MRI of renal oxygenation and function after normothermic ischemia-reperfusion injury

Multiparametric MRI based assessment of kidney injury in a mouse model of ischemia reperfusion injury

Kidney diseases pose a global healthcare burden, with millions requiring renal replacement therapy. Ischemia/reperfusion injury (IRI) is a common pathology of acute kidney injury, causing hypoxia and subsequent inflammation-induced kidney damage. Accurate detection of acute kidney injury due to IRI is crucial for timely intervention. We used longitudinal, multi-parametric magnetic resonance imaging (MRI) employing arterial spin labelling (ASL), diffusion weighted imaging (DWI), and dynamic contrast enhanced (DCE)-MRI to assess IRI induced changes in both the injured and healthy contralateral kidney, in a unilateral IRI mouse model (n = 9). Multi-parametric MRI demonstrated significant differences in kidney volume (p = 0.001), blood flow (p = 0.002), filtration coefficient (p = 0.038), glomerular filtration rate (p = 0.005) and apparent diffusion coefficient (p = 0.048) between the injured kidney and contralateral kidney on day 1 post-IRI surgery. Identification of the injured kidney using principal component analysis including most of the imaging parameters demonstrated an area under the curve (AUC) of 0.97. These results point to the utility of multi-parametric MRI in early detection of IRI-induced kidney damage suggesting that the combination of various MRI parameters may be suitable for monitoring the extent of injury in this model.

[Functional MRI assessment of microstructural and perfusion changes in the kidneys of rats with intrauterine growth restriction]

OBJECTIVES

To explore the value of functional magnetic resonance imaging (MRI) techniques, including intravoxel incoherent motion (IVIM), T1 mapping, and T2 mapping, in assessing the microstructural and perfusion changes in the kidneys of rats with intrauterine growth restriction (IUGR).

METHODS

An IUGR rat model was established through a low-protein diet during pregnancy. Offspring from pregnant rats on a low-protein diet were randomly divided into an IUGR 8-week group and an IUGR 12-week group, while offspring from pregnant rats on a normal diet were divided into a normal 8-week group and a normal 12-week group (n=8 for each group). The apparent diffusion coefficient (ADC), true diffusion coefficient (Dt), pseudo-diffusion coefficient (D*), perfusion fraction (f), T1 value, and T2 value of the renal cortex and medulla were compared, along with serum creatinine and blood urea nitrogen levels among the groups.

RESULTS

The Dt value in the renal medulla was higher in the IUGR 12-week group than in the IUGR 8-week group, and the D* value in the renal medulla was lower in the IUGR 12-week group than in both the normal 12-week group and the IUGR 8-week group (P<0.05). The T1 value in the renal medulla was higher than in the cortex in the IUGR 8-week group, and the T1 value in the renal medulla was higher in the IUGR 12-week group than in both the IUGR 8-week group and the normal 12-week group, with the cortical T1 value in the IUGR 12-week group also being higher than that in the normal 12-week group (P<0.05). The T2 values in the renal medulla were higher than those in the cortex across all groups (P<0.05). There were no significant differences in the T2 values of either the cortex or medulla among the groups (P>0.05). There were no significant differences in serum creatinine and blood urea nitrogen levels among the groups (P>0.05). Glomerular hyperplasia and hypertrophy without significant fibrotic changes were observed in the IUGR 8-week group, whereas glomerular atrophy, cystic stenosis, and interstitial inflammatory infiltration and fibrosis were seen in the IUGR 12-week group.

CONCLUSIONS

IVIM MRI can be used to assess and dynamically observe the microstructural and perfusion damage in the kidneys of IUGR rats. MRI T1 mapping can be used to evaluate kidney damage in IUGR rats, and the combination of MRI T1 mapping and T2 mapping can further differentiate renal fibrosis in IUGR rats.

Assessment of Acute Kidney Injury using MRI

There has been growing interest in using quantitative magnetic resonance imaging (MRI) to describe and understand the pathophysiology of acute kidney injury (AKI). The ability to assess kidney blood flow, perfusion, oxygenation, and changes in tissue microstructure at repeated timepoints is hugely appealing, as this offers new possibilities to describe nature and severity of AKI, track the time-course to recovery or progression to chronic kidney disease (CKD), and may ultimately provide a method to noninvasively assess response to new therapies. This could have significant clinical implications considering that AKI is common (affecting more than 13 million people globally every year), harmful (associated with short and long-term morbidity and mortality), and currently lacks specific treatments. However, this is also a challenging area to study. After the kidney has been affected by an initial insult that leads to AKI, complex coexisting processes ensue, which may recover or can progress to CKD. There are various preclinical models of AKI (from which most of our current understanding derives), and these differ from each other but more importantly from clinical AKI. These aspects are fundamental to interpreting the results of the different AKI studies in which renal MRI has been used, which encompass different settings of AKI and a variety of MRI measures acquired at different timepoints. This review aims to provide a comprehensive description and interpretation of current studies (both preclinical and clinical) in which MRI has been used to assess AKI, and discuss future directions in the field. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY: Stage 3.

Multiparametric Magnetic Resonance Investigations on Acute and Long-Term Kidney Injury

Acute kidney injury (AKI) is a frequent complication of critical illness and carries a significant risk of short- and long-term mortality. The prediction of the progression of AKI to long-term injury has been difficult for renal disease treatment. Radiologists are keen for the early detection of transition from AKI to long-term kidney injury, which would help in the preventive measures. The lack of established methods for early detection of long-term kidney injury underscores the pressing needs of advanced imaging technology that reveals microscopic tissue alterations during the progression of AKI. Fueled by recent advances in data acquisition and post-processing methods of magnetic resonance imaging (MRI), multiparametric MRI is showing great potential as a diagnostic tool for many kidney diseases. Multiparametric MRI studies offer a precious opportunity for real-time noninvasive monitoring of pathological development and progression of AKI to long-term injury. It provides insight into renal vasculature and function (arterial spin labeling, intravoxel incoherent motion), tissue oxygenation (blood oxygen level-dependent), tissue injury and fibrosis (diffusion tensor imaging, diffusion kurtosis imaging, T1 and T2 mapping, quantitative susceptibility mapping). The multiparametric MRI approach is highly promising but the longitudinal investigation on the transition of AKI to irreversible long-term impairment is largely ignored. Further optimization and implementation of renal MR methods in clinical practice will enhance our comprehension of not only AKI but chronic kidney diseases. Novel imaging biomarkers for microscopic renal tissue alterations could be discovered and benefit the preventative interventions. This review explores recent MRI applications on acute and long-term kidney injury while addressing lingering challenges, with emphasis on the potential value of the development of multiparametric MRI for renal imaging on clinical systems. EVIDENCE LEVEL: 1 TECHNICAL EFFICACY: Stage 2.

