Structural organization and biological relevance of oscillatory parathyroid hormone secretion

The Postprandial Calcium Absorption of a Milk-Derived Calcium Permeate - The Acute RENEW Double-Blinded Randomized Controlled Cross-Over Study

BACKGROUND

Studies suggest that dairy-derived calcium supplements have additional beneficial properties compared with other calcium supplements in relation to bone health.

OBJECTIVES

We investigated the postprandial calcium absorption from a milk-derived calcium permeate (CP) compared with calcium carbonate (CC).

METHODS

In this randomized double-blinded cross-over study, 10 healthy postmenopausal females (age 50-65 y) received maltodextrin (placebo), 800 mg calcium from CP or from CC provided in 6 capsules on separate days. A fasting blood sample was collected at baseline, 60, 120, 240, and 360 min after ingestion. At baseline and 360 min, spot-urine samples were collected. Serum-ionized calcium, intact parathyroid hormone, phosphorus, and magnesium were analyzed, as were urinary calcium, phosphorus, and magnesium. A linear mixed model was applied.

RESULTS

Serum-ionized calcium concentration after the CC supplement was higher at 240 min compared with the CP supplement [between-group difference; 95% confidence interval (CI): 0.039 mmol/L; 95% CI: 0.017-0.061; P = 0.00078]. Serum-ionized calcium concentration after the CC supplement was significantly higher than placebo at all postprandial time points except at 60 min. Urinary calcium concentration in 360 min spot urine was higher after intake of CC compared with CP [between-group difference; 95% CI: 2.47 mmol/L; 95% CI: 1.90-3.03; P = 0.0042].

CONCLUSIONS

Postprandial calcium absorption from CP was lower than that of CC, and concurrently, urinary concentration reflected increased serum appearance by CC compared with CP, highlighting different metabolic responses. The long-term and clinical implications should be studied further.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

The effect of bovine parathyroid hormone withdrawal on MC3T3-E1 cell proliferation and phosphorus metabolism

Hypocalcemia and hypophosphatemia are common complications after parathyroidectomy (PTX). Sudden removal of high circulating levels of parathyroid hormone (PTH) causes decreased osteoclastic resorption resulting in a decreased bone remodeling space. These phenomena are likely due to an increased influx of calcium and phosphorus into bone. However, there are currently no data to support this hypothesis. In this study, we found that PTX significantly reduced levels of PTH, calcium and phosphate. Compared with preoperative levels, after 1 year, postoperative PTH, calcium and phosphate levels were 295.6 ± 173.7 pg/mL (P < 0.05), 86.62 ± 15.98 mg/dL (P < 0.05) and 5.56 ± 2.03 mg/dL (P < 0.05), respectively. We investigated continuous bovine PTH administration as well as withdrawal of bovine PTH stimulation in the mouse osteoblast precursor cell line MC3T3-E1. MC3T3-E1 cells were cultured with continuous bovine PTH treatment for 20 days or with transient bovine PTH treatment for 10 days. High doses of continuous bovine PTH exposure strongly reduced cell proliferation, alkaline phosphatase activity and the number of mineralized calcium nodules. However, withdrawal of bovine PTH (100 ng/mL) significantly increased the number of mineralized calcium nodules and caused a rapid decline in calcium and phosphorus content of culture medium. In conclusion, continuous exposure to bovine PTH inhibited osteoblast differentiation and reduced the formation of mineralized nodules. However, this inhibition was removed and mineralized nodule formation resumed with withdrawal of bovine PTH. According to the results of our clinical examinations and in vitro experiments, we hypothesize that the sudden removal of high levels of PTH may cause an increased influx of calcium and phosphorus into bone after PTX.

