Parathyroid hormone (PTH)/PTH-related protein (PTHrP) receptor and its messenger ribonucleic acid in rat aortic vascular smooth muscle cells and UMR osteoblast-like cells: cell-specific regulation by angiotensin-II and PTHrP

PTH-related protein (PTHrP) is produced in vascular smooth muscle, where it is believed to act as a local vasorelaxant by activating either the classical PTH or a unique PTHrP receptor. We used a newly cloned complementary DNA encoding the rat PTH/PTHrP receptor to study the expression of its messenger RNA (mRNA) in primary aortic vascular smooth muscle cells (VSMC) and in UMR-106 osteoblast-like cells under basal conditions and in response to treatment with agonists. Both cell types expressed a 2.4-kilobase PTH/PTHrP receptor mRNA transcript and exhibited hormone-induced desensitization of PTHrP-(1-34)NH2-stimulated cAMP. In VSMC, angiotensin-II, which induces PTHrP expression, also rapidly (30 min) desensitized the cAMP response and down-regulated (75-90%) receptor mRNA within 1 h. Treatment of cells with phorbol 12-myristate 13-acetate (0.1 microM) mimicked these effects, whereas neither PTHrP-(1-34)NH2, forskolin, nor (Bu)2cAMP altered receptor mRNA expression. By contrast, in UMR-106 cells, PTHrP-(1-34)NH2 induced time- and dose-dependent decreases in receptor mRNA that were preceded by pronounced desensitization (cAMP and ligand binding) of cell surface receptors. These effects were mimicked by (Bu)2cAMP and forskolin, but not by phorbol 12-myristate 13-acetate, suggesting that both receptor mRNA down-regulation and receptor desensitization in UMR cells were mediated through a protein kinase-A pathway. We suggest that VSMC and UMR cells express a common receptor, which is subject to cell-specific regulation. Such diversity in the PTH/PTHrP receptor regulatory mechanisms provides a means for restricting the length and duration of the cellular response to hormone in a cell/tissue-specific manner.

Regulated production and intracrine action of 1,25-dihydroxyvitamin D3 in the chick myelomonocytic cell line HD-11

To better understand the extrarenal production of active vitamin D metabolites by cells of the monocyte/macrophage lineage, we investigated the 25-hydroxyvitamin D (25OHD)-1-hydroxylation reaction in the v-myc-transformed chick myelomonocytic cell line HD-11; the 1-hydroxylation reaction in this cell line has a high affinity for 25-hydroxylated vitamin D substrates, is localized to mitochondria, and is associated with cytochrome P450 activity. In this study we demonstrated that the HD-11 cell 1-hydroxylation reaction in vitro is not affected by the majority of extracellular regulatory factors that modulate expression of the renal 25OHD-1-hydroxylase in vivo. A 50% increase in extracellular calcium and phosphate concentrations, physiological inhibitory events for renal 1,25-dihydroxyvitamin D [1,25-(OH)2D] synthesis, did not decrease basal expression of the HD-11 cell 1-hydroxylation reaction, nor did a 50% decrease in extracellular calcium and phosphate concentrations, stimulatory signals for the 1-hydroxylase in vivo, increase 1,25-(OH)2D3 synthesis in vitro. Receptor-saturating concentrations of PTH and PTH-related peptide were similarly without effect. In contrast, the HD-11 1-hydroxylation reaction was significantly stimulated in a dose-dependent fashion by the macrophage stimulatory agents lipopolysaccharide [P < 0.001 at a maximum effective concentration (EC100) of 25 micrograms/ml] and interferon-gamma (P < 0.001 at EC100 of 1000 IU/ml) and by insulin-like growth factor-I (P < 0.01 at EC100 of 15 nM) with the rank order of stimulation being interferon-gamma > lipopolysaccharide > insulin-like growth factor-I. Dexamethasone (> or = 10 nM) and the cytochrome P450 inhibitors (EC100, 20 microM), ketoconazole, clotrimazole, and menadione, all significantly inhibited the HD-11 cell 1-hydroxylation reaction. The naphthoquinone menadione, which blocks electron transfer to the P450-associated enzyme, was the most effective inhibitor of the reaction in both intact cells (3 +/- 1% of basal expression; P < or = 0.002) and after reconstitution of HD-11 cell mitochondrial extracts with a ferredoxin, reductase, O2, and NADPH (5 +/- 1% of basal; P < or = 0.02). We have also shown that 1,25-(OH)2D3 produced from substrate 25OHD3 appears to exert an endogenous (intracrine) inhibitory effect on HD-11 cell growth; incubation of HD-11 cells with a concentration of ketoconazole (10 microM) known to reduce 1,25-(OH)2D3 production by roughly 50% restored 50% of the growth deficit induced by 1,25-(OH)2D3 (EC100, 100 nM).(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanical stimuli induce vascular parathyroid hormone-related protein gene expression in vivo and in vitro

