Intrinsic nitric oxide and superoxide production regulates descending vasa recta contraction

Low dimensional nanomaterials for treating acute kidney injury

Acute kidney injury (AKI) is one of the most common severe complications among hospitalized patients. In the absence of specific drugs to treat AKI, hemodialysis remains the primary clinical treatment for AKI patients. AKI treatment has received significant attention recently due to the excellent drug delivery capabilities of low-dimensional nanomaterials (LDNs) and their unique therapeutic effects. Diverse LDNs have been proposed to treat AKI, with promising results and the potential for future clinical application. This article aims to provide an overview of the pathogenesis of AKI and the recent advances in the treatment of AKI using different types of LDNs. In addition, it is intended to provide theoretical support for the design of LDNs and implications for AKI treatment.

Small molecule compound M12 reduces vascular permeability in obese mice via blocking endothelial TRPV4-Nox2 interaction

Transient receptor potential channel TRPV4 and nicotinamide adenine dinucleotide phosphate oxidase (Nox2) are involved in oxidative stress that increases endothelial permeability. It has been shown that obesity enhances the physical association of TRPV4 and Nox2, but the role of TRPV4-Nox2 association in obesity has not been clarified. In this study we investigated the function of TRPV4-Nox2 complex in reducing oxidative stress and regulating abnormal vascular permeability in obesity. Obesity was induced in mice by feeding a high-fat diet (HFD) for 14 weeks. The physical interaction between TRPV4 and Nox2 was measured using FRET, co-immunoprecipitation and GST pull-down assays. The functional interaction was measured by rhodamine phalloidin, CM-H₂DCFDA in vitro, the fluorescent dye dihydroethidium (DHE) staining assay, and the Evans blue permeability assay in vivo. We demonstrated that TRPV4 physically and functionally associated with Nox2, and this physical association was enhanced in aorta of obese mice. Furthermore, we showed that interrupting TRPV4-Nox2 coupling by TRPV4 knockout, or by treatment with a specific Nox2 inhibitor Nox2 dstat or a specific TRPV4 inhibitor HC067046 significantly attenuated obesity-induced ROS overproduction in aortic endothelial cells, and reversed the abnormal endothelial cytoskeletal structure. In order to discover small molecules disrupting the over-coupling of TPRV4 and Nox2 in obesity, we performed molecular docking analysis and found that compound M12 modulated TRPV4-Nox2 association, reduced ROS production, and finally reversed disruption of the vascular barrier in obesity. Together, this study, for the first time, provides evidence for the TRPV4 physically interacting with Nox2. TRPV4-Nox2 complex is a potential drug target in improving oxidative stress and disruption of the vascular barrier in obesity. Compound M12 targeting TRPV4-Nox2 complex can improve vascular barrier function in obesity.

Protection of kidney function and tissue integrity by pharmacologic use of natriuretic peptides and neprilysin inhibitors

With variable potencies atrial-, brain-type and c-type natriuretic peptides (NP)s, best documented for ANP and its analogues, promote sodium and water excretion, renal blood flow, lipolysis, lower blood pressure, and suppress renin and aldosterone secretion through interaction predominantly with cGMP-coupled NPR-A receptor. Infusion of especially ANP and its analogues up to 50 ng/kg/min in patients with high risk of acute kidney injury (cardiac vascular bypass surgery, intraabdominal surgery, direct kidney surgery) protects kidney function (GFR, plasma flow, medullary flow, albuminuria, renal replacement therapy, tissue injury) at short term and also long term and likely additively with the diuretic furosemide. This documents a pharmacologic potential for the pathway. Neprilysin (NEP, neutral endopeptidase) degrades NPs, in particular ANP, and angiotensin II. The drug LCZ696, a mixture of the neprilysin inhibitor sacubitril and the ANGII-AT1 receptor blocker valsartan, was FDA approved in 2015 and marketed as Entresto®. In preclinical studies of kidney injury, LCZ696 and NPs lowered plasma creatinine, countered hypoxia and oxidative stress, suppressed proinflammatory cytokines, and inhibited fibrosis. Few randomized clinical studies exist and were designed with primary cardiac outcomes. The studies showed that LCZ696/entresto stabilized and improved glomerular filtration rate in patients with chronic kidney disease. LCZ696 is safe to use concerning kidney function and stabilizes or increases GFR. In perspective, combined AT1 and neprilysin inhibition is a promising approach for long-term renal protection in addition to AT1 receptor blockers in acute kidney injury and chronic kidney disease.

