Renal functional reserve in obesity hypertension

Renal Dysfunction Phenotypes in Patients Undergoing Obesity Surgery

Obesity surgery candidates are at an increased risk of kidney injury, but pre-operative evaluation usually neglects kidney function assessment. This study aimed to identify renal dysfunction in candidates for bariatric surgery. To reduce the sources of bias, subjects with diabetes, prediabetes under metformin treatment, neoplastic or inflammatory diseases were excluded. Patients' (n = 192) average body mass index was 41.7 ± 5.4 kg/m². Among these, 51% (n = 94) had creatinine clearance over 140 mL/min, 22.4% (n = 43) had proteinuria over 150 mg/day and 14.6% (n = 28) albuminuria over 30 mg/day. A creatinine clearance higher than 140 mL/min was associated with higher levels of proteinuria and albuminuria. Univariate analysis identified sex, glycated hemoglobin, uric acid, HDL and VLDL cholesterol as being associated with albuminuria, but not with proteinuria. On multivariate analysis, glycated hemoglobin and creatinine clearance as continuous variables were significantly associated with albuminuria. In summary, in our patient population prediabetes, lipid abnormalities and hyperuricemia were associated with albuminuria, but not with proteinuria, suggesting different disease mechanisms might be implicated. Data suggest that in obesity-associated kidney disease, tubulointerstitial injury precedes glomerulopathy. A significant proportion of obesity surgery candidates present clinically relevant albuminuria and proteinuria along with renal hyperfiltration, suggesting that routine pre-operative assessment of these parameters should be considered.

Renal Functional Response-Association With Birth Weight and Kidney Volume

Introduction

Renal functional response (RFR) is the acute increase in glomerular filtration rate (GFR) after a protein load. Low RFR is a marker of single nephron hyperfiltration. Low birth weight (LBW) is associated with reduced number of nephrons, lower kidney function, and smaller kidneys in adults. In the present study, we investigate the associations among LBW, kidney volume, and RFR.

Methods

We studied adults aged 41 to 52 years born with either LBW (≤2300 g) or normal birth weight (NBW; 3500-4000 g). GFR was measured using plasma clearance of iohexol. A stimulated GFR (sGFR) was measured on a separate day after a protein load of 100 g using a commercially available protein powder, and RFR was calculated as delta GFR. Kidney volume was estimated from magnetic resonance imaging (MRI) images using the ellipsoid formula.

Results

A total of 57 women and 48 men participated. The baseline mean ± SD GFR was 118 ± 17 ml/min for men and 98 ± 19 ml/min for women. The overall mean RFR was 8.2 ± 7.4 ml/min, with mean RFR of 8.3 ± 8.0 ml/min and 8.1 ± 6.9 ml/min in men and women, respectively (P = 0.5). No birth-related variables were associated with RFR. Larger kidney volume was associated with higher RFR, 1.9 ml/min per SD higher kidney volume (P = 0.009). Higher GFR per kidney volume was associated with a lower RFR, -3.3ml/min per SD (P < 0.001).

Conclusion

Larger kidney size and lower GFR per kidney volume were associated with higher RFR. Birth weight was not shown to associate with RFR in mainly healthy middle-aged men and women.

The roles of protein load test in quantitatively evaluating renal function after severe unilateral ureteral obstruction in adult rabbits

Background

The accuracy of dynamic renal scintigraphy in evaluating the function of hydronephrotic kidneys is controversial. This study sought to investigate the effects of ureteral obstruction on renal function and the role of the protein load test (PLT) in evaluating hydronephrotic kidneys' function.

Methods

A ureteral obstruction model was generated with New Zealand white rabbits. The baseline glomerular filtration rate (GFR) and the GFR during the PLT were measured by dynamic renal scintigraphy at weeks 6, 9, and 12 after obstruction and the renal reserve GFRs were calculated. The renal GFR differences between different time points and between the PLT and baseline status were compared.

Results

(I) Both the baseline GFRs (t=11.287, P=0.000) and the GFRs during the PLTs (t=16.235, P=0.000) of the hydronephrotic kidneys were significantly lower than those of the contralateral kidneys. (II) The baseline GFRs of the obstructed kidneys were not significantly different 6, 9, and 12 weeks after obstruction (F=0.122, P=0.887), but their net reserve GFRs (F=8.419, P=0.004) and relative reserve GFRs (F=3.770, P=0.049) gradually decreased over time. (III) There was no significant correlation between the net reserve GFRs (r=-0.006, P=0.977) or the relative reserve functions (r=-0.022, P=0.920) of the obstructed kidneys and their baseline GFRs. However, there was a significant negative correlation between the net reserve GFRs (r=-0.590, P=0.002) or the relative reserve functions (r=-0.546, P=0.006) of the obstructed kidneys and the duration of obstruction. (IV) The reserve GFRs of the obstructed kidneys were not proportional to their baseline GFRs.

