Blood oxygen level-dependent magnetic resonance imaging identifies cortical hypoxia in severe renovascular disease

Diffusely Increased FDG Uptake of the Kidney Caused by Severe Renal Artery Stenosis

ABSTRACT

Renal artery stenosis can result in renovascular hypertension and ischemic nephropathy. We describe FDG PET/CT and delayed abdominal FDG PET/MRI findings in a kidney associated with severe renal artery stenosis resulting from renal artery invasion in retroperitoneal alveolar echinococcosis. The renal parenchyma showed diffusely increased activity on both FDG PET/CT and delayed FDG PET/MRI. There was no activity in the renal collecting system on FDG PET/CT and focal urine activity in the renal calyces on delayed FDG PET/MRI. This case indicates that renal artery stenosis should be included in the differential diagnosis of diffuse renal parenchymal FDG uptake.

Magnetic Resonance Imaging in Atherosclerotic Renal Artery Stenosis: The Update and Future Directions from Interventional Perspective

Background

Atherosclerotic renal artery stenosis (ARAS) is a condition where the renal arteries become narrowed due to atherosclerosis, leading to reduced blood flow to the kidneys and various renal complications. The effectiveness of interventional treatments, such as renal artery angioplasty and stenting, remains debated, making patient selection for these procedures challenging.

Summary

This review focuses on the diagnosis and management of ARAS, with a particular emphasis on the potential role of functional magnetic resonance imaging (MRI) in evaluating renal function and mechanisms. By summarizing current diagnostic approaches and outcomes of interventional treatments, the review highlights the importance of informed clinical decision-making in ARAS management. Functional MRI emerges as a promising noninvasive tool to assess renal function, aiding in patient stratification and treatment planning.

Key Messages

The efficacy of interventional treatments for ARAS requires further investigation and careful patient selection. Functional MRI holds promise as a noninvasive means to assess renal function and mechanisms, potentially guiding more effective clinical decisions in ARAS management. Advancing research in diagnostic methods, particularly functional MRI, can enhance our understanding and improve the treatment outcomes for ARAS patients.

Efficacy of Human Embryonic Stem Cells Compared to Adipose Tissue-Derived Human Mesenchymal Stem/Stromal Cells for Repair of Murine Post-Stenotic Kidneys

Clinical translation of mesenchymal stem/stromal cell (MSC) therapy has been impeded by the heterogenous nature and limited replicative potential of adult-derived MSCs. Human embryonic stem cell-derived MSCs (hESC-MSCs) that differentiate from immortal cell lines are phenotypically uniform and have shown promise in-vitro and in many disease models. Similarly, adipose tissue-derived MSCs (MSC(AT)) possess potent reparative properties. How these two cell types compare in efficacy, however, remains unknown. We randomly assigned mice to six groups (n = 7-8 each) that underwent unilateral RAS or a sham procedure (3 groups each). Two weeks post-operation, each mouse was administered either vehicle, MSC(AT)s, or hESC-MSCs (5 × 105 cells) into the aorta. Mice were scanned with micro-MRI to determine renal hemodynamics two weeks later and kidneys then harvested. hESC-MSCs and MSC(AT)s were similarly effective at lowering systolic blood pressure. However, MSC(AT)s more robustly increased renal perfusion, oxygenation, and glomerular filtration rate in the post-stenotic kidney, and more effectively mitigated tubular injury, fibrosis, and vascular remodeling. These observations suggest that MSC(AT) are more effective than hESC-MSC in ameliorating kidney dysfunction and tissue injury distal to RAS. Our findings highlight the importance of tissue source in selection of MSCs for therapeutic purposes and underscore the utility of cell-based therapy for kidney disease.

Perfusion and oxygenation in allografts with transplant renal artery stenosis: Evaluation with functional magnetic resonance imaging

BACKGROUND

Transplant renal artery stenosis (TRAS) has been shown to reduce kidney perfusion leading to post-operative hypertension. We aimed to measure the perfusion and oxygenation changes in TRAS with arterial spin labeling (ASL) and blood oxygen level-dependent (BOLD) imaging, respectively.

METHODS

In this single-center prospective study, a total of seven patients with TRAS and seven age- and sex-matched normal kidney transplant recipients underwent both ASL and BOLD imaging. Moreover, measurements of ASL and BOLD were also performed in five patients after successful angioplasty for TRAS.

RESULTS

Allograft cortical perfusion as measured by ASL in the TRAS group was significantly decreased as compared with normal control group (129.9 ± 46.6 ml/100 g vs. 202.4 ± 47.7 ml/100 g, P = .01). Interestingly, allograft oxygenation as indicated by R2* derived from BOLD in both the cortex (16.42 ± 1.90 Hz vs. 18.25 ± 4.34 Hz, P = .33) and the medulla (30.34 ± 2.35 Hz vs. 30.43 ± 6.85 Hz, P = .97) showed no statistical difference between the TRAS and normal control group. In addition, both cortical and medullary oxygenation remained unchanged despite significantly improved cortical perfusion in those undergone successful angioplasty.

CONCLUSION

Cortical and medullary oxygenation were preserved in the presence of reduced allograft perfusion in clinically significant TRAS. Prospective larger studies are needed to conclusively establish perfusion and oxygenation changes in TRAS.

Managing acute presentations of atheromatous renal artery stenosis

BACKGROUND

Atherosclerotic renovascular disease (ARVD) often follows an asymptomatic chronic course which may be undetected for many years. However, there are certain critical acute presentations associated with ARVD and these require a high index of suspicion for underlying high-grade RAS (renal artery stenosis) to improve patient outcomes. These acute presentations, which include decompensated heart failure syndromes, accelerated hypertension, rapidly declining renal function, and acute kidney injury (AKI), are usually associated with bilateral high-grade RAS (> 70% stenosis), or high-grade RAS in a solitary functioning kidney in which case the contralateral kidney is supplied by a vessel demonstrating renal artery occlusion (RAO). These presentations are typically underrepresented in large, randomized control trials which to date have been largely negative in terms of the conferred benefit of revascularization.

CASE PRESENTATION

Here we describe 9 individual patients with 3 classical presentations including accelerated phase hypertension, heart failure syndromes, AKI and a fourth category of patients who suffered recurrent presentations. We describe their response to renal revascularization. The predominant presentation was that consistent with ischaemic nephropathy all of whom had a positive outcome with revascularization.

CONCLUSION

A high index of suspicion is required for the diagnosis of RAS in these instances so that timely revascularization can be undertaken to restore or preserve renal function and reduce the incidence of hospital admissions for heart failure syndromes.

Hypertension and the kidneys

Hypertension is a leading risk factor for cardiovascular disease and all-cause mortality globally. Hypertension and chronic kidney disease are closely intertwined conditions as hypertension can lead to deteriorating renal function and progressive chronic kidney disease can contribute to worsening hypertension.

In the setting of chronic kidney disease, the pathophysiology of hypertension is complex and involves the interplay of many factors including a reduced number of functioning nephrons, sodium retention and volume expansion, upregulation of the sympathetic nervous system, hormonal factors such as upregulation of the renin–angiotensin–aldosterone system, and endothelial dysfunction. Poorly controlled hypertension can accelerate the progression to end-stage kidney disease.

This review discusses the pathophysiological mechanisms that contribute to hypertension, including sympathetic nervous system activity, the renin–angiotensin–aldosterone system and the role of sodium. In the setting of chronic kidney disease, the relationship with hypertension and renovascular disease as a potential cause and target for therapeutic intervention is briefly reviewed. Finally, treatment options, targets and the long-term cardiovascular benefits of optimal blood pressure control are discussed.

Cortical and medullary oxygenation evaluation of kidneys with renal artery stenosis by BOLD-MRI

Aim

Blood oxygen level–dependent magnetic resonance imaging (BOLD-MRI) can measure deoxyhemoglobin content. This study aims to evaluate the capacity of BOLD-MRI, which is possible to evaluate the oxygenation state of kidneys with renal artery stenosis (RAS).