Assessment of Mitochondrial Function and Oxygen Consumption Measured During Ex Vivo Normothermic Machine Perfusion of Injured Pig Kidneys Helps to Monitor Organ Viability

Donor kidney assessment may improve organ utilisation. Normothermic Machine Perfusion (NMP) has the potential to facilitate this advance. The mechanism of action is not yet determined and we aimed to assess mitochondrial function during NMP. Anaesthetised pigs (n = 6) had one kidney clamped for 60 min. The healthy contralateral kidney was removed and underwent NMP for 8 h (healthy control (HC), n = 6). Following 60 min warm ischaemia the injured kidney underwent HMP for 24 h, followed by NMP for 8 h (n = 6). Mitochondria were extracted from fresh tissue for analysis. Injured kidneys were analysed as two separate groups (IMa, n = 3 and IMb, n = 3). Renal resistance was higher (0.39ï, ± 0.29 vs. 1.65ï, ± 0.85; p = 0.01) and flow was lower (55ï, ± 28 vs. 7ï, ± 4; p = 0.03) during HMP in IMb than IMa. NMP blood flow was higher in IMa versus IMb (2-way ANOVA; p < 0.001) After 60 min NMP, O₂ consumption was significantly lower in IMb versus IMa (p ≤ 0.002). State-3 respiration was significantly different between the groups (37ï, ± 19 vs. 24ï, ± 14 vs. 10ï, ± 8; nmolO₂/min/mg; p = 0.049). Lactate levels were significantly lower in IMa versus IMb (p = 0.028). Mitochondrial respiration levels during NMP may be suggestive of kidney viability. Oxygen consumption, renal blood flow and lactate can differentiate severity of kidney injury during NMP.

Renal pH Imaging Using Chemical Exchange Saturation Transfer (CEST) MRI: Basic Concept

Magnetic Resonance Imaging (MRI) has been actively explored in the last several decades for assessing renal function by providing several physiological information, including glomerular filtration rate, renal plasma flow, tissue oxygenation and water diffusion. Within MRI, the developing field of chemical exchange saturation transfer (CEST) has potential to provide further functional information for diagnosing kidney diseases. Both endogenous produced molecules as well as exogenously administered CEST agents have been exploited for providing functional information related to kidney diseases in preclinical studies. In particular, CEST MRI has been exploited for assessing the acid-base homeostasis in the kidney and for monitoring pH changes in several disease models. This review summarizes several CEST MRI procedures for assessing kidney functionality and pH, for monitoring renal pH changes in different kidney injury models and for evaluating renal allograft rejection.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

Analysis Protocol for Dynamic Contrast Enhanced (DCE) MRI of Renal Perfusion and Filtration

Here we present an analysis protocol for dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) data of the kidneys. It covers comprehensive steps to facilitate signal to contrast agent concentration mapping via T1 mapping and the calculation of renal perfusion and filtration parametric maps using model-free approaches, model free analysis using deconvolution, the Toft's model and a Bayesian approach.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This analysis protocol chapter is complemented by two separate chapters describing the basic concept and experimental procedure.

Dynamic Contrast Enhancement (DCE) MRI-Derived Renal Perfusion and Filtration: Basic Concepts

Dynamic contrast-enhanced (DCE) MRI monitors the transit of contrast agents, typically gadolinium chelates, through the intrarenal regions, the renal cortex, the medulla, and the collecting system. In this way, DCE-MRI reveals the renal uptake and excretion of the contrast agent. An optimal DCE-MRI acquisition protocol involves finding a good compromise between whole-kidney coverage (i.e., 3D imaging), spatial and temporal resolution, and contrast resolution. By analyzing the enhancement of the renal tissues as a function of time, one can determine indirect measures of clinically important single-kidney parameters as the renal blood flow, glomerular filtration rate, and intrarenal blood volumes. Gadolinium-containing contrast agents may be nephrotoxic in patients suffering from severe renal dysfunction, but otherwise DCE-MRI is clearly useful for diagnosis of renal functions and for assessing treatment response and posttransplant rejection.Here we introduce the concept of renal DCE-MRI, describe the existing methods, and provide an overview of preclinical DCE-MRI applications to illustrate the utility of this technique to measure renal perfusion and glomerular filtration rate in animal models.This publication is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction is complemented by two separate publications describing the experimental procedure and data analysis.

Role of Renal Hypoxia in the Progression From Acute Kidney Injury to Chronic Kidney Disease

Over the past 20 years, there has been an increased appreciation of the long-term sequelae of acute kidney injury (AKI) and the potential development of chronic kidney disease (CKD). Several pathophysiologic features have been proposed to mediate AKI to CKD progression including maladaptive alterations in tubular, interstitial, inflammatory, and vascular cells. These alterations likely interact to culminate in the progression to CKD. In this article we focus primarily on evidence of vascular rarefaction secondary to AKI, and the potential mechanisms by which rarefaction occurs in relation to other alterations in tubular and interstitial compartments. We further focus on the potential that rarefaction contributes to renal hypoxia. Consideration of the role of hypoxia in AKI to CKD transition focuses on experimental evidence of persistent renal hypoxia after AKI and experimental maneuvers to evaluate the influence of hypoxia, per se, in progressive disease. Finally, consideration of methods to evaluate hypoxia in patients is provided with the suggestion that noninvasive measurement of renal hypoxia may provide insight into progression in post-AKI patients.

