A prospective trial of phosphate and 1,25-dihydroxyvitamin D3 therapy in symptomatic adults with X-linked hypophosphatemic rickets.

Current treatment of X-linked hypophosphatemia (XLH) employs the combined administration of oral phosphate and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Although this drug regimen significantly improves the clinical course of the disease in children, the value of medical treatment in symptomatic adults with XLH has not been established. We, therefore, investigated the clinical, biochemical, and histological responses to phosphate and 1,25-(OH)2D3 in 16 symptomatic adult patients with XLH followed for a mean of 4.2 yr. Eighty-seven percent of the patients had an improvement in bone or joint pain with therapy. There was a significant increase in mean serum phosphate (from 0.61 +/- 0.03 to 0.77 +/- 0.03 mmol/L) and urinary calcium excretion (from 2.45 +/- 0.38 to 4.39 +/- 0.44 mmol/day) with treatment. Pretreatment bone biopsies demonstrated findings characteristic of osteomalacia, including abnormally increased osteoid volume and decreased mineral apposition rates. Treatment was accompanied by a significant decrease in osteoid thickness as well as a reduction in mean osteoid volume. However, therapy did not completely normalize these parameters. Disease severity, as assessed by histomorphometric parameters, did not correlate with any pretreatment serum or urinary biochemical measurement, but did seem to correlate with symptom score. Although most patients tolerated therapy without difficulty, 1 patient developed tertiary hyperparathyroidism during treatment and renal insufficiency that progressed despite cessation of therapy. This study provides evidence that therapy with oral phosphate and 1,25-(OH)2D3 in symptomatic adults with XLH can result in significant clinical and histomorphometric improvement.     

Combined effects of dexamethasone and 1,25-dihydroxyvitamin D3 on parathyroid hormone secretion in cultured bovine parathyroid cells

The present studies investigate the effects of glucocorticoids on the function of the parathyroid glands using primary cultures of bovine parathyroid cells. Treatment of parathyroid cell cultures with dexamethasone for 48 h caused a dose-dependent stimulation of PTH secretion. The minimal concentration of dexamethasone required for a significant stimulation of PTH secretion was 0.1 nM. The stimulatory effect of dexamethasone on the secretion of PTH was found within 12 h of treatment with 100 nM dexamethasone. The steroids deoxycorticosterone and cortexolone, which do not have glucocorticoid activity were without effect of PTH secretion. Since glucocorticoids may modulate the effects of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] in other tissues, additional studies were performed to evaluate the interactions of glucocorticoids and 1,25-(OH)2D3. Addition of 1,25-(OH)2D3 to parathyroid cell cultures for 48 h significantly suppressed PTH secretion. In the presence of dexamethasone, however, 1,25-(OH)2D3 also significantly decreased PTH secretion, although it did not reduce PTH secretion to control levels. The treatment of parathyroid cell cultures with 100 nM dexamethasone did not affect the parathyroid cell content of 1,25-(OH)2D3 receptors. In summary, these studies indicate that glucocorticoids significantly increase the secretion of PTH in vitro. This stimulatory effect can be inhibited by 1,25-(OH)2D3. The parathyroid gland is an additional site of physiological antagonism of glucocorticoids and 1,25-(OH)2D3.     

Effects of 1,25-dihydroxyvitamin D3 and cortisol on bovine and human parathyroid cells

Incubation of bovine parathyroid cells with 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) decreased both preproparathyroid mRNA levels and parathyroid hormone (PTH) secretion. There was a fall to 56.6 +/- 13.7% (mean +/- S.E.M.) and 65.1 +/- 9.3% in mRNA levels and PTH secretion respectively at 1 nmol 1,25-(OH)2D3/l, and 41.1 +/- 13.6% and 42.0 +/- 12.1% at 10 nmol 1,25-(OH)2D3/l after 24 h. After 48 h in 0.1 nmol 1,25-(OH)2D3/l, mRNA levels had fallen to 35.3 +/- 12.6% and PTH secretion to 32.1 +/- 5.0%. In human adenomatous cells, however, incubation with 1,25-(OH)2D3 (10 nmol/l) had no effect on either mRNA levels or PTH secretion even after 48 h. This lack of sensitivity of adenomatous cells to 1,25-(OH)2D3 was not due to an absence of receptors (3847 +/- 39 receptors/ng cytosolic protein in adenomatous cells compared with 4068 +/- 371 in bovine cells) or receptors being of low affinity. Cortisol (1 mumol/l) caused a reduction in the number of receptors for 1,25-(OH)2D3 in bovine parathyroid cells of approximately 20% within 24 h of incubation, but no change in affinity. This decrease was accompanied by abolition of the response to 1,25-(OH)2D3 and was reversible, in that withdrawal of cortisol for the final 24 h of incubation was sufficient for the response to return, the number of receptors having returned to control values. These results suggest that only a small percentage of receptors for 1,25-(OH)2D3 in bovine parathyroid cells may be functional at any one time.(ABSTRACT TRUNCATED AT 250 WORDS)     

