Phosphorus and protein restriction and parathyroid function in chronic renal failure

Chemical evidence for the tradeoff-in-the-nephron hypothesis to explain secondary hyperparathyroidism

Background

Secondary hyperparathyroidism (SHPT) complicates advanced chronic kidney disease (CKD) and causes skeletal and other morbidity. In animal models of CKD, SHPT was prevented and reversed by reduction of dietary phosphate in proportion to GFR, but the phenomena underlying these observations are not understood. The tradeoff-in-the-nephron hypothesis states that as GFR falls, the phosphate concentration in the distal convoluted tubule ([P]_DCT]) rises, reduces the ionized calcium concentration in that segment ([Ca⁺⁺]_DCT), and thereby induces increased secretion of parathyroid hormone (PTH) to maintain normal calcium reabsorption. In patients with CKD, we previously documented correlations between [PTH] and phosphate excreted per volume of filtrate (E_P/C_cr), a surrogate for [P]_DCT. In the present investigation, we estimated [P]_DCT from physiologic considerations and measurements of phosphaturia, and sought evidence for a specific chemical phenomenon by which increased [P]_DCT could lower [Ca⁺⁺]_DCT and raise [PTH].

Methods and findings

We studied 28 patients (“CKD”) with eGFR of 14–49 mL/min/1.73m² (mean 29.9 ± 9.5) and 27 controls (“CTRL”) with eGFR > 60 mL/min/1.73m² (mean 86.2 ± 10.2). In each subject, total [Ca]_DCT and [P]_DCT were deduced from relevant laboratory data. The Joint Expert Speciation System (JESS) was used to calculate [Ca⁺⁺]_DCT and concentrations of related chemical species under the assumption that a solid phase of amorphous calcium phosphate (Ca₃(PO₄)₂ (am., s.)) could precipitate. Regressions of [PTH] on eGFR, [P]_DCT, and [Ca⁺⁺]_DCT were then examined. At filtrate pH of 6.8 and 7.0, [P]_DCT was found to be the sole determinant of [Ca⁺⁺]_DCT, and precipitation of Ca₃(PO₄)₂ (am., s.) appeared to mediate this result. At pH 6.6, total [Ca]_DCT was the principal determinant of [Ca⁺⁺]_DCT, [P]_DCT was a minor determinant, and precipitation of Ca₃(PO₄)₂ (am., s.) was predicted in no CKD and five CTRL. In CKD, at all three pH values, [PTH] varied directly with [P]_DCT and inversely with [Ca⁺⁺]_DCT, and a reduced [Ca⁺⁺]_DCT was identified at which [PTH] rose unequivocally. Relationships of [PTH] to [Ca⁺⁺]_DCT and to eGFR resembled each other closely.

Conclusions

As [P]_DCT increases, chemical speciation calculations predict reduction of [Ca⁺⁺]_DCT through precipitation of Ca₃(PO₄)₂ (am., s.). [PTH] appears to rise unequivocally if [Ca⁺⁺]_DCT falls sufficiently. These results support the tradeoff-in-the-nephron hypothesis, and they explain why proportional phosphate restriction prevented and reversed SHPT in experimental CKD. Whether equally stringent treatment can be as efficacious in humans warrants investigation.

Recent Advances in the Role of Diet in Bone and Mineral Disorders in Chronic Kidney Disease

PURPOSE OF REVIEW

Chronic kidney disease mineral and bone disease (CKD-MBD) is a common complication of kidney disease and is strongly influenced by diet. The purpose of this manuscript is to review recent advances in the role of diet in CKD-MBD over the last 5 years.

RECENT FINDINGS

Many of the recent studies examining the role of diet in CKD-MBD have focused on the adverse effects of high phosphorus consumption on bone health and metabolism. In general, the studies have shown that high phosphorus consumption worsens markers of bone and mineral metabolism but that eating a diet with a calcium to phosphorus ratio closer to 1:1 can attenuate some of these effects. Recent studies also showed that dietary counseling is efficacious for improving markers of CKD-MBD. High consumption of phosphorus aggravates CKD-MBD. Dietary counseling may ameliorate these effects, for example, by consuming diets with higher calcium to phosphorus ratios.

Acute effects of dietary phosphorus intake on markers of mineral metabolism in hemodialysis patients: post hoc analysis of a randomized crossover trial

Background

Long-term dietary phosphorus excess influences disturbances in mineral metabolism, but it is unclear how rapidly the mineral metabolism responds to short-term dietary change in dialysis populations.

Methods

This was a post hoc analysis of a randomized crossover trial that evaluated the short-term effects of low-phosphorus diets on mineral parameters in hemodialysis patients. Within a 9-day period, we obtained a total of 4 repeated measurements for each participant regarding dietary intake parameters, including calorie, phosphorus, and calcium intake, and markers of mineral metabolism, including phosphate, calcium, intact parathyroid hormone (iPTH), intact fibroblast growth factor 23 (iFGF23), and C-terminal fibroblast growth factor 23 (cFGF23). The correlations between dietary phosphorus intake and serum mineral parameters were assessed by using mixed-effects models.

Results

Thirty-four patients were analyzed. In the fully adjusted model, we found that an increase in dietary phosphorus intake of 100 mg was associated with an increase in serum phosphate of 0.3 mg/dL (95% confidence intervals [CI], 0.2–0.4, p < .001), a decrease in serum calcium of 0.06 mg/dL (95% CI, −0.11 to −0.01, p = .01), an increase in iPTH of 5.4% (95% CI, 1.4–9.3, p = .01), and an increase in iFGF23 of 5.0% (95% CI, 2.0–8.0, p = .001). Dietary phosphorus intake was not related to cFGF23.

Conclusions

Increased dietary phosphorus intake acutely increases serum phosphate, iPTH, and iFGF23 levels and decreases serum calcium levels, highlighting the important role of daily fluctuations of dietary habits in disturbed mineral homeostasis in hemodialysis patients.

Advances in the Detection, Mechanism and Therapy of Chronic Kidney Disease

Chronic Kidney Disease (CKD) is characterized by the gradual loss of renal mass and functions. It has become a global health problem, with hundreds of millions of people being affected. Both its incidence and prevalence are increasing over time. More than $20,000 are spent on each patient per year. The economic burden on the patients, as well as the society, is heavy and their life quality worsen over time. However, there are still limited effective therapeutic strategies for CKD. Patients mainly rely on dialysis and renal transplantation, which cannot prevent all the complications of CKD. Great efforts are needed in understanding the nature of CKD progression as well as developing effective therapeutic methods, including pharmacological agents. This paper reviews three aspects in the research of CKD that may show great interests to those who devote to bioanalysis, biomedicine and drug development, including important endogenous biomarkers quantification, mechanisms underlying CKD progression and current status of CKD therapy.

