Polyamines in the anemia of end-stage renal disease

Metabolites Associated With Uremic Symptoms in Patients With CKD: Findings From the Chronic Renal Insufficiency Cohort (CRIC) Study

RATIONALE & OBJECTIVE

The toxins that contribute to uremic symptoms in patients with chronic kidney disease (CKD) are unknown. We sought to apply complementary statistical modeling approaches to data from untargeted plasma metabolomic profiling to identify solutes associated with uremic symptoms in patients with CKD.

STUDY DESIGN

Cross-sectional.

SETTING & PARTICIPANTS

1,761 Chronic Renal Insufficiency Cohort (CRIC) participants with CKD not treated with dialysis.

PREDICTORS

Measurement of 448 known plasma metabolites.

OUTCOMES

The uremic symptoms of fatigue, anorexia, pruritus, nausea, paresthesia, and pain were assessed by single items on the Kidney Disease Quality of Life-36 instrument.

ANALYTICAL APPROACH

Multivariable adjusted linear regression, least absolute shrinkage and selection operator linear regression, and random forest models were used to identify metabolites associated with symptom severity. After adjustment for multiple comparisons, metabolites selected in at least 2 of the 3 modeling approaches were deemed "overall significant."

RESULTS

Participant mean estimated glomerular filtration rate was 43mL/min/1.73m2, with 44% self-identifying as female and 41% as non-Hispanic Black. The prevalence of uremic symptoms ranged from 22% to 55%. We identified 17 metabolites for which a higher level was associated with greater severity of at least one uremic symptom and 9 metabolites inversely associated with uremic symptom severity. Many of these metabolites exhibited at least a moderate correlation with estimated glomerular filtration rate (Pearson's r≥0.5), and some were also associated with the risk of developing kidney failure or death in multivariable adjusted Cox regression models.

LIMITATIONS

Lack of a second independent cohort for external validation of our findings.

CONCLUSIONS

Metabolomic profiling was used to identify multiple solutes associated with uremic symptoms in adults with CKD, but future validation and mechanistic studies are needed.

PLAIN-LANGUAGE SUMMARY

Individuals living with chronic kidney disease (CKD) often experience symptoms related to CKD, traditionally called uremic symptoms. It is likely that CKD results in alterations in the levels of numerous circulating substances that, in turn, cause uremic symptoms; however, the identity of these solutes is not known. In this study, we used metabolomic profiling in patients with CKD to gain insights into the pathophysiology of uremic symptoms. We identified 26 metabolites whose levels were significantly associated with at least one of the symptoms of fatigue, anorexia, itchiness, nausea, paresthesia, and pain. The results of this study lay the groundwork for future research into the biological causes of symptoms in patients with CKD.

Cardiovascular Consequences of Uremic Metabolites: an Overview of the Involved Signaling Pathways

The crosstalk of the heart with distant organs such as the lung, liver, gut, and kidney has been intensively approached lately. The kidney is involved in (1) the production of systemic relevant products, such as renin, as part of the most essential vasoregulatory system of the human body, and (2) in the clearance of metabolites with systemic and organ effects. Metabolic residue accumulation during kidney dysfunction is known to determine cardiovascular pathologies such as endothelial activation/dysfunction, atherosclerosis, cardiomyocyte apoptosis, cardiac fibrosis, and vascular and valvular calcification, leading to hypertension, arrhythmias, myocardial infarction, and cardiomyopathies. However, this review offers an overview of the uremic metabolites and details their signaling pathways involved in cardiorenal syndrome and the development of heart failure. A holistic view of the metabolites, but more importantly, an exhaustive crosstalk of their known signaling pathways, is important for depicting new therapeutic strategies in the cardiovascular field.

Maybe the various forms of kidney disease are not so mechanistically different?

Sieckmann and colleagues provide evidence of a common abnormality in polyamine metabolism in 11 different rodent models of acute kidney injury and chronic kidney disease, and in human renal transplantation. The abnormality is characterized by downregulation of enzymes involved in polyamine synthesis and/or upregulation of enzymes involved in polyamine metabolism. Therefore, polyamine metabolism is a potential target for development of pharmacotherapies for a broad range of kidney diseases.

Hematological, immunological, and polyamines alterations in the concomitant occurrence of Fasciola gigantica and hepatic leiomyoma in cattle

No previous studies have investigated the polyamines alterations during fascioliasis due to F. gigantica in ruminants. This study was therefore carried out to find out the possible relationship between the extent of liver destruction and leiomyoma and some hematological and immunological parameters and polyamines alterations in F. gigantica infection. Fifty cattle with liver fascioliasis and fifteen healthy cattle were selected for the study. For the histopathological study, liver tissue samples were stained with hematoxylin and eosin (H&E) and Masson's Trichrome methods. The leiomyoma suspected specimens were immunohistochemically stained for smooth muscle actin and desmin. Different hematological parameters were investigated in infected and non-infected animals. Furthermore, levels of putrescine, spermidine, and spermine were measured in homogenized liver samples. Serum IL-4 and TNF-α levels were also evaluated. By histological examination, the lesions were noted in all the infected specimens. These lesions were varied from leiomyoma, chronic catarrhal cholangitis, arteriosclerosis, telangiectasia, and fresh migratory tunnels filled with RBC and eosinophils. Comparison of hemogram results between infected and non-infected groups revealed a significant decrease in red blood cell counts (RBC), mean corpuscular hemoglobin concentration (MCHC), and platelet count (PLT) in infected animals. Also, a significant elevation in mean corpuscular volume (MCV) concentration was detected in infected animals. The putrescine and spermine levels of the infected animals were significantly higher than the non-infected animals. Although spermidine was increased in infected livers, its elevation was not significant. Based on the results, the level of IL-4 and TNF-α was not significantly changed in infected animals. In conclusion, the concurrent occurrence of leiomyoma and fascioliasis due to F. gigantica and polyamines elevation (putrescine and spermine) is reported for the first time. The role of polyamines in the concurrent occurrence of leiomyoma and fascioliasis is an area for future research.

