Renal Decapsulation Prevents Intrinsic Renal Compartment Syndrome in Ischemia-Reperfusion-Induced Acute Kidney Injury: A Physiologic Approach

Local Renal Treatments for Acute Kidney Injury: A Review of Current Progress and Future Translational Opportunities

Acute kidney injury (AKI) constitutes a significant public health concern, with limited therapeutic options to mitigate injury or expedite recovery. A novel therapeutic approach, local renal treatment, encompassing pharmacotherapy and surgical interventions, has exhibited positive outcomes in AKI management. Peri-renal administration, employing various delivery routes, such as the renal artery, intrarenal, and subcapsular sites, has demonstrated superiority over peripheral intravenous infusion. This review evaluates different drug delivery methods, analyzing their benefits and limitations, and proposes potential improvements. Renal decapsulation, particularly with the availability of minimally invasive techniques, emerges as an effective procedure warranting renewed consideration for AKI treatment. The potential synergistic effects of combined drug delivery and renal decapsulation could further advance AKI therapies. Clinical studies have already begun to leverage the benefits of local renal treatments, and with ongoing technological advancements, these modalities are expected to increasingly outperform systemic intravenous therapy.

Renal Sodium Avidity in Heart Failure

BACKGROUND

Increased renal sodium avidity is a hallmark feature of the heart failure syndrome.

SUMMARY

Increased renal sodium avidity refers to the inability of the kidneys to elicit potent natriuresis in response to sodium loading. This eventually causes congestion, which is a major contributor to hospital admissions and mortality in heart failure.

KEY MESSAGES

Important novel concepts such as the renal tamponade hypothesis, accelerated nephron loss, and the role of hypochloremia, the sympathetic nervous system, inflammation, the lymphatic system, and interstitial sodium buffers are involved in the pathophysiology of renal sodium avidity. A good understanding of these concepts is crucially important with respect to treatment recommendations regarding dietary sodium restriction, fluid restriction, rapid up-titration of guideline-directed medical therapies, combination diuretic therapy, natriuresis-guided diuretic therapy, use of hypertonic saline, and ultrafiltration.

Acute kidney injury in patients with burns

Burn injury is associated with a high risk of acute kidney injury (AKI) with a prevalence of AKI among patients with burns of 9-50%. Despite an improvement in burn injury survival in the past decade, AKI in patients with burns is associated with an extremely poor short-term and long-term prognosis, with a mortality of >80% among those with severe AKI. Factors that contribute to the development of AKI in patients with burns include haemodynamic alterations, burn-induced systemic inflammation and apoptosis, haemolysis, rhabdomyolysis, smoke inhalation injury, drug nephrotoxicity and sepsis. Early and late AKI after burn injury differ in their aetiologies and outcomes. Sepsis is the main driver of late AKI in patients with burns and late AKI has been associated with higher mortality than early AKI. Prevention of early AKI involves correction of hypovolaemia and avoidance of nephrotoxic drugs (for example, hydroxocobalamin), whereas prevention of late AKI involves prevention and early recognition of sepsis as well as avoidance of nephrotoxins. Treatment of AKI in patients with burns remains supportive, including prevention of fluid overload, treatment of electrolyte disturbance and use of kidney replacement therapy when indicated.

Pericyte detachment and renal congestion involve interstitial injury and fibrosis in Dahl salt-sensitive rats and humans with heart failure

Congestive heart failure produces fluid volume overload, central and renal venous pressure elevation, and consequently renal congestion, which results in worsening renal function. Pericyte detachment and pericyte-myofibroblast transition (PMT) were linked to renal interstitial fibrosis. Dahl salt-sensitive hypertensive (DahlS) rats are a non-surgical renal congestion model. The relation, however, between renal interstitial damage, pericyte morphology, and PMT in the renal congestion of DahlS rats has not been reported. DahlS rats (8-week-old) were fed normal salt (NS, 0.4% NaCl) or high salt (HS, 4% NaCl), and the left kidney was decapsulated to reduce renal interstitial hydrostatic pressure (RIHP) at 9 weeks old. One week after capsulotomy, both kidneys were analyzed by molecular and histological techniques. Renal pericyte structure was assessed in the body donors with/without venous stasis. Markers of tubulointerstitial damage, interstitial fibrosis, and PMT were upregulated in the right non-decapsulated kidney of DahlS rats fed HS. Renal tubular injury and fibrosis were detected in the HS diet groups in histological analysis. Pericyte detachment was observed in the right non-decapsulated kidney of DahlS rats fed HS by low vacuum-scanning electron microscopy. Decapsulation in DahlS rats fed HS attenuated these findings. Also, renal pericytes detached from the vascular wall in patients with heart failure. These results suggest that pericyte detachment and PMT induced by increased RIHP are responsible for tubulointerstitial injury and fibrosis in DahlS rats and humans with renal congestion. Renal venous congestion and subsequent physiological changes could be therapeutic targets for renal damage in cardiorenal syndrome.

