Urinary oxygen tension and its role in predicting acute kidney injury: A narrative review

Acute kidney injury occurs frequently in the perioperative setting. The renal medulla often endures hypoxia or hypoperfusion and is susceptible to the imbalance between oxygen supply and demand due to the nature of renal blood flow distribution and metabolic rate in the kidney. The current available evidence demonstrated that the urine oxygen pressure is proportional to the variations of renal medullary tissue oxygen pressure. Thus, urine oxygenation can be a candidate for reflecting the change of oxygen in the renal medulla. In this review, we discuss the basic physiology of acute kidney injury, as well as techniques for monitoring urine oxygen tension, confounding factors affecting the reliable measurement of urine oxygen tension, and its clinical use, highlighting its potential role in early detection and prevention of acute kidney injury.

Lipopolysaccharide Pretreatment Prevents Medullary Vascular Congestion following Renal Ischemia by Limiting Early Reperfusion of the Medullary Circulation

BACKGROUND

Vascular congestion of the renal medulla-trapped red blood cells in the medullary microvasculature-is a hallmark finding at autopsy in patients with ischemic acute tubular necrosis. Despite this, the pathogenesis of vascular congestion is not well defined.

METHODS

In this study, to investigate the pathogenesis of vascular congestion and its role in promoting renal injury, we assessed renal vascular congestion and tubular injury after ischemia reperfusion in rats pretreated with low-dose LPS or saline (control). We used laser Doppler flowmetry to determine whether pretreatment with low-dose LPS prevented vascular congestion by altering renal hemodynamics during reperfusion.

RESULTS

We found that vascular congestion originated during the ischemic period in the renal venous circulation. In control animals, the return of blood flow was followed by the development of congestion in the capillary plexus of the outer medulla and severe tubular injury early in reperfusion. Laser Doppler flowmetry indicated that blood flow returned rapidly to the medulla, several minutes before recovery of full cortical perfusion. In contrast, LPS pretreatment prevented both the formation of medullary congestion and its associated tubular injury. Laser Doppler flowmetry in LPS-pretreated rats suggested that limiting early reperfusion of the medulla facilitated this protective effect, because it allowed cortical perfusion to recover and clear congestion from the large cortical veins, which also drain the medulla.

CONCLUSIONS

Blockage of the renal venous vessels and a mismatch in the timing of cortical and medullary reperfusion results in congestion of the outer medulla's capillary plexus and promotes early tubular injury after renal ischemia. These findings indicate that hemodynamics during reperfusion contribute to the renal medulla's susceptibility to ischemic injury.

Medullary Blood Oxygen Level-Dependent MRI Index (R2*) is Associated with Annual Loss of Kidney Function in Moderate CKD

BACKGROUND

The estimated glomerular filtration rate (eGFR) is frequently used to monitor progression of kidney disease. Multiple values have to be obtained, sometimes over years to determine the rate of decline in kidney function. Recent data suggest that functional MRI (fMRI) methods may be able to predict loss of eGFR. In a prior study, baseline data with multi-parametric MRI in individuals with diabetes and moderate CKD was reported. This report extends our prior observations in order to evaluate the temporal variability of the fMRI measurements over 36 months and their association with annual change in eGFR.

METHODS

Twenty-four subjects with moderate CKD completed 3 sets of MRI scans over a 36-month period. Blood oxygenation level-dependent (BOLD), arterial spin labeling perfusion, and diffusion MRI images were acquired using a 3 T scanner. Coefficients of variation was used to evaluate variability between subjects at each time point and temporal variability within each subject. We have conducted mixed effects models to examine the trajectory change in GFR over time using time and MRI variables as fixed effects and baseline intercept as random effect. Associations of MRI image markers with annual change in eGFR were evaluated.

RESULTS

Multi-parametric functional renal MRI techniques in individuals with moderate CKD showed higher temporal variability in R2* of medulla compared to healthy individuals. This was consistent with the significant lower R2* in medulla observed at 36 months compared to baseline values. The results of linear mixed model showing that R2*_Medulla was the only predictor associated with change in eGFR over time. Furthermore, a significant association of medullary R2* with annual loss of eGFR was observed at all the 3 time points.

CONCLUSIONS

The lower R2* values and the higher temporal variability in the renal medulla over time suggest the ability to monitor progressive CKD. These were confirmed by the fact that reduced medullary R2* was associated with higher annual loss in eGFR. These data collectively emphasize the need for inclusion of medulla in the analysis of renal BOLD MRI studies.

Connexin43 is differentially distributed within renal vasculature and mediates profibrotic differentiation in medullary fibroblasts

Connexins (Cxs) form gap junctions for intercellular exchange of inorganic ions and messenger molecules. In the kidney, Cxs play essential roles within its compartments, but data on the precise cellular localization and cell type-related function of their isoforms are scarce. We tested whether Cx43 distribution is restricted to vascular and interstitial cells and whether medullary fibroblasts express Cx43 to coordinate profibrotic signaling. Confocal immunofluorescence techniques, ultrastructural labeling, and functional experiments in cell culture were performed. Cx43 was chiefly expressed in the vasculature but was absent from tubular epithelia. All arterial, arteriolar, and lymphatic endothelia showed continuous Cx43 signal along their borders. In the inner medulla, only the interstitium showed Cx43 signals, which were assigned to fibroblasts and their processes. Cultured Cx43-expressing medullary fibroblasts served to study the role of gap junctions in a profibrotic context. In a dye spreading assay, Cx43-sensitive diffusion of Lucifer yellow was dependent on gap junctional passage. The addition of transforming growth factor-β1 (5 ng/mL for 48 h) activated Cx43 biosynthesis and caused Cx43-sensitive transformation of the fibroblasts into a myofibroblast phenotype. This suggested that Cx43 gap junctional channels enable the coordination of profibrotic signaling between cells of the medullary interstitium. In summary, we demonstrate the presence of Cx43-expressing gap junctions within the two major renal compartments, the vasculature and interstitium. Endothelial Cx43 likely provides functions of an earlier-defined "electrical syncytium" within the vascular wall. Additionally, Cx43 facilitates profibrotic signaling between medullary interstitial fibroblasts.

Diabetes Affects the A1 Adenosine Receptor-Dependent Action of Diadenosine Tetraphosphate (Ap4A) on Cortical and Medullary Renal Blood Flow

Diabetes through adenosine A1 receptor (A1R) and P2 receptors (P2Rs) may lead to disturbances in renal microvasculature. We investigated the renal microvascular response to Ap4A, an agonist of P2Rs, in streptozotocin-induced diabetic rats. Using laser Doppler flowmetry, renal blood perfusion (RBP) was measured during infusion of Ap4A alone or in the presence of A1R antagonist, either DPCPX (8-cyclopentyl-1,3-dipropylxanthine) or 8-cyclopentyltheophylline (CPT). Ap4A induced a biphasic response in RBP: a phase of rapid decrease was followed by a rapid increase, which was transient in diabetic rats but extended for 30 min in nondiabetic rats. Phase of decreased RBP was not affected by DPCPX or CPT in either group. Early and extended increases in RBP were prevented by DPCPX and CPT in nondiabetic rats, while in diabetic rats, the early increase in RBP was not affected by these antagonists. A1R mRNA and protein levels were increased in isolated glomeruli of diabetic rats, but no changes were detected in P2Y1R and P2Y2R mRNA. Presence of unblocked A1R is a prerequisite for the P2R-mediated relaxing effect of Ap4A in nondiabetic conditions, but influence of A1R on P2R-mediated renal vasorelaxation is abolished under diabetic conditions.

Renal hemodynamics and fatty acid uptake: effects of obesity and weight loss

Human studies of renal hemodynamics and metabolism in obesity are insufficient. We hypothesized that renal perfusion and renal free fatty acid (FFA) uptake are higher in subjects with morbid obesity compared with lean subjects and that they both decrease after bariatric surgery. Cortical and medullary hemodynamics and metabolism were measured in 23 morbidly obese women and 15 age- and sex-matched nonobese controls by PET scanning of [¹⁵O]-H₂O (perfusion) and 14(R,S)-[¹⁸F]fluoro-6-thia-heptadecanoate (FFA uptake). Kidney volume and radiodensity were measured by computed tomography, cardiac output by MRI. Obese subjects were re-studied 6 mo after bariatric surgery. Obese subjects had higher renal volume but lower radiodensity, suggesting accumulation of water and/or lipid. Both cardiac output and estimated glomerular filtration rate (eGFR) were increased by ~25% in the obese. Total renal blood flow was higher in the obese [885 (317) (expressed as median and interquartile range) vs. 749 (300) (expressed as means and SD) ml/min of controls, P = 0.049]. In both groups, regional blood perfusion was higher in the cortex than medulla; in either region, FFA uptake was ~50% higher in the obese as a consequence of higher circulating FFA levels. Following weight loss (26 ± 8 kg), total renal blood flow was reduced (P = 0.006). Renal volume, eGFR, cortical and medullary FFA uptake were decreased but not fully normalized. Obesity is associated with renal structural, hemodynamic, and metabolic changes. Six months after bariatric surgery, the hemodynamic changes are reversed and the structural changes are improved. On the contrary, renal FFA uptake remains increased, driven by high substrate availability.