Intestinal dialysis for conservative management of Uremia

PURPOSE OF REVIEW

Renal replacement therapies, such as hemodialysis are invasive and impose significant financial burden as well as burden on quality of life. Conservative and 'gentler' forms of renal replacement therapy for the frail and palliative care patient is an unmet medical need.

RECENT FINDINGS

The treatment of uremia using the gut as a substitute for the kidney has been proposed but is not practiced widely because of proven lack of long-term mortality benefit coupled with complications like edema and hyperchloremia. Mounting evidence showed that endotoxins from gastrointestinal tract are a major source of chronic inflammation in chronic kidney disease (CKD). The high load of nitrogenous waste elimination through the bowel could potentially serve as an alternative modality to remove uremic wastes especially in people who opt for conservative management for end-stage renal disease with some recent studies in Iran and China showing promising benefits in uremia.

SUMMARY

In this review, we will discuss the history, recent evidence and potential of these therapies and their implications in CKD for conservative and easy management of uremia.

Polyamine biomarkers as indicators of human disease

The significant increase of periodontitis, chronic kidney disease (CKD), Alzheimer's disease and cancer can be attributed to an ageing population. Each disease produces a range of biomarkers that can be indicative of disease onset and progression. Biomarkers are defined as cellular (intra/extracellular components and whole cells), biochemical (metabolites, ions and toxins) or molecular (nucleic acids, proteins and lipids) alterations which are measurable in biological media such as human tissues, cells or fluids. An interesting group of biomarkers that merit further investigation are the polyamines. Polyamines are a group of molecules consisting of cadaverine, putrescine, spermine and spermidine and have been implicated in the development of a range of systemic diseases, in part due to their production in periodontitis. Cadaverine and putrescine within the periodontal environment have demonstrated cell signalling interfering abilities, by way of leukocyte migration disruption. The polyamines spermine and spermidine in tumour cells have been shown to inhibit cellular apoptosis, effectively prolonging tumorigenesis and continuation of cancer within the host. Polyamine degradation products such as acrolein have been shown to exacerbate renal damage in CKD patients. Thus, the use of such molecules has merit to be utilized in the early indication of such diseases in patients.

Uremic Toxins Affect Erythropoiesis during the Course of Chronic Kidney Disease: A Review

Chronic kidney disease (CKD) is a global health problem characterized by progressive kidney failure due to uremic toxicity and the complications that arise from it. Anemia consecutive to CKD is one of its most common complications affecting nearly all patients with end-stage renal disease. Anemia is a potential cause of cardiovascular disease, faster deterioration of renal failure and mortality. Erythropoietin (produced by the kidney) and iron (provided from recycled senescent red cells) deficiencies are the main reasons that contribute to CKD-associated anemia. Indeed, accumulation of uremic toxins in blood impairs erythropoietin synthesis, compromising the growth and differentiation of red blood cells in the bone marrow, leading to a subsequent impairment of erythropoiesis. In this review, we mainly focus on the most representative uremic toxins and their effects on the molecular mechanisms underlying anemia of CKD that have been studied so far. Understanding molecular mechanisms leading to anemia due to uremic toxins could lead to the development of new treatments that will specifically target the pathophysiologic processes of anemia consecutive to CKD, such as the newly marketed erythropoiesis-stimulating agents.

The Influence of Inflammation on Anemia in CKD Patients

Anemia is frequently observed in the course of chronic kidney disease (CKD) and it is associated with diminishing the quality of a patient’s life. It also enhances morbidity and mortality and hastens the CKD progression rate. Patients with CKD frequently suffer from a chronic inflammatory state which is related to a vast range of underlying factors. The results of studies have demonstrated that persistent inflammation may contribute to the variability in Hb levels and hyporesponsiveness to erythropoietin stimulating agents (ESA), which are frequently observed in CKD patients. The understanding of the impact of inflammatory cytokines on erythropoietin production and hepcidin synthesis will enable one to unravel the net of interactions of multiple factors involved in the pathogenesis of the anemia of chronic disease. It seems that anti-cytokine and anti-oxidative treatment strategies may be the future of pharmacological interventions aiming at the treatment of inflammation-associated hyporesponsiveness to ESA. The discovery of new therapeutic approaches towards the treatment of anemia in CKD patients has become highly awaited. The treatment of anemia with erythropoietin (EPO) was associated with great benefits for some patients but not all.

