The role of phosphorus in the development of secondary hyperparathyroidism and parathyroid cell proliferation in chronic renal failure

Activation of the Calcium Receptor by Calcimimetic Agents Is Preserved Despite Modest Attenuating Effects of Hyperphosphatemia

BACKGROUND

Phosphorus levels in the range seen clinically among patients undergoing dialysis have been reported to attenuate calcium receptor activation and modify parathyroid hormone (PTH) release from isolated parathyroid glands in vitro. Some clinicians and providers of dialysis thus have suggested that calcimimetic agents are ineffective and should not be used to manage secondary hyperparathyroidism among those undergoing dialysis when serum phosphorus concentrations exceed certain threshold levels.

METHODS

To determine whether hyperphosphatemia diminishes the therapeutic response to calcimimetic agents, we used data from large clinical trials to analyze the effects of etelcalcetide and cinacalcet to lower plasma PTH levels in individuals on hemodialysis who had secondary hyperparathyroidism and varying degrees of hyperphosphatemia.

RESULTS

Plasma PTH levels declined progressively during 26 weeks of treatment with either etelcalcetide or cinacalcet without regard to the degree of hyperphosphatemia at baseline. However, with each calcimimetic agent, the decreases in PTH from baseline were less at each interval of follow-up during the trials among participants with serum phosphorus levels above one of three prespecified threshold values compared with those with serum phosphorus levels below these thresholds.

CONCLUSIONS

These in vivo findings are the first in humans to support the idea that hyperphosphatemia attenuates calcium receptor activation by calcium ions and by calcimimetic agents. The effect of hyperphosphatemia on the responsiveness to calcimimetic agents appears relatively modest, however, and unlikely to be significant therapeutically. The efficacy of treatment with calcimimetic agents for lowering plasma PTH levels among those with secondary hyperparathyroidism remains robust despite substantial elevations in serum phosphorus.

New Insights to the Crosstalk between Vascular and Bone Tissue in Chronic Kidney Disease-Mineral and Bone Disorder

Vasculature plays a key role in bone development and the maintenance of bone tissue throughout life. The two organ systems are not only linked in normal physiology, but also in pathophysiological conditions. The chronic kidney disease–mineral and bone disorder (CKD-MBD) is still the most serious complication to CKD, resulting in increased morbidity and mortality. Current treatment therapies aimed at the phosphate retention and parathyroid hormone disturbances fail to reduce the high cardiovascular mortality in CKD patients, underlining the importance of other factors in the complex syndrome. This review will focus on vascular disease and its interplay with bone disorders in CKD. It will present the very late data showing a direct effect of vascular calcification on bone metabolism, indicating a vascular-bone tissue crosstalk in CKD. The calcified vasculature not only suffers from the systemic effects of CKD but seems to be an active player in the CKD-MBD syndrome impairing bone metabolism and might be a novel target for treatment and prevention.

A mathematical model of parathyroid gland biology

Altered parathyroid gland biology in patients with chronic kidney disease (CKD) is a major contributor to chronic kidney disease-mineral bone disorder (CKD-MBD). This disorder is associated with an increased risk of bone disorders, vascular calcification, and cardiovascular events. Parathyroid hormone (PTH) secretion is primarily regulated by the ionized calcium concentration as well as the phosphate concentration in the extracellular fluid and vitamin D. The metabolic disturbances in patients with CKD lead to alterations in the parathyroid gland biology. A hallmark of CKD is secondary hyperparathyroidism, characterized by an increased production and release of PTH, reduced expression of calcium-sensing and vitamin D receptors on the surface of parathyroid cells, and hyperplasia and hypertrophy of these cells. These alterations happen on different timescales and influence each other, thereby triggering a cascade of negative and positive feedback loops in a highly complex manner. Due to this complexity, mathematical models are a useful tool to break down the patterns of the multidimensional cascade of processes enabling the detailed study of subsystems. Here, we introduce a comprehensive mathematical model that includes the major adaptive mechanisms governing the production, secretion, and degradation of PTH in patients with CKD on hemodialysis. Combined with models for medications targeting the parathyroid gland, it provides a ready-to-use tool to explore treatment strategies. While the model is of particular interest for use in hemodialysis patients with secondary hyperparathyroidism, it has the potential to be applicable to other clinical scenarios such as primary hyperparathyroidism or hypo- and hypercalcemia.

The Use of Vitamin D Metabolites and Analogues in the Treatment of Chronic Kidney Disease

Chronic kidney disease (CKD) and end-stage renal disease (ESRD) are associated with abnormalities in bone and mineral metabolism, known as CKD-bone mineral disorder. CKD and ESRD cause skeletal abnormalities characterized by hyperparathyroidism, mixed uremic osteodystrophy, osteomalacia, adynamic bone disease, and frequently enhanced vascular and ectopic calcification. Hyperparathyroidism and mixed uremic osteodystrophy are the most common manifestations due to phosphate retention, reduced concentrations of 1,25-dihydroxyvitamin D, intestinal calcium absorption, and negative calcium balance. Treatment with 1-hydroxylated vitamin D analogues is useful.