Nitric oxide signalling in kidney regulation and cardiometabolic health

The prevalence of cardiovascular and metabolic disease coupled with kidney dysfunction is increasing worldwide. This triad of disorders is associated with considerable morbidity and mortality as well as a substantial economic burden. Further understanding of the underlying pathophysiological mechanisms is important to develop novel preventive or therapeutic approaches. Among the proposed mechanisms, compromised nitric oxide (NO) bioactivity associated with oxidative stress is considered to be important. NO is a short-lived diatomic signalling molecule that exerts numerous effects on the kidneys, heart and vasculature as well as on peripheral metabolically active organs. The enzymatic L-arginine-dependent NO synthase (NOS) pathway is classically viewed as the main source of endogenous NO formation. However, the function of the NOS system is often compromised in various pathologies including kidney, cardiovascular and metabolic diseases. An alternative pathway, the nitrate-nitrite-NO pathway, enables endogenous or dietary-derived inorganic nitrate and nitrite to be recycled via serial reduction to form bioactive nitrogen species, including NO, independent of the NOS system. Signalling via these nitrogen species is linked with cGMP-dependent and independent mechanisms. Novel approaches to restoring NO homeostasis during NOS deficiency and oxidative stress have potential therapeutic applications in kidney, cardiovascular and metabolic disorders.

Near-drowning: new perspectives for human hypoxic acute kidney injury

Concepts regarding hypoxic acute kidney injury (AKI) are derived from widely used warm ischemia-reflow (WIR) models, characterized by extensive proximal tubular injury and associated with profound inflammation. However, there is ample clinical and experimental data indicating that hypoxic AKI may develop without total cessation of renal blood flow, with a different injury pattern that principally affects medullary thick limbs in the outer medulla. This injury pattern likely reflects an imbalance between blood and oxygen supply and oxygen expenditure, principally for tubular transport. Experimental models of hypoxic AKI other than WIR are based on mismatched oxygen delivery and consumption, particularly within the physiologically hypoxic outer medulla. However, evidence for such circumstances in human AKI is lacking. Recent analysis of the clinical course and laboratory findings of patients following near-drowning (ND) provides a rare glimpse into such a scenario. This observation supports the role of renal hypoxia in the evolution of AKI, as renal impairment could be predicted by the degree of whole-body hypoxia (reflected by lactic acidosis). Furthermore, there was a close association of renal functional impairment with indices of reduced oxygen delivery (respiratory failure and features of intense sympathetic activity) and of enhanced oxygen consumption for active tubular transport (extrapolated from the calculated volume of consumed hypertonic seawater). This unique study in humans supports the concept of renal oxygenation imbalance in hypoxic AKI. The drowning scenario, particularly in seawater, may serve as an archetype of this disorder, resulting from reduced oxygen delivery, combined with intensified oxygen consumption for tubular transport.

Role of Hypoxia in Renal Failure Caused by Nephrotoxins and Hypertonic Solutions

Hypoxia plays a role in the pathogenesis of acute kidney injury under diverse clinical settings, including nephrotoxicity. Although some nephrotoxins exert direct renal parenchymal injury, likely with consequent altered oxygenation, others primarily reduce renal parenchymal oxygenation, leading to hypoxic tubular damage. As outlined in this review, nephrotoxin-related renal hypoxia may result from an altered renal oxygen supply (cyclosporine), enhanced oxygen consumption for tubular transport (agents inducing osmotic diuresis), or their combination (nonsteroidal anti-inflammatory drugs, radiocontrast agents, and others). Most agents causing hypoxic renal injury further supress physiologic low medullary Po₂, in which a limited regional blood supply barely matches the intense regional tubular transport and oxygen consumption. The medullary tubular transport and blood supply are finely matched, securing oxygen sufficiency. Predisposition to hypoxia-mediated nephrotoxicity by medical conditions, such as chronic kidney disease or diabetes, may be explained by malfunctioning of control systems that normally maintain medullary oxygenation. However, this propensity may be diminished by hypoxia-mediated adaptive responses governed by hypoxia-inducible factors. Recent reports have suggested that inhibitors of sodium-glucose cotransporters and the administration of hypertonic saline may be added to the growing list of common therapeutic interventions that intensify medullary hypoxia, and potentially could lead to hypoxic acute kidney injury.

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

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

Short-term hypoxia and vasa recta function in kidney slices

BACKGROUND

Descending vasa recta (DVR) supply the inner part of outer renal medulla an area at risk for hypoxic damages.

OBJECTIVE

We hypothesize increased vasoreactivity after hypoxia/re-oxygenation (H/R) in DVR, which might contribute to the reduced medullary perfusion after an ischemic event.