Conclusions

The functional changes in hydronephrotic kidneys are not sensitively reflected by baseline GFRs. GFRs during the PLTs and the reserve GFRs play important roles in the early and accurate evaluation of the function of obstructed kidneys.

The Aging Kidney-As Influenced by Heavy Metal Exposure and Selenium Supplementation

The aging process in the kidneys has been well studied. It is known that the glomerular filtration rate (GFR) declines with age in subjects older than 50–60 years. However, there is still insufficient knowledge regarding the response of the aged kidney to environmental toxicants such as mercury, cadmium, and lead. Here, we present a review on the functional decline and proposed mechanisms in the aging kidney as influenced by metal pollutants. Due to the prevalence of these toxicants in the environment, human exposure is nearly unavoidable. Further, it is well known that acute and chronic exposures to toxic metals may be detrimental to kidneys of normal adults, thus it may be hypothesized that exposure of individuals with reduced GFR will result in additional reductions in renal function. Individuals with compromised renal function, either from aging or from a combination of aging and disease, may be particularly susceptible to environmental toxicants. The available data appear to show an association between exposure to mercury, cadmium and/or lead and an increase in incidence and severity of renal disease in elderly individuals. Furthermore, some physiological thiols, as well as adequate selenium status, appear to exert a protective action. Further studies providing improved insight into the mechanisms by which nephrotoxic metals are handled by aging kidneys, as well as possibilities of therapeutic protection, are of utmost importance.

The Role of Renal Functional Reserve in Predicting Acute Kidney Injury

Renal functional reserve (RFR) is described as the difference between a glomerular filtration rate (GFR) measured at baseline and after protein stimulation. The percent change in GFR after a protein load varies based on differences in experimental conditions, with the use of an oral meat protein stimulus and a creatinine clearance method to quantify GFR showing the greatest RFR. A decline in RFR has been found in numerous patient groups. Recent investigations have suggested that a lower RFR may be associated with an increased risk of acute kidney injury and eventual chronic kidney disease.

Arterial Hypertension as a Risk Factor for Reduced Glomerular Filtration Rate after Living Kidney Donation

Living kidney donation represents the optimal renal replacement therapy, but recent data suggest an increased long-term renal risk for the donor. Here, we evaluated the risk for reduced estimated glomerular filtration rate (eGFR), death, and major cardiovascular events such as nonfatal myocardial infarction or cerebrovascular event including TIA (transient ischemic attack) and stroke in 225 donors, who underwent pre-donation examinations and live donor nephrectomy between 1985 and 2014 at our center. The median follow-up time was 8.7 years (1.0–29.1). In multivariate analysis, age and arterial hypertension at baseline were significantly associated with a higher risk of adverse renal outcomes, such as (1) eGFR <60 mL/min/1.73 m² (age per year: HR (hazard ratio) 1.05, 95% confidence interval (CI) 1.03–1.08, hypertension: HR 2.25, 95% CI 1.22–3.98), (2) eGFR <60 mL/min/1.73 m² and a decrease of ≥40% from baseline (age: HR 1.08, 95% CI 1.03–1.13, hypertension: HR 4.22, 95% CI 1.72–10.36), and (3) eGFR <45 mL/min/1.73 m² (age: HR 1.12, 95% CI 1.05–1.20, hypertension: HR 5.06, 95% CI 1.49–17.22). In addition, eGFR at time of donation (per mL/min/1.73 m²) was associated with a lower risk of (1) eGFR <60 mL/min/1.73 m² (HR 0.98, 95% CI 0.97–1.00) and (2) eGFR <45 mL/min/1.73 m² (HR 0.95, 95% CI 0.90–1.00). Age was the only significant predictor for death or major cardiovascular event (HR 1.08, 95% CI 1.01–1.16). In conclusion, arterial hypertension, lower eGFR, and age at the time of donation are strong predictors for adverse renal outcomes in living kidney donors.

Inverse correlation between vascular endothelial growth factor back-filtration and capillary filtration pressures

Background

Vascular endothelial growth factor A (VEGF) is an essential growth factor during glomerular development and postnatal homeostasis. VEGF is secreted in high amounts by podocytes into the primary urine, back-filtered across the glomerular capillary wall to act on endothelial cells. So far it has been assumed that VEGF back-filtration is driven at a constant rate exclusively by diffusion.

Methods

In the present work, glomerular VEGF back-filtration was investigated in vivo using a novel extended model based on endothelial fenestrations as surrogate marker for local VEGF concentrations. Single nephron glomerular filtration rate (SNGFR) and/or local filtration flux were manipulated by partial renal mass ablation, tubular ablation, and in transgenic mouse models of systemic or podocytic VEGF overexpression or reduction.

Results

Our study shows positive correlations between VEGF back-filtration and SNGFR as well as effective filtration rate under physiological conditions along individual glomerular capillaries in rodents and humans.

Conclusion

Our results suggest that an additional force drives VEGF back-filtration, potentially regulated by SNGFR.

