renal hemodynamics in aki: in search of new treatment targets · 2017-08-15 · disorders for the...

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Renal Hemodynamics in AKI: In Search of NewTreatment Targets

Martin Matejovic,* Can Ince,† Lakhmir S. Chawla,‡ Roland Blantz,§ Bruce A. Molitoris,|

Mitchell H. Rosner,¶ Mark D. Okusa,¶ John A. Kellum,** and Claudio Ronco,††

the ADQI XIII Work Group

*First Medical Department and Biomedical Centre, Faculty of Medicine in Plzen, Charles University in Prague, TeachingHospital in Plzen, Plzen, Czech Republic; †Department of Intensive Care, Erasmus Medical Center University Hospital,Rotterdam, The Netherlands; ‡Department of Medicine, Division of Intensive Care Medicine and Division of Nephrology,Veterans Affairs Medical Center, Washington, DC; §Nephrology-Hypertension Division, University of California, SanDiego School of Medicine and Veterans Affairs San Diego Healthcare System, San Diego, California; |Department ofMedicine, Division of Nephrology and Department of Cellular and Integrative Physiology, Indiana University School ofMedicine and the Rouderbush Veterans Affairs Medical Center, Indianapolis, Indiana; ¶Division of Nephrology, Center forImmunity, Inflammation and Regenerative Medicine, University of Virginia Health System, Charlottesville, Virginia;**Center for Critical Care Nephrology and Department of Critical Care Medicine, University of Pittsburgh, Pittsburgh,Pennsylvania; and ††Department of Nephrology Dialysis and Transplantation, San Bortolo Hospital and the InternationalRenal Research Institute, Vicenza, Italy

ABSTRACTNovel therapeutic interventions are required to prevent or treat AKI. To expediteprogress in this regard, a consensus conference held by the Acute Dialysis QualityInitiative was convened in April of 2014 to develop recommendations for researchpriorities and future directions. Here, we highlight the concepts related to renalhemodynamics in AKI that are likely to reveal new treatment targets on investiga-tion. Overall, we must better understand the interactions between systemic, totalrenal, and glomerular hemodynamics, including the role of tubuloglomerularfeedback. Furthermore, the net consequences of therapeutic maneuvers aimedat restoring glomerular filtration need to be examined in relation to the nature,magnitude, and duration of the insult. Additionally, microvascular blood flowheterogeneity in AKI is now recognized as a common occurrence; timely interven-tions to preserve the renal microcirculatory flowmay interrupt the downward spiralof injury toward progressive kidney failure and should, therefore, be investigated.Finally, development of techniques that permit an integrative physiologic ap-proach, including direct visualization of renal microvasculature and measurementof oxygen kinetics and mitochondrial function in intact tissue in all nephronsegments, may provide new insights into how the kidney responds to variousinjurious stimuli and allow evaluation of new therapeutic strategies.

J Am Soc Nephrol 26: ccc–ccc, 2015. doi: 10.1681/ASN.2015030234

There is anunmetneed todevelop effectivetreatments aimed at decreasing the mor-bidity and mortality of AKI, which con-tinues to impose a substantial health careburden. AKI is a complex clinical syn-drome encompassing a wide spectrum ofetiologies, such as sepsis, volume deple-

tion, inflammation, ischemia-reperfusion,exogenous and endogenous toxins, sur-gery, obstruction, heart failure, and oth-ers. These various insults often coexistin a given patient, thus making it verychallenging to identify unique patho-physiologic mechanism(s) and universal

relevant therapeutic targets. The 13th AcuteDialysis Quality Initiative (ADQI) Consen-susConferencefocusedonidentifyingmajormechanisms and targets for therapeutics inAKI, and this paper highlights the work-group that specifically focused on hemody-namic mechanisms and targets in AKI. Theconference was held in Charlottesville,Virginia onApril 20, 2014. The consensuspanelmade recommendationson themajorgaps between incomplete understanding ofmechanisms that underlie AKI, target dis-covery, andviableAKItreatments.Given thewide heterogeneity of etiologies for AKI, forthe purpose of research and conceptualiza-tion of the role of renal hemodynamics inAKI, sepsis, ischemia, and nephrotoxicinsults were focused on as major model

Published online ahead of print. Publication dateavailable at www.jasn.org.