Materials and methods

We performed BOLD-MRI for 40 patients with RAS and for 30 healthy volunteers. We then performed post-scan processing and analysis of manually drawn regions of interest to determine R2* values (relaxation rates) for the renal cortex and medulla. We compared R2* values in patients with RAS with those in the control group, and also compared these values for subgroups with varying degrees of stenosis.

Results

Medulla R2* values were higher than cortex R2* values in the control group. There was no significant difference in R2* values for different segments (upper, middle, lower) of the kidneys. Both cortex and medulla R2* values in patients with RAS were significantly higher than corresponding R2* values in the control group (P < 0.05), and BOLD-MRI was more sensitive to changes in the R2* values in the medulla than in the cortex. Among different subgroups in the RAS group, the medulla R2* values were significantly higher in kidneys with severe stenosis than in those with no obvious obstruction, mild stenosis, or moderate stenosis (P < 0.05).

Conclusion

BOLD-MRI is an effective, noninvasive method for evaluating kidney oxygenation, which is important for proper treatment in RAS. It is sufficiently sensitive for detecting medulla ischemia and anoxia of the kidneys.

Role of blood oxygen level-dependent magnetic resonance imaging in studying renal oxygenation changes in renal artery stenosis

AIM

Primary objective of this study was to compare R2* value of the post-stenotic kidney with contralateral kidney, kidneys of essential hypertensive patients, and healthy subjects using blood oxygen level-dependent magnetic resonance imaging (BOLD MRI) technique. The secondary objective was to study the effect of severity of stenosis and viability of kidneys on R2* value.

METHODS

We compared 4 groups of kidneys including 92 with renal artery stenosis, 37 normal contralateral kidneys of unilateral renal artery stenosis patients, 62 kidneys of essential hypertensive patients, and 40 kidneys of healthy controls using BOLD MRI. Deoxyhemoglobin level represented by R2* was calculated before and after giving furosemide and was compared among different groups.

RESULTS

Baseline means cortical R2* value did not differ between groups. Response to furosemide was reduced in stenotic kidneys as compared to essential hypertensive and healthy control groups (p < 0.001). The mean R2* value of the contralateral normal kidney group was not significantly different from the stenotic group. Baseline R2* value and delta R2* values did not differ between different degrees of stenosis. Higher mean cortical R2* was seen in stenotic kidneys which were small (< 7 cm) in size (24.27 ± 5.65 vs 21.7 ± 3.88; p value 0.02) or with poor corticomedullary differentiation (24.64 ± 5.8 vs 20.74 ± 3.34; p value 0.006) as compared to other stenotic kidneys. Similarly, the delta R2* value was also blunted in these small shrunken kidneys (p value < 0.001).

CONCLUSION

R2* values on BOLD MRI are significantly different between kidneys with and without renal artery stenosis and can potentially also predict the utility of revascularization.

Quantification of renal T2 relaxation rate by use of blood oxygen level-dependent magnetic resonance imaging before and after furosemide administration in healthy Beagles

OBJECTIVE

To assess the feasibility of blood oxygen level-dependent (BOLD) MRI for measurement of the renal T2^* relaxation rate (R2^*; proxy for renal oxygenation) before and after furosemide administration and to evaluate the reliability and repeatability of those measurements in healthy dogs.

ANIMALS

8 healthy adult Beagles (4 males and 4 females).

PROCEDURES

Each dog was anesthetized and underwent BOLD MRI before (baseline) and 3 minutes after administration of furosemide (1 mg/kg, IV) twice, with a 1-week interval between scanning sessions. Mapping software was used to process MRI images and measure R2^* and the difference in R2^* (ΔR2^*) before and after furosemide administration. The intraclass correlation coefficient was calculated to assess measurement reliability, and the coefficient of variation and Bland-Altman method were used to assess measurement repeatability.

RESULTS

Mean ± SD baseline R2^* in the renal medulla (24.5 ± 3.8 seconds^-1) was significantly greater than that in the renal cortex (20.6 ± 2.7 seconds^-1). Mean R2^* in the renal cortex (18.6 ± 2.6 seconds^-1) and medulla (17.8 ± 1.5 seconds^-1) decreased significantly after furosemide administration. Mean ΔR2^* in the medulla (6.7 ± 2.4 seconds^-1) was significantly greater than that in the renal cortex (2.1 ± 0.7 seconds^-1). All R2^* and ΔR2^* values had good or excellent reliability and repeatability, except the cortical ΔR2^*, which had poor repeatability.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that BOLD MRI, when performed before and after furosemide administration, was noninvasive and highly reliable and repeatable for dynamic evaluation of renal oxygenation in healthy dogs.

Cell-based regenerative medicine for renovascular disease

Renal artery stenosis (RAS) elicits the development of hypertension and post-stenotic kidney damage, which may become irresponsive to restoration of arterial patency. Rather than mere losses of blood flow or oxygen supply, irreversible intrarenal microvascular rarefaction, tubular injury, and interstitial fibrosis are now attributed to intrinsic pathways activated within the kidney, focusing attention on the kidney parenchyma as a therapeutic target. Several regenerative approaches involving the delivery of reparative cells or products have achieved kidney repair in experimental models of RAS and the delivery of mesenchymal stem/stromal cells (MSCs) has already been translated to human subjects with RAS with promising results. The ongoing development of innovative approaches in kidney disease awaits application, validation, and acceptance as routine clinical treatment to avert kidney damage in RAS.

Evaluation of Renal Tissue Oxygenation Using Blood Oxygen Level-Dependent Magnetic Resonance Imaging in Chronic Kidney Disease

BACKGROUND

Blood oxygen level-dependent magnetic resonance imaging (BOLD-MRI) has been widely used to assess renal oxygenation changes in different kidney diseases in recent years. This study was designed to evaluate and compare renal tissue oxygenation using 2 BOLD-MRI analysis methods, namely, the regional and whole-kidney region of interest (ROI) selection methods.

METHODS

The study ended up with 10 healthy controls and 40 chronic kidney disease (CKD) patients without dialysis. Their renal BOLD-MRI data were analyzed using whole-kidney ROI selection method and compared with regional ROI selection method.

RESULTS

We found the cortical, medullary, and whole-kidney R2* values were significantly higher in CKD patients than those in controls. Compared with the regional ROI selection method, the whole-kidney ROI selection method yielded higher cortical R2* values in both controls and CKD patients. The whole-kidney R2* values of deteriorating renal function group were significantly higher than those in stable renal function group.

CONCLUSIONS

Cortical and medullary oxygenation was decreased significantly in CKD patients compared with the healthy controls, particularly in the medulla. The whole-kidney R2* values were positively correlated with kidney function and inversely correlated with the estimated glomerular filtration rate and effective renal plasma flow. Whole-Kidney R2* value might effectively predict the progression of renal function in patients with CKD.

Inflammation and Oxidative Damage in Ischaemic Renal Disease

Ischaemic renal disease as result of atherosclerotic renovascular disease activates a complex biological response that ultimately leads to fibrosis and chronic kidney disease. Large randomised control trials have shown that renal revascularisation in patients with atherosclerotic renal artery disease does not confer any additional benefit to medical therapy alone. This is likely related to the activation of complex pathways of oxidative stress, inflammatory cytokines and fibrosis due to atherosclerotic disease and hypoxic injury due to reduced renal blood flow. New evidence from pre-clinical trials now indicates a role for specific targeted therapeutic interventions to counteract this complex pathogenesis. This evidence now suggests that the focus for those with atherosclerotic renovascular disease should be a combination of revascularisation and renoprotective therapies that target the renal tissue response to ischaemia, reduce the inflammatory infiltrate and prevent or reduce the fibrosis.

