Sodium reabsorption and distribution of Na+/K+-ATPase during postischemic injury to the renal allograft

Human Kidney Proximal Tubule-Microvascular Model Facilitates High-Throughput Analyses of Structural and Functional Effects of Ischemia-Reperfusion Injury

Kidney ischemia reperfusion injury (IRI) poses a major global healthcare burden, but effective treatments remain elusive. IRI involves a complex interplay of tissue-level structural and functional changes caused by interruptions in blood and filtrate flow and reduced oxygenation. Existing in vitro models poorly replicate the in vivo injury environment and lack means of monitoring tissue function during the injury process. Here, a high-throughput human primary kidney proximal tubule (PT)-microvascular model is described, which facilitates in-depth structural and rapid functional characterization of IRI-induced changes in the tissue barrier. The PREDICT96 (P96) microfluidic platform's user-controlled fluid flow can mimic the conditions of IR to induce pronounced changes in cell structure that resemble clinical and in vivo phenotypes. High-throughput trans-epi/endo-thelial electrical resistance (TEER) sensing is applied to non-invasively track functional changes in the PT-microvascular barrier during the two-stage injury process and over repeated episodes of injury. Notably, ischemia causes an initial increase in tissue TEER followed by a sudden increase in permeability upon reperfusion, and this biphasic response occurs only with the loss of both fluid flow and oxygenation. This study demonstrates the potential of the P96 kidney IRI model to enhance understanding of IRI and fuel therapeutic development.

Sacubitril/valsartan ameliorates renal tubulointerstitial injury through increasing renal plasma flow in a mouse model of type 2 diabetes with aldosterone excess

BACKGROUND

Aldosterone has been assumed to be one of aggravating factors in diabetic kidney disease (DKD). Natriuretic peptides/guanylyl cyclase-A/cGMP signalling has been shown to ameliorate aldosterone-induced renal injury in mice. Sacubitril/valsartan (SAC/VAL) is used clinically for chronic heart failure and hypertension, in part by augmenting natriuretic peptide bioavailability. The effects of SAC/VAL on renal pathophysiology including in DKD, however, have remained unclarified.

METHODS

Eight-week-old male db/db mice fed on a high-salt diet (HSD) were treated with vehicle or aldosterone (0.2 μg/kg/min), and divided into four groups: HSD control, ALDO (aldosterone), ALDO + VAL (valsartan), and ALDO + SAC/VAL group. After 4 weeks, they were analysed for plasma atrial natriuretic peptide (ANP) levels, renal histology, and haemodynamic parameters including glomerular filtration rate (GFR) by FITC-inulin and renal plasma flow (RPF) by para-amino hippuric acid.

RESULTS

The ALDO + SAC/VAL group showed significantly increased plasma ANP concentration and creatinine clearance, and decreased tubulointerstitial fibrosis and neutrophil gelatinase-associated lipocalin expression compared to ALDO and ALDO + VAL groups. SAC/VAL treatment increased GFR and RPF, and suppressed expression of Tgfb1, Il1b, Ccl2, and Lcn2 genes compared to the ALDO group. The percentage of tubulointerstitial fibrotic areas negatively correlated with the RPF and GFR.

CONCLUSION

In a mouse model of type 2 diabetes with aldosterone excess, SAC/VAL increased RPF and GFR, and ameliorated tubulointerstitial fibrosis. Furthermore, RPF negatively correlated well with tubulointerstitial injury, suggesting that the beneficial effects of SAC/VAL could be through increased renal plasma flow with enhanced natriuretic peptide bioavailability.

Elevations in sweat sodium concentration following ischemia-reperfusion injury during passive heat stress

Renal ischemia-reperfusion (I/R) injury results in damage to the renal tubules and causes impairments in sodium [Na+] reabsorption. Given the inability to conduct mechanistic renal I/R injury studies in vivo in humans, eccrine sweat glands have been proposed as a surrogate model given the anatomical and physiological similarities. We tested the hypothesis that sweat Na+ concentration is elevated following I/R injury during passive heat stress. We also tested the hypothesis that I/R injury during heat stress will impair cutaneous microvascular function. Fifteen young healthy adults completed ∼160 min of passive heat stress using a water-perfused suit (50°C). At 60 min of whole body heating, one upper arm was occluded for 20 min followed by a 20-min reperfusion. Sweat was collected from each forearm via an absorbent patch pre- and post-I/R. Following the 20-min reperfusion, cutaneous microvascular function was measured via local heating protocol. Cutaneous vascular conductance (CVC) was calculated as red blood cell flux/mean arterial pressure and normalized to CVC during local heating to 44°C. Na+ concentration was log-transformed and data were reported as a mean change from pre-I/R (95% confidence interval). Changes in sweat sodium concentration from pre-I/R differed between arms post-I/R (experimental arm: +0.97 [+0.67 - 1.27] [LOG] Na+; control arm: +0.68 [+0.38 - 0.99] [LOG] Na+; P < 0.01). However, CVC during the local heating was not different between the experimental (80 ± 10%max) and control arms (78 ± 10%max; P = 0.59). In support of our hypothesis, Na+ concentration was elevated following I/R injury, but likely not accompanied by alterations in cutaneous microvascular function.NEW & NOTEWORTHY In the present study, we have demonstrated that sweat sodium concentration is elevated following ischemia-reperfusion injury during passive heat stress. This does not appear to be mediated by reductions in cutaneous microvascular function or active sweat glands, but may be related to alterations in local sweating responses during heat stress. This study demonstrates a potential use of eccrine sweat glands to understand sodium handling following ischemia-reperfusion injury, particularly given the challenges of in vivo studies of renal ischemia-reperfusion injury in humans.

Renal Hemodynamics, Function, and Oxygenation in Critically Ill Patients and after Major Surgery

This review outlines the available data from the work of our group on renal hemodynamics, function, and oxygenation in patients who are critically ill with acute renal dysfunction, such as those with postoperative AKI, those in early clinical septic shock, in patients undergoing cardiac surgery with cardiopulmonary bypass, or in patients undergoing liver transplantation. We also provide information on renal hemodynamics, function, and oxygenation in patients with chronic renal impairment due to congestive heart failure. This review will argue that, for all of these groups of patients, the common denominator is that renal oxygenation is impaired due to a lower renal oxygen delivery or a pronounced increase in renal oxygen consumption.

Intrarenal oxygenation determines kidney function during the recovery from an ischemic insult

Acute kidney injury (AKI) is a significant clinical problem associated with poor outcome. The kidney, due to its inhomogeneous blood flow, is particularly susceptible to changes in oxygen delivery, and intrarenal hypoxia is a hallmark of AKI and progression to chronic kidney disease. However, the role of intrarenal hypoxia per se in the recovery from an ischemic insult is presently unclear. The present study was designed to investigate 1) the role of systemic hypoxia in the acute progression and recovery of AKI and 2) whether increased intrarenal oxygenation improves recovery from an ischemic insult. Anesthetized male Sprague-Dawley rats were subjected to unilateral warm renal ischemia for 45 min followed by 2 h of reperfusion under systemic hypoxia (10% inspired oxygen), normoxia (21% inspired oxygen), or hyperoxia (60% inspired oxygen). Intrarenal oxygen tension was successfully manipulated by altering the inspired oxygen. Glomerular filtration rate (GFR) before the ischemic insult was independent of intrarenal oxygen tension. GFR during the recovery from the ischemic insult was significantly lower compared with baseline in all groups (3 ± 1%, 13 ± 1%, and 30 ± 11% of baseline for hypoxia, normoxia, and hyperoxia, respectively). However, GFR was significantly higher in hyperoxia than hypoxia (P < 0.05, hypoxia vs. hyperoxia). During recovery, renal blood flow was only reduced in hyperoxia, as a consequence of increased renal vascular resistance. In conclusion, the present study demonstrates that renal function during the recovery from an ischemic insult is dependent on intrarenal oxygen availability, and normobaric hyperoxia treatment has the potential to protect kidney function.

