Uraemia: is urea more important than we think?

What If Not All Metabolites from the Uremic Toxin Generating Pathways Are Toxic? A Hypothesis

The topic of uremic toxicity has received broad attention from the nephrological community over the past few decades. An aspect that is much less often considered is the possibility that the metabolic pathways that generate uremic toxins also may produce molecules that benefit body functions. Here, we discuss this dualism based on the example of tryptophan-derived metabolites, which comprise elements that are mainly toxic, such as indoxyl sulfate, kynurenine and kynurenic acid, but also beneficial compounds, such as indole, melatonin and indole-3-propionic acid, and ambivalent (beneficial for some aspects and harmful for others) compounds such as serotonin. This dualism can also be perceived at the level of the main receptor of the tryptophan-derived metabolites, the aryl hydrocarbon receptor (AHR), which has also been linked to both harm and benefit. We hypothesize that these beneficial effects are the reason why uremic toxin generation remained preserved throughout evolution. This duality is also not unique for the tryptophan-derived metabolites, and in this broader context we discuss the remote sensing and signaling theory (RSST). The RSST proposes that transporters (e.g., organic anion transporter 1—OAT1; ATP-binding cassette transporter G—ABCG2) and drug metabolizing enzymes form a large network of proteins interacting to promote small molecule remote communication at the inter-organ (e.g., gut–liver–heart–brain–kidney) and inter-organismal (e.g., gut microbe–host) levels. These small molecules include gut microbe-derived uremic toxins as well as beneficial molecules such as those discussed here. We emphasize that this positive side of uremic metabolite production needs more attention, and that this dualism especially needs to be considered when assessing and conceiving of therapeutic interventions. These homeostatic considerations are central to the RSST and suggest that interventions be aimed at preserving or restoring the balance between positive and negative components rather than eliminating them all without distinction.

Avenues for post-translational protein modification prevention and therapy

Non-enzymatic post-translational modifications (nPTMs) of proteins have emerged as novel risk factors for the genesis and progression of various diseases. We now have a variety of experimental and established therapeutic strategies to target harmful nPTMs and potentially improve clinical outcomes. Protein carbamylation and glycation are two common and representative nPTMs that have gained considerable attention lately as favorable therapeutic targets with emerging clinical evidence. Protein carbamylation is associated with the occurrence of cardiovascular disease (CVD) and mortality in patients with chronic kidney disease (CKD); and advanced glycation end products (AGEs), a heterogeneous group of molecules produced in a series of glycation reactions, have been linked to various diabetic complications. Therefore, reducing the burden of protein carbamylation and AGEs is an appealing and promising therapeutic approach. This review chapter summarizes potential anti-nPTM therapy options in CKD, CVD, and diabetes along with clinical implications. Using two prime examples-protein carbamylation and AGEs-we discuss the varied preventative and therapeutic options to mitigate these pathologic nPTMs in detail. We provide in-depth case studies on carbamylation in the setting of kidney disease and AGEs in metabolic disorders, with an emphasis on the relevance to reducing adverse clinical outcomes such as CKD progression, cardiovascular events, and mortality. Overall, whether specific efforts to lower carbamylation and AGE burden will yield definitive clinical improvement in humans remains largely to be seen. However, the scientific rationale for such pursuits is demonstrated herein.

Successive Multiple Ionic-Polymer Layer Coatings for Intact Protein Analysis by Capillary Zone Electrophoresis-Mass Spectrometry: Application to Hemoglobin Analysis