Isosorbide dinitrate in nephronophthisis treatment

Nephronophthisis is a progressive disease that affects development of the renal tubules and leads to end stage renal disease. Many affected children have isolated renal disease; however, there can be additional manifestations including heart defects, liver fibrosis, brain malformations, and situs inversus. There is no way to slow or modify the disease. We describe a patient who presented at birth with cholestatic jaundice and decreased kidney function, found by exome sequencing to have two NPHP3 variants. Her clinical status deteriorated rapidly, and two disease-modifying agents were given in hopes of slowing disease progression, the arginine vasopressin type II receptor antagonist tolvaptan to stabilize her renal function and isosorbide dinitrate to manage her poorly controlled hypertension. Tolvaptan therapy initiated at 82 days of life had limited effect on the rate of decline in renal function and was insufficient to abrogate the need for dialysis; however, isosorbide dinitrate therapy led to a dramatic improvement in blood pressure control and allowed for the discontinuation of multiple anti-hypertensive agents. This is the first report of the use of tolvaptan and isosorbide dinitrate for nephronophthisis management. We suggest that isosorbide dinitrate may represent a disease-modifying agent in nephronophthisis treatment.

Generation, clearance, toxicity, and monitoring possibilities of unaccounted uremic toxins for improved dialysis prescriptions

Current dialysis-dosing calculations provide an incomplete assessment of blood purification. They exclude clearances of protein-bound uremic toxins (PB-UTs), such as polyamines, p-cresol sulfate, and indoxyl sulfate, relying solely on the clearance of urea as a surrogate for all molecules accumulating in patients with end-stage renal disease (ESRD). PB-UTs clear differently in dialysis but also during normal renal function. The kidney clears PB toxins via the process of secretion, whereas it clears urea through filtration. Herein, we review the clearance, accumulation, and toxicity of various UTs. We also suggest possible methods for their monitoring toward the ultimate goal of a more comprehensive dialysis prescription. A more inclusive dialysis prescription would retain the kidney-filtration surrogate, urea, and consider at least one PB toxin as a surrogate for UTs cleared through cellular secretion. A more comprehensive assessment of UTs that includes both secretion and filtration is expected to result in a better understanding of ESRD toxicity and consequently, to reduce ESRD mortality.

Biochemical and Clinical Impact of Organic Uremic Retention Solutes: A Comprehensive Update

In this narrative review, the biological/biochemical impact (toxicity) of a large array of known individual uremic retention solutes and groups of solutes is summarized. We classified these compounds along their physico-chemical characteristics as small water-soluble compounds or groups, protein bound compounds and middle molecules. All but one solute (glomerulopressin) affected at least one mechanism with the potential to contribute to the uremic syndrome. In general, several mechanisms were influenced for each individual solute or group of solutes, with some impacting up to 7 different biological systems of the 11 considered. The inflammatory, cardio-vascular and fibrogenic systems were those most frequently affected and they are one by one major actors in the high morbidity and mortality of CKD but also the mechanisms that have most frequently been studied. A scoring system was built with the intention to classify the reviewed compounds according to the experimental evidence of their toxicity (number of systems affected) and overall experimental and clinical evidence. Among the highest globally scoring solutes were 3 small water-soluble compounds [asymmetric dimethylarginine (ADMA); trimethylamine-N-oxide (TMAO); uric acid], 6 protein bound compounds or groups of protein bound compounds [advanced glycation end products (AGEs); p-cresyl sulfate; indoxyl sulfate; indole acetic acid; the kynurenines; phenyl acetic acid;] and 3 middle molecules [β₂-microglobulin; ghrelin; parathyroid hormone). In general, more experimental data were provided for the protein bound molecules but for almost half of them clinical evidence was missing in spite of robust experimental data. The picture emanating is one of a complex disorder, where multiple factors contribute to a multisystem complication profile, so that it seems of not much use to pursue a decrease of concentration of a single compound.

Indole 3-acetic acid, indoxyl sulfate and paracresyl-sulfate do not influence anemia parameters in hemodialysis patients

Background

The main reason for anemia in renal failure patients is the insufficient erythropoietin production by the kidneys. Beside erythropoietin deficiency, in vitro studies have incriminated uremic toxins in the pathophysiology of anemia but clinical data are sparse. In order to assess if indole 3-acetic acid (IAA), indoxyl sulfate (IS), and paracresyl sulfate (PCS) -three protein bound uremic toxins- are clinically implicated in end-stage renal disease anemia we studied the correlation between IAA, IS and PCS plasmatic concentrations with hemoglobin and Erythropoietin Stimulating Agents (ESA) use in hemodialysis patients.

Methods

Between June and July 2014, we conducted an observational cross sectional study in two hemodialysis center. Three statistical approaches were conducted. First, we compared patients treated with ESA and those not treated. Second, we performed linear regression models between IAA, IS, and PCS plasma concentrations and hemoglobin, the ESA dose over hemoglobin ratio (ESA/Hemoglobin) or the ESA resistance index (ERI). Third, we used a polytomous logistic regression model to compare groups of patients with no/low/high ESA dose and low/high hemoglobin statuses.

Results

Overall, 240 patients were included in the study. Mean age ± SD was 67.6 ± 16.0 years, 55.4% were men and 42.5% had diabetes mellitus.

When compared with ESA treated patients, patients with no ESA had higher hemoglobin (mean 11.4 ± 1.1 versus 10.6 ± 1.2 g/dL; p <0.001), higher transferrin saturation (TSAT, 31.1 ± 16.3% versus 23.1 ± 11.5%; p < 0.001), less frequently an IV iron prescription (52.1 versus 65.7%, p = 0.04) and were more frequently treated with hemodiafiltration (53.5 versus 36.7%). In univariate analysis, IAA, IS or PCS plasma concentrations did not differ between the two groups.