Snake bite-induced renal medullary angitiis in a child: A case report

Snake bite envenomation is common in tropical countries during the summer. Snake bite-induced acute kidney injury (AKI) has varied histopathological manifestations such as acute cortical necrosis, acute tubular necrosis (ATN), and acute interstitial nephritis. However, snake bite-induced renal medullary angiitis has rarely been reported. We describe a nine-year-old child with AKI following viperine snake bite and renal biopsy revealed pigment cast nephropathy, ATN and medullary angiitis.

Renal medullary hypoxia during experimental cardiopulmonary bypass: a pilot study

Objectives: To determine the effect of cardiopulmonary bypass (CPB) on renal medullary oxygenation.

Design: Observational.

Setting: Laboratory.

Participants: Pigs ( n=3).

Interventions: Following induction of general anesthesia, a Paratrend™ blood gas probe was placed directly into the left renal medulla. Two animals were subjected to 90 min of CPB, while a third served as a non-CPB control. A probe was also placed in the left renal pelvis of one (CPB) animal to allow direct urine PO2measurements.

Measurements and main results: Medullary hypoxia (PO2B < 65 mmHg) was evident prior to CPB. With the onset of CPB, medullary PO2 further declined to nearly unmeasurable levels; PCO2 and pH were unchanged. Brief circulatory arrest during CPB in one animal resulted in rapid additional PCO2 rise and pH decline that corrected with reperfusion. Following the cessation of CPB, medullary PO2 gradually increased, but remained lower than pre-CPB levels. No changes in medullary PO2 were observed in the sham animal. Renal pelvis urine PO2, but not pH or PCO2, appeared to correlate with medullary values at all times.

Conclusions: Our findings indicate that renal medullary hypoxia is extreme during CPB and may persist following CPB. These data suggest a basis for the vulnerability of the kidney to injury during cardiac surgery. Renal pelvis urine PO2 appears to correlate closely with medullary PO2 and may be a useful tool for studying medullary oxygenation during CPB in humans.

Reproducibility of Kidney Perfusion Measurements With Arterial Spin Labeling at 1.5 Tesla MRI Combined With Semiautomatic Segmentation for Differential Cortical and Medullary Assessment

Magnetic resonance imaging with arterial spin labeling (ASL) is a noninvasive approach to measure organ perfusion. The purpose of this study was to evaluate the reproducibility of ASL kidney perfusion measurements with semiautomatic segmentation, which allows separate quantification of cortical and medullary perfusion. The right kidneys of 14 healthy volunteers were examined 6 times on 2 occasions (3 times at each occasion). There was a 10-minute pause between each examination and a 14-day interval between the 2 occasions. Cortical, medullary, and whole kidney parenchymal perfusion was determined with customized semiautomatic segmentation software. Coefficient of variances (CVs) and intraclass correlations (ICCs) were calculated. Mean whole, cortical, and medullary kidney perfusion was 307.26 ± 25.65, 337.10 ± 34.83, and 279.61 ± 26.73 mL/min/100 g, respectively. On session 1, mean perfusion for the whole kidney, cortex, and medulla was 307.08 ± 26.91, 336.79 ± 36.54, and 279.60 ± 27.81 mL/min/100 g, respectively, and on session 2, 307.45 ± 24.65, 337.41 ± 33.48, and 279.61 ± 25.94 mL/min/100 g, respectively (P > 0.05; R² = 0.60/0.59/0.54). For whole, cortical, and medullary kidney perfusion, the total ICC/CV were 0.97/3.43 ± 0.86%, 0.97/4.19 ± 1.33%, and 0.96/4.12 ± 1.36%, respectively. Measurements did not differ significantly and showed a very good correlation (P > 0.05; R² = 0.75/0.76/0.65). ASL kidney measurements combined with operator-independent semiautomatic segmentation revealed high correlation and low variance of cortical, medullary, and whole kidney perfusion.

Application of Onyx for Renal Arteriovenous Malformation With First Case Report of a Renal Hyperdense Striation Sign: A CARE-Compliant Article

Onyx is an emerging treatment modality for visceral vascular malformations, especially in cases in which delicate nidal penetration of the arteriovenous malformation (AVM) is desired. A computed tomography (CT) image presentation of hyperdense striations along the renal medulla secondary to the tantalum powder has not been previously reported. A 65-year-old woman presented to our institution with intermittent gross hematuria and left flank pain for 10 days. Both CT and conventional angiographies confirmed cirsoid-type renal AVM, which was successfully treated with Onyx. Follow-up CT after treatment revealed presence of hyperdense striations along the renal medulla, which resolved during later image follow-up. Despite its frequent usage in neural intervention, the application of Onyx in visceral AVM is gradually gaining interest, especially in cases in which delicate nidal penetration of the AVM is desired. Renal hyperdense striation sign should be recognized to avoid confusion with embolizer migration, and further studies in patients with renal function impairment may be helpful in understanding its influence of renal function.

Urine-concentrating mechanism in the inner medulla: function of the thin limbs of the loops of Henle

The ability of mammals to produce urine hyperosmotic to plasma requires the generation of a gradient of increasing osmolality along the medulla from the corticomedullary junction to the papilla tip. Countercurrent multiplication apparently establishes this gradient in the outer medulla, where there is substantial transepithelial reabsorption of NaCl from the water-impermeable thick ascending limbs of the loops of Henle. However, this process does not establish the much steeper osmotic gradient in the inner medulla, where there are no thick ascending limbs of the loops of Henle and the water-impermeable ascending thin limbs lack active transepithelial transport of NaCl or any other solute. The mechanism generating the osmotic gradient in the inner medulla remains an unsolved mystery, although it is generally considered to involve countercurrent flows in the tubules and vessels. A possible role for the three-dimensional interactions between these inner medullary tubules and vessels in the concentrating process is suggested by creation of physiologic models that depict the three-dimensional relationships of tubules and vessels and their solute and water permeabilities in rat kidneys and by creation of mathematical models based on biologic phenomena. The current mathematical model, which incorporates experimentally determined or estimated solute and water flows through clearly defined tubular and interstitial compartments, predicts a urine osmolality in good agreement with that observed in moderately antidiuretic rats. The current model provides substantially better predictions than previous models; however, the current model still fails to predict urine osmolalities of maximally concentrating rats.

Oxygen transport in a cross section of the rat inner medulla: impact of heterogeneous distribution of nephrons and vessels

We have developed a highly detailed mathematical model of oxygen transport in a cross section of the upper inner medulla of the rat kidney. The model is used to study the impact of the structured organization of nephrons and vessels revealed in anatomic studies, in which descending vasa recta are found to lie distant from clusters of collecting ducts. Specifically, we formulated a two-dimensional oxygen transport model, in which the positions and physical dimensions of renal tubules and vessels are based on an image obtained by immunochemical techniques (T. Pannabecker and W. Dantzler, Three-dimensional architecture of inner medullary vasa recta, Am. J. Physiol. Renal Physiol. 290 (2006) F1355-F1366). The model represents oxygen diffusion through interstitium and other renal structures, oxygen consumption by the Na(+)/K(+)-ATPase activities of the collecting ducts, and basal metabolic consumption. Model simulations yield marked variations in interstitial PO2, which can be attributed, in large part, to the heterogeneities in the position and physical dimensions of the collecting ducts. Further, results of a sensitivity study suggest that medullary oxygenation is highly sensitive to medullary blood flow, and that, at high active consumption rates, localized patches of tissue may be vulnerable to hypoxic injury.

Silencing of HIF prolyl-hydroxylase 2 gene in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats

BACKGROUND

In response to high salt intake, transcription factor hypoxia-inducible factor (HIF) 1α activates many antihypertensive genes, such as heme oxygenase 1 (HO-1) 1 and cyclooxygenase 2 (COX-2) in the renal medulla, which is an important molecular adaptation to promote extra sodium excretion. We recently showed that high salt inhibited the expression of HIF prolyl-hydroxylase 2 (PHD2), an enzyme that promotes the degradation of HIF-1α, thereby upregulating HIF-1α, and that high salt-induced inhibition in PHD2 and subsequent activation of HIF-1α in the renal medulla was blunted in Dahl salt-sensitive hypertensive rats. This study tested the hypothesis that silencing the PHD2 gene to increase HIF-1α levels in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats.