Absence of renal hypoxia in the subacute phase of severe renal ischemia-reperfusion injury

Tissue hypoxia has been proposed as an important event in renal ischemia-reperfusion injury (IRI), particularly during the period of ischemia and in the immediate hours following reperfusion. However, little is known about renal oxygenation during the subacute phase of IRI. We employed four different methods to assess the temporal and spatial changes in tissue oxygenation during the subacute phase (24 h and 5 days after reperfusion) of a severe form of renal IRI in rats. We hypothesized that the kidney is hypoxic 24 h and 5 days after an hour of bilateral renal ischemia, driven by a disturbed balance between renal oxygen delivery (Do₂) and oxygen consumption (V̇o₂). Renal Do₂ was not significantly reduced in the subacute phase of IRI. In contrast, renal V̇o₂ was 55% less 24 h after reperfusion and 49% less 5 days after reperfusion than after sham ischemia. Inner medullary tissue Po₂, measured by radiotelemetry, was 25 ± 12% (mean ± SE) greater 24 h after ischemia than after sham ischemia. By 5 days after reperfusion, tissue Po₂ was similar to that in rats subjected to sham ischemia. Tissue Po₂ measured by Clark electrode was consistently greater 24 h, but not 5 days, after ischemia than after sham ischemia. Cellular hypoxia, assessed by pimonidazole adduct immunohistochemistry, was largely absent at both time points, and tissue levels of hypoxia-inducible factors were downregulated following renal ischemia. Thus, in this model of severe IRI, tissue hypoxia does not appear to be an obligatory event during the subacute phase, likely because of the markedly reduced oxygen consumption.

Effects of Cardiopulmonary Bypass on Renal Perfusion, Filtration, and Oxygenation in Patients Undergoing Cardiac Surgery

Background

Acute kidney injury is a common complication after cardiac surgery with cardiopulmonary bypass. The authors evaluated the effects of normothermic cardiopulmonary bypass on renal blood flow, glomerular filtration rate, renal oxygen consumption, and renal oxygen supply/demand relationship, i.e., renal oxygenation (primary outcome) in patients undergoing cardiac surgery.

Methods

Eighteen patients with a normal preoperative serum creatinine undergoing cardiac surgery procedures with normothermic cardiopulmonary bypass (2.5 l · min−1 · m−2) were included after informed consent. Systemic and renal hemodynamic variables were measured by pulmonary artery and renal vein catheters before, during, and after cardiopulmonary bypass. Arterial and renal vein blood samples were taken for measurements of renal oxygen delivery and consumption. Renal oxygenation was estimated from the renal oxygen extraction. Urinary N-acetyl-β-d-glucosaminidase was measured before, during, and after cardiopulmonary bypass.

Results

Cardiopulmonary bypass induced a renal vasoconstriction and redistribution of blood flow away from the kidneys, which in combination with hemodilution decreased renal oxygen delivery by 20%, while glomerular filtration rate and renal oxygen consumption were unchanged. Thus, renal oxygen extraction increased by 39 to 45%, indicating a renal oxygen supply/demand mismatch during cardiopulmonary bypass. After weaning from cardiopulmonary bypass, renal oxygenation was further impaired due to hemodilution and an increase in renal oxygen consumption, accompanied by a seven-fold increase in the urinary N-acetyl-β-d-glucosaminidase/creatinine ratio.

Conclusions

Cardiopulmonary bypass impairs renal oxygenation due to renal vasoconstriction and hemodilution during and after cardiopulmonary bypass, accompanied by increased release of a tubular injury marker.

Cerebral Edema in a Child after Preemptive Kidney Transplantation

Dialysis disequilibrium syndrome (DDS) is characterized by acute neurological manifestations in patients undergoing first dialysis treatment. The mechanisms for the development of DDS include the reverse urea effect, transient intracranial acidosis, and idiogenic osmoles which can increase intracellular osmolality and promote water movement into the brain. We present a case of a 4-year-old child with chronic kidney disease who underwent a preemptive living unrelated donor kidney transplant. He had a 24 mEq/L drop in his sodium concentration, 92% reduction in blood urea nitrogen (BUN) concentration, and a 67 mOsm/kg drop in serum osmolality within 18 hours after transplant, with concurrent development of symptomatic and radiologic cerebral edema, similar to that described in DDS. Mental status rapidly returned to baseline after administration of 3% hypertonic saline. This case highlights the risk of cerebral edema in patients who have a high pretransplant BUN. It emphasizes the need for close monitoring of vital signs, mental status, and electrolytes in children undergoing renal transplant. Hypertonic solutions can be used to prevent or manage cerebral edema in these patients when serum osmolality decreases rapidly. Pretransplant dialysis is another consideration to proactively reduce serum hyperosmolality.

Sepsis-associated acute kidney injury: macrohemodynamic and microhemodynamic alterations in the renal circulation

Traditionally, renal ischemia has been regarded as central to the pathogenesis of sepsis-associated acute kidney injury (SA-AKI). Accordingly, hemodynamic management of SA-AKI has emphasized restoration of renal perfusion, whereas, experimentally, ischemia reperfusion models have been emphasized. However, in human beings, SA-AKI usually is accompanied by hyperdynamic circulation. Moreover, clinical and experimental evidence now suggests the importance of inflammatory mechanisms in the development of AKI and microcirculatory dysfunction more than systemic alteration in renal perfusion. In this review, we examine systemic, regional, and microcirculatory hemodynamics in SA-AKI, and attempt to rationalize the hemodynamic management of this condition.

Solid-organ transplantation in older adults: current status and future research

An increasing number of patients older than 65 years are referred for and have access to organ transplantation, and an increasing number of older adults are donating organs. Although short-term outcomes are similar in older versus younger transplant recipients, older donor or recipient age is associated with inferior long-term outcomes. However, age is often a proxy for other factors that might predict poor outcomes more strongly and better identify patients at risk for adverse events. Approaches to transplantation in older adults vary across programs, but despite recent gains in access and the increased use of marginal organs, older patients remain less likely than other groups to receive a transplant, and those who do are highly selected. Moreover, few studies have addressed geriatric issues in transplant patient selection or management, or the implications on health span and disability when patients age to late life with a transplanted organ. This paper summarizes a recent trans-disciplinary workshop held by ASP, in collaboration with NHLBI, NIA, NIAID, NIDDK and AGS, to address issues related to kidney, liver, lung, or heart transplantation in older adults and to propose a research agenda in these areas.

Preactivation of AMPK by metformin may ameliorate the epithelial cell damage caused by renal ischemia

Alterations in epithelial cell polarity and in the subcellular distributions of epithelial ion transport proteins are key molecular consequences of acute kidney injury and intracellular energy depletion. AMP-activated protein kinase (AMPK), a cellular energy sensor, is rapidly activated in response to renal ischemia, and we demonstrate that its activity is upregulated by energy depletion in Madin-Darby canine kidney (MDCK) cells. We hypothesized that AMPK activity may influence the maintenance or recovery of epithelial cell organization in mammalian renal epithelial cells subjected to energy depletion. MDCK cells were ATP depleted through a 1-h incubation with antimycin A and 2-deoxyglucose. Immunofluoresence localization demonstrated that this regimen induces mislocalization of the Na-K-ATPase from its normal residence at the basolateral plasma membrane to intracellular vesicular compartments. When cells were pretreated with the AMPK activator metformin before energy depletion, basolateral localization of Na-K-ATPase was preserved. In MDCK cells in which AMPK expression was stably knocked down with short hairpin RNA, preactivation of AMPK with metformin did not prevent Na-K-ATPase redistribution in response to energy depletion. In vivo studies demonstrate that metformin activated renal AMPK and that treatment with metformin before renal ischemia preserved cellular integrity, preserved Na-K-ATPase localization, and led to reduced levels of neutrophil gelatinase-associated lipocalin, a biomarker of tubular injury. Thus AMPK may play a role in preserving the functional integrity of epithelial plasma membrane domains in the face of energy depletion. Furthermore, pretreatment with an AMPK activator before ischemia may attenuate the severity of renal tubular injury in the context of acute kidney injury.

Acute renal failure is NOT an "acute renal success"--a clinical study on the renal oxygen supply/demand relationship in acute kidney injury

OBJECTIVES

Acute kidney injury occurs frequently after cardiac or major vascular surgery and is believed to be predominantly a consequence of impaired renal oxygenation. However, in patients with acute kidney injury, data on renal oxygen consumption (RVO2), renal blood flow, glomerular filtration, and renal oxygenation, i.e., the renal oxygen supply/demand relationship, are lacking and current views on renal oxygenation in the clinical situation of acute kidney injury are presumptive and largely based on experimental studies.