Physiological system analysis of the kidney by high-temporal-resolution T 2 ∗ monitoring of an oxygenation step response

PURPOSE

Examine the feasibility of characterizing the regulation of renal oxygenation using high-temporal-resolution monitoring of the T 2 ∗ response to a step-like oxygenation stimulus.

METHODS

For T 2 ∗ mapping, multi-echo gradient-echo imaging was used (temporal resolution = 9 seconds). A step-like renal oxygenation challenge was applied involving sequential exposure to hyperoxia (100% O₂ ), hypoxia (10% O₂ + 90% N₂ ), and hyperoxia (100% O₂ ). In vivo experiments were performed in healthy rats (N = 10) and in rats with bilateral ischemia-reperfusion injury (N = 4). To assess the step response of renal oxygenation, a second-order exponential model was used (model parameters: amplitude [A], time delay [Δt], damping constant [D], and period of the oscillation [T]) for renal cortex, outer stripe of the outer medulla, inner stripe of the outer medulla, and inner medulla.

RESULTS

The second-order exponential model permitted us to model the exponential T 2 ∗ recovery and the superimposed T 2 ∗ oscillation following renal oxygenation stimulus. The in vivo experiments revealed a difference in D_{outer medulla} between healthy controls (D < 1, indicating oscillatory recovery) and ischemia-reperfusion injury (D > 1, reflecting aperiodic recovery). The increase in D_{outer medulla} by a factor of 3.7 (outer stripe of the outer medulla) and 10.0 (inner stripe of the outer medulla) suggests that this parameter might be rather sensitive to (patho)physiological oxygenation changes.

CONCLUSION

This study demonstrates the feasibility of monitoring the dynamic oxygenation response of renal tissues to a step-like oxygenation challenge using high-temporal-resolution T 2 ∗ mapping. Our results suggest that the implemented system analysis approach may help to unlock questions regarding regulation of renal oxygenation, with the ultimate goal of providing imaging means for diagnostics and therapy of renal diseases.

Multiparametric Functional Magnetic Resonance Imaging for Evaluating Renal Allograft Injury

Kidney transplantation is the treatment of choice for patients with end-stage renal disease, as it extends survival and increases quality of life in these patients. However, chronic allograft injury continues to be a major problem, and leads to eventual graft loss. Early detection of allograft injury is essential for guiding appropriate intervention to delay or prevent irreversible damage. Several advanced MRI techniques can offer some important information regarding functional changes such as perfusion, diffusion, structural complexity, as well as oxygenation and fibrosis. This review highlights the potential of multiparametric MRI for noninvasive and comprehensive assessment of renal allograft injury.

Epidemiology, outcomes, and management of acute kidney injury in the vascular surgery patient

OBJECTIVE

Conventional clinical wisdom has often been nihilistic regarding the prevention and management of acute kidney injury (AKI), despite its being a frequent and morbid complication associated with both increased mortality and cost. Recent developments have shown that AKI is not inevitable and that changes in management of patients can reduce both the incidence and morbidity of perioperative AKI. The purpose of this narrative review was to review the epidemiology and outcomes of AKI in patients undergoing vascular surgery using current consensus definitions, to discuss some of the novel emerging risk stratification and prevention techniques relevant to the vascular surgery patient, and to describe a standardized perioperative pathway for the prevention of AKI after vascular surgery.

METHODS

We performed a critical review of the literature on AKI in the vascular surgery patient using the PubMed and MEDLINE databases and Google Scholar through September 2017 using web-based search engines. We also searched the guidelines and publications available online from the organizations Kidney Disease: Improving Global Outcomes and the Acute Dialysis Quality Initiative. The search terms used included acute kidney injury, AKI, epidemiology, outcomes, prevention, therapy, and treatment.

RESULTS

The reported epidemiology and outcomes associated with AKI have been evolving since the publication of consensus criteria that allow accurate identification of mild and moderate AKI. The incidence of AKI after major vascular surgery using current criteria is as high as 49%, although there are significant differences, depending on the type of procedure performed. Many tools have become available to assess and to stratify the risk for AKI and to use that information to prevent AKI in the surgical patient. We describe a standardized clinical assessment and management pathway for vascular surgery patients, incorporating current risk assessment and preventive strategies to prevent AKI and to decrease its complications. Patients without any risk factors can be managed in a perioperative fast-track pathway. Those patients with positive risk factors are tested for kidney stress using the urinary biomarker TIMP-2•IGFBP7, and care is then stratified according to the result. Management follows current Kidney Disease: Improving Global Outcomes guidelines.

CONCLUSIONS

AKI is a common postoperative complication among vascular surgery patients and has a significant impact on morbidity, mortality, and cost. Preoperative risk assessment and optimal perioperative management guided by that risk assessment can minimize the consequences associated with postoperative AKI. Adherence to a standardized perioperative pathway designed to reduce risk of AKI after major vascular surgery offers a promising clinical approach to mitigate the incidence and severity of this challenging clinical problem.

Renal hypoxia in kidney disease: Cause or consequence?

Tissue hypoxia has been proposed as an important factor in the pathophysiology of both chronic kidney disease (CKD) and acute kidney injury (AKI), initiating and propagating a vicious cycle of tubular injury, vascular rarefaction, and fibrosis and thus exacerbation of hypoxia. Here, we critically evaluate this proposition by systematically reviewing the literature relevant to the following six questions: (i) Is kidney disease always associated with tissue hypoxia? (ii) Does tissue hypoxia drive signalling cascades that lead to tissue damage and dysfunction? (iii) Does tissue hypoxia per se lead to kidney disease? (iv) Does tissue hypoxia precede pathology? (v) Does tissue hypoxia colocalize with pathology? (vi) Does prevention of tissue hypoxia prevent kidney disease? We conclude that tissue hypoxia is a common feature of both AKI and CKD. Furthermore, at least under in vitro conditions, renal tissue hypoxia drives signalling cascades that lead to tissue damage and dysfunction. Tissue hypoxia itself can lead to renal pathology, independent of other known risk factors for kidney disease. There is also some evidence that tissue hypoxia precedes renal pathology, at least in some forms of kidney disease. However, we have made relatively little progress in determining the spatial relationships between tissue hypoxia and pathological processes (i.e. colocalization) or whether therapies targeted to reduce tissue hypoxia can prevent or delay the progression of renal disease. Thus, the hypothesis that tissue hypoxia is a "common pathway" to both AKI and CKD still remains to be adequately tested.

