Serum Leptin Correlates with Fat Mass but not Dietary Energy Intake in Continuous Ambulatory Peritoneal Dialysis Patients

Objectives

In view of previous studies demonstrating hyperleptinemia in uremic and hemodialysis patients, the aims of the present study were to determine whether serum leptin levels are elevated in peritoneal dialysis (PD) patients, to establish whether leptin is significantly removed by PD, and to elucidate the relationship of plasma leptin to body composition, dietary intake, nutritional indices, and dialysis adequacy.

Design

Cross-sectional analysis of PD patients and matched healthy controls.

Setting

Tertiary-care institutional dialysis center.

Participants

The study included 49 PD patients [35 women and 14 men; median age 63 years, interquartile range (IQR) 49.5 -68.5 yr; body mass index (BMI) 25.5: I: 0.8] and 27 controls (11 men and 16 women; median age 42 years, IQR 34.8 51; BMI 27.2: I: 0.9). For evaluation of leptin clearance, 8 patients receiving nocturnal intermittent PD were also evaluated.

Main Outcome Measures

The primary outcome measure was plasma leptin concentration. Dialysate leptin concentration was also measured in 7 patients.

Results

Serum leptin levels were significantly higher (p < 0.01) in patients (males: median 11 nglmL, IQR 9 19 ng/mL; females: 53 ng/mL, 19.5 -128 ng/mL) compared with controls (males: 5.5 nglmL, 4 9.5 nglmL; females: 12 ng/mL, 9.8 17.3 ng/mL). Leptin levels in both groups correlated positively with BMI (r = 0.64 and 0.60, respectively; p < 0.0001) and with percentage body fat determined by dual-energy x-ray absorptiometry (r = 0.86 and 0.82, respectively; p < 0.01). Dialysis patients exhibited a greater increase in serum leptin for any given increase in BMI. No significant correlation was observed between leptin concentration and residual renal function, dialysis adequacy (Kt/V), dietary protein or caloric intake, or serum levels of albumin, prealbumin, C-reactive protein, glucose, and insulin-like growth factor-1. Although leptin was detectable in peritoneal dialysate after a 6-hour dwell (median 4.2 ng/ mL, IQR 1.1 -8.5 ng/mL, n = 8), serum leptin levels were not appreciably lowered following intermittent PD via an automated cycler (63.9: I: 19.3 ng/mL vs 57.6: I: 20.5 ng/mL, p = NS, n = 8).

Conclusions

Serum leptin levels are elevated in PD patients and are not appreciably cleared by PD. Although hyperleptinemia correlates poorly with dialysis adequacy and protein intake, a strong and significant relationship was maintained between serum leptin and fat mass. Serum leptin could therefore serve as a useful clinical marker of body fat content in PD patients.

Uremic Toxicity: Present State of the Art

The uremic syndrome is a complex mixture of organ dysfunctions, which is attributed to the retention of a myriad of compounds that under normal condition are excreted by the healthy kidneys (uremic toxins). In the area of identification and characterization of uremic toxins and in the knowledge of their pathophysiologic importance, major steps forward have been made during recent years. The present article is a review of several of these steps, especially in the area of information about the compounds that could play a role in the development of cardiovascular complications. It is written by those members of the Uremic Toxins Group, which has been created by the European Society for Artificial Organs (ESAO). Each of the 16 authors has written a state of the art in his/her major area of interest.

Implications of circulating Meteorin-like (Metrnl) level in human subjects with type 2 diabetes

AIMS

Meteorin-like (Metrnl) was recently identified as a novel adipomyokine induced by exercise and cold exposure. Metrnl improves glucose tolerance, increases systemic energy expenditure, induces white adipose browning, and promotes anti-inflammatory gene programs in obese/diabetic mice. However, the relationship of Metrnl with diabetes and cardiometabolic risk variables in humans has not been explored.

METHODS

In 800 subjects (400 patients with type 2 diabetes and 400 non-diabetes), Metrnl concentration was measured with an enzyme-linked immunosorbent assay, and the correlations of Metrnl level with anthropometric parameters, lifestyle factors, body composition values, and laboratory measurements were assessed.

RESULTS

Metrnl concentration was significantly higher in patients with diabetes than in those without diabetes [median (inter-quartile range); diabetes: 1219.9 (1020.6, 1535.6), non-diabetes: 1131.2 (993.1, 1313.6) pg/ml, P < .001]. After adjustment for age and sex, Metrnl level was significantly associated with fasting plasma glucose, blood pressure, lipid profile, and eGFR, but not with BMI or percent body fat. Multiple stepwise regression analysis exhibited that Metrnl level was independently associated with diabetes status (P < .001), eGFR (P < .001), and total cholesterol (P = .026) (R² = 0.127). In multiple logistic regression analysis, the odds ratio for the risk of diabetes was 3.53 (95% confidence interval: 2.04-6.10) in the highest tertile of Metrnl compared to the lowest after adjustment for confounding factors.

CONCLUSIONS

This study is the first to demonstrate that Metrnl level is elevated in human subjects with type 2 diabetes and is inversely related to various cardiometabolic risk factors, including renal function.

Hyperleptinemia positively associated with central arterial stiffness in hemodialysis patients

Objective

Leptin plays a role in stimulating vascular inflammation, vascular smooth muscle hypertrophy, and augmenting blood pressure, which contributes to the pathogenesis of atherosclerosis and leads to arterial stiffness. This vascular damage, measured by carotid-femoral pulse wave velocity (cfPWV), is recognized as an independent predictor of cardiovascular mortality in hemodialysis (HD) patients. The aim of this study was to evaluate the relationship between serum leptin and arterial stiffness in HD patients.

Patients and methods

In 112 of the 126 HD patients were eligible and their biochemical data were collected for analysis. Serum leptin level was measured using a commercial enzyme-linked immunosorbent assay kit. Carotid-femoral pulse wave velocity was measured by a validated tonometry system (SphygmoCor). Those have cfPWV values above 10 m/s are defined as the high arterial stiffness group.