Phosphate Additive Avoidance in Chronic Kidney Disease

IN BRIEF Dietary guidelines for patients with diabetes extend beyond glycemic management to include recommendations for mitigating chronic disease risk. This review summarizes the literature suggesting that excess dietary phosphorus intake may increase the risk of skeletal and cardiovascular disease in patients who are in the early stages of chronic kidney disease (CKD) despite having normal serum phosphorus concentrations. It explores strategies for limiting dietary phosphorus, emphasizing that food additives, as a major source of highly bioavailable dietary phosphorus, may be a suitable target. Although the evidence for restricting phosphorus-based food additives in early CKD is limited, diabetes clinicians should monitor ongoing research aimed at assessing its efficacy.

Patterns in the parathyroid response to sodium bicarbonate infusion test in healthy volunteers

Background. The sodium bicarbonate infusion test evaluates the function of the parathyroid glands. The present study aims to evaluate the range of parathyroid response in healthy individuals and the potential influence of various factors. Methods. Fifty healthy volunteers were subjected to the test. Levels of vitamin D, calcium, albumin, and PTH were measured before infusion. PTH was measured at 3, 5, 10, 30, and 60 minutes after infusion. Results. A curve describing the response of parathyroids to the test was drawn. Twenty percent of the subjects had blunted PTH response. No significant difference was observed between normal and blunted responders concerning age, BMI, baseline PTH, or calcium levels. Nonetheless, there was a significant difference in vitamin D levels (P = 0.024). Interpretation. The test is easy to perform and may be used for everyday screening. It has to be clarified whether our observations are, at least partly, produced due to the presence of individuals with a constitutively blunted response or if low levels of vitamin D decrease the ability of the parathyroids to respond. Whichever the case, PTH response of normal individuals to sodium bicarbonate infusion test is more varied than previously thought and vitamin D levels influence it.

Heart failure in hypophosphatemic rickets: complications from high-dose phosphate therapy

OBJECTIVE

To report a rare case of hypophosphatemic rickets (HR) leading to extensive cardiac complications.

METHODS

We present the clinical course and autopsy findings of a patient with HR, treated with chronic phosphate-only therapy as a child, who subsequently developed tertiary hyperparathyroidism leading to extensive cardiac calcifications and complications. We also review the literature on the pathophysiology of calcifications from HR.

RESULTS

A 34-year-old man was diagnosed with HR at 4 years of age after presenting with growth delay and leg bowing. Family history was negative for the disease. He was initiated on high-dose phosphate therapy (2-6 g of elemental phosphorus/day) with sporadic calcitriol use between 4-18 years of age. For 6 years he received phosphate-only therapy. Subsequently, he developed nephrocalcinosis, heart valve calcifications, severe calcific coronary artery disease, heart block, and congestive heart failure. At a young age, he required an aortic valve replacement and a biventricular pacemaker that was subsequently upgraded to an implantable cardioverter defibrillator. Autopsy showed extensive endocardial, myocardial, and coronary artery calcifications.

CONCLUSION

Cardiac calcification is a known sequela of tertiary hyperparathyroidism when it occurs in patients with renal failure, but it is rarely seen in HR due to high phosphate therapy. Phosphate alone should never be used to treat HR; high doses, even with calcitriol, should be avoided. It is important to be cognizant of high-dose phosphate effects and to consider parathyroidectomy for autonomous function, if needed. This case emphasizes the importance of appropriate therapy, monitoring, and management of patients with HR.

Analysis of α-klotho, fibroblast growth factor-, vitamin-D and calcium-sensing receptor in 70 patients with secondary hyperparathyroidism

BACKGROUND/AIMS

Secondary hyperparathyroidism (sHPT) is known as a very common complication in patients with chronic kidney disease, and G-protein-coupled calcium-sensing receptor (CaSR), Vitamin D receptor (VDR) and Fibroblast growth factor receptor (FGFR)/Klotho complexes seem to be involved in its development.

METHODS

Hyperplastic parathyroid glands from 70 sHPT patients and normal parathyroid tissue from 7 patients were obtained during parathyroidectomy. Conventional morphological and immunohistochemical analysis of parathyroid glands was performed after dividing each slide in a 3x3 array.

RESULTS

The presence of lipocytes in the normal parathyroid gland and tissue architecture (nodal in patients with sHPT) allows for discrimination between normal parathyroid glands and parathyroid glands of patients with sHPT. Protein expression of Klotho, FGFR, CaSR and VDR was higher in the normal parathyroid glands compared to the sHPT group (p<0.001, p=0.07, p =0.01 and p=0.001). The variability of each protein expression within each tissue slide was high. Therefore correlations between the different immunohistochemical variables were analyzed for each of the nine fields and than analyzed for all patients. Using this analysis, a highly significant positive correlation could be found between the expression of FGFR and VDR (p=0.0004). Interestingly, in terms of VDR we found a shift to a more mixed nuclear/cytoplasmic staining in the HPT group compared to normal parathyroid gland cells, which showed solitary nuclear staining for VDR (p>0.05).