Red blood cell antibody-induced anemia causes differential degrees of tissue hypoxia in kidney and brain

Moderate anemia is associated with increased mortality and morbidity, including acute kidney injury (AKI), in surgical patients. A red blood cell (RBC)-specific antibody model was utilized to determine whether moderate subacute anemia could result in tissue hypoxia as a potential mechanism of injury. Cardiovascular and hypoxic cellular responses were measured in transgenic mice capable of expressing hypoxia-inducible factor-1α (HIF-1α)/luciferase activity in vivo. Antibody-mediated anemia was associated with mild intravascular hemolysis (6 h) and splenic RBC sequestration ( day 4), resulting in a nadir hemoglobin concentration of 89 ± 13 g/l on day 4. At this time point, renal tissue oxygen tension (P_tO₂) was decreased in anemic mice relative to controls (13.1 ± 4.3 vs. 20.8 ± 3.7 mmHg, P < 0.001). Renal tissue hypoxia was associated with an increase in HIF/luciferase expression in vivo ( P = 0.04) and a 20-fold relative increase in renal erythropoietin mRNA transcription ( P < 0.001) but no increase in renal blood flow ( P = 0.67). By contrast, brain P_tO₂ was maintained in anemic mice relative to controls (22.7 ± 5.2 vs. 23.4 ± 9.8 mmHg, P = 0.59) in part because of an increase in internal carotid artery blood flow (80%, P < 0.001) and preserved cerebrovascular reactivity. Despite these adaptive changes, an increase in brain HIF-dependent mRNA levels was observed (erythropoietin: P < 0.001; heme oxygenase-1: P = 0.01), providing evidence for subtle cerebral tissue hypoxia in anemic mice. These data demonstrate that moderate subacute anemia causes significant renal tissue hypoxia, whereas adaptive cerebrovascular responses limit the degree of cerebral tissue hypoxia. Further studies are required to assess whether hypoxia is a mechanism for acute kidney injury associated with anemia.

Unilateral nephrectomy diminishes ischemic acute kidney injury through enhanced perfusion and reduced pro-inflammatory and pro-fibrotic responses

While unilateral nephrectomy (UNx) is suggested to protect against ischemia-reperfusion injury (IRI) in the remaining kidney, the mechanisms underlying this protection remain to be elucidated. In this study, functional MRI was employed in a renal IRI rat model to reveal global and regional changes in renal filtration, perfusion, oxygenation and sodium handling, and microarray and pathway analyses were conducted to identify protective molecular mechanisms. Wistar rats were randomized to either UNx or sham UNx immediately prior to 37 minutes of unilateral renal artery clamping or sham operation under sevoflurane anesthesia. MRI was performed 24 hours after reperfusion. Blood and renal tissue were harvested. RNA was isolated for microarray analysis and QPCR validation of gene expression results. The perfusion (T1 value) was significantly enhanced in the medulla of the post-ischemic kidney following UNx. UNx decreased the expression of fibrogenic genes, i.a. Col1a1, Fn1 and Tgfb1 in the post-ischemic kidney. This was associated with a marked decrease in markers of activated myofibroblasts (Acta2/α-Sma and Cdh11) and macrophages (Ccr2). This was most likely facilitated by down-regulation of Pdgfra, thus inhibiting pericyte-myofibroblast differentiation, chemokine production (Ccl2/Mcp1) and macrophage infiltration. UNx reduced ischemic histopathologic injury. UNx may exert renoprotective effects against IRI through increased perfusion in the renal medulla and alleviation of the acute pro-inflammatory and pro-fibrotic responses possibly through decreased myofibroblast activation. The identified pathways involved may serve as potential therapeutic targets and should be taken into account in experimental models of IRI.

Perioperative Acute Kidney Injury: Risk Factors and Predictive Strategies

Acute kidney injury (AKI) is a common complication in surgical patients and is associated with increases in mortality, an increased risk for chronic kidney disease and hemodialysis after discharge, and increased cost. Better understanding of the risk factors that contribute to perioperative AKI has led to improved AKI prediction and will eventually lead to improved prevention of AKI, mitigation of injury when AKI occurs, and enhanced recovery in patients who sustain AKI. The development of advanced clinical prediction scores for AKI, new imaging techniques, and novel biomarkers for early detection of AKI provides new tools toward these ends.

Noninvasive evaluation of renal pH homeostasis after ischemia reperfusion injury by CEST-MRI

Acute kidney injury (AKI) in mice caused by sustained ischemia followed by reperfusion is associated with acute tubular necrosis and renal dysfunctional blood flow. Although the principal role of the kidney is the maintenance of acid-base balance, current imaging approaches are unable to assess this important parameter, and clinical biomarkers are not robust enough in evaluating the severity of kidney damage. Therefore, novel noninvasive imaging approaches are needed to assess the acid-base homeostasis in vivo. This study investigates the usefulness of MRI-chemical exchange saturation transfer (CEST) pH imaging (through iopamidol injection) in characterizing moderate and severe AKI in mice following unilateral ischemia reperfusion injury. Moderate (20 min) and severe (40 min) ischemia were induced in Balb/C mice, which were imaged at several time points thereafter (Days 0, 1, 2, 7). A significant increase of renal pH values was observed as early as one day after the ischemia reperfusion damage for both moderate and severe ischemia. MRI-CEST pH imaging distinguished the evolution of moderate from severe AKI. A recovery of normal renal pH values was observed for moderate AKI, whereas a persisting renal pH increase was observed for severe AKI on Day 7. Renal filtration fraction was significantly lower for clamped kidneys (0.54-0.57) in comparison to contralateral kidneys (0.84-0.86) following impairment of glomerular filtration. The severe AKI group showed a reduced filtration fraction even after 7 days (0.38 for the clamped kidneys). Notably, renal pH values were significantly correlated with the histopathological score. In conclusion, MRI-CEST pH mapping is a valid tool for the noninvasive evaluation of both acid-base balance and renal filtration in patients with ischemia reperfusion injury.