Correspondence: Dr. Mitchell H. Rosner, Divisionof Nephrology, Center for Immunity, Inflammationand Regenerative Medicine, University of VirginiaHealth System, Box 800133 HSC, Charlottesville,VA 22908. Email: [email protected]

Copyright © 2015 by the American Society ofNephrology

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mailto:[email protected]

mailto:[email protected]

disorders for the discussion of the path-ophysiology and treatment targets re-lated to renal macrocirculatory andmicrocirculatory alterations. We presentrecommendations for future researchto bridge gaps in knowledge as well aspotential targets for therapy, whichshould hopefully result in effective ther-apeutic interventions for AKI.

METHODS

The 13th ADQI Consensus Conferenceon Therapeutic Targets of Human AKIheld in Charlottesville, Virginia in Aprilof 2014 (www.adqi.net) was attended byan international group of experts andfocused on an objective scientific reviewof the current literature, developmentof a consensus of opinion, with evidencewhere possible, to distill current litera-ture, and articulate a research agenda toaddress important unanswered questions.Similar to other ADQI meetings, a mod-ified Delphi approach was followed.Details of the methods can be foundin the supplement of the introductionand summary by Okusa et al.1 in thisissue of the Journal of the AmericanSociety of Nephrology.

RESULTS

Total Renal Blood Flow in AKIGlobal renal ischemia caused by compro-mised kidney perfusion has traditionallybeen recognized as one of the most com-mon pathogenic factors leading to AKI inacutely ill patients. However, studies inlarge mammals, including humans, sug-gest thatglobal renalhypoperfusioncannotbe invoked as the sole etiology of AKI.2–5

The causative contribution of renal bloodflow (RBF) to kidney dysfunction is par-ticularly puzzling in sepsis, and there iswidespread disagreement as to whetherRBF is reduced, normal, or even in-creased.2,3,6–8 In the few large animaland clinical studies, the patterns ofRBF in AKI and its relation with sys-temic hemodynamics are highly vari-able.3,6 Absence of benefit of vasodilatortherapy in human sepsis–associated AKI9

may be explained by variability in bloodflow. Unfortunately, more specificknowledge about the relationship be-tween RBF and AKI in sepsis is limitedby the lack of reliable methods to moni-tor continuousRBF inhumanAKI.Never-theless, recent studies allow for severalimportant insights. First, renal circula-tory changes might behave differently insepsis with AKI as opposed to sepsisalone.3 Second, renal hemodynamics insubjects with sepsis and AKI cannot bepredicted reliably from systemic hemo-dynamics.3 Third, sepsis-associated AKImay be accompanied by dissociation be-tween systemic and renal vascular resis-tance. These differences in systemic andrenal vascular resistances may relate toboth species differences and most im-portantly, the time periods of observa-tions after the induction of sepsis.3,6

Regardless the pattern of total RBF, keyquestions are whether the interplay be-tween the changes in global renal flow,renal oxygenation, and function is caus-ally linked, just epiphenomena, or evenprotective in most common types ofAKI (i.e., sepsis and surgery) and whattheir role is in different phases of AKI(i.e., early versus established AKI). Ofcritical importance is that an isolateddetermination of RBFoutside the contextof the complex interaction with glomer-ular and peritubular microcirculationand the tissue energy state may not berepresentative of downstream processesandmay downplay the existence of wide-spread, albeit patchy renal tissue ische-mia (see below). Thus, total RBF is oflimited value in predicting AKI, and itscausative role in AKI is difficult to deter-mine given the complexities of the renalmicrocirculation.

Renal MicrocirculationGlomerular Hemodynamics in AKIThe reduction in GFR is a hallmark ofAKI. The precise knowledge of glomer-ular hemodynamics and its determinantsduring AKI may have therapeutic impli-cations. Animal studies show that, insevere prerenal insults (e.g., profoundhypovolemia, low BP, and reduced car-diac output), there is a homogenous andintense reduction in single-nephron

GFR in virtually all nephrons, primarilyas a result of reduced renal plasma flow(RPF) and Kf.10 In contrast to this sce-nario, glomerular dynamics in thecourse of septic AKI are controver-sial.7,8,11 The widely held concept of afall in transcapillary hydraulic pressurecaused by afferent arteriolar vasocon-striction leading to the reduction inGFR in sepsis has been largely derivedfrom rodents challenged with large en-dotoxin bolus.7,8 Recent experimentaldata from models of hyperdynamic sep-sis in sheep questioned this paradigm,and data introduced the concept of de-creased rather than increased glomeru-lar vascular resistance, at least in earlysepsis in ruminants.11,12 Although thedecline in GFR is mainly attributableto a profound lowering of the intraglo-merular capillary pressure, the potentialcontributions of elevation of Bowman’spressure secondary to tubular obstruc-tion and/or increased tubular flow inhyperfiltrating nephrons, impaired hy-draulic permeability, and reduction of fil-tration surface area should be addressedby other studies in relevant animalmodels.Collectively, it is conceivable that a com-parison of determinants of hypofiltrationamong progressors and nonprogressors ofAKI as well as precise (semi)continuousmonitoring of changes in GFR over ex-tended periods of time should allowmore sensitive investigations of the roleof intrarenal hemodynamics during thedevelopment of AKI.