Therapeutic drug monitoring-guided continuous infusion of piperacillin/tazobactam significantly improves pharmacokinetic target attainment in critically ill patients: a retrospective analysis of four years of clinical experience

PURPOSE

Standard dosing and intermittent bolus application (IB) are important risk factors for pharmacokinetic (PK) target non-attainment during empirical treatment with β-lactams in critically ill patients, particularly in those with sepsis and septic shock. We assessed the effect of therapeutic drug monitoring-guided (TDM), continuous infusion (CI) and individual dosing of piperacillin/tazobactam (PIP) on PK-target attainment in critically ill patients.

METHODS

This is a retrospective, single-center analysis of a database including 484 patients [933 serum concentrations (SC)] with severe infections, sepsis and septic shock who received TDM-guided CI of PIP in the intensive care unit (ICU) of an academic teaching hospital. The PK-target was defined as a PIP SC between 33 and 64 mg/L [fT > 2-4 times the epidemiological cutoff value (ECOFF) of Pseudomonas aeruginosa (PSA)].

RESULTS

PK-target attainment with standard dosing (initial dose) was observed in 166 patients (34.3%), whereas only 49 patients (10.1%) demonstrated target non-attainment. The minimum PK-target of ≥ 33 mg/L was overall realized in 89.9% (n = 435/484) of patients after the first PIP dose including 146 patients (30.2%) with potentially harmful SCs ≥ 100 mg/L. Subsequent TDM-guided dose adjustments significantly enhanced PK-target attainment to 280 patients (62.4%) and significantly reduced the fraction of potentially overdosed (≥ 100 mg/L) patients to 4.5% (n = 20/449). Renal replacement therapy (RRT) resulted in a relevant reduction of PIP clearance (CL_PIP): no RRT CL_PIP 6.8/6.3 L/h (median/IQR) [SCs n = 752, patients n = 405], continuous veno-venous hemodialysis (CVVHD) CL_PIP 4.3/2.6 L/h [SCs n = 160, n = 71 patients], intermittent hemodialysis (iHD) CL_PIP 2.6/2.3 L/h [SCs n = 21, n = 8 patients]). A body mass index (BMI) of > 40 kg/m² significantly increased CL_PIP 9.6/7.7 L/h [SC n = 43, n = 18 patients] in these patients. Age was significantly associated with supratherapeutic PIP concentrations (p < 0.0005), whereas high CrCL led to non-target attainment (p < 0.0005). Patients with target attainment (33-64 mg/L) within the first 24 h exhibited the lowest hospital mortality rates (13.9% [n = 23/166], p < 0.005). Those with target non-attainment demonstrated higher mortality rates (≤ 32 mg/L; 20.8% [n = 10/49] ≥ 64 mg/L; 29.4% [n = 79/269]).

CONCLUSION

TDM-guided CI of PIP is safe in critically ill patients and improves PK-target attainment. Exposure to defined PK-targets impacts patient mortality while lower and higher than intended SCs may influence the outcome of critically ill patients. Renal function and renal replacement therapy are main determinants of PK-target attainment. These results are only valid for CI of PIP and not for prolonged or intermittent bolus administration of PIP.

A maternal low-protein diet and neonatal overnutrition result in similar changes to glomerular morphology and renal cortical oxidative stress measures in male Wistar rats

There is a strong correlation between inadequate gestational and postpartum nutrition and the occurrence of cardiovascular diseases. The present study investigated the effects of a maternal low-protein diet and neonatal overfeeding on the oxidative balance and morphology of the renal cortex of male Wistar rats. Two independent protocols were used. First, pregnant Wistar rats received diets containing either 17% (normal protein) or 8% (low protein) casein throughout pregnancy and lactation. Second, the litter size was reduced by one-third on the third postnatal day to induce overnourishment in offspring. At 30 days, the oxidative balance and morphology of the renal cortex were analyzed. There was a small but significant increase in renal corpuscle area in the low protein (LP, 5%) and overnutrition (ON, 8%) groups. Glomerular tuft area also increased in LP (6%) and ON (9%), as did glomerular cellularity (LP, +11%; ON, +12%). In the oxidative stress analyses, both nutritional insults significantly elevated lipid peroxidation (LP, +18%; ON, +135%) and protein oxidation (LP, +40%; ON, +65%) while significantly reducing nonenzymatic antioxidant defenses, measured as reduced glutathione (LP, –32%; ON, –45%) and total thiol content (LP, –28%; ON, –24%). We also observed a decrease in superoxide dismutase (LP, –78%; ON, –51%), catalase (LP, –18%; ON, –61%), and glutathione S-transferase (only in ON, –44%) activities. Our results demonstrate that nutritional insults, even those of a very different nature, during perinatal development can result in similar changes in oxidative parameters and glomerular morphology in the renal cortex.

Association between metabolic parameters and glomerular hyperfiltration in a representative Korean population without chronic kidney disease

Aims

To investigate associations of glomerular hyperfiltration with other metabolic factors in a nationally representative dataset.