DESIGN

Prospective, two-group comparative study.

SETTING

Cardiothoracic intensive care unit of a tertiary center.

PATIENTS

Postcardiac surgery patients with (n = 12) and without (n = 37) acute kidney injury were compared with respect to renal blood flow, glomerular filtration, RVO2, and renal oxygenation.

INTERVENTIONS

None

MEASUREMENTS AND MAIN RESULTS

Data on systemic hemodynamics (pulmonary artery catheter) and renal variables were obtained during two 30-min periods. Renal blood flow was measured using two independent techniques: the renal vein thermodilution technique and the infusion clearance of paraaminohippuric acid, corrected for renal extraction of paraaminohippuric acid. The filtration fraction was measured by the renal extraction of Cr-EDTA and the renal sodium resorption was measured as the difference between filtered and excreted sodium. Renal oxygenation was estimated from the renal oxygen extraction. Cardiac index and mean arterial pressure did not differ between the two groups. In the acute kidney injury group, glomerular filtration (-57%), renal blood flow (-40%), filtration fraction (-26%), and sodium resorption (-59%) were lower, renal vascular resistance (52%) and renal oxygen extraction (68%) were higher, whereas there was no difference in renal oxygen consumption between groups. Renal oxygen consumption for one unit of reabsorbed sodium was 2.4 times higher in acute kidney injury.

CONCLUSIONS

Renal oxygenation is severely impaired in acute kidney injury after cardiac surgery, despite the decrease in glomerular filtration and tubular workload. This was caused by a combination of renal vasoconstriction and tubular sodium resorption at a high oxygen demand.

Renal plasma flow and glomerular filtration rate during acute kidney injury in man

During acute kidney injury (AKI), lowered glomerular filtration rate (GFR) is believed to be consequent to reduced renal plasma flow (RPF). We aimed to systematically evaluate the evidence for such an association. Using specific search terms, we systematically interrogated the Pub Med electronic reference database for studies of human AKI where renal plasma or blood flow and GFR were measured; older articles were then identified by screening bibliographies of retrieved reports. We identified 22 articles describing 250 patients (203 native kidney, 47 in renal allograft). Of these studies, 8 articles (110 patients) estimated effective renal plasma flow (ERPF) by clearance techniques and 14 articles (140 patients) estimated true renal plasma flow (TRPF). Mean RPF was 272 mL/min (95% CI 213-331) and GFR 13.9 mL/min (9.9-17.9). Mean TRPF was significantly greater than mean ERPF (344 vs. 180, p=0.004) despite lower mean GFR (8.8 vs. 20.4, p=0.002). There was no significant association between RPF and GFR between studies. Eleven studies presented individual patient data (76 patients: 49 TRPF, 27 ERPF); here, individual patient ERPF was associated with GFR (r2=0.52), but TRPF was not. During AKI in man, there is only a limited association between ERPF and GFR, and no detectable association between TRPF and GFR.

Biological modulation of renal ischemia-reperfusion injury

PURPOSE OF REVIEW

Biological modulation of renal ischemia-reperfusion injury holds the potential to reduce the incidence of early graft dysfunction and to safely expand the donor pool with kidneys that have suffered prolonged ischemic injury before organ recovery.

RECENT FINDINGS

In the current review, we will discuss clinical studies that compare kidney transplant recipients with and without early graft dysfunction in order to elucidate the pathophysiology of ischemic acute allograft injury. We will specifically review the mechanisms leading to depression of the glomerular filtration rate and activation of the innate immune system in response to tissue injury.

SUMMARY

We conclude that the pathophysiology of delayed graft function after kidney transplantation is complex and shares broad similarity with rodent models of ischemic acute kidney injury. Given the lack of specific therapies to prevent delayed graft function in transplant recipients, comprehensive efforts should be initiated to translate the promising findings obtained in small animal models into clinical interventions that attenuate ischemic acute kidney injury after transplantation.

Bone marrow-derived endothelial progenitor cells and endothelial cells may contribute to endothelial repair in the kidney immediately after ischemia-reperfusion

In ischemic acute kidney injury, renal blood flow is decreased. We have previously shown that reperfused, transplanted kidneys exhibited ischemic injury to vascular endothelium and that preservation of peritubular capillary endothelial integrity may be critical to recovery from ischemic injury. We hypothesized that bone marrow-derived (BMD) endothelial progenitor cells (EPCs) might play an important role in renal functional recovery after ischemia. We tested this hypothesis in recipients of cadaveric renal allografts before and for 2 weeks after transplantation. We found that the numbers of circulating CD34-positive EPCs and CD146-positive endothelial cells (ECs) decreased immediately after ischemia-reperfusion. In renal allograft tissues obtained 1 hr after reperfusion, CD34-positive cells were more frequently observed along the endothelial lining of peritubular capillaries compared with non-ischemic controls. Moreover, 0-17.5% of peritubular capillary ECs were of recipient origin. In contrast, only 0.1-0.7% of tubule cells were of recipient origin. Repeat graft biopsy samples obtained 35 and 73 days after transplant did not contain capillary ECs of recipient origin, whereas 1.4% and 12.1% of tubule cells, respectively, were of recipient origin. These findings suggest that BMD EPCs and ECs may contribute to endothelial repair immediately after ischemia-reperfusion.

A fatal case of cerebral oedema with hyponatraemia and massive polyuria after renal transplantation

We report the case of a child who died from severe cerebral oedema in the context of hyponatraemia and extreme polyuria immediately after renal transplantation. The patient was treated according to a standard post-transplantation protocol, receiving 0.45% saline solution for urine output replacement. The case highlights the dangers of massive fluid therapy in the context of polyuria and, therefore, the need for intensive monitoring.

Acute kidney injury in children

Acute kidney injury (AKI) (previously called acute renal failure) is characterized by a reversible increase in the blood concentration of creatinine and nitrogenous waste products and by the inability of the kidney to regulate fluid and electrolyte homeostasis appropriately. The incidence of AKI in children appears to be increasing, and the etiology of AKI over the past decades has shifted from primary renal disease to multifactorial causes, particularly in hospitalized children. Genetic factors may predispose some children to AKI. Renal injury can be divided into pre-renal failure, intrinsic renal disease including vascular insults, and obstructive uropathies. The pathophysiology of hypoxia/ischemia-induced AKI is not well understood, but significant progress in elucidating the cellular, biochemical and molecular events has been made over the past several years. The history, physical examination, and laboratory studies, including urinalysis and radiographic studies, can establish the likely cause(s) of AKI. Many interventions such as 'renal-dose dopamine' and diuretic therapy have been shown not to alter the course of AKI. The prognosis of AKI is highly dependent on the underlying etiology of the AKI. Children who have suffered AKI from any cause are at risk for late development of kidney disease several years after the initial insult. Therapeutic interventions in AKI have been largely disappointing, likely due to the complex nature of the pathophysiology of AKI, the fact that the serum creatinine concentration is an insensitive measure of kidney function, and because of co-morbid factors in treated patients. Improved understanding of the pathophysiology of AKI, early biomarkers of AKI, and better classification of AKI are needed for the development of successful therapeutic strategies for the treatment of AKI.

Renal organic anion transporter 1 is maldistributed and diminishes in proximal tubule cells but increases in vasculature after ischemia and reperfusion

Renal solute clearances are reduced in ischemic acute kidney injury. However, the mechanisms explaining how solute clearance is impaired have not been clarified. Recently, we reported that cadaveric renal allografts exhibit maldistribution of organic anion transporter 1 (OAT1) in proximal tubule cells after ischemia and reperfusion, resulting in impairment of PAH clearance. In the present study, we characterized renal OAT1 in detail after ischemia-reperfusion using a rat model. We analyzed renal OAT1 using confocal microscopy with a three-dimensional reconstruction of serial optical images, Western blot, and quantitative real-time RT-PCR. OAT1 was distributed to basolateral membranes of proximal tubule cells in controls. With ischemia, OAT1 decreased in basolateral membrane, especially in the lateral membrane domain, and appeared diffusely in cytoplasm. After reperfusion following 60-min ischemia, OAT1 often formed cytoplasmic aggregates. The staining for OAT1 started reappearing in lateral membrane domain 1 h after reperfusion. The basolateral membrane staining was relatively well discernable at 240 h of reperfusion. Of note, a distinct increase in OAT1 expression was noted in vasculature early after ischemia and after reperfusion. The total amount of OAT1 protein expression in the kidney diminished after ischemia-reperfusion in a duration-dependent manner until 72 h, when they began to recover. However, even at 240 h, the amount of OAT1 did not reach control levels. The kidney tissues tended to show a remarkable but transient increase in mRNA expression for OAT1 at 5 min of ischemia. Our findings may provide insights of renal OAT1 in its cellular localization and response during ischemic acute kidney injury and recovery from it.

