Human renal response to furosemide: Simultaneous oxygenation and perfusion measurements in cortex and medulla

The physiological basis of renal nuclear medicine

Renal physiology underpins renal nuclear medicine, both academic and clinical. Clearance, an important concept in renal physiology, comprises tissue uptake rate of tracer (tissue clearance), disappearance rate from plasma (plasma clearance), appearance rate in urine (urinary clearance) and disappearance rate from tissue. In clinical research, steady-state plasma clearances of para-amino-hippurate and inulin have been widely used to measure renal blood flow (RBF) and glomerular filtration rate (GFR), respectively. Routinely, GFR is measured at non-steady state as plasma clearance of a filtration agent, such as technetium-99m diethylenetriaminepentaacetic acid. Scaled to three-dimensional whole body metrics rather than body surface area, GFR in women is higher than in men but declines faster with age. Age-related decline is predominantly from nephron loss. Tubular function determines parenchymal transit time, which is important in renography, and the route of uptake of technetium-99m dimercaptosuccinic acid, which is via filtration. Resistance to flow is defined according to the pressure-flow relationship but in renography, only transit time can be measured, which, being equal to urine flow divided by collecting system volume, introduces further uncertainty because the volume is also unmeasurable. Tubuloglomerular feedback governs RBF and GFR, is regulated by the macula densa, mediated by adenosine and renin, and can be manipulated with proximal tubular sodium-glucose cotransporter-2 inhibitors. Other determinants of renal haemodynamics include prostaglandins, nitric oxide and dopamine, while protein meal and amino acid infusion are used to measure renal functional reserve. In conclusion, for measuring renal responses to exogenous agents, steady-state para-amino-hippurate and inulin clearances should be replaced with rubidium-82 and gallium-68 EDTA for measuring RBF and GFR.

Mitochondrial bioenergetics: coupling of transport to tubular mitochondrial metabolism

PURPOSE OF REVIEW

Renal tubules have robust active transport and mitochondrial metabolism, which are functionally coupled to maintain energy homeostasis. Here, I review the current literature and our recent efforts to examine mitochondrial adaptation to different transport activities in renal tubules.

RECENT FINDINGS

The advance of extracellular flux analysis (EFA) allows real-time assessments of mitochondrial respiration, glycolysis, and oxidation of energy substrates. We applied EFA assays to freshly isolated mouse proximal tubules, thick ascending limbs (TALs), and distal convoluted tubules (DCTs) and successfully differentiated their unique metabolic features. We found that TALs and DCTs adjusted their mitochondrial bioenergetics and biogenesis in response to acute and chronic alterations of transport activity. Based on the literature and our recent findings, I discuss working models and mechanisms underlying acute and chronic tubular adaptations to transport activity. The potential roles of peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), AMP-activated protein kinase (AMPK), and uncoupling protein 2 (UCP2) are discussed.

SUMMARY

Mitochondria in renal tubules are highly plastic to accommodate different transport activities. Understanding the mechanisms may improve the treatment of renal tubulopathies.

Evaluation of renal tubular function by multiparametric functional MRI in early diabetes

Purpose To evaluate the tubular function in an alloxan-induced type 1 diabetes mellitus (DM) rabbit model measured by renal oxygenation (R2*), oxygen extraction fraction (OEF), and renal blood flow (RBF) using blood oxygenation level dependent, asymmetric spin echo, and arterial spin labeling MRI. Methods Twenty-six rabbits were randomized into the 3-day DM group (n = 13) and the 7-day DM group (n = 13). We performed pairs of multiparametric MRIs (before and after furosemide injection) at baseline and 3/7 days post-DM, and scored pathological kidney injury. We performed statistical analyses using non-parametric, chi-square, and Spearman correlation tests. Results At baseline, medullary R2* significantly decreased by 24.97% and 16.74% in the outer and inner stripes of the outer medulla (OS and IS, p = 0.006 and 0.003, respectively) after furosemide administration. While the corresponding OEF decreased by 15.91% for OS and 16.67% for IS (both p = 0.003), and no significant change in medullary RBF was observed (p > 0.05). In the 3-day DM group, the decrease of medullary R2* and OEF post-furosemide became unremarkable, suggesting tubular dysfunction. We noticed similar changes in the 7-day DM group. Correlation analysis showed pathological tubular injury score significantly correlated with medullary ∆R2* (post-furosemide - pre-furosemide difference, r = 0.82 for OS and 0.82 for IS) and ∆OEF (r = 0.82 for OS and 0.82 for IS) (p < 0.001, respectively). Conclusion: The combination of medullary OEF and R2* in response to furosemide could detect renal tubular dysfunction in early DM.

Transport activity regulates mitochondrial bioenergetics and biogenesis in renal tubules

Renal tubules are featured with copious mitochondria and robust transport activity. Mutations in mitochondrial genes cause congenital renal tubulopathies, and changes in transport activity affect mitochondrial morphology, suggesting mitochondrial function and transport activity are tightly coupled. Current methods of using bulk kidney tissues or cultured cells to study mitochondrial bioenergetics are limited. Here, we optimized an extracellular flux analysis (EFA) to study mitochondrial respiration and energy metabolism using microdissected mouse renal tubule segments. EFA detects mitochondrial respiration and glycolysis by measuring oxygen consumption and extracellular acidification rates, respectively. We show that both measurements positively correlate with sample sizes of a few centimeter-length renal tubules. The thick ascending limbs (TALs) and distal convoluted tubules (DCTs) critically utilize glucose/pyruvate as energy substrates, whereas proximal tubules (PTs) are significantly much less so. Acute inhibition of TALs' transport activity by ouabain treatment reduces basal and ATP-linked mitochondrial respiration. Chronic inhibition of transport activity by 2-week furosemide treatment or deletion of with-no-lysine kinase 4 (Wnk4) decreases maximal mitochondrial capacity. In addition, chronic inhibition downregulates mitochondrial DNA mass and mitochondrial length/density in TALs and DCTs. Conversely, gain-of-function Wnk4 mutation increases maximal mitochondrial capacity and mitochondrial length/density without increasing mitochondrial DNA mass. In conclusion, EFA is a sensitive and reliable method to investigate mitochondrial functions in isolated renal tubules. Transport activity tightly regulates mitochondrial bioenergetics and biogenesis to meet the energy demand in renal tubules. The system allows future investigation into whether and how mitochondria contribute to tubular remodeling adapted to changes in transport activity.