Strategies for Phosphate Control in Patients With CKD

Hyperphosphatemia is a common complication in patients with chronic kidney disease (CKD), particularly in those requiring renal replacement therapy. The importance of controlling serum phosphate has long been recognized based on observational epidemiological studies that linked increased phosphate levels to adverse outcomes and higher mortality risk. Experimental data further supported the role of phosphate in the development of bone and cardiovascular diseases. Recent advances in our understanding of the mechanisms involved in phosphate homeostasis have made it clear that the serum phosphate concentration depends on a complex interplay among the kidneys, intestinal tract, and bone, and is tightly regulated by a complex endocrine system. Moreover, the source of dietary phosphate and the use of phosphate-based additives in industrialized foods are additional factors that are of particular importance in CKD. Not surprisingly, the management of hyperphosphatemia is difficult, and, despite a multifaceted approach, it remains unsuccessful in many patients. An additional issue is the fact that the supposedly beneficial effect of phosphate lowering on hard clinical outcomes in interventional trials is a matter of ongoing debate. In this review, we discuss currently available treatment approaches for controlling hyperphosphatemia, including dietary phosphate restriction, reduction of intestinal phosphate absorption, phosphate removal by dialysis, and management of renal osteodystrophy, with particular focus on practical challenges and limitations, and on potential benefits and harms.

Tradeoff-in-the-Nephron: A Theory to Explain the Primacy of Phosphate in the Pathogenesis of Secondary Hyperparathyroidism

Chronic kidney disease (CKD) causes secondary hyperparathyroidism (SHPT). The cardinal features of SHPT are persistence of normocalcemia as CKD progresses and dependence of the parathyroid hormone concentration ([PTH]) on phosphate influx (IP). The tradeoff-in-the-nephron hypothesis integrates these features. It states that as the glomerular filtration rate (GFR) falls, the phosphate concentration ([P]CDN) rises in the cortical distal nephron, the calcium concentration ([Ca]CDN) in that segment falls, and [PTH] rises to maintain normal calcium reabsorption per volume of filtrate (TRCa/GFR). In a clinical study, we set GFR equal to creatinine clearance (Ccr) and IP equal to the urinary excretion rate of phosphorus (EP). We employed EP/Ccr as a surrogate for [P]CDN. We showed that TRCa/Ccr was high in patients with primary hyperparathyroidism (PHPT) and normal in those with SHPT despite comparably increased [PTH] in each group. In subjects with SHPT, we examined regressions of [PTH] on EP/Ccr before and after treatment with sevelamer carbonate or a placebo. All regressions were significant, and ∆[PTH] correlated with ∆EP/Ccr in each treatment cohort. We concluded that [P]CDN determines [PTH] in CKD. This inference explains the cardinal features of SHPT, much of the evidence on which other pathogenic theories are based, and many ancillary observations.

Diets for patients with chronic kidney disease, should we reconsider?

Here we revisit how dietary factors could affect the treatment of patients with complications of chronic kidney disease (CKD), bringing to the attention of the reader the most recent developments in the field. We will briefly discuss five CKD-induced complications that are substantially improved by dietary manipulation: 1) metabolic acidosis and the progression of CKD; 2) improving the diet to take advantage of the benefits of angiotensin converting enzyme inhibitors (ACEi) on slowing the progression of CKD; 3) the diet and mineral bone disorders in CKD; 4) the safety of nutritional methods utilizing dietary protein restriction; and 5) evidence that new strategies can treat the loss of lean body mass that is commonly present in patients with CKD.

Bone loss in chronic kidney disease: Quantity or quality?

Chronic kidney disease (CKD) patients experience bone loss and fracture because of a specific CKD-related systemic disorder known as CKD-mineral bone disorder (CKD-MBD). The bone turnover, mineralization, and volume (TMV) system describes the morphological bone lesions in renal osteodystrophy related to CKD-MBD. Bone turnover and bone volume are defined as high, normal, or low, and bone mineralization is classified as normal or abnormal. All types of bone histology related to TMV are responsible for both bone quantity and bone quality losses in CKD patients. This review focuses on current bone quantity and bone quality losses in CKD patients and finally discusses potential therapeutic measures.

Pathophysiology of parathyroid hyperplasia in chronic kidney disease: preclinical and clinical basis for parathyroid intervention

Secondary hyperparathyroidism is characterised by excessive secretion of parathyroid hormone and parathyroid hyperplasia, resulting in both skeletal and extraskeletal consequences. Recent basic and clinical studies have brought considerable advances in our understanding of the pathophysiology of parathyroid hyperplasia and have also provided practical therapeutic approaches, especially with regard to indications for parathyroid intervention. In this context, it is quite important to recognize the development of nodular hyperplasia, because the cells in nodular hyperplasia are usually resistant to calcitriol treatment. Patients with nodular hyperplasia should undergo parathyroid intervention including percutaneous ethanol injection therapy (PEIT). Selective PEIT of the parathyroid gland is an effective approach in which the enlarged parathyroid gland with nodular hyperplasia is 'selectively' destroyed by ethanol injection, and other glands with diffuse hyperplasia are then managed by medical therapy. With a more focused attention to applying parathyroid intervention, we can expect significant improvement in the management of secondary hyperparathyroidism in dialysis patients.

Prevention and control of phosphate retention/hyperphosphatemia in CKD-MBD: what is normal, when to start, and how to treat?