METHODS

Live kidney slices (200μm) from SD rats were used for functional experiments. TUNEL assay and H&E staining were used to estimate slice viability. Kidney slices were treated with carbogen or hypoxia (1% O2) for 60 or 90 min and vasoreactivity to Ang II (10-7 M) was recorded by DIC microscopy after re-oxygenation with carbogen. Expression of NOS and NADPH enzymes mRNA were determined in iron-perfusion isolated VR.

RESULTS

Percentage of apoptotic cells increased in control and H/R after 90 min in the medulla. Ang II- induced constriction of DVR was reduced after 90 min in control (compared to 60 min), but not after H/R. NOS enzymes mRNA expression levels decreased over 90 min hypoxia.

CONCLUSIONS

Increased reactivity of DVR to Ang II after H/R compared to control (90 min) suggest a role of DVR in renal ischemia/reperfusion injury.

Adaptive responses of rat descending vasa recta to ischemia

tested whether rat descending vasa recta (DVR) undergo regulatory adaptations after the kidney is exposed to ischemia. Left kidneys (LK) were subjected to 30-min renal artery cross clamp. After 48 h, the postischemic LK and contralateral right kidney (RK) were harvested for study. When compared with DVR isolated from either sham-operated LK or the contralateral RK, postischemic LK DVR markedly increased their NO generation. The selective inducible NOS (iNOS) inhibitor 1400W blocked the NO response. Immunoblots from outer medullary homogenates showed a parallel 2.6-fold increase in iNOS expression ( P = 0.01). Microperfused postischemic LK DVR exposed to angiotensin II (ANG II, 10 nM), constricted less than those from the contralateral RK, and constricted more when exposed to 1400W (10 µM). Resting membrane potentials of pericytes from postischemic LK DVR pericytes were hyperpolarized relative to contralateral RK pericytes (62.0 ± 1.6 vs. 51.8 ± 2.2 mV, respectively, P < 0.05) or those from sham-operated LK (54.9 ± 2.1 mV, P < 0.05). Blockade of NO generation with 1400W did not repolarize postischemic pericytes (62.5 ± 1.4 vs. 61.1 ± 3.4 mV); however, control pericytes were hyperpolarized by exposure to NO donation from S-nitroso- N-acetyl- dl-penicillamine (51.5 ± 2.9 to 62.1 ± 1.4 mV, P < 0.05). We conclude that postischemic adaptations intrinsic to the DVR wall occur after ischemia. A rise in 1400W sensitive NO generation and iNOS expression occurs that is associated with diminished contractile responses to ANG II. Pericyte hyperpolarization occurs that is not explained by the rise in ambient NO generation within the DVR wall.

High salt induces autocrine actions of ET-1 on inner medullary collecting duct NO production via upregulated ETB receptor expression

The collecting duct endothelin-1 (ET-1), endothelin B (ETB) receptor, and nitric oxide synthase-1 (NOS1) pathways are critical for regulation of fluid-electrolyte balance and blood pressure control during high-salt feeding. ET-1, ETB receptor, and NOS1 are highly expressed in the inner medullary collecting duct (IMCD) and vasa recta, suggesting that there may be cross talk or paracrine signaling between the vasa recta and IMCD. The purpose of this study was to test the hypothesis that endothelial cell-derived ET-1 (paracrine) and collecting duct-derived ET-1 (autocrine) promote IMCD nitric oxide (NO) production through activation of the ETB receptor during high-salt feeding. We determined that after 7 days of a high-salt diet (HS7), there was a shift to 100% ETB expression in IMCDs, as well as a twofold increase in nitrite production (a metabolite of NO), and this increase could be prevented by acute inhibition of the ETB receptor. ETB receptor blockade or NOS1 inhibition also prevented the ET-1-dependent decrease in ion transport from primary IMCDs, as determined by transepithelial resistance. IMCD were also isolated from vascular endothelial ET-1 knockout mice (VEETKO), collecting duct ET-1 KO (CDET-1KO), and flox controls. Nitrite production by IMCD from VEETKO and flox mice was similarly increased twofold with HS7. However, IMCD NO production from CDET-1KO mice was significantly blunted with HS7 compared with flox control. Taken together, these data indicate that during high-salt feeding, the autocrine actions of ET-1 via upregulation of the ETB receptor are critical for IMCD NO production, facilitating inhibition of ion reabsorption.