Parathyroid hormone pulsatility: physiological and clinical aspects

Parathyroid hormone (PTH) secretion is characterized by an ultradian rhythm with tonic and pulsatile components. In healthy subjects, the majority of PTH is secreted in tonic fashion, whereas approximately 30% is secreted in low-amplitude and high-frequency bursts occurring every 10–20 min, superimposed on tonic secretion. Changes in the ultradian PTH secretion were shown to occur in patients with primary and secondary osteoporosis, with skeletal effects depending on the reciprocal modifications of pulsatile and tonic components. Indeed, pathophysiology of spontaneous PTH secretion remains an area potentially suitable to be explored, particularly in those conditions such as secondary forms of osteoporosis, in which conventional biochemical and densitometric parameters may not always give reliable diagnostic and therapeutic indications. This review will highlight the literature data supporting the hypothesis that changes of ultradian PTH secretion may be correlated with skeletal fragility in primary and secondary osteoporosis.

Conjugated linoleic acid is related to bone mineral density but does not affect parathyroid hormone in men

The relationships between conjugated linoleic acid (CLA) status, bone, body composition, and the effect of CLA on calciotropic hormones are unclear. A cross-sectional study was designed to examine the association between c9, t11 CLA status in erythrocyte membranes (RBC) and body composition. This preceded a dose-response trial investigating if c9, t11 CLA affected parathyroid hormone (PTH). It was hypothesized that (1) higher c9, t11 CLA status in RBC will be associated with a lower fat and higher bone mass and that (2) PTH will be reduced by 30% after supplementation of c9, t11 CLA. Fifty-four men (age, 19-53 years) were included in the cross-sectional analysis, of which 31 were studied in the dose-response trial and randomized to 1 of 3 groups: placebo (n = 10), 1.5 g/d (n = 11), or 3.0 g/d (n = 10) of c9, t11 CLA for 16 weeks. Men with RBC c9, t11 CLA status above the median had higher whole body bone mineral density (BMD) (1.359 ± 0.024 vs 1.287 ± 0.023 g/cm(2); P = .04) and whole body lean mass (WBL) percentage (78.8% ± 0.9% vs 75.3% ± 1.0%; P = .01), whereas body mass index (24.8 ± 0.5 kg/m(2) vs 27.3 ± 0.9 kg/m(2); P = .01) and whole body fat mass percentage (17.3% ± 0.9% vs 21.3% ± 1.1%; P = .007) were lower. In regression analysis, RBC c9, t11 CLA status accounted for a significant proportion (r(2) = 0.10) of the variation in whole body BMD (P = .03). There were no time or treatment differences among any bone or biomarkers of bone metabolism including PTH. These findings indicate that RBC c9, t11 CLA status, a reflection of long-term (~4 months) dietary CLA intake, positively relates to BMD. However, c9, t11 CLA supplementation does not appear to affect PTH in healthy men.

Calcium homeostasis may influence resting energy expenditure with effects most apparent in early pubertal girls

AIM

Perturbations in dietary and hormonal components of the calciotropic network may be mediated through the influence of calcium homoeostasis on resting energy expenditure (REE). We investigated the association of dietary and hormonal factors involved in the regulation of calcium homoeostasis with REE in girls.

METHODS

Thirty-six girls aged 7-11 years participated. REE was assessed by indirect calorimetry, and body composition, dietary intake (calcium, vitamins D and K, phosphorus) and serum hormones (PTH, osteocalcin, 25OHD) were evaluated by DXA, 24 h recall and serum assay, respectively.

RESULTS

  A positive association between vitamin K and REE and an inverse association of parathyroid hormone (PTH) with REE (p = 0.05) were observed. PTH and REE were positively related in those having normal adiposity (p = 0.03) and inversely related in those with excess adiposity (p = 0.01). The association of REE with vitamin K intake was evident in lean individuals (p = 0.001), but was null in those with excess adiposity.

CONCLUSION

Decreased calciotropic hormone levels along with increased related nutrient intakes were associated with greater REE, although these relationships differed according to adiposity. The physiologic response to the diet and subsequent energy partitioning needs to be considered in the context of puberty. In particular, regulation and signalling of the calciotropic network during pubertal maturation warrant investigation.