In the linear model, IAA plasma concentration was not associated with hemoglobin, but was negatively associated with ESA/Hb (p = 0.02; R = 0.18) and with the ERI (p = 0.03; R = 0.17). IS was associated with none of the three anemia parameters. PCS was positively associated with hemoglobin (p = 0.03; R = 0.14), but negatively with ESA/Hb (p = 0.03; R = 0.17) and the ERI (p = 0.02; R = 0.19). In multivariate analysis, the association of IAA concentration with ESA/Hb or ERI was not statistically significant, neither was the association of PCS with ESA/Hb or ERI. Identically, in the subgroup of 76 patients with no inflammation (CRP <5 mg/L) and no iron deficiency (TSAT >20%) linear regression between IAA, IS or PCS and any anemia parameter did not reach significance.

In the third model, univariate analysis showed no intergroup significant differences for IAA and IS. Regarding PCS, the Low Hb/High ESA group had lower concentrations. However, when we compared PCS with the other significant characteristics of the five groups to the Low Hb/high ESA (our reference group), the polytomous logistic regression model didn’t show any significant difference for PCS.

Conclusions

In our study, using three different statistical models, we were unable to show any correlation between IAA, IS and PCS plasmatic concentrations and any anemia parameter in hemodialysis patients. Indolic uremic toxins and PCS have no or a very low effect on anemia parameters.

Gut microbiome in chronic kidney disease: challenges and opportunities

More than 100 trillion microbial cells that reside in the human gut heavily influence nutrition, metabolism, and immune function of the host. Gut dysbiosis, seen commonly in patients with chronic kidney disease (CKD), results from qualitative and quantitative changes in host microbiome profile and disruption of gut barrier function. Alterations in gut microbiota and a myriad of host responses have been implicated in progression of CKD, increased cardiovascular risk, uremic toxicity, and inflammation. We present a discussion of dysbiosis, various uremic toxins produced from dysbiotic gut microbiome, and their roles in CKD progression and complications. We also review the gut microbiome in renal transplant, highlighting the role of commensal microbes in alteration of immune responses to transplantation, and conclude with therapeutic interventions that aim to restore intestinal dysbiosis.

Role of the Gut Microbiome in Uremia: A Potential Therapeutic Target

Also known as the "second human genome," the gut microbiome plays important roles in both the maintenance of health and the pathogenesis of disease. The symbiotic relationship between host and microbiome is disturbed due to the proliferation of dysbiotic bacteria in patients with chronic kidney disease (CKD). Fermentation of protein and amino acids by gut bacteria generates excess amounts of potentially toxic compounds such as ammonia, amines, thiols, phenols, and indoles, but the generation of short-chain fatty acids is reduced. Impaired intestinal barrier function in patients with CKD permits translocation of gut-derived uremic toxins into the systemic circulation, contributing to the progression of CKD, cardiovascular disease, insulin resistance, and protein-energy wasting. The field of microbiome research is still nascent, but is evolving rapidly. Establishing symbiosis to treat uremic syndrome is a novel concept, but if proved effective, it will have a significant impact on the management of patients with CKD.

Dietary protein metabolism by gut microbiota and its consequences for chronic kidney disease patients

The breakdown of proteins and peptides by colonic microorganisms yields a great diversity of end products, including short-chain fatty acids, ammonia, amines, phenols, indoles, thiols, CO2, H2 and H2S, many of which have toxic properties. An increase of the dietary protein load in healthy individuals results in enhanced generation of these toxins, many of which are rapidly cleared by the kidneys. In this regard, the impact upon the colonic microbiota of controlled changes in the dietary protein has not been examined in chronic kidney disease patients. This review focuses on the impact of dietary proteins on the intestinal microbiota and its possible consequences for chronic kidney disease patients.

Pathophysiology of anemia in chronic kidney diseases: A review

Backgroud. Anemia is one of the laboratory and clinical findings of chronic kidney diseases (CKD). The presence of anemia in patients with CKD has a wide range of clinically important consequences. Some of the symptoms that were previously attributed to reduced renal function are, in fact, a consequence of anemia. Anemia contributes to increased cardiac output, the development of left ventricular hypertrophy, angina, and congestive heart failure. According to current knowledge, anemia also contributes to the progression of CKD and is one of the factors that contribute to the high morbidity and mortality in patients with chronic renal failure and their reduced survival.

METHODS

MEDLINE search was performed to collect both original and review articles addressing anemia in CKD, pathophysiology of renal anemia, erythropoiesis, erythropoietin, iron metabolism, inflammation, malnutrition, drugs, renal replacement therapy and anemia management

CONCLUSION

The present review summarized current knowledge in the field of the pathophysiology of renel anemia. Understanding the pathophysiology of anemia in CKD is crucial for the optimal treatment of anemia according to recent clinical practice guidelines and recommendation, and correct recognition of causes of resistence to treatment of erythropoietin stimulating agents (ESA).