Regardless of the nature of glomerularloss of filtration capacity, another keyquestion remains of what the net conse-quence is of strategies aimed at enhancingGFR. The reversal of functional vasocon-striction and restoration of GFR withhemodynamic optimization strategiesmay be beneficial during early, transientprerenal events, such as volume depletionand lowBP, because the reduction inRPFis caused by systemic or extrarenalevents. In most cases, if systemic alter-ations can be corrected, the RPF and GFRin the kidney will be improved or nor-malized, because this is a normal renalresponse to systemic changes. However,during AKI with tubular epithelial cellinjury and vasoconstriction, the net

2 Journal of the American Society of Nephrology J Am Soc Nephrol 26: ccc–ccc, 2015


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effects of a hemodynamic optimizationapproach are far less straightforward.This renal vasoconstriction is in re-sponse to intrarenal events, such astubular injury, and mediated largely bytubuloglomerular feedback activation.Under these conditions, such reductionin GFRmay contribute to a form of renalhibernation that acts to protect injuredtubules from further injury by reducingthe filtered load and thereby, the reab-sorptive workload. Studies performed inrat models of renal ischemia and reper-fusion have shown thatmaneuvers aimedat ameliorating the decrease in GFR may,in fact, produce more tubular damage inthe long run.13,14 In addition, transgenicmice lacking adenosine A1 receptors,through which tubuloglomerular feed-back is modulated, were more sensitiveto ischemia-reperfusion injury.15 Thus,although restoration of RBF and GFRmight be a logical strategy in early prere-nal states, hypofiltration mediated by thepersistence of the tubuloglomerularfeedback system may be protective inthe presence of significant renal in-jury.16 It must be stressed, however, thatthe importance of tubuloglomerular feed-back in mediating the link between prom-inent decline in GFR and focal tubularinjury has been suggested for ischemia-reperfusion and nephrotoxic AKI17,18 butremains unproven in models of septic AKIand clinical AKI caused by multiple in-sults. Finally, internephron heterogeneity,the focal distribution of lesions and acces-sibility limited to a subgroup of superfi-cial nephrons, should be kept in mindwhen interpreting the results of micro-puncture techniques. Combinations ofmicropuncture with new imagingtechniques, such as multiphoton mi-croscopy, may offer new opportunitiesin studying real–time acute renal patho-physiology.19 In summary, uncer-tainty about the nature, time course,and magnitude of glomerular hemo-dynamics, its regulation, and how itcan be effectively modulated repre-sents an important gap of knowledgethat may hinder the appropriate designof hemodynamically oriented clinicaltrial. The relationship and timing ofglomerular hemodynamic changes to

evidence of major tubular damagemust also be better correlated and un-derstood. It has been shown repeatedlythat tubuloglomerular feedback adaptstemporally, and the reduction of RBFduring acute tubular injury may not bepersistent and may only be during theacute injury phase. Clearly, the totalreduction in GFR cannot be explainedentirely by renal vasoconstriction andreductions in RPF rates. Other factors,including reduced glomerular perme-ability coefficient, tubular backleak ofsolutes, and tubular obstruction, remaincritical factors to the total reduction inGFR. In fact, renal vasoconstriction maybe transient and an appropriate re-sponse to tubular injury, inducing aform of renal hibernation or protectionof injured tubules. Absolute reductionsin RBF are probably more dramatic andquantitatively significant during prere-nal states, which one encounters in se-vere volume depletion or congestiveheart failure, although the latter hasnot been proven unequivocally. Again,species- and model-specific differ-ences in glomerular dynamics mustseriously be taken into considerationwhen extrapolating the findings tohumans.