PTH-related peptide (PTHrP), the factor mediating the syndrome of humoral hypercalcemia of malignancy, is also expressed in smooth muscle cells (SMC) of the urinary bladder and uterus in response to mechanical distention and fetal occupancy, respectively. Vascular SMC also produce PTHrP, and its expression is induced by serum and vasoconstrictors, such as angiotensin-II. To determine whether mechanical distension affected vascular PTHrP gene expression, the abdominal aorta of adult male rats was balloon-distended, and aortae were collected at various times after the intervention. PTHrP mRNA was determined by competitive reverse transcriptase-polymerase chain reaction, using sequential dilutions of a cloned internally truncated PTHrP RNA fragment as standard. The molar concentration of PTHrP mRNA was obtained by extrapolating at a standard/wild-type band intensity ratio of 1:1. Aortic PTHrP mRNA was induced from a basal level of 19, to 22, 46, 36, 13, 12, 22, and 20 attamoles/mg total RNA 1, 2, 12, 24, and 48 h and 7 and 48 days after balloon distension, respectively. To determine whether mechanical events directly regulate vascular PTHrP gene expression, primary rat aortic SMC were plated and placed on a rocking device at 20 oscillations/min to create a gentle flowing motion of the culture medium. Rocking induced PTHrP mRNA of SMC exposed to either serum-free medium or 10% serum by 2.5-and 4.0-fold at 4 h, and 2.9- and 3.7-fold at 24 h, respectively. These effects were oscillation rate dependent, potentiated by angiotensin-II, and specific, as similar changes were not observed in alpha-actin mRNA content. Flow motion-induced PTHrP mRNA at 24 h was partially decreased by 10(-6) M colchicine (which inhibits microtubule assembly), but not by cytochalasin-E (which disrupts actin polymerization). As PTHrP is a known vasorelaxant, we propose that mechanical events induce the release of PTHrP by SMC, possibly to serve as a compliance factor or an agent for vascular remodeling.

A role for nitric oxide in the regulated expression of the 25-hydroxy-vitamin D-1-hydroxylation reaction in the chick myelomonocytic cell line HD-11

We have recently described the existence of a cytochrome P450-associated, mitochondrial-based 25-hydroxyvitamin D (25-OHD)-1-hydroxylation reaction in the chick macrophage-like cell line HD-11. Considering that this reaction is regulated by the same set of factors (ie. interferon-gamma, lipopolysaccharide, and glucocorticoids) that modulate expression of the macrophage nitric oxide synthase (mac NOS), we investigated the possibility that endogenous nitric oxide (NO) production may be linked to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D) synthesis by HD-11 cells in vitro. To test this hypothesis we investigated the effects excluding from the extracellular medium the essential amino acid L-arginine, substrate for endogenous NO production, on the basal and stimulated expression of the HD-11 cell 25-OHD-1-hydroxylation reaction. Depletion of L-arginine from the extracellular medium for as little as 6 h resulted in a significant decrease (p < 0.02) in basal 1,25-(OH)2D synthesis; after 15 h in an L-arginine-free environment hormone production was reduced to < 10% of basal levels without any adverse affect on cell viability. Reintroduction of L-arginine, but not D-arginine, into the extracellular medium restored 1,25-(OH)2D3 synthetic capacity fully if done after < or = 6 h of incubation in the absence of L-arginine. Competitive inhibition of NOS with Nw-nitro-L-arginine methyl ester (p < 0.002) and Nw-nitro-L-arginine (p < 0.02) significantly inhibited 1,25-(OH)2D synthesis, indicating that macrophage NO generating capacity is functionally linked to endogenous synthesis of the active vitamin D metabolite.

Effects of N-terminal, midregion, and C-terminal parathyroid hormone-related peptides on adenosine 3',5'-monophosphate and cytoplasmic free calcium in rat aortic smooth muscle cells and UMR-106 osteoblast-like cells