Effects of furosemide, acetazolamide and amiloride on renal cortical and medullary tissue oxygenation in non-anaesthetised healthy sheep

It has been proposed that diuretics can improve renal tissue oxygenation through inhibition of tubular sodium reabsorption and reduced metabolic demand. However, the impact of clinically used diuretic drugs on the renal cortical and medullary microcirculation is unclear. Therefore, we examined the effects of three commonly used diuretics, at clinically relevant doses, on renal cortical and medullary perfusion and oxygenation in non-anaesthetised healthy sheep. Merino ewes received acetazolamide (250 mg; n = 9), furosemide (20 mg; n = 10) or amiloride (10 mg; n = 7) intravenously. Systemic and renal haemodynamics, renal cortical and medullary tissue perfusion andP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ , and renal function were then monitored for up to 8 h post-treatment. The peak diuretic response occurred 2 h (99.4 ± 14.8 mL/h) after acetazolamide, at which stage cortical and medullary tissue perfusion andP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ were not significantly different from their baseline levels. The peak diuretic response to furosemide occurred at 1 h (196.5 ± 12.3 mL/h) post-treatment but there were no significant changes in cortical and medullary tissue oxygenation during this period. However, cortical tissueP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ fell from 40.1 ± 3.8 mmHg at baseline to 17.2 ± 4.4 mmHg at 3 h and to 20.5 ± 5.3 mmHg at 6 h after furosemide administration. Amiloride did not produce a diuretic response and was not associated with significant changes in cortical or medullary tissue oxygenation. In conclusion, clinically relevant doses of diuretic agents did not improve regional renal tissue oxygenation in healthy animals during the 8 h experimentation period. On the contrary, rebound renal cortical hypoxia may develop after dissipation of furosemide-induced diuresis.

Transport activity regulates mitochondrial bioenergetics and biogenesis in renal tubules

Renal tubules are featured with copious mitochondria and robust transport activity. Mutations in mitochondrial genes cause congenital renal tubulopathies, and changes in transport activity affect mitochondrial morphology, suggesting mitochondrial function and transport activity are tightly coupled. Current methods of using bulk kidney tissues or cultured cells to study mitochondrial bioenergetics are limited. Here, we optimized an extracellular flux analysis (EFA) to study mitochondrial respiration and energy metabolism using microdissected mouse renal tubule segments. EFA detects mitochondrial respiration and glycolysis by measuring oxygen consumption and extracellular acidification rates, respectively. We show that both measurements positively correlate with sample sizes of a few centimeter-length renal tubules. The thick ascending limbs (TALs) and distal convoluted tubules (DCTs) predominantly utilize glucose/pyruvate as energy substrates, whereas proximal tubules (PTs) are significantly much less so. Acute inhibition of TALs' transport activity by ouabain treatment reduces basal and ATP-linked mitochondrial respiration. Chronic inhibition of transport activity by 2-week furosemide treatment or deletion of with-no-lysine kinase 4 (Wnk4) decreases maximal mitochondrial capacity. In addition, chronic inhibition downregulates mitochondrial DNA mass and mitochondrial length/density in TALs and DCTs. Conversely, gain-of-function Wnk4 mutation increases maximal mitochondrial capacity and mitochondrial length/density without increasing mitochondrial DNA mass. In conclusion, EFA is a sensitive and reliable method to investigate mitochondrial functions in isolated renal tubules. Transport activity tightly regulates mitochondrial bioenergetics and biogenesis to meet the energy demand in renal tubules. The system allows future investigation into whether and how mitochondria contribute to tubular remodeling adapted to changes in transport activity.

Key points

A positive correlation between salt reabsorption and oxygen consumption in mammalian kidneys hints at a potential interaction between transport activity and mitochondrial respiration in renal tubules.Renal tubules are heterogeneous in transport activity and mitochondrial metabolism, and traditional assays using bulk kidney tissues cannot provide segment-specific information.Here, we applied an extracellular flux analysis to investigate mitochondrial respiration and energy metabolism in isolated renal tubules. This assay is sensitive in detecting oxygen consumption and acid production in centimeter-length renal tubules and reliably recapitulates segment-specific metabolic features.Acute inhibition of transport activity reduces basal and ATP-linked mitochondrial respirations without changing maximal mitochondrial respiratory capacity. Chronic alterations of transport activity further adjust maximal mitochondrial respiratory capacity via regulating mitochondrial biogenesis or non-transcriptional mechanisms.Our findings support the concept that renal tubular cells finely adjust mitochondrial bioenergetics and biogenesis to match the new steady state of transport activity.