Acarbose treatment lowers generation and serum concentrations of the protein-bound solute p-cresol: a pilot study

Several protein-bound uremic retention solutes (including p-cresol) originate from colonic bacterial fermentation of protein. Higher colonic availability of carbohydrates drives this process towards lower production of toxic metabolites. Small intestinal alpha-glucosidase inhibitors like Acarbose (Glucobay) enhance the amount of undigested carbohydrates reaching the colon. We studied the effect of Acarbose on generation and serum concentrations of p-cresol. Nine healthy volunteers (age 25 (22-36) years) with a creatinine clearance of 89.6 ml/min/1.73 m(2) (85.5-116.4) were treated with Acarbose for 3 weeks. Dose was gradually increased to reach 300 mg/day after 1 week. Blood sampling, 24-h urine and stool collections on 3 consecutive days were performed before and during the last days of the treatment period. p-Cresol generation was estimated from mean 24-h urinary elimination. Gastrointestinal side effects, if present, were mild to moderate. Serum concentrations of p-cresol declined significantly after Acarbose treatment (before: 1.14 mg/l (0.93-3.03); after: 1.11 mg/l (0.31-1.82); P=0.047). Urinary excretion of p-cresol, reflecting its colonic generation rate, was significantly lower after treatment (before: 29.93 mg/day (6.79-75.19); after: 10.54 mg/day (1.08-30.85); P=0.031). The fecal excretion of nitrogen increased after treatment (before: 1.04 g/day (0.47-2.29); after: 1.99 g/day (0.76-3.08); P=0.047). This pilot study suggests that Acarbose treatment lowers generation and serum concentrations of the protein-bound uremic solute p-cresol. Although further confirmation is warranted, the data may point to a novel treatment option for chronic kidney disease patients in view of the potential toxic effects of p-cresol and related substances.

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.

Effect of valsartan on erythropoietin and hemoglobin levels in stage III-IV chronic kidney disease patients

Angiotensin-converting enzyme inhibitors (ACEIs) were accepted as a potential cause of inadequate epoetin response in chronic kidney disease (CKD) patients. We aimed to determine the effects of valsartan, an angiotensin receptor blocker (ARB), on serum ertyhropoietin levels and on certain biochemical and haematological parameters in hypertensive CKD patients. Twenty-two stage III-IV CKD patients (mean age; 56.8 +/- 8.9 years, 12 male 10 female) were included in the study. Before initiating the treatment, current anti-hypertensive treatments (if any) were discontinued, and blood samples were collected after a washout period of 3 weeks. Valsartan 80 mg/day was started, and additional anti-hypertensive agents were given according to study protocol if needed. One way Anova and paired t-tests were used for statistical comparisons. Serum blood urea nitrogen (BUN), creatinine, uric acid, potassium, haemoglobin and erythropoietin values were measured, and glomerular filtration rates were calculated before and 3, 6 and 90 days after valsartan treatment, a significant reduction in EPO level was observed at 3rd (19.6 +/- 24.0 vs. 13.8 +/- 8.5, p = 0.010), 6th (12.1 +/- 7.6, p = 0.009), and 90th days (8.3 +/- 5.4, p = 0.007). When pre-treatment values were compared with 90th day results, no significant change was observed in terms of hgb, htc, serum BUN, creatinine, uric acid, potassium, and GFR values. In conclusion, valsartan, an ARB, did not decrease haemoglobin levels in stage III-IV CKD patients despite significant reduction in serum erythropoietinlevels, so ARBs may be preferred to ACEIs in CKD patients when indicated.

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.

Determination of renal porphyrin handling in rats suffering from different kinds of chronic renal failure (CRF): uranyl nitrate (UN) induced fibrosis or 5/6-nephrectomy (5/6NX)

The renal handling of porphyrins is reported to be a sensitive marker for chronic renal failure (CRF) for two reasons: heme is synthesised in proximal tubules and porphyrins are reabsorbed in the renal proximal tubule by apical peptide transporter PEPT 2. Two different models of CRF in female Wistar rats have been used for investigation of renal porphyrin handling: (1) single administration of uranyl nitrate (UN; 0.5 mg/100 g b.wt.) and (2) 5/6 nephrectomy (5/6NX). Renal clearance experiments were performed at weeks 2 and 10 after the onset of CRF. The concentrations of porphyrin intermediates (uroporphyrin I and III, coproporphyrin I and II, heptaporphyrin, and pentaporphyrin) were measured by HPLC with fluorescence detection. Both after UN and 5/6NX a significant reduction of body weight occurred. The kidney weight was enhanced 2 weeks after UN compared to controls (+31%). After 5/6NX, the weight of the remnant kidney was 44% (2nd week) and 140% (10th week) higher compared to one control kidney. Urine volumes and GFR were significantly reduced at week 2 and 10 after 5/6NX, but at week 10 after UN values were comparable to controls. Two weeks after UN and 5/6NX the concentrations of heptaporphyrin was moderately decreased in renal tissue whereas after 10 weeks the concentrations of most porphyrins were increased in the kidney. The plasma levels of free porphyrins were only slightly enhanced (week 2). The renal excretion of porphyrins was initially slightly reduced in both models, whereas it increases 10 weeks after UN, but it remained reduced 10 weeks after 5/6NX. UN induces tubulointerstitial fibrosis including atrophic glomeruli, whereas 5/6NX was characterized by distinct proteinuria, dilated tubules containing hyaline casts. A modulation of porphyrin metabolism in the kidney seems first of all to be responsible for UN effect on renal porphyrin handling. Summing up the 5/6NX results, both reduction in intact renal tissue mass and a modification of enzymes involved in heme biosynthesis by uraemic toxins are responsible for accumulation of porphyrins in renal tissue. After 5/6NX reduced excretion of porphyrins into urine and enhanced porphyrin concentrations in the kidney indicate more a damage of renal porphyrin biosynthesis than changes in their reabsorption.