N-Terminal analogs of PTH-related protein (PTHrP) and PTH bind to a common receptor and exhibit similar biological properties. However, recent studies suggest that certain midregion and C-terminal PTHrP peptides have activities distinct from those of PTH in the placenta and in osteoclasts, respectively. In this study we determined the biological activities of full-length recombinant PTHrP-(1-141) and several synthetic N-terminal, midregion, and C-terminal PTHrP fragments in two PTHrP-producing cell types. Peptides were tested for their ability to stimulate cAMP production and raise intracellular free calcium ([Ca2+]i) in primary rat aortic smooth muscle cells (VSMC) and UMR-106 rat osteoblast-like (UMR) cells. In UMR cells PTHrP-(1-34)NH2, PTHrP-(1-141), and bovine PTH-(1-34) all increased cAMP (approximately 50 fold) and [Ca2+]i (180 nM). By contrast, in VSMC, these N-terminal peptides increased cAMP (3-fold) but had no detectable effect on [Ca2+]i. PTHrP-(1-34) and PTHrP-(1-141) significantly blunted the angiotensin II-induced rise in cAMP (but not the calcium signal) consistent with the concept that PTHrP opposes angiotensin II activity in VSMC. PTHrP-(67-86)NH2, PTHrP-(107-138)NH2, and PTHrP-(107-111)NH2 had no effect on either cAMP or [Ca2+]i in either cell type. VSMC and UMR-106 cells both expressed a 2.5-kilobase PTH/PTHrP receptor messenger RNA (mRNA) transcript. However, high affinity specific binding of 125I-labeled [Tyr36] PTHrP-(1-36)NH2 was detected in UMR cells but not in VSMC. We conclude that the PTH-like, N terminus of the PTHrP molecule is critical in induction of cAMP and [Ca2+]i pathways in UMR cells, and for cAMP stimulation in VSMC. In addition, PTHrP, like other established vasodilators, signals in VSMC mainly (if not exclusively) by increasing the production of cAMP.

Exogenous calciferol (vitamin D) and vitamin D endocrine status among elderly nursing home residents in the New York City area

OBJECTIVE

To determine the role and relative importance of sources of exogenous calciferol (vitamin D) in maintaining vitamin D endocrine status in the mid-winter and early spring in a representative sample of institutionalized elderly persons in the New York City area.

DESIGN

Cross-sectional survey.

SETTING

A privately-run urban nursing home and the long-term care unit of a suburban public hospital.

PARTICIPANTS

Residents aged 60 years and older scheduled for a routine annual physical examination and an additional group of individuals ascertained by random sampling. Those with a history of anti-convulsant or glucocorticoid use, liver disease, chronic renal disease (or serum creatinine > 1.5 mg/dL), parathyroid disease, Paget's disease, gastric surgery, or pharmacological vitamin D use were excluded. Of 301 sampled individuals, 221 were found eligible to participate, and 109 were successfully enrolled.

RESULTS

The average vitamin D intake was 379 IU/day (range 55-1006 IU/day) and total vitamin D intake was below the Recommended Dietary Allowance in 16% of subjects. Fifty percent of total vitamin D intake came from fortified milk, and 26% came from vitamin supplements. Vitamin supplement use was not associated with low dietary intake. Among subjects taking a supplement containing 400 IU/day, none had serum calcidiol levels below 15 ng/mL, while among subjects with vitamin D intake between 200 and 400 IU/day, 46% had serum calcidiol levels below 15 ng/mL and 14% had calcidiol levels below 10 ng/mL. Vitamin D intake from non-supplement sources (but not from supplements) appeared to have a negative association with serum calcitriol levels.

CONCLUSIONS

Many nursing home residents may require vitamin supplements in order to achieve optimal levels of calciferol replacement. The choice of a vehicle for calciferol replacement may affect calcitriol levels.

Subcellular localization and partial purification of the 25-hydroxyvitamin D3 1-hydroxylation reaction in the chick myelomonocytic cell line HD-11

Hypercalcemia in human granuloma-forming diseases like sarcoidosis results from the endogenous overproduction of 1,25-dihydroxyvitamin D [1,25-(OH)2D] by disease-activated tissue macrophages. The recent identification of an immortalized chick myelomonocytic cell line, HD-11, that constitutively expresses a 25-hydroxyvitamin D (25-OHD) 1-hydroxylation reaction has alleviated dependence on studying primary macrophage cultures with no replicative potential in vitro. In these experiments we established conditions for the maximal expression of the HD-11 cell 25-OHD3-1-hydroxylation reaction and localized this activity to the mitochondrial fraction. On a per cell basis, the activity of HD-11 cell 25-OHD3 1-hydroxylation reaction was comparable to that in primary cultures of chick renal tubular epithelial cells, which express the authentic renal 25-OHD3 1-hydroxylase. Maximal product yield was achieved after incubation of HD-11 cells with 200 nM 25-OHD3 for 3 h. Although adherent monolayers possessed 3- to 4-fold more capacity for hormone production than cells in suspension, suspended cells exhibited easily detectable 25-OHD3 catalytic activity (0.58 +/- 0.08 pmol per 10(6) cells per h; +/- SEM), 50% of which remained solubilized in a sonicate of suspended cells cleared of nuclei and plasma membrane. Subcellular localization disclosed 91% of the residual activity to be concentrated in the mitochondrial subfraction. A detergent-solubilized extract of this mitochondrial subfraction contained 1.9 +/- 0.3 pmol 1,25-(OH)2D3 synthetic capacity per mg protein. The catalytic activity (1-hydroxylase activity) was concentrated 20.2-fold after chromatography on octyl-amino agarose and was associated with 0.054 nmol cytochrome P450 per mg protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II regulates parathyroid hormone-related protein expression in cultured rat aortic smooth muscle cells through transcriptional and post-transcriptional mechanisms