Adsorption of analytes, e.g., proteins, often interfere with separation in CE, due to the relatively large surface of the narrow capillary. Coatings often are applied to prevent adsorption and to determine the electroosmotic flow (EOF), which is of major importance for the separation in CE. Successive multiple ionic-polymer layer (SMIL) coatings are frequently used for protein analysis in capillary electrophoresis resulting in high separation efficiency and repeatability. Here, the coating procedure of a five-layer SMIL coating is described using quaternized diethylaminoethyl dextran (DEAEDq) as polycation and poly(methacrylic acid) (PMA) as polyanion. Depending on the analyte, different polyions may be used to increase separation efficiency. However, the coating procedure remains the same.To demonstrate the applicability of SMIL coatings in CE-MS, human hemoglobin was measured in a BGE containing 2 M acetic acid. DEAEDq-PMA coating was found to be the most suitable for hemoglobin analysis due to relatively low reversed electroosmotic mobility leading to increased electrophoretic resolution of closely related proteoforms. Thereby, not only alpha and beta subunit of the hemoglobin could be separated, but also positional isoforms of glycated and carbamylated species were separated within 24 min.

Prerenal Azotemia: Differentiation of Hyperureagenesis from Renal Hypoperfusion Using Urinary Urea Nitrogen Data

Blood urea nitrogen (BUN) rises disproportionately to serum creatinine in patients with prerenal azotemia whether due to impaired hemodynamics or excessive ureagenesis. To determine whether urinary urea nitrogen excretion rates can distinguish between these causes of hyperuremia we performed a cross-sectional observational study to compare urinary urea nitrogen excretion rates in a highly selected group of patients with prerenal azotemia. Patients who had stable serum creatinine levels, BUN: serum creatinine ratios exceeding 20:1, and progressive azotemia were identified from the hospital laboratory data base. Using conventional clinicolaboratory criteria, 27 patients were diagnosed with either renal hypoperfusion (group I; n = 17) or hyperureagenesis ((group II; n = 10). Random urine sampling for electrolytes, osmolality, creatinine, and urea nitrogen was followed by 24 h collection for creatinine clearance and urinary urea nitrogen. There were no significant differences in age, gender, absolute levels of BUN, or BUN: serum creatinine ratios between the groups. Creatinine clearance (ml/min/1.73 m2) (ml/s/1.73 m2) was lower in group I than in group II (21 ± 16 vs. 36 ± 13; p<0.05) (0.35 ± 0.27 vs. 0.60 ± 0.22; p < 0.05). Twenty-four hour urinary urea nitrogen levels were significantly different (group I, 4.8 ± 2.9 vs. group II, 13.6 ± 3.2 gm; p < 0.001) (group I, 171 ± 300 vs. group II, 486 ± 114 mmol; p < 0.001). Random urine urea excretion indices were less discriminating but nevertheless still capable of separating the groups. Timed as well as random urine urea nitrogen determinations may assist in differentiating prerenal azotemia due to renal hypoperfusion from hyperureagenesis. Differentiation of these causes of prerenal azotemia might prevent iatrogenic overhydration of patients with azotemia incorrectly attributed to hemodynamic disturbances.

Acute poisoning of silver gulls (Larus novaehollandiae) following urea fertilizer spillage

Two episodes of accidental urea toxicosis are described in wild silver gulls (Larus novaehollandiae) following spillage of fertilizer grade urea at a commercial shipping facility near Perth, Western Australia. In both cases, urea spillage had been seen to contaminate freshwater wash-down pools on the wharves where ships were being unloaded and gulls were seen to be drinking and washing in the pools nearby the spillages. Affected birds were found moribund or dead. Necropsy and histopathological findings were non-specific and consisted of mild to moderate congestion of visceral organs and brain. Analysis of a water sample collected during Case 1 revealed a very high urea concentration of 4.124 mol/l (pH 5.5), and fluid from the proventriculus of two birds had urea concentrations of 382 and 308 mmol/l, respectively. Nine birds were examined during the second episode (Case 2) and, from heparinized heart blood samples collected (n = 5), the mean plasma urea (288 +/- 92.0 mmol/l), ammonia (43.9 +/- 34.2 mmol/l) and uric acid (7.45 +/- 1.99 mmol/l) concentrations were markedly elevated above the reference ranges for all bird species. Proventricular contents (n = 7) similarly contained high concentrations of urea (394 +/- 203 mmol/l) and ammonia (9.3 +/- 15 mmol/l). The probable mechanisms of urea and ammonia toxicity in these birds are discussed.