Methods

We analyzed cross-sectional data from 15,918 subjects with estimated glomerular filtration rate (eGFR) >60 ml/min/1.73 m² and urine albumin creation ratio (ACR) <30 mg/g, who participated in the 5th and 6th Korea National Health and Nutrition Examination Surveys. Hyperfiltration was defined as eGFR (CKD-EPI equation) exceeding the age- and sex-specific 95th percentile for healthy control subjects.

Results

Prevalence of hyperfiltration was 5.2% and that among normal, prediabetic, and diabetic subjects was 4.9%, 5.6%, and 7.3%, respectively, after adjusting for age, sex, and body weight (p for trend = 0.008). In a multiple logistic regression analysis, hyperfiltration was associated with a body mass index ≥30 kg/m² [odds ratio (OR) = 3.461, p<0.001], waist circumference 85 cm (men) or 80 cm (women) (OR = 1.425, p = 0.015), systolic blood pressure 120–129 mmHg (OR = 1.644, p = 0.022), fasting plasma glucose 140 mg/dL (OR = 1.695, p = 0.033) and t serum triglyceride level 500 mg/dL (OR = 2.988, p = 0.001), and was independently associated with the ACR (B = 0.053, p<0.001).

Conclusions

In a general Korean population, both hyperfiltration and ACR were associated with similar metabolic parameters, and hyperfiltration correlated independently with a high ACR. Longitudinal studies are needed to further explore risks of hyperfiltration and microalbuminuria.

Haemodynamic or metabolic stimulation tests to reveal the renal functional response: requiem or revival?

Renal stimulation tests document the dynamic response of the glomerular filtration rate (GFR) after a single or a combination of stimuli, such as an intravenous infusion of dopamine or amino acids or an oral protein meal. The increment of the GFR above the unstimulated state has formerly been called the renal functional reserve (RFR). Although the concept of a renal reserve capacity has not withstood scientific scrutiny, the literature documenting renal stimulation merits renewed interest. An absent or a blunted response of the GFR after a stimulus indicates lost or diseased nephrons. This information is valuable in preventing, diagnosing and prognosticating acute kidney injury and pregnancy-related renal events as well as chronic kidney disease. However, before renal function testing is universally practiced, some shortcomings must be addressed. First, a common nomenclature should be decided upon. The expression of RFR should be replaced by renal functional response. Second, a simple protocol must be developed and propagated. Third, we suggest designing prospective studies linking a defective stimulatory response to emergence of renal injury biomarkers, to histological or morphological renal abnormalities and to adverse renal outcomes in different renal syndromes.

Use of stress tests in evaluating kidney disease

PURPOSE OF REVIEW

The kidney, like most other organs, has a reserve capacity that can be utilized in times of increased physiologic demand. The ability to quantify this renal reserve function across various parts of the nephron (glomerular and tubular) has been an area of increased investigation over the past several years. In this review, we discuss several techniques that have been developed to interrogate the maximal physiologic capacity of the injured kidney.

RECENT FINDINGS

Although protein loading has been established as an ideal method to investigate glomerular filtration capacity in healthy kidneys, other methods such as the antagonism of the renin-angiotensin-aldosterone system have demonstrated promise as a method to determine underlying glomerular disease in those with acute kidney injury and other comorbidities (e.g., congestive heart failure and chronic kidney disease). The furosemide stress test has been demonstrated to be a useful clinical tool to ascertain tubular integrity in the setting of acute kidney injury.

SUMMARY

Although various methods to interrogate the reserve capacity of the several nephron segments (glomerulus and tubules) have been investigated, none of these techniques have had wide-spread clinical implementation. Further research into acute kidney injury stress testing is warranted.

Glomerular and Tubular Kidney Stress Test: New Tools for a Deeper Evaluation of Kidney Function

Kidney function has been traditionally identified with glomerular filtration rate (GFR). However, the kidney has multiple functions beyond filtration and a careful evaluation of its whole spectrum of activities should be carried out. Furthermore, the kidney presents a functional reserve capacity both at glomerular and tubular level. In presence of appropriate stimuli, patients with intact nephron mass can increase their GFR and tubular secretion. The difference between maximal capacity and baseline function represents the renal functional reserve (RFR). Glomerular and tubular kidney stress test may help to evaluate RFR and improve the clinicians' ability to better diagnose and prognosticate both acute and chronic kidney disease. © 2016 S. Karger AG, Basel.