Peritubular Microcirculation in AKIThe postglomerular peritubular micro-vasculature has been increasingly recog-nizedasacrucial compartment indifferentforms ofAKI.20–22 Both ischemia and sep-sis have profound effects on the integrityand function of peritubular capillaries.23

Rodent models of ischemia-reperfusionshow compromised peritubular perfusioncharacterized by sluggish and retrogradeblood flow that develops within minutesafter reperfusion24,25 followed by restora-tion of normal flow within the first4 hours, only to dramatically worsen overthe next 20 hours.21,26 Similarly, rodentmodels of acute endotoxemia suggestthat cortical peritubular capillaries areamong the first renal structures in-jured.27–30 Interestingly, despite an appar-ent full recovery of renal function at48 hours, functional capillary density re-covered only partially.27 Moreover, areasof compromised cortical microvascular

perfusion have been shown to correlatewith renal tubular cell stress in corre-sponding regions, suggesting importantlinks between altered peritubular micro-circulation and epithelial cell dysfunc-tion.27 These findings were corroboratedby a study by Gupta et al.,31 in whichquantitative two–photon intravital mi-croscopy revealed markedly reducedperitubular capillary blood flow in anendotoxemia model in rats. Addition-ally, renal ischemia and reperfusionwere associated with reduced capillaryblood flow and loss of glycocalyx integ-rity in human kidney transplantation.32

The resulting tubular hypoxic stressserves as a robust stimulus for the ampli-fication of local inflammatory and profi-brotic factors, oxidative stress, andpossibly, adaptive cellular metabolicdownregulation with reprioritizationof energy pathways because of hyp-oxia.33–35 These data collectively supportthe emerging evidence that tubular hyp-oxia and inflammation resulting from re-nal microvascular dysfunction are criticalcontributing steps in the progression ofAKI. Peritubular capillaries are not onlyamong the first structure affected dur-ing an acute insult, but also, their dam-age may determine both functional andstructural reversibility of AKI. Indeed,microvascular injury may persist evenafter resolution of the initial insult, re-sulting in chronic microvascular alter-ations, fibrogenesis, and rarefaction ofcells.36 In fact, persistent peritubularcapillary failure and subsequent mi-crovascular dropout predispose survi-vors of AKI to development of CKDand increase their risk for recurrentAKI.36,37

Mechanisms of Peritubular CapillaryDysfunctionThe unique microvascular architectureof the kidney (Figure 1) and its role as aset of resistors in series as well as in par-allel make the study of renal microvas-culature challenging but essential tounderstanding the pathogenesis ofAKI. Having both series and parallel com-ponents allows for continued flow if mi-crovascular obstruction or constrictionoccurs in certain areas. This likely is the

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underlying reason for the focal natureof cortical injury during ischemia-reperfusion and injury during sepsis.The mechanisms that contribute to peri-tubular capillary dysfunction in both ische-mia and sepsis may be multifactorial.Because peritubular capillaries are derivedfrom the efferent glomerular arterioles,any disturbance of glomerular bloodflow will impair peritubular perfusion.In addition, because efferent arterioles inthe middle and inner cortex of the kidneysupply not only their parent nephron,zonal peritubular ischemia involvingseveral adjacent nephrons might occur.Furthermechanisms includedirect inflam-matory injury of endothelial cells andactivation of the epithelial-endothelialaxis by inflammatory signals releasedfrom tubular cells exposed to toxicity offiltered danger signals.23 As a result, thebalance is strongly tipped toward in-creased microvascular permeability, en-dothelial cell inflammation, imbalance

betweenvasodilating and vasoconstrictingfactors, and activation of coagulation.The intricate molecular pathwayswithin the local microenvironment ofendothelial cells fall outside the scopeof this work, and the reader is referredto recent in–depth reviews.23,38

Interestingly, microcirculatory perfu-sion defects are not uniform throughoutthe kidney during acute injury. Persistentperfusion deficits and diminished tissueoxygenation have been shown to be ofgreater magnitude in the highly vulnerableoutermedulla comparedwith thecortex inavariety of experimental models, includingtotal ischemia, radiocontrast nephropathy,and nonsteroidal anti–inflammatory drug–and other drug–induced AKI etiologies, in-cluding fluid therapy.23,39 The renal cortexmicrocirculation is significantly com-promised in sepsis models3,40 and hu-mans,41 where inhomogeneous patchyareas of microischemia can occur (Fig-ure 2). These heterogeneous areas of

hypoxia and normal oxygenation de-fine the nature of hemodynamic alter-ations leading to inflammation, becauseit is expected that there is increasedreactive oxygen species production asso-ciated with hypoxia-normoxia interac-tions.42 The increased incidence of AKIin patients with CKD is likely, in part,because of the existing underlying mi-crovascular alterations and marginalareas of oxygenation existing in pa-tients with CKD before the new ische-mic or septic event.23 In endotoxemicrats, microvascular hypoxic areas wereidentified using a partial pressure of ox-ygen (pO2) histogram analysis. How-ever, mean microcirculatory pO2showed no changes; a demonstrationof the overall heterogeneity of ischemiais in Johannes et al.43 Similar resultswere obtained in an ischemia-reperfusionmodel of AKI, where mean tissue pO2measured using oxygen electrodesshowed no change, whereas the hypoxia–sensitive pimonidazole adduct immu-nohistochemistry was able to identifymicroheterogenous hypoxic areas.44