MRI-based quantification of whole-organ renal metabolic rate of oxygen

During the early stages of diabetes, kidney oxygen utilization increases. The mismatch between oxygen demand and supply contributes to tissue hypoxia, a key driver of chronic kidney disease. Thus, whole-organ renal metabolic rate of oxygen (rMRO2 ) is a potentially valuable biomarker of kidney function. The key parameters required to determine rMRO2 include the renal blood flow rate (RBF) in the feeding artery and oxygen saturation in the draining renal vein (SvO2 ). However, there is currently no noninvasive method to quantify rMRO2 in absolute physiologic units. Here, a new MRI pulse sequence, Kidney Metabolism of Oxygen via T2 and Interleaved Velocity Encoding (K-MOTIVE), is described, along with evaluation of its performance in the human kidney in vivo. K-MOTIVE interleaves a phase-contrast module before a background-suppressed T2 -prepared balanced steady-state-free-precession (bSSFP) readout to measure RBF and SvO2 in a single breath-hold period of 22 s, yielding rMRO2 via Fick's principle. Variants of K-MOTIVE to evaluate alternative bSSFP readout strategies were studied. Kidney mass was manually determined from multislice gradient recalled echo images. Healthy subjects were recruited to quantify rMRO2 of the left kidney at 3-T field strength (N = 15). Assessments of repeat reproducibility and comparisons with individual measurements of RBF and SvO2 were performed, and the method's sensitivity was evaluated with a high-protein meal challenge (N = 8). K-MOTIVE yielded the following metabolic parameters: T2  = 157 ± 19 ms; SvO2  = 92% ± 6%; RBF = 400 ± 110 mL/min; and rMRO2  = 114 ± 117(μmol O2 /min)/100 g tissue. Reproducibility studies of T2 and RBF (parameters directly measured by K-MOTIVE) resulted in coefficients of variation less than 10% and intraclass correlation coefficients more than 0.75. The high-protein meal elicited an increase in rMRO2 , which was corroborated by serum biomarkers. The K-MOTIVE sequence measures SvO2 and RBF, the parameters necessary to quantify whole-organ rMRO2 , in a single breath-hold. The present work demonstrates that rMRO2 quantification is feasible with good reproducibility. rMRO2 is a potentially valuable physiological biomarker.

Novel Approaches in Chronic Renal Failure without Renal Replacement Therapy: A Review

Chronic kidney disease (CKD) is characterized by renal parenchymal damage leading to a reduction in the glomerular filtration rate. The inflammatory response plays a pivotal role in the tissue damage contributing to renal failure. Current therapeutic options encompass dietary control, mineral salt regulation, and management of blood pressure, blood glucose, and fatty acid levels. However, they do not effectively halt the progression of renal damage. This review critically examines novel therapeutic avenues aimed at ameliorating inflammation, mitigating extracellular matrix accumulation, and fostering renal tissue regeneration in the context of CKD. Understanding the mechanisms sustaining a proinflammatory and profibrotic state may offer the potential for targeted pharmacological interventions. This, in turn, could pave the way for combination therapies capable of reversing renal damage in CKD. The non-replacement phase of CKD currently faces a dearth of efficacious therapeutic options. Future directions encompass exploring vaptans as diuretics to inhibit water absorption, investigating antifibrotic agents, antioxidants, and exploring regenerative treatment modalities, such as stem cell therapy and novel probiotics. Moreover, this review identifies pharmaceutical agents capable of mitigating renal parenchymal damage attributed to CKD, targeting molecular-level signaling pathways (TGF-β, Smad, and Nrf2) that predominate in the inflammatory processes of renal fibrogenic cells.

Furosemide exacerbated the impairment of renal function, oxygenation and medullary damage in a rat model of renal ischemia/reperfusion induced AKI

BACKGROUND

Perioperative acute kidney injury (AKI) caused by ischemia-reperfusion (IR) is a significant contributor to mortality and morbidity after major surgery. Furosemide is commonly used in postoperative patients to promote diuresis and reduce tissue edema. However, the effects of furosemide on renal microcirculation, oxygenation and function are poorly understood during perioperative period following ischemic insult. Herein, we investigated the effects of furosemide in rats subjected IR insult.

METHODS

24 Wistar albino rats were divided into 4 groups, with 6 in each; Sham-operated Control (C), Control + Furosemide (C + F), ischemia/reperfusion (IR), and IR + F. After induction of anesthesia (BL), supra-aortic occlusion was applied to IR and IR + F groups for 45 min followed by ongoing reperfusion for 15 min (T1) and 2 h (T2). Furosemide infusion was initiated simultaneously in the intervention groups after ischemia. Renal blood flow (RBF), vascular resistance (RVR), oxygen delivery (DO_2ren) and consumption (VO_2ren), sodium reabsorption (TNa⁺), oxygen utilization efficiency (VO₂/TNa⁺), cortical (CμO₂) and medullary (MμO₂) microvascular oxygen pressures, urine output (UO) and creatinine clearance (Ccr) were measured. Biomarkers of inflammation, oxidative and nitrosative stress were measured and kidneys were harvested for histological analysis.

RESULTS

IR significantly decreased RBF, mainly by increasing RVR, which was exacerbated in the IR + F group at T2 (2198 ± 879 vs 4233 ± 2636 dyne/s/cm⁵, p = 0.07). CμO₂ (61.6 ± 6.8 vs 86 ± 6.6 mmHg) and MμO₂ (51.1 ± 4.1 vs 68.7 ± 4.9 mmHg, p < 0.05) were both reduced after IR and did not improve by furosemide. Moreover, VO₂/TNa⁺ increased in the IR + F group at T2 with respect to the IR group (IR: 3.3 ± 2 vs IR + F: 8.2 ± 10 p = 0.07) suggesting a possible deterioration of oxygen utilization. Ccr did not change, but plasma creatinine increased significantly in IR + F groups. Histopathology revealed widespread damage both in the cortex and medulla in IR, IR + F and C + F groups.