Phosphate retention and, later, hyperphosphatemia are key contributors to chronic kidney disease (CKD)-mineral and bone disorder (MBD). Phosphate homeostatic mechanisms maintain normal phosphorus levels until late-stage CKD, because of early increases in parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23). Increased serum phosphorus, and these other mineral abnormalities, individually and collectively contribute to bone disease, vascular calcification, and cardiovascular disease. Earlier phosphate control may, therefore, help reduce the early clinical consequences of CKD-MBD, and help control hyperphosphatemia and secondary hyperparathyroidism in late-stage CKD. Indeed, it is now widely accepted that achieving normal phosphorus levels is associated with distinct clinical benefits. This therapeutic goal is achievable in CKD stages 3 to 5 but more difficult in dialysis patients. Currently, phosphate control is only initiated when hyperphosphatemia occurs, but a potentially beneficial and simple approach may be to intervene earlier, for example, when tubular phosphate reabsorption is substantially diminished. Early CKD-MBD management includes dietary phosphate restriction, phosphate binder therapy, and vitamin D supplementation. Directly treating phosphorus may be the most beneficial approach because this can reduce serum phosphorus, PTH, and FGF-23. This involves dietary measures, but these are not always sufficient, and it can be more effective to also consider phosphate binder use. Vitamin D sterols can improve vitamin D deficiency and PTH levels but may worsen phosphate retention and increase FGF-23 levels, and thus, may also require concomitant phosphate binder therapy. This article discusses when and how to optimize phosphate control to provide the best clinical outcomes in CKD-MBD patients.

Prescribed dietary phosphate restriction and survival among hemodialysis patients

BACKGROUND AND OBJECTIVES

Hyperphosphatemia is common among hemodialysis patients. Although prescribed dietary phosphate restriction is a recommended therapy, little is known about the long-term effects on survival.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We conducted a post hoc analysis of data from the Hemodialysis Study (n = 1751). Prescribed dietary phosphate was recorded at baseline and annually thereafter. Marginal structural proportional hazard models were fit to estimate the adjusted association between dietary phosphate restriction and mortality in the setting of time-dependent confounding.

RESULTS

At baseline, prescribed daily phosphate was restricted to levels ≤ 870, 871 to 999, 1000, 1001 to 2000 mg, and not restricted in 300, 314, 307, 297, and 533 participants, respectively. More restrictive prescribed dietary phosphate was associated with poorer indices of nutritional status on baseline analyses and a persistently greater need for nutritional supplementation but not longitudinal changes in caloric or protein intake. On marginal structural analysis, there was a stepwise trend toward greater survival with more liberal phosphate prescription, which reached statistical significance among subjects prescribed 1001 to 2000 mg/d and those with no specified phosphate restriction: hazard ratios (95% CIs) 0.73 (0.54 to 0.97) and 0.71 (0.55 to 0.92), respectively. Subgroup analysis suggested a more pronounced survival benefit of liberal dietary phosphate prescription among nonblacks, participants without hyperphosphatemia, and those not receiving activated vitamin D.

CONCLUSIONS

Prescribed dietary phosphate restriction is not associated with improved survival among prevalent hemodialysis patients, and increased level of restriction may be associated with greater mortality particularly in some subgroups.

Dietary phosphorus restriction in advanced chronic kidney disease: merits, challenges, and emerging strategies

Hyperphosphatemia is an independent risk factor for mortality in patients on maintenance dialysis. Since phosphorus clearance by standard three times-weekly dialysis is insufficient to balance ongoing dietary phosphorus intake, strategies to prevent absorption of dietary phosphorus are essential for attenuating increased serum levels. Dietary phosphorus binders are used widely for this purpose but dietary phosphorus restriction is relatively underutilized, most likely because of the logistical complexity of instituting and monitoring a low phosphorus diet, and for fear of worsening protein-energy wasting, which itself is a potent risk factor for mortality. In this review, we propose sustainable strategies for reducing phosphorus intake while avoiding exacerbation of protein-energy wasting. The approach is based on recognition of the dissociation between protein and phosphorus content in phosphorus-rich processed foods and the varying phosphorus bioavailability in different dietary sources. Controlling serum phosphate levels is among the most challenging aspects of day-to-day dialysis care but integration of sensible dietary interventions will likely improve phosphorus control.

Potential Benefits of Renal Diets on Cardiovascular Risk Factors in Chronic Kidney Disease Patients

Dietary manipulation, including protein, phosphorus, and sodium restriction, when coupled with the vegetarian nature of the renal diet and ketoacid supplementation can potentially exert a cardiovascular protective effect in chronic renal failure patients by acting on both traditional and nontraditional cardiovascular risk factors. Blood pressure control may be favored by the reduction of sodium intake and by the vegetarian nature of the diet, which is very important also for lowering serum cholesterol and improving plasma lipid profile. The low protein and phosphorus intake has a crucial role for reducing proteinuria and preventing and reversing hyperphosphatemia and secondary hyperparathyroidism, which are major causes of the vascular calcifications, cardiac damage, and mortality risk of uremic patients. The reduction of nitrogenous waste products and lowering of serum PTH levels may also help ameliorate insulin sensitivity and metabolic control in diabetic patients, as well as increase the responsiveness to erythropoietin therapy, thus allowing greater control of anemia. Protein-restricted diets may have also anti-inflammatory and anti-oxidant properties. Thus, putting aside the still debatable effects on the progression of renal disease and the more admitted effects on uremic signs and symptoms, it is possible that a proper nutritional treatment early in the course of renal disease may be useful also to reduce the cardiovascular risk in the renal patient. However, conclusive data cannot yet be drawn because quality studies are lacking in this field; future studies should be planned to assess the effect of renal diets on hard outcomes, as cardiovascular events or mortality.

Effect of sevelamer hydrochloride on bone in experimental uremic rats

Hyperphosphatemia in dialysis patients is known to cause secondary hyperparathyroidism and high-turnover bone disease. Sevelamer hydrochloride (sevelamer) is a nonabsorbed, calcium-free phosphate-binder. We determined the effect of sevelamer on parathyroid hormone (PTH)-induced high bone turnover. Rats were sham-operated or 5/6-nephrectomized (Nx) and fed a phosphate loading diet for 16 weeks or 5/6-nephrectomized and fed a phosphate loading diet for 8 weeks and then fed the same diet containing 3% sevelamer for the subsequent 8 weeks (Nx-S). Sevelamer significantly reduced serum PTH. The relative osteoid volume (OV/BV), osteoid surface (OS/BS), eroded surface (ES/BS), mineral appositional rate (MAR), volume-referent bone formation rate (BFR/TV), and bone-referent bone formation rate (BFR/BV) were measured for vertebral bone histomorphometric analysis. All parameters were statistically higher in the Nx rats than in the sham-operated control rats. The administration of sevelamer attenuated increases in OV/BV, ES/BS, BFR/TV, and BFR/BV. For femur histomorphometric analysis, the porosity area (%) (PoAr/CtAr), osteoid surface on the periosteal surface, osteoid surface on the endocortical surface (OS/Es), mineral appositional rate on the periosteal surface, mineral appositional rate on the endocortical surface, bone formation rate on the periosteal surface, and bone formation rate on the endocortical surface (Es BFR) were calculated. All parameters were higher in the Nx group than in the control group. Sevelamer inhibited the elevation of PoAr/CtAr, OS/Es, and Es BFR. Our findings suggest that the decrease in PTH by sevelamer may be beneficial in the treatment of high PTH-induced bone disease.