Results

Among the participants, thirty-eight of them who were in the high arterial stiffness group, had a higher prevalence of diabetes mellitus (p = 0.002), age (p = 0.029), body mass index (BMI, p = 0.018), body fat mass (p = 0.001), hemoglobin (p = 0.040), and serum leptin levels (P<0.001). Multivariable logistic regression analysis showed that leptin (odds ratio [OR] 1.09; 95% confidence interval [CI] 1.04–1.14; p <0.001), diabetes (OR 7.17; CI 1.39–37.00; p = 0.019), body fat mass (OR 1.16; CI 1.02–1.33; p = 0.027); and hemoglobin (OR 2.11; CI 1.15–3.87; p = 0.015) were independently associated with arterial stiffness in HD patients.

Conclusion

In our study, hyperleptinemia was positively correlated to the high cfPWV and thus was related to high arterial stiffness in HD patients.

Obesity, Oxidative Stress, Adipose Tissue Dysfunction, and the Associated Health Risks: Causes and Therapeutic Strategies

Obesity is gaining acceptance as a serious primary health burden that impairs the quality of life because of its associated complications, including diabetes, cardiovascular diseases, cancer, asthma, sleep disorders, hepatic dysfunction, renal dysfunction, and infertility. It is a complex metabolic disorder with a multifactorial origin. Growing evidence suggests that oxidative stress plays a role as the critical factor linking obesity with its associated complications. Obesity per se can induce systemic oxidative stress through various biochemical mechanisms, such as superoxide generation from NADPH oxidases, oxidative phosphorylation, glyceraldehyde auto-oxidation, protein kinase C activation, and polyol and hexosamine pathways. Other factors that also contribute to oxidative stress in obesity include hyperleptinemia, low antioxidant defense, chronic inflammation, and postprandial reactive oxygen species generation. In addition, recent studies suggest that adipose tissue plays a critical role in regulating the pathophysiological mechanisms of obesity and its related co-morbidities. To establish an adequate platform for the prevention of obesity and its associated health risks, understanding the factors that contribute to the cause of obesity is necessary. The most current list of obesity determinants includes genetic factors, dietary intake, physical activity, environmental and socioeconomic factors, eating disorders, and societal influences. On the basis of the currently identified predominant determinants of obesity, a broad range of strategies have been recommended to reduce the prevalence of obesity, such as regular physical activity, ad libitum food intake limiting to certain micronutrients, increased dietary intake of fruits and vegetables, and meal replacements. This review aims to highlight recent findings regarding the role of oxidative stress in the pathogenesis of obesity and its associated risk factors, the role of dysfunctional adipose tissue in development of these risk factors, and potential strategies to regulate body weight loss/gain for better health benefits.

Diet induced obesity in rats reduces NHE3 and Na(+) /K(+) -ATPase expression in the kidney

The consumption of a high fat diet (HFD) is associated with proteinuria and altered sodium handling and excretion, which can lead to kidney disease. In the proximal tubule, the Na(+) /H(+) Exchanger 3 (NHE3) is responsible for normal protein reabsorption and the reabsorption of approximately 70% of the renal sodium load. It is the Na(+) /K(+) -ATPase that provides the driving force for the reabsorption of sodium and its exit across the basolateral membrane. This study investigates the effects that consumption of a HFD for 12 weeks has on NHE3 and Na(+) /K(+) -ATPase expression in the kidney. Western blot analysis identified a significant reduction in NHE3 and its modulator, phosphorylated protein kinase B, in renal lysate from obese rats. In the obese rats, a reduction in NHE3 expression in the proximal tubule may impact on the acidification of endosomes which are responsible for albumin uptake, suggesting a key role for the exchanger in protein endocytosis in obesity. Western blot analysis identified a reduction in Na(+) /K(+) -ATPase which could also potentially impact on albumin uptake and sodium reabsorption. This study demonstrates that consumption of a HFD for 12 weeks reduces renal NHE3 and Na(+) /K(+) -ATPase expression, an effect that may contribute to the albuminuria associated with obesity. Furthermore the reduction in these transporters is not likely to contribute to the reduced sodium excretion in obesity. These data highlight a potential link between NHE3 and Na(+) /K(+) -ATPase in the pathophysiological changes in renal protein handling observed in obesity.

Hyperleptinemia Is a Risk Factor for the Development of Central Arterial Stiffness in Kidney Transplant Patients

BACKGROUND

Arterial stiffness could cause adverse outcomes in kidney transplant (KT) patients. Leptin has a role in influencing vascular smooth muscle that may contribute to atherosclerosis. The aim of this study was to evaluate the relationship between fasting serum leptin concentration and carotid-femoral pulse wave velocity (cfPWV) in KT patients.

MATERIALS AND METHODS

Fasting blood samples were obtained from 55 KT patients and 65 subjects from the outpatient department were enrolled as the control group. The cfPWV values of >10 m/s were used to define as the high arterial stiffness group and <10 m/s as the low arterial stiffness group. The predictive ability of leptin for arterial stiffness of KT was assessed using receiver operating characteristic (ROC) curve and multivariate logistic regression analyses.

RESULTS

Kidney transplant patients had lower hemoglobin, but higher blood urea nitrogen, creatinine, total cholesterol, diastolic blood pressure, intact parathyroid hormone levels, and leptin levels than controls. Although cfPWV levels were higher in KT patients, there is no difference of cfPWV levels between KT patients and control (P = .595). Fifteen KT patients (27.3%) were defined in the high arterial stiffness group, and serum leptin level was higher in the high arterial stiffness group compared with the low arterial stiffness group in KT patients (P < .001). Multivariate logistic regression analysis showed that leptin (odds ratio: 1.044, 95% confidence interval [CI]: 1.016-1.072, P = .002) was an independent predictor of arterial stiffness in KT patients. The sensitivity, specificity, positive predictive value, negative predictive value, and area under the ROC curve predicting arterial stiffness in KT patients were 73.33%, 87.5%, 68.7%, 89.7%, and 0.828 (95% CI: 0.703-0.917, P < .001), and the leptin cut-off value was 74.14 ng/mL.