Diminished NO generation by injured endothelium and loss of macula densa nNOS may contribute to sustained acute kidney injury after ischemia-reperfusion

In postischemic acute kidney injury (AKI) or acute renal failure, a dissipation of glomerular filtration pressure is associated with an altered renal vascular tone and reactivity, as well as a loss of vascular autoregulation. To test the hypothesis that renal nitric oxide (NO) generation reflects endothelial damage in the kidney after ischemia-reperfusion, we quantified the urinary NO levels and identified the site of its generation in postischemic AKI. Subjects were 50 recipients of cadaveric renal allografts: 15 with sustained AKI and 35 with recovering renal function. Urine and blood samples were obtained after transplant, and intraoperative allograft biopsies were performed to examine NO synthases (NOSs) in the kidney. In the sustained AKI group, urinary nitrite and nitrate excretion (in mumol/g urine creatinine) was lower (12.3 +/- 1.8 and 10.0 +/- 1.4 on postoperative days 0 and 3) than in the recovery group [20.0 +/- 3.6 and 35.1 +/- 5.3 (P < 0.005 vs. sustained AKI on days 0 and 3) on postoperative days 0 and 3]. Endothelial NOS expression diminished from the peritubular capillaries of 6 of 7 subjects in the sustained AKI group but from only 6 of 16 subjects in the recovery group. No differences were observed in the inducible NOS staining pattern between the two groups. Neuronal NOS staining was rarely observed in the macula densae of subjects but was prominent in control tissues. These findings suggest that a diminished NO generation by injured endothelium and loss of macula densa neuronal NOS could impair the vasodilatory ability of the renal vasculature and contribute to the reduction in the glomerular filtration rate in postischemic AKI.

Preservation of peritubular capillary endothelial integrity and increasing pericytes may be critical to recovery from postischemic acute kidney injury

Decreased renal blood flow following an ischemic insult contributes to a reduction in glomerular filtration. However, little is known about the underlying cellular or subcellular mechanisms mediating reduced renal blood flow in human ischemic acute kidney injury (AKI) or acute renal failure (ARF). To examine renal vascular injury following ischemia, intraoperative graft biopsies were performed after reperfusion in 21 cadaveric renal allografts. Confocal fluorescence microscopy was utilized to examine vascular smooth muscle and endothelial cell integrity as well as peritubular interstitial pericytes in the biopsies. The reperfused, transplanted kidneys exhibited postischemic injury to the renal vasculature, as demonstrated by disorganization/disarray of the actin cytoskeleton in vascular smooth muscle cells and disappearance of von Willebrand factor from vascular endothelial cells. Damage to peritubular capillary endothelial cells was more severe in subjects destined to have sustained ARF than in those with rapid recovery of their graft function. In addition, peritubular pericytes/myofibroblasts were more pronounced in recipients destined to recover than those with sustained ARF. Taken together, these data suggest damage to the renal vasculature occurs after ischemia-reperfusion in human kidneys. Preservation of peritubular capillary endothelial integrity and increasing pericytes may be critical to recovery from postischemic AKI.

The L1 cell adhesion molecule is a potential biomarker of human distal nephron injury in acute tubular necrosis

The L1 cell adhesion molecule (CD171) is a multidomain membrane glycoprotein of the immunoglobulin superfamily. We evaluated its expression in human acute kidney injury and assessed its use as a tissue and urinary marker of acute tubular injury. Using immunohistochemical studies with antibodies to the extracellular or cytoplasmic domains, we compared L1 expression in normal kidneys in 24 biopsies taken from patients with acute tubular necrosis. L1 was found at the basolateral and the lateral membrane in all epithelial cells of the collecting duct in the normal kidney except for intercalated cells. In acute tubular necrosis, L1 lost its polarized distribution being found in both the basolateral and apical domains of the collecting duct. Further, it was induced in thick ascending limb and distal tubule cells. Apically expressed L1 found only when the cytoplasmic domain antibody was used in biopsy specimens of patients with acute tubular necrosis. The levels of urinary L1, normalized for creatinine, were significantly higher in all 24 patients with acute tubular necrosis compared to five patients with prerenal azotemia or to six patients with other causes of acute kidney injury. Our study shows that a soluble form of human L1 can be detected in the urine of patients with acute tubular necrosis and that this may be a marker of distal nephron injury.

Alteration in Renal Organic Anion Transporter 1 After Ischemia/Reperfusion in Cadaveric Renal Allografts

We have previously shown that postischemic injury to renal allografts results in profound impairment of p-aminohippuric acid (PAH) extraction. To elucidate the cellular integrity of the human organic anion transporter 1 (hOAT1) in postischemic acute renal failure (ARF), immunohistochemical analysis of hOAT1 was performed in cadaveric renal allografts using confocal microscopy for three-dimensional reconstruction of serial optical images. Biopsy samples were obtained from 10 cadaveric renal allografts 1 hr after reperfusion during transplant operation. Control tissues were obtained from four living donors of healthy kidneys immediately before an arterial clamp was applied to the renal artery. Control tissues demonstrated hOAT1 distributed to basolateral membrane of proximal tubule cells. In contrast, maldistribution of hOAT1 to cytoplasm and/or diminution of the protein was noted in cadaveric allografts. Characteristics of maldistribution were variable: disappearance of lateral distribution, diffuse cytoplasmic aggregates, apical cytoplasmic aggregates, and disappearance of the staining. In addition, iothalamate and PAH clearances were performed on posttransplant days 3–7 in 18 recipients of a cadaveric renal allograft. PAH clearance was depressed <250 ml/min in all but three subjects. We conclude that reperfused, transplanted kidneys exhibit maldistribution of hOAT1 in proximal tubule cells, resulting in impairment of PAH clearance. This manuscript contains online supplemental material at http://www.jhc.org . Please visit this article online to view these materials.

Evolution of renal function and Na+, K +-ATPase expression during ischaemia-reperfusion injury in rat kidney

The aim of the present work was to study the effects of an unilateral ischaemic-reperfusion injury on Na+, K+-ATPase activity, alpha1 and beta1 subunits protein and mRNA abundance and ATP content in cortical and medullary tissues from postischaemic and contralateral kidneys. Right renal artery was clamped for 40 min followed by 24 and 48 h of reperfusion. Postischaemic and contralateral renal function was studied cannulating the ureter of each kidney. Postischaemic kidneys after 24 (IR24) and 48 (IR48) hours of reperfusion presented a significant dysfunction. Na+, K+-ATPase alpha1 subunit abundance increased in IR24 and IR48 cortical tissue and beta1 subunit decreased in IR48. In IR24 medullary tissue, alpha1 abundance increased and returned to control values in IR48 while beta1 abundance was decreased in both periods. Forty minutes of ischaemia without reperfusion (I40) promoted an increment in alpha1 mRNA in cortex and medulla that normalised after 24 h of reperfusion. beta1 mRNA was decreased in IR24 medullas. No changes were observed in contralateral kidneys. This work provides evidences that after an ischaemic insult alpha1 and beta1 protein subunit abundance and mRNA levels are independently regulated. After ischaemic-reperfusion injury, cortical and medullary tissue showed a different pattern of response. Although ATP and Na+, K+-ATPase activity returned to control values, postischemic kidney showed an abnormal function after 48 h of reflow.