Interplay between local versus soluble transforming growth factor-beta and fibrin scaffolds: role of cells and impact on human mesenchymal stem cell chondrogenesis

Structural extracellular matrix molecules gain increasing attention as scaffolds for cartilage tissue engineering owing to their natural role as a growth factor repository. We recently observed that a collagen-type I/III (Col-I/III) matrix, human recombinant transforming growth factor-beta (TGF-β) protein, and fibrin hydrogel (FG) combined to a biphasic construct provided sufficient long-term TGF-β support to drive in vitro chondrogenesis of human mesenchymal stem cells (hMSC). Here we ask whether FG and Col-I/III can both retain TGF-β, describe the influence of cell seeding on TGF-β release, and compare the molecular path of hMSC chondrogenic differentiation under soluble versus local TGF-β supply. Release of growth factor from scaffolds augmented with increasing amounts of TGF-β was analyzed over 7 days and chondrogenesis was assessed over 42 days. Low TGF-β release rates from Col-I/III as opposed to higher release from FG indicated that both molecules retained TGF-β, with Col-I/III being the superior storage component. Cell seeding enhanced TGF-β retention in FG by about threefold and almost stopped release beyond 24 h. TGF-β remained bioactive and supported MSC chondrogenesis without impairing the amount of proteoglycan and collagen-type II deposition per cell and per construct compared to standard scaffold-free MSC pellets supplied with soluble TGF-β. Local TGF-β, however, mediated lower cell content, less collagen-type X relative to collagen-type II deposition and no matrix metalloproteinase-13 up-regulation. In conclusion, cells quickly halted release of local TGF-β from FG, turning FG and Col-I/III into attractive TGF-β repositories capable to drive full hMSC chondrogenesis, but via a modulated differentiation pathway. Since only part of the changes was reproduced by transient soluble TGF-β supply, release kinetics alone could not explain the molecular differences, suggesting that local TGF-β acts distinct from its soluble counterpart.

Forearm bone mineral density varies as a function of adiposity in inuit women 40-90 years of age during the vitamin D-synthesizing period

Aging Inuit women are at increased risk for low vitamin D status due to habitation at higher latitudes, darker skin, and ongoing nutrition transition. Lower serum 25-hydroxyvitamin D (25[OH]D) concentration and higher risk of fracture have been separately reported in Inuit women, with particular relevance to postmenopausal women. We evaluated vitamin D status, forearm bone mineral density (fBMD), and nutrition in Inuit women ≥40 years. Women (n = 568) were randomly selected to participate in the 2007-2008 International Polar Year Inuit Health Survey from 36 Arctic communities. fBMD was measured using peripheral dual-energy X-ray absorptiometry. Dietary intakes were derived from 24 h recall and food-frequency questionnaires. Fasting serum 25(OH)D, parathyroid hormone, and osteocalcin (OC) were measured using a LIAISON(®) automated analyzer. The weighted prevalence of women having 25(OH)D concentration below 37.5, 50, and 75 nmol/L was 7.2 %, 17.6 %, and 48.6 %, respectively, with older women having better status. The dietary density of most nutrients increased with age, as did traditional food intake. fBMD was low in 3 (1.4 %) premenopausal (Z score < -2) and 107 (29.6 %) postmenopausal (T score < -1.5) women. Regression revealed that either weight, body mass index, or percent body fat significantly predicted fBMD in premenopausal women, in addition to age and OC in postmenopausal women. Women ≥50 years have higher vitamin D status and more nutrient-dense diets than women 40-49 years. While measures of adiposity predicted fBMD in all women, additional predictors after menopause included age and bone turnover.