Phosphate and the parathyroid

The phosphate (Pi) retention in patients with chronic kidney disease leads to secondary hyperparathyroidism (2HPT). 2HPT is the physiological response of the parathyroid not only to Pi retention but also to decreased synthesis of 1,25(OH)(2) vitamin D, and the attendant hypocalcemia. 2HPT is characterized by increased PTH synthesis, secretion, and parathyroid cell proliferation. Extracellular fluid (ECF) Ca(2+) is recognized by the parathyroid calcium receptor and a small decrease in the ECF Ca(2+) results in relaxation of the calcium receptor and allows the unrestrained secretion and synthesis of PTH and in the longer term, parathyroid cell proliferation. Both 1,25(OH)(2) vitamin D and fibroblast growth factor 23 inhibit PTH gene expression and secretion. Secondary hyperparathyroidism can initially be controlled by a single therapeutic intervention, such as a Pi-restricted diet, a calcimimetic, or an active vitamin D analog. In this review we discuss the mechanisms whereby Pi regulates the parathyroid. Pi has a direct effect on the parathyroid which requires intact parathyroid tissue architecture. The effect of Pi, as of Ca(2+), on PTH gene expression is post-transcriptional and involves the regulated interaction of parathyroid cytosolic proteins to a defined cis acting sequence in the PTH mRNA. Changes in serum Ca(2+) or Pi regulate the activity of trans acting interacting proteins in the parathyroid, which alters their binding to a defined 26 nucleotide cis acting instability sequence in the PTH mRNA 3'-untranslated region. The trans factors are either stabilizing or destabilizing factors and their regulated binding to the PTH cis acting element determines the PTH mRNA half-life. The responses of the parathyroid to changes in serum Pi are now being revealed but the sensing mechanisms remain a mystery.

Phosphate is a uremic toxin

Hyperphosphatemia is one of the more prevalent metabolic disturbances in kidney failure. Phosphate can be considered a uremic toxin based on the accumulation of phosphate during chronic kidney disease, the effects of phosphate on biological systems, and the adverse effects of hyperphosphatemia. The renal clearance of phosphate is maintained until later stages of chronic kidney disease, when the remaining nephrons are no longer able to excrete sufficient phosphate to offset dietary phosphate absorption. Clearance of phosphate by conventional forms of dialysis is insufficient to prevent hyperphosphatemia in most endstage kidney-disease patients. Phosphate contributes to metabolic disturbances such as hyperparathyroidism, vitamin D resistance, and hypocalemia. In combination with these and other factors, hyperphosphatemia damages many organs, including the parathyroid glands, bones, and most importantly the cardiovascular system. Elevated phosphorus is associated with arterial and valvular calcification, arteriosclerosis, and an increased risk of cardiovascular death. Importantly, the adverse effects of hyperphosphatemia are partially preventable with the effective treatments available today.

Protein restriction for children with chronic renal failure

BACKGROUND

Protein restriction has been prescribed for some time for patients with chronic kidney disease (CKD). The effect of a low protein diet on delaying the progression to end-stage kidney disease (ESKD) in children and its impact on growth and nutrition remains unresolved.

OBJECTIVES

To determine the efficacy of protein restricted diet in delaying the start of maintenance dialysis and maintaining nutrition in children.

SEARCH STRATEGY

Cochrane Renal group trials register, the Cochrane Central register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, reference lists of nephrology textbooks, review articles and relevant trials.

SELECTION CRITERIA

Randomised control trials (RCTs) comparing un-restricted protein diet versus a protein restricted diet (to safe amounts recommended by WHO), with follow-up period of at least six months were included.

DATA COLLECTION AND ANALYSIS

Two authors independently selected studies. Statistical analysis were performed using the random effects model. For dichotomous outcome results are expressed as relative risk (RR) with 95% confidence intervals (CI). Where continuous scales of measurement are used to assess the effects of treatment, the mean difference (WMD) was used.

MAIN RESULTS

Two studies (250 children) were identified, 124 received a protein restricted diet and 126 a control diet. No significant differences was found in the number of renal deaths (RR 1.12, 95% CI 0.54 to 2.33), progression of kidney disease (creatinine clearance at two years: WMD 1.47, 95% CI -1.19 to 4.14) or growth (weight - WMD -0.13, 95% CI -1.10 to 0.84; height - WMD -1.99, 95% CI -4.84 to 0.86).

AUTHORS' CONCLUSIONS

Reducing protein intake does not appear to have significant impact in delaying the progression to ESKD in children.

Effect of L-carnitine supplementation on secondary hyperparathyroidism and bone metabolism in hemodialyzed patients

The aim of our work was to test the influence of L-carnitine supplementation on secondary hyperparathyroidism and bone metabolism in hemodialyzed patients in a randomized study. Eighty-three chronically hemodialyzed patients were observed; 44 were supplemented with L-carnitine (15 mg/kg intravenously after each hemodialysis for 6 months), while 39 took placebo. Levels of free carnitine (CAR), calcium (Ca), inorganic phosphate (P), Ca x P product, parathormone (PTH), bone-specific alkaline phosphatase (b-ALP), osteocalcin (OC), and osteoprotegerin (OPG) were monitored. In comparison with pretreatment values, changes of some selected parameters occurred in the supplemented patients after 6 months (data are expressed as medians; NS, nonsignificant change): PTH, 186.0 vs. 135.5 ng/L (NS); b-ALP, 13.9 vs. 13.2 microg/L (P < 0.05); OC, 78.3 vs. 68.8 microg/L (NS); OPG, 144.0 vs. 182.0 ng/L (P < 0.05). In the controls, there were the following changes: PTH, 148.0 vs. 207.0 ng/L (NS); b-ALP, 15.2 vs. 13.2 microg/L (P < 0.05); OC, 62.7 vs. 79.8 microg/L (P < 0.05); OPG, 140.0 vs. 164.0 ng/L (NS). A significant correlation was found between CAR and OPG changes (r = 0.51, P < 0.001) in the supplemented patients. The supplementation led to a significant increase of serum OPG concentration. Nevertheless, we observed only nonsignificant tendencies to correction of secondary hyperparathyroidism and reduction of bone turnover in hemodialyzed patients supplemented with L-carnitine in contrast to controls. At this point, the use of L-carnitine does not seem to be justified.