低血磷性佝偻病／骨软化症患者急性磷负荷对血磷和甲状旁腺激素水平的影响

目的比较低血磷性佝偻病／骨软化症（hypophosphatemic rickets／osteomalacia,HR／OM）患者与正常人磷负荷前后血磷（Pi）、甲状旁腺激素（iPTH）水平变化情况。方法14例HR／OM患者（低磷组）及8名健康志愿者（对照组）纳入本研究。对受试者行中性磷负荷试验,测定两组服磷前后血Pi、iPTH的水平。结果对照组服磷前血iFTH均处于正常水平;而低磷组中有7例正常,另7例高于正常。低磷组在磷负荷后30分钟至150分钟的血iPTH水平均显著高于基线水平（P〈0.05）,而对照组仅在磷负荷后30分钟显著高于基线水平。结论HR／OM患者基础iPTH水平存在一定程度不均一性,其甲状旁腺对于磷负荷可能表现为“亢进”反应。     

Insulin modulates the stimulation of renal 1,25-dihydroxyvitamin D3 production by parathyroid hormone

Previous studies have shown that there is an impairment in renal production of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), the major biologically active metabolite of vitamin D3, in diabetes. This impairment is not due to a deficiency in the parathyroid hormone (PTH), a major stimulator of renal 1,25(OH)2D3 production. Therefore, we have investigated the capacity of PTH to stimulate 1,25(OH)2D3 production in insulin deficiency and with insulin replacement. Experiments were performed in rats fed a 0.6% calcium, vitamin D sufficient diet for 2 weeks. Thyroparathyroidectomy was performed on all rats. Rats to be rendered diabetic were injected with streptozotocin immediately after surgery. In non-diabetic rats, PTH administration significantly increased renal 1,25(OH)2D3 production (11 +/- 2 vs 46 +/- 5 pg/min/g; P less than 0.05). In diabetic rats, however, PTH caused only a modest increase in 1,25(OH)2D3 production (11 +/- 1 vs 19 +/- 4 pg/min/g; P less than 0.05). With insulin replacement, PTH stimulation of 1,25(OH)2D3 production was markedly increased over that seen in diabetic rats (48 +/- 12 vs 19 +/- 4 pg/min/g; P less than 0.05). PTH was equally effective in raising serum calcium, depressing serum phosphorus and tubular reabsorption of phosphate in non-diabetic as well as in diabetic rats. These results demonstrate that insulin is necessary for the maximal stimulation of renal 1,25(OH)2D3 production by PTH. However, insulin is not necessary for PTH action in terms of renal handling of phosphate and inducing hypercalcaemia. These results suggest multiple pathways for the action of PTH, only some of which are insulin requiring.     

Stimulation of 1,25-dihydroxyvitamin D production by parathyroid hormone and hypocalcemia in man

The serum levels of 1.25-dihydroxycholecalciferol [1,25(OH)2D3] were increased in five patients with primary hyperparathyroidism [60 +/- 13 (SD) pg/ml; normal value, 33 +/- 15 (SD) pg/ml] but fell rapidly after parathyroidectomy to values of 23 +/- 9 (SD) pg/ml. This was accompanied by parallel decreases in the serum concentrations of calcium and immunoreactive parathyroid hormone. Over the following 5--35 days, the serum 1,25(OH)2D3 concentrations increased markedly to levels of 59 +/- 17 (SD) pg/ml, which could most likely be explained by a stimulatory effect of the hypocalcemia per se on the renal production of 1,25(OH)2D3.     