METHODS

PHD2 short hairpin RNA (shRNA) plasmids were transfected into the renal medulla in uninephrectomized Dahl S rats. Renal function and blood pressure were then measured.

RESULTS

PHD2 shRNA reduced PHD2 levels by >60% and significantly increased HIF-1α protein levels and the expression of HIF-1α target genes HO-1 and COX-2 by >3-fold in the renal medulla. Functionally, pressure natriuresis was remarkably enhanced, urinary sodium excretion was doubled after acute intravenous sodium loading, and chronic high salt-induced sodium retention was remarkably decreased, and as a result, salt-sensitive hypertension was significantly attenuated in PHD2 shRNA rats compared with control rats.

CONCLUSIONS

Impaired PHD2 response to high salt intake in the renal medulla may represent a novel mechanism for hypertension in Dahl S rats, and inhibition of PHD2 in the renal medulla could be a therapeutic approach for salt-sensitive hypertension.

Comparative physiology and architecture associated with the mammalian urine concentrating mechanism: role of inner medullary water and urea transport pathways in the rodent medulla

Comparative studies of renal structure and function have potential to provide insights into the urine-concentrating mechanism of the mammalian kidney. This review focuses on the tubular transport pathways for water and urea that play key roles in fluid and solute movements between various compartments of the rodent renal inner medulla. Information on aquaporin water channel and urea transporter expression has increased our understanding of functional segmentation of medullary thin limbs of Henle's loops, collecting ducts, and vasa recta. A more complete understanding of membrane transporters and medullary architecture has identified new and potentially significant interactions between these structures and the interstitium. These interactions are now being introduced into our concept of how the inner medullary urine-concentrating mechanism works. A variety of regulatory pathways lead directly or indirectly to variable patterns of fluid and solute movements among the interstitial and tissue compartments. Animals with the ability to produce highly concentrated urine, such as desert species, are considered to exemplify tubular structure and function that optimize urine concentration. These species may provide unique insights into the urine-concentrating process.(1)

Architecture of vasa recta in the renal inner medulla of the desert rodent Dipodomys merriami: potential impact on the urine concentrating mechanism

We hypothesize that the inner medulla of the kangaroo rat Dipodomys merriami, a desert rodent that concentrates its urine to over 6,000 mosmol/kg H(2)O, provides unique examples of architectural features necessary for production of highly concentrated urine. To investigate this architecture, inner medullary vascular segments in the outer inner medulla were assessed with immunofluorescence and digital reconstructions from tissue sections. Descending vasa recta (DVR) expressing the urea transporter UT-B and the water channel aquaporin 1 lie at the periphery of groups of collecting ducts (CDs) that coalesce in their descent through the inner medulla. Ascending vasa recta (AVR) lie inside and outside groups of CDs. DVR peel away from vascular bundles at a uniform rate as they descend the inner medulla, and feed into networks of AVR that are associated with organized clusters of CDs. These AVR form interstitial nodal spaces, with each space composed of a single CD, two AVR, and one or more ascending thin limbs or prebend segments, an architecture that may lead to solute compartmentation and fluid fluxes essential to the urine concentrating mechanism. Although we have identified several apparent differences, the tubulovascular architecture of the kangaroo rat inner medulla is remarkably similar to that of the Munich Wistar rat at the level of our analyses. More detailed studies are required for identifying interspecies functional differences.

The regulation of blood perfusion in the renal cortex and medulla by reactive oxygen species and nitric oxide in the anaesthetised rat

AIMS

The regulation of blood flow through the renal medulla is important in determining blood pressure, and its dysregulation in pathophysiological states, such as oxidative stress, may contribute to the development of hypertension. This investigation examined the hypothesis that reactive oxygen species has both direct and indirect actions, via scavenging NO, to determine the degree of blood perfusion through the renal medulla.

METHODS

Groups of male Wistar rats received a renal interstitial infusion of either tempol, a superoxide dismutase (SOD) mimetic, or tempol plus catalase (tem + cat), or diethyldithio-carbamic acid (DETC) a SOD inhibitor, or L-NAME alone or L-NAME followed by DETC.

RESULTS

Medullary blood perfusion (MBP) increased by 16 ± 1% (P < 0.05) following the renal infusion of tempol and by 35 ± 4%% (P < 0.05) when tem + cat was infused. Cortical blood perfusion (CBP) was unchanged during the administration of tempol and tem + cat. The renal interstitial infusion of DETC reduced CBP by 13 ± 2%, (P < 0.05) and MBP by 22 ± 3% (P < 0.05). Infusion of L-NAME to block NOS did not change CBP but decreased MBP by 12 ± 4%, which was (P < 0.05) less than the reduction obtained with DETC. Administration of DETC in the presence of L-NAME reduced CBP and MBP by 17 and 14%, respectively, the latter response being approximately half that obtained when only DETC was infused.

CONCLUSIONS

These findings demonstrated that both reactive oxygen species and NO determined the level of MBP. The findings support the hypothesis that reactive oxygen species can act both indirectly, via scavenging of NO, and directly via H(2)O(2) to modulate blood perfusion in the medulla.

Non-invasive investigation of kidney disease in type 1 diabetes by magnetic resonance imaging

AIMS/HYPOTHESIS

Pathophysiological abnormalities in early diabetic nephropathy are poorly understood. We employed MRI to characterise renal perfusion, tissue oxygenation and kidney size in non-diabetic volunteers and type 1 diabetic patients without and with early renal disease.

METHODS

We studied ten control participants (C; age 40.0 [range 31-54] years), nine longstanding normotensive type 1 diabetic patients (T1Normo; age 40.1 [31-50] years, estimated glomerular filtration rate [eGFR] 83.4 ± 10.6 ml min(-1) 1.73 m(-2)) and eight microalbuminuric type 1 diabetic patients (T1Micro; age 42.4 [33-52] years, eGFR 71.6 ± 13.7 ml min(-1) 1.73 m(-2)). Six microalbuminuric patients were restudied after 4 weeks without renin-angiotensin-aldosterone system inhibitors. Phase contrast angiography and kidney blood oxygen level dependent (BOLD) (R(2)(*)) MRI were performed, before and during water diuresis. Contrast-enhanced MRI was performed at baseline urine flow rate. Renal artery flow, renal vascular resistance (RVR), cortical and medullary volumes, and R(2)(*) were determined.

RESULTS

Renal cortical and medullary volumes were similar in all groups (cortex: C 108 ± 16, T1Normo 112 ± 21, T1Micro 111 ± 10 cm(3)/1.73 m(2); medulla: C 35 ± 14, T1Normo 29 ± 10, 33 ± 6 cm(3)/1.73 m(2)). RVR increased from control to normoalbuminuric to microalbuminuric type 1 diabetic patients (C 0.061 ± 0.018, T1Normo 0.077 ± 0.014, T1Micro 0.093 ± 0.024 mmHg ml(-1) min(-1) 1.73 m(-2), ANOVA p = 0.012). RVR correlated inversely with eGFR in normoalbuminuric, but not in microalbuminuric diabetic patients. Renal artery flow was lower in the whole diabetes cohort (control 740 ± 205 vs diabetes 591 ± 128 ml min(-1) 1.73 m(-2), p = 0.035).

CONCLUSIONS/INTERPRETATION

Cortical and medullary volumes remain normal in early diabetic nephropathy. Decreased renal flow in longstanding normoalbuminuric type 1 diabetic patients may reflect intrarenal vascular stiffening, whereas in the microalbuminuric patients it may also reflect increased intraglomerular pressure.

Changes in renal cortical and medullary perfusion in a patient with renal vein thrombosis

Dynamic renal perfusion computerized tomographic (CT) scan was performed to test the cortical and medullary perfusion in a patient with unilateral renal vein thrombosis secondary to idiopathic focal and segmental glomerulosclerosis (FSGS). Forty mL of Iohexol was injected intravenously. Multiple fixed repeated axial renal CT scan cuts at specific intervals, over the mid pole, were recorded over 400 seconds. Radio density was measured over the aorta, cortex and medulla during that period. Graphs for the radio contrast density against time were plotted. Aortic, cortical and medullary perfusions were calculated by estimating the slopes of the curves. Based on the CT scan findings, perfusion of different parts of the kidney was measured. The reduction in kidney function with renal vein thrombosis seems to be secondary to hypoperfusion of renal cortex and medulla. Further studies are required to confirm this observation. The blood flow to the kidney im-proved within four days after therapy with anticoagulation and pulse steroids. The sequences of events that take place need further studies for validation.