Altered renal tubular expression of the complement inhibitor Crry permits complement activation after ischemia/reperfusion

Ischemia/reperfusion (I/R) of several organs results in complement activation, but the kidney is unique in that activation after I/R occurs only via the alternative pathway. We hypothesized that selective activation of this pathway after renal I/R could occur either because of a loss of complement inhibition or from increased local synthesis of complement factors. We examined the relationship between renal complement activation after I/R and the levels and localization of intrinsic membrane complement inhibitors. We found that loss of polarity of complement receptor 1-related protein y (Crry) in the tubular epithelium preceded activation of the alternative pathway along the basolateral aspect of the tubular cells. Heterozygous gene-targeted mice that expressed lower amounts of Crry were more sensitive to ischemic injury. Furthermore, inhibition of Crry expressed by proximal tubular epithelial cells in vitro resulted in alternative pathway-mediated injury to the cells. Thus, altered expression of a complement inhibitor within the tubular epithelium appears to be a critical factor permitting activation of the alternative pathway of complement after I/R. Increased C3 mRNA and decreased factor H mRNA were also detected in the outer medulla after I/R, suggesting that altered synthesis of these factors might further contribute to complement activation in this location.

Tight junction biology and kidney dysfunction

The epithelial tight junction (TJ) has three major functions. As a "gate," it serves as a regulatory barrier separating and maintaining biological fluid compartments of different composition. As a "fence," it generates and maintains the apicobasal polarity of cells that form the confluent epithelium. Finally, the TJ proteins form a trafficking and signaling platform that regulates cell growth, proliferation, differentiation, and dedifferentiation. Six examples are selected that illustrate the emerging link between TJ dysfunction and kidney disease. First, the glomerular slit diaphragm (GSD) is evolved, in part, from the TJ and, on maturation, exhibits all three functions of the TJ. GSD dysfunction leads to proteinuria and, in some instances, podocyte dedifferentiation and proliferation. Second, accumulating evidence supports epithelial-mesenchymal transformation (EMT) as a major player in renal fibrosis, the final common pathway that leads to end-stage renal failure. EMT is characterized by a loss of cell-cell contact and apicobasal polarity, which are hallmarks of TJ dysfunction. Third, in autosomal dominant polycystic kidney disease, mutations of the polycystins may disrupt their known interactions with the apical junction complex, of which the TJ is a major component. This can lead to disturbances in epithelial polarity regulation with consequent abnormal tubulogenesis and cyst formation. Fourth, evidence for epithelial barrier and polarity dysregulation in the pathogenesis of ischemic acute renal failure will be summarized. Fifth, the association between mutations of paracellin-1, the first TJ channel identified, and clinical disorders of magnesium and calcium wasting and bovine renal fibrosis will be used to highlight an integral TJ protein that can serve multiple TJ functions. Finally, the role of WNK4 protein kinase in shunting chloride across the TJ of the distal nephron will be addressed.

Multiple functions of Na,K-ATPase in epithelial cells

The Na,K-adenosine triphosphatase (ATPase), or sodium pump, has been well studied for its role in the regulation of ion homeostasis in mammalian cells. Recent studies suggest that Na,K-ATPase might have multiple functions such as a role in the regulation of tight junction structure and function, induction of polarity, regulation of actin dynamics, control of cell movement, and cell signaling. These functions appear to be modulated by Na,K-ATPase enzyme activity as well as protein-protein interactions of the alpha and beta subunits. In this review we attempt to differentiate functions associated with enzyme activity and subunit interactions. In addition, the consequence of impaired Na,K-ATPase function or reduced subunit expression levels in kidney diseases such as cancer, tubulointerstitial fibrosis, and ischemic nephropathy are discussed.

Effect of acetaminophen on the membrane anchoring of Na+, K+ATPase of rat renal cortical cells

In previous works we reported that the administration of a toxic dose of acetaminophen (APAP) induces acute renal failure (ARF) and promotes changes on Na(+), K(+)ATPase distribution in renal proximal plasma membranes. In the present work, we analyzed if APAP could promote the dissociation of Na(+), K(+)ATPase from its membrane anchorage. The participation of calpain activation was also evaluated. We analyzed the Triton X-100 extractability of Na(+), K(+)ATPase in freshly isolated cortical cell suspensions incubated with different APAP concentrations (0.1, 1, 10 and 100 mM). Both alpha(1) and beta(1) subunits were studied by Western blot. APAP promoted the increment of both subunits abundance in the Triton-soluble fraction. Calpain activation was detected in the membrane fractions of cells incubated with APAP. Incubation with APAP 0.1, 1 and 10 mM did not promote an increment in LDH release compared with controls, while APAP 100 mM promoted an increased LDH release. Our results show that incubation of proximal cells with sublethal and lethal APAP concentrations promotes the detachment of Na(+), K(+)ATPase from its membrane anchoring. Inhibition of calpain activation by SJA 7029 protected against APAP-induced membrane damage but not against APAP-induced increase of the Triton X-100 extractability of Na(+), K(+)ATPase.

Cellular and molecular derangements in acute tubular necrosis

PURPOSE OF REVIEW

Acute tubular necrosis secondary to ischemic acute renal failure remains a common clinical problem with serious consequences and unsatisfactory therapeutic options. The purpose of this review is to summarize recent advances that have provided an improved understanding of the underlying cellular and molecular derangements, and have resulted in the design of novel therapeutic approaches.

RECENT FINDINGS

Sophisticated morphologic studies have identified apoptosis and vascular changes as significant novel findings in human acute tubular necrosis. Promising roles for inhibitors of apoptosis have been proposed. Activation of tubuloglomerular feedback, previously thought to contribute to acute tubular necrosis, has now emerged as a potentially beneficial phenomenon. The role of reactive oxygen molecules has been further elucidated, and novel antioxidants and iron chelators have been identified. Genome-wide screening techniques have identified the molecular mechanisms underlying the regeneration and repair processes, and have provided clues towards accelerating recovery from acute renal failure. An improved understanding of the role of inflammation has suggested strategies to target this previously underappreciated aspect of acute tubular necrosis.

SUMMARY

The cellular and molecular tools of modern science have provided critical new insights into the roles of apoptosis, oxidant and iron-mediated injury, endothelial changes, regeneration, and the inflammatory response in the pathogenesis of acute tubular necrosis. Novel strategies that modulate these pathways hold tremendous promise for the proactive treatment of human acute renal failure.

Ischemic acute renal failure: An inflammatory disease?

Inflammation plays a major role in the pathophysiology of acute renal failure resulting from ischemia. In this review, we discuss the contribution of endothelial and epithelial cells and leukocytes to this inflammatory response. The roles of cytokines/chemokines in the injury and recovery phase are reviewed. The ability of the mouse kidney to be protected by prior exposure to ischemia or urinary tract obstruction is discussed as a potential model to emulate as we search for pharmacologic agents that will serve to protect the kidney against injury. Understanding the inflammatory response prevalent in ischemic kidney injury will facilitate identification of molecular targets for therapeutic intervention.

Inflammatory cells in ischemic acute renal failure

Ischemic acute renal failure (ARF) is increasingly recognized as involving a complex cascade of mechanisms with both acute and chronic consequences. Attention to nontraditional mediators of ARF such as inflammatory pathways and microvascular events has yielded new paradigms and avenues of research. The initiation phase of renal ischemia/reperfusion (I/R) injury damage involves microvascular hemodynamic changes characterized by red blood cell sludging with platelets and leukocytes. Blocking leukocyte-endothelial interactions has yielded significant protection from renal I/R injury in experimental models. However, experiments focusing on the role of the neutrophil have led to a modest expectation of its role in ARF. Recent studies have found that T cells directly mediate renal injury in experimental I/R injury. The CD4+ T cell, working both via interferon-gamma (IFN-gamma) and costimulatory molecules appears to be an important modulator of ARF. The B cell has recently been implicated in ARF. Little is known about the role for the macrophage. Finally, resident kidney cells likely contribute to the inflammatory pathogenesis of I/R damage and protection/repair, but how, and to what extent they are involved is not known. New tools to modulate inflammatory cells, particularly mononuclear leukocytes, hold promise for clinical trials in ARF.