CONCLUSION

Renal microvascular oxygenation, renal function, renal vascular resistance, oxygen utilization and damage were not improved by furosemide administration after IR insult. Our study suggests that furosemide may cause additional structural and functional impairment to the kidney following ischemic injury and should be used with caution.

GLP-1 Promotes Cortical and Medullary Perfusion in the Human Kidney and Maintains Renal Oxygenation During NaCl Loading

Background GLP-1 (glucagon-like peptide-1) receptor agonists exert beneficial long-term effects on cardiovascular and renal outcomes. In humans, the natriuretic effect of GLP-1 depends on GLP-1 receptor interaction, is accompanied by suppression of angiotensin II, and is independent of changes in renal plasma flow. In rodents, angiotensin II constricts vasa recta and lowers medullary perfusion. The current randomized, controlled, crossover study was designed to test the hypothesis that GLP-1 increases renal medullary perfusion in healthy humans. Methods and Results Healthy male participants (n=10, aged 27±4 years) ingested a fixed sodium intake for 4 days and were examined twice during a 1-hour infusion of either GLP-1 (1.5 pmol/kg per minute) or placebo together with infusion of 0.9% NaCl (750 mL/h). Interleaved measurements of renal arterial blood flow, oxygenation (R₂*), and perfusion were acquired in the renal cortex and medulla during infusions, using magnetic resonance imaging. GLP-1 infusion increased medullary perfusion (32±7%, P<0.001) and cortical perfusion (13±4%, P<0.001) compared with placebo. Here, NaCl infusion decreased medullary perfusion (-5±2%, P=0.007), whereas cortical perfusion remained unchanged. R₂* values increased by 3±2% (P=0.025) in the medulla and 4±1% (P=0.008) in the cortex during placebo, indicative of decreased oxygenation, but remained unchanged during GLP-1. Blood flow in the renal artery was not altered significantly by either intervention. Conclusions GLP-1 increases predominantly medullary but also cortical perfusion in the healthy human kidney and maintains renal oxygenation during NaCl loading. In perspective, suppression of angiotensin II by GLP-1 may account for the increase in regional perfusion. Registration URL: https://www.clinicaltrials.gov; Unique identifier: NCT04337268.

Magnetic resonance imaging during warm ex vivo kidney perfusion

BACKGROUND

The shortage of donor organs for transplantation remains a worldwide problem. The utilization of suboptimal deceased donors enlarges the pool of potential organs, yet consequently, clinicians face the difficult decision of whether these sub-optimal organs are of sufficient quality for transplantation. Novel technologies could play a pivotal role in making pre-transplant organ assessment more objective and reliable.

METHODS

Ex vivo normothermic machine perfusion (NMP) at temperatures around 35-37°C allows organ quality assessment in a near-physiological environment. Advanced magnetic resonance imaging (MRI) techniques convey unique information about an organ's structural and functional integrity. The concept of applying magnetic resonance imaging during renal normothermic machine perfusion is novel in both renal and radiological research and we have developed the first MRI-compatible NMP setup for human-sized kidneys.

RESULTS

We were able to obtain a detailed and real-time view of ongoing processes inside renal grafts during ex vivo perfusion. This new technique can visualize structural abnormalities, quantify regional flow distribution, renal metabolism, and local oxygen availability, and track the distribution of ex vivo administered cellular therapy.

CONCLUSION

This platform allows for advanced pre-transplant organ assessment, provides a new realistic tool for studies into renal physiology and metabolism, and may facilitate therapeutic tracing of pharmacological and cellular interventions to an isolated kidney.

Continuous bladder urinary oxygen tension as a new tool to monitor medullary oxygenation in the critically ill

Acute kidney injury (AKI) is common in the critically ill. Inadequate renal medullary tissue oxygenation has been linked to its pathogenesis. Moreover, renal medullary tissue hypoxia can be detected before biochemical evidence of AKI in large mammalian models of critical illness. This justifies medullary hypoxia as a pathophysiological biomarker for early detection of impending AKI, thereby providing an opportunity to avert its evolution. Evidence from both animal and human studies supports the view that non-invasively measured bladder urinary oxygen tension (PuO₂) can provide a reliable estimate of renal medullary tissue oxygen tension (tPO₂), which can only be measured invasively. Furthermore, therapies that modify medullary tPO₂ produce corresponding changes in bladder PuO₂. Clinical studies have shown that bladder PuO₂ correlates with cardiac output, and that it increases in response to elevated cardiopulmonary bypass (CPB) flow and mean arterial pressure. Clinical observational studies in patients undergoing cardiac surgery involving CPB have shown that bladder PuO₂ has prognostic value for subsequent AKI. Thus, continuous bladder PuO₂ holds promise as a new clinical tool for monitoring the adequacy of renal medullary oxygenation, with its implications for the recognition and prevention of medullary hypoxia and thus AKI.

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.

Decreased renal perfusion during acute kidney injury in critical COVID-19 assessed by magnetic resonance imaging: a prospective case control study

BACKGROUND

Renal hypoperfusion has been suggested to contribute to the development of acute kidney injury (AKI) in critical COVID-19. However, limited data exist to support this. We aim to investigate the differences in renal perfusion, oxygenation and water diffusion using multiparametric magnetic resonance imaging in critically ill COVID-19 patients with and without AKI.

METHODS

A prospective case-control study where patients without prior kidney disease treated in intensive care for respiratory failure due to COVID-19 were examined. Kidney Disease: Improving Global Outcomes Creatinine criteria were used for group allocation. Main comparisons were tested using Mann-Whitney U test.