Drug Insight: renal indications of calcimimetics

Calcimimetics suppress the secretion of parathyroid hormone by sensitizing the parathyroid calcium receptor to serum calcium. Cinacalcet (Sensipar/Mimpara), Amgen Inc., Thousand Oaks, CA), the first-in-class calcimimetic agent approved for treatment of secondary hyperparathyroidism in dialysis patients, is, in association with higher dose of a calcium-based oral phosphate binder, a well-tolerated and effective alternative to standard treatments such as vitamin D derivatives in association with a non-calcium-based oral phosphate binder. Here, we present an overview of evidence in support of this assertion. We extend our discussion to encompass other indications for calcimimetics -- secondary hyperparathyroidism in predialysis chronic kidney disease patients, hypercalcemic hyperparathyroidism in renal transplant recipients, primary hyperparathyroidism, and hypercalcemia associated with parathyroid carcinoma -- as well as providing guidance on optimal usage of this drug.

Dietary protein restriction benefits patients with chronic kidney disease (Review Article)

The prevalence of chronic kidney disease (CKD) is rapidly increasing so every strategy should be used to avoid the complications of CKD. Most CKD symptoms or uraemia are caused by protein intolerance; symptoms arise because the patient is unable to excrete metabolic products of dietary protein and the ions contained in protein-rich foods. Consequently, CKD patients accumulate salt, phosphates, uric acid and many nitrogen-containing metabolic products, and secondary problems of metabolic acidosis, bone disease and insulin resistance become prominent. These problems can be avoided with dietary planning. Protein-restricted diets do not produce malnutrition and with these diets even patients with advanced CKD maintain body weight, serum albumin and normal electrolyte values. Non-compliance is a problem, but this can be detected using standard techniques to provide the patient with appropriate responses. The role of dietary protein restriction in the progression of CKD has not been proven, but it can reduce albuminuria and will prevent uraemic symptoms. Until a means of preventing kidney disease or progression is found, safe methods of management such as dietary manipulation should be available for CKD patients.

Uremic toxins: a new focus on an old subject

The uremic syndrome is characterized by an accumulation of uremic toxins due to inadequate kidney function. The European Uremic Toxin (EUTox) Work Group has listed 90 compounds considered to be uremic toxins. Sixty-eight have a molecular weight less than 500 Da, 12 exceed 12,000 Da, and 10 have a molecular weight between 500 and 12,000 Da. Twenty-five solutes (28%) are protein bound. The kinetics of urea removal is not representative of other molecules such as protein-bound solutes or the middle molecules, making Kt/V misleading. Clearances of urea, even in well-dialyzed patients, amount to only one-sixth of physiological clearance. In contrast to native kidney function, the removal of uremic toxins in dialysis is achieved by a one-step membrane-based process and is intermittent. The resulting sawtooth plasma concentrations of uremic toxins contrast with the continuous function of native kidneys, which provides constant solute clearances and mass removal rates. Our increasing knowledge of uremic toxins will help guide future treatment strategies to remove them.

Viewpoint: How Do Calcimimetics Fit Into the Management of Parathyroid Hormone, Calcium, and Phosphate Disturbances in Dialysis Patients?

As suggested by its American brand name (Sensipar), the calcimimetic cinacalcet sensitizes the parathyroid cells to the extracellular calcium signal, suppressing parathyroid hormone (PTH) release and synthesis and preventing parathyroid cell proliferation. This primary PTH suppression decreases the release of calcium and phosphate from bone without increasing intestinal absorption of calcium and phosphate. Therefore cinacalcet decreases the risk of hypercalcemia and hyperphosphatemia in contrast to 1alpha-OH vitamin D derivatives. Compared with calcium-containing oral phosphate binder (OPB), it increases the risk of hypocalcemia and may decrease the PTH-mediated phosphaturia in predialysis patients. This justifies its combined use with calcium-containing OPB in order to prevent hypocalcemia and enhance the hypophosphatemic effect of the latter, while improving PTH suppression. The National Kidney Foundation (NKF) Kidney Disease Outcomes Quality Initiative (K/DOQI) has recommended restriction of supplemental elemental calcium to 1.5 g/day, a recommendation that we believe should be revised. No pathophysiologic or randomized trial data have yet evidenced the absolute necessity for systematically using 1alpha-OH vitamin D derivatives and noncalcium-containing OPB rather than higher doses of calcium-containing OPB alone in uremic patients without vitamin D insufficiency. In patients with hyperparathyroidism as severe as in the "Treat to Goal Study," the Durham study showed that a calcium carbonate dose more than three times the K/DOQI limit could decrease PTH into the recommended range, with the advantage of a lower calcium-phosphate product compared with the combination of calcitriol and noncalcium OPB. Besides the efficient PTH suppression associated with lower calcium-phosphate product and a good gastrointestinal tolerance, long-term data suggest that cinacalcet may decrease the risk of parathyroidectomy and fracture, while high bone turnover lesions are improved. However, no long-term data on bone mineral density and cardiovascular calcification and complications are yet available. Such studies, along with those comparing cinacalcet and 1alpha-OH vitamin D-based approaches to hyperparathyroidism, are needed.

Low water-soluble uremic toxins

The uremic syndrome is the result of the retention of solutes, which under normal conditions are cleared by the healthy kidneys. Uremic retention products are arbitrarily subdivided according to their molecular weight. Low-molecular-weight molecules are characterized by a molecular weight below 500 D. The purpose of the present publication is to review the main water soluble, nonprotein bound uremic retention solutes, together with their main toxic effects. We will consecutively discuss creatinine, glomerulopressin, the guanidines, the methylamines, myo-inositol, oxalate, phenylacetyl-glutamine, phosphate, the polyamines, pseudouridine, the purines, the trihalomethanes, and urea per se.