Kidney function changes in acute heart failure: a practical approach to interpretation and management

Worsening kidney function (WKF) is common in patients with acute heart failure (AHF) syndromes. Although WKF has traditionally been associated with worse outcomes on a population level, serum creatinine concentrations vary greatly during episodes of worsening heart failure, with substantial individual heterogeneity in terms of their clinical meaning. Consequently, interpreting such changes within the appropriate clinical context is essential to unravel the pathophysiology of kidney function changes and appropriately interpret their clinical meaning. This article aims to provide a critical overview of WKF in AHF, aiming to provide physicians with some tips and tricks to appropriately interpret kidney function changes in the context of AHF.

Extravasation of Blood and Blood Toxicity Drives Tubular Injury from RBC Trapping in Ischemic AKI

Red blood cell (RBC) trapping is common in ischemic acute kidney injury (AKI) and presents as densely packed RBCs that accumulate within and engorge the kidney medullary circulation. In this study, we tested the hypothesis that "RBC trapping directly promotes tubular injury independent of extending ischemia time." Studies were performed on rats. Red blood cell congestion and tubular injury were compared between renal arterial clamping, venous clamping, and venous clamping of blood-free kidneys. Vessels were occluded for either 15 or 45 min with and without reperfusion. We found that RBC trapping in the medullary capillaries occurred rapidly following reperfusion from renal arterial clamping and that this was associated with extravasation of blood from congested vessels, uptake of blood proteins by the tubules, and marked tubular injury. To determine if this injury was due to blood toxicity or an extension of ischemia time, we compared renal venous and arterial clamping without reperfusion. Venous clamping resulted in RBC trapping and marked tubular injury within 45 min of ischemia. Conversely, despite the same ischemia time, RBC trapping and tubular injury were minimal following arterial clamping without reperfusion. Confirming the role of blood toward tubular injury, injury was markedly reduced in blood-free kidneys with venous clamping. Our data demonstrate that RBC trapping results in the rapid extravasation and uptake of blood components by tubular cells, causing toxic tubular injury. Tubular toxicity from extravasation of blood following RBC trapping appears to be a major component of tubular injury in ischemic AKI, which has not previously been recognized.

ALTERATION IN SHEAR WAVE ELASTOGRAPHY IS ASSOCIATED WITH ACUTE KIDNEY INJURY: A PROSPECTIVE OBSERVATIONAL PILOT STUDY

ABSTRACT

Background: Kidney stiffness could change during kidney disease. We hypothesize that acute kidney injury (AKI) would increase renal stiffness. Therefore, evaluating kidney Young's modulus (YM; a measure of tissue stiffness) using shear wave elastography (SWE) might help to diagnose AKI. Methods: This research was divided into two studies. Study A: Male C57BL/6 mice were used to observe kidney YM changes induced by sepsis-associated AKI, which was established by cecal ligation and puncture (CLP). Study B included 54 consecutive critically ill patients with or without AKI. Changes in renal YM were observed. Results: Study A: CLP mice showed a significantly higher kidney YM compared with the sham group. The YM gradually increased from CLP 0 hours to CLP 24 hours, and presented a fair relationship with the renal tubular injury score ( R2 = 0.71) and serum creatinine ( R2 = 0.73). Study B: YM was easily accessible, and the intraclass correlation coefficient ranged from 0.62 to 0.84. Kidney YM was higher in AKI patients and gradually increased from non-AKI to AKI III patients. Furthermore, the YM in the upper, middle, and lower poles of the renal cortex presented a fair relationship with kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin ( R2 ranging from 0.4 to 0.58), and the areas under the curve of the above five indicators for the diagnosis of AKI were 0.7, 0.73, 0.70, 0.74, and 0.79, respectively. Conclusion: SWE-derived estimates of renal stiffness are higher in AKI patients and sepsis-associated AKI mice. However, it has no advantage over NGAL and KIM-1. Trial Registration: Chinese Clinical Trial Registry No: ChiCTR2200061725. Retrospectively registered July 1, 2022, https://www.chictr.org.cn/showproj.aspx?proj=169359 .