Pathophysiology of ischaemic acute renal failure

This chapter summarizes the pathophysiology of ischaemic acute renal failure from both the experimental and clinical points of view. Traditionally, the abrupt fall in glomerular filtration rate (GFR) is thought to be due to an interplay of haemodynamic and tubular abnormalities. The intrarenal haemodynamic alterations include renal vasoconstriction, leukocyte-endothelium interactions and loss of blood flow and GFR autoregulation. During recent years it has become evident that pronounced outer medulary ischaemia makes an important contribution. In severe and prolonged ischaemia, the tubular epithelial cells can undergo either sublethal or lethal cell damage. Cell death occurs by necrosis and apoptosis. The different mechanisms of post-ischaemic cell damage are discussed. The post-ischaemic kidney also shows a dramatic capacity for recovery. During this recovery phase some of the damaged cells undergo de-differentiation--which is an important step in regeneration of the tubular epithelium. Recent evidence points to the possibility that infiltration of the kidney with bone-marrow-derived stem cells contributes to the repair process. The molecular mechanisms and the effect of growth factors are summarized.

Role of Na-K-ATPase in the assembly of tight junctions

Na-K-ATPase, also known as the sodium pump, is a crucial enzyme that regulates intracellular sodium homeostasis in mammalian cells. In epithelial cells Na-K-ATPase function is also involved in the formation of tight junctions through RhoA GTPase and stress fibers. In this review, a new two-step model for the assembly of tight junctions is proposed: step 1, an E-cadherin-dependent formation of partial tight junction strands and of the circumferential actin ring; and step 2, active actin polymerization-dependent tethering of tight junction strands to form functional tight junctions, an event requiring normal function of Na-K-ATPase in epithelial cells. A new role for stress fibers in the assembly of tight junctions is proposed. Also, implications of Na-K-ATPase function on tight junction assembly in diseases such as cancer, ischemia, hypomagnesemia, and polycystic kidney disease are discussed.

A randomized, placebo-controlled trial of IGF-1 for delayed graft function: a human model to study postischemic ARF

BACKGROUND

Insulin-like growth factor (IGF-1) has been shown in animal models to accelerate recovery from acute renal failure (ARF). However, a therapeutic trial of recombinant human (rh) IGF-1 in patients with ARF in the intensive care unit (ICU) failed to demonstrate efficacy [1]. Such patients often had multiple organ failure, recurrent renal injury, and a delay of several days before commencing treatment.

METHODS

To circumvent these confounding factors, we randomized recipients of cadaveric renal allografts to immediate (<5 hours) rhIGF-1 versus placebo therapy (100 mg/kg subcutaneously twice a day for 6 days). Preliminary observations 3 hours posttransplantation in an additional 44 patients revealed a creatinine clearance < or = 20 mL/min to predict protracted ARF. Thus, this value was used to determine study eligibility.

RESULTS

Creatinine clearance prior to commencing treatment was not significantly different between the two groups (8 +/- 5 mL/min for IGF-1 and 7 +/- 6 mL/min for placebo; P = 0.39). Inulin clearance on day 7, the primary outcome measure, was 21 +/- 22 mL/min and 19 +/- 19 mL/min in the IGF-1 (N = 19) and placebo (N = 24) groups, respectively (P = 0.67). Secondary outcome measures, including nadir serum creatinines after 6 weeks and need for dialysis, also did not differ between the two groups. We performed an analysis of statistical power using the placebo arm of the trial. Defining a twofold increase above placebo in day 7 glomerular filtration rate (GFR) as of meaningful biologic significance, we determined that the modest sample size used in the present study is adequate.

CONCLUSION

We, thus, conclude that (1) IGF-1 treatment is unlikely to benefit ARF and (2) the transplanted kidney is a good model to screen new agents for ARF that have demonstrated promise in animal trials.

Membrane repolarization is delayed in proximal tubules after ischemia-reperfusion: possible role of microtubule-organizing centers

We have previously shown that microtubule-organizing centers (MTOCs) attach to the apical network of intermediate filaments (IFs) in epithelial cells in culture and in epithelia in vivo. Because that attachment is important for the architecture of microtubules (MTs) in epithelia, we analyzed whether chemical anoxia in LLC-PK1 and CACO-2 cells or unilateral ischemia-reperfusion in rat kidney (performed under fluorane anesthesia) had an effect on the binding and distribution of MTOCs. In culture, we found that chemical anoxia induces MTOC detachment from IFs by morphological and biochemical criteria. In reperfused rat proximal tubules, noncentrosomal MTOCs were fully detached from the cytoskeleton and scattered throughout the cytoplasm at 3 days after reperfusion, when brush borders were mostly reassembled. At that time, MTs were also fully reassembled but, as expected, lacked their normal apicobasal orientation. Two apical membrane markers expressed in S2 and S3 segments were depolarized at the same stage. At 8 days after reperfusion, membrane polarity, MTOCs, and MTs were back to normal. Na+-K+-ATPase was also found redistributed, not to the apical domain but rather to an intracellular compartment, as described by others (Alejandro VS, Nelson W, Huie P, Sibley RK, Dafoe D, Kuo P, Scandling JD Jr., and Myers BD. Kidney Int 48: 1308-1315, 1995). The prolonged depolarization of the apical membrane may have implications in the pathophysiology of acute renal failure.

Differentiation between acute cyclosporine nephrotoxicity and acute tubular necrosis using enalaprilat renal scintigraphy in rats

RATIONALE AND OBJECTIVES

Acute cyclosporine (CsA) nephrotoxicity cannot be easily differentiated from other renal parenchymal complications, such as acute tubular necrosis (ATN), that cause renal function impairment at the early posttransplantation period. The purpose of this study was to differentiate acute CsA nephrotoxicity from ATN using enalaprilat renal scintigraphy in rats.

METHODS

Twenty-six rats were divided into 4 experimental groups: CsA group, who were treated with CsA (50 mg/kg/d) for 2 days; ATN group, who received clamping of both renal arteries for 45 minutes; vehicle group, who were treated with olive oil (1 mL/kg/d) for 2 days; and sham-operated group, who received the same surgical procedure as ATN group without clamping of renal arteries. The baseline study was performed with 300 microCi of technetium-99m diethylenetriaminepentaacetic acid and enalaprilat scintigraphy with 2 mCi of technetium-99m diethylenetriaminepentaacetic acid 5 minutes after intravenous enalaprilat injection (30 microg/kg). The changes of renogram grade and the renal function indices such as T(max), T(1/2), residual cortical activity, and mean transit time between 2 studies were analyzed. Immediately after renal scintigraphy, blood urea nitrogen and serum creatinine levels were measured and renal tissues stained by periodic acid Schiff reaction were examined in each group.

RESULTS

Blood urea nitrogen and serum creatinine levels in the CsA and ATN groups were higher than their control groups (P < 0.05). Histologic study revealed severe ischemic necrosis of tubular epithelium in ATN group, but the other groups remained with essentially normal morphology. After enalaprilat injection, renal function indices became improved in CsA group, whereas they deteriorated in ATN group. The renogram grade was decreased in CsA group and increased or unchanged in ATN group after enalaprilat injection. The T(max), residual cortical activity, and mean transit time ratio were statistically different between the 2 groups on enalaprilat study (P < 0.05).

CONCLUSIONS

These results suggest that enalaprilat renal scintigraphy could be used clinically in differentiating acute CsA nephrotoxicity from ATN after renal transplantation.

A modified University of Wisconsin preservation solution with high-NA+ low-K+ content reduces reperfusion injury of the pig kidney graft

BACKGROUND

Ischemia-reperfusion injury has been associated with both early and late effects on allografts in the form of delayed graft function and decreased graft survival. Recent studies demonstrated that functional parameters were influenced by cold storage conditions and particularly the ratio of Na+:K+ of the preservation solution.

METHODS

We have extended this study to examine whether the high-Na+ low-K+ formulation of Belzer's solution (HEH) was efficient in an autotransplanted pig kidney model when compared with the classical low-Na+ high-K+ University of Wisconsin solution and the new high-Na+ low-K+ Celsior solution. Kidneys were harvested, cold flushed, and preserved for 24, 48, or 72 hr with HEH, Celsior solution, or University of Wisconsin solution and autotransplanted. Renal function was determined on days 1, 3, 7, and 14, and at 4 to 16 weeks after autotransplantation. Histologic changes and cell infiltration were assessed on kidney biopsy specimens taken after reperfusion (30-40 min), at days 5 and 14, and at 4 to 5 and 10 to 12 weeks after surgery. Peripheral benzodiazepine receptor (PBR), a structural mitochondrial protein, was also studied.