Nutrition in hemodialysis patients previously on a supplemented very low protein diet

BACKGROUND

Nutritional safety of protein-restricted diets in patients with chronic renal failure is controversial. In the present study, we have assessed the evolution of nutritional status after initiation of hemodialysis in patients previously treated by a supplemented very low protein diet (SVLPD).

METHODS

Nutritional data were prospectively collected during the first year of hemodialysis from 15 consecutive patients treated with a SVLPD (0.3 g protein/kg/day supplemented with essential amino acids, calcium, iron, and vitamins) and compared to 15 age- and gender-matched end-stage renal disease (ESRD) patients previously on a less-restricted diet (0.90 +/- 0.21 g protein/kg/day) who started hemodialysis during the same period. Dual-energy x-ray absorptiometry (DEXA) was used to assess body composition at 0, 6, and 12 months. Hemodialysis prescriptions, biologic data and 3-day food records were collected every 3 months.

RESULTS

Protein intake was higher than 1.2 g/kg/day in both groups as soon as 3 months after the start of hemodialysis. Albumin and prealbumin increased significantly during the first 6 months in all patients. Body mass index (BMI) increased in all patients (+0.97 +/- 1.31 kg/m2; P < 0.001) reflecting a gain in fat mass in the overall population (+2.36 +/- 2.94 kg/m2; P < 0.001) while lean body mass remained stable overall.

CONCLUSION

Once on hemodialysis, SVLPD patients rapidly increased protein intake. Nutritional status improved in all patients, with a gain in fat mass in all, and a gain in lean body mass in SVLPD men only. These data indicate that treatment with a SVLPD prior to hemodialysis initiation is nutritionally safe.

Improved Strategies for the Treatment of Renal Osteodystrophy

Renal osteodystrophy (ROD) encompasses several distinct skeletal complications resulting from the metabolic abnormalities associated with chronic kidney disease (CKD). Other manifestations of altered calcium and phosphorus homeostasis include an increased risk of cardiovascular mortality and morbidity. Hyperphosphatemia and secondary hyperparathyroidism (HPT) are associated with these adverse events, and ultimately can lead to ROD. Dietary restriction of phosphorus, use of phosphate binding agents, and vitamin D therapy have been the mainstay of therapy for HPT. Recently, several new therapeutic agents have become available for the management of HPT. The nonelemental phosphate binder, sevelamer hydrochloride (Renagel®) has provided an alternative to calcium-containing binders for the management of hyperphosphatemia. This agent also decreases calcium load and minimizes the risk of hypercalcemia and skin and soft tissue calcifications. Research in the area of vitamin D receptors has lead to the development of vitamin D analogs including paricalcitol (Zemplar®) and doxercalciferol (Hectorol®) as alternative agents for HPT. Potential benefits of these analogs include a lower risk of hypercalcemia and hyperphosphatemia compared with calcitriol, although further evaluation is warranted as these agents are used more in clinical practice. Calcimimetics are also in the pipeline as potential agents for management of HPT by inhibition of parathyroid hormone secretion. These new developments in the management of metabolic disorders of CKD provide pharmacotherapeutic alternatives to improve patient outcomes and prevent/manage ROD. The challenge is to determine appropriate guidelines for use of these agents at all stages of CKD.

Role of dietary phosphorus in the progression of renal failure

Dietary phosphorus is thought to be a factor that impairs the residual renal function in patients with chronic renal failure. To determine the effect of dietary phosphorus on the prognosis of chronic renal failure, low-phosphorus milk was prepared from normal cow's milk using boehmite, a synthetic phosphate-ion absorbent. Regular diet, normal cow's milk, and low-phosphorus milk were then given to 5/6-nephrectomized rats and the serum levels of inorganic phosphorus, calcium, creatinine, and blood urine nitrogen in the rats in each group were compared. The serum levels of inorganic phosphorus and calcium were not different among the groups, despite a significant difference in phosphorus intakes. On the other hand, serum levels of creatinine (Cr) and blood urine nitrogen (BUN) in the rats fed low-phosphorus milk were significantly lower (Cr, 0.54+/-0.054mg/dl; BUN, 29.2+/-3.90mg/dl) than those in the rats fed a regular diet (Cr, 0.64+/-0.057mg/dl; BUN, 37.4+/-3.55mg/dl) or normal milk (Cr, 0.61+/-0.040mg/dl; BUN, 34.5+/-3.59mg/dl). No beneficial effect of protein restriction was observed when residual renal functions in rats fed a regular diet and those fed normal milk were compared. The results suggest that dietary phosphorus plays a major role in the progression of renal failure.

Pathogenesis of parathyroid dysfunction in end-stage renal disease

The parathyroid functions to maintain normal calcium and phosphate homeostasis and is central to normal bone physiology. In end-stage renal disease (ESRD), there is a failure of these normal homeostatic mechanisms with the frequent development of secondary hyperparathyroidism, which contributes to the pathogenesis of renal bone disease. The phosphate retention of ESRD, together with the reduced serum calcium and 1,25-dihydroxycholecalciferol vitamin D(3) (1,25[OH](2)D(3)) concentrations are the known factors that determine the progression to secondary hyperparathyroidism. 1,25(OH)(2)D(3) markedly decreases parathyroid hormone (PTH) gene transcription, whereas the effects of calcium and phosphate are on PTH mRNA stability, PTH secretion, and parathyroid cell proliferation. The mechanisms of these effects are discussed in this review.

Hyperplasia of the parathyroid gland without secondary hyperparathyroidism

BACKGROUND

Low dietary phosphorus (P) prevents parathyroid gland (PTG) hyperplasia and the development of secondary hyperparathyroidism (SH) in uremic rats. The present study explores the effects of P restriction on parathyroid hormone (PTH) synthesis and secretion and PT cell growth in rats with established SH and PTG hyperplasia.

METHODS

Normal and 5/6 nephrectomized rats were fed a high P (0.8%) diet. After two weeks, the normal rats and half of the uremic rats were sacrificed (U-HP) while the remaining uremic rats were switched to a low P (0.2%) diet (U-HP-LP).

RESULTS

High dietary P induced a significant increase in serum P, PTH, and PTG weight, but not ionized calcium compared to normal animals fed the same diet (N-HP). P restriction returned serum P and PTH to normal levels by one week. In contrast, PTG size did not regress and glands remained enlarged for up to eight weeks with no evidence of apoptosis. Ribonuclease protection assay and metabolic labeling studies demonstrated similar PTH/actin mRNA ratios and 35S-labeled PTH among the three groups. Intracellular intact PTH was higher in U-HP and U-HP-LP rats compared to N-HP animals with no differences between the two uremic groups. PTG-PTH content correlated only with PTG weight, and serum PTH only with serum P. The PTG secretory response to calcium remained intact.

