The renin-angiotensin system and the third mechanism of renal blood flow autoregulation

Renal MRI: From Nephron to NMR Signal

Renal diseases pose a significant socio-economic burden on healthcare systems. The development of better diagnostics and prognostics is well-recognized as a key strategy to resolve these challenges. Central to these developments are MRI biomarkers, due to their potential for monitoring of early pathophysiological changes, renal disease progression or treatment effects. The surge in renal MRI involves major cross-domain initiatives, large clinical studies, and educational programs. In parallel with these translational efforts, the need for greater (patho)physiological specificity remains, to enable engagement with clinical nephrologists and increase the associated health impact. The ISMRM 2022 Member Initiated Symposium (MIS) on renal MRI spotlighted this issue with the goal of inspiring more solutions from the ISMRM community. This work is a summary of the MIS presentations devoted to: 1) educating imaging scientists and clinicians on renal (patho)physiology and demands from clinical nephrologists, 2) elucidating the connection of MRI parameters with renal physiology, 3) presenting the current state of leading MR surrogates in assessing renal structure and functions as well as their next generation of innovation, and 4) describing the potential of these imaging markers for providing clinically meaningful renal characterization to guide or supplement clinical decision making. We hope to continue momentum of recent years and introduce new entrants to the development process, connecting (patho)physiology with (bio)physics, and conceiving new clinical applications. We envision this process to benefit from cross-disciplinary collaboration and analogous efforts in other body organs, but also to maximally leverage the unique opportunities of renal physiology. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 2.

Mediators of Regional Kidney Perfusion during Surgical Pneumo-Peritoneum Creation and the Risk of Acute Kidney Injury—A Review of Basic Physiology

Acute kidney injury (AKI), especially if recurring, represents a risk factor for future chronic kidney disease. In intensive care units, increased intra-abdominal pressure is well-recognized as a significant contributor to AKI. However, the importance of transiently increased intra-abdominal pressures procedures is less commonly appreciated during laparoscopic surgery, the use of which has rapidly increased over the last few decades. Unlike the well-known autoregulation of the renal cortical circulation, medulla perfusion is modulated via partially independent regulatory mechanisms and strongly impacted by changes in venous and lymphatic pressures. In our review paper, we will provide a comprehensive overview of this evolving topic, covering a broad range from basic pathophysiology up to and including current clinical relevance and examples. Key regulators of oxidative stress such as ischemia-reperfusion injury, the activation of inflammatory response and humoral changes interacting with procedural pneumo-peritoneum formation and AKI risk will be recounted. Moreover, we present an in-depth review of the interaction of pneumo-peritoneum formation with general anesthetic agents and animal models of congestive heart failure. A better understanding of the relationship between pneumo-peritoneum formation and renal perfusion will support basic and clinical research, leading to improved clinical care and collaboration among specialists.

Prevention of Contrast-Induced Nephropathy by Inferior Vena Cava Ultrasonography-Guided Hydration in Chronic Heart Failure Patients

INTRODUCTION

Contrast-induced nephropathy (CIN) is a common complication resulting from the administration of contrast media. This study was designed to determine whether inferior vena cava (IVC) ultrasonography (IVCU)-guided hydration can reduce the risk of CIN in chronic heart failure patients undergoing coronary angiography or coronary angiography with percutaneous coronary intervention compared with standard hydration.

METHODS

This prospective clinical trial enrolled 207 chronic heart failure patients from February 2016 to November 2017, who were randomly assigned to either the IVCU-guided hydration group (n = 104) or the routine hydration group (n = 103). In the IVCU-guided group, the hydration infusion rate was set according to the IVC diameter determined by IVCU, while the control group received intravenous infusion of 0.9% saline at 0.5 mL/(kg·h). Serum Cr was measured before and 48-72 h after the procedure. All patients were followed up for 18 months. The incidence of nephropathy and major adverse cardiovascular or cerebrovascular events (MACCEs) was also compared between the 2 groups.

RESULTS

Statistically significant difference between the 2 groups regarding the occurrence of CIN was observed (12.5 vs. 29.1%, p = 0.004). The hydration volume of the IVCU-guided group was significantly higher than that of the routine group (p < 0.001). In addition, patients receiving IVCU-guided hydration had significantly lower risk of developing MACCEs than patients in the control group during the 18-month follow-up (14.4 vs. 27.2%, p = 0.027).

CONCLUSION

Our findings support that IVCU-guided hydration is superior to standard hydration in prevention of CIN and may substantially reduce longtime composite major adverse events.