In addition to these effects, the patchydistribution of microvascular changesmight affect both the glomerular andperitubular circulations. As a result,zones of nephrons with glomerular hy-pofiltration and peritubular capillaryocclusion might coexist with clustersof normal or even hyperfiltrating at–risk nephrons. Hypoxic zones mightcoexist with intact regions with pre-served tissue pO2. Hypothetically, thehypoxic nephrovascular units may sig-nal surrounding functional nephronsin a paracrine but undefined fashion,thereby disseminating the injuriousstimulus. In summary, such hypoxicareas in the cortex have been shown tocoexist in the presence of normal or evensupranormal arterial RBF. These find-ings have important consequences forclinical monitoring, because diagnostictools will have to be developed that allowfor simultaneous and dynamic determi-nation of total RBF, differential zoneflow, and tissue oxygenation.45 Withoutthis, we will find it very difficult to trans-late preclinical data into therapeutic suc-cess given the heterogeneity of the disease

Figure 1. (A) Schematic representation of the microvasculature within the kidney. Thecoloredgradient represents theoxygen tension inmmHgacross thekidney.Note the lackofarterial blood in the outermedullary region. AA, afferent arteriole; EF, efferent arteriole; IA,interlobular artery; IM, innermedulla; ISOM, inner stripeof theoutermedulla;OSOM,outerstripeof theoutermedulla.Modified fromworkbyAird,60withpermission. (B)Drawings andthree–dimensional two–photon images of glomerular and cortical tubular areas. Glomer-ular: Intravial two–photon three–dimensional micrograph showing fluorescent albumin(yellow) localized within the capillary loops of a glomerulus (center) and the microvasculaturesurrounding proximal tubules (PTs). Peritubular: Intravital two–photon three–dimensional mi-crograph showing a large molecular weight fluorescent dextran (red) localized within the peri-tubular capillaries (red) surrounding both PTs and distal tubules (DTs). Hoechst 33342labels the nuclei of all cells types (cyan), with brighter binding occurring within the DTs.Note that the space between tubular structures, known as the interstitial space, islargely filled by the peritubular microvasculature under physiologic conditions. Scalebar, 20 mm.



syndrome. Currently, various functionalmagnetic resonance imaging (MRI) tech-niques (e.g., cine phase–contrast MRI, ar-terial spin labeling, and blood oxygenlevel–dependent MRI) and to some ex-tent, contrast-enhanced ultrasound arethe only clinically available research toolsallowing noninvasive quantification andassessment of renal perfusion and intra-organ flow distribution during AKI.46

Although compromised renal peri-tubular microcirculation seems to have acentral and possibly causative role in thepathogenesis of AKI, often profound re-duction in GFR is unlikely to be solely aneffect of the focal nature of microvascularchanges, indicating a central role for alteredglomerular hemodynamics. Hypotheti-cally, increased filtration of danger signalsin areas of initially preserved nephrovas-cular units can lead to subcellular renaltubular cell injury/activation, thus drivingthe activation of the tubuloglomerularfeedback mechanism. However, there isno clear proof showing the traffickingbetween tubular and glomerular compart-ments linking tubular function to renalhemodynamics in human AKI. Along thesame vein, a question remains whetherthe renal microvascular alterations arealways the primary and predominantevent leading to tubular stress and in-jury.47 Alternatively, tubular cell injurymight trigger reflex microvascular vaso-constriction at the single-nephron level.This reasoning is supported by observa-tions derived from models of single–nephron tubular obstruction48 andlocalized microinjection of Escherichiacoli into early proximal tubule lu-mens.49–51 Therefore, there seems tobe a bidirectional and possibly syner-gistic crosstalk within the complextubule-capillary microenvironment,with the sequence of events dependingon the nature of insult.23 Although mi-crocirculatory changes may lead totubular injury, the reverse may alsobe true.