Parathyroid hormone-related protein (PTHrP), a tumor product responsible for malignancy-associated hypercalcemia, is also produced in many normal tissues, including vascular smooth muscle cells (SMC). As PTHrP exhibits vasodilatory properties, we postulated that other vasoactive agents may control PTHrP gene expression in SMC. Addition of angiotensin II to serum-deprived SMC resulted in a marked induction of PTHrP mRNA by 2 h, with a peak (6-10-fold) at 4-6 h. Angiotensin II effects on PTHrP gene expression were inhibited by saralasin, an angiotensin II receptor antagonist, and blocked by actinomycin D and cycloheximide, suggesting a requirement for gene transcription and protein synthesis. Nuclear run-off assays revealed a 3-fold increase in PTHrP gene transcription 1 h after angiotensin II treatment. Angiotensin II also prolonged PTHrP mRNA half-life by 2-3-fold. Angiotensin-induced PTHrP mRNA is partially dependent on cyclooxygenase products and protein kinase C activation. Other vasoconstrictor substances, including serotonin and bradykinin, also stimulated PTHrP expression, whereas the vasodilator atrial natriuretic peptide did not. Addition of recombinant PTHrP-(1-141) significantly inhibited angiotensin II-induced SMC DNA synthesis. PTHrP expression is increased by angiotensin II through transcriptional and post-transcriptional mechanisms. In addition, PTHrP modulates the effect of angiotensin II on SMC proliferation. This suggests that PTHrP acts locally in SMC, possibly to oppose the vasoactive and/or growth-promoting effects of vasoconstrictor agents such as angiotensin II.

Regulation of avian calbindin-D28K gene expression in primary chick kidney cells: importance of posttranscriptional mechanisms and calcium ion concentration

Vitamin D-dependent calcium-binding protein (calbindin-D28K), is regulated by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], and several other factors in a tissue-specific manner, but the controlling mechanisms are still poorly understood. In this study we examined the relative contributions of transcriptional and posttranscriptional mechanisms in the 1,25-(OH)2D3 control of calbindin-D28K mRNA expression in primary chick kidney cells and investigated the effect of extracellular Ca2+ on calbindin-D28K gene expression in the presence and absence of hormone. 1,25-(OH)2D3 treatment (10(-8) M) of cells grown in serum-free medium resulted in a marked 20- to 30-fold increase in calbindin-D28K mRNA peaking at 12-18 h, which then rapidly declined to basal levels by 24 h. The abrupt decline in mRNA appeared to be associated with a reduction in size of the calbindin-D28K transcripts. Nuclear run-off assays showed only a slight (1.5-fold) increase in calbindin-D28K gene transcription 2 h after 1,25-(OH)2D3, whereas parallel assays clearly demonstrated a marked (7-fold) induction in the rate of metallothionein gene transcription 2 h after treatment of chick kidney cells with 10 microM zinc. The induction of calbindin-D mRNA by 1,25-(OH)2D3 required ongoing protein synthesis, since it was blocked by cycloheximide. Calbindin-D28K mRNA was stable for 12 h in the presence of actinomycin-D in both vitamin D-deficient and 1,25-(OH)2D3-treated cells. Both basal and 1,25-(OH)2D3-induced calbindin-D28K mRNA were modulated by the extracellular Ca2+, with maximum expression occurring at 1-2 mM. We conclude that 1,25-(OH)2D3 induces kidney calbindin-D28K mRNA by producing a small increase in its transcriptional rate, which is accompanied by pronounced posttranscriptional effects(s). The striking modulation of calbindin-D28K expression by extracellular Ca2+ is consistent with a putative role for this protein in the regulation of this ion in the kidney cell.

A 7-34 analog of the parathyroid hormone-related protein has potent antagonist and partial agonist activity in vivo

The biological properties of a new synthetic analog of parathyroid hormone-related protein [PTHrP-(7-34)NH2] were examined in vivo using a well characterized thyroparathyroidectomized (TPTX) rat model. The phosphaturic and urine cyclic AMP response induced by infusion of PTHrP-(1-34)NH2 (0.16 nmol/h) was inhibited by 70% (P less than 0.01, n = 6) by co-infusion of PTHrP-(7-34)NH2 at a 10-fold molar excess (1.6 nmol/h). The 7-34 PTHrP analog also antagonized the PTHrP-(1-34)NH2-induced hypercalcemia and rises in blood 1,25-dihydroxyvitamin D concentrations. However, when infused alone at a higher dose rate (8 nmol/h), PTHrP-(7-34)NH2 displayed significant PTH agonist activity. This profile contrasts to that of [Tyr-34]bPTH-(7-34)NH2 which is comparatively less potent (10-20-fold) with respect to its antagonist activity but has no appreciable agonist activity in vivo.