CONCLUSION

Serum fasting leptin level could predict the development of central arterial stiffness of KT patients.

Adipocytokines in renal transplant recipients

In the last two decades, perceptions about the role of body fat have changed. Adipocytes modulate endocrine and immune homeostasis by synthesizing hundreds of hormones, known as adipocytokines. Many studies have been investigating the influences and effects of these adipocytokines and suggest that they are modulated by the nutritional and immunologic milieu. Kidney transplant recipients (KTRs) are a unique and relevant population in which the function of adipocytokines can be examined, given their altered nutritional and immune status and subsequent dysregulation of adipocytokine metabolism. In this review, we summarize the recent findings about four specific adipocytokines and their respective roles in KTRs. We decided to evaluate the most widely described adipocytokines, including leptin, adiponectin, visfatin and resistin. Increasing evidence suggests that these adipocytokines may lead to cardiovascular events and metabolic changes in the general population and may also increase mortality and graft loss rate in KTRs. In addition, we present findings on the interrelationship between serum adipocytokine levels and nutritional and immunologic status, and mechanisms by which adipocytokines modulate morbidity and outcomes in KTRs.

Acute leptin exposure reduces megalin expression and upregulates TGFβ1 in cultured renal proximal tubule cells

Increased leptin concentrations observed in obesity can lead to proteinuria, suggesting that leptin may play a role in obesity-related kidney disease. Obesity reduces activation of AMP-activated protein kinase (AMPK) and increases transforming growth factor-β1 (TGF-β1) expression in the kidney, leading to albuminuria. Thus we investigated if elevated leptin altered AMPK and TGF-β1 signaling in proximal tubule cells (PTCs). In opossum kidney (OK) PTCs Western blot analysis demonstrated that leptin upregulates TGF-β1 secretion (0.50 µg/ml) and phosphorylated AMPKα (at 0.25, and 0.50 µg/ml), and downregulates megalin expression at all concentrations (0.05-0.50 µg/ml). Using the AMPK inhibitor, Compound C, leptin exposure regulated TGF-β1 expression and secretion in PTCs via an AMPK mediated pathway. In addition, elevated leptin exposure (0.50 µg/ml) reduced albumin handling in OK cells independently of megalin expression. This study demonstrates that leptin upregulates TGF-β1, reduces megalin, and reduces albumin handling in PTCs by an AMPK mediated pathway.

The peptidic middle molecules: is molecular weight doing the trick?

Chronic kidney disease (CKD) is characterized by a gradual endogenous intoxication caused by the progressive accumulation of bioactive compounds that in normal conditions would be excreted and/or metabolized by the kidney. Uremic toxicity now is understood as one of the potential causes for the excess of cardiovascular disease and mortality observed in CKD. An important family of uremic toxins is that of the peptidic middle molecules, with a molecular weight ranging between 500 and 60,000 Da, which makes them, as a consequence, difficult to remove in the process of dialysis unless the dialyzer pore size is large enough. This review provides an overview of the main and best-characterized peptidic middle molecules and their role as potential culprits of the cardiometabolic complications inherent to CKD patients.

Short term exposure to elevated levels of leptin reduces proximal tubule cell metabolic activity

Leptin plays a pathophysiological role in the kidney, however, its acute effects on the proximal tubule cells (PTCs) are unknown. In opossum kidney (OK) cells in vitro, Western blot analysis identified that exposure to leptin increases the phosphorylation of the mitogen-activated protein kinase (MAPK) p44/42 and the mammalian target of rapamycin (mTOR). Importantly leptin (0.05, 0.10, 0.25 and 0.50 μg/ml) significantly reduced the metabolic activity of PTCs, and significantly decreased protein content per cell. Investigation of the role of p44/42 and mTOR on metabolic activity and protein content per cell, demonstrated that in the presence of MAPK inhibitor U0126 and mTOR inhibitor Ku-63794, that the mTOR pathway is responsible for the reduction in PTC metabolic activity in response to leptin. However, p44/42 and mTOR play no role the reduced protein content per cell in OKs exposed to leptin. Therefore, leptin modulates metabolic activity in PTCs via an mTOR regulated pathway.

Adipokines as a link between obesity and chronic kidney disease

Adipocytes secrete a number of bioactive adipokines that activate a variety of cell signaling pathways in central and peripheral tissues. Obesity is associated with the altered production of many adipokines and is linked to a number of pathologies. As an increase in body weight is directly associated with an increased risk for developing chronic kidney disease (CKD), there is significant interest in the link between obesity and renal dysfunction. Altered levels of the adipokines leptin, adiponectin, resistin, and visfatin can decrease the glomerular filtration rate and increase albuminuria, which are pathophysiological changes typical of CKD. Specifically, exposure of the glomerulus to altered adipokine levels can increase its permeability, fuse the podocytes, and cause mesangial cell hypertrophy, all of which alter the glomerular filtration rate. In addition, the adipokines leptin and adiponectin can act on tubular networks. Thus, adipokines can act on multiple cell types in the development of renal pathophysiology. Importantly, most studies have been performed using in vitro models, with future studies in vivo required to further elucidate the specific roles that adipokines play in the development and progression of CKD.