CONCLUSIONS

CaSR, VDR and an impaired Klotho-FGFR-axis seem to be the major players in the development of sHPT. Whether the detected correlation between FGFR and VDR and the shift to a more mixed nuclear/cytoplasmic staining of VDR will yield new insights into the pathogenesis of the disease has to be evaluated in further studies.

Effect of an oral glucose load on PTH, 250HD3, calcium, and phosphorus homeostasis in postmenopausal women

INTRODUCTION. Previous studies particularly in children and neonates have shown that serum calcium declines and parathyroid hormone (PTH) rises during an oral glucose load. However, there is not a general agreement in this regard. This study was carried out to evaluate the effects of an oral glucose load on calcium and phosphorus homeostasis in postmenopausal women along with serum insulin, PTH, and 250HD3 changes. PATIENTS AND METHODS. After an overnight fasting, an oral glucose tolerance test was performed in 50 postmenopausal women; and glucose, insulin, PTH, and D3 were measured at baseline and every 30 min during the 2 hours of the test. RESULTS. Serum glucose and insulin increased as expected and reached their peak values at 60 and 90 min, respectively. PTH and phosphorus decreased significantly and the maximum decline was observed at 30 and 120 min after glucose load (p < 0.0001), respectively. Serum calcium, magnesium, and D3 levels showed no significant changes at any time measured. Serum PTH values had a significant negative correlation with glucose and insulin values (p = 0.026 and p = 0.031, respectively). Serum D3 also correlated negatively with glucose (p = 0.002). CONCLUSION. Our study shows that an oral glucose load induced hyperglycemia/hyperinsulinemia promotes a significant decline in serum PTH and phosphorus levels without changes in calcium or 250HD3 in postmenopausal women.

New therapies: calcimimetics, phosphate binders and vitamin D receptor activators

At present, new compounds are available to treat secondary hyperparathyroidism, namely calcimimetics, novel phosphorus binders and also novel vitamin D receptor activators. Calcimimetics increase the sensitivity of the parathyroid gland to calcium through spatial configurational changes of the calcium-sensing receptor. In addition, experimental studies have demonstrated that calcimimetics also upregulate both the calcium-sensing receptor and the vitamin D receptor. They are efficacious in children, though the experience in paediatric chronic kidney disease is still limited. Sevelamer, lanthanum carbonate and magnesium iron hydroxycarbonate are novel phosphorus binders available on the market. Several studies have demonstrated their efficacy and safety up to 6 years, though costs are the main limitation for a wider use. Since almost all the experience available on the new phosphorus binders comes from its use in adults, studies on children are needed in order to confirm the efficacy and safety of these products. Other new salts and polymers are also being developed. New vitamin D receptor activators, such as paricalcitol, are as effective at suppressing parathyroid hormone (PTH) as the traditional vitamin D receptor activators used for the past two decades, but they have a better and safer profile, showing fewer calcaemic and phosphoraemic effects while preserving the desirable effects of the vitamin D receptor activators on the cardiovascular system, hypertension, inflammation and fibrosis. Their use in children with chronic kidney disease has revealed similar responses to those of adults. The novel compounds discussed in this review should facilitate and improve the management of mineral and bone disorders in children with chronic kidney disease.

Evolutionary changes of astroglia in Elasmobranchii comparing to amniotes: a study based on three immunohistochemical markers (GFAP, S-100, and glutamine synthetase)

This paper supplements former studies on elasmobranch species with an immunohistochemical investigation into glutamine synthetase and S-100 protein, in addition to GFAP, and extends its scope to the representatives of almost every group of Elasmobranchii: squalomorph sharks, galeomorph sharks, skates (Rajiformes) and rays (Torpediniformes and Myliobatifomes). More glial elements were labeled by S-100 protein, and even more so by using glutamine synthetase immunostaining than by GFAP: more astrocytes (mainly non-perivascular ones) were detected in the telencephalon of sharks, skates and rays. Only the markers S-100 and glutamine synthetase, but not GFAP, characterized the Bergmann-glia of skates and rays and astrocyte-like non-ependymal cells in Squalus acanthias. Another squalomorph shark species, Pristiophorus cirratus, however, had GFAP immunopositive astrocytes. Of all the species studied, the greatest number of GFAP positive astrocytes could be observed in Mobula japanica (order Myliobatiformes), in each major brain part. According to anatomical location, perivascular glia comprised varied types, including even a location in Mobula, which can also be found in mammals. Remnants of radial glia were found in confined areas of skates, less so in rays. In the rhombencephalon and in the spinal cord modified ependymoglia predominated in every group. In conclusion, there was no meaningful difference between the astroglial architectures of squalomorph and galeomorph sharks. The difference in the astroglial structure between sharks and batoids, however, was confined to the telencephalon and mesencephalon, and did not take place in the rhombencephalon, the latter structure being quite similar in all the species studied. The appearance of astrocytes in the relatively thin-walled shark telencephalon, however, indicates that the brain thickening promoted the preponderance of astrocytes rather than their appearance itself. Although the evolutionary changes of astroglia had some similarities in Elasmobranchii and Amniota, there was one meaningful difference: in Elasmobranchii astrocytes did not prevail in conservative brain regions as they did in the progressive brain regions.