Monitoring kidney size to interpret MRI-based assessment of renal oxygenation in acute pathophysiological scenarios

AIM

Tissue hypoxia is an early key feature of acute kidney injury. Assessment of renal oxygenation using magnetic resonance imaging (MRI) markers T₂ and T₂ * enables insights into renal pathophysiology. This assessment can be confounded by changes in the blood and tubular volume fractions, occurring upon pathological insults. These changes are mirrored by changes in kidney size (KS). Here, we used dynamic MRI to monitor KS for physiological interpretation of T₂ * and T₂ changes in acute pathophysiological scenarios.

METHODS

KS was determined from T₂ *, T₂ mapping in rats. Six interventions that acutely alter renal tissue oxygenation were performed directly within the scanner, including interventions that change the blood and/or tubular volume. A biophysical model was used to estimate changes in O₂ saturation of hemoglobin from changes in T₂ * and KS.

RESULTS

Upon aortic occlusion KS decreased; this correlated with a decrease in T₂ *, T₂ . Upon renal vein occlusion KS increased; this negatively correlated with a decrease in T₂ *, T₂ . Upon simultaneous occlusion of both vessels KS remained unchanged; there was no correlation with decreased T₂ *, T₂ . Hypoxemia induced mild reductions in KS and T₂ *, T₂ . Administration of an X-ray contrast medium induced sustained KS increase, with an initial increase in T₂ *, T₂ followed by a decrease. Furosemide caused T₂ *, T₂ elevation and a minor increase in KS. Model calculations yielded physiologically plausible calibration ratios for T₂ *.

CONCLUSION

Monitoring KS allows physiological interpretation of acute renal oxygenation changes obtained by T₂ *, T₂ . KS monitoring should accompany MRI-oximetry, for new insights into renal pathophysiology and swift translation into human studies.

Perirenal Fat and Kidney Function Deterioration in Patients With Acute Decompensated Heart Failure

BACKGROUND AND OBJECTIVES

The thick perirenal fat pad can induce high intracapsular pressure and cause compression of the renal vasculature and resultant congestive nephropathy. The current study investigated the association of perirenal fat thickness with kidney dysfunction in patients with acute decompensated heart failure (ADHF).

METHODS

Data from 266 patients hospitalized with ADHF were analyzed. Patients were divided into two groups according to the glomerular filtration rate (GFR) at admission (preserved kidney function [GFR ≥60 mL/min/1.73 m²] and reduced kidney function [GFR <60 mL/min/1.73 m²] groups). Right and left posterior perirenal fat thicknesses were measured using computed tomography, and their average values were calculated. Associated factors with reduced kidney function was assessed by logistic regression model, presenting with odds ratio (OR) and confidence interval (CI).

RESULTS

Increasing age (OR, 1.08; 95% CI, 1.04-1.12; p<0.001), diabetes mellitus (OR, 2.46; 95% CI, 1.18-5.12; p<0.017), increased log N-terminal pro-B-type natriuretic peptide (NT-proBNP) (OR, 1.82; 95% CI, 1.32-2.52; p<0.001), and increased average perirenal fat thickness (OR, 1.11; 95% CI, 1.06-1.16; p<0.001) were independently associated with reduced kidney function. In the subgroup analyses, patients over 70 years old, the ratio of mitral-to-mitral annular velocity >15, elevated log NT-proBNP had a significantly higher association with increased perirenal fat thickness with reduced kidney function.

CONCLUSIONS

Thick perirenal fat pads were independently associated with kidney function deterioration in patients hospitalized with ADHF.