Comparative renal physiology of exotic species

In this article the osmoregulatory, acid-base homeostasis, and excretory functions of the renal system of invertebrates and vertebrates are reviewed. The mammalian renal system is the most highly evolved in terms of the range of functions performed by the kidneys. Renal physiology in other animals can be very different, and a sound knowledge of these differences is important for understanding health and disease processes that involve the kidneys, as well as ion and water homeostasis. Many animals rely on multiple organs along with the kidneys to maintain osmotic, ionic, and pH balance. Some animals rely heavily on postrenal modification of urine to conserve water and salt balance; this can influence the interpretation of disease signs and treatment modalities.

Influence of standard haemodialysis treatment on transcription of human serum- and glucocorticoid-inducible kinase SGK1 and taurine transporter TAUT in blood leukocytes

BACKGROUND

Standard haemodialysis (HD) rapidly alters osmolality and composition of extracellular fluid and, thus, challenges cell volume constancy. Cell volume-sensitive genes upregulated by osmotic cell shrinkage include those encoding for taurine transporter TAUT as well as for serum- and glucocorticoid-inducible kinase SGK1.

METHODS

Six HD patients were haemodialysed for 4 h with high-flux dialysers. Blood was drawn from the arterial section of the fistula immediately prior to start of HD and subsequently after 60, 120 and 240 min of HD treatment and, in addition, 120 min after HD treatment. Taurine plasma concentrations ([taurine]p) and erythrocytic taurine content ([taurine]e) were determined by high-performance liquid chromatography. SGK1 and TAUT transcript levels in leukocytes were quantified by real-time polymerase chain reaction.

RESULTS

The [taurine]p was significantly higher in HD patients before HD treatment when compared with healthy controls and it decreased significantly during 4 h of HD. The ratio of SGK1/GAPDH and of TAUT/GAPDH transcript levels increased significantly by 50% or 27%, respectively, during HD.

CONCLUSIONS

Standard HD treatment decreases plasma taurine concentration and upregulates leukocyte SGK1 and TAUT transcription. As SGK1 is a potent regulator of ion channels and transporters in nervous system, heart muscle and epithelial cells, the deranged regulation of SGK1 may contribute to acute side effects of HD treatment.

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.

New insights in uremic toxins

The retention in the body of compounds, which normally are secreted into the urine results in a clinical picture, called the uremic syndrome. The retention compounds responsible for the uremic syndrome are called uremic toxins. Only a few of the uremic retention solutes fully conform to a true definition of uremic toxins. Uremic patients develop atheromatotic vascular disease more frequently and earlier than the general population. The classical risk factors seem to be less important. Other factors have been suggested to be at play, and among those uremic toxins are mentioned as potential culprits. The identification, classification and characterization of the solutes responsible for vascular problems seems of utmost importance but is far from complete due to a lack of standardization and organization. The European Uremic Toxin Work Group (EUTox) has as a primary aim to discuss, analyze and offer guidelines in matters related to the identification, characterization, analytical determination and evaluation of biological activity of uremic retention solutes. The final aim remains the development of new strategies to reduce the concentration of the most active uremic solutes. These activities will at first be concentrated on reducing factors influencing cardiovascular morbidity and mortality.