Comparison of total parathyroidectomy without autotransplantation and without thymectomy versus total parathyroidectomy with autotransplantation and with thymectomy for secondary hyperparathyroidism: TOPAR PILOT-Trial

BACKGROUND

Secondary hyperparathyroidism (sHPT) is common in patients with chronic renal failure. Despite the initiation of new therapeutic agents, several patients will require parathyroidectomy (PTX). Total PTX with autotransplantation of parathyroid tissue (TPTX+AT) and subtotal parathyroidectomy (SPTX) are currently considered as standard surgical procedures in the treatment of sHPT. Recurrencerates after TPTX+AT or SPTX are between 10% and 12% (median follow up: 36 months). Recent retrospective studies demonstrated a lower rate of recurrent sHPT of 0-4% after PTX without autotransplantation and thymectomy (TPTX) with no higher morbidity when compared to the standard procedures. The observed superiority of TPTX is flawed due to different definitions of outcomes, varying follow up periods and different surgical treatment strategies (with and without thymectomy).

METHODS/DESIGN

Patients with sHPT (intact parathyroid hormone > 10 times above the upper limit of normal) on long term dialysis (>12 months) will be randomized either to TPTX or TPTX+AT and followed for 36 months. Outcome parameters are recurrence rates of sHPT, frequencies of reoperations due to refractory hypoparathyroidism or recurrent/persistent hyperparathyroidism, postoperative morbidity and mortality and quality of life. 50 patients per group will be randomized in order to obtain relevant frequencies of outcome parameters that will form the basis for a large scale confirmatory multicentred randomized controlled trial.

DISCUSSION

sHPT is a disease with a high incidence in patients with chronic renal failure. Even a small difference in outcomes will be of clinical relevance. To assess sufficient data about the rate of recurrent sHPT after both methods, a multicentred, randomized controlled trial (MRCT) under standardized conditions is mandatory. Due to the existing uncertainties the calculated number of patients necessary in each treatment arm (n > 4000) makes it impossible to perform this study as a confirmatory trial. Therefore estimates of different outcomes are performed using a pilot MRCT comparing 50 versus 50 randomized patients in order to establish a hypothesis that can be tested thereafter. If TPTX proves to have a lower rate of recurrent sHPT, no relevant disadvantages and no higher morbidity than TPTX+AT, current surgical practice may be changed.

TRIAL REGISTRATION

International Standard Randomized Controlled Trial Number Registration (ISRCTN86202793).

Effect of manipulating serum phosphorus with phosphate binder on circulating PTH and FGF23 in renal failure rats

Phosphorus directly controls parathyroid hormone (PTH) synthesis and secretion. Serum levels of the novel phosphate-regulating hormone, fibroblast growth factor 23 (FGF23), are positively correlated with hyperphosphatemia in patients with chronic renal insufficiency (CRI). We proposed that changes in serum PTH and FGF23 levels might be associated with changes in serum phosphorus levels caused by the phosphate binder sevelamer hydrochloride (sevelamer, i.e. crosslinked poly[allylamine hydrochloride]). Rats were fed a diet containing adenine for 4 weeks to establish CRI. Animals were then offered either a normal diet or a diet containing 1 or 3% sevelamer for 8 weeks continuously, or intermittently with sevelamer diet or a normal diet offered for alternating 2-week periods. Changes in the serum levels of phosphorus, calcium, PTH, FGF23, and 1alpha,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) were monitored over time. Adenine-treated rats developed severe CRI, with markedly elevated serum levels of phosphorus, PTH and FGF23, and reduced levels of serum 1,25(OH)(2)D(3). Continuous treatment with sevelamer suppressed these increases throughout the study period. Serum phosphorus, PTH, and FGF23 levels decreased rapidly when sevelamer treatments commenced and recovered rapidly once they were discontinued. However, the changes in serum FGF23 levels began after the onset of changes in serum phosphorus and PTH levels. In conclusion, circulating PTH, and FGF23 levels can be promptly manipulated through the control of serum phosphorus levels. Moreover, phosphate-binder treatment can effectively inhibit the elevation of serum FGF23 levels, as well as PTH levels, under conditions of CRI.