Monitoring Renal Hemodynamics and Oxygenation by Invasive Probes: Experimental Protocol

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 methods to study control of renal hemodynamics and tissue oxygenation by means of invasive probes in anesthetized rats. Step-by-step protocols are provided for two setups, one for experiments in laboratories for integrative physiology and the other for experiments within small-animal magnetic resonance scanners.This publication is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This experimental protocol chapter is complemented by a separate chapter describing the basic concepts of quantitatively assessing renal perfusion and oxygenation with invasive probes.

Quantitative Assessment of Renal Perfusion and Oxygenation by Invasive Probes: Basic Concepts

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 basic principles of methodology to quantify renal hemodynamics and tissue oxygenation by means of invasive probes in experimental animals. Advantages and disadvantages of the various methods are discussed in the context of the heterogeneity of renal tissue perfusion and oxygenation.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by a separate chapter describing the experimental procedure and data analysis.

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.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Renal Autocrine and Paracrine Signaling: A Story of Self-protection

Autocrine and paracrine signaling in the kidney adds an extra level of diversity and complexity to renal physiology. The extensive scientific production on the topic precludes easy understanding of the fundamental purpose of the vast number of molecules and systems that influence the renal function. This systematic review provides the broader pen strokes for a collected image of renal paracrine signaling. First, we recapitulate the essence of each paracrine system one by one. Thereafter the single components are merged into an overarching physiological concept. The presented survey shows that despite the diversity in the web of paracrine factors, the collected effect on renal function may not be complicated after all. In essence, paracrine activation provides an intelligent system that perceives minor perturbations and reacts with a coordinated and integrated tissue response that relieves the work load from the renal epithelia and favors diuresis and natriuresis. We suggest that the overall function of paracrine signaling is reno-protection and argue that renal paracrine signaling and self-regulation are two sides of the same coin. Thus local paracrine signaling is an intrinsic function of the kidney, and the overall renal effect of changes in blood pressure, volume load, and systemic hormones will always be tinted by its paracrine status.

Left ventricular end-diastolic pressure-guided hydration for the prevention of contrast-induced acute kidney injury in patients with stable ischemic heart disease: the LAKESIDE trial

OBJECTIVES

Contrast-induced acute kidney injury (CI-AKI) is a serious complication in patients undergoing diagnostic cardiac angiography or percutaneous coronary intervention. We aimed to evaluate the preventive effects of left ventricular end-diastolic pressure (LVEDP)-guided hydration for the prevention of CI-AKI in patients with chronic kidney disease undergoing cardiac catheterization.

METHODS

This prospective randomized single-blind clinical trial enrolled 114 eligible patients with an estimated glomerular filtration rate (eGFR) of 15 < eGFR ≤ 60 mL/min/1.73 m² [according to the level-modified Modification of Diet in Renal Disease formula (MDRD)] and stable ischemic heart disease undergoing coronary procedures. The patients were randomly allocated 1:1 into the LVEDP-guided hydration group (n = 57) or the standard hydration group (n = 57). CI-AKI was defined as a greater than 25% or greater than 0.5 mg/dL (44.2 mmol/L) increase in the serum creatinine concentration compared with the baseline value. Hydration with 0.9% sodium chloride at a rate of 1 mL/kg/h (0.5 mL/kg/h if left ventricular ejection fraction < 40%) within 12 h was given to all the patients in both groups before the procedure. In the LVEDP-guided group, the hydration infusion rate was adjusted according to the LVEDP level during and after the procedure.

RESULTS

The incidence of CI-AKI was 7.01% (4/57) in the LVEDP-guided group vs 3.84% (2/52) in the standard hydration group (summary odds ratio 0.53, 95% CI 0.093-3.022; P = 0.463). Major adverse cardiac events, hemodialysis, or related deaths occurred in neither of the groups during hospitalization or the 30-day follow-up.

CONCLUSIONS

In the present study, LVEDP-guided fluid administration, by comparison with standard hydration, failed to offer protection against the risk of CI-AKI in patients with renal insufficiency undergoing coronary angiography with or without percutaneous coronary intervention.

Safe Hydration Volume to Prevent Contrast-induced Acute Kidney Injury and Worsening Heart Failure in Patients With Heart Failure and Preserved Ejection Fraction After Cardiac Catheterization