Impact of season and diet on vitamin D status of African American and Caucasian children

Seasonal variation of vitamin D status and adequacy of dietary vitamin D and impact of race on maintaining vitamin D sufficiency was assessed in 140 healthy 6- to 12-year-old African American (AA) and Caucasian (C) children residing in Pittsburgh, Pennsylvania during summer and winter. Vitamin D insufficiency was not rare in either group (AA vs C, summer, 17.2% vs 14.3%, nonsignificant; winter, 34.1% vs 32.5%, nonsignificant) despite a mean dietary intake of vitamin D above the American Academy of Pediatrics (AAP) recommended intake (400 IU/d; AA vs C, summer, 421 vs 456 IU/d, nonsignificant; winter, 507 vs 432 IU/d, nonsignificant). Race/season and dietary vitamin D were predictors of serum 25-hydroxyvitamin D [25(OH)D] concentrations. However, dietary vitamin D influenced 25(OH)D only in Caucasians during winter. Current AAP recommended daily intake for vitamin D is inadequate for maintaining vitamin D sufficiency in children.

Per-1 is a specific clock gene regulated by parathyroid hormone (PTH) signaling in osteoblasts and is functional for the transcriptional events induced by PTH

Per-1 is one of the clock genes and is known to regulate various biological events including bone mass determination. Parathyroid hormone is anabolic to bone while the mechanism of its action is not fully understood. Here, we examined the role of PTH on Per-1 gene expression under osteoblast specific PTH signaling. Constitutively active PTH receptor (caPPR) expressed specifically in osteoblasts in transgenic mice activates Per-1 gene expression in bone. This is specific as expression of other clock gene Bmal-1 is not affected by caPPR over-expression. Per-1 is also expressed in osteoblastic cell line. Interestingly, Per-1 expression is required for PTH signaling-induced CRE dependent transcription. This is forming a positive feed back loop in the anabolic action of PTH signaling and Per-1 in bone. These data indicate that PTH singling in osteoblasts activates Per-1 gene expression in vivo in association with its anabolic action in bone at least in part through the regulation of transcriptional events.

Circadian regulation of renal function

Urinary excretion of water and all major electrolytes exhibit robust circadian oscillations. The 24-h periodicity has been well documented for several important determinants of urine formation, including renal blood flow, glomerular filtration, tubular reabsorption, and tubular secretion. Disturbance of the renal circadian rhythms is increasingly recognized as a risk factor for hypertension, polyuria, and other diseases and may contribute to renal fibrosis. The origin of these rhythms has been attributed to the reactive response of the kidney to circadian changes in volume and/or in the composition of extracellular fluids that are entrained by rest/activity and feeding/fasting cycles. However, numerous studies have shown that most of the renal excretory rhythms persist for long periods of time, even in the absence of periodic environmental cues. These observations led to the hypothesis of the existence of a self-sustained mechanism, enabling the kidney to anticipate various predictable circadian challenges to homeostasis. The molecular basis of this mechanism remained unknown until the recent discovery of the mammalian circadian clock made of a system of autoregulatory transcriptional/translational feedback loops, which have been found in all tissues studied, including the kidney. Here, we present a review of the growing evidence showing the involvement of the molecular clock in the generation of renal excretory rhythms.

Molecular clock is involved in predictive circadian adjustment of renal function

Renal excretion of water and major electrolytes exhibits a significant circadian rhythm. This functional periodicity is believed to result, at least in part, from circadian changes in secretion/reabsorption capacities of the distal nephron and collecting ducts. Here, we studied the molecular mechanisms underlying circadian rhythms in the distal nephron segments, i.e., distal convoluted tubule (DCT) and connecting tubule (CNT) and the cortical collecting duct (CCD). Temporal expression analysis performed on microdissected mouse DCT/CNT or CCD revealed a marked circadian rhythmicity in the expression of a large number of genes crucially involved in various homeostatic functions of the kidney. This analysis also revealed that both DCT/CNT and CCD possess an intrinsic circadian timing system characterized by robust oscillations in the expression of circadian core clock genes (clock, bma11, npas2, per, cry, nr1d1) and clock-controlled Par bZip transcriptional factors dbp, hlf, and tef. The clock knockout mice or mice devoid of dbp/hlf/tef (triple knockout) exhibit significant changes in renal expression of several key regulators of water or sodium balance (vasopressin V2 receptor, aquaporin-2, aquaporin-4, alphaENaC). Functionally, the loss of clock leads to a complex phenotype characterized by partial diabetes insipidus, dysregulation of sodium excretion rhythms, and a significant decrease in blood pressure. Collectively, this study uncovers a major role of molecular clock in renal function.