Detection of renal hypoxia configuration in patients with lupus nephritis: a primary study using blood oxygen level-dependent MR imaging

BACKGROUND

Renal microstructure and function are closely associated with oxygenation homeostasis. Analyzing renal blood oxygen level‒dependent (BOLD) magnetic resonance imaging (MRI) examination results will provide information on the biological status of the kidneys. The current study was performed to explore the hypoxia mode of the entire renal parenchyma in patients with lupus nephritis (LN).

METHODS

A total of 23 adult patients with LN and 18 healthy volunteers were recruited. R²* values were acquired using BOLD MRI analysis. The narrow rectangular region of interest was used to explore the hypoxia configuration in entire depths of renal parenchyma. Acquired sequential R²* data were fitted using four categories of mathematic functions. The tendency of R²* data in both patients with LN and healthy volunteers was also compared using repeated-measures analysis of variance.

RESULTS

R²* data from the superficial cortex to deep medulla displayed two patterns called a sharp uptrend style and a flat uptrend style. After sequential R²* data were fitted individually with the use of four mathematic formulas, the multiple-compartment Gaussian function showed the highest goodness of fit. Compared with two categories of R²* value styles, the R²* tendency of entire parenchyma in patients with LN was different from that in healthy volunteers.

CONCLUSIONS

Deep renal medullary oxygenation was not always overtly lower than oxygenation in the superficial renal cortical zone. The manifestation of renal parenchyma oxygenation could be described using a Gaussian function model. Deoxygenation tolerance was damaged in patients with LN.

Stem Cell Therapy for Microvascular Injury Associated with Ischemic Nephropathy

Ischemic nephropathy reflects progressive loss of kidney function due to large vessel atherosclerotic occlusive disease. Recent studies indicate that this process is characterized by microvascular rarefaction, increased tissue hypoxia and activation of inflammatory processes of tissue injury. This review summarizes the rationale and application of functional MR imaging to evaluate tissue oxygenation in human subjects that defines the limits of renal adaptation to reduction in blood flow, development of increasingly severe tissue hypoxia and recruitment of inflammatory injury pathways in ischemic nephropathy. Human mesenchymal stromal/stem cells (MSC) are capable of modifying angiogenic pathways and immune responses, but the potency of these effects vary between individuals and various clinical characteristics including age and chronic kidney disease and levels of hypoxia. We summarize recently completed first-in-human studies applying intrarenal infusion of autologous adipose-derived MSC in human subjects with ischemic nephropathy that demonstrate a rise in blood flow and reduction in tissue hypoxia consistent with partial repair of microvascular injury, even without restoring main renal arterial blood flow. Inflammatory biomarkers in the renal vein of post-stenotic kidneys fell after MSC infusion. These changes were associated with modest but significant dose-related increments in kidney function. These data provide support a role for autologous MSC in repair of microvascular injury associated with tissue hypoxia.

The Role of Hypoxia in Ischemic Chronic Kidney Disease

A gradually developing reduction in renal blood flow from atherosclerotic renovascular disease results in loss of kidney volume and a decrease in glomerular filtration rate that eventually becomes irreversible. Whether this process fundamentally reflects tissue hypoxia has been difficult to establish. Studies of human renovascular disease have indicated that reductions in blood flow of up to 30% to 40% can be tolerated with preservation of normal oxygenation and structural integrity. These observations are consistent with remarkable stability of poststenotic kidney function during sustained medical antihypertensive drug therapy in moderate renovascular disease. With more severe and sustained reductions, however, cortical oxygenation decreases and the magnitude of medullary hypoxia expands. These changes are associated with increasing renal venous levels of inflammatory cytokines, angiogenic markers, and infiltration of inflammatory cells, including tissue macrophages and T cells. Although restoring large-vessel blood flow can improve oxygenation, some of these processes reflect microvascular rarefication, remain activated, and do not depend on hemodynamic factors alone. Elucidation of tissue injury pathways associated with hypoxia opens the possibility of adjunctive therapeutic measures beyond renal revascularization. These include cell-based regeneration, mitochondrial protection, and/or angiogenic cytokine therapy to restore or preserve renal function in ischemic nephropathy.

Lipoprotein (a) as a residual risk factor for atherosclerotic renal artery stenosis in hypertensive patients: a hospital-based cross-sectional study

Background

Low-density lipoprotein cholesterol (LDL-c) has been proven to be a risk factor for atherosclerotic cardiovascular disease (CVD), while lipoprotein (a) (Lp(a)) is a residual risk factor for CVD, even though LDL-c is well controlled by statin use. Importantly, the role of Lp(a) in atherosclerotic renal artery stenosis (ARAS) is still unknown.

Methods

For this hospital-based cross-sectional study, patients who simultaneously underwent coronary and renal angiography were examined. ARAS was defined as a 50% reduction in the cross-sectional (two-dimensional plane) area of the renal artery. Data were collected and compared between ARAS and non-ARAS groups, including clinical history and metabolite profiles. Univariate analysis, three tertile LDL-c-based stratified analysis, and multivariate-adjusted logistic analysis were conducted, revealing a correlation between Lp(a) and ARAS.

Results

A total of 170 hypertensive patients were included in this study, 85 with ARAS and 85 with non-RAS. Baseline information indicated comparability between the two groups. In the univariate and multivariate analysis, common risk factors for atherosclerosis were not significantly different. Stratified analysis of LDL-c revealed a significant increase in the incidence of ARAS in patients who had high Lp(a) concentrations at low LDL-c levels (odds ratio (OR): 4.77, 95% confidence interval (CI): 1.04–21.79, P = 0.044). Further logistic analysis with adjusted covariates also confirmed the result, indicating that high Lp(a) levels were independently associated with ARAS (adjusted OR (aOR): 6.14, 95%CI: 1.03–36.47, P = 0.046). This relationship increased with increasing Lp(a) concentration based on a curve fitting graph. These results were not present in the low and intermediate LDL-c-level groups.

Conclusion

In hypertensive patients who present low LDL-c, high Lp(a) was significantly associated with atherosclerotic renal artery stenosis and thus is a residual risk factor.

Coexisting renal artery stenosis and metabolic syndrome magnifies mitochondrial damage, aggravating poststenotic kidney injury in pigs

OBJECTIVE

Renovascular disease (RVD) produces chronic underperfusion of the renal parenchyma and progressive ischemic injury. Metabolic abnormalities often accompany renal ischemia, and are linked to poorer renal outcomes. However, the mechanisms of injury in kidneys exposed to the ischemic and metabolic components of RVD are incompletely understood. We hypothesized that coexisting renal artery stenosis (RAS) and metabolic syndrome (MetS) would exacerbate mitochondrial damage, aggravating poststenotic kidney injury in swine.

METHODS

Domestic pigs were studied after 16 weeks of either standard diet (Lean) or high-fat/high-fructose (MetS) with or without superimposed RAS (n = 6 each). Single-kidney renal blood flow (RBF) and glomerular filtration rate (GFR) were assessed in vivo with multidetector-CT, and renal tubular mitochondrial structure, homeostasis and function and renal injury ex vivo.

RESULTS

Both RAS groups achieved significant stenosis. Single-kidney RBF and GFR were higher in MetS compared with Lean, but decreased in Lean+RAS and MetS+RAS vs. their respective controls. MetS and RAS further induced changes in mitochondrial structure, dynamics, and function, and their interaction (diet × ischemia) decreased matrix density, mitophagy, and ATP production, and lead to greater renal fibrosis.

CONCLUSION

Coexisting RAS and MetS synergistically aggravate mitochondrial structural damage and dysfunction, which may contribute to structural injury and dysfunction in the poststenotic kidney. These observations suggest that mitochondrial damage precedes loss of renal function in experimental RVD, and position mitochondria as novel therapeutic targets in these patients.