[Change of blood volume in the kidney cortical and medulla vessels in aging process]

We studied the volumes of arterial and venous vessels (cortical and medulla) of a kidney, excepting microcirculation system. The research covered 76 corrosion preparations of kidneys of people, aged from 21 till 85 years, died from diseases which could not affect on morfofunctional condition of kidneys and their vascular system. We have found out that morphological changes in kidneys at aging are characterised, basically, by progressing nephrosclerosis. The major factor provoking development of sclerous changes in a kidney is the age changes of kidney arterial vessels. It is proved that structural changes in venous and arterial vessels in involution are characterised by gradual decrease of volume of cortical substances vessels, where the considerable part of blood proceeds.

Recovery from renal ischemia-reperfusion injury is associated with altered renal hemodynamics, blunted pressure natriuresis, and sodium-sensitive hypertension

The present studies evaluated intrarenal hemodynamics, pressure natriuresis, and arterial blood pressure in rats following recovery from renal ischemia-reperfusion (I/R) injury. Acute I/R injury, induced by 40 min of bilateral renal arterial occlusion, resulted in an increase in plasma creatinine that resolved within a week. Following 5 wk of recovery on a 0.4% NaCl diet, the pressure-natriuresis response was assessed in anesthetized rats in which the kidney was denervated and extrarenal hormones were administered intravenously. Increasing renal perfusion pressure (RPP) from 107 to 141 mmHg resulted in a fourfold increase in urine flow and sodium excretion in sham control rats. In comparison, pressure diuresis and natriuresis were significantly attenuated in post-I/R rats. In sham rats, glomerular filtration rate (GFR) averaged 1.6 +/- 0.2 mlxmin(-1)xg kidney weight(-1) and renal blood flow (RBF) averaged 7.8 +/- 0.7 mlxmin(-1)xg kidney weight(-1) at RPP of 129 mmHg. Renal cortical blood flow, measured by laser-Doppler flowmetry, was well autoregulated whereas medullary blood flow and renal interstitial hydrostatic pressure increased directly with elevated RPP in sham rats. In contrast, GFR and RBF were significantly reduced whereas medullary perfusion and interstitial pressure demonstrated an attenuated response to RPP in post-I/R rats. Further experiments demonstrated that conscious I/R rats develop hypertension when sodium intake is increased. The present data indicate that the pressure-natriuretic-diuretic response in I/R rats is blunted because of a decrease in GFR and RBF and the depressed pressure-dependent increase in medullary blood flow and interstitial pressure.

EFFECTS OF MITRAL REGURGITATION ON THE REFLEX DIURESIS TO PULMONARY LYMPHATIC OBSTRUCTION IN RABBITS

Increasing the extravascular fluid of the airways acutely by obstructing pulmonary lymph drainage causes a reflex diuresis mediated by neuronal nitric oxide synthase in the renal medulla. The authors examined this reflex in rabbits with a chronic increase in extravascular fluid of the airways resulting from surgically induced mitral regurgitation. Intact rabbits served as controls. Renal neuronal (nNOS) and endothelial (eNOS) nitric oxide synthase expressions were also examined. The reflex was absent in rabbits with mitral regurgitation. There were significant increases in medullary and cortical nNOS mRNA compared to controls. The observed changes in mRNA levels correlated with nNOS protein levels. eNOS mRNA was unaffected.

Renal medullary effects of transient prehypertensive treatment in young spontaneously hypertensive rats

AIM

Transient angiotensin II receptor blockade (ARB) leads to prolonged blood pressure (BP) lowering, but the underlying mechanism remains uncertain. Long-term BP control is regulated by the medullary microcirculation with the pericyte as contractile cell. We hypothesize that the prolonged BP effect is caused by increased medullary blood flow (MBF) associated with structural alterations based on reduced medullary pericyte number.

METHODS

Four-week-old spontaneously hypertensive rats (SHR) were treated for 4 weeks with losartan (SHR-Los: 20 mg kg(-1) day(-1)), hydralazine (SHR-Hyd: 15 mg kg(-1) day(-1)), losartan and pan-caspase inhibitor zVAD (SHR-Los + 1 mg kg(-1) day(-1) zVAD), losartan and glycogen synthase kinase-3beta (GSK) inhibitor valproate (SHR-Los + 10 mg kg(-1) day(-1) Val) or placebo. BP, MBF and pericyte number were determined under and after treatment (8 and 12 weeks). Apoptotic pericytes were determined with alpha-actin and TUNEL double staining. Sodium concentration was determined in renal medulla and urine.

RESULTS

Antihypertensive treatment equipotently reduced BP at 8 weeks of age. After drug withdrawal (12 weeks of age) BP reduction was restricted to SHR-Los (SHR-Los: 153 +/- 5, SHR-Hyd: 177 +/- 2, SHR: 184 +/- 3 mmHg). Simultaneously, MBF was increased and pericyte number reduced, while medullary and urinary sodium concentration increased. Transient ARB in combination with zVAD or valproate resulted in more medullary pericytes and higher BP (SHR-Los/zVAD: 164 +/- 7; SHR-Los/Val: 168 +/- 6 mmHg) compared with transient ARB alone.

CONCLUSION

After drug withdrawal, transient ARB leads to increased MBF and is associated with a reduction in medullary pericytes. This may be associated with pericyte apoptosis as anti-apoptosis during transient ARB increases pericyte number and BP.

Contribution of endothelin A receptors in endothelin 1-dependent natriuresis in female rats

Renal medullary endothelin B receptors contribute to blood pressure regulation by facilitating salt excretion. Premenopausal females have relatively less hypertension than males; therefore, we examined whether there is a sex difference in the natriuretic response to renal medullary infusion of endothelin peptides in the rat. All of the experiments were conducted in anesthetized wild-type (wt) or endothelin B-deficient (sl/sl) rats. Infusion of endothelin 1 (ET-1) significantly increased sodium excretion (U(Na)V) in female, but not male, wt rats (Delta U(Na)V: 0.41+/-0.07 versus -0.04+/-0.06 micromol/min, respectively). The endothelin B receptor agonist sarafotoxin 6c produced similar increases in U(Na)V in both male (Delta 0.58+/-0.15 micromol/min) and female (Delta 0.67+/-0.18 micromol/min) wt rats. Surprisingly, ET-1 markedly increased U(Na)V in female (Delta 0.70+/-0.11 micromol/min) but not male sl/sl rats (Delta 0.00+/-0.05 micromol/min). ET-1 had no effect on medullary blood flow in females, although medullary blood flow was significantly reduced to a similar extent in males of both strains. These results suggest that the lack of a natriuretic response to ET-1 in male rats is because of reductions in medullary blood flow. Treatment with ABT-627, an endothelin A receptor antagonist, or N(G)-propyl-L-arginine, an NO synthase 1 inhibitor, prevented the increase in U(Na)V observed in female rats. Gonadectomy eliminated the sex difference in the U(Na)V and medullary blood flow response to ET-1. These findings demonstrate that there is no sex difference in endothelin B-dependent natriuresis, and the endothelin A receptor contributes to ET-1-dependent natriuresis in female rats, an effect that requires NO synthase 1. These findings provide a possible mechanism for why premenopausal women are more resistant to salt-dependent hypertension.

Intrarenal vasodilator systems: NO, prostaglandins and bradykinin. An integrative approach

Intrarenal microcirculation is under hormonal, paracrine and neural control. Of particular interest is circulation in the renal medulla: its perfusion seems critical for long term control of arterial pressure. Exposure of the organism to adverse conditions often leads to activation of vasopressor factors, such as renin/angiotensin, renal sympathetic input or vasopressin; this helps maintain arterial pressure but endangers renal circulation. Fortunately, it is protected by intrarenal vasodilators: nitric oxide, prostaglandins, bradykinin and others. The potency of NO to oppose intrarenal vasoconstrictors may differ between individual factors: it is substantial in the case of renal sympathetic input whereas the constrictor influence of angiotensin II in the medulla seems to be offset mostly by intrarenal prostaglandins. Although these are commonly regarded as intrarenal vasodilators, our new data show that this is so only in the renal medulla. In the cortex they exert modest vasoconstriction, probably mediated by EP3 receptors. The role of bradykinin as intrarenal vasodilator is not yet known in sufficient detail, its effect is most pronounced in the inner medulla. The source of vasoactive kinins is uncertain, they could reach intrarenal microvasculature from the sites of synthesis in tubular cells but the synthesis in the vessels themselves cannot be excluded.