Overview of the physiology and pathophysiology of leptin with special emphasis on its role in the kidney

The adipocyte product leptin is a pleiotropic adipokine and hormone, with a role extending beyond appetite suppression and increased energy expenditure. This review summarizes the biology of the leptin system and the roles of its different receptors in a multitude of cellular functions in different organs, with special emphasis on the kidney. Leptin's physiological functions as well as deleterious effects in states of leptin deficiency or hyperleptinemia are emphasized. Chronic hyperleptinemia can increase blood pressure through the sympathetic nervous system and renal salt retention. The concept of selective leptin resistance in obesity is emerging, whereby leptin's effect on appetite and energy expenditure is blunted, with a concomitant increase in leptin's other effects as a result of the accompanying hyperleptinemia. The divergence in response likely is explained by different receptors and post-receptor activating mechanisms. Chronic kidney disease is a known cause of hyperleptinemia. There is an emerging view that the effect of hyperleptinemia on the kidney can contribute to the development and/or progression of chronic kidney disease in selective resistance states such as in obesity or type 2 diabetes mellitus. The mechanisms of renal injury are likely the result of exaggerated and undesirable hemodynamic influences as well as profibrotic effects.

Normal and pathologic concentrations of uremic toxins

An updated review of the existing knowledge regarding uremic toxins facilitates the design of experimental studies. We performed a literature search and found 621 articles about uremic toxicity published after a 2003 review of this topic. Eighty-seven records provided serum or blood measurements of one or more solutes in patients with CKD. These records described 32 previously known uremic toxins and 56 newly reported solutes. The articles most frequently reported concentrations of β2-microglobulin, indoxyl sulfate, homocysteine, uric acid, and parathyroid hormone. We found most solutes (59%) in only one report. Compared with previous results, more recent articles reported higher uremic concentrations of many solutes, including carboxymethyllysine, cystatin C, and parathyroid hormone. However, five solutes had uremic concentrations less than 10% of the originally reported values. Furthermore, the uremic concentrations of four solutes did not exceed their respective normal concentrations, although they had been previously described as uremic retention solutes. In summary, this review extends the classification of uremic retention solutes and their normal and uremic concentrations, and it should aid the design of experiments to study the biologic effects of these solutes in CKD.

Leptin and lipid metabolism in chronic kidney failure

OBJECTIVE

In the general population, leptin has been associated with atherosclerosis and has been shown to interfere with lipoprotein profiles. Patients with chronic renal failure are at increased risk of cardiovascular disease and display alterations in both lipoprotein and leptin levels. The aim of this study was to investigate the relationship between leptin and the lipoprotein profile in non-dialyzed patients with chronic kidney disease (CKD).

MATERIAL AND METHODS

Leptin and lipid and lipoprotein concentrations were studied in 73 CKD patients and in 68 healthy controls in a cross-sectional case-control design.

RESULTS

The mean leptin levels were increased in the CKD patients (24.0 (SD 37.1) ng/mL) compared to those in controls (9.0 (SD 8.5) ng/mL) (p = 0.008). Also, the ratio between leptin levels and body mass index (leptin/BMI) was increased in CKD patients (mean 0.80 (SD 1.03)) compared to that in controls (0.31 (SD 0.24)) (p = 0.001). In linear regression analysis, leptin independently predicted total cholesterol and triglycerides in CKD patients (p = 0.010 and p = 0.001, respectively) and ratio between total and HDL cholesterol (Chol/HDL) in controls (p = 0.024). Furthermore, in CKD patients, the leptin/BMI predicted the variation in total cholesterol and triglycerides (p = 0.010 and p = 0.002, respectively).

CONCLUSIONS

Leptin concentrations and leptin/BMI were elevated in CKD patients compared to those in controls. Leptin levels in both study groups, and leptin/BMI in the CKD group, were associated with atherogenic lipid profiles, which may contribute to the elevated cardiovascular risk that has been linked to hyperleptinaemia.

Role of adipose tissue in determining muscle mass in patients with chronic kidney disease

OBJECTIVE

Malnutrition is a powerful predictor of mortality in chronic kidney disease (CKD). However, its etiology is unclear. We hypothesized that the adipocyte-derived proteins leptin and adiponectin, inflammation (as measured by C-reactive protein, CRP), and insulin resistance (as measured by homeostasis model assessment, HOMA), implicated in the malnutrition-inflammation complex syndrome commonly seen in maintenance dialysis patients, would be associated with the loss of muscle mass in earlier stages of CKD. Arm muscle area was used as an indicator of muscle mass.

PATIENTS AND SETTING

The Modification of Diet in Renal Disease Study cohort of people with CKD stages 3 and 4 was used for analysis (N = 780).

MAIN OUTCOME MEASURES

Regression models were carried out to examine the relationships of leptin, adiponectin, CRP, and HOMA with arm muscle area (the main study outcome).

RESULTS

Arm muscle area was 39 +/- 15 cm(2) (mean +/- SD), and adiponectin levels were 13 +/- 7 microg/mL. Median and interquartile range (IQR) concentrations were: 9.0 (13.6) ng/mL for leptin, 2.3 (4.9) mg/L for CRP, and 2.4 (2.0) for HOMA. Higher leptin (beta coefficient and 95% confidence interval, -6.9 [-8.7 to -5.1], P < .001) and higher CRP (beta coefficient and 95% confidence interval, -2.7 [-3.9 to -1.4], P < .001) were associated with lower arm muscle area. There was a trend toward lower arm muscle area with higher adiponectin (P = .07), but no association with HOMA (P = .80).

CONCLUSION

Leptin and CRP were associated with lower muscle mass in subjects with CKD at stages 3 and 4. Further studies are needed to understand the mechanisms underlying these associations, and to develop targeted interventions for this patient population.

Inconsistency of reported uremic toxin concentrations

Discrepancies in reported uremic toxin concentrations were evaluated for 78 retention solutes. For this analysis, 378 publications were screened. Up to eight publications per toxin were retained. The highest and the lowest reported concentrations, as well as the median reported concentration were registered. The ratio between the highest and the lowest (H/L) concentrations and, for some solutes, also the ratio between the highest and the median (H/M) concentrations were calculated. The compounds were arbitrarily subdivided into three groups based on their H/L ratio: group A, H/L < 3 (n = 33); group B, 3 < H/L < 8.5 (n = 20); and group C, H/L > 8.5 (n = 25). Solutes of groups A and B showed a low to intermediate scatter, suggesting a homogeneity of reported data. Group C showed a more substantial scatter. For at least 10 compounds of group C, extremely divergent concentrations were registered (H/M > 5.5) using scatter plot analysis. For all solutes of groups A and B, the highest reported concentration could be used as a reference. For some solutes of group C and for the compounds showing a divergent scatter analysis, however, more refined directives should be followed.