Selection of Patients for Angioplasty for Treatment of Atherosclerotic Renovascular Disease: Predicting Responsive Patients

Atherosclerotic renal artery stenosis (ARAS) can cause secondary hypertension, progressive decline in renal function, and cardiac complications. Recent randomized controlled trials including the Cardiovascular Outcomes in Renal Atherosclerotic Lesions study have not reported the benefit of renal artery stenting compared with medical therapy alone to improve renal function or reduce cardiovascular and renal events in the enrolled patients with ARAS. However, observational evidence indicating the benefits of angioplasty in the selected high-risk patients with ARAS has been increasing. Thus, the timely correction of stenosis through angioplasty may have a beneficial effect in selected patients. However, optimal patient selection for angioplasty has been debated and can be challenging at times. Clinicians must identify the responsive patients who would benefit from angioplasty through risk stratification and the prediction of outcomes. Efforts have been made for the determination of predictors that can identify the subgroups of patients who would benefit from angioplasty. Lower age, more severe stenosis, preserved renal perfusion, and absence of diabetes or generalized atherosclerosis have been reported as the predictors for the improvement of hypertension after angioplasty. Global renal ischemia, rapidly declining renal function over 6-12 months, progressive shrinkage of the affected kidney, lower resistive index, and lower levels of albuminuria have been reported as predictors of improved or preserved renal function after angioplasty. This review discusses the identification of ARAS patients who will potentially respond well to angioplasty.

Atherosclerotic-nephropathy: an updated narrative review

The increased prevalence of chronic kidney disease (CKD) in elderly patients recognizes, as main cause, the long-term exposure to atherosclerosis and hypertension. Chronic ischemic damage due to critical renal arterial stenosis induces oxidative stress and intra-renal inflammation, resulting in fibrosis and microvascular remodelling, that is the histological picture of atherosclerotic renal vascular disease (ARVD). The concomitant presence of a long history of hypertension may generate intimal thickening and luminal narrowing of renal arteries and arterioles, glomerulosclerosis, interstitial fibrosis and tubular atrophy, more typically expression of hypertensive nephropathy. These complex mechanisms contribute to the development of CKD and the progression to End Stage Kidney Disease. In elderly CKD patients, the distinction among these nephropathies may be problematic; therefore, ischemic and hypertensive nephropathies can be joined in a unique clinical syndrome defined as atherosclerotic nephropathy. The availability of novel diagnostic procedures, such as intra-vascular ultrasound and BOLD-MRI, in addition to traditional imaging, have opened new scenarios, because these tools allow to identify ischemic lesions responsive to renal revascularization. Indeed, although trials have deflated the role of renal revascularization on the renal outcomes, it should be still used to avoid dialysis initiation and/or to reduce blood pressure in selected elderly patients at high risk. Nonetheless, lifestyle modifications (smoking cessation, increased physical activity), statins and antiplatelet use, as well as cautious use of renin-angiotensin system inhibitors, remain the main therapeutic approach aimed at slowing the renal damage progression. Mesenchymal stem cells and Micro-RNA are promising target of anti-fibrotic therapy, which might provide potential benefit in ARVD patients, though safety and efficacy profile in humans is unknown too.

A Boolean Model of Microvascular Rarefaction to Predict Treatment Outcomes in Renal Disease

Despite advances in renovascular disease (RVD) research, gaps remain between experimental and clinical outcomes, translation of results, and the understanding of pathophysiological mechanisms. A predictive tool to indicate support (or lack of) for biological findings may aid clinical translation of therapies. We created a Boolean model of RVD and hypothesized that it would predict outcomes observed in our previous studies using a translational swine model of RVD. Our studies have focused on developing treatments to halt renal microvascular (MV) rarefaction in RVD, a major feature of renal injury. A network topology of 20 factors involved in renal MV rarefaction that allowed simulation of 5 previously tested treatments was created. Each factor was assigned a function based upon its interactions with other variables and assumed to be “on” or “off”. Simulations of interventions were performed until outcomes reached a steady state and analyzed to determine pathological processes that were activated, inactivated, or unchanged vs. RVD with no intervention. Boolean simulations mimicked the results of our previous studies, confirming the importance of MV integrity on treatment outcomes in RVD. Furthermore, our study supports the potential application of a mathematical tool to predict therapeutic feasibility, which may guide the design of future studies for RVD.

Renovascular Hypertension

Renovascular disease (RVD) is a major cause of secondary hypertension. Atherosclerotic renal artery stenosis is the most common type of RVD followed by fibromuscular dysplasia. It has long been recognized as the prototype of angiotensin-dependent hypertension. However, the mechanisms underlying the physiopathology of hypertensive occlusive vascular renal disease are complex and distinction between the different causes of RVD should be made. Recognition of these distinct types of RVD with different degrees of renal occlusive disease is important for management. The greatest challenge is to individualize and implement the best approach for each patient in the setting of widely different comorbidities.

Non-invasive evaluation of renal structure and function of healthy individuals with multiparametric MRI: Effects of sex and age

Clinically, when applying multiparametric magnetic resonance imaging (MRI) examinations in renal diseases, assessment of renal structure and function has to account for age- and sex-related effects. The aim of this study was to investigate the influence of age and sex on multiparametric MRI assessment of renal structure and function in healthy human beings. Studies on 33 healthy volunteers were performed using multiparametric MRI on a 3.0-Tesla MR scanner, including T1-weighted imaging, blood oxygen level-dependent MRI (BOLD MRI), diffusion-weighted imaging (DWI) and diffusion tensor imaging (DTI). Our results revealed that the mean renal cortical thickness (RCT), ratio of cortex to parenchyma (CPR), and cortical R2* values were higher in males than in females. The cortical R2* value was higher in older group than in younger group (18.57 ± 0.99 vs 17.53 ± 0.58, p = 0.001); there was no significant difference in medullary R2* between the older and younger groups (38.18 ± 2.96 vs 36.45 ± 2.47, p = 0.077). The parenchymal thickness (PT) and medullary fractional anisotropy (FA) were lower in older group than in younger group (1.547 ± 0.06 vs 1.604 ± 0.05, p = 0.005 and 0.343 ± 0.03 vs 0.371 ± 0.03, p = 0.016, respectively). Pearson's correlation analysis showed that PT and medullary FA were inversely related with age (r = -0.483, p = 0.004; r = -0.446, p = 0.009) while cortical R2* values was positively related (r = 0.511, p = 0.002, respectively). The medullary apparent diffusion coefficient (ADC) value had a significant association with PT (r = 0.359, p = 0.04). This study indicated that multiparametric renal MRI parameters are age and sex dependent.

Tissue hypoxia, inflammation, and loss of glomerular filtration rate in human atherosclerotic renovascular disease

The relationships between renal blood flow (RBF), tissue oxygenation, and inflammatory injury in atherosclerotic renovascular disease (ARVD) are poorly understood. We sought to correlate RBF and tissue hypoxia with glomerular filtration rate (GFR) in 48 kidneys from patients with ARVD stratified by single kidney iothalamate GFR (sGFR). Oxygenation was assessed by blood oxygenation level dependent magnetic resonance imaging (BOLD MRI), which provides an index for the levels of deoxyhemoglobin within a defined volume of tissue (R2*). sGFR correlated with RBF and with the severity of vascular stenosis as estimated by duplex velocities. Higher cortical R2* and fractional hypoxia and higher levels of renal vein neutrophil-gelatinase-associated-lipocalin (NGAL) and monocyte-chemoattractant protein-1 (MCP-1) were observed at lower GFR, with an abrupt inflection below 20 ml/min. Renal vein MCP-1 levels correlated with cortical R2* and with fractional hypoxia. Correlations between cortical R2* and RBF in the highest sGFR stratum (mean sGFR 51 ± 12 ml/min; R = -0.8) were degraded in the lowest sGFR stratum (mean sGFR 8 ± 3 ml/min; R = -0.1). Changes in fractional hypoxia after furosemide were also absent in the lowest sGFR stratum. These data demonstrate relative stability of renal oxygenation with moderate reductions in RBF and GFR but identify a transition to overt hypoxia and inflammatory cytokine release with severely reduced GFR. Tissue oxygenation and RBF were less correlated in the setting of reduced sGFR, consistent with variable oxygen consumption or a shift to alternative mechanisms of tissue injury. Identifying transitions in tissue oxygenation may facilitate targeted therapy in ARVD.