The novel nitric oxide donor PDNO attenuates ovine ischemia-reperfusion induced renal failure

Background

Renal ischemia-reperfusion injury is a common cause of acute kidney injury in intensive care and surgery. Recently, novel organic mononitrites of 1,2-propanediol (PDNO) were synthesized and shown to rapidly and controllably deploy nitric oxide in the circulation when administered intravenously. We hypothesized that intravenous infusion of PDNO during renal ischemia reperfusion would improve post-ischemic renal function and microcirculation.

Methods

Sixteen sheep were anesthetized, mechanically ventilated, and surgically instrumented. The left renal artery was clamped for 90 min, and the effects of ischemia were studied for a total of 8 h. Fifteen minutes prior to the release of the clamp, intravenous infusions of PDNO (n = 8) or vehicle (1,2 propanediol + inorganic nitrite, n = 8) were initiated (180 nmol/kg/min for 30 min, thereafter 60 nmol/kg/min for the remainder of the experiment).

Results

Renal artery blood flow, cortical and medullary perfusion, and diuresis and creatinine clearance decreased in the left kidney post ischemia. However, in the sheep treated with PDNO, diuresis and creatinine clearance in the left kidney were significantly higher post ischemia compared to vehicle-treated animals (1.7 ± 0.5 vs 0.7 ± 0.3 ml/kg/h, p = 0.04 and 7.5 ± 2.1 vs 1.7 ± 0.6 ml/min, p = 0.02, respectively). Left renal medullary perfusion and oxygen uptake were higher in the PDNO group (73 ± 9 vs 37 ± 5% of baseline, p = 0.004 and 2.6 ± 0.4 vs 1.6 ± 0.3 ml/min, p = 0.02, respectively). PDNO significantly increased renal oxygen consumption and reduced the oxygen utilization for sodium reabsorption (p = 0.03 for both). Mean arterial blood pressure was significantly reduced by PDNO (83 ± 3 vs 94 ± 3 mmHg, p = 0.02) but was still within normal limits. Total renal blood flow was not affected, and there were no signs of increased blood methemoglobin concentrations or tachyphylaxis.

Conclusions

The novel nitric oxide donor PDNO improved renal function after ischemia. PDNO also prevented the persistent reduction in medullary perfusion during reperfusion and improved renal oxygen utilization without severe side effects.

In situ lactate dehydrogenase activity: a novel renal cortical imaging biomarker of tubular injury?

Renal ischemia-reperfusion injury is the state of which a tissue experiences injury after a phase of restrictive blood supply and recirculation. Ischemia-reperfusion injury (I/R-I) is a leading cause of acute kidney injury (AKI) in several disease states, including kidney transplantation, sepsis, and hypovolemic shock. The most common methods to evaluate AKI are creatinine clearance, plasma creatinine, blood urea nitrogen, or renal histology. However, currently, there are no precise methods to directly assess renal injury state noninvasively. Hyperpolarized 13C-pyruvate MRI enables noninvasive accurate quantification of the in vivo conversion of pyruvate to lactate, alanine, and bicarbonate. In the present study, we investigated the in situ alterations of metabolic conversion of pyruvate to lactate, alanine, and bicarbonate in a unilateral I/R-I rat model with 30 min and 60 min of ischemia followed by 24 h of reperfusion. The pyruvate conversion was unaltered compared with sham in the 30 min I/R-I group, while a significant reduced metabolic conversion was found in the postischemic kidney after 60 min of ischemia. This indicates that after 30 min of ischemia, the kidney maintains normal metabolic function in spite of decreased kidney function, whereas the postischemic kidney after 60 min of ischemia show a generally reduced metabolic enzyme activity concomitant with a reduced kidney function. We have confidence that these findings can have a high prognostic value in prediction of kidney injury and the outcome of renal injury.

Recent advances in renal hypoxia: insights from bench experiments and computer simulations

The availability of oxygen in renal tissue is determined by the complex interactions among a host of processes, including renal blood flow, glomerular filtration, arterial-to-venous oxygen shunting, medullary architecture, Na(+) transport, and oxygen consumption. When this delicate balance is disrupted, the kidney may become susceptible to hypoxic injury. Indeed, renal hypoxia has been implicated as one of the major causes of acute kidney injury and chronic kidney diseases. This review highlights recent advances in our understanding of renal hypoxia; some of these studies were published in response to a recent Call for Papers of this journal: Renal Hypoxia.

High-resolution renal perfusion mapping using contrast-enhanced ultrasonography in ischemia-reperfusion injury monitors changes in renal microperfusion

Alterations in renal microperfusion play an important role in the development of acute kidney injury with long-term consequences. Here we used contrast-enhanced ultrasonography as a novel method for depicting intrarenal distribution of blood flow. After infusion of microbubble contrast agent, bubbles were collapsed in the kidney and postbubble destruction refilling was measured in various regions of the kidney. Local perfusion was monitored in vivo at 15, 30, 45, 60 minutes and 24 hours after 28 minutes of bilateral ischemia in 12 mice. High-resolution, pixel-by-pixel analysis was performed on each imaging clip using customized software, yielding parametric perfusion maps of the kidney, representing relative blood volume in each pixel. These perfusion maps revealed that outer medullary perfusion decreased disproportionately to the reduction in the cortical and inner medullary perfusion after ischemia. Outer medullary perfusion was significantly decreased by 69% at 60 minutes postischemia and remained significantly less (40%) than preischemic levels at 24 hours postischemia. Thus, contrast-enhanced ultrasonography with high-resolution parametric perfusion maps can monitor changes in renal microvascular perfusion in space and time in mice. This novel technique can be translated to clinical use in man.

Role of nitric oxide in kidney and liver (as distance organ) function in bilateral renal ischemia-reperfusion: Effect of L-Arginine and NG-nitro-L-Arginine methyl ester

Background:

Renal ischemia-reperfusion (RIR) is a major cause of renal dysfunction that acts through different mechanisms. We investigated the role of L-Arginine as an endogenous nitric oxide (NO) precursor and NG-nitro-L-Arginine methyl ester (L-NAME) as an NO inhibitor on kidney and liver function in RIR model.