Erythropoietin: physiology and pharmacology update

This minireview is an update of a 1997 review on erythropoietin (EPO) in this journal. EPO is a 30,400-dalton glycoprotein that regulates red cell production. In the human, EPO is produced by peritubular cells in the kidneys of the adult and in hepatocytes in the fetus. Small amounts of extra-renal EPO are produced by the liver in adult human subjects. EPO binds to an erythroid progenitor cell surface receptor that includes a p66 chain, and, when activated, the p66 protein becomes dimerized. EPO receptor activation induces a JAK2 tyrosine kinase, which leads to tyrosine phosphorylation of the EPO receptor and several proteins. EPO receptor binding leads to intracellular activation of the Ras/mitogen-activated kinase pathway, which is involved with cell proliferation, phosphatidylinositol 3-kinase, and STATS 1, 3, 5A, and 5B transcriptional factors. EPO acts primarily to rescue erythroid cells from apoptosis (programmed cell death) to increase their survival. EPO acts synergistically with several growth factors (SCF, GM-CSF, 1L-3, and IGF-1) to cause maturation and proliferation of erythroid progenitor cells (primarily colony-forming unit-E). Oxygen-dependent regulation of EPO gene expression is postulated to be controlled by a hypoxia-inducible transcription factor (HIF-1alpha). Hypoxia-inducible EPO production is controlled by a 50-bp hypoxia-inducible enhancer that is approximately 120 bp 3' to the polyadenylation site. Hypoxia signal transduction pathways involve kinases A and C, phospholipase A(2), and transcription factors ATF-1 and CREB-1. A model has been proposed for adenosine activation of EPO production that involves protein kinases A and C and the phospholipase A(2) pathway. Other effects of EPO include a hematocrit-independent, vasoconstriction-dependent hypertension, increased endothelin production, upregulation of tissue renin, change in vascular tissue prostaglandins production, stimulation of angiogenesis, and stimulation of endothelial and vascular smooth muscle cell proliferation. Recombinant human EPO (rHuEPO) is currently being used to treat patients with anemias associated with chronic renal failure, AIDS patients with anemia due to treatment with zidovudine, nonmyeloid malignancies in patients treated with chemotherapeutic agents, perioperative surgical patients, and autologous blood donation. A novel erythropoiesis-stimulating factor (NESP, darbepoetin) has been synthesized and when compared with rHuEPO, NESP has a higher carbohydrate content (52% vs 40%), a longer plasma half-life, the amino acid sequence differs from that of native human EPO at five positions, and has been reported to maintain hemoglobin levels just as effectively in patients with chronic renal failure as rHuEPO at less frequent dosing. The use of rHuEPO and darbepoetin to enhance athletic performance is officially banned by most sports-governing bodies because the excessive erythrocytosis can lead to increased thrombogenicity and can cause deep vein, coronary, and cerebral thromboses.

Unexpectedly high prevalence of posttransplant anemia in pediatric and young adult renal transplant recipients

BACKGROUND

Although posttransplant anemia (PTA) is recognized as a common problem in adult renal transplant recipients, few pediatric studies have been published.

METHODS

In this retrospective cohort study of 162 pediatric renal transplant recipients treated at Stanford University, the authors sought to determine the prevalence, severity, and the predictive factors of PTA. Anemia was defined as a hematocrit (HCT) level greater than 2 SD below published means for age or as erythropoietin dependency to maintain a normal HCT.

RESULTS

Sixty-seven percent of pediatric renal transplant recipients were anemic at the time of transplantation. The prevalence of anemia increased to 84.3% in the first month posttransplant. From 6 months to 60 months posttransplant, the prevalence of anemia remained high at 64.2% to 82.2%. Only 4 patients (2.5%) were never anemic. Iron depletion was detected in 19 of 26 and 23 of 23 anemic patients 12 and 60 months posttransplant, respectively. Serum erythropoietin levels were low relative to hematocrit levels in 38 of 56 anemic patients. Logistic regression at 3 months posttransplant showed that discharge hematocrit level (P < 0.0001), calcium (P = 0.0004), and cyclosporine dose (P = 0.0002) correlated with anemia. Creatinine clearance (P = 0.002) and white blood cell count (P = 0.004) correlated with anemia at 12 months posttransplant, but only creatinine clearance (P = 0.011) correlated with anemia 60 months posttransplant.

CONCLUSION

Nearly all pediatric renal transplant recipients experience PTA. However, few children less than 2 years of age were anemic during the first year posttransplant. Antirejection therapy, bone disease, iron depletion, and creatinine clearance appear to play pivotal roles in the development of PTA in children.

Polymeric protein-polyamine conjugates: a new class of uremic toxins affecting erythropoiesis

BACKGROUND

Preliminary evidence on the accumulation of polyamine-protein conjugates (PPCs) was obtained in uremic patients. The presence of these substances in the plasma of hemodialysis (HD) patients was evaluated, and their possible contribution to uremic anemia was investigated by testing the effect of PPC synthesized in vitro on erythroid cell proliferation.