Obesity-related glomerulopathy: clinical and pathologic characteristics and pathogenesis

The prevalence of obesity-related glomerulopathy is increasing in parallel with the worldwide obesity epidemic. Glomerular hypertrophy and adaptive focal segmental glomerulosclerosis define the condition pathologically. The glomerulus enlarges in response to obesity-induced increases in glomerular filtration rate, renal plasma flow, filtration fraction and tubular sodium reabsorption. Normal insulin/phosphatidylinositol 3-kinase/Akt and mTOR signalling are critical for podocyte hypertrophy and adaptation. Adipokines and ectopic lipid accumulation in the kidney promote insulin resistance of podocytes and maladaptive responses to cope with the mechanical forces of renal hyperfiltration. Although most patients have stable or slowly progressive proteinuria, up to one-third develop progressive renal failure and end-stage renal disease. Renin-angiotensin-aldosterone blockade is effective in the short-term but weight loss by hypocaloric diet or bariatric surgery has induced more consistent and dramatic antiproteinuric effects and reversal of hyperfiltration. Altered fatty acid and cholesterol metabolism are increasingly recognized as key mediators of renal lipid accumulation, inflammation, oxidative stress and fibrosis. Newer therapies directed to lipid metabolism, including SREBP antagonists, PPARα agonists, FXR and TGR5 agonists, and LXR agonists, hold therapeutic promise.

Renal Stress Testing in the Assessment of Kidney Disease

As part of human evolutionary development, many human organ systems have innate mechanisms to adapt to increased "work demand" or stress. This reserve capacity can be informative and is used commonly in cardiology to assess cardiac function (e.g., treadmill test). Similarly, the kidney possesses reserve capacity, which can be demonstrated in at least 2 of the following renal domains: glomerular and tubular. When appropriate stimulants are used, healthy patients with intact kidneys can significantly increase their glomerular filtration rate and their tubular secretion. This approach has been used to develop diagnostics for the assessment of renal function. This article reviews both glomerular and tubular kidney stress tests and their respective diagnostic utility.

Effect of Acetazolamide on Obesity-Induced Glomerular Hyperfiltration: A Randomized Controlled Trial

AIMS

Obesity is an important risk factor for the development of chronic kidney disease. One of the major factors involved in the pathogenesis of obesity-associated kidney disease is glomerular hyperfiltration. Increasing salt-delivery to the macula densa is expected to decrease glomerular filtration rate (GFR) by activating tubuloglomerular feedback. Acetazolamide, a carbonic anhydrase inhibitor which inhibits salt reabsorption in the proximal tubule, increases distal salt delivery. Its effects on obesity-related glomerular hyperfiltration have not previously been studied. The aim of this investigation was to evaluate whether administration of acetazolamide to obese non diabetic subjects reduces glomerular hyperfiltration.

MATERIALS AND METHODS

The study was performed using a randomized double-blind crossover design. Obese non-diabetic men with glomerular hyperfiltration were randomized to receive intravenously either acetazolamide or furosemide at equipotent doses. Twelve subjects received the allocated medications. Two weeks later, the same subjects received the drug which they had not received during the first study. Inulin clearance, p-aminohippuric acid clearance and fractional lithium excretion were measured before and after medications administration. The primary end point was a decrease in GFR, measured as inulin clearance.

RESULTS

GFR decreased by 21% following acetazolamide and did not decrease following furosemide. Renal vascular resistance increased by 12% following acetazolamide, while it remained unchanged following furosemide administration. Natriuresis increased similarly following acetazolamide and furosemide administration. Sodium balance was similar in both groups.

CONCLUSIONS

Intravenous acetazolamide decreased GFR in obese non-diabetic men with glomerular hyperfiltration. Furosemide, administered at equipotent dose, did not affect GFR, suggesting that acetazolamide reduced glomerular hyperfiltration by activating tubuloglomerular feedback.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01146288.

Renal blood flow using arterial spin labelling MRI and calculated filtration fraction in healthy adult kidney donors Pre-nephrectomy and post-nephrectomy

OBJECTIVES

Renal plasma flow (RPF) (derived from renal blood flow, RBF) and glomerular filtration rate (GFR) allow the determination of the filtration fraction (FF), which may have a role as a non-invasive renal biomarker. This is a hypothesis-generating pilot study assessing the effect of nephrectomy on renal function in healthy kidney donors.

METHODS

Eight living kidney donors underwent arterial spin labelling (ASL) magnetic resonance imaging (MRI) and GFR measurement prior to and 1 year after nephrectomy. Chromium-51 labelled ethylenediamine tetraacetic acid ((51)Cr-EDTA) with multi-blood sampling was undertaken and GFR calculated. The RBF and GFR obtained were used to calculate FF.

RESULTS

All donors showed an increase in single kidney GFR of 24 - 75 %, and all but two showed an increase in FF (-7 to +52 %) after nephrectomy. The increase in RBF, and hence RPF, post-nephrectomy was not as great as the increase in GFR in seven out of eight donors. As with any pilot study, the small number of donors and their relatively narrow age range are potential limiting factors.

CONCLUSIONS

The ability to measure RBF, and hence RPF, non-invasively, coupled with GFR measurement, allows calculation of FF, a biomarker that might provide a sensitive indicator of loss of renal reserve in potential donors.

KEY POINTS

• Non-invasive MRI measured renal blood flow and calculated renal plasma flow. • Effect of nephrectomy on blood flow and filtration in donors is presented. • Calculated filtration fraction may be a useful new kidney biomarker.