RESULTS

Cold storage in HEH resulted in reduction of delayed graft function and renal damage, with a decrease in interstitial inflammation. HEH reduced interstitial fibrosis, tubular atrophy, and improved PBR expression.

CONCLUSION

This study suggests that cold preservation in HEH has a beneficial action in in vivo renal preservation and reduces tubular necrosis, interstitial inflammation, and fibrosis in these groups. In addition, PBR detection was correlated to the level of preservation integrity.

Dedifferentiation and proliferation of surviving epithelial cells in acute renal failure

In contrast to the heart or brain, the kidney can completely recover from an ischemic or toxic insult that results in cell death. During recovery from ischemia/reperfusion injury, surviving tubular epithelial cells dedifferentiate and proliferate, eventually replacing the irreversibly injured tubular epithelial cells and restoring tubular integrity. Repair of the kidney parallels kidney organogenesis in the high rate of DNA synthesis and apoptosis and in patterns of gene expression. As has been shown by proliferating cell nuclear antigen and 5-bromo 2'-deoxyuridine labeling studies and, in unpublished studies, by counting mitotic spindles identified by labeling with antitubulin antibody, the proliferative response is rapid and extensive, involving many of the remaining cells of the proximal tubule. This extensive proliferative capacity is interpreted to reflect the intrinsic ability of the surviving epithelial cell to adapt to the loss of adjacent cells by dedifferentiating and proliferating. Adhesion molecules likely play important roles in the regulation of renal epithelial cell migration, proliferation, and differentiation, as do cytokines and chemokines. Better understanding of all of the characteristics resulting in dedifferentiation and proliferation of the proximal tubule epithelial cell and cell-cell and cell-matrix interactions important for this repair function will lead to novel approaches to therapies designed to facilitate the processes of recovery in humans.

Urinary actin, interleukin-6, and interleukin-8 may predict sustained ARF after ischemic injury in renal allografts

BACKGROUND

Cellular damage and inflammation after ischemia contribute to sustained acute renal failure (ARF).

METHODS

To quantify cellular damage and inflammation in postischemic ARF and identify markers of renal functional outcome, urine specimens from 40 renal allograft recipients, including 30 cadaveric (9 "sustained ARF" and 21 "recovery" subjects) and 10 living donor allografts ("LD"), were analyzed for actin, gamma-glutamyl transpeptidase (GGTP), lactate dehydrogenase (LDH), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha) and interleukin-8 (IL-8) during the first posttransplant week.

RESULTS

On day 0, urinary actin, GGTP, IL-6, and IL-8 were elevated in recipients destined to have sustained ARF compared with those destined to recover. Median values per gram of urine creatinine in the sustained ARF, recovery, and LD groups were 263.9, 0.0, and 0.0 microg for actin; 5000.0, 892.9, and 5555.6 U for GGTP; 193.1, 27.2, and 10.5 ng for IL-6; and 382.0, 17.8, and 18.5 ng for IL-8, respectively. In contrast, urinary LDH and TNF-alpha increased in recipients with recovering function compared with those who had sustained ARF. The corresponding median values were 36.7 and 16.3 U (recovery versus sustained ARF) for LDH, and 18.4 and 7.6 ng (LD versus sustained ARF) for TNF-alpha. Computational analyses using the Receiver Operating Characteristic Curve found that elevated urinary actin, IL-6, and IL-8 on day 0 were strong predictors of sustained ARF, where the calculated areas under the curve were 0.75, 0.91, and 0.82, respectively.

CONCLUSION

Increased urinary actin, IL-6, and IL-8 may be useful markers for the prediction of sustained ARF after ischemia.

Dissociation of spectrin-ankyrin complex as a basis for loss of Na-K-ATPase polarity after ischemia

The polarized distribution of Na-K-ATPase at the basolateral membranes of renal tubule epithelial cells is maintained via a tethering interaction with the underlying spectrin-ankyrin cytoskeleton. In this study, we have explored the mechanism underlying the loss of Na-K-ATPase polarity after ischemic injury in Madin-Darby canine kidney (MDCK) cells, utilizing a novel antibody raised against a recently described kidney-specific isoform of ankyrin. In control MDCK cells, ankyrin was colocalized with Na-K-ATPase at the basolateral membrane. ATP depletion resulted in a duration-dependent mislocation of Na-K-ATPase and ankyrin throughout the cytoplasm. Colocalization studies showed a partial overlap between the distribution of ankyrin and Na-K-ATPase at all periods after ATP depletion. By immunoprecipitation with anti-ankyrin antibody, the mislocated Na-K-ATPase remained bound to ankyrin at all time points after ATP depletion. However, the interaction between ankyrin and spectrin was markedly diminished within 3 h of ATP depletion and was completely lost after 6 h. In solution binding assays using a fusion peptide of glutathione S-transferase with the ankyrin binding domain of Na-K-ATPase, a complex with ankyrin was detected at all time points after ATP depletion, but spectrin was lost from the complex in a duration-dependent manner. The loss of spectrin binding was not attributable to spectrin degradation but was associated with hyperphosphorylation of ankyrin. The results suggest that a dissociation of the membrane-cytoskeleton complex at the spectrin-ankyrin interface may contribute to the loss of Na-K-ATPase polarity after ischemic injury and reaffirm a critical adapter role for ankyrin in the normal maintenance of Na-K-ATPase polarity.

Modulation of peripheral-type benzodiazepine receptor levels in a reperfusion injury pig kidney-graft model

BACKGROUND

Ischemia-reperfusion injury is associated with an increased risk of acute rejection, delayed graft function, or chronic graft dysfunction. Mitochondria play a central role in this process.

METHODS

Using an autotransplantation pig kidney model, both early (40 min and 7 days) and late (2-16 weeks) changes in renal function and morphology were determined after different periods of cold ischemia in University of Wisconsin or Euro-Collins solutions. We have also investigated the expression of the peripheral-type benzodiazepine receptor (PBR), which is also critical in maintaining outer mitochondrial membrane stability.

RESULTS

Function of the kidneys was better preserved after 1 hr and 24 hr than after 48 hr and 72 hr in Euro-Collins and University of Wisconsin solutions. Medulla injury was reduced in 1 hr-preserved and 24 hr-preserved groups. PBR was found to be present in epithelial cells of the deep cortical and outer medulla in both normal human and well-preserved pig kidneys. PBR expression was modulated by ischemia-reperfusion injury and the concurrent tubular injury and repair processes.

CONCLUSION

This study indicates that PBR expression correlates with the quality of kidney preservation and might serve as an index of kidney and mitochondria viability. Moreover, these data suggest that PBR might be involved in membrane biogenesis during reperfusion. In addition, considering the identical localization of PBR in human and pig kidneys, these findings could have a direct application in human clinical settings of kidney pathology.

Ischemia induces alterations in actin filaments in renal vascular smooth muscle cells

Although altered renal vascular reactivity is known to occur after ischemia, the structural basis explaining the phenomenon has not been clarified. To evaluate for structural damage to the renal vasculature in ischemic acute renal failure (ARF), F-actin in the renal vasculature of rat kidneys and cultured vascular smooth muscle cells was examined using confocal fluorescence microscopy. The left renal artery was clamped for 15 or 45 min in Sprague-Dawley rats. In other experimental groups, 45 min of renal arterial clamping was followed by 1 or 3 h of reperfusion. Control kidneys were procured without any preceding interventional procedure. F-actin was labeled with either fluorescein or Texas red-conjugated phalloidin. Serial optical sections were collected by confocal microscopy, and image volumes were rendered three dimensionally. The degree of cytoskeletal damage in the vasculature was assessed by semiquantitative scoring of the staining for F-actin. Disorganization/disarray of F-actin, reflected by disruption and clumping of the actin filaments, was observed in arteries, arterioles, and the vasa rectae of the kidney after ischemia or ischemia-reperfusion. Smooth muscle cells from arteries and arterioles showed significant damage to F-actin after either 15 or 45 min of ischemia in a duration-dependent manner. The actin cytoskeleton tended to recover from damage from 45 min of ischemia 1 and 3 h after reperfusion. The vasa rectae did not demonstrate significant damage to F-actin after 15- or 45-min ischemia. However, significant damage to the vasa rectae was manifest 3 h after the reperfusion following 45 min of ischemia. In summary, disorganization/disarray of F-actin in vascular smooth muscle cells of the kidney was observed after ischemia or ischemia-reperfusion. A similar finding was observed in cultured vascular smooth muscle cells. We suggest that this disorganization of the actin cytoskeleton may play a contributory role in the loss of autoregulation of renal blood flow and the aberrant vascular reactivity in postischemic ARF.