METHODS

Plasma PPC was measured by high-performance liquid chromatography. The in vitro synthesis of PPC from human plasma was carried out by means of the enzyme transglutaminase in the presence of either [3H]-labeled or unlabeled spermidine (SPD). After gel filtration chromatography and detection of the fractions containing [3H]SPD, the latter were tested for their effect on mononuclear bone marrow cell proliferation.

RESULTS

In three out of four patients examined, mainly SPD-protein conjugates (SPD-PC) were observed to accumulate during HD. The levels ranged from 0.17 to 4.93 pmol/mg proteins before dialysis, and these values increased at 30 minutes and at the end of the dialysis up to levels 11.90 pmol/mg. SPD-PC levels in healthy controls were 1.46 +/- 0.82. SPD-PCs synthesized in vitro were recovered in two main fractions showing a molecular weight of> 100 kD (peak 1) and of approximately 30 to 50 kD (peak 3), respectively. The SPD-PC contained in peak 1 showed the greatest inhibitory effect on colony-forming units-erythroid (CFU-E) proliferation without any appreciable effect on burst-forming units-erythroid (BFU-E).

CONCLUSION

We demonstrate that SPD-PC can accumulate in HD patients. These substances, which affect CFU-E proliferation, can be considered as an at yet unrevealed class of uremic toxins contributing to the onset of the uremic anemia.

Apoptosis of leukocytes: basic concepts and implications in uremia

Circulating blood leukocytes have short life expectancies and end their lives by committing programmed cell death or apoptosis. Apoptosis is an active form of cell death that is initiated by a number of stimuli and is intricately regulated. Apoptosis in both excessive and reduced amounts has pathological implications. Evidence suggests that apoptosis may play a role in the pathophysiology of immune dysfunction in uremia. Indeed, accelerated programmed cell death has been observed in lymphocytes, monocytes, and polymorphonuclear leukocytes among patients with chronic renal failure. This may be due in part to the retention of uremic toxins. The aim of this article is to review the evidence for accelerated leukocyte apoptosis, key regulatory apoptotic pathways, and the possible role of this highly organized process in the pathogenesis of immune dysfunction in uremia.

Adequacy of dialysis and differences in hematocrit among dialysis facilities

Despite the clearly established relationship between adequacy of dialysis and response to erythropoietin, recent guidelines on anemia management in end-stage renal disease (ESRD) omit mention of dialysis adequacy while advocating the use of large amounts of intravenous iron. To determine the relative effects of adequacy of dialysis and intravenous iron on hematocrit, we studied 309 hemodialysis patients and analyzed data from 141 hemodialysis facilities in New York State (ESRD Network 2), as well as data from all 18 ESRD Networks in the United States, for the last quarter of 1997. Among the 309 subjects, mean hematocrit differed between quartiles of urea reduction ratio (URR; F statistic = 4; P: = 0.008). Patients with URRs greater than 70% were 2.6 times more likely to have hematocrits greater than 33% (odds ratio, 2.6; 95% confidence interval [CI], 1.3 to 5.3; P: = 0.009) after adjustment for other factors. Mean dialysis facility (n = 141) hematocrits correlated directly with mean URRs (r = 0.32; P: = 0.001). Facilities with a mean URR greater than 70% were three times more likely to have a mean hematocrit greater than 33% (odds ratio, 3; 95% CI, 1.2 to 7.5; P: = 0.02). The percentage of patients in each of the 18 ESRD Networks with hematocrits of 33% or greater correlated inversely with the percentage of patients administered intravenous iron (r = -0.53; P: = 0.03) after adjustment for dose of erythropoietin. We conclude that adequacy of dialysis predicts the response to erythropoietin at both patient and dialysis facility levels. Patients with low hematocrits primarily because of inadequate dialysis may inappropriately be administered excess intravenous iron intended as a corrective measure.

The removal of uremic toxins

Three major groups of uremic solutes can be characterized: the small water-soluble compounds, the middle molecules, and the protein-bound compounds. Whereas small water-soluble compounds are quite easily removed by conventional hemodialysis, this is not the case for many other molecules with different physicochemical characteristics. Continuous ambulatory peritoneal dialysis (CAPD) is often characterized by better removal of those compounds. Urea and creatinine are small water-soluble compounds and the most current markers of retention and removal, but they do not exert much toxicity. This is also the case for many other small water-soluble compounds. Removal pattern by dialysis of urea and creatinine is markedly different from that of many other uremic solutes with proven toxicity. Whereas middle molecules are removed better by dialyzers containing membranes with a larger pore size, it is not clear whether this removal is sufficient to prevent the related complications. Larger pore size has virtually no effect on the removal of protein-bound toxins. Therefore, at present, the current dialytic methods do not offer many possibilities to remove protein-bound compounds. Nutritional and environmental factors as well as the residual renal function may influence the concentration of uremic toxins in the body fluids.