CONCLUSIONS

In established chief-cell hyperplasia, P restriction restores normal serum PTH levels without affecting PTG hyperplasia, PTH synthesis, PTG cytosolic PTH or the PTH secretory response to calcium, suggesting an impaired exocytosis of PTH.

Importance of low serum intact parathyroid hormone as a predictor of mortality in hemodialysis and peritoneal dialysis patients: 14 years of prospective observation

Excess parathyroid hormone (PTH) has long been considered detrimental to the health of patients with end-stage renal disease. PTH has been implicated as a multisystem uremic toxin, and hyperparathyroidism can be a debilitating complication in dialyzed patients. We have studied prospectively the relationship of enrollment serum intact PTH and various demographic characteristics and other biochemical parameters to all-cause mortality in 345 hemodialysis (HD) and 277 peritoneal dialysis (PD) patients. We monitored the patients for 14 years. Observed survival and survival after adjustment for age, race, gender, months on dialysis at enrollment, diabetic status, and nutritional markers were significantly better for patients with enrollment PTH greater than 200 pg/mL than for patients with PTH 65 to 199 pg/mL and patients with PTH less than 65 pg/mL. Enrollment serum PTH was an independent predictor of survival in HD and PD patients. For HD patients, age and months on HD at enrollment were associated inversely with PTH level, whereas black race, creatinine, and phosphorus were associated directly with PTH. For PD patients, age, diabetes, and months on PD at enrollment were inverse predictors, whereas black race, albumin, creatinine, and phosphorus were associated positively with PTH. Lower than expected levels of PTH in uremic patients is associated with increased mortality. We hypothesize that inadequate protein intake or phosphorus intake or both result in impaired development of the expected secondary hyperparathyroidism and in the excess mortality risk inherent with malnutrition.

Phosphate, the renal tubule, and the musculoskeletal system

A component of ATP, phosphate is at the hub of the energy-related mechanisms operative in muscle cells. Together with calcium, phosphate is involved in bone tissue mineralization: thus, a chronic alteration in the metabolism of phosphate can induce bone and joint disorders. Diagnosis of chronic hypophosphatemia. Serum phosphate, calcium, and creatinine should be assayed simultaneously. Serum calcium is increased in hypophosphatemia caused by hyperparathyroidism and decreased in osteomalacia. Urinary phosphate excretion should be measured in patients with a normal serum calcium level and a serum phosphate level lower than 0.80 mmol/L. A decrease in urinary phosphate excretion to less than 10 mmol/24 h strongly suggests a gastrointestinal disorder, such as malabsorption, antacid use, or chronic alcohol abuse. In patients with a urinary phosphate excretion greater than 20 mmol/24 h, the maximal rate of tubular reabsorption of phosphate (TmPO4) and the ratio of TmPO4 over glomerular filtration rate (GFR) should be determined to look for phosphate diabetes. Manifestations and causes of phosphate diabetes in adults. Moderately severe phosphate diabetes in adults manifests as chronic fatigue, depression, spinal pain, and polyarthralgia, with osteoporosis ascribable to increased bone resorption. Although many cases are idiopathic, investigations should be done to look for X-linked vitamin D-resistant rickets missed during childhood, a mesenchymatous tumor, or Fanconi's syndrome with renal wasting of phosphate, glucose, and amino acids. Management of phosphate diabetes. Phosphate supplementation and, in patients with normal urinary calcium excretion, calcitriol produce some improvement in the symptoms and increase the bone mineral density. Whether dipyramidole is clinically effective remains unclear.

Are supplemented low-protein diets nutritionally safe?

In patients with chronic renal failure (CRF), the reduction of dietary protein intake may correct uremic symptoms, slow the rate of progression of renal failure, and delay the onset on dialysis. Concerns have been made on the nutritional consequences of protein-restricted diets. Over 15 years, 239 patients were treated with a very-low-protein diet providing 0.3 g vegetable protein/kg/day supplemented (SLPD) with essential amino acids and keto analogs. Many adverse consequences of uremia were corrected by this regimen, such as metabolic acidosis, secondary hyperparathyroidism, resistance to insulin, decreased Na(+)-K(+)-ATPase activity. A joint physician-dietitian monitoring contributed to the maintenance or obtention of a satisfactory nutritional status, even in patients at risk, diabetics, patients with the nephrotic syndrome and with renal allograft chronic rejection. The outcome of these patients when treated by hemodialysis or transplantation was favorable, their nutritional status being preserved. Results from the present study and results of other studies show that SLPD can be used in patients with advanced CRF without adverse effects in carefully selected and monitored patients.

Renagel: reducing serum phosphorus in haemodialysis patients

Approximately half of all dialysis patients experience persistent hyperphosphataemia, which causes significant problems. Renagel is a calcium-free, aluminium-free phosphate binder which reduces serum phosphorus in haemodialysis patients by reducing absorption of dietary phosphorus. This article reviews the management of hyperphosphataemia and presents some Renagel clinical trial data.

Reduced parathyroid functional mass after successful kidney transplantation

BACKGROUND

Chronic uremia is responsible for secondary hyperparathyroidism (HPT II). Parathyroid secretion usually tends to normalize after kidney transplantation (KT), but the parameters of the reversibility of HPT II remain poorly defined, particularly the intrinsic mechanisms underlying the improvement of parathyroid function.

METHODS

The kinetic functional parameters of the ionized calcium (iCa)/parathormone (PTH) relationship curve were studied in 11 patients with mild to moderate HPT II one and six months after successful KT. Hypercalcemia and hypocalcemia were induced, respectively, by CaCl2 and Na2-ethylenediaminetetraacetic acid (Na2-EDTA) infusions.

RESULTS

The mean glomerular filtration rate remained stable during follow-up. Basal PTH decreased from 195 +/- 54 pg/ml before KT to 70 +/- 12 pg/ml six months later (P < 0. 005). During the tests, mean PTH levels decreased significantly between the two measured times for all iCa levels, indicating an improved parathyroid function. An analysis of the kinetic parameters of the curves showed significant decreases of the mean maximal and minimal PTH levels, respectively, from 340 +/- 91 to 220 +/- 30 pg/ml (P = 0.03) and from 25 +/- 6 to 15 +/- 5 pg/ml (P = 0.005). On the other hand, no change was noted in the parathyroid-cell calcium-sensitivity parameters (slope, set point) assessed using two different approaches, either the entire curve or the limited calcium-mediated suppression curve.