Few studies have investigated the efficacy and safety of hydration to prevent contrast-induced acute kidney injury (CI-AKI) and worsening heart failure (WHF) after cardiac catheterization in heart failure and preserved ejection fraction (HFpEF; HF and EF ≥50%) patients. We recruited 1206 patients with HFpEF undergoing cardiac catheterization with periprocedural hydration volume/weight (HV/W) ratio data and investigated the relationship between hydration volumes and risk of CI-AKI and WHF. Incidence of CI-AKI was not significantly reduced in individuals with higher HV/W [quartile (Q) 1, Q2, Q3, and Q4: 9.7%, 10.2%, 12.7%, and 12.2%, respectively; P = 0.219]. Multivariate analysis indicated that higher HV/W ratios were not associated with decreased CI-AKI risks [Q2 vs. Q1: odds ratio (OR), 0.95; Q3 vs. Q1: OR, 1.07; Q4 vs. Q1: OR, 0.92; all P > 0.05]. According to multivariate analysis, higher HV/W significantly increased the WHF risk (Q4 vs. Q1: adjusted OR, 8.13 and 95% confidence interval, 1.03-64.02; P = 0.047). CI-AKI and WHF were associated with a significantly increased risk of long-term mortality (mean follow-up, 2.33 years). For HFpEF patients, an excessively high hydration volume might not be associated with lower risk of CI-AKI but may increase the risk of postprocedure WHF.

Establishment and evaluation of a reversible two-kidney, one-clip renovascular hypertensive rat model

The aim of the present study was to establish and evaluate a novel and reversible two-kidney, one-clip renovascular hypertensive rat model with a titanium vascular clip. A total of 40 male Sprague-Dawley rats were evenly and randomly divided into a sham-operated group, and 3, 7, 12 and 28D groups (namely removing the vascular clip in the renovascular hypertensive model after 3, 7, 12 and 28 days, respectively). The systolic blood pressure (SBP) and plasma renin activity (PRA) were measured, and color duplex imaging was conducted before placing the clips, as well as before and after removing them. After placing the vascular clips, SBP and PRA in the 3, 7, 12 and 28D groups were significantly increased (SBP: Sham-operated vs. 3D groups, P=0.020; 3 vs. 7D groups, P=0.008; 7 vs. 28D groups, P=0.019; 12 vs. 28D groups, P=0.039, and between other groups P<0.001. PRA: 3 vs. 7D groups, P=0.001; 7 vs. 12D groups, P=0.004; 12 vs. 28D groups, P=0.040, and between other groups, P<0.001). After removing the clips, SBP were significantly reduced in the 3 and 7D groups (P=0.023, 0.040, 0.066 and 0.314 in the 3, 7, 12 and 28D groups, respectively), but were still significantly higher than that before placing clips in the 7, 12 and 28D groups (P=0.067, P=0.005, P<0.001 and P<0.001 in the 3, 7, 12 and 28D groups, respectively). After removing the clips, PRA was significantly reduced in each group (P<0.001, P<0.001, P=0.012 and P=0.049 in 3, 7, 12 and 28D groups, respectively), but still higher than that before placing the clips (P<0.001, P=0.001, P=0.001 and P=0.003 in 3, 7, 12 and 28D groups, respectively). Vascular imaging also indicates this model has a reversible property. In conclusion, a reversible renovascular hypertension rat model is feasible, and provides a basis for research on clinical ischemic nephropathy and renal artery revascularization.

Prevention of contrast-induced acute kidney injury in patients undergoing cardiovascular procedures-a systematic review and network meta-analysis

Background

Interventional diagnostic and therapeutic procedures requiring intravascular iodinated contrast steadily increase patient exposure to the risks of contrast-induced acute kidney injury (CIAKI), which is associated with death, nonfatal cardiovascular events, and prolonged hospitalization. The aim of this study was to investigate the efficacy of pharmacological and non-pharmacological treatments for CIAKI prevention in patients undergoing cardiovascular invasive procedures with iodinated contrast.

Methods and findings

MEDLINE, Google Scholar, EMBASE and Cochrane databases as well as abstracts and presentations from major cardiovascular and nephrology meetings were searched, up to 22 April 2016. Eligible studies were randomized trials comparing strategies to prevent CIAKI (alone or in combination) when added to saline versus each other, saline, placebo, or no treatment in patients undergoing cardiovascular invasive procedures with administration of iodinated contrast. Two reviewers independently extracted trial-level data including number of patients, duration of follow-up, and outcomes. Eighteen strategies aimed at CIAKI prevention were identified. The primary outcome was the occurrence of CIAKI. Secondary outcomes were mortality, myocardial infarction, dialysis and heart failure. The data were pooled using network meta-analysis. Treatment estimates were calculated as odds ratios (ORs) with 95% credible intervals (CrI). 147 RCTs involving 33,463 patients were eligible. Saline plus N-acetylcysteine (OR 0.72, 95%CrI 0.57–0.88), ascorbic acid (0.59, 0.34–0.95), sodium bicarbonate plus N-acetylcysteine (0.59, 0.36–0.89), probucol (0.42, 0.15–0.91), methylxanthines (0.39, 0.20–0.66), statin (0.36, 0.21–0.59), device-guided matched hydration (0.35, 0.12–0.79), prostaglandins (0.26, 0.08–0.62) and trimetazidine (0.26, 0.09–0.59) were associated with lower odds of CIAKI compared to saline. Methylxanthines (0.12, 0.01–0.94) or left ventricular end-diastolic pressure-guided hydration (0.09, 0.01–0.59) were associated with lower mortality compared to saline.