Targets to Protect theNephrovascular UnitsThe maintenance of peritubular micro-circulation, thus, seems to be an impor-tant therapeutic target to improve the

Figure2. (A)Histopathologyof humankidneybiopsy from thecortical area in apatientwithAKI. Note the enlarged interstitial space with markedly reduced peritubular capillaries,ongoing cellular plugging of existing capillaries, deteriorating tubules, and an ischemicshrunken glomerulus. This is a later stage of ischemic AKI likely resulting in microvasculardropout,CKDdevelopment, and/oraccelerationofCKDprogression.Reprinted fromSteve



acute and chronic renal outcomes inpatients with AKI. However, multipleknowledge gaps need to be addressed toimprove our capacity to transfer basicscience knowledge into clinical practice.Many of pathways implicated in renalmicrovascular failure may serve dualroles—damaging endothelial and tubu-lar cells acutely but triggering adaptiveresponse or even enhancing tissue repairlater. Future studies must dissect thesedual mechanisms. Furthermore, limitedblood flow can modulate downstreammolecular mechanisms (evoking localinflammatory response) or act directlyas executioners of cell death. Likewise,augmenting the blood flowmay increaseinflux of danger molecules and amplifythe danger response. Dissection ofwhich phenomenon prevails in whichphase of AKI might dictate the treat-ment efficacy. In addition, any thera-peutic intervention aimed at restoringglomerular perfusion and filtrationmay be counterproductive unless ac-companied by the restoration of peri-tubular circulation and oxygenation.We also need to better understand the ef-fect and mechanisms of aging and majorcomorbidities on renal vascular vulnerabil-ity to insults and how these mechanismsaffect the effectiveness of novel therapies.Another recognized problem is that themajority of animal models copy neitherthe entire spectrum of clinical AKI (com-bination of relevant insults; e.g., hypovole-mia, sepsis, and toxicity) nor relevant AKIpopulation (predisposing comorbidities).

Therapeutic strategies that target themicrovasculature in AKI can be broadlydivided into several categories (Table 1).Oxidative stress–mediated cellular injuryhas been implicated in the pathogenesisof AKI as a central element of the networkof danger response, affecting bothendothelial and epithelial functions aswell as cellular energetics.33 Therapeuticinterventions aimed at maintaining

the homeostatic balance between oxygenradicals, nitric oxide, and oxygen havebeen shown to be effective in severalstudies in rodent models of septicAKI27–31 and ischemia-reperfusion in-jury52 as well as in large animal modelsof septic AKI.53,54 The lack of effectiveinducible nitric oxide synthase inhibi-tors for clinical use can be consideredan important area for research.

Recent discoveries of the critical physi-ologic role of endothelial glycocalyx incontrolling vascular permeability and lim-iting interaction between endothelium andcirculating cells as well as its role in prop-agating inflammation and tissue edema ifinjured give hope for the development ofnew strategies aimed at enhancement ofvascular integrity and amelioration of vas-cular leak in AKI.55 Another emerging pos-sibility to act therapeutically on endothelialvascular integrity involves Toll–like recep-tor 4–dependent activation of endothelialcells56 and modulation thrombomodulin–driven pathways.57 From the long-termperspective, postinsult recovery of the peri-tubular capillary network may preventchronic tubulointestinal hypoxia and fibro-sis.58 Therefore, the therapeutic potential ofstrategies targeting vascular repair and re-generation, such as angiogenic factors, andstem or progenitor cell populations is wor-thy of further preclinical testing.

In addition to identifying new treat-ment targets, there is also a pressingneed to assess more completely the con-sequences of current therapeutics. Mostinterventions that are used in the resus-citation of critically ill patients mayinfluence the renal microcirculation in adeleterious manner. For example, theexact mechanisms by which differentfluid compositions (such as normal salineand colloid or buffered solutions) affectrenal microcirculation are not well un-derstood, and emerging experimentaland clinical data suggest that use of in-appropriate fluids or fluid replacement

strategies may exert deleterious effects onthe microcirculation and renal out-comes.40,59 Strategies allowing real-timeevaluation of microcirculatory fluid andvasopressors responsiveness might be ex-pected to optimize resuscitation effective-ness while limiting the potential to causeharm.

Research AgendaWe have developed the following re-search agenda in relation to the under-standing and treatment of AKI from arenal hemodynamic point of view. Issuesthat require further investigation arelisted.

(1) The identification of pathway(s) im-plicated in renal microvascular stressthat should be targeted for thera-peutic intervention (functional ver-sus structural changes).

(2) Understanding the nature, time course,and magnitude of glomerular hemo-dynamics and the effect of bidirec-tional tubuloglomerular feedbacksignal on injury and recovery in var-ious AKI models. Development ofanimal models depicting the phe-nomenon of uncoupling severe de-pression of glomerular filtration andlimited focal structural tubular celldysfunction.