The impact of hyperfiltration on the diabetic kidney

More than two decades ago, hyperfiltration (HF) in diabetes was postulated to be a maladaptive response observed early in the course of diabetic kidney disease (DKD), which may eventually predispose to irreversible damage to nephrons and development of progressive renal disease. Despite this, the potential mechanisms leading to renal HF in diabetes are not fully understood, although several hypotheses have been proposed, including alterations in glomerular haemodynamic function and tubulo-glomerular feedback. Furthermore, the role of HF as a causative factor in renal disease progression is still unclear and warrants further prospective longer-term studies. Although HF has been entrenched as the first stage in the classic albuminuric pathway to end-stage renal disease in DKD, and HF has been shown to predict the progression of albuminuria in many, but not all studies, the concept that HF predisposes to the development of chronic kidney disease (CKD) stage 3, that is, glomerular filtration rate (GFR) decline to<60mL/min/1.73m(2), remains to be proved. Further long-term studies of GFR gradients therefore are required to establish whether HF ultimately leads to decreased kidney function, after adjustment for glycaemic control and other confounders. Whether reversal of HF with therapeutic agents is protective against reducing the risk of development of albuminuria and renal impairment is also worth investigating in prospective randomized trials.

Renal functional reserve and renal recovery after acute kidney injury

Renal functional reserve (RFR) represents the capacity of the kidney to increase glomerular filtration rate (GFR) in response to certain physiological or pathological stimuli or conditions. Once baseline GFR is determined, RFR can be assessed clinically after an oral protein load or intravenous amino acid infusion. In clinical practice, baseline GFR displays variable levels due to diet or other factors. RFR is the difference between peak 'stress' GFR induced by the test (p.o. or i.v.) and the baseline GFR. In clinical scenarios where hyperfiltration is present (high baseline GFR due to pregnancy, hypertension or diabetic nephropathy, in solitary kidney or kidney donors), RFR may be fully or partially used to achieve normal or supranormal renal function. Since commonly used renal function markers, such as GFR, may remain within normal ranges until 50% of nephrons are lost or in patients with a single remnant kidney, the RFR test may represent a sensitive and early way to assess the functional decline in the kidney. RFR assessment may become an important tool to evaluate the ability of the kidney to recover completely or partially after a kidney attack. In case of healing with a defect and progressive fibrosis, recovery may appear complete clinically, but a reduced RFR may be a sign of a maladaptive repair or subclinical loss of renal mass. Thus, a reduction in RFR may represent the equivalent of renal frailty or susceptibility to insults. The main aim of this article is to review the concept of RFR, its utility in different clinical scenarios, and future perspective for its use.

Obesity in kidney disease: A heavyweight opponent

Obesity is an important worldwide challenge that must be faced in most developed and developing countries because of unhealthy nutritional habits. The consequences of obesity and being overweight are observed in different organs, but the kidney is one of the most affected. Excess adipose tissue causes hemodynamic alterations in the kidney that can result in renal disease. However, obesity is also commonly associated with other comorbidities such as chronic inflammation, hypertension and diabetes. This association of several aggravating factors is still a matter of concern in clinical and basic research because the pathophysiologic mechanisms surrounding chronic kidney disease development in obese patients remain unclear. This review will discuss the consequences of obesity in the context of renal injury.

Proximal tubular hypertrophy and enlarged glomerular and proximal tubular urinary space in obese subjects with proteinuria

Background

Obesity is associated with glomerular hyperfiltration, increased proximal tubular sodium reabsorption, glomerular enlargement and renal hypertrophy. A single experimental study reported an increased glomerular urinary space in obese dogs. Whether proximal tubular volume is increased in obese subjects and whether their glomerular and tubular urinary spaces are enlarged is unknown.

Objective

To determine whether proximal tubules and glomerular and tubular urinary space are enlarged in obese subjects with proteinuria and glomerular hyperfiltration.

Methods

Kidney biopsies from 11 non-diabetic obese with proteinuria and 14 non-diabetic lean patients with a creatinine clearance above 50 ml/min and with mild or no interstitial fibrosis were retrospectively analyzed using morphometric methods. The cross-sectional area of the proximal tubular epithelium and lumen, the volume of the glomerular tuft and of Bowman’s space and the nuclei number per tubular profile were estimated.

Results

Creatinine clearance was higher in the obese than in the lean group (P=0.03). Proteinuria was similarly increased in both groups. Compared to the lean group, the obese group displayed a 104% higher glomerular tuft volume (P=0.001), a 94% higher Bowman’s space volume (P=0.003), a 33% higher cross-sectional area of the proximal tubular epithelium (P=0.02) and a 54% higher cross-sectional area of the proximal tubular lumen (P=0.01). The nuclei number per proximal tubular profile was similar in both groups, suggesting that the increase in tubular volume is due to hypertrophy and not to hyperplasia.