Descending Vasa Recta Endothelial Membrane Potential Response Requires Pericyte Communication

Using dual-cell electrophysiological recording, we examined the routes for equilibration of membrane potential between the pericytes and endothelia that comprise the descending vasa recta (DVR) wall. We measured equilibration between pericytes in intact vessels, between pericytes and endothelium in intact vessels and between pericytes physically separated from the endothelium. Dual pericyte recording on the abluminal surface of DVR showed that both resting potential and subsequent time-dependent voltage fluctuations after vasoconstrictor stimulation remained closely equilibrated, regardless of the agonist employed (angiotensin II, vasopressin or endothelin 1). When pericytes where removed from the vessel wall but retained physical contact with one another, membrane potential responses were also highly coordinated. In contrast, responses of pericytes varied independently when they were isolated from both the endothelium and from contact with one another. When pericytes and endothelium were in contact, their resting potentials were similar and their temporal responses to stimulation were highly coordinated. After completely isolating pericytes from the endothelium, their mean resting potentials became discordant. Finally, complete endothelial isolation eliminated all membrane potential responses to angiotensin II. We conclude that cell-to-cell transmission through the endothelium is not needed for pericytes to equilibrate their membrane potentials. AngII dependent responses of DVR endothelia may originate from gap junction coupling to pericytes rather than via receptor dependent signaling in the endothelium, per se.

Renal endothelial injury and microvascular dysfunction in acute kidney injury

The kidney is comprised of heterogeneous cell populations that function together to perform a number of tightly controlled, complex and interdependent processes. Renal endothelial cells contribute to vascular tone, regulation of blood flow to local tissue beds, modulation of coagulation and inflammation, and vascular permeability. Both ischemia and sepsis have profound effects on the renal endothelium, resulting in microvascular dysregulation resulting in continued ischemia and further injury. In recent years, the concept of the vascular endothelium as an organ that is both the source of and target for inflammatory injury has become widely appreciated. Here we revisit the renal endothelium in the light of ever evolving molecular advances.

Vitamin D deficiency is a potential risk factor for contrast-induced nephropathy

Vitamin D deficiency (VDD) is widespread in the general population. Iodinated (IC) or gadolinium-based contrast media (Gd) may decrease renal function in high-risk patients. This study tested the hypothesis that VDD is a predisposing factor for IC- or Gd-induced nephrotoxicity. To this end, male Wistar rats were fed standard (SD) or vitamin D-free diet for 30 days. IC (diatrizoate), Gd (gadoterate meglumine), or 0.9% saline was then administered intravenously and six groups were obtained as the following: SD plus 0.9% saline (Sham-SD), SD plus IC (SD+IC), SD plus Gd (SD+Gd), vitamin D-free diet for 30 days plus 0.9% saline (Sham-VDD30), vitamin D-free diet for 30 days plus IC (VDD30+IC), and vitamin D-free diet for 30 days plus Gd (VDD30+Gd). Renal hemodynamics, redox status, histological, and immunoblot analysis were evaluated 48 h after contrast media (CM) or vehicle infusion. VDD rats showed lower levels of total serum 25-hydroxyvitamin D [25(OH)D], similar plasma calcium and phosphorus concentration, and higher renal renin and angiotensinogen protein expression compared with rats fed SD. IC or Gd infusion did not affect inulin clearance-based estimated glomerular filtration rate (GFR) in rats fed SD but significantly decreased GFR in rats fed vitamin D-free diet. Both CM increased renal angiotensinogen, and the interaction between VDD and CM triggered lower renal endothelial nitric oxide synthase abundance and higher renal thiobarbituric acid reactive substances-to-glutathione ratio (an index of oxidative stress) on VDD30+IC and VDD30+Gd groups. Conversely, worsening of renal function was not accompanied by abnormalities on kidney structure. Additionally, rats on a VDD for 60 days displayed a greater fall in GFR after CM administration. Collectively, our findings suggest that VDD is a potential risk factor for IC- or Gd-induced nephrotoxicity most likely due to imbalance in intrarenal vasoactive substances and oxidative stress.

Inhalation of Hydrogen Gas Is Beneficial for Preventing Contrast-Induced Acute Kidney Injury in Rats

Background: The present study aimed at investigating the effect of a novel antioxidant, hydrogen (H2) gas, on the severity of contrast-induced acute kidney injury (CIAKI) in a rat model. Methods: CIAKI was induced in rats by intravenous injection of a contrast medium, Ioversol, in addition to reagents inhibiting prostaglandin and nitric oxide synthesis. During the injection of these reagents, the rats inhaled H2 gas or control gas. Results: One day after the injection, serum levels of urea nitrogen were significantly lower in H2 gas-inhaling CIAKI rats (17.6 ± 2.3 mg/dl) than those in control gas-treated CIAKI rats (36.0 ± 7.3 mg/dl), although they both were elevated as compared to untreated rats (14.9 ± 0.9 mg/dl). Consistently, creatinine clearance in H2 gas-treated CIAKI rats was higher than that in control gas-treated counterparts. Renal histological analysis revealed that the formation of proteinaceous casts and tubular necrosis was improved by H2 gas inhalation. Mechanistic analyses showed that inhalation of H2 gas significantly reduced renal cell apoptosis, expression of cleaved caspase 3, and expression of an oxidative stress marker, 8-hydroxydeoxyguanosine, in injured kidneys. Conclusion: Results suggest that H2 gas inhalation is effective in ameliorating the severity of CIAKI in rats by reducing renal cell apoptosis and oxidative stress. © 2015 S. Karger AG, Basel.