Conclusions

Currently recommended treatment with saline as the only measure to prevent CIAKI during cardiovascular procedures may not represent the optimal strategy. Vasodilators, when added to saline, may significantly reduce the odds of CIAKI following cardiovascular procedures.

Acute effects of ferumoxytol on regulation of renal hemodynamics and oxygenation

The superparamagnetic iron oxide nanoparticle ferumoxytol is increasingly used as intravascular contrast agent in magnetic resonance imaging (MRI). This study details the impact of ferumoxytol on regulation of renal hemodynamics and oxygenation. In 10 anesthetized rats, a single intravenous injection of isotonic saline (used as volume control) was followed by three consecutive injections of ferumoxytol to achieve cumulative doses of 6, 10, and 41 mg Fe/kg body mass. Arterial blood pressure, renal blood flow, renal cortical and medullary perfusion and oxygen tension were continuously measured. Regulation of renal hemodynamics and oxygenation was characterized by dedicated interventions: brief periods of suprarenal aortic occlusion, hypoxia, and hyperoxia. None of the three doses of ferumoxytol resulted in significant changes in any of the measured parameters as compared to saline. Ferumoxytol did not significantly alter regulation of renal hemodynamics and oxygenation as studied by aortic occlusion and hypoxia. The only significant effect of ferumoxytol at the highest dose was a blunting of the hyperoxia-induced increase in arterial pressure. Taken together, ferumoxytol has only marginal effects on the regulation of renal hemodynamics and oxygenation. This makes ferumoxytol a prime candidate as contrast agent for renal MRI including the assessment of renal blood volume fraction.

Excessively High Hydration Volume May Not Be Associated With Decreased Risk of Contrast-Induced Acute Kidney Injury After Percutaneous Coronary Intervention in Patients With Renal Insufficiency

BACKGROUND

No well-defined protocols currently exist regarding the optimal rate and duration of normal saline administration to prevent contrast-induced acute kidney injury (CI-AKI) in patients with renal insufficiency.

METHODS AND RESULTS

Hydration volume ratios (hydration volume/weight; HV/W) were calculated in 1406 patients with renal insufficiency (estimated glomerular filtration rate [eGFR], <90 mL/min per 1.73 m(2)) undergoing percutaneous coronary intervention (PCI) with routine speed hydration (1 or 0.5 mL/kg per hour). We investigated the relationship between hydration volume, risk of CI-AKI (increase in serum creatinine ≥0.5 mg/dL or 25% within 48-72 hours), and prognosis. Mean follow-up duration was 2.85±0.88 years. Individuals with higher HV/W were more likely to develop CI-AKI (quartiles: Q1, Q2, Q3, and Q4: 4.3%, 6.6%, 10.9%, and 15.0%, respectively; P<0.001). After adjusting 12 confounders, including age, sex, eGFR, anemia, emergent PCI, diabetes mellitus, chronic heart failure, diuretics, contrast volume, lesions, smoking status, and number of stents, multivariate analysis showed that a higher HV/W ratio was not associated with a decreased CI-AKI risk (Q2 vs Q1: adjusted odds ratio [OR], 1.13; Q3 vs Q1: adjusted OR, 1.51; Q4 vs Q1: adjusted OR, 1.87; all P>0.05) and even increased CI-AKI risk (HV/W >25 mL/kg: adjusted OR, 2.11; 95% CI, 1.24-3.59; P=0.006). Additionally, higher HV/W was significantly associated with an increased risk of death (Q4 vs Q1: adjusted hazard ratio, 3.44; 95% CI, 1.20-9.88; P=0.022).

CONCLUSIONS

Excessively high hydration volume at routine speed might be associated with increased risk of CI-AKI and death post-PCI in patients with renal insufficiency.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Detailing the relation between renal T2* and renal tissue pO2 using an integrated approach of parametric magnetic resonance imaging and invasive physiological measurements

OBJECTIVES

This study was designed to detail the relation between renal T2* and renal tissue pO2 using an integrated approach that combines parametric magnetic resonance imaging (MRI) and quantitative physiological measurements (MR-PHYSIOL).