RESULTS

Nineteen patients were examined, ten with AKI and nine without AKI. Patients with AKI were examined in median 1 [0-2] day after criteria fulfillment. Age and baseline Plasma-Creatinine were similar in both groups. Total renal blood flow was lower in patients with AKI compared with patients without (median 645 quartile range [423-753] vs. 859 [746-920] ml/min, p = 0.037). Regional perfusion was reduced in both cortex (76 [51-112] vs. 146 [123-169] ml/100 g/min, p = 0.015) and medulla (28 [18-47] vs. 47 [38-73] ml/100 g/min, p = 0.03). Renal venous saturation was similar in both groups (72% [64-75] vs. 72% [63-84], ns.), as was regional oxygenation (R₂*) in cortex (17 [16-19] vs. 17 [16-18] 1/s, ns.) and medulla (29 [24-39] vs. 27 [23-29] 1/s, ns.).

CONCLUSIONS

In critically ill COVID-19 patients with AKI, the total, cortical and medullary renal blood flows were reduced compared with similar patients without AKI, whereas no differences in renal oxygenation were demonstrable in this setting. Trial registration ClinicalTrials ID: NCT02765191 , registered May 6 2014 and updated May 7 2020.

Kidney oxygenation, perfusion and blood flow in people with and without type 1 diabetes

Background

We used magnetic resonance imaging (MRI) to study kidney energetics in persons with and without type 1 diabetes (T1D).

Methods

In a cross-sectional study, 15 persons with T1D and albuminuria and 15 non-diabetic controls (CONs) underwent multiparametric MRI (3 Tesla Philips Scanner) to quantify renal cortical and medullary oxygenation (R₂*, higher values correspond to higher deoxyhaemoglobin concentration), renal perfusion (arterial spin labelling) and renal artery blood flow (phase contrast). Analyses were adjusted for age, sex, systolic blood pressure, plasma haemoglobin, body mass index and estimated glomerular filtration rate (eGFR).

Results

Participants with T1D had a higher median (Q1; Q3) urine albumin creatinine ratio (UACR) than CONs [46 (21; 58) versus 4 (3; 6) mg/g; P < .0001] and a lower mean ± SD eGFR (73 ± 32 mL/min/1.73 m² versus 88 ± 15 mL/min/1.73 m²;  P = .12), although not significantly. Mean medullary R₂* was lower in T1D (34 ± 6/s versus 38 ± 5/s; P < .01) corresponding to a higher oxygenation. R₂* was not different in the cortex. Cortical perfusion was lower in T1D (163 ± 40 versus 224 ± 49 mL/100 g/min; P < .001). Renal artery blood flow was lower in T1D than in CONs (360 ± 130 versus 430 ± 113 mL/min; P = .05). In T1D, lower cortical oxygenation and renal artery blood flow were both associated with higher UACR and lower eGFR (P < .05).

Conclusions

Participants with T1D and albuminuria exhibited higher medullary oxygenation than CONs, despite lower cortical perfusion and renal artery blood flow. This might reflect perturbed kidney energetics leading to a higher setpoint of medullary oxygenation in T1D. Lower cortical oxygenation and renal artery blood flow were associated with higher UACR and lower eGFR in T1D.

Hidden in Plain Sight: Does Medullary Red Blood Cell Congestion Provide the Explanation for Ischemic Acute Kidney Injury?

Acute kidney injury (AKI) represents a sudden reduction in renal function and is a major clinical problem with a high mortality rate. Despite decades of research, there are currently no direct therapies for AKI. The failure of therapeutic approaches identified in rodents to translate to human beings has led to questions regarding the appropriateness of these models. Our recent data indicate that there are two distinct processes driving tubular injury in the commonly used rat model of warm bilateral renal ischemia reperfusion injury, which often is used to mimic ischemic AKI. One results from the period of warm ischemia, manifesting as sublethal injury and coagulative necrosis of the proximal tubules in the renal cortex. This is the predominate type of injury observed 24 hours after reperfusion and the most well studied. The other results from red blood cell congestion of the outer medullary vasculature. This type of injury manifests as cell sloughing, along with the later formation of heme casts that fill distal nephron segments. Cell sloughing from congestion is most prominent in the early hours after reperfusion and often is masked by regeneration of the tubular epithelium by 24 hours postischemia. In this review, we argue that injury from outer medullary red blood cell congestion reflects the pathology observed in human kidneys and likely is representative of injury in most cases of ischemic AKI after shock. Greater focus on this congestive injury is likely to lead to improved translation in AKI.

Hip Position Acutely Affects Oxygenation and Perfusion of Kidney Grafts as Measured by Functional Magnetic Resonance Imaging Methods-The Bent Knee Study

Background: Kidney perfusion and oxygenation are two important determinants of kidney graft function. In kidney transplantation, repeated graft hypoperfusion may occur during hip flexion, for example in the sitting position, due to the progressive development of fibrotic tissue around iliac arteries. The aim of this study was to assess the changes in oxygenation and perfusion of kidney grafts during hip flexion and extension using a new functional magnetic resonance imaging (fMRI) protocol. Methods: Nineteen kidney graft recipients prospectively underwent MRI on a 3T scanner including diffusion-weighted, blood oxygenation level dependent (BOLD), and arterial spin labeling sequences in hip positions 0° and >90° before and after intravenous administration of 20 mg furosemide. Results: Unexpectedly, graft perfusion values were significantly higher in flexed compared to neutral hip position. Main diffusion-derived parameters were not affected by hip position. BOLD-derived cortico-medullary R2^* ratio was significantly modified during hip flexion suggesting an intrarenal redistribution of the oxygenation in favor of the medulla and to the detriment of the cortex. Furthermore, the increase in medullary oxygenation induced by furosemide was significantly blunted during hip flexion (p < 0.001). Conclusion: Hip flexion has an acute impact on perfusion and tissue oxygenation in kidney grafts. Whether these position-dependent changes affect the long-term function and outcome of kidney transplants needs further investigation.