Renal blood oxygenation level-dependent magnetic resonance imaging to measure renal tissue oxygenation: a statement paper and systematic review

Tissue hypoxia plays a key role in the development and progression of many kidney diseases. Blood oxygenation level-dependent magnetic resonance imaging (BOLD-MRI) is the most promising imaging technique to monitor renal tissue oxygenation in humans. BOLD-MRI measures renal tissue deoxyhaemoglobin levels voxel by voxel. Increases in its outcome measure R2* (transverse relaxation rate expressed as per second) correspond to higher deoxyhaemoglobin concentrations and suggest lower oxygenation, whereas decreases in R2* indicate higher oxygenation. BOLD-MRI has been validated against micropuncture techniques in animals. Its reproducibility has been demonstrated in humans, provided that physiological and technical conditions are standardized. BOLD-MRI has shown that patients suffering from chronic kidney disease (CKD) or kidneys with severe renal artery stenosis have lower tissue oxygenation than controls. Additionally, CKD patients with the lowest cortical oxygenation have the worst renal outcome. Finally, BOLD-MRI has been used to assess the influence of drugs on renal tissue oxygenation, and may offer the possibility to identify drugs with nephroprotective or nephrotoxic effects at an early stage. Unfortunately, different methods are used to prepare patients, acquire MRI data and analyse the BOLD images. International efforts such as the European Cooperation in Science and Technology (COST) action 'Magnetic Resonance Imaging Biomarkers for Chronic Kidney Disease' (PARENCHIMA) are aiming to harmonize this process, to facilitate the introduction of this technique in clinical practice in the near future. This article represents an extensive overview of the studies performed in this field, summarizes the strengths and weaknesses of the technique, provides recommendations about patient preparation, image acquisition and analysis, and suggests clinical applications and future developments.

Magnetization Transfer Magnetic Resonance Imaging Noninvasively Detects Renal Fibrosis in Swine Atherosclerotic Renal Artery Stenosis at 3.0 T

OBJECTIVES

Renal fibrosis is a useful biomarker for diagnosis and evaluation of therapeutic interventions of renal diseases but often requires invasive testing. Magnetization transfer magnetic resonance imaging (MT-MRI), which evaluates the presence of macromolecules, offers a noninvasive tool to probe renal fibrosis in murine renal artery stenosis (RAS) at 16.4 T. In this study, we aimed to identify appropriate imaging parameters for collagen detection at 3.0 T MRI and to test the utility of MT-MRI in measuring renal fibrosis in a swine model of atherosclerotic RAS (ARAS).

MATERIALS AND METHODS

To select the appropriate offset frequency, an MT-MRI study was performed on a phantom containing 0% to 40% collagen I and III with offset frequencies from -1600 to +1600 Hz and other MT parameters empirically set as pulse width at 16 milliseconds and flip angle at 800 degrees. Then selected MT parameters were used in vivo on pigs 12 weeks after sham (n = 8) or RAS (n = 10) surgeries. The ARAS pigs were fed with high-cholesterol diet to induce atherosclerosis. The MT ratio (MTR) was compared with ex vivo renal fibrosis measured using Sirius-red staining.

RESULTS

Offset frequencies at 600 and 1000 Hz were selected for collagen detection without direct saturation of free water signal, and subsequently applied in vivo. The ARAS kidneys showed mild cortical and medullary fibrosis by Sirius-red staining. The cortical and medullary MTRs at 600 and 1000 Hz were both increased. Renal fibrosis measured ex vivo showed good linear correlations with MTR at 600 (cortex: Pearson correlation coefficient r = 0.87, P < 0.001; medulla: r = 0.70, P = 0.001) and 1000 Hz (cortex: r = 0.75, P < 0.001; medulla: r = 0.83, P < 0.001).

CONCLUSIONS

Magnetization transfer magnetic resonance imaging can noninvasively detect renal fibrosis in the stenotic swine kidney at 3.0 T. Therefore, MT-MRI may potentially be clinically applicable and useful for detection and monitoring of renal pathology in subjects with RAS.

Total Renal Artery Occlusion: Recovery of Function After Revascularization

Current trends in managing atherosclerotic renal artery stenosis favor medical therapy, on account of negative results from prospective trials of revascularization, such as CORAL and ASTRAL. One result of this trend has been encountering occasional patients with progressive disease, sometimes leading to total arterial occlusion. We illustrate a case of accelerated hypertension with complete renal artery occlusion in which the patient recovered function after surgical bypass and we review the clinical approach used and the advanced imaging modalities available to us. A high index of suspicion and careful radiologic imaging play important roles in selecting patients who may have residual function and may benefit from revascularization. This case illustrates an example whereby restoring renal artery perfusion for carefully selected patients can be life changing, with recovery of kidney function and improved blood pressure, pill burden, and overall quality of life.

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.

Serum calcification propensity is associated with renal tissue oxygenation and resistive index in patients with arterial hypertension or chronic kidney disease

BACKGROUND

Arterial calcifications increase arterial stiffness and are associated with a faster decline of kidney function in patients with arterial hypertension (AH) and/or chronic kidney disease (CKD). Yet the underlying mechanisms linking arterial calcifications, vascular stiffness and renal function decline are incompletely understood. A novel in-vitro blood test evaluates the propensity of patient's serum to prevent the formation of calcifications by measuring the maturation time of calciprotein particles (CPP) [transformation time of amorphous calcium phosphate-containing primary CPP to crystalline hydroxyapatite-containing secondary CPP (T50)]. We hypothesized that a high arterial stiffness and a high propensity to calcify may be associated with high renal vascular resistance and low renal tissue oxygenation.

METHODS

T50 was measured in patients with AH and a preserved renal function, in CKD patients and in healthy controls, a lower T50 indicating a higher risk of calcification. Pulse wave velocity (PWV) was assessed as a measure of arterial stiffness, and renal resistive index was measured by renal Doppler ultrasound. Renal tissue oxygenation was measured by blood oxygenation level-dependent MRI using the mean R2 values of the cortex, the medulla and layers of renal parenchyma. A high R2 value corresponds to a low tissue oxygenation.

RESULTS

Mean T50 was 246 ± 129 min in 58 CKD patients, 275 ± 111 min in 48 AH patients and 324 ± 96 min in 39 healthy controls (Panova = 0.008). In multivariable adjusted linear regression analysis, serum T50 correlated negatively with circulating calcium and phosphate levels, mean cortical and medullary R2, PWV, renal resistive index and being hypertensive. PWV was positively associated with R2 levels of outer and inner layers of renal parenchyma.

CONCLUSION

The current study shows that hypertensive patients with preserved renal function as well as CKD patients have a higher risk of calcification than controls. High arterial stiffness and calcification propensity are linked to low renal tissue oxygenation and perfusion in hypertensive and CKD patients. These results provide new insights on the relationships among arterial stiffness, renal tissue oxygenation and the risk of developing CKD.

