplantation. One may propose, along this line, the persistence of a pretransplant hyperparathyroidism1,2, the post-transplant administration of oral hydroxide antacids3, the post-transplant steroid therapy4 a deficiency in vitamin D active metabolites4,5, a tubular dysfunction6, or an abnormality in intestinal phosphate absorption7. In an attempt to analyse the respective importance of these factors, serial measurements of biochemical and radioimmunological parameters have been performed in the twelve subjects transplanted during the year 1976 in the Department of Pediatric Nephrology (Pr. M. Broyer) and considered as successful kidney transplantation.

The Calcium Receptor in Health and Disease

The recent cloning of a G-protein-coupled, extracellular calcium [(Ca2+)e]-sensing receptor (CaRG) from the parathyroid, kidney and brain of several species has clarified the molecular mechanisms underlying Ca2+-sensing by parathyroid and other cell types. It has long been suspected that such a receptor existed on parathyroid cells, coupled to intracellular second messengers through guanine nucleotide regulatory (G) protein which is able to recognize and respond to (Ca2+)e. Recently, functional screening of a cDNA library constructed from bovine parathyroid mRNA led to the isolation of a 5.3-kb clone expressing maximal Ca2+-stimulated Cl– currents in oocytes. This 5.3-kb cDNA encodes a protein of 1,085 amino acids with three principal predicted structural domains. The CaRG protein is present in chief parathyroid cells, in C cells of the thyroid, in the cortical thick ascending limb (TAL) and collecting duct of the kidney, and in discrete brain areas. CaRG may play several physiological roles. It is a central element in the control of both parathyroid and calcitonin secretion by (Ca2+)e. Moreover, functional evidence for its participation in the regulation of renal Ca2+ reabsorption in TAL and water reabsorption in the collecting duct has been obtained. Mutations of the CaRG gene are responsible for hereditary and familial parathyroid disorders, and a decrease in CaRG expression has been documented in primary and secondary uremic hyperparathyroidism. The expression of CaRG in several additional organs and tissues allows speculation on the potential involvement in other pathologies.

Plasma vitamin D metabolites in a patient with sporadic hypophosphataemic osteomalacia (adult-onset type).

Abstract. Sporadic hypophosphataemic osteomalacia (adult‐onset type) was demonstrated in a 40‐year‐old man on the basis of severe osteomalacia, hypophos‐phataemia, hyperphosphaturia and glycinuria. Plasma immunoreactive parathyroid hormone (iPTH) concentration was 9‐3 ng prot./ml (normal range: 4–8 ng prot./ml). Plasma 25‐hydroxy‐vitamin D and 24, 25‐dihydroxy‐vitamin D concentrations were 11 and 2–4 ng/ml respectively. Basal lα, 25‐dihydroxy‐vitamin D concentrations were slightly elevated (116 and 96 pg/ml) and increased to 240 pg/ml after 3 days on a low‐phosphorus diet. The patient was put on oral treatment with 25‐hydroxycholecalciferol (100 μ per day) and phosphorus (1500 mg per day). On the 4th month on treatment, a clinical improvement was apparent. Plasma 25(OH) D was 44 ng/ml, plasma l,25 (OH)2D was 256 pg/ml. However, plasma phosphorus remained low (0.77 mmol/l). On the 9th month of treatment a radiological improvement was evident despite a persistent hypophosphataemia (0.68 mmol/I). These facts suggest in our patient the existence of a vitamin D‐independent renal phosphorus leak.