Conclusions

Obesity-related glomerular hyperfiltration is associated with proximal tubular epithelial hypertrophy and increased glomerular and tubular urinary space volume in subjects with proteinuria. The expanded glomerular and urinary space is probably a direct consequence of glomerular hyperfiltration. These effects may be involved in the pathogenesis of obesity-related renal disease.

Glomerular hyperfiltration: definitions, mechanisms and clinical implications

Glomerular hyperfiltration is a phenomenon that can occur in various clinical conditions including kidney disease. No single definition of glomerular hyperfiltration has been agreed upon, and the pathophysiological mechanisms, which are likely to vary with the underlying disease, are not well explored. Glomerular hyperfiltration can be caused by afferent arteriolar vasodilation as seen in patients with diabetes or after a high-protein meal, and/or by efferent arteriolar vasoconstriction owing to activation of the renin-angiotensin-aldosterone system, thus leading to glomerular hypertension. Glomerular hypertrophy and increased glomerular pressure might be both a cause and a consequence of renal injury; understanding the renal adaptations to injury is therefore important to prevent further damage. In this Review, we discuss the current concepts of glomerular hyperfiltration and the renal hemodynamic changes associated with this condition. A physiological state of glomerular hyperfiltration occurs during pregnancy and after consumption of high-protein meals. The various diseases that have been associated with glomerular hyperfiltration, either per nephron or per total kidney, include diabetes mellitus, polycystic kidney disease, secondary focal segmental glomerulosclerosis caused by a reduction in renal mass, sickle cell anemia, high altitude renal syndrome and obesity. A better understanding of the mechanisms involved in glomerular hyperfiltration could enable the development of new strategies to prevent progression of kidney disease.

Renal function and severity of bright liver. Relationship with insulin resistance, intrarenal resistive index, and glomerular filtration rate

AIMS

Relationships of renal function and liver disease are described in acute and chronic liver failure. The aim of the study is to investigate which relationship, if any, is present between severity of non-alcoholic fatty liver disease (NAFLD), assessed by bright liver score (BLS) versus mild-moderate renal insufficiency assessed by glomerular filtration rate (GFR) and by ultrasound intra-renal arterial resistive index (RRI). Moreover, which difference, if any, can be found in NAFLD patients with normal versus increased transaminases.

PATIENTS

The study enrolled 323 NAFLD and 176 non-NAFLD consecutive patients, comparable for age, gender distribution, GFR, and RRI referred to a university clinical day hospital after an ultrasound diagnosis of bright liver, for clinical-nutritional counselling. Personalized computerized mediterranean diet, physical activity increase, and smoking withdrawal integrated counselling were provided.

RESULTS

In NAFLD patients, homoeostasis model (HOMA) has a significant correlation with BLS. According to the severity of BLS, grade II-III versus grade I patients have significantly higher values of HOMA, body mass index (BMI), triglycerides, and longitudinal right liver length. By odds ratio, more severe BLS, increased HOMA, and transaminases are associated with lower GFR. Increased transaminases are associated with higher grades of BLS, HOMA, and BMI. By multiple linear regression waist-to-hip ratio, RRI, and BLS, as significant independent factors (p < 0.0001), explain significantly variance to GFR. This is not observed in normal control group, in which only RRI is a factor explaining GFR.

CONCLUSION

Greater RRI, abdominal obesity, and greater BLS account for a lower GFR in NAFLD patients suggesting the hypothesis that inter-related factors can be operating early in the natural history of obesity-related kidney and liver disease.

Protein-enriched diet increases water absorption via the aquaporin-2 water channels in healthy humans

BACKGROUND

According to animal experiments, a protein-enriched diet increased renal absorption of sodium and water. We wanted to test the hypothesis that a protein-enriched diet would increase the expression of the aquaporin-2 water channels and the epithelial sodium channels in the distal part of the nephron using biomarkers for the activity of the two channels.

METHODS

We performed a randomized, placebo controlled crossover study in 13 healthy humans to examine the effect of a protein-enriched diet on renal handling of water and sodium during baseline condition and during hypertonic saline infusion. We measured the effect of the protein-enriched diet on urinary excretions of aquaporin-2 (u-AQP2), the beta-fraction of the epithelial sodium channels (u-ENaC(beta)), free water clearance (C(H2O)), fractional excretion of sodium and vasoactive hormones.

RESULTS

During baseline conditions, u-AQP2 increased, and C(H2O) decreased during the protein-enriched diet, whereas u-ENaC(beta) was unchanged, although the urinary sodium excretion increased. During hypertonic saline infusion, the response in the effect variables did not deviate between protein-enriched and normal diet. Plasma concentrations of angiotensin II and aldosterone increased as well as pulse rate. Vasopressin in plasma was unchanged, and prostaglandin E(2) fell during the protein-enriched diet.

CONCLUSIONS

The protein-enriched diet increased water absorption via an increased transport via the aquaporin-2 water channels. The increased u-AQP2 might be due to a reduced prostaglandin level. The increase in renal sodium excretion seems to be mediated in another part of the nephron than the epithelial sodium channels.