Methods of Blood Oxygen Level-Dependent Magnetic Resonance Imaging Analysis for Evaluating Renal Oxygenation

Blood oxygen level-dependent magnetic resonance imaging (BOLD MRI) has recently been utilized as a noninvasive tool for evaluating renal oxygenation. Several methods have been proposed for analyzing BOLD images. Regional ROI selection is the earliest and most widely used method for BOLD analysis. In the last 20 years, many investigators have used this method to evaluate cortical and medullary oxygenation in patients with ischemic nephropathy, hypertensive nephropathy, diabetic nephropathy, chronic kidney disease (CKD), acute kidney injury and renal allograft rejection. However, clinical trials of BOLD MRI using regional ROI selection revealed that it was difficult to distinguish the renal cortico-medullary zones with this method, and that it was susceptible to observer variability. To overcome these deficiencies, several new methods were proposed for analyzing BOLD images, including the compartmental approach, fractional hypoxia method, concentric objects (CO) method and twelve-layer concentric objects (TLCO) method. The compartmental approach provides an algorithm to judge whether the pixel belongs to the cortex or medulla. Fractional kidney hypoxia, measured by using BOLD MRI, was negatively correlated with renal blood flow, tissue perfusion and glomerular filtration rate (GFR) in patients with atherosclerotic renal artery stenosis. The CO method divides the renal parenchyma into six or twelve layers of thickness in each coronal slice of BOLD images and provides a R2* radial profile curve. The slope of the R2* curve associated positively with eGFR in CKD patients. Indeed, each method invariably has advantages and disadvantages, and there is generally no consensus method so far. Undoubtedly, analytic approaches for BOLD MRI with better reproducibility would assist clinicians in monitoring the degree of kidney hypoxia and thus facilitating timely reversal of tissue hypoxia.

In Vivo Photoacoustic Imaging of Brain Injury and Rehabilitation by High-Efficient Near-Infrared Dye Labeled Mesenchymal Stem Cells with Enhanced Brain Barrier Permeability

Stem cell migration and interaction with pathology are critical to understand the complexity and status of disease recovery progress. However, the dynamic visualization still remains a great challenge due to imaging technical limitation, cell labeling difficulty, or blood-brain barrier (BBB). Herein, fast photoacoustic tomography (PAT) with optical molecular probes is applied to noninvasively monitor traumatic brain injury (TBI) and its rehabilitation. The vascular distribution and TBI hemorrhage are clearly imaged, longitudinally monitored, and quantified. Bone mesenchymal stem cells (BMSCs) labeled with modified Prussian blue particles (PBPs), excellent near-infrared dyes and photoacoustic contrasts, are intravenously injected to the mice for improved observation and efficient therapy. BMSCs are demonstrated to be capable of overcoming BBB with enhanced delivery of PBPs to the brain parenchyma. Notably, the versatile BMSCs are observed by PAT to home to the damage region and repair the ruptured vasculature. Moreover, the wound treated by BMSCs exhibits much faster recovery speed than that without treatment. These findings can potentially provide a new noninvasive and high-resolution approach to image TBI, monitor recovery process, and especially trace BMSCs. This study will stimulate extensive researches on brain diseases and provide promising strategies of dye labeled BMSCs in regenerative medicine.

Current Concepts in the Treatment of Renovascular Hypertension

Renovascular disease (RVD) remains a major cause of secondary and treatment-resistant hypertension. Most cases are related either to fibromuscular or atherosclerotic lesions, but a variety of other causes including arterial dissection, stent occlusion, and embolic disease can produce the same syndrome. Recent studies emphasize the kidney's tolerance to moderate flow reduction during antihypertensive drug therapy and the relative safety of medical therapy to control blood pressure. Several prospective trials in moderate RVD fail to identify major benefits from endovascular revascularization for moderate atherosclerotic disease. However, high-risk and progressive renovascular syndromes are recognized to be relatively refractory to medical therapy only and respond better to combining renal revascularization with ongoing medical therapy. Clinicians caring for complex hypertension should be familiar with pathogenic pathways, imaging techniques, and a rational approach to managing renovascular hypertension in the current era.

Reduced cortical oxygenation predicts a progressive decline of renal function in patients with chronic kidney disease

Renal tissue hypoxia is a final pathway in the development and progression of chronic kidney disease (CKD), but whether renal oxygenation predicts renal function decline in humans has not been proven. Therefore, we performed a prospective study and measured renal tissue oxygenation by blood oxygenation level-dependent magnetic resonance imaging (BOLD-MRI) in 112 patients with CKD, 47 with hypertension without CKD, and 24 healthy control individuals. Images were analyzed with the twelve-layer concentric objects method that divided the renal parenchyma in 12 layers of equal thickness and reports the mean R2* value of each layer (a high R2* corresponds to low oxygenation), along with the change in R2* between layers called the R2* slope. Serum creatinine values were collected to calculate the yearly change in estimated glomerular function rate (MDRD eGFR). Follow up was three years. The change in eGFR in CKD, hypertensive and control individuals was -2.0, 0.5 and -0.2 ml/min/1.73m²/year, respectively. In multivariable regression analysis adjusted for age, sex, diabetes, RAS-blockers, eGFR, and proteinuria the yearly eGFR change correlated negatively with baseline 24 hour proteinuria and the mean R2* value of the cortical layers, and positively with the R2* slope, but not with the other covariates. Patients with CKD and high outer R2* or a flat R2* slope were three times more likely to develop an adverse renal outcome (renal replacement therapy or over a 30% increase in serum creatinine). Thus, low cortical oxygenation is an independent predictor of renal function decline. This finding should stimulate studies exploring the therapeutic impact of improving renal oxygenation on renal disease progression.

Elevated urinary podocyte-derived extracellular microvesicles in renovascular hypertensive patients

BACKGROUND

An increased number of podocyte-derived extracellular vesicles (pEVs) may reflect podocyte injury in renal disease. Elevated glomerular pressure and other insults may injure podocytes, yet it remains unclear whether the numbers of pEVs are altered in hypertensive patients. We tested the hypothesis that urinary pEV levels would be elevated in patients with renovascular hypertension (RVH) compared with essential hypertension (EH) or healthy volunteers (HVs).

METHODS

We prospectively enrolled patients with EH ( n = 30) or RVH ( n = 31) to study renal blood flow (RBF) and cortical perfusion using multidetector computed tomography under controlled condition (regulated sodium intake and renin-angiotensin blockade). After isolation from urine samples, pEVs (nephrin and podocalyxin positive) were characterized by flow cytometry. Fourteen RVH patients were studied again 3 months after stenting or continued medical therapy. HVs ( n = 15) served as controls.

RESULTS

The fraction of pEV among urinary EVs was elevated in RVH compared with HVs and EH (11.4 ± 6.4, 6.8 ± 3.4 and 6.3 ± 3.7%, respectively; P < 0.001) and remained unchanged after 3 additional months of therapy and after controlling for clinical parameters. However, eGFR- and age-adjusted pEV levels did not correlate with any clinical or renal parameters.

CONCLUSIONS

In hypertensive patients under controlled conditions, urinary pEV levels are elevated in patients with RVH and low eGFR compared with patients with EH and relatively preserved renal function. These pEVs may reflect podocyte injury secondary to kidney damage, and their levels might represent a novel therapeutic target.

Blood Oxygenation Level-Dependent MRI to Assess Renal Oxygenation in Renal Diseases: Progresses and Challenges

BOLD-MRI (blood oxygenation-level dependent magnetic resonance imaging) allows non-invasive measurement of renal tissue oxygenation in humans, without the need for contrast products. BOLD-MRI uses the fact that magnetic properties of hemoglobin depend of its oxygenated state:: the higher local deoxyhemoglobin, the higher the so called apparent relaxation rate R2^* (sec^-1), and the lower local tissue oxygen content. Several factors other than deoxyhemoglobin (such as hydration status, dietary sodium intake, and susceptibility effects) influence the BOLD signal, and need to be taken into account when interpreting results. The last 5 years have witnessed important improvements in the standardization of these factors, and the appearance of new, highly reproducible analysis techniques of BOLD-images, that are reviewed in this article. Using these new BOLD-MRI analysis techniques, it has recently been shown that persons suffering from chronic kidney diseases (CKD) have lower cortical oxygenation than normotensive controls, thus confirming the chronic hypoxia hypothesis. The acute alterations in R2^* after the administration of furosemide are smaller in CKD, and represent an estimate of the oxygen-dependent tubular transport of sodium. BOLD-MRI-alone or in combination with other functional MRI methods- can be used to monitor the renal effects of drugs, and is increasingly used in the preclinical setting. The near future will tell whether or not BOLD-MRI represents a new tool to predict renal function decline an adverse renal outcome.