Parametric MRI Detects Aristolochic Acid Induced Acute Kidney Injury

Exposure to aristolochic acid (AA) is of increased concern due to carcinogenic and nephrotoxic effects, and incidence of aristolochic acid nephropathy (AAN) is increasing. This study characterizes renal alterations during the acute phase of AAN using parametric magnetic resonance imaging (MRI). An AAN and a control group of male Wistar rats received administration of aristolochic acid I (AAI) and polyethylene glycol (PEG), respectively, for six days. Both groups underwent MRI before and 2, 4 and 6 days after AAI or PEG administration. T₂ relaxation times and apparent diffusion coefficients (ADCs) were determined for four renal layers. Serum creatinine levels (sCr) and blood urea nitrogen (BUN) were measured. Tubular injury scores (TIS) were evaluated based on histologic findings. Increased T₂ values were detected since day 2 in the AAN group, but decreased ADCs and increased sCr levels and BUN were not detected until day 4. Significant linear correlations were observed between T₂ of the cortex and the outer stripe of outer medulla and TIS. Our results demonstrate that parametric MRI facilitates early detection of renal injury induced by AAI in a rat model. T₂ mapping may be a valuable tool for assessing kidney injury during the acute phase of AAN.

Renal Compression in Heart Failure: The Renal Tamponade Hypothesis

Renal dysfunction is one of the strongest predictors of outcome in heart failure. Several studies have revealed that both reduced perfusion and increased congestion (and central venous pressure) contribute to worsening renal function in heart failure. This paper proposes a novel factor in the link between cardiac and renal dysfunction: "renal tamponade" or compression of renal structures caused by the limited space for expansion. This space can be limited either by the rigid renal capsule that encloses the renal interstitial tissue or by the layer of fat around the kidneys or by the peritoneal space exerting pressure on the retroperitoneal kidneys. Renal decapsulation in animal models of heart failure and acute renal ischemia has been shown effective in alleviating pressure-related injury within the kidney itself, thus supporting this concept and making it a potentially interesting novel treatment in heart failure.

Plasma Metabolites-Based Prediction in Cardiac Surgery-Associated Acute Kidney Injury

Background

Cardiac surgery–associated acute kidney injury (CSA‐AKI) is a common postoperative complication following cardiac surgery. Currently, there are no reliable methods for the early prediction of CSA‐AKI in hospitalized patients. This study developed and evaluated the diagnostic use of metabolomics‐based biomarkers in patients with CSA‐AKI.

Methods and Results

A total of 214 individuals (122 patients with acute kidney injury [AKI], 92 patients without AKI as controls) were enrolled in this study. Plasma samples were analyzed by liquid chromatography tandem mass spectrometry using untargeted and targeted metabolomic approaches. Time‐dependent effects of selected metabolites were investigated in an AKI swine model. Multiple machine learning algorithms were used to identify plasma metabolites positively associated with CSA‐AKI. Metabolomic analyses from plasma samples taken within 24 hours following cardiac surgery were useful for distinguishing patients with AKI from controls without AKI. Gluconic acid, fumaric acid, and pseudouridine were significantly upregulated in patients with AKI. A random forest model constructed with selected clinical parameters and metabolites exhibited excellent discriminative ability (area under curve, 0.939; 95% CI, 0.879–0.998). In the AKI swine model, plasma levels of the 3 discriminating metabolites increased in a time‐dependent manner (R², 0.480–0.945). Use of this AKI predictive model was then confirmed in the validation cohort (area under curve, 0.972; 95% CI, 0.947–0.996). The predictive model remained robust when tested in a subset of patients with early‐stage AKI in the validation cohort (area under curve, 0.943; 95% CI, 0.883–1.000).

Conclusions

High‐resolution metabolomics is sufficiently powerful for developing novel biomarkers. Plasma levels of 3 metabolites were useful for the early identification of CSA‐AKI.

Pathophysiology of COVID-19-associated acute kidney injury

Although respiratory failure and hypoxaemia are the main manifestations of COVID-19, kidney involvement is also common. Available evidence supports a number of potential pathophysiological pathways through which acute kidney injury (AKI) can develop in the context of SARS-CoV-2 infection. Histopathological findings have highlighted both similarities and differences between AKI in patients with COVID-19 and in those with AKI in non-COVID-related sepsis. Acute tubular injury is common, although it is often mild, despite markedly reduced kidney function. Systemic haemodynamic instability very likely contributes to tubular injury. Despite descriptions of COVID-19 as a cytokine storm syndrome, levels of circulating cytokines are often lower in patients with COVID-19 than in patients with acute respiratory distress syndrome with causes other than COVID-19. Tissue inflammation and local immune cell infiltration have been repeatedly observed and might have a critical role in kidney injury, as might endothelial injury and microvascular thrombi. Findings of high viral load in patients who have died with AKI suggest a contribution of viral invasion in the kidneys, although the issue of renal tropism remains controversial. An impaired type I interferon response has also been reported in patients with severe COVID-19. In light of these observations, the potential pathophysiological mechanisms of COVID-19-associated AKI may provide insights into therapeutic strategies.