A cute plasma ghrelin and leptin responses to oral feeding or intraperitoneal hypertonic glucose-based dialysate in patients with chronic renal failure

BACKGROUND

Chronic renal failure (CRF) is associated with increased plasma levels of ghrelin and leptin, but the regulation of the secretion of these hormones has been insufficiently studied, in this setting. The aim of this study was to analyze the acute effects of oral feeding or intraperitoneal 3.86% glucose-based dialysate infusion on plasma ghrelin and leptin levels in patients with CRF undergoing peritoneal dialysis (PD).

METHODS

Following a crossover design, 10 patients and eight healthy controls underwent a standardized oral intake, a 3.86% glucose-based dialysate PD exchange (patients) and placebo oral intake. We scrutinized acute changes in plasma ghrelin, leptin, glucose, insulin, and growth hormone (GH) levels.

RESULTS

In patients, total ghrelin decreased modestly immediately after oral feeding (nadir 90.6% of baseline, range 85.1, 94.5, P= 0.03) or the PD exchange test (92.2%, range 58.7, 101.9, P= 0.05) (median). Response to oral feeding was markedly blunted when compared with healthy individuals (73.8%, range 56.1, 89.1, P= 0.007) (P < 0.005 vs. patients). Plasma acyl-ghrelin had a less marked but more persistent decay after the PD exchange test (nadir 80.4%, range 55.1, 96.3, P= 0.02) than after oral intake (64.4%, range 45.6, 82.3, P= 0.005); again, changes were more intense in normal controls (47.4%, range 32.1, 67.3, P= 0.01) (P < 0.05 vs. patients). Leptin levels decreased slightly (P < 0.05) after the PD exchange in patients, but did not respond acutely to oral feeding in patients or controls.

CONCLUSION

Ghrelin secretion is partially refractory to the acute inhibitory effect of oral feeding in patients with CRF undergoing PD therapy. A 3.86% glucose-based PD exchange results in a significant decrease of plasma ghrelin levels. Plasma leptin levels are not acutely affected by oral feeding in patients with CRF or healthy individuals.

Leptin, ghrelin, and proinflammatory cytokines: compounds with nutritional impact in chronic kidney disease?

Metabolic and nutritional derangements are prominent features of the uremic syndrome. Recent evidence suggest that several large-molecular-weight molecules that often are elevated in uremia, such as leptin, ghrelin, and proinflammatory cytokines, may have nutritional impact in this patient group. On the basis of present knowledge, these compounds could be regarded as suspected but not established uremic toxins. The discovery of the ob gene, its product leptin, and cerebral leptin receptors has undoubtedly widened our understanding of obesity and the underlying molecular and physiologic mechanisms that regulate food intake and body weight. Moreover, the recent discovery of leptin receptor isoforms in several peripheral organs suggests that leptin besides having a central function also has several important peripheral biological functions. Because uremic patients in general have an inappropriate elevation of circulatory leptin, further research is necessary to determine the potential biological effects of elevated leptin levels in end-stage renal disease. Also, because many symptoms and findings prevalent in the uremic syndrome are known to be associated with elevated levels of proinflammatory cytokines, such as interleukin-6, future studies are needed to evaluate the role of specific anti-inflammatory treatment strategies in malnourished uremic patients.

Acidosis downregulates leptin production from cultured adipocytes through a glucose transport-dependent post-transcriptional mechanism

Metabolic acidosis, a common feature of uremia, has a well documented wasting effect on skeletal muscle. In contrast, the effect of metabolic acidosis on adipose tissue is unknown. Serum levels of the adipocyte hormone leptin have been shown to be lower in acidotic uremic rats when compared with uremic controls. This study investigated the effect of acidosis on leptin protein secretion and leptin gene expression. This was studied in vitro by means of 3T3-L1 cultured adipocytes. Leptin secretion was decreased at an acid pH of 7.1 compared with a control pH of 7.5 (1277 versus 1950 pg/well/48 h, P < 0.05). In contrast, acidosis did not affect leptin mRNA content. Glucose transport was reduced by 39% at pH 7.1 at 24 h, which was comparable in magnitude with the inhibition of leptin secretion at the same pH. The glucose transport inhibitors cytochalasin B (0.5 to 50 micro M) and phloretin (0.05 to 0.25 mM) mimicked the effect of acidosis and reduced leptin secretion in a dose-dependent fashion (P < 0.02). Dose-response curves for the inhibition of glucose uptake showed that decreasing glucose transport to the same extent as with acid was sufficient to drive down leptin secretion, independently of changes of leptin mRNA. Acid decreases leptin secretion from 3T3-L1 adipocytes through a post-transcriptional mechanism via changes in glucose transport. This starvation-like response may be physiologically important in conditions such as uremia to prevent excessive energy expenditure.

Protein and energy nutrition among adult patients treated with chronic peritoneal dialysis

Protein-energy malnutrition (PEM) in adult patients treated with chronic peritoneal dialysis (CPD), which is highly prevalent and frequently severe in its manifestation, poses a significant therapeutic dilemma. The causes of PEM include inflammation, low nutrient intake, nutrient losses during dialysis, metabolic acidemia, coexisting illnesses, and possibly the endocrine disorders of uremia. Treatment strategies for PEM in CPD patients include the following: attempt to treat the potentially reversible causes of anorexia, increase nutrient intake (by nutritional counseling, oral food supplements, consideration of appetite stimulants and intraperitonial amino acid solutions), and the correction of metabolic acidosis. Coexisting illnesses engendering PEM should be treated. Experimental evidence suggests that such agents as anabolic steroids, human growth hormone, insulin-like growth factor-I, and L-carnitine may engender positive protein balance in these individuals. Finally, the use of anti-inflammatory agents to improve the nutritional status of malnourished CPD patients remains to be defined. There is a need to carry out clinical trials that examine whether an improvement in the nutritional status of CPD patients is associated with an improvement in their mortality, morbidity and/or quality of life.