Nephrectomy elicits impact of age and BMI on renal hemodynamics: lower postdonation reserve capacity in older or overweight kidney donors

Renal functional reserve could be relevant for the maintenance of renal function after kidney donation. Low-dose dopamine induces renal vasodilation with a rise in glomerular filtration rate (GFR) in healthy subjects and is thought to be a reflection of reserve capacity (RC). Older age and higher body mass index (BMI) may be associated with reduced RC. We therefore investigated RC in 178 consecutive living kidney donors (39% males, age 48 +/- 11 years, BMI 25.5 +/- 4.1). RC was determined as the rise in GFR ((125)I-iothalamate), 4 months before and 2 months after donor nephrectomy. Before donor nephrectomy, GFR was 114 +/- 20 mL/min, with a reduction to 72 +/- 12 mL/min after donor nephrectomy. The dopamine-induced rise in GFR of 11 +/- 10% was reduced to 5 +/- 7% after donor nephrectomy (p < 0.001). Before donor nephrectomy, older age and higher BMI did not affect reserve capacity. After donor nephrectomy, the response of GFR to dopamine independently and negatively correlated with older age and higher BMI. Moreover, postdonation reserve capacity was absent in obese donors. The presence of overweight had more impact on loss of RC in younger donors. In conclusion, donor nephrectomy unmasked an age- and overweight-induced loss of reserve capacity. Younger donors with obesity should be carefully monitored.

New aspects of the relationship among hypertension, obesity, and the kidneys

Recent studies have highlighted the association between obesity and chronic renal disease. Overweight has been shown to be a survival advantage in patients on maintenance hemodialysis therapy. However, in patients with preterminal renal failure, obesity may contribute to the progression of renal disease. Data collected from the Prevention of Renal and Vascular End-stage Disease study suggest that changes in weight directly influence albuminuria, hinting that albumin excretion could be used in the general population as a surrogate marker for risk of developing chronic kidney disease. The causal pathways linking excess weight to chronic kidney disease and hypertension are briefly reviewed in this article, and therapeutic approaches to combat this growing health problem are highlighted.

Obesity-induced glomerular hyperfiltration: its involvement in the pathogenesis of tubular sodium reabsorption

BACKGROUND

Obesity is associated with hypertension and glomerular hyperfiltration. A major mechanism responsible for the obesity-associated hypertension is renal salt retention. An increased glomerular filtration fraction (FF) is expected to raise postglomerular oncotic pressure and to increase proximal tubular sodium reabsorption. The aim of the present study was to verify whether obesity-associated hyperfiltration leads to increased postglomerular oncotic pressure and increased proximal sodium reabsorption.

METHODS

Twelve obese subjects (BMI >36) and 19 lean subjects participated in the study. They underwent measurement of glomerular filtration rate (GFR), renal plasma flow (RPF) and fractional excretion of lithium (FE Li).

RESULTS

GFR, RPF and FF were 61%, 28% and 29% higher, respectively, in the obese than in the control group (P < 0.00001 for GFR, P < 0.005 for RPF and P < 0.00005 for FF). Half of the obese group had increased FF with increased GFR, while the other half had normal FF with high-normal or increased GFR. Postglomerular oncotic pressure was 13% higher (P < 0.03) and FE Li was 33% lower (P < 0.005) in the obese group with high FF than in the lean group. Postglomerular oncotic pressure and FE Li were normal in the obese group with normal FF.

CONCLUSIONS

These results suggest that glomerular hyperfiltration may lead to increased proximal tubular sodium reabsorption in the obese.

Obesity-hypertension: an ongoing pandemic

Considerable evidence has suggested that excessive weight gain is the most common cause of arterial hypertension. This association has been observed in several populations, in different regions of the world. Obesity-hypertension, a term that underscores the link between these two deleterious conditions, is an important public health challenge, because of its high frequency and concomitant risk of cardiovascular and kidney diseases. The obesity-hypertension pandemic imposes a considerable economic burden on societies, directly reflecting on healthcare system costs. Increased renal sodium reabsorption and blood volume expansion are central features in the development of obesity-hypertension. Overweight is also associated with increased sympathetic activity. Leptin, a protein expressed in and secreted by adipocytes, is the main factor linking obesity, increased sympathetic nervous system activity and hypertension. The renin-angiotensin-aldosterone system has also been causally implicated in obesity-hypertension, because angiotensinogen is expressed in and secreted by adipose tissue. Hypoadiponectinemia, high circulating levels of free fatty acids and increased vascular production of endothelin-1 (ET-1) have been reported as potential mechanisms for obesity-hypertension. Lifestyle changes are effective in obesity-hypertension control, though pharmacological treatment is frequently necessary. Despite the consistency of the mechanistic approach in explaining the causal relation between hypertension and obesity, there is yet no evidence that one class of drug is superior to the others in controlling obesity-hypertension. In this review, we present the current knowledge and research in obesity-hypertension, exploring the epidemiologic evidence of the association, its probable pathophysiological mechanisms and treatment issues.