Insulin permits parathyroid hormone stimulation of 1,25-dihydroxyvitamin D3 production in cultured kidney cells

Primary cultures of chick kidney cells in serum free medium respond to PTH with increased production of 1,25(OH)2D3 only when exposed to insulin. The response of 1,25(OH)2D3 is maximal at 5 ng bPTH (1-34) per ml and decreases at higher hormone concentrations. Increased 1,25(OH)2D3 synthesis is not evident after 30 minutes exposure to bPTH and is maximal at 4-6 hours of treatment. Insulin does not increase the cyclic AMP response to PTH suggesting that whatever permissive role it is playing occurs beyond the generation of cyclic AMP.     

Effects of gonadal suppression on the regulation of parathyroid hormone and 1,25-dihydroxyvitamin D secretion in women.

Although a causal association between estrogen deficiency and bone loss has been established for many years, the mechanism by which estrogen deficiency leads to bone loss is unclear. Estrogen deficiency could induce bone loss either by a direct effect on bone cells to modify the production of bone-resorbing cytokines or by altering the production or response to calcium regulatory hormones such as PTH and 1,25-dihydroxyvitamin D. To assess the effects of ovarian hormones on calcium regulatory hormones, we evaluated the ability of calcium to suppress PTH secretion and the ability of PTH to increase serum 1,25-dihydroxyvitamin D and whole blood ionic calcium levels in women before and after GnRH analog-induced ovarian suppression. Sixteen women with endometriosis underwent i.v. infusion of calcium (1.1 mg calcium gluconate/cc in 5% dextrose) at a rate of 4 cc/kg x h (n = 7) or human PTH-(1-34) (Parathar) at a dose of 0.55 U/kg x h (n = 9) before and after 6 months of suppression of ovarian function with the GnRH analog nafarelin acetate (200 microg, intranasally, twice daily). Initial infusions were performed between days 6-10 of the menstrual cycle. Serum PTH and whole blood ionic calcium levels were measured at -20, -10, and 0 min and then every 10 min for 2 h during i.v. calcium infusions. Whole blood ionic calcium and 1,25-dihydroxyvitamin D levels were measured every 6 h for 24 h during i.v. human PTH-(1-34) infusions. Serum estradiol levels were markedly suppressed by nafarelin therapy in both groups of women. The relationship between whole blood ionic calcium and serum PTH levels was similar before and during nafarelin-induced ovarian suppression. The net change and rate of rise in serum 1,25-dihydroxyvitamin D levels in response to PTH infusion were similar before and during nafarelin therapy. Peak whole blood ionic calcium and incremental increases in ionic calcium in response to PTH were similar before and during nafarelin therapy. Our data suggest that ovarian suppression does not alter the regulation of PTH secretion in response to calcium, the ability of PTH to stimulate 1,25-dihydroxyvitamin D formation, or the skeletal sensitivity to PTH. These findings suggest that alterations in calcium regulatory hormones by estrogen deficiency are unlikely to play a major role in the pathogenesis of estrogen deficiency bone loss.     

A comparison between 1,25-dihydroxy-22-oxavitamin D3 and 1,25-dihydroxyvitamin D3 regarding suppression of parathyroid hormone secretion and calcemic action

1alpha,25-dihydroxy-22-oxavitamin D(3) (maxacalcitol;OCT), a vitamin D analogue with reduced calcemic activity, showed potency 10 times greater than 1alpha,25 (OH) (2)D(3) in differentiation induction of HL-60 cells. In addition, OCT showed immunomodulating activity, anti-proliferative effect and a suppressive effect on PTH secretion in vitro. OCT suppressed the expression of PTH mRNA in both normal and nephrectomized rats, and up-regulated VDR in the parathyroid gland in uremic rats to the same degree as 1alpha,25 (OH) (2)D(3). The results of a comparison between OCT and 1alpha,25 (OH) (2)D(3) indicate that the dissociation between efficacy and side effects with OCT is greater than with 1alpha,25 (OH) (2)D(3) in uremic rats. In future, we expect that OCT would contribute a great deal to the dialysis field in clinical.     