Oral Feeding Acutely Down-Regulates Serum PTH in Hemodialysis Patients

BACKGROUND

Changes in serum parathyroid hormone (PTH) within minutes are known only to be mediated by changes in ionized calcium. Recent animal studies show ingestion of a low phosphorus meal can lower serum PTH within 15 min, before changes in serum ionized calcium or phosphorus occur, suggesting a rapid gastrointestinal signal may regulate PTH.

METHODS

Eight hemodialysis patients with secondary hyperparathyroidism were admitted twice to a metabolic unit and ate a high and low phosphorus meal after an overnight fast. Serum PTH, total and ionized calcium, phosphorus, pH, and glucose were measured at 0, 15, 30, 60, 120 and 240 min. In the second protocol, we examined the possible role of volume or glucose changes in rapid PTH suppression by administering intravenous saline and glucose after an overnight fast to 6 patients, with similar testing.

RESULTS

Intact PTH decreased 24% from 419 +/- 331 at baseline to 312 +/- 221 pg/ml (p = 0.002) 15 min after a meal. Total and ionized calcium and pH did not change, glucose rose by 15 min, and phosphorus changed only after 60-90 min. During the second protocol, saline and glucose infusions failed to change PTH.

CONCLUSIONS

In dialysis patients, a glucose-containing meal, with or without phosphorus, rapidly suppresses serum PTH approximately 25% within 15 min. This effect is not mediated by changes in ionized calcium, phosphorus, pH, glucose, or insulin. These data suggest there may be an as yet unknown enteral signal that rapidly suppresses PTH.

Pharmacological and clinical properties of calcimimetics: calcium receptor activators that afford an innovative approach to controlling hyperparathyroidism

Circulating levels of calcium ion (Ca2+) are maintained within a narrow physiological range mainly by the action of parathyroid hormone (PTH) secreted from parathyroid gland (PTG) cells. PTG cells can sense small fluctuations in plasma Ca2+ levels by virtue of a cell surface Ca2+ receptor (CaR) that belongs to the superfamily of G protein-coupled receptors (GPCR). Compounds that activate the CaR and inhibit PTH secretion are termed 'calcimimetics' because they mimic or potentiate the effects of extracellular Ca2+ on PTG cell function. Preclinical studies with NPS R-568, a first generation calcimimetic compound that acts as a positive allosteric modulator of the CaR, have demonstrated that oral administration decreases serum levels of PTH and calcium, with a leftward shift in the set-point for calcium-regulated PTH secretion in normal rats. NPS R-568 also suppresses the elevation of serum PTH levels and PTG hyperplasia and can improve bone mineral density (BMD) and strength in rats with chronic renal insufficiency (CRI). Clinical trials with cinacalcet hydrochloride (cinacalcet), a compound with an improved metabolic profile, have shown that long-term treatment continues to suppress the elevation of serum levels of calcium and PTH in patients with primary hyperparathyroidism (1HPT). Furthermore, clinical trials in patients with uncontrolled secondary hyperparathyroidism (2HPT) have demonstrated that cinacalcet not only lowers serum PTH levels, but also the serum phosphorus and calcium x phosphorus product; these are a hallmark of an increased risk of cardiovascular disease and mortality in dialysis patients with end-stage renal disease. Indeed, cinacalcet has already been approved for marketing in several countries. Calcimimetic compounds like cinacalcet have great potential as an innovative medical approach to manage 1HPT and 2HPT.

Calcium agonists in hyperparathyroidism

Primary hyperparathyroidism (PHPT), in addition to cancer, represents an important cause of hypercalcaemia in the general population. Furthermore, hypercalcaemia, in the course of uraemic HPT, represents the late stage of chronic renal failure refractory to therapy. Neck surgery is still the only curative approach for these forms of HPT and medical treatment rarely exhibits an effective control on HPT and HPT-dependent hypercalcaemia. Moreover, some HPT patients may not undergo neck surgery due to the presence of other concomitant disorders. Therefore, more effective therapeutic approaches are needed than the commonly used 'palliative' treatments. The identification of a specific membrane receptor able to bind extracellular calcium on cells of the parathyroid and other tissues has allowed the development of new molecules acting through this receptor to reduce both parathyroid hormone secretion and the rate of parathyroid cell proliferation. Consequently, they may substantially contribute to the regulation of bloodstream calcium levels in HPT patients. Preliminary results obtained in clinical trials are encouraging, demonstrating a good efficacy and safety of such drugs. However, more in vitro and in vivo, as well as long-term clinical studies, will be necessary before they can be commonly used as therapeutical molecules in the clinical practice.

Laboratory screening for hyperparathyroidism

INTRODUCTION

The clinical syndrome produced by excess parathyroid hormone (PTH) is referred to as hyperparathyroidism (HPT). Autonomous growth of PTH producing cells is defined as primary hyperparathyroidism (pHPT). In its classic form pHPT is characterized by painful bones, kidney stones, abdominal groans, psychic moans, fatigue overtones and hypercalcemia. Chronic stimulation of the parathyroid glands secondary to low circulating calcium level results in secondary hyperparathyroidism (sHPT). Tertiary hyperparathyroidism (tHPT) results from prolonged secondary hyperparathyroidism when the glands take on an autonomous function manifested by hypercalcemia and high PTH levels despite resolution of the original stimulus.