Reactive oxygen species as important determinants of medullary flow, sodium excretion, and hypertension

The physiological evidence linking the production of superoxide, hydrogen peroxide, and nitric oxide in the renal medullary thick ascending limb of Henle (mTAL) to regulation of medullary blood flow, sodium homeostasis, and long-term control of blood pressure is summarized in this review. Data obtained largely from rats indicate that experimentally induced elevations of either superoxide or hydrogen peroxide in the renal medulla result in reduction of medullary blood flow, enhanced Na(+) reabsorption, and hypertension. A shift in the redox balance between nitric oxide and reactive oxygen species (ROS) is found to occur naturally in the Dahl salt-sensitive (SS) rat model, where selective reduction of ROS production in the renal medulla reduces salt-induced hypertension. Excess medullary production of ROS in SS rats emanates from the medullary thick ascending limbs of Henle [from both the mitochondria and membrane NAD(P)H oxidases] in response to increased delivery and reabsorption of excess sodium and water. There is evidence that ROS and perhaps other mediators such as ATP diffuse from the mTAL to surrounding vasa recta capillaries, resulting in medullary ischemia, which thereby contributes to hypertension.

Molecular mechanisms of renal cellular nephrotoxicity due to radiocontrast media

Modern iodinated radiocontrast media are all based on the triiodinated benzene ring with various chemical modifications having been made over the last few decades in order to reduce their toxicity. However, CIN remains a problem especially in patients with pre-existing renal failure. In vitro studies have demonstrated that all RCM are cytotoxic. RCM administration in vivo may lead to a decrease in renal medullary oxygenation leading to the generation of reactive oxygen species that may cause harmful effects to renal tissue. In addition, endothelin and adenosine release and decreased nitric oxide levels may worsen the hypoxic milieu. In vitro cell culture studies together with sparse in vivo rat model data have shown that important cell signalling pathways are affected by RCM. In particular, the prosurvival and proproliferative kinases Akt and ERK1/2 have been shown to be dephosphorylated (deactivated), whilst proinflammatory/cell death molecules such as the p38 and JNK kinases and the transcription factor NF- κ B may be activated by RCM, accompanied by activation of apoptotic mediators such as caspases. Increasing our knowledge of the mechanisms of RCM action may help to develop future therapies for CIN.

Chronic activation of vasopressin V2 receptor signalling lowers renal medullary oxygen levels in rats

AIM

In the present study, we aimed to elucidate the effects of chronic vasopressin administration on renal medullary oxygen levels.

METHODS

Adult Sprague Dawley or vasopressin-deficient Brattleboro rats were treated with the vasopressin V2 receptor agonist, desmopressin (5 ng/h; 3d), or its vehicle via osmotic minipumps. Immunostaining for pimonidazole and the transcription factor HIF-1α (hypoxia-inducible factor-1α) were used to identify hypoxic areas. Activation of HIF-target gene expression following desmopressin treatment was studied by microarray analysis.

RESULTS

Pimonidazole staining was detected in the outer and inner medulla of desmopressin-treated rats, whereas staining in control animals was weak or absent. HIF-1α immunostaining demonstrated nuclear accumulation in the papilla of desmopressin-treated animals, whereas no staining was observed in the controls. Gene expression analysis revealed significant enrichment of HIF-target genes in the group of desmopressin-regulated gene products (P = 2.6*10(-21) ). Regulated products included insulin-like growth factor binding proteins 1 and 3, angiopoietin 2, fibronectin, cathepsin D, hexokinase 2 and cyclooxygenase 2.

CONCLUSION

Our results demonstrate that an activation of the renal urine concentrating mechanism by desmopressin causes renal medullary hypoxia and an upregulation of hypoxia-inducible gene expression.