1,25-dihydroxyvitamin D resistance,rickets, and alopecia

Two unrelated kindreds with four affected children having 1,25-dihydroxyvitamin D resistance, rickets, and alopecia are described. The children exhibited early onset of severe rickets with hypocalcemia, hypophosphatemia, elevated serum alkaline phosphatase levels, and secondary hyperparathyroidism. Radiography showed diffuse demineralization and classic changes of rickets. All affected children had total-body alopecia. Serum levels of 1,25-dihydroxyvitamin D₃ were elevated and rose to extremely high values during treatment, with no apparent change in the mineral disorder. However, secondary hyperparathyroidism and hypophosphatemia did remit during treatment despite persistently low calcium levels. Skin biopsy was performed in the parents and affected children in one kindred. Analysis of 1,25-dihydroxyvitamin D₃ receptors in cultured fibroblasts Indicated apparent normal receptors in the parents and undetectable receptors In both affected children. After long periods of treatment with vitamin D metabolites and mineral replacement, healing took place in the older child in each kindred. These data suggest that the healing occurred spontaneously as the children reached seven to nine years of age rather than as a result of the treatment. The biochemical lesion in these children appeared to be a genetically transmitted defect in the 1,25-dihydroxyvitamin D₃ receptor. The mechanisms by which healing was initiated and maintained remain to be elucidated.     

The mobilization of bone mineral by 1,25-dihydroxyvitamin D3 in hypophosphatemic rats.

Administration of 1,25-dihydroxyvitamin D3 to thyroparathyroidectomized, hypophosphatemic rats on a low-phosphorus diet increases serum inorganic phosphorus and calcium concentrations. Experiments utilizing such rats treated with 45Ca and 32P ten days before use revealed that the rise in serum phosphorus in response in 1,25-dihydroxy-vitamin D3 is derived at least in part from bone mineral. This suggests that under conditions of hypophosphatemia, 1,25-dihydroxyvitamin D3 can mobilize bone in the absence of parathyroid hormone.     

Effect of experimental human magnesium depletion on parathyroid hormone secretion and 1,25-dihydroxyvitamin D metabolism

Magnesium (Mg) deficiency in man may result in hypocalcemia, impaired PTH secretion, and low serum concentrations of 1,25-dihydroxyvitamin D [1,25-(OH)2D]. To determine whether these changes are due to selective Mg depletion, we studied 26 normal subjects before and after a 3-week low Mg (less than 1 meq/day) diet. This diet induced Mg deficiency, as demonstrated by a fall in pre- to postdiet serum Mg levels from 0.80 +/- 0.01 to 0.61 +/- 0.02 mmol/L (P less than 0.001), an increase in Mg retention from 11 +/- 4% to 62 +/- 4% (P less than 0.001), and a fall in red blood cell free Mg2+ from 205 +/- 10 to 162 +/- 7 microM (P less than 0.001). Serum calcium (Ca) fell significantly from 2.36 +/- 0.02 to 2.31 +/- 0.03 mmol/L (P less than 0.05), and serum 1,25-(OH)2D fell from 55 +/- 4 to 43 +/- 3 pmol/L (P less than 0.05). PTH secretion was impaired, as demonstrated by a fall or no change in serum PTH in 20 of 26 subjects despite a fall in the serum Ca and Mg. In addition, an iv injection of Mg in eight subjects after the diet resulted in a significant rise in PTH from 15 +/- 2 to 19 +/- 2 ng/L (P less than 0.01), whereas a similar injection given to six of the subjects before the diet resulted in a significant fall from 28 +/- 5 to 13 +/- 3 ng/L (P less than 0.001). The fall in serum 1,25-(OH)2D may be due to both the decrease in PTH secretion and a renal resistance to PTH. PTH resistance was suggested, as no increase in serum 1,25-(OH)2D was observed in the six subjects in which the PTH concentration rose by mean of 68% after the diet. Also, the rise in serum 1,25-(OH)2D after a 6-h human PTH-(1-34) infusion was significantly less after Mg deprivation. The results demonstrate that mild Mg depletion can impair mineral homeostasis and may be implicated as risk factor for osteoporosis in disorders such as chronic alcoholism and diabetes mellitus, in which Mg deficiency and osteoporosis are both common.     