Materials and Methods:

Fifty-eight Wistar rats were randomly assigned to four groups. Groups 1 (sham-operated, n = 13) received a single dose of saline (4 ml/kg, i.p.) and 2 (Ischemia [Isch], n = 14) received a single dose of saline (4 ml/kg, i.p.). Groups 3 (Isch + L-NAME, n = 15) received a single dose of L-NAME (20 mg/kg, i.p.) and 4 (Isch + L-Arginine n = 16) received a single dose of L-Arginine (300 mg/kg, i.p.), After 2 h, renal failure was induced by clamping both renal pedicles for 45 min, followed by 24-h reperfusion in Groups 2–4. Finally, blood samples were obtained, and kidney tissue samples were subjected for pathology investigations.

Results:

The body weight decreased, and the serum levels of blood urea nitrogen (BUN) and creatinine (Cr), and kidney tissue damage score (KTDS) increased significantly in the Isch and Isch + L-NAME groups compared with the sham group while L-Arginine improved weight reduction (P < 0.05), and it reduced the serum levels of BUN and Cr, and KTDS when compared with the Isch and Isch + L-NAME groups. Kidney weight increased significantly in all groups compared with the sham group. L-Arginine reduced the liver tissue level of malondialdehyde and increased alkaline phosphatase.

Conclusion:

L-Arginine as an NO precursor can improve kidney function against RIR. It also improves oxidative stress in liver tissue.

Acute Kidney Injury in the Surgical Patient

Perioperative acute kidney injury (AKI) is a common, morbid, and costly surgical complication. Current efforts to understand and manage AKI in surgical patients focus on prevention, mitigation of further injury when AKI has occurred, treatment of associated conditions, and facilitation of renal recovery. Lesser severity AKI is now understood to be much more common, and more morbid, than was previously thought. The ability to detect AKI within hours of onset would be helpful in protecting the kidney and in preserving renal function, and several imaging and biomarker modalities are currently being evaluated.

Recent advances in renal hemodynamics: insights from bench experiments and computer simulations

It has been long known that the kidney plays an essential role in the control of body fluids and blood pressure and that impairment of renal function may lead to the development of diseases such as hypertension (Guyton AC, Coleman TG, Granger Annu Rev Physiol 34: 13-46, 1972). In this review, we highlight recent advances in our understanding of renal hemodynamics, obtained from experimental and theoretical studies. Some of these studies were published in response to a recent Call for Papers of this journal: Renal Hemodynamics: Integrating with the Nephron and Beyond.

How bold is blood oxygenation level-dependent (BOLD) magnetic resonance imaging of the kidney? Opportunities, challenges and future directions

Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. Yet, in vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Many of the established approaches are invasive, hence not applicable in humans. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) offers an alternative. BOLD-MRI is non-invasive and indicative of renal tissue oxygenation. Nonetheless, recent (pre-) clinical studies revived the question as to how bold renal BOLD-MRI really is. This review aimed to deliver some answers. It is designed to inspire the renal physiology, nephrology and imaging communities to foster explorations into the assessment of renal oxygenation and haemodynamics by exploiting the powers of MRI. For this purpose, the specifics of renal oxygenation and perfusion are outlined. The fundamentals of BOLD-MRI are summarized. The link between tissue oxygenation and the oxygenation-sensitive MR biomarker T2∗ is outlined. The merits and limitations of renal BOLD-MRI in animal and human studies are surveyed together with their clinical implications. Explorations into detailing the relation between renal T2∗ and renal tissue partial pressure of oxygen (pO2 ) are discussed with a focus on factors confounding the T2∗ vs. tissue pO2 relation. Multi-modality in vivo approaches suitable for detailing the role of the confounding factors that govern T2∗ are considered. A schematic approach describing the link between renal perfusion, oxygenation, tissue compartments and renal T2∗ is proposed. Future directions of MRI assessment of renal oxygenation and perfusion are explored.

Blood oxygenation level-dependent MRI for assessment of renal oxygenation

Blood oxygen level-dependent magnetic resonance imaging (BOLD MRI) has recently emerged as an important noninvasive technique to assess intrarenal oxygenation under physiologic and pathophysiologic conditions. Although this tool represents a major addition to our armamentarium of methodologies to investigate the role of hypoxia in the pathogenesis of acute kidney injury and progressive chronic kidney disease, numerous technical limitations confound interpretation of data derived from this approach. BOLD MRI has been utilized to assess intrarenal oxygenation in numerous experimental models of kidney disease and in human subjects with diabetic and nondiabetic chronic kidney disease, acute kidney injury, renal allograft rejection, contrast-associated nephropathy, and obstructive uropathy. However, confidence in conclusions based on data derived from BOLD MRI measurements will require continuing advances and technical refinements in the use of this technique.

Renal transplantation induces mitochondrial uncoupling, increased kidney oxygen consumption, and decreased kidney oxygen tension

Hypoxia is an acknowledged pathway to renal injury and ischemia-reperfusion (I/R) and is known to reduce renal oxygen tension (Po2). We hypothesized that renal I/R increases oxidative damage and induces mitochondrial uncoupling, resulting in increased oxygen consumption and hence kidney hypoxia. Lewis rats underwent syngenic renal transplantation (TX) and contralateral nephrectomy. Controls were uninephrectomized (1K-CON) or left untreated (2K-CON). After 7 days, urinary excretion of protein and thiobarbituric acid-reactive substances were measured, and after 14 days glomerular filtration rate (GFR), renal blood flow, whole kidney Qo2, cortical Po2, kidney cortex mitochondrial uncoupling, renal oxidative damage, and tubulointerstitial injury were assessed. TX, compared with 1K-CON, resulted in mitochondrial uncoupling mediated via uncoupling protein-2 (16 ± 3.3 vs. 0.9 ± 0.4 pmol O2 · s(-1)· mg protein(-1), P < 0.05) and increased whole kidney Qo2 (55 ± 16 vs. 33 ± 10 μmol O2/min, P < 0.05). Corticomedullary Po2 was lower in TX compared with 1K-CON (30 ± 13 vs. 47 ± 4 μM, P < 0.05) whereas no significant difference was observed between 2K-CON and 1K-CON rats. Proteinuria, oxidative damage, and the tubulointerstitial injury score were not significantly different in 1K-CON and TX. Treatment of donors for 5 days with mito-TEMPO reduced mitochondrial uncoupling but did not affect renal hemodynamics, Qo2, Po2, or injury. Collectively, our results demonstrate increased mitochondrial uncoupling as an early event after experimental renal transplantation associated with increased oxygen consumption and kidney hypoxia in the absence of increases in markers of damage.