Differential regulation of ET(A) and ET(B) in the renal tissue of rats with compensated and decompensated heart failure

Endothelin-1 (ET-1) exerts its biological actions through two receptor subtypes: endothelin-A (ETA) receptor and endothelin-B (ETB) receptor. We demonstrated previously that ET-1 induces systemic and renal cortical vasoconstriction via ETA whereas ETB mediates medullary vasodilation. Congestive heart failure (CHF) is characterized by increased vascular resistance and impaired renal hemodynamic and excretory function. While the pathophysiological effects of ET-1 in CHF are well established, the status of ETA and ETB in the kidney is poorly characterized. The present study examined the immunostaining and localization of ETA and ETB in the renal cortex and medulla of rats with experimental CHF induced by aorto-caval fistula. Rats with CHF were further subdivided, based on their daily urinary sodium excretion, into rats with compensated (urinary sodium excretion > 1200 microEq/day) and decompensated CHF (urinary sodium excretion < 200 microEq/day). ETA is predominantly localized to the cortex mainly in the peritubular capillaries, and is upregulated in rats with compensated and decompensated CHF compared with sham controls. In contrast, ETB is preferentially expressed in the outer and inner medulla, mainly in the vasa recta, the thick ascending limb of Henle's loop and the collecting duct. While compensated CHF is associated with upregulation of ETB in the collecting duct and vasa recta, decompensated CHF is accompanied with enhanced ETB abundance in the vasa recta and remarkable downregulation of this receptor subtype in the collecting duct. The findings suggest that upregulation of ETA may lead to a decrease in cortical blood flow while upregulation of ETB in the vasa recta probably contributes to the preservation of medullary blood flow. Furthermore, downregulation of ETB in the collecting duct, only in rats with decompensated CHF, could contribute to sodium retention in that subgroup.

Blockade of renal medullary bradykinin B2 receptors increases tubular sodium reabsorption in rats fed a normal-salt diet

The present study was performed to test the hypothesis that under normal physiological conditions and/or during augmentation of kinin levels, intrarenal kinins act on medullary bradykinin B(2) (BKB(2)) receptors to acutely increase papillary blood flow (PBF) and therefore Na(+) excretion. We determined the effect of acute inner medullary interstitial (IMI) BKB(2) receptor blockade on renal hemodynamics and excretory function in rats fed either a normal (0.23%)- or a low (0.08%)-NaCl diet. For each NaCl diet, two groups of rats were studied. Baseline renal hemodynamic and excretory function were determined during IMI infusion of 0.9% NaCl into the left kidney. The infusion was then either changed to HOE-140 (100 microg.kg(-1).h(-1), treated group) or maintained with 0.9% NaCl (time control group), and the parameters were again determined. In rats fed a normal-salt diet, HOE-140 infusion decreased left kidney Na(+) excretion (urinary Na(+) extraction rate) and fractional Na(+) excretion by 40 +/- 5% and 40 +/- 4%, respectively (P < 0.01), but did not alter glomerular filtration rate, inner medullary blood flow (PBF), or cortical blood flow. In rats fed a low-salt diet, HOE-140 infusion did not alter renal regional hemodynamics or excretory function. We conclude that in rats fed a normal-salt diet, kinins act tonically via medullary BKB(2) receptors to increase Na(+) excretion independent of changes in inner medullary blood flow.

Quantitative analysis of functional reconstructions reveals lateral and axial zonation in the renal inner medulla

Three-dimensional functional reconstructions of descending thin limbs (DTLs) and ascending thin limbs (ATLs) of loops of Henle, descending vasa recta (DVR), ascending vasa recta (AVR), and collecting ducts (CDs) permit quantitative definition of lateral and axial zones of probable functional significance in rat inner medulla (IM). CD clusters form the organizing motif for loops of Henle and vasa recta in the initial 3.0-3.5 mm of the IM. Using Euclidean distance mapping, we defined the lateral boundary of each cluster by pixels lying maximally distant from any CD. DTLs and DVR lie almost precisely on this independently defined boundary, placing them in the intercluster interstitium maximally distant from any CD. ATLs and AVR lie in a nearly uniform pattern throughout intercluster and intracluster regions, which we further differentiated by a polygon around CDs in each cluster. Loops associated with individual CD clusters show a similar axial exponential decrease as all loops together in the IM. Because approximately 3.0-3.5 mm below the IM base CD clusters cease to form the organizing motif, all DTLs lack aquaporin 1 (AQP1), and all vasa recta are fenestrated, we have designated the first 3.0-3.5 mm of the IM the "outer zone" (OZ) and the final 1.5-2.0 mm the "inner zone" (IZ). We further subdivided these into OZ-1, OZ-2, IZ-1, and IZ-2 on the basis of the presence of completely AQP1-null DTLs only in the first 1 mm and on broad transverse loop bends only in the final 0.5 mm. These transverse segments expand surface area for probable NaCl efflux around loop bends from approximately 40% to approximately 140% of CD surface area in the final 100 microm of the papilla.

Peritubular endothelium: The Achilles heel of the kidney?

The development of renal ischemia has been postulated to be a main cause of the progressive nature of kidney diseases. In recent years, it has become clear that inappropriate and sustained activation of the endothelium could mediate this phenomenon. Endothelial activation will result in leucostasis and can compromise peritubular flow. The associated sustained redox signaling will also accelerate the development of endothelial senescence. In addition, risk factors for renal disease progression can reduce endothelial repair. In the course of these events, loss of capillary structure and rarefaction develops, which drives the further development of nephron loss. In this mini review, the evidence for this pathophysiological concept as well as the possibility to detect such endothelial activation in the clinical arena is summarized.

Heme oxygenase-1 induction improves ischemic renal failure: role of nitric oxide and peroxynitrite

The present study evaluated the effects of heme oxygenase-1 (HO-1) induction on the changes in renal outer medullary nitric oxide (NO) and peroxynitrite levels during 45-min renal ischemia and 30-min reperfusion in anesthetized rats. Glomerular filtration rate (GFR), outer medullary blood flow (OMBF), HO and nitric oxide synthase (NOS) isoform expression, and renal low-molecular-weight thiols (-SH) were also determined. During ischemia significant increases in NO levels and peroxynitrite signal were observed (from 832.1 +/- 129.3 to 2,928.6 +/- 502.0 nM and from 3.8 +/- 0.7 to 9.0 +/- 1.6 nA before and during ischemia, respectively) that dropped to preischemic levels during reperfusion. OMBF and -SH significantly decreased after 30 min of reperfusion. Twenty-four hours later, an acute renal failure was observed (GFR 923.0 +/- 66.0 and 253.6 +/- 55.3 microl.min(-1).g kidney wt(-1) in sham-operated and ischemic kidneys, respectively; P < 0.05). The induction of HO-1 (CoCl(2) 60 mg/kg sc, 24 h before ischemia) decreased basal NO concentration (99.7 +/- 41.0 nM), although endothelial and neuronal NOS expression were slightly increased. CoCl(2) administration also blunted the ischemic increase in NO and peroxynitrite (maximum values of 1,315.6 +/- 445.6 nM and 6.3 +/- 0.5 nA, respectively; P < 0.05), preserving postischemic OMBF and GFR (686.4 +/- 45.2 microl.min(-1).g kidney wt(-1)). These beneficial effects of CoCl(2) on ischemic acute renal failure seem to be due to HO-1 induction, because they were abolished by stannous mesoporphyrin, a HO inhibitor. In conclusion, HO-1 induction has a protective effect on ischemic renal failure that seems to be partially mediated by decreasing the excessive production of NO with the subsequent reduction in peroxynitrite formation observed during ischemia.

Delayed recovery of renal regional blood flow in diabetic mice subjected to acute ischemic kidney injury

Ischemic acute kidney injury in experimental diabetes mellitus (DM) is associated with a more severe deterioration in renal function than shown in nondiabetic animals. We evaluated whether the early recovery phase from acute kidney injury is associated with a more prolonged and sustained decrease in renal perfusion in diabetic mice, which could contribute to the impaired recovery of renal function. Perfusion to the renal cortex and medulla was evaluated by laser-Doppler flowmetry in 10- to 12-wk-old anesthetized mice with type 2 DM (db/db), heterozygous mice (db/m), and nondiabetic (control) mice (C57BL/6J). After baseline measurements were obtained, the right renal artery was clampedfor 20 min followed by reperfusion for 60 min. The data demonstrated that, in all three groups studied, the reperfusion phase was characterized by a significant increase in the medullary-to-cortical blood flow ratio. Moreover, during recovery from ischemia, there was a marked prolongation in the time (in min) required to reach peak reperfusion in the cortex (db/db: 20.7 +/- 4.0, db/m: 12.92 +/- 1.9, C57BL/6J: 9.3 +/- 1.3) and the medulla (db/db: 20.8 +/- 3.2, db/m: 12.88 +/- 1.89, C57BL/6J: 11.2 +/- 1.2). Additionally, the slope of the recovery phase was lower in db/db mice (cortex: 61.9 +/- 23.1%/min, medulla: 16.3 +/- 3.6%/min) than in C57BL/6J mice (cortex: 202.2 +/- 41.6%/min, medulla: 42.1 +/- 7.2%/min). Our findings indicate that renal ischemia is associated with a redistribution of blood flow from cortex to medulla, not related to DM. Furthermore, renal ischemia in db/db mice results in a marked impairment in reperfusion of the renal cortex and medulla during the early postischemic period.