Calcium, vitamin D, and parathyroid hormone status in young white and black women: association with racial differences in bone mass

The etiology of the racial disparity in bone mass and fracture rate is unknown. Since the PTH-vitamin D endocrine system is a major regulator of calcium metabolism and bone turnover, this cross-sectional study examined the relationship of radial and lumbar bone density to vitamin D metabolite and PTH concentrations and to calcium intake and excretion in 67 white and 70 black highly comparable, healthy, premenopausal women. Bone density at both radial and lumbar sites was higher in blacks than in whites. Serum 25-hydroxyvitamin D was slightly but not statistically significantly (P = 0.08), lower in blacks than in whites, but there were no racial differences in 1,25-dihydroxyvitamin D, PTH, or renal tubular maximum for reabsorption of phosphate. The mean 25-hydroxyvitamin D concentration in blacks was well within the normal range and was not associated with evidence of secondary hyperparathyroidism. There were no correlations of bone density to vitamin D or PTH concentrations. Although there were no racial differences in dietary intake of calcium and vitamin D or in sodium excretion, 24-h urinary calcium excretion was significantly lower in blacks than in whites, and calcium excretion was inversely associated with radial bone density. In contrast to previous reports, in healthy, normal weight, premenopausal black women there is no evidence of vitamin D deficiency or secondary hyperparathyroidism, suggesting that factors other than the vitamin D-PTH axis are responsible for racial differences in bone mass.

Constitutive expression of a vitamin D 1-hydroxylase in a myelomonocytic cell line: a model for studying 1,25-dihydroxyvitamin D production in vitro

The capacity of the v-myc-transformed, chicken myelomonocytic cell line HD-11 to metabolize 25-hydroxyvitamin D3 (25-OHD3) was examined. HD-11 cells produced and secreted a metabolite of 25-OHD3 that was bound with high affinity by receptor for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. On normal-phase HPLC, this metabolite cochromatographed with authentic 1,25-(OH)2D3 in both hexane- and methylene chloride-based solvent systems. The 25-OHD3 1-hydroxylation reaction was substrate saturable with a Km of 73 nM 25-OHD3 and a maximal velocity of 167 fmol per 10(6) cells per h. This reaction was inhibited by ketoconazole, a recognized inhibitor of cytochrome P450 mixed-function oxidases including the authentic, renal 25-OHD3 1-hydroxylase. On the other hand, HD-11 cell 1,25-(OH)2D3 production was not affected by the antioxidant DPPD, a known inhibitor of free radical-generated 1,25-(OH)2D3. In addition to synthesizing 1,25-(OH)2D3, this monocyte-macrophage cell line also has the potential to be a target for the hormone; HD-11 cells express high-affinity receptor for 1,25-(OH)2D3 (Kin = 0.06 nM).

Vitamin D supplementation in elderly nursing home residents increases 25(OH)D but not 1,25(OH)2D

Vitamin D metabolism in elderly individuals can be compromised by several mechanisms. We previously described reduced concentrations of 1,25-dihydroxyvitamin D [1,25(OH)2D] in 30% of elderly nursing home residents. The present study assesses the effect of vitamin D supplementation on 25-hydroxyvitamin D [25(OH)D] and 1,25(OH)2D. We performed a double-blind study in which 30 elderly nursing home residents were randomly given either 50 micrograms vitamin D or a placebo daily for 6 wk. Vitamin D metabolites, immunometrically assayed parathyroid hormone (IRMA-PTH), ionized calcium, and bone Gla hormone (BGP) were measured in serum at baseline and biweekly for 6 wk. Serum 25(OH)D concentrations increased significantly (P less than 0.0001) over the 6 wk in the treatment group but were unchanged in the placebo group. Serum 1,25(OH)2D, ionized calcium, BGP, and PTH were not significantly altered by the supplement. We conclude that vitamin D supplementation results in an increase in circulating 25(OH)D but not 1,25(OH)2D; however, the long-term effect on bone mineral metabolism remains unclear.

Nephrolithiasis and bone involvement in primary hyperparathyroidism

PURPOSE

The purpose of this study was to compare patients with primary hyperparathyroidism with and without nephrolithiasis with regard to (1) biochemical profile, and (2) presence and extent of bone involvement.

PATIENTS AND METHODS

Of 70 unselected patients enrolled in a longitudinal study on the natural history of primary hyperparathyroidism, 62 who underwent complete bone densitometry evaluation were considered. The patients had mild hypercalcemia (2.77 +/- 0.02 mmol/L), as well as elevated parathyroid hormone levels by mid-molecule, N-terminal, and immunoradiometric assays. Bone densitometry was assessed by dual-photon absorptiometry of the lumbar spine and femoral neck, and single-photon absorptiometry of the forearm.