REVIEW

The paper reviews the physiologic regulation of PTH secretion and types and forms of HPT. Calcium homeostasis is discussed, emphasizing interactions of PTH, PO4 and vitamin D that can lead to HPT. In addition, the paper reviews the contribution of serum calcium, chloride, phosphorus and PTH levels to the diagnosis of HPT, the role of urinary calcium in the diagnosis of familial benign hypocalciuric hypercalcemia (FBHH), and the role of alkaline phosphatase and bone mass measurements as markers of severity of hyperparathyroid bone disease.

CONCLUSIONS

It is concluded that the diagnosis of hyperparathyroidism can be made with a very high confidence rate by documenting an increased serum PTH level with an increased ionized or total calcium level in pHPT, increased serum PTH level with low or normal calcium level and an underlying renal failure or vitamin D deficiency in sHPT. Early management of HPT is important because many of the nonspecific complains, or classic symptoms, or metabolic conditions often improve after proper control of hyperparathyroidism.

A moderately low phosphate intake may provide health benefits analogous to those conferred by UV light - a further advantage of vegan diets

Although exposure to ultraviolet light is often viewed as pathogenic owing to its role in the genesis of skin cancer and skin aging, there is growing epidemiological evidence that such exposure may decrease risk for a number of more serious cancers, may have a favorable impact on blood pressure and vascular health, and may help to prevent certain autoimmune disorders - in addition to its well-known influence on bone density. Most likely, these health benefits are reflective of improved vitamin D status. Increased synthesis or intake of vitamin D can be expected to down-regulate parathyroid hormone (PTH), and to increase autocrine synthesis of its active metabolite calcitriol in certain tissues; these effects, in turn, may impact cancer risk, vascular health, immune regulation, and bone density through a variety of mechanisms. Presumably, a truly adequate supplemental intake of vitamin D - manyfold higher than the grossly inadequate current RDA - could replicate the benefits of optimal UV exposure, without however damaging the skin. Diets moderately low in bioavailable phosphate - like many vegan diets - might be expected to have a complementary impact on disease risks, inasmuch as serum phosphate suppresses renal calcitriol synthesis while up-regulating that of PTH. A proviso is that the impact of dietary phosphorus on bone health is more equivocal than that of vitamin D. Increased intakes of calcium, on the other hand, down-regulate the production of both PTH and calcitriol - the latter effect may explain why the impact of dietary calcium on cancer risk (excepting colon cancer), hypertension, and autoimmunity is not clearly positive. An overview suggests that a vegan diet supplemented with high-dose vitamin D should increase both systemic and autocrine calcitriol production while suppressing PTH secretion, and thus should represent a highly effective way to achieve the wide-ranging health protection conferred by optimal UV exposure.

Vitamin D analogs in uremia: integrating medical and nutritional issues

A system of regulatory molecules interacts at the cellular level to control and coordinate the many metabolic pathways that constitute normal mineral metabolism. Alterations that occur in uremia profoundly disrupt this intricate system of regulation. A lack of control poses serious consequences for patients with chronic renal disease, and restoring some level of control represents a significant treatment goal. To achieve adequate treatment, it is necessary to correct aberrations in the metabolism of the major regulatory molecules, parathyroid hormone, vitamin D, calcium, and phosphorus. The use of vitamin D hormone replacement therapy is one important part of this strategy, and the availability of newer vitamin D compounds may prove to be especially beneficial. The effective use of these compounds, nevertheless, depends on the coordinated efforts of each member of the health care team to design and implement an integrated treatment protocol that recognizes all aspects of intervention.

Cinacalcet HCl: a calcimimetic agent for the management of primary and secondary hyperparathyroidism

Cinacalcet HCl (AMG 073) is an investigational oral calcimimetic drug currently being evaluated for the treatment of primary and secondary hyperparathyroidism (HPT). Calcimimetics bind to the calcium-sensing receptors of the parathyroid glands and lower the sensitivity for receptor activation by extracellular calcium, thereby diminishing parathyroid hormone release. Cinacalcet HCl has demonstrated efficacy in controlling the hypercalcaemia of severe primary HPT and in reducing parathyroid hormone levels in patients with secondary HPT. Asymptomatic dose-dependent hypocalcaemia has occurred in some clinical trials. This drug has a favourable pharmacokinetic profile compared to its precursors and will prove useful as an additional/alternative agent in patients with primary and secondary HPT.