Review on uremic toxins: classification, concentration, and interindividual variability

BACKGROUND

The choice of the correct concentration of potential uremic toxins for in vitro, ex vivo, and in vivo experiments remains a major area of concern; errors at this level might result in incorrect decisions regarding therpeutic correction of uremia and related clinical complications.

METHODS

An encyclopedic list of uremic retention solutes was composed, containing their mean normal concentration (CN), their highest mean/median uremic concentration (CU), their highest concentration ever reported in uremia (CMAX), and their molecular weight. A literature search of 857 publications on uremic toxicity resulted in the selection of data reported in 55 publications on 90 compounds, published between 1968 and 2002.

RESULTS

For all compounds, CU and/or CMAX exceeded CN. Molecular weight was lower than 500 D for 68 compounds; of the remaining 22 middle molecules, 12 exceeded 12,000 D. CU ranged from 32.0 ng/L (methionine-enkephalin) up to 2.3 g/L (urea). CU in the ng/L range was found especially for the middle molecules (10/22; 45.5%), compared with 2/68 (2.9%) for a molecular weight <500 D (P < 0.002). Twenty-five solutes (27.8%) were protein bound. Most of them had a molecular weight <500 D except for leptin and retinol-binding protein. The ratio CU/CN, an index of the concentration range over which toxicity is exerted, exceeded 15 in the case of 20 compounds. The highest values were registered for several guanidines, protein-bound compounds, and middle molecules, to a large extent compounds with known toxicity. A ratio of CMAX/CU <4, pointing to a Gaussian distribution, was found for the majority of the compounds (74/90; 82%). For some compounds, however, this ratio largely exceeded 4 [e.g., for leptin (6.81) or indole-3-acetic acid (10.37)], pointing to other influencing factors than renal function, such as gender, genetic predisposition, proteolytic breakdown, posttranslation modification, general condition, or nutritional status.

CONCLUSION

Concentrations of retention solutes in uremia vary over a broad range, from nanograms per liter to grams per liter. Low concentrations are found especially for the middle molecules. A substantial number of molecules are protein bound and/or middle molecules, and many of these exert toxicity and are characterized by a high range of toxic over normal concentration (CU/CN ratio). Hence, uremic retention is a complex problem that concerns many more solutes than the current markers of urea and creatinine alone. This list provides a basis for systematic analytic approaches to map the relative importance of the enlisted families of toxins.

Factors contributing to higher hematocrit levels in hemodialysis patients not receiving recombinant human erythropoietin

Recombinant human erythropoietin (rHuEPO) is used to correct anemia in the majority of hemodialysis patients, but a few patients can maintain greater hematocrits without the use of rHuEPO. We aim to investigate which factors stimulate erythropoiesis, other than rHuEPO, in hemodialysis patients. One hundred fifty-eight patients undergoing regular hemodialysis treatment participated in a cross-sectional study. To keep the target hematocrit of 30%, 133 patients (84%) were administered rHuEPO, but 25 patients (16%) did not need rHuEPO. Mean hematocrits were 33.4% +/- 4.6% in patients who did not need rHuEPO and 30.9% +/- 4.0% in those administered rHuEPO. In the analysis of factors contributing to the lack of requirement of rHuEPO with multivariate logistic regression analysis, years on dialysis therapy and body mass index (BMI) were determined to be independent factors with odds ratios of 1.12 (95% confidence interval [CI], 1.02 to 1.23; P = 0.02) and 1.36 (95% CI, 1.13 to 1.63; P = 0.001), respectively. Neither serum erythropoietin level, albumin concentration, nor normalized protein catabolic rate contributed to the lack of requirement of rHuEPO. BMI correlated closely with log serum leptin level (r = 0.55; P < 0.0001), and log serum leptin level correlated inversely with rHuEPO dose (r = -0.18; P = 0.03). These results indicate that 16% of hemodialysis patients could maintain greater hematocrits without the administration of rHuEPO, and independently contributing factors were greater BMI and more years on hemodialysis therapy. Regarding nutritional parameters, a relatively greater BMI, possibly through effects of greater leptin levels, may stimulate erythropoiesis in uremic patients even in the absence of sufficient erythropoietin production.

Critical care issues in the patient with chronic renal failure

The metabolic abnormalities associated with chronic renal failure and complications of the dialysis procedure present unique challenges in critical care medicine. Understanding how renal failure impacts the development and management of cardiovascular disease, bleeding tendencies, infection, and malnutrition is necessary to provide optimal care for these patients. The recognition of ESRD as a state of chronic inflammation and increased oxidative stress ultimately should lead to more effective treatment approaches for several of the comorbid conditions common in this patient population.