RESULTS

Eleven of the 62 patients (18%) had nephrolithiasis. There was no difference in serum parathyroid hormone levels, calcium, phosphorus, serum 25-hydroxyvitamin D, and 1,25-dihydroxyvitamin D3 between those with and without kidney stones. Total daily urinary calcium excretion was higher among those who formed stones (8.2 +/- 1.0 mmol versus 6.1 +/- 0.4 mmol, p less than 0.05), but not when expressed per mmol of creatinine (0.72 +/- 0.07 versus 0.69 +/- 0.04). Urinary hydroxyproline was also higher in patients who formed stones (58 +/- 11 mg/24 hours versus 37 +/- 2 mg/24 hours; p less than 0.05). Hypercalciuria occurred in 39% of the entire cohort (n = 24), and in 33% (n = 17) of those without stones. Only 29% (n = 7) of those with hypercalciuria had nephrolithiasis. Calcium excretion correlated positively with serum 1,25-dihydroxyvitamin D3 (r = +0.32, p less than 0.05), and negatively with forearm bone mineral density (all patients: r = -0.34, p less than 0.05; hypercalciuric patients: r = -0.53, p less than 0.05). Circulating 1,25-dihydroxyvitamin D3 levels were elevated in a similar proportion of (1) all patients (31%, n = 19); (2) those with nephrolithiasis (27%); and (3) those without stones (31%). Bone mineral density was less than 80% of normal in 61% of patients, but forearm, femoral neck, and lumbar spine density were indistinguishable among those with and without stones.

CONCLUSIONS

Cortical bone demineralization occurs to the same extent and frequency in patients with and without nephrolithiasis, and these two subgroups share similar biochemical and bone densitometric profiles. The pathophysiologic events leading to renal and skeletal involvement in primary hyperparathyroidism may be less selective than previously believed, as evidenced by: (1) increased urinary hydroxyproline in patients with nephrolithiasis, and (2) documentation that urinary calcium excretion reflects not only vitamin D status, but bone resorption was well.

The synthetic human parathyroid hormone-related protein is inhibited by a parathyroid hormone antagonist in rats in vivo

The structure of a novel protein, parathyroid hormone-related protein (PTHrP), secreted by human tumors associated with hypercalcemia has recently been determined. Administration of a synthetic fragment of this protein in vivo reproduces features of the clinical paraneoplastic syndrome of humoral hypercalcemia of malignancy and produces biologic responses closely similar to those obtained with parathyroid hormone (PTH). A PTH antagonist designed to reversibly occupy PTH receptors inhibited major actions of the tumor peptide in vivo, including phosphaturia, urinary cAMP excretion, and increased serum ionized calcium. These studies indicate that PTHrP and PTH mediate their bioactivities through shared receptors in vivo and establish a potential specific mechanism-based approach utilizing PTH antagonists for the therapy of tumor-associated hypercalcemia.

Suppression of parathyroid hormone secretion with oral calcium in normal subjects and patients with primary hyperparathyroidism

Exquisite sensitivity of normal parathyroid glands to small changes in ambient calcium concentrations and impaired sensitivity in primary hyperparathyroidism have been shown in vitro. Using an assay for PTH that detects rapid changes in PTH secretion (N-terminal-specific RIA; normal range, less than 3-33 pg/mL), we determined PTH suppressibility in response to a standardized dose of oral calcium in normal subjects and patients with primary hyperparathyroidism. Nine normal subjects were given oral calcium (25 mg/kg), and blood was analyzed half-hourly for 3 h for calcium and N-terminal PTH (N-PTH). Serum calcium rose by 0.34 +/- 0.06 mg/dL (0.085 +/- 0.015 mmol/L), and N-PTH levels declined rapidly from 15.3 +/- 1.4 to 4.2 +/- 1.1 pg/mL (-73 +/- 6%; P less than 0.01). In six subjects N-PTH concentrations became undetectable. Nine patients with primary hyperparathyroidism were tested in the same manner. Serum calcium rose by 0.53 +/- 0.1 mg/dL (0.13 +/- 0.025 mmol/L), and N-PTH levels declined less, from 66 +/- 14 to 52 +/- 12 pg/mL (-21 +/- 4%; P less than 0.05). In none of the patients was the PTH reduced to less than 20 pg/mL. These results illustrate in vivo that the PTH response to oral calcium in primary hyperparathyroidism is markedly different from that in normal subjects.

Useful clinical assays for vitamin D metabolites

Advances in our understanding of the physiology of vitamin D and its importance in health and disease have depended on the accurate measurement of its metabolites in blood. Assays that were once cumbersome and insensitive are now performed easily, are highly sensitive, reproducible, and relatively inexpensive. The availability of these modern techniques has facilitated the clinical diagnosis and treatment of patients.