Renal pericytes: regulators of medullary blood flow

Regulation of medullary blood flow (MBF) is essential in maintaining normal kidney function. Blood flow to the medulla is supplied by the descending vasa recta (DVR), which arise from the efferent arterioles of juxtamedullary glomeruli. DVR are composed of a continuous endothelium, intercalated with smooth muscle-like cells called pericytes. Pericytes have been shown to alter the diameter of isolated and in situ DVR in response to vasoactive stimuli that are transmitted via a network of autocrine and paracrine signalling pathways. Vasoactive stimuli can be released by neighbouring tubular epithelial, endothelial, red blood cells and neuronal cells in response to changes in NaCl transport and oxygen tension. The experimentally described sensitivity of pericytes to these stimuli strongly suggests their leading role in the phenomenon of MBF autoregulation. Because the debate on autoregulation of MBF fervently continues, we discuss the evidence favouring a physiological role for pericytes in the regulation of MBF and describe their potential role in tubulo-vascular cross-talk in this region of the kidney. Our review also considers current methods used to explore pericyte activity and function in the renal medulla.

Intake of Gnetum africanum and Dacryodes edulis, imbalance of oxidant/antioxidant status and prevalence of diabetic retinopathy in central Africans

OBJECTIVE

To estimate the prevalence of DR and to correlate cardiometabolic, sociodemographic, and oxidant/antioxidant imbalance data to the prevalence of DR.

DESIGN

This case-control study included type 2 DM (T2 DM) patients with DR (n = 66), T2 DM patients without DR (N = 84), and healthy controls (n = 45) without DR, in Kinshasa town. Diet, albuminemia, serum vitamins, and 8-isoprostane were examined.

RESULTS

No intake of safou (OR = 2.7 95% CI 1.2-5.8; P = 0.014), low serum albumin <4.5 g/dL (OR-2.9 95% CI 1.4-5.9; P = 0.003), no intake of fumbwa (OR = 2.8 95% CI 1.2-6.5; P = 0.014), high 8-isoprostane (OR = 14.3 95% CI 4.5-46; P<0.0001), DM duration ≥ 5 years (OR = 3.8 95% CI 1.6-9.1; P = 0.003), and low serum vitamin C (OR = 4.5 95% CI 1.3-15.5; P = 0.016) were identified as the significant independent determinants of DR.

CONCLUSION

The important role of oxidant/antioxidant status imbalance and diet is demonstrated in DR.

Iodinated contrast media cause endothelial damage leading to vasoconstriction of human and rat vasa recta

Contrast-induced acute kidney injury is an important clinical event with a worldwide increasing number of cases. Medullary hypoperfusion and hypoxia due to constriction of vasa recta are main factors in the pathophysiology of acute kidney injury. However, the mechanism of contrast media (CM)-induced vessel constriction is not known. We tested the hypothesis that vasa recta constriction is a consequence of endothelial dysfunction due to the cytotoxicity of CM. Human and rat descending vasa recta (DVR) were isolated and perfused with CM, and the luminal diameter was analyzed. For morphological analysis of the endothelium, renal arteries were CM perfused and then processed for electron microscopy. Transcellular electrical resistance was used to estimate CM-induced changes in the permeability of human umbilical vein endothelial cell (HUVEC) layers. Perfusion with CM constricted human and rat DRV (to 54.3 and 50.9% of initial diameter, respectively). This was blunted by adrenomedullin (77.7 and 77.1%, respectively). The ANG II response was enhanced by CM in rat DVR (reduction to 15.6 and 35.0% of initial diameter, respectively). Adrenomedullin blunted this effect (67.5%). CM led to endothelial damage of renal arteries characterized by a ragged surface, with sharply protruding intimal folds, spindle-like shape, and bulging in the lumen. These phenomena were reduced by adrenomedullin. The permeability of HUVEC cell layers was increased by CM, and this went along with increased myosin light chain phosporylation. Again, adremonedullin reduced the CM effect. Our study suggests that the constrictor effect of CM on the renal medullary microvasculature is a consequence of endothelial cell damage and the resulting endothelial dysfunction.

Medullary thick ascending limb buffer vasoconstriction of renal outer-medullary vasa recta in salt-resistant but not salt-sensitive rats