MATERIALS AND METHODS

Experiments were performed in 21 male Wistar rats. In vivo modulation of renal hemodynamics and oxygenation was achieved by brief periods of aortic occlusion, hypoxia, and hyperoxia. Renal perfusion pressure (RPP), renal blood flow (RBF), local cortical and medullary tissue pO2, and blood flux were simultaneously recorded together with T2*, T2 mapping, and magnetic resonance-based kidney size measurements (MR-PHYSIOL). Magnetic resonance imaging was carried out on a 9.4-T small-animal magnetic resonance system. Relative changes in the invasive quantitative parameters were correlated with relative changes in the parameters derived from MRI using Spearman analysis and Pearson analysis.

RESULTS

Changes in T2* qualitatively reflected tissue pO2 changes induced by the interventions. T2* versus pO2 Spearman rank correlations were significant for all interventions, yet quantitative translation of T2*/pO2 correlations obtained for one intervention to another intervention proved not appropriate. The closest T2*/pO2 correlation was found for hypoxia and recovery. The interlayer comparison revealed closest T2*/pO2 correlations for the outer medulla and showed that extrapolation of results obtained for one renal layer to other renal layers must be made with due caution. For T2* to RBF relation, significant Spearman correlations were deduced for all renal layers and for all interventions. T2*/RBF correlations for the cortex and outer medulla were even superior to those between T2* and tissue pO2. The closest T2*/RBF correlation occurred during hypoxia and recovery. Close correlations were observed between T2* and kidney size during hypoxia and recovery and for occlusion and recovery. In both cases, kidney size correlated well with renal vascular conductance, as did renal vascular conductance with T2*. Our findings indicate that changes in T2* qualitatively mirror changes in renal tissue pO2 but are also associated with confounding factors including vascular volume fraction and tubular volume fraction.

CONCLUSIONS

Our results demonstrate that MR-PHYSIOL is instrumental to detail the link between renal tissue pO2 and T2* in vivo. Unravelling the link between regional renal T2* and tissue pO2, including the role of the T2* confounding parameters vascular and tubular volume fraction and oxy-hemoglobin dissociation curve, requires further research. These explorations are essential before the quantitative capabilities of parametric MRI can be translated from experimental research to improved clinical understanding of hemodynamics/oxygenation in kidney disorders.

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

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

Haemodynamic-guided fluid administration for the prevention of contrast-induced acute kidney injury: the POSEIDON randomised controlled trial

BACKGROUND

The administration of intravenous fluid remains the cornerstone treatment for the prevention of contrast-induced acute kidney injury. However, no well-defined protocols exist to guide fluid administration in this treatment. We aimed to establish the efficacy of a new fluid protocol to prevent contrast-induced acute kidney injury.

METHODS

In this randomised, parallel-group, comparator-controlled, single-blind phase 3 trial, we assessed the efficacy of a new fluid protocol based on the left ventricular end-diastolic pressure for the prevention of contrast-induced acute kidney injury in patients undergoing cardiac catheterisation. The primary outcome was the occurrence of contrast-induced acute kidney injury, which was defined as a greater than 25% or greater than 0·5 mg/dL increase in serum creatinine concentration. Between Oct 10, 2010, and July 17, 2012, 396 patients aged 18 years or older undergoing cardiac catheterisation with an estimated glomerular filtration rate of 60 mL/min per 1·73 m(2) or less and one or more of several risk factors (diabetes mellitus, history of congestive heart failure, hypertension, or age older than 75 years) were randomly allocated in a 1:1 ratio to left ventricular end-diastolic pressure-guided volume expansion (n=196) or the control group (n=200) who received a standard fluid administration protocol. Four computer-generated concealed randomisation schedules, each with permuted block sizes of 4, were used for randomisation, and participants were allocated to the next sequential randomisation number by sealed opaque envelopes. Patients and laboratory personnel were masked to treatment assignment, but the physicians who did the procedures were not masked. Both groups received intravenous 0·9% sodium chloride at 3 mL/kg for 1 h before cardiac catheterisation. Analyses were by intention to treat. Adverse events were assessed at 30 days and 6 months and all such events were classified by staff who were masked to treatment assignment. This trial is registered with ClinicalTrials.gov, number NCT01218828.

FINDINGS

Contrast-induced acute kidney injury occurred less frequently in patients in the left ventricular end-diastolic pressure-guided group (6·7% [12/178]) than in the control group (16·3% [28/172]; relative risk 0·41, 95% CI 0·22-0·79; p=0·005). Hydration treatment was terminated prematurely because of shortness of breath in three patients in each group.

INTERPRETATION

Left ventricular end-diastolic pressure-guided fluid administration seems to be safe and effective in preventing contrast-induced acute kidney injury in patients undergoing cardiac catheterisation.