Acute renal failure

Acute renal failure is characterized by an increase in the blood concentration of creatinine and nitrogenous waste products and by the inability of the kidney to appropriately regulate fluid and electrolyte homeostasis. There are many different causes of acute renal failure in children, including prerenal disease, intrinsic renal failure, which includes ischemic hypoxic insults, and obstructive uropathy. This review will focus on hypoxic/ischemic acute renal failure, the most common causes of hospital acquired acute renal failure in children. This review will briefly discuss the epidemiology and incidence of acute renal failure in pediatric patients and review new insights into the pathogenesis of acute renal failure. including hemodynamic alterations induced by alterations in nitric oxide and endothelin metabolism, the role of the inflammatory response, and alteration in polarity in the acute renal failure. The therapy of acute renal failure has changed substantially during the past few years. Controlled trials (in adults) to test the efficacy of "renal dose" dopamine have shown that it is ineffective, and hemofiltration has become increasingly popular as a choice of therapy for acute renal failure.

Maintenance and recovery stages of postischemic acute renal failure in humans

Postischemic injury in 38 recipients of 7-day-old cadaveric renal allografts was classified into sustained (n = 15) or recovering (n = 23) acute renal failure (ARF) according to the prevailing inulin clearance. Recipients of long-standing allografts that functioned optimally (n = 16) and living transplant donors undergoing nephrectomy (n = 10) served as functional and structural controls, respectively. A combination of physiological and morphometric techniques were used to evaluate glomerular filtration rate and its determinants 1-3 h after reperfusion and again on day 7 to elucidate the mechanism for persistent hypofiltration in ARF that is sustained. Glomerular filtration rate in the sustained ARF group on day 7 was depressed by 90% (mean +/- SD); the corresponding fall in renal plasma flow was proportionately less. Neither plasma oncotic pressure nor the single-nephron ultrafiltration coefficient differed between the sustained ARF and the control group, however. A model of glomerular ultrafiltration and a sensitivity analysis were used to compute the prevailing transcapillary hydraulic pressure gradient (DeltaP), the only remaining determinant of DeltaP. This revealed that DeltaP varied between 27 and 28 mmHg in sustained ARF and 32-38 mmHg in recovering ARF on day 7 vs. 47-54 mmHg in controls. Sustained ARF was associated with persistent tubular dilatation. We conclude that depression of DeltaP, perhaps due partially to elevated tubule pressure, is the predominant cause of hypofiltration in the maintenance stage of ARF that is sustained for 7 days.

Renal function and cortical (Na(+)+K(+))-ATPase activity, abundance and distribution after ischaemia-reperfusion in rats

The effects of ischaemic injury and reperfusion on renal function, cortical ATP content, alkaline phosphatase activity and (Na(+)+K(+))-ATPase activity and abundance in cortical homogenates and isolated basolateral and apical membranes were examined. Rats were submitted to 5 or 40 min of right renal artery occlusion and 60 min of reperfusion. Renal function of the ischaemic-reperfused kidney was studied by conventional clearance techniques. Our results show that 1 h of reperfusion after a short period of renal ischaemia (5 min) allows the complete restoration of the biochemical features of cortical cells and functional properties of the injured kidney. A longer period of ischaemia, such as 40 min, followed by 1 h of reperfusion showed functional and biochemical alterations. ATP recovered from 26% after 40 min of ischaemia to 50% of control values after 1 h reperfusion. However, renal function was strongly impaired. Brush border integrity was compromised, as suggested by AP excretion and actin appearance in urine. Although total cortical (Na(+)+K(+))-ATPase activity was not different from controls, its distribution in isolated apical and basolateral membranes was abnormal. Remarkably, we detected an increase in alpha-subunit protein abundance that may suggest that (Na(+)+K(+))-ATPase synthesis is promoted by ischaemia-reperfusion. This increase may play an important role in the pathophysiology of ischaemic acute renal failure.

Difficulties in understanding human "acute tubular necrosis": limited data and flawed animal models

This review summarizes the current understanding of the renal biopsy in "acute tubular necrosis" and the attempts to mimic this phenomenon in animal models. Paradoxically, only very limited necrosis is present in the biopsy of patients with this condition and differences in biopsies of patients with sustained and recovering renal failure cannot be clearly defined. The small amount of material examined, the variation in timing of the biopsy, the ability of the nephron to recover from sublethal injury, and the complexity of the clinical situation compound the difficulties in understanding this condition. Morphological findings in the animal studies are not equivalent to those in the human biopsy of "acute tubular necrosis," because they either have too much proximal tubular necrosis (ischemia-reflow model) or show severe injury to distal nephron segments (distal nephron model), the degree of which has not been clearly documented, as yet, in human material. The direct relevance of animal models in part may be tested by new noninvasive methods that define and quantify excreted proteins that reflect nephron injury or measure the status of renal oxygenation by radiological imaging techniques. Finally, it may be time to re-examine the morphology of "acute tubular necrosis," utilizing new techniques that illustrate induction of heat shock proteins, sublethal and apoptotic cellular injury, and alteration of gene expression.

Expression of multidrug resistance P-glycoprotein in kidney allografts from cyclosporine A-treated patients

BACKGROUND

The multidrug resistance (MDR) gene product P-glycoprotein (P-gp) is a transmembrane efflux pump for hydrophobic, potentially toxic compounds, including the immunosuppressant cyclosporine A (CsA). We have previously shown that CsA increases P-gp expression in proximal tubule and endothelial cells in vitro. The aim of the present study was to investigate the in vivo relevance of these observations in renal allograft biopsies from CsA-treated patients.

METHODS

P-gp expression was determined by immunohistochemistry of paraffin sections using two different monoclonal antibodies (UIC2 and MRK16). Biopsies were taken from CsA-treated renal transplant patients with different histopathological diagnoses (N = 79) and were compared with biopsies from normal human kidneys (N = 13) or with allograft biopsies from patients under a CsA-free immunosuppression (N = 15). Moreover, biopsies from 10 donor kidneys before implantation and during rejection episodes ("zero biopsies") were investigated.

RESULTS

P-gp expression in biopsies with acute tubular necrosis (ATN; N = 10) after CsA treatment was significantly higher in arterial endothelia, proximal tubules, and epithelial cells of Bowman's capsule (BC), whereas P-gp was sparsely induced in CsA nephrotoxicity (N = 19) compared with controls. Acute cellular (N = 30) and vascular rejection (N = 10) or chronic allograft nephropathy (N = 10) after CsA was associated with strong P-gp expression in infiltrating leukocytes and increased P-gp expression in arterial endothelia, proximal tubules, and BC. In contrast, biopsies of patients treated with a CsA-free immunosuppression regimen did not show increases in P-gp expression compared with controls. Zero biopsies showed a weak, homogeneous, nonpolarized expression of P-gp in tubules and an increased expression of P-gp after CsA therapy in the brush border, arterial endothelia, and BC.

CONCLUSIONS

CsA treatment was associated with increased P-gp expression in parenchymal cells of kidney transplants with ATN, acute or chronic transplant rejection, but P-gp was not increased in patients with CsA nephrotoxicity. This indicates that CsA induces its own detoxification by P-gp and that inadequate up-regulation of P-gp in renal parenchymal cells contributes to CsA nephrotoxicity. Increased expression of P-gp in infiltrating leukocytes correlated with the severity of allograft rejection, suggesting that P-gp may decrease the immunosuppressive efficacy of CsA. Thus, individual differences in the P-gp induction response of CsA-exposed renal parenchymal cells and/or infiltrating leukocytes may predispose to either CsA nephrotoxicity or rejection, respectively.