Parathyroid hormone effects on serum 1,25-dihydroxyvitamin D levels in patients with X-linked hypophosphatemic rickets: evidence for abnormal 25-hydroxyvitamin D-1-hydroxylase activity

Patients with X-linked hypophosphatemic rickets (XLH) have normal or marginally low serum 1,25-dihydroxyvitamin D [1,25-(OH)2D] levels despite manifesting hypophosphatemia and phosphate depletion, which increase 1,25-(OH)2D production in many animal species. These data are consistent with the possibility that regulation of vitamin D metabolism is abnormal in XLH. However, controversy concerning the role of phosphate in the regulation of 25-hydroxyvitamin D-1-hydroxylase activity in man has raised doubt about this proposed defect. The presence of a defect in vitamin D metabolism could be established if hormonal or metabolic factors, other than hypophosphatemia, were unable to stimulate 25-hydroxyvitamin D-1-hydroxylase activity normally in patients with XLH. Thus, we compared the effects of parathyroid hormone infusion on serum 1,25-(OH)2D levels in patients with XLH and normals. In response to iv infusion of parathyroid extract (200 U at 0915 and 1700 h), the serum 1,25-(OH)2D concentration increased 218% above base line (from 34.0 +/- 3.0 to 108.8 +/- 2.5 pg/ml) in normals and only 68% (from 30.6 +/- 3.0 to 48.8 +/- 5.5 pg/ml) in patients with XLH. The disparate response occurred in spite of an equivalent increase in urinary cAMP excretion in the normals (from 3.00 +/- 0.14 to 8.70 +/- 0.25 mumol/g creatinine . 24 h) and XLH patients (from 3.10 +/- 0.39 to 8.30 +/- 1.0 mumol/g creatinine . 24 h) as well as equivalent decreases in the renal tubular maximum for the reabsorption of phosphate per liter glomerular filtrate (1.2 +/- 0.1 and 0.9 +/- 0.2 mg/dl, respectively). These observations support the possibility that regulation of vitamin D metabolism is abnormal in XLH.     

Infusions of parathyroid hormone in ruminants: hypercalcemia and reduced plasma 1,25-dihydroxyvitamin D concentrations

The relationship between infused synthetic bovine PTH-(1-34) and plasma concentrations of minerals and vitamin D metabolites was studied in eight calves (150-230 kg) and two thyroparathyroidectomized goats. Calves were infused iv with saline for 15-20 h. Then, calves were infused with one of three types of solution for an additional 35-h period. Three of the eight calves received 3 ng/kg X min (group H), three received 0.75 ng/kg X min (group L), and the remaining two calves received control saline over a 33-h period (group C). Blood samples were taken every 4-6 h. Plasma calcium, phosphorus, hydroxyproline, and 1,25-dihydroxyvitamin D [1,25-(OH)2D] remained relatively constant in control calves. PTH infusions into calves in group H resulted in an increase in plasma calcium from 2.4 to a plateau of 3.0 mmol/liter. PTH infusion caused no change in plasma phosphorus, but increased urinary excretion of phosphorus. Infusion of PTH caused a moderate increase in urinary calcium excretion, followed by pronounced calciuria after PTH withdrawal. Plasma concentrations of 1,25-(OH)2D decreased from about 30 pg/ml at the start of infusion to undetectable levels (less than 5 pg/ml) at the end of the infusion and for 30 h thereafter. Similar, but less pronounced, changes in plasma calcium and 1,25-(OH)2D concentration were observed in group L. Hypocalcemia and hypophosphatemia developed in the two lactating goats after thyroparathyroidectomy, and plasma 1,25-(OH)2D concentrations were decreased. PTH infusion (3 ng/kg X min) corrected the hypocalcemia and hypophosphatemia and markedly raised plasma 1,25-(OH)2D concentrations. When calcium chloride was infused in addition to PTH, the resulting hypercalcemia (3 mmol/liter) was associated with a marked reduction in plasma 1,25-(OH)2D. We conclude that the concentration of calcium in plasma has the major regulatory role on plasma 1,25-(OH)2D concentrations in ruminant species when potentially conflicting signals, such as hypercalcemia and high PTH concentrations, are present simultaneously.     