Reliable kidney size determination by magnetic resonance imaging in pathophysiological settings

AIM

Kidney diseases constitute a major health challenge, which requires noninvasive imaging to complement conventional approaches to diagnosis and monitoring. Several renal pathologies are associated with changes in kidney size, offering an opportunity for magnetic resonance imaging (MRI) biomarkers of disease. This work uses dynamic MRI and an automated bean-shaped model (ABSM) for longitudinal quantification of pathophysiologically relevant changes in kidney size.

METHODS

A geometry-based ABSM was developed for kidney size measurements in rats using parametric MRI (T₂ , T₂ * mapping). The ABSM approach was applied to longitudinal renal size quantification using occlusion of the (a) suprarenal aorta or (b) the renal vein, (c) increase in renal pelvis and intratubular pressure and (d) injection of an X-ray contrast medium into the thoracic aorta to induce pathophysiologically relevant changes in kidney size.

RESULTS

The ABSM yielded renal size measurements with accuracy and precision equivalent to the manual segmentation, with >70-fold time savings. The automated method could detect a ~7% reduction (aortic occlusion) and a ~5%, a ~2% and a ~6% increase in kidney size (venous occlusion, pelvis and intratubular pressure increase and injection of X-ray contrast medium, respectively). These measurements were not affected by reduced image quality following administration of ferumoxytol.

CONCLUSION

Dynamic MRI in conjunction with renal segmentation using an ABSM supports longitudinal quantification of changes in kidney size in pathophysiologically relevant experimental setups mimicking realistic clinical scenarios. This can potentially be instrumental for developing MRI-based diagnostic tools for various kidney disorders and for gaining new insight into mechanisms of renal pathophysiology.

The role of oxidative stress and hypoxia in renal disease

Oxygen is required to sustain aerobic organisms. Reactive oxygen species (ROS) are constantly released during mitochondrial oxygen consumption for energy production. Any imbalance between ROS production and its scavenger system induces oxidative stress. Oxidative stress, a critical contributor to tissue damage, is well-known to be associated with various diseases. The kidney is susceptible to hypoxia, and renal hypoxia is a common final pathway to end stage kidney disease, regardless of the underlying cause. Renal hypoxia aggravates oxidative stress, and elevated oxidative stress, in turn, exacerbates renal hypoxia. Oxidative stress is also enhanced in chronic kidney disease, especially diabetic kidney disease, through various mechanisms. Thus, the vicious cycle between oxidative stress and renal hypoxia critically contributes to the progression of renal injury. This review examines recent evidence connecting chronic hypoxia and oxidative stress in renal disease and subsequently describes several promising therapeutic approaches against oxidative stress.

Pathophysiological and molecular mechanisms involved in renal congestion in a novel rat model

Increased central venous pressure in congestive heart failure causes renal dysfunction; however, the underlying mechanisms are unclear. We created a rat renal congestion model and investigated the effect of renal congestion on hemodynamics and molecular mechanisms. The inferior vena cava (IVC) between the renal veins was ligated by suture in male Sprague-Dawley rats to increase upstream IVC pressure and induce congestion in the left kidney only. Left kidney congestion reduced renal blood flow, glomerular filtration rate, and increased renal interstitial hydrostatic pressure. Tubulointerstitial and glomerular injury and medullary thick ascending limb hypoxia were observed only in the congestive kidneys. Molecules related to extracellular matrix expansion, tubular injury, and focal adhesion were upregulated in microarray analysis. Renal decapsulation ameliorated the tubulointerstitial injury. Electron microscopy captured pericyte detachment in the congestive kidneys. Transgelin and platelet-derived growth factor receptors, as indicators of pericyte-myofibroblast transition, were upregulated in the pericytes and the adjacent interstitium. With the compression of the peritubular capillaries and tubules, hypoxia and physical stress induce pericyte detachment, which could result in extracellular matrix expansion and tubular injury in renal congestion.