Magnetic resonance imaging assessment of renal flow distribution patterns during ex vivo normothermic machine perfusion in porcine and human kidneys

Acceptance criteria of deceased donor organs have gradually been extended toward suboptimal quality, posing an urgent need for more objective pre‐transplant organ assessment. Ex vivo normothermic machine perfusion (NMP) combined with magnetic resonance imaging (MRI) could assist clinicians in deciding whether a donor kidney is suitable for transplantation. Aim of this study was to characterize the regional distribution of perfusate flow during NMP, to better understand how ex vivo kidney assessment protocols should eventually be designed. Nine porcine and 4 human discarded kidneys underwent 3 h of NMP in an MRI‐compatible perfusion setup. Arterial spin labeling scans were performed every 15 min, resulting in perfusion‐weighted images that visualize intrarenal flow distribution. At the start of NMP, all kidneys were mainly centrally perfused and it took time for the outer cortex to reach its physiological dominant perfusion state. Calculated corticomedullary ratios based on the perfusion maps reached a physiological range comparable to in vivo observations, but only after 1 to 2 h after the start of NMP. Before that, the functionally important renal cortex appeared severely underperfused. Our findings suggest that early functional NMP quality assessment markers may not reflect actual physiology and should therefore be interpreted with caution.

Acute effects of dapagliflozin on renal oxygenation and perfusion in type 1 diabetes with albuminuria: A randomised, double-blind, placebo-controlled crossover trial

BACKGROUND

Inhibitors of the sodium-glucose cotransporter 2 (SGLT2) slow the progression of diabetic kidney disease, possibly by reducing the proximal tubule transport workload with subsequent improvement of renal oxygenation. We aimed to test this hypothesis in individuals with type 1 diabetes and albuminuria.

METHODS

A randomised, double-blind, placebo-controlled, crossover trial with a single 50 mg dose of the SGLT2 inhibitor dapagliflozin and placebo in random order, separated by a two-week washout period. Magnetic resonance imaging (MRI) was used to assess renal R2* (a low value corresponds to a high tissue oxygenation), renal perfusion (arterial spin labelling) and renal artery flow (phase contrast imaging) at baseline, three- and six hours from tablet ingestion. Exploratory outcomes, including baroreflex sensitivity, peripheral blood oxygen saturation, peripheral blood mononuclear cell mitochondrial oxygen consumption rate, and biomarkers of inflammation were evaluated at baseline and 12 h from medication. The study is registered in the EU Clinical Trials Register (EudraCT 2019-004,557-92), on ClinicalTrials.gov (NCT04193566), and is completed.

FINDINGS

Between February 3, 2020 and October 23, 2020, 31 individuals were screened, and 19 eligible individuals were randomised. Three dropped out before receiving any of the interventions and one dropped out after receiving only placebo. We included 15 individuals (33% female) in the per-protocol analysis with a mean age of 58 (SD 14) years, median urinary albumin creatinine ratio of 46 [IQR 21-58] mg/g and an eGFR of 73 (32) ml/min/1·73m2. The mean changes in renal cortical R2* from baseline to six hours were for dapagliflozin -1·1 (SD 0·7) s-1 and for placebo +1·3 (0·7) s-1, resulting in a difference between interventions of -2·3 s-1 [95% CI -4·0 to -0·6]; p = 0·012. No between-intervention differences were found in any other MRI outcomes, physiological parameters or exploratory outcomes. There were no adverse events.

INTERPRETATION

A single dose of 50 mg dapagliflozin acutely improved renal cortical R2* without changing renal perfusion or blood flow. This suggests improved renal cortical oxygenation due to a reduced tubular transport workload in the proximal tubules. Such improved oxygenation may in part explain the long-term beneficial renal effects seen with SGLT2 inhibitors, but it remains to be determined whether the observed effects can be achieved with lower doses, with chronic treatment and if they occur in type 2 diabetes as well.

Basic principles and new advances in kidney imaging

Over the past few years, clinical renal imaging has seen great advances, allowing assessments of kidney structure and morphology, perfusion, function and metabolism, and oxygenation, as well as microstructure and the interstitium. Medical imaging is becoming increasingly important in the evaluation of kidney physiology and pathophysiology, showing promise in management of patients with renal disease, in particular with regard to diagnosis, classification, and prediction of disease development and progression, monitoring response to therapy, detection of drug toxicity, and patient selection for clinical trials. A variety of imaging modalities, ranging from routine to advanced tools, are currently available to probe the kidney both spatially and temporally, particularly ultrasonography, computed tomography, positron emission tomography, renal scintigraphy, and multiparametric magnetic resonance imaging. Given that the range is broad and varied, kidney imaging techniques should be chosen based on the clinical question and the specific underlying pathologic mechanism, taking into account contraindications and possible adverse effects. Integration of various modalities providing complementary information will likely provide the greatest insight into renal pathophysiology. This review aims to highlight major recent advances in key tools that are currently available or potentially relevant for clinical kidney imaging, with a focus on non-oncological applications. The review also outlines the context of use, limitations, and advantages of various techniques, and highlights gaps to be filled with future development and clinical adoption.