Chronic kidney disease: issues and establishing programs and clinics for improved patient outcomes

The spectrum of chronic kidney disease (CKD) extends from the point at which there is slight kidney damage, but normal function, to the point at which patients require either a renal transplant or renal replacement therapy to survive. Epidemiological studies suggest there are approximately 20,000,000 patients with various stages of CKD. These patients have many comorbidities, including cardiovascular disease, hypertension, diabetes, anemia, nutritional and metabolic derangements, and fluid overload. Unfortunately, evidence shows that current CKD care in the United States is suboptimal, and late referral to a nephrologist is often the rule and not the exception. Roles of primary care physicians (PCPs) and nephrologists in the care of patients with CKD remain undefined. Several studies have suggested that care provided by multidisciplinary nephrology teams can improve patient outcomes. Currently, there are published evidence-based clinical practice guidelines for anemia management, nutritional therapy, and vascular access placement, with other CKD guidelines under development. The intent of this review includes providing compelling evidence for earlier screening, identification, and management of patients with CKD; showing that current CKD care is suboptimal; encouraging the development of multidisciplinary teams that provide collaborative care to patients with CKD, suggesting roles for PCPs and nephrologists in the care of these patients; describing CKD initiatives from national organizations; and providing a comprehensive checklist that can guide the development of CKD clinics and programs.

Pharmacological and clinical trial data on a novel phosphate-binding polymer (sevelamer hydrochloride), a medicine for hyperphosphatemia in hemodialysis patients

Hyperphosphatemia is one of the major complications of hemodialysis patients and plays a key role in the pathogenesis of cardiovascular calcification and secondary hyperparathyroidism. Dietary phosphate restriction and removal of phosphate by dialysis are insufficient to control hyperphosphatemia. Therefore, almost all patients undergoing hemodialysis should take oral phosphate binders. Sevelamer hydrochloride (sevelamer) is a novel phosphate-binding polymer that contains neither aluminum nor calcium, and it is not absorbed from the gastrointestinal tract. In rat models with progressive chronic renal insufficiency, in addition to lowering effects on serum levels of phosphorus, calcium x phosphorus product, and parathyroid hormone, dietary treatment of sevelamer can prevent parathyroid hyperplasia, vascular calcification, high turnover bone lesion, and renal functional deterioration. In clinical studies with hemodialysis patients, sevelamer lowers serum phosphorus and calcium x phosphorus product without any incidence of hypercalcemia. Switching calcium-containing phosphate binders to sevelamer can decrease the percentage of hypoparathyroidism and hyperparathyroidism by negative calcium balance and increased dosage of vitamin D, respectively. Sevelamer also decreases serum low-density lipoprotein cholesterol levels by its bile acid-binding capacity. A long-term clinical study has demonstrated that the progression of coronary and aortic calcification in hemodialysis patients is attenuated by sevelamer. Thus, sevelamer offers the promise of impacting cardiac calcification and thereby reducing morbidity and mortality of hemodialysis patients.

Persistent downregulation of calcium-sensing receptor mRNA in rat parathyroids when severe secondary hyperparathyroidism is reversed by an isogenic kidney transplantation

Experimental severe secondary hyperparathyroidism (HPT) is reversed within 1 wk after reversal of uremia by an isogenic kidney transplantation (KT) in the uremic rats. Abnormal parathyroid hormone (PTH) secretion in uremia is related to downregulation of CaR and vitamin D receptor (VDR) in the parathyroid glands (PG). The aim of this investigation was to examine the expression of CaR and VDR genes after reversal of uremia and HPT in KT rats. 5/6 nephrectomized rats were kept on a normal or high-phosphorus (hP) diet for 8 wk to induce severe HPT (n = 8 in each group). In another group of seven uremic hP rats, uremia was reversed by an isogenic KT and PG were harvested within 1 wk posttransplant. Plasma urea, creatinine, total calcium, phosphorus, and PTH levels were measured. Parathyroid CaR and VDR mRNA were measured by quantitative PCR. Uremic hP rats had significantly elevated levels of creatinine, urea, and phosphorus (P < 0.001) and developed significant hypocalcemia (plasma calcium 1.83 +/- 0.2 mmol/L; P < 0.001) compared with normal control rats. After KT, the levels were normalized from day 3 to 7: creatinine from 0.117 +/- 0.016 to 0.050 +/- 0.002 mmol/L; urea from 23 +/- 4 to 7 +/- 0.3 mmol/L; phosphorus from 3.9 +/- 0.6 to 1.5 +/- 0.06 mmol/L; calcium from 1.8 +/- 0.2 to 2.5 +/- 0.02 mmol/L. Plasma PTH levels fell from 849 +/- 224 to a normal level of 38 +/- 9 pg/ml (P < 0.01). In uremic rats on a standard diet, CaR mRNA was similar to that of normal control rats, whereas VDR mRNA was significantly decreased. In uremic rats kept on hP diet, CaR mRNA was significantly decreased to 26 +/- 7% of control rats (P = 0.01) and VDR mRNA reduced to 36 +/- 11% (P < 0.01). In KT, previously hP uremic rats, both CaR mRNA and VDR mRNA remained severely reduced (CaR, 39 +/- 7%; VDR, 9 +/- 3%; P < 0.01) compared with normal rats. In conclusion, circulating plasma PTH levels normalized rapidly after KT, despite persisting downregulation of CaR and VDR gene expression. This indicates that upregulation of CaR mRNA and VDR mRNA is not necessary to induce the rapid normalization of PTH secretion from hyperplastic parathyroid glands.