Urea percentiles in children with chronic renal failure. Data from the ItalKid project

In chronic renal failure high serum urea levels (sUrea) are correlated with the onset of uremic symptoms. Urea has generally been considered relatively non-toxic, functioning more as a surrogate for other toxic solutes; however, it has been recently reported that it can contribute to uremic toxicity. Clinically sUrea are often difficult to interpret because of the wide range of kidney functions. To obtain a practical and easily accessible tool to evaluate sUrea, we have produced percentile curves for different ranges of chronic renal failure, defined with creatinine clearance ( C(Cr)) obtained with the Schwartz formula. Data were obtained from the Italian Pediatric Registry of Chronic Renal Failure (ItalKid); its inclusion criteria are: (1) C(Cr )<75 ml/min per 1.73 m(2), (2) age <20 years at time of registration, and (3) conservative treatment. To obtain the percentiles, the following patients were excluded: patients with an underlying disease, a concomitant treatment, or a disorder that could affect urea metabolism, per se, and/or food intake, and patients aged <2 years. The study group included 690 subjects (mean age 9.56+/-4.54 years, 485 males). In total, 2,085 observations (C(Cr )and sUrea) were available for the construction of the percentile curves. A median of 258 (range 99-380) observations was obtained for each of the eight different categories of C(Cr )(intervals of 10 ml/min per 1.73 m(2)). The 10th, 25th, 50th, 75th, and 90th percentiles were calculated and a graph was produced. Patients with the highest urea percentiles showed significantly higher plasma levels of phosphorus and parathyroid hormone and significantly lower hemoglobin concentrations and bicarbonate levels. Our percentile curves may help to identify subjects with inappropriate sUrea for a given C(Cr).

The other side of the coin: impact of toxin generation and nutrition on the uremic syndrome

Both the morbidity of the uremic syndrome and the generation of uremic toxins are attributed to malnutrition. If protein intake and catabolism result in the generation of solutes, then nutritional intake should be related directly to toxicity. On the other hand, inadequate nutrition has been linked to inflammation and mortality. It remains difficult to reconcile these two lines of thought. Several possibilities exist that might account for this apparent paradox: 1) not all nutritional and protein degradation products are toxic; 2) toxins generated from increased protein intake are removed if protein intake is linked to dialysis dose; 3) albumin acts as a buffer for toxicity-hypoalbuminemia favors liberation of protein-bound toxins from their binding sites, enhancing their toxicity; 4) solutes generated from tissue breakdown are more toxic than those generated by nutritional protein; 5) both high and low concentrations of solutes have a negative impact; 6) toxic compounds unrelated to protein breakdown are specific causes of malnutrition and inflammation; 7) and/or residual renal function plays a key role in the elimination of compounds discussed under possibility 6. Thus the uremic syndrome should be considered as a potentially fatal interaction among inflammation, malnutrition, low levels of albumin in the plasma, accumulated protein-bound solutes and generation of nonnutritionally related toxins. Not only optimal dialysis, but also optimal nutritional intake and optimal utilization of these nutrients should help neutralize this chain of events.

Dissociation between dialysis adequacy and Kt/V

Since the initiation of dialysis, nephrologists have sought an index (or indices) for the adequacy of toxic solute removal. This quest has been characterized by a gradual shift in thinking, ending with a preference for dynamic parameters such as clearances normalized for body size (Kt/V). The threshold Kt/V, however, has changed over the years. While present guidelines suggest 1.2 with single-pool kinetics, higher levels might be proposed in the future. In spite of the known relation between Kt/V and survival, the accuracy of this parameter as a representative of the removal of the whole spectrum of compounds that are responsible for uremia is problematic. Kt/V only assesses the removal of a water-soluble compound from the body water through mostly hydrophilic membranes to the dialysate water. Furthermore, the small size of urea means that convective and/or diffusive transfer through a given semipermeable membrane is unlikely to be representative of larger molecules, especially if dialyzers with a small pore size are applied. Urea kinetics are also poorly representative of the removal of small protein-bound molecules and intracellular solutes with cell membrane-limited clearance. Finally, it should be realized that the Kt/V concept has been developed in a specific population, that is, a group of renal failure patients with few comorbidities, submitted to short intermittent hemodialysis with small-pore bioincompatible membranes very likely using dialysate of lower quality than that used today. Kt/V might well become less accurate and useful in predicting outcomes as different dialysis conditions are pursued, such as dialysis with biocompatible and/or large-pore membranes, (ultra) pure dialysate, alternative time frames, high levels of convection, and/or in populations with a different distribution of body mass.