The anemia in continuous ambulatory peritoneal dialysis patients is related to Kt/V index

Anemia in chronic renal failure causes a number of serious problems to the patient. As a result, it is imperative to make use of all rational options to alleviate it. The present study addresses the question, the answer to which is not yet known, whether or not the degree of anemia depends on the efficacy of continuous ambulatory peritoneal dialysis (CAPD) and, if so, what the importance of peritoneal clearance and residual renal function is. A significant correlation between the hematocrit (Hct) and the total weekly Kt/V index (tKt/V) (r = 0.61, p<0.01), total weekly creatinine clearance (tCLCR) (r = 0.50, p<0.05), and residual glomerular filtration rate (r = 0.43, p<0.05) was demonstrated in a group of 22 CAPD patients. Stepwise regression analysis showed that of all the variables monitored, Hct depends exclusively on tKt/V (p<0.01, r2 = 0.37). The value of Hct in a group of patients with a tKt/V <2.3 (n = 15) was 28.9+/-1.2% (arithmetic mean +/- SEM) while in a group with a tKt/V > or =2.3 (n = 7), it was 35.1+/-1.9% (p<0.01). On dividing tKt/V and tCLCR into their peritoneal and renal components, a significant correlation between Hct and renal Kt/V (r = 0.47, p<0.05) was found; stepwise regression analysis identified renal Kt/V (p<0.01) and peritoneal Kt/V (p<0.05), with R2 = 0.38 as major variables with an effect on Hct. The authors conclude the efficacy of blood purification is another factor affecting renal anemia in CAPD patients. The relationship between anemia and blood purification is best expressed using the Kt/V index. The Kt/V provided by one's own kidneys seems to be of greater importance for anemia than the Kt/V provided by peritoneal dialysis. The results provide the basis for prospective interventional studies.

A quest for erythropoietin over nine decades

The major research accomplishments of the author are described from the time of his PhD thesis work on the mechanism of cobalt polycythemia to the present day. His early work on the quest for the cell that produces erythropoietin (Epo) to his current work on oxygen sensing and signal transduction pathways involved in erythropoietin gene expression are reported. He describes his main research interest in the mechanism of cobalt polycythemia between 1954 and 1962 and his research on how hormones such as the glucocorticoids function in the regulation of erythropoiesis (1956-1962). His major findings during this period were the discovery that hydrocortisone and corticosterone stimulated erythropoiesis (1958) and that cobalt increased erythropoietin production in the isolated perfused dog kidney (1961). He describes how he was led astray in some of his early studies on the cells in the kidney that produce erythropoietin, because of the less-developed technology available to him at that time; and how in situ hybridization and other molecular biology techniques enabled him to confirm some of the earlier work in mice by other investigators that interstitial cells in the kidney were the site of production of erythropoietin in the primate. His work in the controversial area of the mechanism of the anemia of end-stage renal disease is described in detail, as it pertains to Epo deficiency and suppressed erythroid progenitor cell response to Epo. He also discusses his recent work on signal transduction pathways (hypoxia, nitric oxide, adenosine, and C kinase) in oxygen sensing and Epo gene expression.

Mechanisms of uremic inhibition of phagocyte reactive species production: characterization of the role of p-cresol

It is generally recognized that the uremic syndrome results in a depression of immune function, but the uremic solutes responsible remain largely unidentified. In this study, the effect of 18 known uremic retention solutes, including urea and creatinine, on hexose monophosphate shunt (HMS)-dependent glucose-1-C14 utilization (G1C-U), chemiluminescence production (CL-P) and flow cytometric parameters (FCP) of respiratory burst and phagocytosis were evaluated in granulocytes and/or monocytes. Among the compounds studied, only p-cresol depressed whole blood respiratory burst reactivity (G1C-U, CL-P) dose dependently at concentrations currently encountered in end-stage renal disease (ESRD) (P < 0.05 from 5 micrograms/ml on). The effect of p-cresol was enhanced by increasing incubation times from 10 to 120 minutes. HMS activity of isolated packed erythrocytes remained unaffected. FCP of respiratory burst activity (Bursttest, expressed as log fluorescence units, LFU) revealed a marked depression in the presence of p-cresol (from 700 +/- 167 to 291 +/- 128 LFU for granulocytes, from 278 +/- 102 to 146 +/- 52 LFU for monocytes, P < 0.01), whereas particle ingestion (Phagotest) remained unaffected. Cell-free myeloperoxidase activity was also markedly depressed in the presence of p-cresol. Polarity based HPLC-elution of a standard solution containing all the solutes studied, using a gradient from 100% formic acid to 100% methanol during 60 minutes, revealed elution of p-cresol after 46.6 minutes, pointing to its relative hydrophobicity. Conjugation of p-cresol to p-cresylsulfate anihilated the depressive effect of p-cresol on granulocyte function, and at the same time caused a shift in HPLC-elution pattern to a less lipophilic range.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of the responses to recombinant human erythropoietin in normal and uremic subjects

The erythropoietic response to graded doses of recombinant human erythropoietin (epoetin alfa) was assessed in 24 hemodialysis patients by quantitative ferrokinetic studies, and measurement of the reticulocyte count and plasma levels of transferrin receptor protein. These responses were compared to those of 22 normal subjects. Epoetin alfa was given intravenously at 15, 50 or 150 U/kg every other day for four injections. Three patients with chronic renal failure were restudied after renal function was restored following renal transplantation. The results of these three different measurements of erythroid function showed that the acute response to recombinant human erythropoietin was similar in normal subjects and patients with renal failure. We conclude that chronic uremia does not alter the responsiveness to erythropoietin in vivo.