FUNDING

Kaiser Permanente Southern California regional research committee grant.

Oxidative stress in hypertension: role of the kidney

SIGNIFICANCE

Renal oxidative stress can be a cause, a consequence, or more often a potentiating factor for hypertension. Increased reactive oxygen species (ROS) in the kidney have been reported in multiple models of hypertension and related to renal vasoconstriction and alterations of renal function. Nicotinamide adenine dinucleotide phosphate oxidase is the central source of ROS in the hypertensive kidney, but a defective antioxidant system also can contribute.

RECENT ADVANCES

Superoxide has been identified as the principal ROS implicated for vascular and tubular dysfunction, but hydrogen peroxide (H2O2) has been implicated in diminishing preglomerular vascular reactivity, and promoting medullary blood flow and pressure natriuresis in hypertensive animals.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Increased renal ROS have been implicated in renal vasoconstriction, renin release, activation of renal afferent nerves, augmented contraction, and myogenic responses of afferent arterioles, enhanced tubuloglomerular feedback, dysfunction of glomerular cells, and proteinuria. Inhibition of ROS with antioxidants, superoxide dismutase mimetics, or blockers of the renin-angiotensin-aldosterone system or genetic deletion of one of the components of the signaling cascade often attenuates or delays the onset of hypertension and preserves the renal structure and function. Novel approaches are required to dampen the renal oxidative stress pathways to reduced O2(-•) rather than H2O2 selectivity and/or to enhance the endogenous antioxidant pathways to susceptible subjects to prevent the development and renal-damaging effects of hypertension.

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

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

Proof of principle: hydration by low-osmolar mannitol-glucose solution alleviates undesirable renal effects of an iso-osmolar contrast medium in rats

OBJECTIVE

Saline infusion is widely used to prevent contrast media (CM)-induced acute kidney injury, because it fosters diuresis. Osmodiuretics have a stronger diuretic effect than saline, yet previous trials indicate that osmodiuretic mannitol tends to promote rather than to prevent CM-induced acute kidney injury. However, these studies used hypertonic mannitol solutions that will result in rebound volume contraction. We hypothesize that combining the osmodiuretic effects of a nonhypertonic mannitol solution with sustained volume expansion alleviates undesirable renal effects of CM.

MATERIALS AND METHODS

Forty-four anesthetized rats were studied by 4 protocols. Urine flow rate, urine viscosity, and glomerular filtration rate (GFR) were measured. Intravenous infusions of hydration solutions were initiated 60 minutes before CM administration and continued throughout the observation period. Hydration by a 3.2% mannitol and 3.2% glucose solution infused at 12 mL/kg per hour (Mannit-Gluc regimen) was compared with a standard regimen of isotonic saline at 4 mL/kg per hour (NaCl regimen); greater infusion rates are required for the Mannit-Gluc regimen because of the profound diuretic effect of mannitol. Two CM were studied: iso-osmolar iodixanol (320 mg I/mL) and low-osmolar iopromide (370 mg I/mL), they were administered as 1.5-mL bolus injection into the thoracic aorta.

RESULTS

The Mannit-Gluc regimen resulted in higher urine flow rates than the standard NaCl regimen, yet maintained a good volume status. By virtue of its stronger diuretic effect, the Mannit-Gluc regimen greatly diminished the increase in urine viscosity and completely prevented the transient decrease in GFR caused by iodixanol with the NaCl regimen. After iopromide, the differences between the hydration regimens were much less, as iopromide increased urine flow rates much more than iodixanol, thus resulting in a much smaller increase in viscosity than iodixanol and no decrease in GFR even with the NaCl regimen.

CONCLUSION

This proof of principle study shows that a hydration regimen that combines the osmodiuretic effect of a low-osmolar mannitol-glucose solution with sustained volume expansion is effective in reducing high urine viscosity and preventing GFR reduction caused by iso-osmolar iodixanol. For low-osmolar CM, the beneficial effects seem negligible, because these compounds per se exert greater osmodiuretic action.

Contrast-induced kidney injury: mechanisms, risk factors, and prevention

In general, iodinated contrast media (CM) are tolerated well, and CM use is steadily increasing. Acute kidney injury is the leading life-threatening side effect of CM. Here, we highlight endpoints used to assess CM-induced acute kidney injury (CIAKI), CM types, risk factors, and CIAKI prevention. Moreover, we put forward a unifying theory as to how CIAKI comes about; the kidney medulla's unique hyperosmolar environment concentrates CM in the tubules and vasculature. Highly concentrated CM in the tubules and vessels increases fluid viscosity. Thus, flow through medullary tubules and vessels decreases. Reducing the flow rate will increase the contact time of cytotoxic CM with the tubular epithelial cells and vascular endothelium, and thereby damage cells and generate oxygen radicals. As a result, medullary vasoconstriction takes place, causing hypoxia. Moreover, the glomerular filtration rate declines due to congestion of highly viscous tubular fluid. Effective prevention aims at reducing the medullary concentration of CM, thereby diminishing fluid viscosity. This is achieved by generous hydration using isotonic electrolyte solutions. Even forced diuresis may prove efficient if accompanied by adequate volume supplementation. Limiting the CM dose is the most effective measure to diminish fluid viscosity and to reduce cytotoxic effects.