We have demonstrated previously that paracrine signaling occurs between medullary thick ascending limb (mTAL) and the contractile pericytes of outer-medullary vasa recta (VR), termed "tubular-vascular cross-talk." The aim of the current study was to determine whether tubular-vascular cross-talk has a functional effect on vasoconstrictor responses to angiotensin II and to determine whether this is altered in the Dahl salt-sensitive (SS) rat. Studies were performed on salt-resistant consomic SS.13 Brown Norway (BN) and SS rats using a novel outer medullary tissue strip preparation in which freshly isolated VRs within VR bundles were perfused either alone or in combination with nearby mTAL. In VRs from SS.13BN rats, angiotensin II (1 µmol/L) increased VR bundle intracellular Ca2+ concentration 19±9 nmol/L (n=8) and reduced focal diameter in perfused VRs by -20±7% (n=5). In the presence of nearby mTAL, however, VR bundle intracellular Ca2+ concentration (-9±8 nmol/L; n=8) and VR diameter (-1±4%, n=7) in SS.13BN rats were unchanged by angiotensin II. In contrast, in Dahl SS rats, angiotensin II resulted in rapid and sustained increase in VR bundle intracellular Ca2+ concentration (89±48 nmol/L, n=7; 50±24%, n=8) and a reduction in VR diameter of (-17±7%, n=7; -11±4%, n=5) in both isolated VRs and VRs with nearby mTAL, respectively. In VRs with mTAL from SS13BN rats, inhibiton of purinergic receptors resulted in an increase in VR bundle intracellular Ca2+ concentration, indicating that purinergic signaling buffers vasoconstriction. Importantly, our in vitro data were able to predict medullary blood flow responses to angiotensin II in SS and SS.13BN rats in vivo. We conclude that paracrine signaling from mTAL buffers angiotensin II vasoconstriction in Dahl salt-resistant SS.13BN rats but not SS rats.

Enhanced vascular neuronal nitric-oxide synthase-derived nitric-oxide production underlies the pressor response caused by peripheral N-methyl-D-aspartate receptor activation in conscious rats

Although the N-methyl-D-aspartate (NMDA) receptor (NMDAR) obligatory unit NMDAR1 is expressed in the vasculature and myocardium, the impact of peripheral NMDAR activation on blood pressure (BP) has received little attention. We demonstrate, for the first time, dose-related pressor responses elicited by systemic NMDA (125, 250, 500, and 1000 μg/kg) in conscious rats. The pressor response was peripheral NMDAR-mediated because: 1) it persisted after ganglion blockade (hexamethonium; 5 mg/kg i.v.); 2) it was attenuated by the selective NMDAR blocker DL-2-amino-5-phosphonopentanoic acid (5 mg/kg, i.v.) or the glycine/NMDAR antagonist R-(+)-3-amino-1-hydroxypyrrolid-2-one [R-(+)-HA-966; 10 mg/kg i.v.]; and 3) NMDA (1.25-10 mM) increased contractile force of rat aorta in vitro. It is noteworthy that ex vivo studies revealed enhanced nitric oxide (NO) and reactive oxygen species (ROS) generation in vascular tissues collected at the peak of the NMDAR-mediated pressor response. Pharmacological, ex vivo, and in vitro findings demonstrated attenuation of the NMDAR-mediated increases in BP and vascular NO and ROS by the nonselective NO synthase (NOS) inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride (10 mg/kg i.v.) or the neuronal NOS (nNOS) inhibitor N(ω)-propyl-L-arginine hydrochloride (150 μg/kg i.p.) but not by the endothelial NOS inhibitor N(5)-(1-iminoethyl)-L-ornithine (4 or 10 mg/kg i.v.). Furthermore, R-(+)-HA-966 attenuated NMDA-evoked generation of vascular NO and ROS. The findings suggest a pivotal role for enhanced vascular nNOS-derived NO in ROS generation and in the subsequent pressor response elicited by peripheral NMDAR in conscious rats.

Reactive Oxygen Species Modulation of Na/K-ATPase Regulates Fibrosis and Renal Proximal Tubular Sodium Handling

The Na/K-ATPase is the primary force regulating renal sodium handling and plays a key role in both ion homeostasis and blood pressure regulation. Recently, cardiotonic steroids (CTS)-mediated Na/K-ATPase signaling has been shown to regulate fibrosis, renal proximal tubule (RPT) sodium reabsorption, and experimental Dahl salt-sensitive hypertension in response to a high-salt diet. Reactive oxygen species (ROS) are an important modulator of nephron ion transport. As there is limited knowledge regarding the role of ROS-mediated fibrosis and RPT sodium reabsorption through the Na/K-ATPase, the focus of this review is to examine the possible role of ROS in the regulation of Na/K-ATPase activity, its signaling, fibrosis, and RPT sodium reabsorption.