(3) The identification of an optimal timewindow for interventions as well asdevelopment of novel real–time mon-itoring methods to ensure that emerg-ing therapies reach their specificcellular targets within the renalvasculature.

(4) An integrative preclinical approachinvolving simultaneous determinationof RBF, direct visualization of micro-circulation and oxygen kinetics, andassessment of mitochondrial func-tion and mediators involved in reg-ulation of renal hemodynamicsrepresents a comprehensive moni-toring platform to identify AKI, in-dividualize therapy, and follow theresponse to therapy. Imaging themicrocirculation will need to take

Bonsid (Nephropath, Little Rock, AK), with permission. (B) Shown is a speckle imagingperfusionmapof the surfaceof a rat kidney61 at baseline, during septic shock, andafterfluidresuscitation after a 30-minute delay. Fluid resuscitation corrected systemic hemodynamicvariables but induced heterogeneous areas of hypoperfusion in the renal cortex. Reprintedfrom Legrand et al.,40 with permission.



Table 1. Promising hemodynamic–based therapeutic targets to treat AKI

Alteration andTarget

Setting Mechanisms Potential Downside Stage

Global RBFMAP Shock states, sepsis Improved early renal perfusion Vasopressor load, medullary

hypoxia, tubular workloadTwo small clinical studies,

one phase 3 clinicalANP low dose Cardiac surgery Improved renal perfusion Ineffective in late AKI,

hypotension in large doseOne small RCT

Renal (de)congestion Heart failure, sepsis,ICU AKI

Improved renal perfusion,tissue edema

Induction of prerenalresponse

Observational trials

GlomerularhemodynamicsSelective renal

adenosine1 receptorsagonists

I/R, sepsis AKI Activation oftubuloglomerularfeedback, prevention oftubular cell danger loadand medullary hypoxia

Reduced cortical perfusion Mice I/R, sepsis AKI

Selective renaladenosine1 receptorsantagonists

Radiocontrast AKI,nephrotoxic AKI

Suppression oftubuloglomerularfeedback, improved GFRand postglomerularperfusion

Medullary hypoxia, tubularworkload

Small animals studies

Angiotensin II Early hyperemicsepsis AKI

Increased glomerularfiltration pressure

Systemic effects, tubulardanger load, renal ischemia

Sheep hyperdynamic sepsisand AKI

Vasopressin/terlipressin

Early sepsis AKI Increased glomerularfiltration pressure

Systemic effects, tubulardanger load, renal ischemia

Small clinical, post hoc RCTanalysis (VASST),preclinical large animals

Glomerularinflammation

Sepsis AKI Glomerular endothelialprotection

Ineffectiveness of anti-TNFstrategies in human trials

TNFR1 knockout mice

PeritubularmicrocirculationNOS-targeted

therapy (e.g.,iNOS inhibition,eNOSpreservation)

Sepsis AKI, I/R Preserved microvascularperfusion, suppression oflocal inflammation,oxidative stress andbioenergetic failure

Unclear timing, bothdamaging and protectiveconsequences

Preclinical studies, limitedhuman evidence

RNOS-targetedtherapy

Sepsis AKI, I/R Preserved microvascularperfusion, suppression oflocal inflammation andbioenergetic failure

Unclear timing, variety ofdrugs, both damaging andprotective consequences

Preclinical studies

IkB kinase inhibition Sepsis AKI Attenuation of iNOS whileincreasing eNOS

Biphasic roles—limitinginflammation early, limitingrecovery later on?Proapoptotic effects?

Mice sepsis andendotoxemia

Endothelin-1antagonism

I/R, sepsis,progression to CKD

Improved renalmicrocirculation, reductionin oxidative stress andinflammation

Role of other receptors andtheir distribution unknown

Mouse I/R, porcineendotoxemia

Vascular integrityToll–like receptors 4

manipulationI/R, sepsis, inflammation Limiting DAMPS–induced

vascular injuryBiphasic roles—limiting

inflammation early, limitingrecovery later on? Immunesuppression?

Small animal I/R, Tx,endotoxemia

S1P1R agonists Prevention of I/R Prevention of endothelialbarrier dysfunction

Unclear Human trial in Tx ongoing,mice I/R, cell culture



into account the heterogeneity of themicrocirculation and its dysfunction.

(5) New technologies that are mostlikely to improve our understandingin the clinical research setting in-cludefunctional MRI (allowing non-invasive quantification and assessmentof intraorgan distribution of renal per-fusion and oxygen kinetics during AKI)and contrast-enhanced ultrasound.