Modulation of the myogenic response in renal blood flow autoregulation by NO depends on endothelial nitric oxide synthase (eNOS), but not neuronal or inducible NOS

Nitric oxide (NO) blunts the myogenic response (MR) in renal blood flow (RBF) autoregulation. We sought to clarify the roles of NO synthase (NOS) isoforms, i.e. neuronal NOS (nNOS) from macula densa, endothelial NOS (eNOS) from the endothelium, and inducible NOS (iNOS) from smooth muscle or mesangium. RBF autoregulation was studied in rats and knockout (ko) mice in response to a rapid rise in renal artery pressure (RAP). The autoregulatory rise in renal vascular resistance within the first 6 s was interpreted as MR, from ∼6 to ∼30 s as tubuloglomerular feedback (TGF), and ∼30 to ∼100 s as the third regulatory mechanism. In rats, the nNOS inhibitor SMTC did not significantly affect MR (67 ± 4 vs. 57 ± 4 units). Inhibition of all NOS isoforms by l-NAME in the same animals markedly augmented MR to 78 ± 4 units. The same was found when SMTC was combined with angiotensin II to reproduce the hypertension and vasoconstriction seen with l-NAME (58 ± 3 vs. 54 ± 7 units, l-NAME 81 ± 2 units), or when SMTC was replaced by the nNOS inhibitor NPA (57 ± 5 vs. 56 ± 7 units, l-NAME 79 ± 4 units) or by the iNOS inhibitor 1400W (50 ± 1 vs. 55 ± 4 units, l-NAME 81 ± 3 units). nNOS-ko mice showed the same autoregulation as wild-types (MR 36 ± 4 vs. 38 ± 3 units) and the same response to l-NAME (111 ± 9 vs. 114 ± 10 units). eNOS-ko had similar autoregulation as wild-types (44 ± 8 vs. 33 ± 4 units), but failed to respond to l-NAME (37 ± 7 vs. 78 ± 16 units). We conclude that the attenuating effect of NO on MR depends on eNOS, but not on nNOS or iNOS. In eNOS-ko mice MR is depressed by NO-independent means.

Effect of mean arterial pressure, haemoglobin and blood transfusion during cardiopulmonary bypass on post-operative acute kidney injury

BACKGROUND

Acute kidney injury (AKI) after cardiac surgery is a common and serious condition carrying significant costs. During cardiopulmonary bypass (CPB) surgery, modifiable factors may contribute to post-operative AKI. Their avoidance might be a potential target for nephroprotection.

METHODS

The objective of the present study was to identify and determine whether intraoperative hypotension, anaemia, or their combination, red blood cell transfusion or vasopressor use are independent risk factors for post-operative AKI defined by the RIFLE (renal Risk, Injury, Failure, Loss of renal function and End-stage renal disease) classification and other thresholds using a mixed logistic multivariate model.

RESULTS

We analysed 381 468 mean arterial pressure (MAP) measurements from 920 consecutive on-pump cardiac surgery patients. Overall, 19.5% developed AKI which was associated with an 8.2-fold increase in-hospital mortality. Haemoglobin concentration was an independent risk factor for AKI {odds ratio [OR] 1.16 per g/dL decrease [95% confidence interval (CI) 1.05-1.31]; P = 0.018} with systemic arterial oxygen saturation and pressure values not adding further strength to such an association. MAP alone or vasopressor administration was not independently associated with AKI but volume of red blood cell transfusion was, with its effect being apparent at a haemoglobin level of >8 g/dL (>5 mmol/L). In patients with severe anaemia (<25th percentile of lowest haemoglobin), the independent effect of hypotension (>75th percentile of area under the curve MAP <50 mmHg) on AKI was more pronounced [OR 3.36 (95% CI 1.34-8.41); P = 0.010].

CONCLUSION

Intraoperative avoidance of the extremes of anaemia, especially during severe hypotension and avoidance of transfusion in patients with haemoglobin levels >8 g/dL (>5 mmol/L) may help decrease AKI in patients undergoing cardiac surgery and represent targets for future controlled interventions.