Binding and metabolism of 1,25-dihydroxyvitamin D3 in cultured bovine parathyroid cells.

Several laboratories, including ours, have reported that receptors for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] are decreased in parathyroid glands of uremic animals and patients. To elucidate the factors involved in receptor regulation in this tissue, we have characterized the receptor in primary cultures of bovine parathyroid cells. Extracts from these cells contain a single binding component that binds 1,25-(OH)2D3 with a Kd of 58 pM and sediments in sucrose density gradients at 3.4S, indicating the continued expression of the vitamin D receptor in these cells. Labeling of the intact parathyroid cells with tritiated 1,25-(OH)2D3 was maximal by 2 h, and binding affinity by this method was estimated to be 22 pM. Longer incubation of the cells with tritiated 1,25-(OH)2D3 resulted in a loss of specific binding to 10% maximal by 12 h. The decrease in binding correlated temporally with degradation of 1,25-(OH)2D3 in the medium. This metabolic activity was absent in vitamin D-deficient cells and was first detectable 3-4 h after the addition of 1,25-(OH)2D3, indicating that 1,25-(OH)2D3 induces its own metabolism in parathyroid cells. Replenishment of the cultures after 12 h with fresh tritiated 1,25-(OH)2D3 restored maximal binding, demonstrating that the loss of binding was not due to down-regulation of receptor. Inclusion of the cytochrome P450 inhibitor ketoconazole did not alter maximal binding at 2 h, but blocked both the metabolism of 1,25-(OH)2D3 and the decrease in binding after 3 h. In contrast to other cell types, such as osteosarcoma cells, no homologous up-regulation was seen in cultured parathyroid cells even after 12 h in the presence of 0.5 nM 1,25-(OH)2D3. Furthermore, receptor levels in preparations from cells treated for 20 h with unlabeled 1,25-(OH)2D3 at concentrations of 0.1, 1.0, and 10 nM were not different from controls. Thus, it appears that the vitamin D receptors in parathyroid cell cultures are not up-regulated by their ligand.     

Effects of growth hormone replacement therapy on 1,25-dihydroxyvitamin D and calcium metabolism

To elucidate the changes in mineral metabolism and blood concentrations of calciotropic hormones which accompany GH therapy, we studied 12 GH-deficient children for 5 days before and for 1 week after high dose (5 IU/day) GH therapy, and again at 1 month, 3 months, and 1 yr of replacement therapy (0.1 IU/kg to a maximum dose of 2 IU three times weekly). All responded with acceleration of height velocity, and bone ages advanced appropriately. Fasting serum ionized calcium levels did not change: 4.11 +/- 0.06 (SEM) mg/dl before, 4.19 +/- 0.05 for the week of high dose therapy, and 4.20 +/- 0.14 during replacement therapy. Likewise, fasting serum parthormone did not vary: 38.9 +/- 2.6 muleq/ml before to 44.1 +/- 9.2 at 1 yr. Twenty four-hour nephrogenous cyclic AMP (NcAMP) did not vary over the first week (1.2 +/- 0.7 nmol/dl glomerular filtrate before, 1.3 +/- 0.4 after 1 week), but increased to 5.3 +/- 1.9 after 1 yr (alpha less than 0.001). The response of ionized calcium and parathormone to a standardized disodium EDTA infusion of 50 mg/kg also did not change. The mean fasting serum calcitonin level was not different before therapy (29.4 +/- 2.8 pg/ml), after 1 week (21.5 +/- 1.8), or after 1 yr (42.4 +/- 11.0). However, the mean serum 1,25-dihydroxyvitamin D concentration rose from 33.1 +/- 3.3 pg/ml before therapy to 68.3 +/- 12.3 on the seventh day of high dose therapy (alpha less than 0.01), returning to pretherapy values by 1 month. We conclude that high dose GH therapy in GH-deficient children raises 1,25-dihydroxyvitamin D concentration acutely, but that long term, physiological replacement therapy does not cause such an effect. Because NcAMP excretion rose in the absence of an increase in serum parathormone concentration, we conclude that GH sensitizes the kidney to a cAMP-mediated effect of parathormone.