Reversible (Patho)Physiologically Relevant Test Interventions: Rationale and Examples

Renal tissue hypoperfusion and hypoxia are early key elements in the pathophysiology of acute kidney injury of various origins, and may also promote progression from acute injury to chronic kidney disease. Here we describe test interventions that are used to study the control of renal hemodynamics and oxygenation in experimental animals in the context of kidney-specific control of hemodynamics and oxygenation. The rationale behind the use of the individual tests, the physiological responses of renal hemodynamics and oxygenation, the use in preclinical studies, and the possible application in humans are discussed.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.

Altered renal medullary blood flow: A key factor or a parallel event in control of sodium excretion and blood pressure?

In the context of the ongoing debate on the mechanism of blood pressure (BP) regulation and pathophysiology of arterial hypertension ("renocentric" vs "neural" concepts), attention is focused on the putative regulatory role of changes in renal medullary blood flow (MBF). Experimental evidence is analysed with regard to the question whether an elevation of BP and renal perfusion pressure (RPP) is likely to increase MBF due to its impaired autoregulation. It is concluded that such increases have been clearly documented only in rats with extracellular fluid volume expansion. A possible translation of this finding to BP regulation in health and hypertension in humans may only be a matter of speculation. Within the "renocentric" theory, the key event leading to restoration of initial BP level is pressure natriuresis. Its relation to elevation of renal interstitial hydrostatic pressure and to the phenomenon of "wash-out" of renal medullary solutes by increasing MBF is discussed. We also assessed the validity of data supporting the putative mechanism of short-term restoration of elevated BP owing to the release of a vasodilator lipid (medullipin) by the medulla. The structure of the proposed medullary lipid is still undefined, and there is no sound evidence on its mediatory role in lowering elevated BP level. In conclusion, MBF change can hardly be regarded as a crucial event in the regulation of BP: it can be involved in the control of sodium excretion and BP only in some circumstances, although its contributory role cannot be excluded.

Peristenotic Collateral Circulation in Atherosclerotic Renovascular Disease: Association With Kidney Function and Response to Treatment

The significance of peristenotic collateral circulation (PCC) development around a stenotic renal artery is unknown. We tested the hypothesis that PCC is linked to loss of kidney function and recovery potential in patients with atherosclerotic renovascular disease (ARVD). Thirty-four patients with ARVD were assigned to medical-therapy with or without revascularization based on clinical indications. The PCC was visualized using multidetector computed tomography and defined relative to segmental arteries in patients with essential hypertension. PCC number before and 3 months after treatment was correlated with various renal parameters. Thirty-four stenotic kidneys from 30 patients were analyzed. PCC number correlated inversely with kidney volume. ARVD-stenotic kidneys with baseline PCC (collateral ARVD [C-ARVD], n=13) associated with elevated 24-hour urine protein and stenotic kidney vein level of tumor necrosis factor-α, lower single-kidney volume and blood flow, and greater hypoxia than in stenotic kidneys with no PCC (no collateral ARVD [NC-ARVD], n=17). Revascularization (but not medical-therapy alone) improved stenotic kidney function and reduced inflammation in both NC-ARVD and C-ARVD. In C-ARVD, revascularization also increased stenotic kidney volume, blood flow, and oxygenation to levels comparable to NC-ARVD, and induced PCC regression. However, revascularization improved systolic blood pressure, plasma renin activity, and filtration fraction only in NC-ARVD. Therefore, patients with C-ARVD have greater kidney dysfunction, atrophy, hypoxia, and inflammation compared with patients with NC-ARVD, suggesting that PCC does not effectively protect the stenotic kidney in ARVD. Renal artery revascularization improved in C-ARVD stenotic kidney function, but not hypertension or renin-angiotensin system activation. These observations may help direct management of patients with ARVD.

The role of renal hypoxia in the pathogenesis of diabetic kidney disease: a promising target for newer renoprotective agents including SGLT2 inhibitors?

Diabetic kidney disease is the most common cause of end-stage kidney disease and poses a major global health problem. Finding new, safe, and effective strategies to halt this disease has proven to be challenging. In part that is because the underlying mechanisms are complex and not fully understood. However, in recent years, evidence has accumulated suggesting that chronic hypoxia may be the primary pathophysiological pathway driving diabetic kidney disease and chronic kidney disease of other etiologies and was called the chronic hypoxia hypothesis. Hypoxia is the result of a mismatch between oxygen delivery and oxygen demand. The primary determinant of oxygen delivery is renal perfusion (blood flow per tissue mass), whereas the main driver of oxygen demand is active sodium reabsorption. Diabetes mellitus is thought to compromise the oxygen balance by impairing oxygen delivery owing to hyperglycemia-associated microvascular damage and exacerbate oxygen demand owing to increased sodium reabsorption as a result of sodium-glucose cotransporter upregulation and glomerular hyperfiltration. The resultant hypoxic injury creates a vicious cycle of capillary damage, inflammation, deposition of the extracellular matrix, and, ultimately, fibrosis and nephron loss. This review will frame the role of chronic hypoxia in the pathogenesis of diabetic kidney disease and its prospect as a promising therapeutic target. We will outline the cellular mechanisms of hypoxia and evidence for renal hypoxia in animal and human studies. In addition, we will highlight the promise of newer imaging modalities including blood oxygenation level-dependent magnetic resonance imaging and discuss salutary interventions such as sodium-glucose cotransporter 2 inhibition that (may) protect the kidney through amelioration of renal hypoxia.