Carbamylated hemoglobin: a potential marker for the adequacy of hemodialysis therapy in end-stage renal failure

Urea can dissociate in vivo to form isocyanic acid which can react with hemoglobin to form carbamylated hemoglobin. Previous work has shown that formation of carbamylated hemoglobin depends upon both the severity and the duration of renal failure. To determine whether carbamylated hemoglobin can be used as an assessment of the adequacy of hemodialysis treatment, we prospectively studied 55 stable patients who regularly attended our hospital dialysis program. Carbamylated hemoglobin was greater in those patients with a Kt/V of < or = 1.1 compared to those with a Kt/V of > 1.1 (120 +/- 8 micrograms VH/gHb versus 99 +/- 7, P < 0.01), and there was a negative correlation with Kt/V (r = -0.37, P = 0.007). There were positive correlations between carbamylated hemoglobin and the time-averaged urea concentration (r = 0.4, P = 0.004), and a negative correlation with the urea reduction ratio (r = -0.37, P = 0.01). Carbamylated hemoglobin may therefore be a useful marker of the degree of uremia, just as glycosylated hemoglobin is used in the assessment of patients with diabetes mellitus.

Opposite effects of urea on hemoglobin-oxygen affinity in anemia of chronic renal failure

We studied the action of urea on the spin-spin relaxation rate of 2,3-diphosphoglycerate (2,3-DPG) phosphorus atoms in normal and uremic erythrocytes. At concentrations from 10 to 60 mM, urea increased the relaxation rates of 2,3-DPG P-3 phosphorus atoms. This evidenced a stronger binding of 2,3-DPG to hemoglobin (Hb), suggesting that the deoxyform of Hb was stabilized. This hypothesis was confirmed by measurements of the association constant of oxygen to hemoglobin (K) in normal erythrocytes in presence of urea concentrations in the range of those observed in uremic patients (30 mM). Indeed, the observed decrease in K suggests that the T structure of hemoglobin is stabilized. By contrast, with higher urea concentrations (120 mM), measurements of P50 showed an increase in the hemoglobin affinity for oxygen (decrease in P50). Moreover, the relaxation rates of 2,3-DPG P-3 phosphorus atoms were not modified, which is consistent with the simultaneous increase of K. This may be attributed to the formation of carbamylated hemoglobin in presence of urea. These results suggest two opposite effects of urea on Hb-O2 affinity: the first reinforces 2,3-DPG-Hb binding and leads to a decrease in O2 affinity; the second, mediated by carbamylation of Hb, hinders the binding of 2,3-DPG and increases the O2 affinity. These findings are consistent with the fact that, despite the presence of carbamylated hemoglobin, uremic patients do not present increased Hb-O2 affinity.

Abnormal glycine betaine content of the blood and urine of diabetic and renal patients

In normal human plasma the concentrations of the renal osmolyte, glycine betaine, are usually between 20 and 70 mumol/l, in adult males (median 44 mumol/l) higher than in females (34 mumol/l). Concentrations are lower in renal disease (median 28 mumol/l) and normal in diabetes. Urinary excretion of glycine betaine shows no sex difference and is frequently elevated both in renal disease and in diabetes (medians: normal, 6.2, renal 12.3 and diabetes, 39.7 mmol/mol creatinine). The elevation in diabetes does not strongly correlate with known renal disease, nor with either urinary microalbumin or plasma creatinine. There is no correlation with glycated haemoglobin. The positive correlation with the excretions of another renal osmolyte, sorbitol, was highly significant in diabetic subjects. In the diabetic group there was also a significant negative correlation between plasma glycine betaine and urine microalbumin.