CONCLUSION

Improvement of the parathyroid function between the first and sixth months post-KT seems mainly attributable to a reduction of the parathyroid functional mass.

Effect of rate of calcium reduction and a hypocalcemic clamp on parathyroid hormone secretion: a study in dogs

BACKGROUND

The parathyroid hormone (PTH) calcium curve is used to evaluate parathyroid function in clinical studies. However, unanswered questions remain about whether PTH secretion is affected by the rate of calcium reduction and how the maximal PTH response to hypocalcemia is best determined. We performed studies in normal dogs to determine whether (a) the rate of calcium reduction affected the PTH response to hypocalcemia and (b) the reduction in PTH values during a hypocalcemic clamp from the peak PTH value observed during the nadir of hypocalcemia was due to a depletion of stored PTH.

METHODS

Fast (30 min) and slow (120 min) ethylenediamine-tetraacetic acid (EDTA) infusions were used to induce similar reductions in ionized calcium. In the fast EDTA infusion group, serum calcium was maintained at the hypocalcemic 30-minute value for an additional 90 minutes (hypocalcemic clamp). To determine whether the reduction in PTH values during the hypocalcemic clamp represented depletion of PTH stores, three subgroups were studied. Serum calcium was rapidly reduced from established hypocalcemic levels in the fast-infusion group at 30 and 60 minutes (after 30 min of a hypocalcemic clamp) and in the slow-infusion group at 120 minutes.

RESULTS

At the end of the fast and slow EDTA infusions, serum ionized calcium values were not different (0.84 +/- 0.02 vs. 0.82 +/- 0.03 mM), but PTH values were greater in the fast-infusion group (246 +/- 19 vs. 194 +/- 13 pg/ml, P < 0.05). During the hypocalcemic clamp, PTH rapidly decreased (P < 0.05) to value of approximately 60% of the peak PTH value obtained at 30 minutes. A rapid reduction in serum calcium from established hypocalcemic levels at 30 minutes did not stimulate PTH further, but also PTH values did not decrease as they did when a hypocalcemic clamp was started at 30 minutes. At 60 minutes, the reduction in serum calcium increased (P < 0.05) PTH to peak values similar to those before the hypocalcemic clamp. The reduction in serum calcium at 120 minutes in the slow EDTA infusion group increased PTH values from 224 +/- 11 to 302 +/- 30 pg/ml (P < 0.05).

CONCLUSIONS

These results suggest that (a) the reduction in PTH values during the hypocalcemic clamp may not represent a depletion of PTH stores. (b) The use of PTH values from the hypocalcemic clamp as the maximal PTH may underestimate the maximal secretory capacity of the parathyroid glands and also would change the analysis of the PTH-calcium curve, and (c) the PTH response to similar reductions in serum calcium may be less for slow than fast reductions in serum calcium.

Bone mass and dynamic parathyroid function according to bone histology in nondialyzed uremic patients after long-term protein and phosphorus restriction

One year of a very low protein diet (VLPD) can reverse secondary hyperparathyroidism in uremic patients. We studied bone histology, bone mineral density (BMD), and dynamic parathyroid function (calcium/PTH curves) in 16 nondialyzed patients with advanced renal failure who had been receiving a VLPD for a mean of 5 yr (mean protein intake, 0.34 +/- 0.12 mg/kg x day; mean phosphorus intake, 8.2 +/- 2.1 mg/kg x day) and daily supplementation with essential amino acids and their ketoanalogs (1000 IU vitamin D2 and 1-2 g calcium carbonate). Three patients exhibited a high bone formation rate (BFR), 7 patients had normal bone remodeling, and 6 patients had a low BFR, including 2 with osteomalacia and 4 with adynamic bone disease without aluminum overload. A longer diet duration and lower caloric intake were associated with low BFR. More than half of the patients exhibited moderate or severe osteoporosis at the appendicular skeleton. The t score of femur BMD explained 65% of the BFR variance. Patients with a low BFR had a dynamic parathyroid function similar to that of patients with a normal BFR, except they had a lower capacity to buffer a calcium load, whereas patients with a high BFR had a higher basal PTH/maximum PTH and a steeper calcium/PTH curve slope; the calcium set-point was identical in the three groups.

Predictors of short-term changes in serum intact parathyroid hormone levels in hemodialysis patients: role of phosphorus, calcium, and gender

Several factors have been identified as important in the pathogenesis of secondary hyperparathyroidism in end-stage renal disease, including serum calcium, phosphorus, and calcitriol. To examine the independent effects of key factors, we prospectively studied 52 new hemodialysis patients with mild secondary hyperparathyroidism (PTH, 110-670 pg/mL) treated with a standardized regimen of calcium supplements, phosphorus binders, and no vitamin D derivatives. We used simple and multivariable linear regression analysis to examine the relationship between changes in PTH (deltaPTH) levels observed over a 4-week period and various biochemical and demographic variables. By simple linear regression we found that changes in serum phosphorus (r2 = 0.31; beta = 41.6; P = 0.0001), initial phosphorus concentration (r2 = 0.15; beta = 33.4; P = 0.005), initial PTH level (r2 = 0.29; beta = 0.58; P = 0.0001), changes in serum calcium (r2 = 0.12; beta = -74.0; P = 0.01), and gender (r2 = 0.07; beta = 76.1; P = 0.05) were significantly associated with deltaPTH. However, upon multivariable regression analysis, only the changes in phosphorus (partial r2 = 0.31; beta = 37.0; P = 0.0001), initial PTH level (partial r2 = 0.23; beta = 0.50; P = 0.0001), and gender (partial r2 = 0.05; beta = 63.1; P = 0.02) remained significantly associated with deltaPTH. Neither the serum concentration of 1,25-dihydroxyvitamin D3, bicarbonate, aluminum, or albumin nor changes in the serum bicarbonate concentration, the presence of diabetes, KT/V, or age were significantly associated with the deltaPTH. Our findings are consistent with independent effects of phosphorus and gender on parathyroid gland function in patients with dialysis-dependent renal failure through mechanisms that remain to be defined.