Haploinsufficiency of Runx2 results in bone formation decrease and different BSP expression pattern changes in two transgenic mouse models

Runx2 has been identified as "a master gene" for the differentiation of osteoblasts and Runx2-deficient mice has demonstrated a complete absence of mature osteoblast and ossification. To further characterize the Runx2 responsive elements within the bone sialoprotein (BSP) promoter and further investigate into the role of Runx2 haploinsufficiency in osteoblast differentiation, mBSP9.0Luc mice and mBSP4.8Luc mice were crossed with Runx2-deficient mice respectively. Luciferase assay, micro CT scan, and histological analysis were performed using tissues isolated from mBSP9.0luc/Runx2+/- mice, mBSP4.8luc/Runx2+/- mice and their corresponding Runx2+/+ littermates. Alkaline phosphatase activity, mineralization assays and RT-PCR analysis using calvarial osteoblasts isolated from these transgenic mice were also performed. Luciferase assay demonstrated an early increase in luciferase expression in mBSP9.0luc/Runx2+/- mice before the expression level of luciferase dramatically decreased and turned lower than that in their control littermates in later stages. In contrast, luciferase expression in mBSP4.8luc/Runx2+/- failed to show such an early increase. Micro CT scan and histological analysis showed that BMD and trabecular bone volume were decreased and bone formation was delayed in Runx2+/- mice. Furthermore, mineralization assay and semi-quantitative RT-PCR assay demonstrated a gene-dose-dependent decrease in bone nodule formation and bone marker genes expression levels in cultured calvarial osteoblasts derived from Runx2 knockout mice. Reconstitution of Runx2-null cells with Runx2 vector partially rescued the osteoblast function defects. In conclusion, the 9.0 kb BSP promoter demonstrated a higher tissue-specific regulation of the BSP gene by Runx2 in vivo and full Runx2 gene dose is essential for osteoblast differentiation and normal bone formation.

Variation in serum and plasma PTH levels in second-generation assays in hemodialysis patients: a cross-sectional study

BACKGROUND

Previous reports show that parathyroid hormone (PTH) concentrations may vary widely depending on the assay used to assess PTH. In this cross-sectional study, we aim to determine the usefulness of standardizing blood handling for optimal interpretation of PTH in patients with chronic kidney disease.

STUDY DESIGN

Diagnostic test study.

SETTING & PARTICIPANTS

Predialysis blood was sampled in 34 long-term hemodialysis patients at a single academic medical center.

INDEX TEST

PTH was measured by using 6 different automated second-generation assays (Elecsys, Advia Centaur, LIAISON, Immulite, Architect, and Access assays), 3 blood specimen types (serum, EDTA plasma, and citrate plasma), and 2 consecutive days of measurement (after thawing and 18 hours later with samples having been let at room temperature).

REFERENCE TEST

None.

RESULTS

A mixed statistical analysis model showed that the nature of the assay (P < 0.001) and nature of the blood sample (P < 0.001) significantly influenced variability in PTH concentrations, whereas day of measurement (day 1 or 2) did not (P = 0.5). Most PTH variability was caused by observations (96.8%), then manufacturer's kit (2.5%), and last, specimen type (0.7%). PTH concentrations measured in citrate plasma were lower with every assay method used than those observed in serum or EDTA plasma. The interaction between manufacturer and specimen type was of moderate statistical significance (P = 0.04). To evaluate the potential clinical consequence of PTH measure variability, we classified patients according to Kidney Disease Outcomes Quality Initiative cutoff values (PTH < 150 pg/mL; PTH, 150 to 300 pg/mL; and PTH > 300 pg/mL). Overall, statistical classification agreement was moderate to high for comparison between assays and high to very high between different blood samples and between days of measurement. However, we found that up to 11 of 34 patients were classified in different categories with some assays (LIAISON versus Architect) and up to 7 of 34 in different categories with different blood specimen type (citrate plasma versus serum [corrected] in LIAISON assay).