Role of renal oxygenation and mitochondrial function in the pathophysiology of acute kidney injury

There are unique features of renal oxygenation that render the kidney susceptible to oxygen demand-supply mismatch and hypoxia. Renal oxygen consumption by oxidative metabolism is closely coupled to and driven by tubular transport, which is linked to the filtered solute load and glomerular filtration rate (GFR). In turn, filtered solute load and GFR are dependent on the renal blood flow. Hence, changes in renal blood flow increase oxygen delivery but also increase oxygen demand (consumption) simultaneously by increasing the tubular workload of solute transport. The renal blood flow to different regions of kidney is also inhomogeneous, increasing the oxygen demand-supply mismatch in particular areas such as the outer medulla which become more susceptible to injury. Thus, tubular transport and oxidative metabolism by mi ochondria are closely coupled in the kidney and are the principal determinants of intrarenal oxygenation. Here we review the published literature characterizing renal oxygenation and mitochondrial function in ischemic and sepsis-associated acute kidney injury (AKI). However, the coupling of transport and metabolism in AKI has not been examined. This is a potentially fruitful area of research that should become increasingly active given the emerging data linking renal oxygenation and hypoxia to acute and chronic dysfunction in the kidney.

How has urinary proteomics contributed to the discovery of early biomarkers of acute kidney injury?

In the past decade, analysis of the urinary proteome (urinary proteomics) has intensified in response to the need for novel biomarkers that support early diagnosis of kidney diseases. In particular, this also applies to acute kidney injury, which is a heterogeneous complex syndrome with a still-increasing incidence at the intensive care unit. Unfortunately, this major need remains largely unmet to date. The current report aims to explain why attempts to implement urinary proteomic-discovered acute kidney injury diagnostic candidates in the intensive care unit setting have not yet led to success. Subsequently, some key notes are provided that should enhance the chance of translating selected urinary proteomic candidates to valuable tools for the nephrologist and intensivist in the near future.

Functional imaging of acute kidney injury at 3 Tesla: investigating multiple parameters using DCE-MRI and a two-compartment filtration model

OBJECT

To investigate how MR-based parameters reflect functional changes in kidneys with acute kidney injury (AKI) using dynamic contrast enhanced MRI and a two-compartment renal filtration model.

MATERIALS AND METHODS

MRI data of eight male Lewis rats were analyzed retrospectively. Five animals were subjected to AKI, three native rats served as control. All animals underwent perfusion imaging by dynamic contrast-enhanced MRI. Renal blood volume, glomerular filtration rate (GFR) as well as plasma and tubular mean transit times were estimated from regions-of-interest drawn in the renal cortex. Differences between healthy kidneys and kidneys subjected to AKI were analyzed using a paired t-test.

RESULTS

Significant differences between ischemic and healthy kidneys could only be detected for the glomerular filtration rate. For all other calculated parameters, differences were present, however not significant. In rats with AKI, average single kidney GFR was 0.66 ± 0.37 ml/min for contralateral and 0.26 ± 0.12 ml/ min for diseased kidneys (P = 0.0254). For the healthy control group, the average GFR was 0.39 ± 0.06 ml/min and 0.41 ± 0.11 ml/min, respectively. Differences between diseased kidneys of AKI rats and ipsilateral kidneys of the healthy control group were significant (P = 0.0381).

CONCLUSION

Significant differences of functional parameters reflecting damage of the renal tissue of kidneys with AKI compared to the contralateral, healthy kidneys could only be detected by GFR. GFR might be a useful parameter that allows for a spatially resolved detection of abnormal changes of renal tissue by AKI.

Renal oxygenation in acute renal ischemia-reperfusion injury

Tissue hypoxia has been demonstrated, in both the renal cortex and medulla, during the acute phase of reperfusion after ischemia induced by occlusion of the aorta upstream from the kidney. However, there are also recent clinical observations indicating relatively well preserved oxygenation in the nonfunctional transplanted kidney. To test whether severe acute kidney injury can occur in the absence of widespread renal tissue hypoxia, we measured cortical and inner medullary tissue Po2 as well as total renal O2 delivery (Do2) and O2 consumption (Vo2) during the first 2 h of reperfusion after 60 min of occlusion of the renal artery in anesthetized rats. To perform this experiment, we used a new method for measuring kidney Do2 and Vo2 that relies on implantation of fluorescence optodes in the femoral artery and renal vein. We were unable to detect reductions in renal cortical or inner medullary tissue Po2 during reperfusion after ischemia localized to the kidney. This is likely explained by the observation that Vo2 (-57%) was reduced by at least as much as Do2 (-45%), due to a large reduction in glomerular filtration (-94%). However, localized tissue hypoxia, as evidence by pimonidazole adduct immunohistochemistry, was detected in kidneys subjected to ischemia and reperfusion, particularly in, but not exclusive to, the outer medulla. Thus, cellular hypoxia, particularly in the outer medulla, may still be present during reperfusion even when reductions in tissue Po2 are not detected in the cortex or inner medulla.

Renal perfusion in acute kidney injury with DCE-MRI: deconvolution analysis versus two-compartment filtration model

PURPOSE

To investigate the results of different pharmacokinetic models of a quantitative analysis of renal blood flow (RBF) in acute kidney injury using deconvolution analysis and a two-compartment renal filtration model.

MATERIALS AND METHODS

MRI data of ten male Lewis rats were analyzed retrospectively. Six animals were subjected to unilateral acute kidney injury and underwent perfusion imaging by dynamic contrast-enhanced MRI (DCE-MRI). Renal blood flow was estimated from regions-of-interest depicting the cortex in the DCE-MRI perfusion maps. The perfusion models were compared by a paired t-test and Bland-Altman plots.