Descending vasa recta endothelia express inward rectifier potassium channels

Descending vasa recta (DVR) are capillary-sized microvessels that supply blood flow to the renal medulla. They are composed of contractile pericytes and endothelial cells. In this study, we used the whole cell patch-clamp method to determine whether inward rectifier potassium channels (K(IR)) exist in the endothelia, affect membrane potential, and modulate intracellular Ca(2+) concentration ([Ca(2+)](cyt)). The endothelium was accessed for electrophysiology by removing abluminal pericytes from collagenase-digested vessels. K(IR) currents were recorded using symmetrical 140 mM K(+) solutions that served to maximize currents and eliminate cell-to-cell coupling by closing gap junctions. Large, inwardly rectifying currents were observed at membrane potentials below the equilibrium potential for K(+). Ba(2+) potently inhibited those currents in a voltage-dependent manner, with affinity k = 0.18, 0.33, 0.60, and 1.20 microM at -160, -120, -80, and -40 mV, respectively. Cs(+) also blocked those currents with k = 20, 48, 253, and 1,856 microM at -160, -120, -80, and -40 mV, respectively. In the presence of 1 mM ouabain, increasing extracellular K(+) concentration from 5 to 10 mM hyperpolarized endothelial membrane potential by 15 mV and raised endothelial [Ca(2+)](cyt). Both the K(+)-induced membrane hyperpolarization and the [Ca(2+)](cyt) elevation were reversed by Ba(2+). Immunochemical staining verified that both pericytes and endothelial cells of DVR express K(IR)2.1, K(IR)2.2, and K(IR)2.3 subunits. We conclude that strong, inwardly rectifying K(IR)2.x isoforms are expressed in DVR and mediate K(+)-induced hyperpolarization of the endothelium.

Enhanced Superoxide Production in Renal Outer Medulla of Dahl Salt-Sensitive Rats Reduces Nitric Oxide Tubular-Vascular Cross-Talk

Studies were conducted to determine whether the diffusion of NO from the renal medullary thick ascending limb (mTAL) to the contractile pericytes of surrounding vasa recta was reduced and, conversely, whether diffusion of oxygen free radicals was enhanced in the salt-sensitive Dahl S rat (SS/Mcwi). Angiotensin II ([Ang II] 1 μmol/L)–stimulated NO and superoxide (O 2 ·− ) production were imaged by fluorescence microscopy in thin tissue strips from the inner stripe of the outer medulla. In prehypertensive SS/Mcwi rats and a genetically designed salt-resistant control strain (consomic SS-13 BN ), Ang II failed to increase either NO or O 2 ·− in pericytes of isolated vasa recta. Ang II stimulation resulted in production of NO in epithelial cells of the mTAL that diffused to vasa recta pericytes of SS-13 BN rats but not in SS/Mcwi rats except when tissues were preincubated with the superoxide scavenger TIRON (1 mmol/L). Ang II resulted in a greater increase of O 2 ·− in the mTAL of SS/Mcwi compared with SS.13 BN mTAL. The O 2 ·− diffused to adjoining pericytes in tissue strips only in SS/Mcwi rats but not in control SS-13 BN rats. Diffusion of Ang II-stimulated O 2 ·− from mTAL to vasa recta pericytes was absent when tissue strips from SS/Mcwi rats were treated with the NO donor DETA-NONOate (20 μmol/L). We conclude that the SS/Mcwi rat exhibits increased production of O 2 ·− in mTAL that diffuses to surrounding vasa recta and attenuates NO cross-talk. Diffusion of O 2 ·− from mTAL to surrounding tissue could contribute to reduced bioavailability of NO, reductions of medullary blood flow, and interstitial fibrosis in the outer medulla of SS/Mcwi rats.

Role of NO and COX pathways in mediation of adenosine A1 receptor-induced renal vasoconstriction

The mechanism of adenosine A1 receptor-induced intrarenal vasoconstriction is unclear; it depends on sodium intake and may be mediated by changing the intrarenal activity of the nitric oxide (NO) and/or cyclooxygenase (COX) pathway of arachidonic acid metabolism. The effects of 2-chloro-N(6)-cyclopentyl-adenosine (CCPA), a selective A1 receptor agonist, on renal hemodynamics were examined in anesthetized rats maintained on high sodium (HS) or low sodium (LS) diet. Total renal (i.e., cortical) blood flow (RBF) as well as superficial cortical (CBF), outer medullary (OMBF), and inner medullary (IMBF) flows were determined by laser-Doppler. In HS rats, suprarenal aortic infusions of 8-40 nmol/kg/hr CCPA decreased IMBF (15%) and other perfusion indices (22%-27%); in LS rats, IMBF increased 3% (insignificant) and other indices decreased 13%-24%. In LS rats, pretreatment with N-nitro-L-arginine methyl ester prevented the A1 receptor-mediated decrease in RBF and CBF but not OMBF; the response in IMBF was not altered. Pretreatment with indomethacin prevented the decreases in RBF, CBF, and OMBF and did not change the response of IMBF. Thus, within the cortex the vasoconstriction that follows A1 receptor activation results both from inhibition of NO synthesis and from stimulation of vasoconstrictor products of the COX pathway. In the outer medulla, the latter products seem exclusively responsible for CCPA-induced vasoconstriction. The observation that in LS rats IMBF was not affected by stimulation of adenosine A1 receptors suggests that limiting salt intake may help protect medullary perfusion against vasoconstrictor stimuli which have the potential to disturb long-term control of arterial pressure.

N-acetylcysteine attenuates NSAID-induced rat renal failure by restoring intrarenal prostaglandin synthesis

BACKGROUND

Renal failure is a threatening side-effect of NSAID administration, consequent to NSAID-mediated abrogation of prostaglandin synthesis and resultant renal ischaemia. N-acetylcysteine (NAC) has renoprotective properties. We examined effects of NAC in a rat model of NSAID-induced renal failure.

METHODS

Renal failure was generated in 80 rats by 6-day water deprivation and 3-day 15 mg/kg/day diclofenac injection. The rats were concomitantly treated, or not, by NAC, 40 mg/kg/day. Renal function was evaluated by cystatin C, creatinine and urea. Intrarenal blood flow was measured by laser Doppler. The kidneys were subjected to pathological examination or evaluation of intrarenal NO, H2O2 and PGE2.

RESULTS

NAC significantly attenuated deterioration of renal function in diclofenac-treated rats: cystatin C dropped from 2.8+/-0.35 to 2.2+/-0.67 mg/l, P=0.016; creatinine from 1.2+/-0.97 to 0.96+/-0.19 mg/dl, P=0.02; urea from 208.4+/-57.9 to 157.6+/-33.7 mg/dl, P=0.028. Diclofenac-inflicted hystopathological damage was significantly reduced following NAC treatment. Intrarenal medullar blood flow dropped by 51+/-12.4% in diclofenac-treated rats, but only by 14+/-3.39% in those receiving NAC after diclofenac injection (P<0.001). H2O2 was elevated in renal tissues of diclofenac-receiving rats, while decreased in NAC-treated animals. PGE2 release by diclofenac-treated rats dropped significantly, but was restored after NAC administration both in renal cortices (144.7+/-10.4 vs 19.7+/-1.5 pmol/ml, P<0.001) and medullae (148.5+/-7.3 vs 66.6+/-7.3 pmol/ml, P<0.001).

CONCLUSIONS

In this model of renal failure induced by NSAID administration combined with water deprivation, NAC treatment successfully attenuated the deterioration of renal function by inducing renal vasodilatation, decreasing oxidative stress via inhibition of intrarenal ROS content and restoration of intrarenal PGE2 release back to the basal levels.