LIMITATIONS

This is a cross-sectional study that used single lots of reagents. There currently is no reference method for the measurement of PTH and no recombinant PTH standard for PTH assay.

CONCLUSION

PTH variability caused by the nature of the assay and/or blood specimen type is large enough to potentially influence clinical decision making. A specified collection method therefore should be used for PTH measurements. In routine practice, we recommend serum PTH over EDTA or citrate plasma.

Skeletal remodeling in health and disease

The use of genetically manipulated mouse models, gene and protein discovery and the cataloguing of genetic mutations have each allowed us to obtain new insights into skeletal morphogenesis and remodeling. These techniques have made it possible to identify molecules that are obligatory for specific cellular functions, and to exploit these molecules for therapeutic purposes. New insights into the pathophysiology of diseases have also enabled us to understand molecular defects in a way that was not possible a decade ago. This review summarizes our current understanding of the carefully orchestrated cross-talk between cells of the bone marrow and between bone cells and the brain through which bone is constantly remodeled during adult life. It also highlights molecular aberrations that cause bone cells to become dysfunctional, as well as therapeutic options and opportunities to counteract skeletal loss.

Anabolic agents and the bone morphogenetic protein pathway

A major unmet need in the medical field today is the availability of suitable treatments for the ever-increasing incidence of osteoporosis and the treatment of bone deficit conditions. Although therapies exist which prevent bone loss, the options are extremely limited for patients once a substantial loss of skeletal bone mass has occurred. Patients who have reduced bone mass are predisposed to fractures and further morbidity. The FDA recently approved PTH (1-34) (Teriparatide) for the treatment of postmenopausal osteoporosis after both preclinical animal and clinical human studies indicated it induces bone formation. This is the only approved bone anabolic agent available but unfortunately it has limited use, it is relatively expensive and difficult to administer. Consequently, the discovery of low cost orally available bone anabolic agents is critical for the future treatment of bone loss conditions. The intricate process of bone formation is co-ordinated by the action of many different bone growth factors, some stored in bone matrix and others released into the bone microenvironment from surrounding cells. Although all these factors play important roles, the bone morphogenetic proteins (BMPs) clearly play a central role in both bone cartilage formation and repair. Recent research into the regulation of the BMP pathway has led to the discovery of a number of small molecular weight compounds as candidate bone anabolic agents. These agents may usher in a new wave of more innovative and versatile treatments for osteoporosis as well as orthopedic and dental indications.

Therapeutic use of calcimimetics

It has long been recognized that the secretion of PTH by chief cells in the parathyroid gland is regulated by extracellular ionized calcium. The molecular mechanism by which extracellular Ca2+ performs this feat was deduced by the cloning of the extracellular calcium-sensing receptor (CaSR) in 1993 in the laboratories of Brown and Hebert. The CaSR is a G protein-coupled cell surface receptor that belongs to family 3 of the GPCR superfamily. The CaSR senses the extracellular ionic activity of the divalent minerals Ca2+ and Mg2+ and translates this information, via a complex array of cellular signaling pathways, to modify cell and tissue function. Genetic studies have demonstrated that the activity of this receptor determines the steady-state plasma calcium concentration in humans by regulating key elements in the calcium homeostatic system. CaSR agonists (calcimimetics) and antagonists (calcilytics) have been identified and have provided both current and potential therapies for a variety of disorders. Calcimimetics can effectively reduce PTH secretion in all forms of hyperparathyroidism. They are likely to become a major therapy for secondary hyperparathyroidism associated with renal failure and for treatment of certain patients with primary hyperparathyroidism. On the therapeutic horizon are calcilytics that can transiently increase PTH and may prove useful in the treatment of osteoporosis.