(6) Because of the inaccessibility of thekidney under clinical conditions, fu-ture research will need to determineto what extent distant microcircu-latory alterations (e.g., sublingualmicrocirculation) reflect microcir-culatory defects occurring in the kidney.

(7) To bridge the gap between preclin-ical and clinical studies, bettermodels of AKI that recapitulate thecomplex nature of human AKI, in-cluding myriad etiologies, are needed.Mechanistic studies are required to

better understand how comorbidities(such as CKD) alter the renal hemo-dynamic response, enhance the in-trinsic susceptibility of the (micro)vascular system to insults, and affectthe effectiveness of novel therapies.

ACKNOWLEDGMENTS

Insights related to this paper have been

obtained with Dutch Kidney Foundation

Grants C09-2290 (to C.I.) and 14OIP11 (to

C.I.). M.M. was supported by National

Sustainability Program INo. LO1503 and by

the project No. CZ.1.07/2.3.00/30.0061 co-

financed by the European Social Fund and

the state budget of the Czech Republic.

A complete list of participants is provided

in Supplemental Appendix.

DISCLOSURESM.M., L.S.C., R.B., B.A.M.,M.H.R.,M.D.O., and

C.R. report no conflicts of interest. C.I. has received

honoraria and independent research grants from

Fresenius-Kabi, Baxter Health Care, and AM

Pharma. C.I. has developed sidestream dark field

(SDF) imaging and is listed as inventor on related

patents commercialized by MicroVision Medical

(MVM) under a license from the Academic

Medical Center. C.I. has been a consultant for

MVM in the past but has not been involved with

this company for .5 years, except that he still

holds shares. Braedius Medical, a company

owned by a relative of C.I., has developed and

designed a handheld microscope called Cyto-

Cam-IDF (incident dark field illumination) im-

aging. C.I. has no financial relation with Braedius

Medical of any sort (i.e., never owned shares or

received consultancy or speaker fees from Brae-

dius Medical). J.A.K. has received consulting fees

from Abbott, Aethlon, Alere, Alung, AM Pharma,

Astute Medical, Atox Bio, Baxter, Cytosorbents,

venBio, Gambro, Grifols, Roche, Spectral

Diagnostics, Sangart, and Siemens. J.A.K. has

also received research grants from Alere, Astute

Medical, Atox Bio, Bard, Baxter, Cytosorbents,

Gambro, Grifols, Kaneka, and Spectral Diagnos-

tics and has licensed technologies through the

University of Pittsburgh to Astute Medical, Cyto-

sorbents, and Spectral Diagnostics.

Table 1. Continued

Alteration andTarget

Setting Mechanisms Potential Downside Stage

Vasculotrophicstrategies(VEGF, EPC, MSC,angiopoietin-1)

Microvascularregeneration

Limiting vascular dropout Safety limits? Small animal I/R

ENT inhibition I/R A2B adenosinereceptors–mediatedprevention ofpost–ischemic no reflowphenomenon

Unclear Mice I/R

Solublethrombomodulin

I/R Attenuated endothelialpermeability, cellularadhesion

Unclear Rat I/R

OthersSupplementedresuscitation

fluids(ethylpyruvateanalog)

Sepsis, shock,inflammation

Glycocalyx-protectinginterventions, antioxidanteffects

Unclear Small animal sepsis

HIF activators I/R Tolerance to tissue hypoxia Only pretreatment?Risk of fibrogenesis?

Small animal models

Because multiple mechanisms are involved in the development of microvascular dysfunction, it is unlikely that a single-pathway intervention would be effective.Evaluation of effectiveness of combined therapies compared with single therapy and determination of the optimal timing of the components of combinationtherapy are required. MAP, mean arterial pressure; ANP, atrial natriuretic factor; NOS, nitrogen oxygen species; iNOS, inducible nitric oxide synthase; eNOS,endothelial nitric oxide synthase; RNOS, reactive nitrogen oxygen species; IkB, inhibitor of kappa B kinase; S1P1R, sphingosine-1 phosphate 1 receptor; VEGF,vascular endothelial growth factor; EPC, endothelial progenitor cell; MSC,mesenchymal stem cell; ENT, equilibrative nucleoside transporter; HIF, hypoxia-inducedfactor; ICU, intensive care unit; I/R, ischemia-reperfusion; DAMPS, damage-associated molecular pattern molecules; A2B, adenosine A2B receptor;RCT, randomized controlled trial; VASST, vasopressin and septic shock trial; TNFR1, tumor necrosis factor receptor 1; Tx, transplantation.



http://jasn.asnjournals.org/lookup/suppl/doi:10.1681/ASN.2015030234/-/DCSupplemental

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