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

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

Pathophysiology of renal tissue damage by iodinated contrast media

1. The present review focuses on the cytotoxic effects of iodinated contrast media (CM) that are shared by all types of CM. 2. Although the clinical nephrotoxicity of CM has been progressively improved, all currently available CM still possess a level of cytotoxicity, which is probably caused by iodine. 3. The toxicity caused by specific CM properties, such as osmolarity, viscosity and ionic strength, can be differentiated from the cytotoxicity common to all CM in studies using cell culture, isolated blood vessels and isolated tubules. 4. The cytotoxicity induced by CM leads to apoptosis and cell death of endothelial and tubular cells and may be initiated by cell membrane damage, together with oxidative stress. 5. Cell damage may be aggravated by factors such as tissue hypoperfusion and hypoxia, properties of individual CM, such as ionic strength, high osmolarity and/or viscosity, and clinically unfavourable conditions. 6. Clinically detectable renal failure may result from the summation of all these factors.

Modulation of outer medullary NaCl transport and oxygenation by nitric oxide and superoxide

We expanded our region-based model of water and solute exchanges in the rat outer medulla to incorporate the transport of nitric oxide (NO) and superoxide (O(2)(-)) and to examine the impact of NO-O(2)(-) interactions on medullary thick ascending limb (mTAL) NaCl reabsorption and oxygen (O(2)) consumption, under both physiological and pathological conditions. Our results suggest that NaCl transport and the concentrating capacity of the outer medulla are substantially modulated by basal levels of NO and O(2)(-). Moreover, the effect of each solute on NaCl reabsorption cannot be considered in isolation, given the feedback loops resulting from three-way interactions between O(2), NO, and O(2)(-). Notwithstanding vasoactive effects, our model predicts that in the absence of O(2)(-)-mediated stimulation of NaCl active transport, the outer medullary concentrating capacity (evaluated as the collecting duct fluid osmolality at the outer-inner medullary junction) would be ∼40% lower. Conversely, without NO-induced inhibition of NaCl active transport, the outer medullary concentrating capacity would increase by ∼70%, but only if that anaerobic metabolism can provide up to half the maximal energy requirements of the outer medulla. The model suggests that in addition to scavenging NO, O(2)(-) modulates NO levels indirectly via its stimulation of mTAL metabolism, leading to reduction of O(2) as a substrate for NO. When O(2)(-) levels are raised 10-fold, as in hypertensive animals, mTAL NaCl reabsorption is significantly enhanced, even as the inefficient use of O(2) exacerbates hypoxia in the outer medulla. Conversely, an increase in tubular and vascular flows is predicted to substantially reduce mTAL NaCl reabsorption. In conclusion, our model suggests that the complex interactions between NO, O(2)(-), and O(2) significantly impact the O(2) balance and NaCl reabsorption in the outer medulla.

Cellular mechanisms underlying nitric oxide-induced vasodilation of descending vasa recta

It has been observed that vasoactivity of explanted descending vasa recta (DVR) is modulated by intrinsic nitric oxide (NO) and superoxide (O(2)(-)) production (Cao C, Edwards A, Sendeski M, Lee-Kwon W, Cui L, Cai CY, Patzak A, Pallone TL. Am J Physiol Renal Physiol 299: F1056-F1064, 2010). To elucidate the cellular mechanisms by which NO, O(2)(-) and hydrogen peroxide (H(2)O(2)) modulate DVR pericyte cytosolic Ca(2+) concentration ([Ca](cyt)) and vasoactivity, we expanded our mathematical model of Ca(2+) signaling in pericytes. We incorporated simulations of the pathways that translate an increase in [Ca](cyt) to the activation of myosin light chain (MLC) kinase and cell contraction, as well as the kinetics of NO and reactive oxygen species formation and their effects on [Ca](cyt) and MLC phosphorylation. The model reproduced experimentally observed trends of DVR vasoactivity that accompany exposure to N(ω)-nitro-L-arginine methyl ester, 8-Br-cGMP, Tempol, and H(2)O(2). Our results suggest that under resting conditions, NO-induced activation of cGMP maintains low levels of [Ca](cyt) and MLC phosphorylation to minimize basal tone. This results from stimulation of Ca(2+) uptake from the cytosol into the SR via SERCA pumps, Ca(2+) efflux into the extracellular space via plasma membrane Ca(2+) pumps, and MLC phosphatase (MLCP) activity. We predict that basal concentrations of O(2)(-) and H(2)O(2) have negligible effects on Ca(2+) signaling and MLC phosphorylation. At concentrations above 1 nM, O(2)(-) is predicted to modulate [Ca(cyt)] and MCLP activity mostly by reducing NO bioavailability. The DVR vasoconstriction that is induced by high concentrations of H(2)O(2) can be explained by H(2)O(2)-mediated downregulation of MLCP and SERCA activity. We conclude that intrinsic generation of NO by the DVR wall may be sufficient to inhibit vasoconstriction by maintaining suppression of MLC phosphorylation.