Probing renal blood volume with magnetic resonance imaging

Damage to the kidney substantially reduces life expectancy. Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. In vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) is sensitive to changes in the effective transversal relaxation time (T₂ *) in vivo, and is non-invasive and indicative of renal tissue oxygenation. However, the renal T₂ * to tissue pO₂ relationship is not governed exclusively by renal blood oxygenation, but is affected by physiological confounders with alterations in renal blood volume fraction (BVf) being of particular relevance. To decipher this interference probing renal BVf is essential for the pursuit of renal MR oximetry. Superparamagnetic iron oxide nanoparticle (USPIO) preparations can be used as MRI visible blood pool markers for detailing alterations in BVf. This review promotes the opportunities of MRI-based assessment of renal BVf. Following an outline on the specifics of renal oxygenation and perfusion, changes in renal BVf upon interventions and their potential impact on renal T₂ * are discussed. We also describe the basic principles of renal BVf assessment using ferumoxytol-enhanced MRI in the equilibrium concentration regimen. We demonstrate that ferumoxytol does not alter control of renal haemodynamics and oxygenation. Preclinical applications of ferumoxytol enhanced renal MRI as well as considerations for its clinical implementation for examining renal BVf changes are provided alongside practical considerations. Finally, we explore the future directions of MRI-based assessment of renal BVf.

Consensus-based technical recommendations for clinical translation of renal ASL MRI

OBJECTIVES

This study aimed at developing technical recommendations for the acquisition, processing and analysis of renal ASL data in the human kidney at 1.5 T and 3 T field strengths that can promote standardization of renal perfusion measurements and facilitate the comparability of results across scanners and in multi-centre clinical studies.

METHODS

An international panel of 23 renal ASL experts followed a modified Delphi process, including on-line surveys and two in-person meetings, to formulate a series of consensus statements regarding patient preparation, hardware, acquisition protocol, analysis steps and data reporting.

RESULTS

Fifty-nine statements achieved consensus, while agreement could not be reached on two statements related to patient preparation. As a default protocol, the panel recommends pseudo-continuous (PCASL) or flow-sensitive alternating inversion recovery (FAIR) labelling with a single-slice spin-echo EPI readout with background suppression and a simple but robust quantification model.

DISCUSSION

This approach is considered robust and reproducible and can provide renal perfusion images of adequate quality and SNR for most applications. If extended kidney coverage is desirable, a 2D multislice readout is recommended. These recommendations are based on current available evidence and expert opinion. Nonetheless they are expected to be updated as more data become available, since the renal ASL literature is rapidly expanding.

Consensus-based technical recommendations for clinical translation of renal ASL MRI

Objectives

This study aimed at developing technical recommendations for the acquisition, processing and analysis of renal ASL data in the human kidney at 1.5 T and 3 T field strengths that can promote standardization of renal perfusion measurements and facilitate the comparability of results across scanners and in multi-centre clinical studies.

Methods

An international panel of 23 renal ASL experts followed a modified Delphi process, including on-line surveys and two in-person meetings, to formulate a series of consensus statements regarding patient preparation, hardware, acquisition protocol, analysis steps and data reporting.

Results

Fifty-nine statements achieved consensus, while agreement could not be reached on two statements related to patient preparation. As a default protocol, the panel recommends pseudo-continuous (PCASL) or flow-sensitive alternating inversion recovery (FAIR) labelling with a single-slice spin-echo EPI readout with background suppression and a simple but robust quantification model.

Discussion

This approach is considered robust and reproducible and can provide renal perfusion images of adequate quality and SNR for most applications. If extended kidney coverage is desirable, a 2D multislice readout is recommended. These recommendations are based on current available evidence and expert opinion. Nonetheless they are expected to be updated as more data become available, since the renal ASL literature is rapidly expanding.

Electronic supplementary material

The online version of this article (10.1007/s10334-019-00800-z) contains supplementary material, which is available to authorized users.

Human renal response to furosemide: Simultaneous oxygenation and perfusion measurements in cortex and medulla

Aim

Disturbances of renal medullary perfusion and metabolism have been implicated in the pathogenesis of kidney disease and hypertension. Furosemide, a loop diuretic, is widely used to prevent renal medullary hypoxia in acute kidney disease by uncoupling sodium metabolism, but its effects on medullary perfusion in humans are unknown. We performed quantitative imaging of both renal perfusion and oxygenation using Magnetic Resonance Imaging (MRI) before and during furosemide. Based on the literature, we hypothesized that furosemide would increase medullary oxygenation, decrease medullary perfusion, but cause minor changes (<10%) in renal artery flow (RAF).

Methods

Interleaved measurements of RAF, oxygenation (T₂*) and perfusion by arterial spin labelling in the renal cortex and medulla of 9 healthy subjects were acquired before and after an injection of 20 mg furosemide. They were preceded by measurements made during isometric exercise (5 minutes handgrip bouts), which are known to induce changes in renal hemodynamics, that served as a control for the sensitivity of the hemodynamic MRI measurements. Experiments were repeated on a second day to establish that the measurements and the induced changes were reproducible.

Results

After furosemide, T₂* values in the medulla increased by 53% (P < 0.01) while RAF and perfusion remained constant. After hand‐grip exercise, T₂* values in renal medulla increased by 22% ± 9% despite a drop in medullary perfusion of 7.2% ± 4.7% and a decrease in renal arterial flow of 17.5% ± 1.7% (P < 0.05). Mean coefficients of variation between repeated measurements for all parameters were 7%.

Conclusion

Furosemide induced the anticipated increase in renal medullary oxygenation, attributable exclusively to a decrease in renal oxygen consumption, since no change of RAF, cortical or medullary perfusion could be demonstrated. All measures and the induced changes were reproducible.