Early changes with diabetes in renal medullary hemodynamics as evaluated by fiberoptic probes and BOLD magnetic resonance imaging

OBJECTIVE

We sought to evaluate the influence of streptozotocin (STZ)-induced diabetes on renal outer medullary pO2 and blood flow by invasive microprobes and to demonstrate feasibility that blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) can monitor these changes.

MATERIALS AND METHODS

A total of 60 Wistar-Furth rats were used. Diabetes was induced by STZ in 48. Animals were divided into OxyLite group (n=30) and BOLD MRI groups (n=30) each with a 5 subgroups of 6 animals: control and 2, 5, 14, and 28 days after induction of diabetes. Outer renal medullary oxygen tension and blood flow were measured by the combined OxyLite/OxyFlo probes.

RESULTS

Both OxyLite and BOLD MRI showed a significant increase in the renal hypoxia levels after STZ at all time points. However, no changes were observed in the outer renal medullary oxygen tension and blood flow between diabetic and control groups.

CONCLUSIONS

These preliminary results suggest that hypoxic changes can be detected as early as 2 days in rat kidneys with diabetes by BOLD MRI and that these early changes are not dependent on blood flow.

Role of UTB urea transporters in the urine concentrating mechanism of the rat kidney

A mathematical model of the renal medulla of the rat kidney was used to investigate urine concentrating mechanism function in animals lacking the UTB urea transporter. The UTB transporter is believed to mediate countercurrent urea exchange between descending vasa recta (DVR) and ascending vasa recta (AVR) by facilitating urea transport across DVR endothelia. The model represents the outer medulla (OM) and inner medulla (IM), with the actions of the cortex incorporated via boundary conditions. Blood flow in the model vasculature is divided into plasma and red blood cell compartments. In the base-case model configuration tubular dimensions and transport parameters are based on, or estimated from, experimental measurements or immunohistochemical evidence in wild-type rats. The base-case model configuration generated an osmolality gradient along the cortico-medullary axis that is consistent with measurements from rats in a moderately antidiuretic state. When expression of UTB was eliminated in the model, model results indicated that, relative to wild-type, the OM cortico-medullary osmolality gradient and the net urea flow through the OM were little affected by absence of UTB transporter. However, because urea transfer from AVR to DVR was much reduced, urea trapping by countercurrent exchange was significantly compromised. Consequently, urine urea concentration and osmolality were decreased by 12% and 8.9% from base case, respectively, with most of the reduction attributable to the impaired IM concentrating mechanism. These results indicate that the in vivo urine concentrating defect in knockout mouse, reported by Yang et al. (J Biol Chem 277(12), 10633-10637, 2002), is not attributable to an OM concentrating mechanism defect, but that reduced urea trapping by long vasa recta plays a significant role in compromising the concentrating mechanism of the IM. Moreover, model results are in general agreement with the explanation of knockout renal function proposed by Yang et al.

Experimental strategies to improve in vitro models of renal ischemia

Ischemia has elicited a great deal of interest among the scientific community due to its role in life-threatening pathologies such as cancer, stroke, acute renal failure, and myocardial infarction. Oxygen deprivation (hypoxia) associated with ischemia has recently become a subject of intense scrutiny. New investigators may find it challenging to induce hypoxic injury in vitro. Researchers may not always be aware of the experimental barriers that contribute to this phenomenon. Furthermore, ischemia is associated with other major insults, such as excess carbon dioxide (hypercapnia), nutrient deprivation, and accumulation of cellular wastes. Ideally, these conditions should also be incorporated into in vitro models. Therefore, the motivation behind this review is to: i. delineate major in vivo ischemic insults; ii. identify and explain critical in vitro parameters that need to be considered when simulating ischemic pathologies; iii. provide recommendations to improve experiments; and as a result, iv. enhance the validity of in vitro results for understanding clinical ischemic pathologies. Undoubtedly, it is not possible to completely replicate the in vivo environment in an ex vivo model system. In fact, the primary goal of many in vitro studies is to elucidate the role of specific stimuli during in vivo pathological events. This review will present methodologies that may be implemented to improve the applicability of in vitro models for understanding the complex pathological mechanisms of ischemia. Finally, although these topics will be discussed within the context of renal ischemia, many are pertinent for cellular models of other organ systems and pathologies.

Renal tissue NO and intrarenal haemodynamics during experimental variations of NO content in anaesthetised rats

Direct renal nitric oxide (NO) measurements were infrequent and no simultaneous measurements of renal cortical and medullary NO and local perfusion. Large-surface NO electrodes were placed in renal cortex and medulla of anaesthetised rats; simultaneously, renal blood flow (RBF, index of cortical perfusion) and medullary laser-Doppler flux (MBF) were determined. NO synthase inhibitors: nonselective (L-NAME) or selective for neuronal NOS (nNOS) (S-methyl-thiocitrulline, SMTC), and NO donor (SNAP), were used to manipulate tissue NO. Baseline tissue NO was significantly higher in medulla (703+/-49 NM) than in cortex (231+/-17 nM). Minimal cortical and medullary NO current measured after maximal L-NAME dose (2.4 mg kg(-1) i.v.) was taken as tissue NO zero kevel. This dose decreased RBF and MBF significantly (-43%). SMTC, 1.2 mg kg(-1) h(-1) i.v., significantly decreased tissue NO by 105+/-32 nM in cortex and 546+/-64 nM in medulla, RBF and MBF decreased 30% and 20%, respectively. Renal artery infusion of SNAP, 0.24 mg kg(-1) min(-1) significantly increased tissue NO by 139+/-18 nM in cortex and 948+/-110 nM in medulla. Since inhibition of nNOS decreased medullary NO by 80% and MBF by 20% only, this isoform has probably minor role in the maintenance of medullary perfusion.

Loss of renal function and microvascular blood flow after suprarenal aortic clamping and reperfusion (SPACR) above the superior mesenteric artery is greatly augmented compared with SPACR above the renal arteries

OBJECTIVE

Renal insufficiency continues to be a complication that can affect patients after treatment for suprarenal aneurysms and renal artery occlusive disease. To our knowledge, no data are available showing that suprarenal aortic clamping and reperfusion (SRACR) above the renal arteries (renal-SRACR) preserves renal function compared with SRACR above the superior mesenteric artery (SMA-SRACR). This study examined the hypothesis that SMA-SRACR-induced downregulation of renal blood flow and function is more severe than renal-SRACR owing to the addition of systemic oxygen-derived free radical (ODFR) release.

METHODS

Male Sprague-Dawley rats (about 350 g) were anesthetized and microdialysis probes or laser Doppler fibers were inserted into the renal cortex (depth of 2 mm) and into the renal medulla (depth of 4 mm). Laser Doppler blood flow was continuously monitored, and the microdialysis probes were connected to a syringe pump and perfused in vivo at 3 microL/min with lactated Ringer's solution.

RESULTS

SMA-SRACR and Renal-SRACR decreased medullary and cortical blood flow and nitric oxide (NO) synthesis. SMA-SRACR downregulated cortical inducible NO synthase, whereas renal-SRACR did not. The cortex and medulla responded to the decreased blood flow and NO synthesis by increasing in prostaglandin E2 synthesis, which was due to increased cyclooxygenase-2 content. Superoxide dismutase restored SMA-SRACR (but not renal-SRACR) cortical and medullary NO synthesis, suggesting that ODFRs generated during mesenteric ischemia-reperfusion were one of the systemic mechanisms contributing to decreased renal NO synthesis in the SMA-SRACR model. The 90% decrease in creatinine clearance after SMA-SRACR was greater than the 60% decrease after renal-SRACR.

CONCLUSIONS

These data show that NO is important in maintaining renal cortical and medullary blood flow and NO synthesis after renal and SMA-SRACR. These data also suggest that in addition to the renal ischemia-reperfusion caused by both models, SMA SRACR induces mesenteric ischemia-reperfusion, resulting in the generation of ODFRs, which contribute to decreased renal cortical and medullary NO synthesis. Maintaining splanchnic blood flow or attempting to keep SRACR below the SMA level may be helpful in developing strategies to minimize the renal injury after SRACR.