Serum leptin concentration in peritoneal dialysis patients: determinants, longitudinal evolution and circadian rhythm

The serum concentration of leptin, an hormone secreted by adipocytes, is increased in obese and chronic renal failure patients. To determine the influence of peritoneal dialysis (PD) therapy on serum leptin levels, we analyzed its concentration in 23 patients on PD for an average of 26.8 +/- 7.1 months and compared it to that of 18 patients with chronic renal failure (creatinine clearance: 49.1 +/- 8.8 ml/min) and of 35 healthy control subjects. Leptin level was also reevaluated in 11 PD patients 9.3 +/- 1 months after the initial analysis. Finally, circadian leptin production was determined in 4 patients on automated PD (APD). Serum leptin was significantly higher in PD (31.9 +/- 7.8 ng/ml) than in chronic renal failure patients (15.2 +/- 5.9 ng/ml) and in healthy control subjects (9.6 +/- 1.1 ng/ml). Serum leptin level was significantly correlated with BMI in all three groups (except in PD males) and with the percentage of fat mass in both male and female PD patients. It did not correlate in PD patients with serum albumin concentration, free fat mass, residual diuresis, time on PD and characteristics of peritoneal permeability. At the second determination, serum leptin level had significantly increased in the PD patients although their respective BMI and serum creatinine concentration had remained virtually unchanged. Finally, in APD patients, the highest leptin level was observed at 08.00 a.m. These results demonstrate that serum leptin level is increased in PD patients and that it progressively rises under PD therapy. The circadian leptin production is delayed in APD patients probably suggesting a negative effect of the nocturnal glucose load on the regulation of its secretion.

Increases in serum leptin levels during peritoneal dialysis are associated with inflammation and a decrease in lean body mass

Leptin, secreted from fat cells, functions as a lipostat mechanism through modulation of satiety signals. Markedly elevated leptin levels have been documented in uremic patients, especially in those who are treated by peritoneal dialysis (PD). However, the role of hyperleptinemia in uremic patients is not clear, and it is not known whether elevated leptin levels contribute to uremic anorexia and changes in body composition. In this prospective study, serum leptin, C-reactive protein (CRP), plasma insulin, and body composition (dual-energy x-ray absorptiometry) were measured in 36 patients (53 +/- 1 yr) close to start and after about 1 yr of PD. In addition, markers of dialysis adequacy and urea kinetics were followed during treatment with PD. During PD, the total body fat mass (20.5 +/- 1.0 to 22.9 +/- 1.3 kg; P < 0.01), truncal fat mass (11.5 +/- 0.7 to 13. 2 +/- 0.9 kg; P < 0.001), and serum leptin levels (20.1 +/- 3.8 to 35.6 +/- 6.8 ng/ml; P < 0.01) increased markedly, especially in patients with diabetes mellitus. Twenty-five PD patients that lost lean body mass during PD had significantly (P < 0.05) elevated initial CRP levels (14 +/- 2 mg/L) compared to 11 patients (<10 mg/L) who gained lean body mass during PD. A significant increase in serum leptin levels (20.9 +/- 4.2 to 42.7 +/- 4.0 ng/ml; P < 0.001) was observed in those patients who lost lean body mass, whereas no such change (18.4 +/- 8.4 to 19.2 +/- 6.4 ng/ml) was observed in the patients that gained lean body mass during PD treatment. The present longitudinal results demonstrate that serum leptin level and body fat content increase markedly during PD, especially in diabetic patients. Patients that lost lean body mass during PD had higher initial CRP levels and increased their serum leptin levels significantly during PD compared to those patients that gained lean body mass. Additional studies are therefore needed to elucidate the role of hyperleptinemia and inflammation in causing anorexia, protein-malnutrition, and changes in body composition during treatment with PD.

Physiological roles of the leptin endocrine system: differences between mice and humans

Leptin is a 16-kDa cytokine secreted in humans primarily but not exclusively by adipose tissues. Its concentration in blood is usually proportional to body fat mass, but is higher in women than in men not only because of a different distribution of and greater fat mass in women, but also because testosterone reduces its level in men. Leptin features in different ways during the life span. It is synthesized in the ovary, transported in the oocyte, and made by both fetus and placenta, particularly during the last month of gestation. It is made by the lactating mammary gland and ingested by the newborn infant in its milk. The prime importance of leptin is realized at puberty when it is necessary for progression to a normal adult reproductive status in females. Fasting and chronic undernutrition result in a lower level of leptin in the blood. Lack of leptin results in hunger, ensuring that the individual eat to survive, and also inhibition of reproduction, until such time as food and fat stores are adequate to supply energy for pregnancy and lactation. Thus, leptin is important for survival of the individual and survival of the species. Although an extremely rare genetic absence of leptin induces hyperphagia and obesity in humans, as it does in mice, there appears to be little role for leptin in humans in ensuring that fat stores are not in excess of adequate, that is, in preventing obesity. The mouse differs from humans in many respects, in particular in the far more drastic ways it conserves energy when it very rapidly adapts to lack of food. These include not only suppression of reproduction but also lowering of its body temperature (torpor), suppressing its thyroid function, suppressing its growth, and increasing secretion of stress hormones (from the adrenal). This review concentrates on roles of leptin in human physiology and pathophysiology but also discusses why some observations on actions of leptin in mice are not applicable to humans.

Hyperleptinemia in uremic patients undergoing conservative management, peritoneal dialysis, and hemodialysis: A comparative analysis

We performed a cross-sectional study in a wide sample of patients with chronic renal failure undergoing conservative therapy (CTh) (n = 79), peritoneal dialysis (PD) (n = 75), and hemodialysis (HD) (n = 51), with the aim of analyzing the impact of the different modes of therapy on serum leptin levels. We used a multivariate approach, taking into consideration the potential effects of other epidemiological, dialysis-related, nutritional, and hormonal factors on serum leptin. Leptin levels were higher in patients treated with PD (median, 36 ng/mL) than in those undergoing CTh (10.8 ng/mL) or HD (5.4 ng/mL) (P < 0.0005). This difference persisted after controlling for gender, body mass index, and fasting insulin levels, suggesting that imbalances in these factors may only partially explain the differences found between the three modes of therapy. Leptin levels showed a significant negative correlation with peritoneal protein losses in PD patients but were poorly associated with factors such as proteinuria, daily peritoneal glucose absorption (PD), renal function, or adequacy of dialysis. Leptin and insulin-like growth factor-I (IGF-I) were significantly correlated in PD patients, but the study design did not allow for establishing a meaning for this correlation. In conclusion, serum leptin levels are increased in PD patients when compared with CTh or HD patients. Differences in gender distribution, fat mass, and insulin levels may partially explain these findings, but other undefined factors also may have a role in producing these results.