Renal Arterial Disease and Hypertension

Renal artery disease produces a spectrum of progressive clinical manifestations ranging from minor degrees of hypertension to circulatory congestion and kidney failure. Moderate reductions in renal blood flow do not induce tissue hypoxia or damage, making medical therapy for renovascular hypertension feasible. Several prospective trials indicate that optimized medical therapy using agents that block the renin-angiotensin system should be the initial management. Evidence of progressive disease and/or treatment failure should allow recognition of high-risk subsets that benefit from renal revascularization. Severe reductions in kidney blood flow ultimately activate inflammatory pathways that do not reverse with restoring blood flow alone.

Approach to atherosclerotic renovascular disease: 2016

The management of atherosclerotic renal artery stenosis in patients with hypertension or impaired renal function remains a clinical dilemma. The current general consensus, supported by the results of the Angioplasty and Stenting for Renal Atherosclerotic Lesions and Cardiovascular Outcomes for Renal Artery Lesions trials, argues strongly against endovascular intervention in favor of optimal medical management. We discuss the limitations and implications of the contemporary clinical trials and present our approach and formulate clear recommendations to help with the management of patients with atherosclerotic narrowing of the renal artery.

Atherosclerotic renal artery stenosis is associated with elevated cell cycle arrest markers related to reduced renal blood flow and postcontrast hypoxia

BACKGROUND

Atherosclerotic renal artery stenosis (ARAS) reduces renal blood flow (RBF), ultimately leading to kidney hypoxia and inflammation. Insulin-like growth factor binding protein-7 (IGFBP-7) and tissue inhibitor of metalloproteinases-2 (TIMP-2) are biomarkers of cell cycle arrest, often increased in ischemic conditions and predictive of acute kidney injury (AKI). This study sought to examine the relationships between renal vein levels of IGFBP-7, TIMP-2, reductions in RBF and postcontrast hypoxia as measured by blood oxygen level-dependent (BOLD) magnetic resonance imaging.

METHODS

Renal vein levels of IGFBP-7 and TIMP-2 were obtained in an ARAS cohort (n= 29) scheduled for renal artery stenting and essential hypertensive (EH) healthy controls (n = 32). Cortical and medullary RBFs were measured by multidetector computed tomography (CT) immediately before renal artery stenting and 3 months later. BOLD imaging was performed before and 3 months after stenting in all patients, and a subgroup (N = 12) underwent repeat BOLD imaging 24 h after CT/stenting to examine postcontrast/procedure levels of hypoxia.

RESULTS

Preintervention IGFBP-7 and TIMP-2 levels were elevated in ARAS compared with EH (18.5 ± 2.0 versus 15.7 ± 1.5 and 97.4 ± 23.1 versus 62.7 ± 9.2 ng/mL, respectively; P< 0.0001); baseline IGFBP-7 correlated inversely with hypoxia developing 24 h after contrast injection (r = -0.73, P< 0.0001) and with prestent cortical blood flow (r = -0.59, P= 0.004).

CONCLUSION

These data demonstrate elevated IGFBP-7 and TIMP-2 levels in ARAS as a function of the degree of reduced RBF. Elevated baseline IGFBP-7 levels were associated with protection against postimaging hypoxia, consistent with 'ischemic preconditioning'. Despite contrast injection and stenting, AKI in these high-risk ARAS subjects with elevated IGFBP-7/TIMP-2 was rare and did not affect long-term kidney function.

Urinary Mitochondrial DNA Copy Number Identifies Chronic Renal Injury in Hypertensive Patients

Mitochondrial injury contributes to renal dysfunction in several models of renal disease, but its involvement in human hypertension remains unknown. Fragments of the mitochondrial genome released from dying cells are considered surrogate markers of mitochondrial injury. We hypothesized that hypertension would be associated with increased urine mitochondrial DNA (mtDNA) copy numbers. We prospectively measured systemic and urinary copy number of the mtDNA genes cytochrome-c oxidase-3 and NADH dehydrogenase subunit-1 by quantitative polymerase chain reaction in essential (n=25) and renovascular (RVH, n=34) hypertensive patients and compared them with healthy volunteers (n=22). Urinary kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin served as indices of renal injury. Renal blood flow and oxygenation were assessed by multidetector computed tomography and blood oxygen level-dependent magnetic resonance imaging. Blood pressure, urinary neutrophil gelatinase-associated lipocalin, and kidney injury molecule-1 were similarly elevated in essential hypertension and RVH, and estimated glomerular filtration rate was lower in RVH versus healthy volunteers and essential hypertension. Renal blood flow was lower in RVH compared with essential hypertension. Urinary mtDNA copy number was higher in hypertension compared with healthy volunteers, directly correlated with urinary neutrophil gelatinase-associated lipocalin and kidney injury molecule-1 and inversely with estimated glomerular filtration rate. In RVH, urinary mtDNA copy number correlated directly with intrarenal hypoxia. Furthermore, in an additional validation cohort, urinary mtDNA copy number was higher in RVH compared with healthy volunteers (n=10 each). The change in serum creatinine levels and estimated glomerular filtration rate 3 months after medical therapy without or with revascularization correlated with the change in urinary mtDNA. Therefore, elevated urinary mtDNA copy numbers in hypertensive patients correlated with markers of renal injury and dysfunction, implicating mitochondrial injury in kidney damage in human hypertension.

Metabolomics of renal venous plasma from individuals with unilateral renal artery stenosis and essential hypertension

Objective:

To compare the metabolite profiles of venous effluent from both kidneys of individuals with unilateral atherosclerotic renal artery stenosis (ARAS) in order to directly examine how impaired renal blood flow impacts small-molecule handling in humans.

Methods:

We applied liquid chromatography–mass spectrometry based metabolite profiling to venous plasma obtained from the stenotic (STK) and contralateral (CLK) kidneys of ARAS patients (n = 16), and both the kidneys of essential hypertensive controls (n = 11). Study samples were acquired during a 3-day protocol that included iothalamate clearance measurements, radiographic kidney phenotyping (Duplex ultrasound, multidetector computed tomography, and blood-oxygen-level-dependent MRI), and controlled sodium and caloric intake and antihypertensive treatment.

Results:

Partial least squares-discriminant analysis demonstrated clear separation of essential hypertensive kidney metabolite profiles versus STK and CLK metabolite profiles, but no separation between metabolite profiles of STK and CLK samples. All of the discriminating metabolites were similarly elevated in the STK and CLK samples, likely reflecting the lower glomerular filtration rate in the ARAS versus essential hypertensive individuals (mean 66.1 versus 89.2 ml/min per 1.73 m²). In a paired analysis within the ARAS group, no metabolite was significantly altered in STK compared with CLK samples; notably, creatinine was the same in STK and CLK samples (STK/CLK ratio = 1.0, P = 0.9). Results were unchanged in an examination of ARAS patients in the bottom half of renal tissue perfusion or oxygenation.

Conclusion:

Metabolite profiling does not differentiate venous effluent from STKs or CLKs in individuals with unilateral ARAS, despite the measurable loss of kidney volume and blood flow on the affected side. These findings are consistent with the kidney's ability to adapt to ARAS to maintain a range of metabolic functions.