Diabetes-induced alterations in renal medullary microcirculation and metabolism

Diabetes-induced renal complications, i.e. diabetes nephropathy, are a major cause of morbidity and mortality. The exact mechanisms mediating the negative influence of hyperglycemia on renal function are unclear, although several hypotheses have been postulated. Cellular mechanisms include glucose-induced excessive formation of reactive oxygen species, increased glucose flux through polyol pathway and pentose phosphate shunt, formation of advanced glycation end-products and activation of protein kinase C and NADPH oxidase. However, the renal effects in vivo of each and every one of these mechanisms are less clear, although recent studies have shown several major alterations predominantly in the renal medulla as a result of sustained hyperglycemia. Already during normal conditions, the renal medulla has a remarkably low oxygen tension (PO2) and a high degree of non-oxygen dependent energy metabolism. Alterations in either blood perfusion or oxygen delivery to the medullary region will have significant effects on both regional metabolism and total kidney function. Recently, sustained hyperglycemia has been shown to induce a pronounced reduction in preferentially renal medullary PO2. This review will present the current knowledge of diabetes-induced alterations in renal medullary metabolism and function, but also discuss future targets for prevention of diabetic nephropathy.

Renal oxygen delivery: matching delivery to metabolic demand

The kidneys are second only to the heart in terms of O2 consumption; however, relative to other organs, the kidneys receive a very high blood flow and oxygen extraction in the healthy kidney is low. Despite low arterial-venous O2 extraction, the kidneys are particularly susceptible to hypoxic injury and much interest surrounds the role of renal hypoxia in the development and progression of both acute and chronic renal disease. Numerous regulatory mechanisms have been identified that act to maintain renal parenchymal oxygenation within homeostatic limits in the in vivo kidney. However, the processes by which many of these mechanisms act to modulate renal oxygenation and the factors that influence these processes remain poorly understood. A number of such mechanisms specific to the kidney are reviewed herein, including the relationship between renal blood flow and O2 consumption, pre- and post-glomerular arterial-venous O2 shunting, tubulovascular cross-talk, the differential control of regional kidney blood flow and the tubuloglomerular feedback mechanism. The roles of these mechanisms in the control of renal oxygenation, as well as how dysfunction of these mechanisms may lead to renal hypoxia, are discussed.

Specific features and roles of renal circulation: angiotensin II revisited

The status of intrarenal circulation determines in part renal excretion, affects body fluid homeostasis and has a role in long term control of arterial blood pressure. The vascular resistance in the renal cortex and medulla is determined by interaction of a vast array of vasoactive hormones and paracrine factors; among these the role of constrictor angiotensin II and dilator prostaglandins and nitric oxide may appear to be dominating. The focus of this review and underlying studies is on the mechanisms whereby the microcirculation of the renal medulla is protected against the vasoconstrictor action of angiotensin II. In anaesthetized normal rats the three mentioned active agents or their inhibitors were applied and total renal blood flow and cortical, outer- and inner medullary laser-Doppler fluxes were determined; in some studies renal tissue nitric oxide was measured using selective electrodes. We conclude that angiotensin II, acting via AT1 receptors, constricts the renal cortical vasculature; in the medulla its action is effectively buffered by prostaglandin E2 but most probably not by nitric oxide.

Cytochrome P-450 monooxygenases in control of renal haemodynamics and arterial pressure in anaesthetized rats

The renal regulatory role of cytochrome P450 dependent metabolites of arachidonic acid (AA), vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE), was examined in anaesthetised rats. We measured renal artery flow (RBF), cortical (CBF) and medullary (MBF) perfusion (laser-Doppler) and medullary tissue nitric oxide (NO, selective electrode), after non-selective inhibition of CYP-450 pathway with 1-aminobenzotriazole (ABT, 10 mg/kg i.v.) or after selective inhibition of 20-HETE synthesis with HET0016 (Taisho Co, Yoshino-cho, Japan), infused into renal artery at 0.3 mg/kg/h or into renal medulla at rates increasing from 0.15 to 1.5 mg/kg/h. ABT caused significant (by 13.7%) decrease in RBF without changing MBF. Renal arterial HET0016 increased MBF (not RBF or CBF) from 152+/-12 to 174+/-12 perfusion units (+16%, P<0.001), while medullary tissue nitric oxide was significantly increased (P<0.001). After renal medullary HET0016, renal perfusion indices were significantly higher than after HET0016 solvent (beta-cyclodextrin). Total renal blood flow seems to be under vasodilator control of EETs whereas renal medullary perfusion under tonic suppression by 20-HETE. The data document, for the first in the whole kidney studies, the functional antagonism of 20-HETE and NO.

Intravenous Tamm-Horsfall protein polyps: report of a case in association with a hematoma that mimicked a renal neoplasm

Tamm-Horsfall protein (THP) is a glycoprotein produced only in the thick ascending limb of the loop of Henle. Its primary physiological function is unknown, but it may have a role in host defense against infectious organisms. THP is the primary scaffolding protein in all varieties of tubular casts. Under certain conditions, THP may be extruded from tubular lumens into the interstitium and lymphatic channels. It even may be found within lymph nodes sampled for staging of neoplastic conditions. THP deposits were described in lumens of large veins. The pathogenetic basis of this finding is not known, but obstruction of renal outflow was suggested, and several cases were associated with macroscopic hematuria. We report a case of intravenous THP polyposis in which, in addition to abundant hemorrhage, there was formation of a hematoma. This measured 12 cm in diameter and caused clinical concern for the possibility of renal cell carcinoma. Although the cause of the hematoma was not apparent, the association with striking intravenous polyps of THP is noteworthy because this represents the first association of intravenous THP polyps with a large intraparenchymal hematoma.

A model of nitric oxide tubulovascular cross talk in a renal outer medullary cross section

We developed a two-dimensional model of NO transport in a cross section of the inner stripe (IS) of the rat outer medulla to determine whether tubular and vascular generation of NO result in significant NO concentration (C(NO)) differences between the periphery and the center of vascular bundles and thereby affect medullary blood flow distribution. Following the approach of Layton and Layton (Layton AT, Layton HE. Am J Physiol Renal Physiol 289: F1346-F1366, 2006), the structural heterogeneity of the IS was incorporated in a representative unit consisting of four concentric regions centered on a vascular bundle. Our model suggests that the diffusion distance of NO in the interstitium is limited to a few micrometers. We predict that, under basal conditions, epithelial NO generation raises the average C(NO) in pericytes surrounding peripheral descending vasa recta (DVR) by a few nanomoles relative to that in pericytes surrounding central DVR. The short descending limbs and long ascending limbs are found to exert the greatest effect on C(NO) in pericytes; long descending limbs and short ascending limbs only have a moderate effect, whereas outer medullary collecting ducts, which are situated far from the vascular bundle center, do not affect pericyte C(NO). Our results suggest that selective stimulation of epithelial NO production should significantly raise the periphery-to-center DVR diameter ratio, thereby increasing the outer medulla-to-inner medulla blood flow ratio. However, concomitant increases in epithelial superoxide (O(2)(-)) production would counteract this effect. This model confirms the importance of NO and O(2)(-) interactions in mediating tubulovascular cross talk.

Regional vascular responses to ATP and ATP analogues in the rabbit kidney in vivo: roles for adenosine receptors and prostanoids

BACKGROUND AND PURPOSE

Our knowledge of the effects of P2-receptor activation on renal vascular tone comes mostly from in vitro models. We aimed to characterise the pharmacology of ATP in the renal circulation in vivo.

EXPERIMENTAL APPROACH

In pentobarbitone anaesthetized rabbits, we examined total renal and medullary vascular responses to ATP (0.2 and 0.8 mg kg(-1)), beta, gamma-methylene ATP (beta, gamma-mATP, 7 and 170 microg kg(-1)), alpha, beta-mATP (0.2 and 2 microg kg(-1)) and adenosine (2 and 6 microg kg(-1)) using transit-time ultrasound and laser Doppler flowmetry, respectively. We also determined whether adenosine receptors, NO or prostanoids contribute to the actions of the purinoceptor agonists.

KEY RESULTS

Renal arterial boluses of ATP, beta,gamma-mATP, and adenosine produced biphasic changes; ischaemia followed by hyperaemia, in total renal and medullary blood flow. alpha,beta-mATP induced only ischaemia. The adenosine receptor antagonist 8-(p-sulphophenyl)theophylline reduced the responses to adenosine and the hyperaemic responses to ATP and beta,gamma-mATP only. NO synthase inhibition (Nomega-nitro-L-arginine) did not significantly alter responses to the P2 receptor agonists. Subsequent cyclooxygenase inhibition (ibuprofen) reduced the ATP- and beta, gamma-mATP-induced increases in renal blood flow. All other responses remained unchanged.

CONCLUSIONS AND IMPLICATIONS

In the rabbit kidney in vivo, alpha, beta-mATP sensitive receptors mediate vasoconstriction. beta,gamma-mATP and ATP induce vasodilation at least partly through adenosine receptors. ATP induced renal vasodilatation is independent of NO and partly dependent on prostanoids in the bulk of the kidney, but not in the vasculature controlling medullary blood flow.