Extracellular calcium modulates vitamin D-dependent calbindin-D28K gene expression in chick kidney cells

The effect of extracellular calcium ion (Ca2+) concentration on 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-induction of vitamin D-dependent calcium-binding protein (calbindin-D28K) and its mRNA levels was examined in primary chick kidney cells in vitro. When exposed to normal medium Ca2+ (1.0 mM), 1,25-(OH)2D3 increased calbindin-D28K mRNA, as measured by Northern analysis, by 4-10 fold over basal levels by 12 to 24 h after addition of hormone. In the presence of 0.5 mM Ca2+, 1,25-(OH)2D3 induced calbindin-D28K mRNA by only 2 fold, whereas, when cells were exposed to 2 mM Ca2+, the induction was 10-15 fold. This calcium modulation of 1,25-(OH)2D3 induction was also observed at the level of calbindin-D28K protein concentrations as measured by radioimmunoassay. The alterations in medium Ca2+ were not associated with any change in the rate of total RNA or protein synthesis. These studies suggest that both Ca2+ and 1,25-(OH)2D3 participate in the regulation of calbindin-D28K gene expression in the kidney.

Abnormalities in parathyroid hormone secretion and 1,25-dihydroxyvitamin D3 formation in women with osteoporosis

We investigated the parathyroid hormone-1,25-dihydroxyvitamin D3 (1,25(OH)2D) axis in osteoporosis by administering phosphate to 8 postmenopausal women with osteoporosis (49 to 78 years old) and to 10 normal women matched for age (50 to 74 years). All subjects responded with a similar increase in the serum phosphorus concentration (women with osteoporosis, 1.15 +/- 0.06 to 1.79 +/- 0.09 mmol per liter; controls, 1.14 +/- 0.05 to 1.73 +/- 0.08 mmol per liter) and a fall in the ionized calcium concentration (women with osteoporosis, 1.12 +/- 0.03 to 1.06 +/- 0.03 mmol per liter; controls, 1.17 +/- 0.01 to 1.11 +/- 0.02 mmol per liter). Parathyroid hormone levels rose 2.5-fold in the control group (15.4 +/- 2.2 to 37.9 +/- 6.1 pg per milliliter) but increased by only 43 percent in the group with osteoporosis (14.8 +/- 2.8 to 21.2 +/- 4.1 pg per milliliter), an increase similar to that previously reported in young normal subjects (53 percent). In healthy older and younger subjects, the levels of 1,25(OH)2D did not change; in the subjects with osteoporosis, however, they decreased significantly (50 percent). We conclude that older women require a greater parathyroid hormone stimulus than younger women to maintain vitamin D homeostasis, because of an age-related decline in the formation of 1,25(OH)2D in response to parathyroid hormone, and that in osteoporosis the age-appropriate parathyroid hormone response to the same hypocalcemic signal is diminished. Our results are consistent with the presence of an abnormality in parathyroid hormone secretory function in osteoporosis in addition to the universal decline in 1,25(OH)2D responsiveness associated with aging.

Bone mineral density in women with type II diabetes mellitus

Bone mineral density (BMD) was assessed in 28 women with type II diabetes mellitus and compared to 207 age-matched nondiabetic women. Mean BMD, as measured by dual-photon absorptiometry, 1.12 +/- 0.3 g/cm2 (+/- SEM), was similar to the mean BMD of control subjects, 1.06 +/- 0.1 g/cm2. Only 1 of the 28 diabetic patients had a BMD less than 0.95 g/cm2 ("fracture threshold"), whereas 25% of the control subjects had a BMD below that level. When diabetic and control subjects were matched for weight as well as age, the data continued to show similar BMD among both groups. Moreover, the disparity between the proportion of weight-matched controls (25%) and diabetic subjects (1 of 28) with a BMD below the fracture threshold persisted. Among the group of 17 diabetic subjects receiving insulin, there was a positive relationship between BMD and insulin dose. There was no significant relationship between BMD, duration of diabetes, or hemoglobin Alc. Thus, women with type II diabetes are not at increased risk for diminished BMD and may be protected against bone loss.