RESULTS

No significant difference was found between the two compartment model and the deconvolution analysis (P=0.2807). Differences between healthy and diseased kidney in the AKI model were significant for both methods (P<0.05). A Bland-Altman plot showed no systematic errors, and values were equally distributed around the mean difference between the methods lying within the range of 1.96 standard deviations.

CONCLUSION

Both quantification strategies could detect the kidneys that were impaired by AKI. When just aiming at RBF as a marker, a deconvolution analysis can provide similar values as the 2CFM. If functional parameters beyond RBF like glomerular filtration rate are needed, the 2CFM should be employed.

Renal oxygenation and haemodynamics in acute kidney injury and chronic kidney disease

Acute kidney injury (AKI) is a major burden on health systems and may arise from multiple initiating insults, including ischaemia-reperfusion injury, cardiovascular surgery, radiocontrast administration and sepsis. Similarly, the incidence and prevalence of chronic kidney disease (CKD) continues to increase, with significant morbidity and mortality. Moreover, an increasing number of AKI patients survive to develop CKD and end-stage renal disease. Although the mechanisms for the development of AKI and progression to CKD remain poorly understood, initial impairment of oxygen balance likely constitutes a common pathway, causing renal tissue hypoxia and ATP starvation that, in turn, induce extracellular matrix production, collagen deposition and fibrosis. Thus, possible future strategies for one or both conditions may involve dopamine, loop diuretics, atrial natriuretic peptide and inhibitors of inducible nitric oxide synthase, substances that target kidney oxygen consumption and regulators of renal oxygenation, such as nitric oxide and heme oxygenase-1.

Assessing intrarenal nonperfusion and vascular leakage in acute kidney injury with multinuclear (1) H/(19) F MRI and perfluorocarbon nanoparticles

PURPOSE

We sought to develop a unique sensor-reporter approach for functional kidney imaging that employs circulating perfluorocarbon nanoparticles and multinuclear (1) H/(19) F MRI.

METHODS

(19) F spin density weighted and T1 weighted images were used to generate quantitative functional mappings of both healthy and ischemia-reperfusion (acute kidney injury) injured mouse kidneys. (1) H blood-oxygenation-level-dependent (BOLD) MRI was also employed as a supplementary approach to facilitate the comprehensive analysis of renal circulation and its pathological changes in acute kidney injury.

RESULTS

Heterogeneous blood volume distributions and intrarenal oxygenation gradients were confirmed in healthy kidneys by (19) F MRI. In a mouse model of acute kidney injury, (19) F MRI, in conjunction with blood-oxygenation-level-dependent MRI, sensitively delineated renal vascular damage and recovery. In the cortico-medullary junction region, we observed 25% lower (19) F signal (P < 0.05) and 70% longer (1) H T2* (P < 0.01) in injured kidneys compared with contralateral kidneys at 24 h after initial ischemia-reperfusion injury. We also detected 71% higher (19) F signal (P < 0.01) and 40% lower (1) H T2* (P < 0.05) in the renal medulla region of injured kidneys compared with contralateral uninjured kidneys.

CONCLUSION

Integrated (1) H/(19) F MRI using perfluorocarbon nanoparticles provides a multiparametric readout of regional perfusion defects in acutely injured kidneys.

Linking non-invasive parametric MRI with invasive physiological measurements (MR-PHYSIOL): towards a hybrid and integrated approach for investigation of acute kidney injury in rats

Acute kidney injury of various origins shares a common link in the pathophysiological chain of events: imbalance between renal medullary oxygen delivery and oxygen demand. For in vivo assessment of kidney haemodynamics and oxygenation in animals, quantitative but invasive physiological methods are established. A very limited number of studies attempted to link these invasive methods with parametric Magnetic Resonance Imaging (MRI) of the kidney. Moreover, the validity of parametric MRI (pMRI) as a surrogate marker for renal tissue perfusion and renal oxygenation has not been systematically examined yet. For this reason, we set out to combine invasive techniques and non-invasive MRI in an integrated hybrid setup (MR-PHYSIOL) with the ultimate goal to calibrate, monitor and interpret parametric MR and physiological parameters by means of standardized interventions. Here we present a first report on the current status of this multi-modality approach. For this purpose, we first highlight key characteristics of renal perfusion and oxygenation. Second, concepts for in vivo characterization of renal perfusion and oxygenation are surveyed together with the capabilities of MRI for probing blood oxygenation-dependent tissue stages. Practical concerns evoked by the use of strong magnetic fields in MRI and interferences between MRI and invasive physiological probes are discussed. Technical solutions that balance the needs of in vivo physiological measurements together with the constraints dictated by small bore MR scanners are presented. An early implementation of the integrated MR-PHYSIOL approach is demonstrated including brief interventions of hypoxia and hyperoxia.

High temporal resolution parametric MRI monitoring of the initial ischemia/reperfusion phase in experimental acute kidney injury

Ischemia/reperfusion (I/R) injury, a consequence of kidney hypoperfusion or temporary interruption of blood flow is a common cause of acute kidney injury (AKI). There is an unmet need to better understand the mechanisms operative during the initial phase of ischemic AKI. Non-invasive in vivo parametric magnetic resonance imaging (MRI) may elucidate spatio-temporal pathophysiological changes in the kidney by monitoring the MR relaxation parameters T₂* and T₂, which are known to be sensitive to blood oxygenation. The aim of our study was to establish the technical feasibility of fast continuous T₂*/T₂ mapping throughout renal I/R. MRI was combined with a remotely controlled I/R model and a segmentation model based semi-automated quantitative analysis. This technique enabled the detailed assessment of in vivo changes in all kidney regions during ischemia and early reperfusion. Significant changes in T₂* and T₂ were observed shortly after induction of renal ischemia and during the initial reperfusion phase. Our study demonstrated for the first time that continuous and high temporal resolution parametric MRI is feasible for in-vivo monitoring and characterization of I/R induced AKI in rats. This technique may help in the identification of the timeline of key events responsible for development of renal damage in hypoperfusion-induced AKI.