Regulation of bone remodeling: impact of novel therapies

The kidneys serve as both an endocrine organ and as a target of endocrine action, with the aim of controlling mineral and water balance. Hormones and other key metabolites regulate mineral homeostasis by altering gene function directly or by initiating a sequence of events, leading ultimately to a change in enzyme function. Two of these hormones, parathyroid hormone (PTH) and calcitriol (the active form of vitamin D), interact in multiple tissues in the body to regulate the flux of calcium and phosphorus between extra- and intracellular compartments. Changes in the concentration of PTH and vitamin D, or the interaction of these with other factors, lead to the aberrant regulation of calcium and phosphorus. Among other effects, this aberrant regulation leads to pathologic changes in bone metabolism. The pathology of renal bone disease varies along a spectrum from disorders of low turnover to those of high turnover. This spectrum reflects the results of therapeutic intervention, hormone balances, and other causes. Effective management of renal bone disease therefore requires thorough evaluation of relevant risk factors, measurement of biochemical markers of bone remodeling, and determination of the physical status of bone tissue either by bone mineral density or bone biopsy. Subsequent therapeutic intervention with newer vitamin D compounds, novel phosphate binders, calcimimetics, and the use of alternative dialysis modalities offer hope in normalizing bone remodeling and mineral balance. The human skeleton functions in two capacities: the storage of minerals and structural support of the body. It is the only tissue that behaves as both a major source and a sink of calcium (Ca) and phosphorus (P). Healthy bone is a composite of a collagenous matrix embedded with crystals of hydroxyapatite. On the surface of bone and within the calcified matrix are specialized cells that build and maintain the tissue, and facilitate the movement of Ca and P into and out of serum. Bone undergoes remodeling in response to either damage from mechanical strain or as part of the normal cycle of bone renewal. The process involves distinct steps of cellular activation, bone resorption, and subsequent bone formation. It is a relatively slow process that takes several months, and at any one time occurs at many different sites along the bone surface. Systemic factors, such as PTH and vitamin D, regulate the resorption and formation of bone, and thus the systemic movement of Ca and P. However, during conditions of stress and disease, other factors may also play a role. In patients with chronic renal disease, the balance of Ca and P is profoundly disturbed. This disruption and the compensatory changes that occur in response alter the normal processes of bone metabolism.

Vitamin D analogs in uremia: integrating medical and nutritional issues

A system of regulatory molecules interacts at the cellular level to control and coordinate the many metabolic pathways that constitute normal mineral metabolism. Alterations that occur in uremia profoundly disrupt this intricate system of regulation. A lack of control poses serious consequences for patients with chronic renal disease, and restoring some level of control represents a significant treatment goal. To achieve adequate treatment, it is necessary to correct aberrations in the metabolism of the major regulatory molecules, parathyroid hormone, vitamin D, calcium, and phosphorus. The use of vitamin D hormone replacement therapy is one important part of this strategy, and the availability of newer vitamin D compounds may prove to be especially beneficial. The effective use of these compounds, nevertheless, depends on the coordinated efforts of each member of the health care team to design and implement an integrated treatment protocol that recognizes all aspects of intervention.

Re-evaluation of risks associated with hyperphosphatemia and hyperparathyroidism in dialysis patients: recommendations for a change in management

Hyperphosphatemia is a predictable consequence of chronic renal failure and is present in most patients on dialysis. Traditionally, the risk associated with elevated serum phosphorus has focused on its impact on renal osteodystrophy. A growing body of evidence, however, suggests that abnormalities in serum phosphorus, calcium-phosphorus product (CaxP), and parathyroid hormone (PTH) levels are resulting in vascular and visceral calcification, thereby contributing to the substantially increased risk of cardiovascular death in this population. In this analysis, we review in detail the literature that describes these associations. We show that the current treatment paradigm for serum phosphorus and secondary hyperparathyroidism is ineffective for a large segment of dialysis patients. Currently, 60% of hemodialysis patients have phosphorus greater than 5.5 mg/dL, and 40% have CaxP greater than 60 mg(2)/dL(2). It is our belief that prevention of uremic calcification, cardiac death, and vascular disease should assume primary importance when evaluating the risks associated with elevated levels of phosphorus, CaxP, and PTH. We recommend that target levels should become 9.2 to 9.6 mg/dL for calcium, 2.5 to 5.5 mg/dL for phosphorus, less than 55 mg(2)/dL(2) for CaxP product, and 100 to 200 pg/mL for intact PTH.