Angiotensin inhibition and longevity: a question of hydration

With the advancement of medical and investigative science, it is somewhat surprising that although it is possible to stabilise medical patients with hypertension and the associated kidney dysfunction, obesity, diabetes and even cancer, there is still no clear method of significantly reducing these chronic disease pathologies, and thus, extending life expectancy. There is one hormone common to these pathologies, the antagonism of which goes some way to clinical improvements, and this is angiotensin, which is released during hypovolaemia. Angiotensin antagonists are used to treat many of these pathologies, and it has been shown in the obesity literature that angiotensin antagonists decrease weight, but also increase the drinking of water. Increased cellular hydration, and hence, improved mitochondrial metabolism could be one of the mechanisms for the reduction in weight seen in these studies, as well as for reducing the other pathologies, all showing metabolic dysfunction. It appears that the application of straightforward physiological regulation might be an appropriate medical approach to the prevention of hypertension, kidney disease, obesity, diabetes and cancer, and thus, to an increased life expectancy.

Connecting tubule glomerular feedback antagonizes tubuloglomerular feedback in vivo

In vitro experiments showed that the connecting tubule (CNT) sends a signal that dilates the afferent arteriole (Af-Art) when Na(+) reabsorption in the CNT lumen increases. We call this process CNT glomerular feedback (CTGF) to differentiate it from tubuloglomerular feedback (TGF), which is a cross talk between the macula densa (MD) and the Af-Art. In TGF, the MD signals the Af-Art to constrict when NaCl transport by the MD is enhanced by increased luminal NaCl. CTGF is mediated by CNT Na(+) transport via epithelial Na(+) channels (ENaC). However, we do not know whether CTGF occurs in vivo or whether it opposes the increase in Af-Art resistance caused by TGF. We hypothesized that CTGF occurs in vivo and opposes TGF. To test our hypothesis, we conducted in vivo micropuncture of individual rat nephrons, measuring stop-flow pressure (P(SF)) as an index of glomerular filtration pressure. To test whether activation of CTGF opposes TGF, we used benzamil to block CNT Na(+) transport and thus CTGF. CTGF inhibition with the ENaC blocker benzamil (1 μM) potentiated the decrease in P(SF) at 40 and 80 nl/min. Next, we tested whether we could augment CTGF by inhibiting NaCl reabsorption in the distal convoluted tubule with hydrochlorothiazide (HCTZ, 1 mM) to enhance NaCl delivery to the CNT. In the presence of HCTZ, benzamil potentiated the decrease in P(SF) at 20, 40, and 80 nl/min. We concluded that in vivo CTGF occurs and opposes the vasoconstrictor effect of TGF.

Enhanced myogenic response in the afferent arteriole of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHRs) have normal glomerular capillary pressure even though renal perfusion pressure is higher, suggesting that preglomerular vessels exhibit abnormally high resistance. This may be due to increased superoxide (O(2)(-)) production, which contributes to the vasoconstriction in hypertension. We tested the hypothesis that the myogenic response of the afferent arteriole (Af-Art) is exaggerated in SHRs because of increased levels of reactive oxygen species (ROS). Single Af-Arts were microdissected from kidneys of SHRs and Wistar-Kyoto (WKY) rats and microperfused in vitro. When perfusion pressure in the Af-Art was increased stepwise from 60 to 140 mmHg, the luminal diameter decreased by 8.4 + or - 2.9% in WKY Af-Arts but fell by 29.3 + or - 5.6% in SHR Af-Arts. To test whether ROS production is enhanced during myogenic response in SHRs, we measured chloromethyl-dichlorodihydrofluorescein diacetate acetyl ester (CM-H(2)DCFDA) florescence before and after increasing intraluminal pressure from 60 to 140 mmHg. Pressure-induced increases in ROS were fourfold greater in SHR Af-Arts compared with WKY Af-Arts (SHR, 48.0 + or - 2.2%; and WKY, 12.2 + or - 0.3%). To test whether O(2)(-) contributes to the myogenic response in SHRs, either the membrane-permeant O(2)(-) scavenger Tempol or the nox2-based NADPH oxidase (NOX2) inhibitor gp91ds-tat were added to the Af-Art lumen and bath and the myogenic response was tested before and after treatment. Both Tempol (10(-4) M) and gp91ds-tat (10(-5) M) significantly attenuated the pressure-induced constriction in SHR Af-Arts but not in WKY Af-Arts. We conclude that 1) pressure-induced constriction is exaggerated in SHR Af-Arts, 2) NOX2-derived O(2)(-) may contribute to the enhanced myogenic response, and 3) O(2)(-) exerts little influence on the myogenic response under normotensive conditions.