Chronic renal ischemia in humans: can cell therapy repair the kidney in occlusive renovascular disease?

Occlusive renovascular disease caused by atherosclerotic renal artery stenosis (ARAS) elicits complex biological responses that eventually lead to loss of kidney function. Recent studies indicate a complex interplay of oxidative stress, endothelial dysfunction, and activation of fibrogenic and inflammatory cytokines as a result of atherosclerosis, hypoxia, and renal hypoperfusion in this disorder. Human studies emphasize the limits of the kidney adaptation to reduced blood flow, eventually leading to renal hypoxia with activation of inflammatory and fibrogenic pathways. Several randomized prospective clinical trials show that stent revascularization alone in patients with atherosclerotic renal artery stenosis provides little additional benefit to medical therapy once these processes have developed and solidified. Experimental data now support developing adjunctive cell-based measures to support angiogenesis and anti-inflammatory renal repair mechanisms. These data encourage the study of endothelial progenitor cells and/or mesenchymal stem/stromal cells for the repair of damaged kidney tissue.

Magnetic Resonance Imaging-Derived Renal Oxygenation and Perfusion During Continuous, Steady-State Angiotensin-II Infusion in Healthy Humans

Background

The role of kidney hypoxia is considered pivotal in the progression of chronic kidney disease. A widely used method to assess kidney oxygenation is blood oxygen level dependent (BOLD)–magnetic resonance imaging (MRI), but its interpretation remains problematic. The BOLD‐MRI signal is the result of kidney oxygen consumption (a proxy of glomerular filtration) and supply (ie, glomerular perfusion). Therefore, we hypothesized that with pharmacological modulation of kidney blood flow, renal oxygenation, as assessed by BOLD‐MRI, correlates to filtration fraction (ie, glomerular filtration rate/effective renal plasma flow) in healthy humans.

Methods and Results

Eight healthy volunteers were subjected to continuous angiotensin‐II infusion at 0.3, 0.9, and 3.0 ng/kg per minute. At each dose, renal oxygenation and blood flow were assessed using BOLD and phase‐contrast MRI. Subsequently, “gold standard” glomerular filtration rate/effective renal plasma flow measurements were performed under the same conditions. Renal plasma flow decreased dose dependently from 660±146 to 467±103 mL/min per 1.73 m² (F[3, 21]=33.3, P<0.001). Glomerular filtration rate decreased from 121±23 to 110±18 mL/min per 1.73 m² (F[1.8, 2.4]=6.4, P=0.013). Cortical transverse relaxation rate (R2*; increases in R2* represent decreases in oxygenation) increased by 7.2±3.8% (F[3, 21]=7.37, P=0.001); medullar R2* did not change. Cortical R2* related to filtration fraction (R² 0.46, P<0.001).

Conclusions

By direct comparison between “gold standard” kidney function measurements and BOLD MRI, we showed that cortical oxygenation measured by BOLD MRI relates poorly to glomerular filtration rate but is associated with filtration fraction. For future studies, there may be a need to include renal plasma flow measurements when employing renal BOLD‐MRI.

Hypoxia: The Force that Drives Chronic Kidney Disease

In the United States the prevalence of end-stage renal disease (ESRD) reached epidemic proportions in 2012 with over 600,000 patients being treated. The rates of ESRD among the elderly are disproportionally high. Consequently, as life expectancy increases and the baby-boom generation reaches retirement age, the already heavy burden imposed by ESRD on the US health care system is set to increase dramatically. ESRD represents the terminal stage of chronic kidney disease (CKD). A large body of evidence indicating that CKD is driven by renal tissue hypoxia has led to the development of therapeutic strategies that increase kidney oxygenation and the contention that chronic hypoxia is the final common pathway to end-stage renal failure. Numerous studies have demonstrated that one of the most potent means by which hypoxic conditions within the kidney produce CKD is by inducing a sustained inflammatory attack by infiltrating leukocytes. Indispensable to this attack is the acquisition by leukocytes of an adhesive phenotype. It was thought that this process resulted exclusively from leukocytes responding to cytokines released from ischemic renal endothelium. However, recently it has been demonstrated that leukocytes also become activated independent of the hypoxic response of endothelial cells. It was found that this endothelium-independent mechanism involves leukocytes directly sensing hypoxia and responding by transcriptional induction of the genes that encode the β2-integrin family of adhesion molecules. This induction likely maintains the long-term inflammation by which hypoxia drives the pathogenesis of CKD. Consequently, targeting these transcriptional mechanisms would appear to represent a promising new therapeutic strategy.

Paradigm Shifts in Atherosclerotic Renovascular Disease: Where Are We Now?

Results of recent clinical trials and experimental studies indicate that whereas atherosclerotic renovascular disease can accelerate both systemic hypertension and tissue injury in the poststenotic kidney, restoring vessel patency alone is insufficient to recover kidney function for most subjects. Kidney injury in atherosclerotic renovascular disease reflects complex interactions among vascular rarefication, oxidative stress injury, and recruitment of inflammatory cellular elements that ultimately produce fibrosis. Classic paradigms for simply restoring blood flow are shifting to implementation of therapy targeting mitochondria and cell-based functions to allow regeneration of vascular, glomerular, and tubular structures sufficient to recover, or at least stabilize, renal function. These developments offer exciting possibilities of repair and regeneration of kidney tissue that may limit progressive CKD in atherosclerotic renovascular disease and may apply to other conditions in which inflammatory injury is a major common pathway.

Determination of Single-Kidney Glomerular Filtration Rate in Human Subjects by Using CT

PURPOSE

To test the hypothesis that computed tomography (CT)-derived measurements of single-kidney glomerular filtration rate (GFR) obtained in human subjects with 64-section CT agree with those obtained with iothalamate clearance, a rigorous reference standard.

MATERIALS AND METHODS

The institutional review board approved this HIPAA-compliant study, and written informed consent was obtained. Ninety-six patients (age range, 51-73 years; 46 men, 50 women) with essential (n = 56) or renovascular (n = 40) hypertension were prospectively studied in controlled conditions (involving sodium intake and renin-angiotensin blockade). Single-kidney perfusion, volume, and GFR were measured by using multidetector CT time-attenuation curves and were compared with GFR measured by using iothalamate clearance, as assigned to the right and left kidney according to relative volumes. The reproducibility of CT GFR over a 3-month period (n = 21) was assessed in patients with renal artery stenosis who were undergoing stable medical treatment. Statistical analysis included the t test, Wilcoxon signed rank test, linear regression, and Bland-Altman analysis.

RESULTS

CT GFR values were similar to those of iothalamate clearance (mean ± standard deviation, 38.2 mL/min ± 18 vs 41.6 mL/min ± 17; P = .062). Stenotic kidney CT GFR in patients with renal artery stenosis was lower than contralateral kidney GFR or essential hypertension single-kidney GFR (mean, 23.1 mL/min ± 13 vs 36.9 mL/min ± 17 [P = .0008] and 45.2 mL/min ± 16 [P = .019], respectively), as was iothalamate clearance (mean, 26.9 mL/min ± 14 vs 38.5 mL/min ± 15 [P = .0004] and 49.0 mL/min ± 14 [P = .001], respectively). CT GFR correlated well with iothalamate GFR (linear regression, CT GFR = 0.88*iothalamate GFR, r(2) = 0.89, P < .0001), and Bland-Altman analysis was used to confirm the agreement. CT GFR was also moderately reproducible in medically treated patients with renal artery stenosis (concordance coefficient correlation, 0.835) but was unaffected by revascularization (mean, 25.3 mL/min ± 15.2 vs 30.3 mL/min ± 18.5; P = .097).

CONCLUSION

CT assessments of single-kidney GFR are reproducible and agree well with a reference standard. CT can be useful to obtain minimally invasive estimates of bilateral single-kidney function in human subjects.