Tissue-specific regulation of avian vitamin D-dependent calcium-binding protein 28-kDa mRNA by 1,25-dihydroxyvitamin D3

We have studied the regulation, by 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), of vitamin D-dependent calcium-binding protein (28-kDa CaBP) mRNA in chick tissues in vivo. Northern analysis of poly(A)+ RNA was carried out using, as hybridization probes, synthetic oligonucleotides complementary to chick 28-kDa CaBP mRNA. In vitamin D-deficient chicks, 28-kDa CaBP mRNA was virtually undetectable in intestine, was clearly detectable in kidney, and present at the highest levels in cerebellum. After a single intravenous dose of 500 ng of 1,25-(OH)2D3, intestinal 28-kDa CaBP mRNA levels were increased 50-fold, kidney levels were increased 4-fold, and cerebellum levels were unchanged. Increased levels of 28-kDa CaBP mRNA were appreciated 2 h after induction and were maximal at 12 h. Pretreatment of vitamin D-deficient chicks with actinomycin D had little effect on the acute phase of the 1,25-(OH)2D3 induction of 28-kDa CaBP mRNA in intestine but blunted the induction in kidney. Pretreatment with cycloheximide caused a delayed response to 1,25-(OH)2D3 in the intestine, although control (noninhibition) levels of 28-kDa CaBP mRNA were present 12 h after hormone administration. By contrast, in the kidney, cycloheximide pretreatment resulted in an increased steady-state (vitamin D-deficient) level of 28-kDa CaBP mRNA, but completely abolished the induction of 1,25-(OH)2D3. Our studies indicate that, whereas 1,25-(OH)2D3 does not regulate 28-kDa CaBP mRNA levels in the brain, the hormone modulates 28-kDa CaBP gene expression in intestine and kidney in a tissue-specific manner, by acting through both transcriptional and post-transcriptional mechanisms.

Immunocytochemical localization of the 1,25-dihydroxyvitamin D3 receptor in target cells

We have used a monoclonal antibody (9A7) against the purified avian 1,25-dihydroxyvitamin D3 receptor to develop an immunocytochemical technique for visualization of the protein in fixed tissues and cultured cells. In Bouin's-fixed, chick intestine, 1,25-dihydroxyvitamin D3 receptor-like immunoreactivity was localized mainly in nuclei of epithelial cells and was more abundant in the crypt than in the villar cells. Receptor staining was low or undetectable in liver hepatocytes but was present in nuclei of cells lining the hepatic sinusoids. In rat brain, receptor-like immunoreactivity was abundant and widely distributed, but did not always coincide with the presence of vitamin D-dependent calcium-binding protein; 1,25-dihydroxyvitamin D3 receptor was absent from cerebellar Purkinje cells that contained abundant calcium-binding protein. In disaggregated rat bone cells, receptor immunoreactivity was present in mononuclear cells including osteoblasts and fibroblasts but was absent from osteoclasts. Two separate clones of osteoblast-like, rat osteosarcoma cells, shown in previous studies to be either receptor positive (17/2.8) or negative (24.1), demonstrated nuclear immunoreactivity in exact concordance with receptor levels as determined by ligand binding. The phenomenon of hormone-induced up-regulation of receptor was visualized in receptor-positive 3T6 fibroblasts by demonstration of markedly enhanced nuclear reactivity in cells treated with 10(-7) M 1,25-dihydroxyvitamin D3 for 48 h. Our studies demonstrate the feasibility of the immunocytochemical approach to visualize the 1,25-dihydroxyvitamin D3 receptor in target tissues and show that it is predominantly a nuclear protein in the relatively unoccupied and fully activated states. Moreover, the vitamin D-dependent calcium binding is not a universal marker for 1,25-dihydroxyvitamin D3 action. Rather, our observations suggest that the expression of the 1,25-dihydroxyvitamin D3 receptor may be connected with the state of cellular differentiation.

Animal model of primary hyperparathyroidism

An experimental model of hyperparathyroidism was developed in the rat to simulate primary hyperparathyroidism in humans. In this model thyroparathyroidectomized (TPTX) or parathyroidectomized (PTX) animals were infused for 6 days with an amount of bovine synthetic parathyroid hormone (PTH)-(1-34) fragment to restore plasma calcium levels to normal (0.7 U X h-1) or with PTH at twofold (1.4 U X h-1) or threefold (2.1 U X h-1) this basal level. Animals infused with 2.1 U X h-1 of bovine PTH-(1-34) exhibited hypercalcemia, hypophosphatemia, a reduction in theoretical renal threshold for phosphate and an increase in 1,25-dihydroxyvitamin D plasma levels that were approximately threefold the control value. In addition, these animals demonstrated nephrocalcinosis and changes of bone histology that were typical of the findings in patients with primary hyperparathyroidism. In contrast, in animals infused at 1.4 U X h-1, plasma calcium, phosphate, and theoretical renal threshold for phosphate remained within normal limits, but plasma 1,25-dihydroxyvitamin D was increased above control, suggesting that increased activity of 1 alpha-hydroxylase may be the most sensitive index of increased PTH levels. This animal model permits sustained elevation of PTH plasma levels at basal or pathologically elevated levels and should provide an effective means by which to evaluate the consequences of chronic hyperparathyroidism on epithelial function, bone, and other organ systems.