Leptin in peritoneal dialysate from continuous ambulatory peritoneal dialysis patients

The adipocyte-derived hormone leptin is the 16-kd product of the ob gene that regulates food intake and body weight. Plasma leptin level is elevated in patients with chronic renal failure, partly because of impaired clearance through the kidney. In this study, we examined whether leptin is cleared into peritoneal dialysate in patients with end-stage renal disease treated by continuous ambulatory peritoneal dialysis (CAPD). The subjects were 46 CAPD patients and 67 age- and gender-matched healthy subjects. Leptin concentration in peritoneal dialysate from CAPD patients was measurable by a sensitive enzyme-linked immunosorbent assay (ELISA), and the daily loss of leptin by the peritoneal route was estimated to correspond to the amount contained in approximately 2 L plasma. Dialysate leptin concentration correlated positively with plasma leptin level and with percent body fat measured by dual-energy X-ray absorptiometry. The dialysate-to-plasma (D/P) ratio of leptin concentration was twice higher than expected from its molecular weight. D/P ratios of beta2-microglobulin, albumin, and transferrin showed strong correlations with each other (r = 0.768 to 0.801), whereas the correlation between D/P ratios of leptin and beta2-microglobulin was less impressive (r = 0.378). This was also the case with the relationship between apparent peritoneal clearances of these macromolecules, suggesting that dialysate leptin had some origins other than passive transport of plasma leptin. To test the hypothesis that abdominal visceral fat may contribute to the unexpectedly raised peritoneal dialysate leptin concentration, multiple regression analysis was performed. Leptin concentration in peritoneal dialysate showed significant association with plasma leptin level and D/P ratio of beta2-microglobulin, and it also showed an independent association with abdominal visceral fat but not with subcutaneous fat assessed by ultrasonography. These results showed that peritoneal dialysate from CAPD patients contained a significant amount of leptin, which derived presumably from both plasma and local visceral fat tissue.

Peritoneal clearance of leptin in continuous ambulatory peritoneal dialysis

Leptin is a 16-kd protein that increases energy expenditure and limits food intake. Serum leptin (S-leptin) is elevated in dialysis patients, and little data have been reported on leptin clearance (Cl) during dialysis. We analyzed the peritoneal dialysis (PD) Cl of leptin in 15 continuous ambulatory peritoneal dialysis (CAPD) patients and compared the results to beta(2)-microglobulin (beta(2)-m), urea, and creatinine PD Cl. S-leptin was significantly elevated (Kruskal-Wallis, P < 0.005) in CAPD women (58.4 +/- 42.4 [SE] microg/L, n = 5) as compared with CAPD men (13.9 +/- 7.1, n = 10) and with healthy women (11.0 +/- 1.4, n = 13) and men (5.1 +/- 0. 9, n = 14). Correlations were found between percent of fat mass and S-leptin (P < 0.05); between S-leptin and the 24-hour PD leptin (P < 0.05); and between dialysate-to-plasma (D/P) beta(2)-m and D/P leptin (P < 0.01). PD leptin Cl (1.80 +/- 0.43 mL/min/1.73 m(2)) was higher than beta(2)-m Cl (1.22 +/- 0.31) (P < 0.01), but reduced as compared with urea Cl (8.84 +/- 1.20) (P < 0.005) and creatinine Cl (7.71 +/- 0.99) (P < 0.005). These results indicate that leptin is eliminated through the peritoneum membrane. However, peritoneal leptin clearance, as beta(2)-m, appears to be clearly restricted as compared with peritoneal transport of smaller molecules. Hence, leptin could use the same diffusion transport pathway as beta(2)-m. In addition, leptin, which has a higher molecular weight than beta(2)-m, was significantly more eliminated into the peritoneal dialysate. More studies are necessary to clarify whether this is an active leptin elimination process by peritoneal secretion or by a different restriction coefficient of diffusion through the peritoneum membrane.

Haemodialysis and peritoneal dialysis: metabolic alterations and nutritional status

In dialysis patients, malnutrition is an independent factor causing morbidity and mortality. Both inadequate alimentation and metabolic alterations, which involve nitrogen and energy metabolism, contribute to malnutrition. Future research must address the treatment of anorexia and inflammation-induced catabolism, as well as the evaluation of nutritional supplementation techniques and anabolic drugs.

Plasma leptin concentrations are only transiently increased in nephrectomized rats

Leptin is an adipocyte-secreted hormone that has effects on appetite and energy expenditure. Several studies have shown that end-stage renal disease results in elevated plasma leptin concentrations and that the kidney is responsible for most of leptin elimination in rodents. Leptin metabolism was investigated in rats that underwent unilateral nephrectomy to experimentally limit renal elimination function. Within 4 h of nephrectomy, plasma leptin concentrations increased from 2.9 +/- 0.8 to 5.8 +/- 1.0 & microg/l but thereafter rapidly (<24 h) decreased to prenephrectomy concentrations, despite continued elevated plasma creatinine levels. Sham-operated rats maintained presurgical concentrations of leptin and creatinine throughout the experiment. Kinetic studies of 125I-labeled leptin elimination showed that fractional catabolic rates and half-lives of leptin in circulation were similar at 48 h in nephrectomized and sham-operated rats, suggesting that production of leptin was unchanged after nephrectomy. Excretion of 125I derived from leptin in urine of nephrectomized rats was similar to that of sham-operated rats, and residual radioactivity was increased in the remaining kidneys excised from nephrectomized rats. These results demonstrate that 1) leptin concentrations are quickly restored to presurgical levels in nephrectomized rats, and 2) it is leptin elimination, not leptin production, that compensates to maintain leptin concentrations. Rapid metabolic adaptation of remaining renal tissue may explain the restoration of normal leptin elimination in nephrectomized rats.