Renal oxidative stress in medullary thick ascending limbs produced by elevated NaCl and glucose

Reactive Oxygen Species (ROS) in Metabolic Disease-Don't Shoot the Metabolic Messenger

Reactive oxygen species (ROS) are widely considered key to pathogenesis in chronic metabolic disease. Consequently, much attention is rightly focused on minimising oxidative damage. However, for ROS production to be most effectively modulated, it is crucial to first appreciate that ROS do not solely function as pathological mediators. There are >90 gene products specifically evolved to generate, handle, and tightly buffer the cellular concentration of ROS. Therefore, it is likely that ROS plays a role as integral homeostatic signalling components and only become toxic in extremis. This review explores these commonly overlooked normal physiological functions, including how ROS are generated in response to environmental or hormonal stimuli, the mechanisms by which the signals are propagated and regulated, and how the cell effectively brings the signal to an end after an appropriate duration. In the course of this, several specific and better-characterised signalling mechanisms that rely upon ROS are explored, and the threshold at which ROS cross from beneficial signalling molecules to pathology mediators is discussed.

Association of Serum Uric Acid Level and Thyroid Function in Chronic Kidney Disease: A Hospital-Based Cross-Sectional Study From Northeast India

BACKGROUND

Although the prevalence of thyroid dysfunction and hyperuricemia are independently high in patients with chronic kidney disease (CKD), there are limited data showing the association of serum uric acid and thyroid function in those with CKD.

AIM AND OBJECTIVES

The aim of this study was to observe the alteration of both the serum uric acid level and thyroid function in CKD patients and to find the association between both.

MATERIALS AND METHODS

This observational cross-sectional study was conducted in a tertiary care hospital over a period of one year in Northeast India. A total of 50 CKD patients were enrolled. Their demographic profiles were studied. Serum urea, creatinine, thyroid-stimulating hormone (TSH), total triiodothyronine (TT3), free triiodothyronine (FT3), total tetraiodothyronine (TT4), and free tetraiodothyronine (FT4) levels were measured to establish the correlation of serum uric acid along with each of the parameters separately. A p-value of <0.05 was considered statistically significant.

RESULTS

In the CKD patients studied, serum uric acid exhibited positive correlations with serum creatinine (p = 0.001, r = 0.67), serum urea (p = 0.001, r = 0.69), and serum TSH levels (p = 0.001, r = 0.5). Conversely, serum uric acid showed negative correlations with serum TT4 (p = 0.001, r = -0.74), TT3 (p = 0.001, r = -0.6), FT4 (p = 0.001, r = -0.53), and FT3 (p = 0.001, r = -0.58) levels.

CONCLUSION

 There was a significant positive correlation between uric acid and TSH levels in CKD patients. Thus, early estimation of both parameters should be considered in CKD patients.

Protein Quality Control of NKCC2 in Bartter Syndrome and Blood Pressure Regulation

Mutations in NKCC2 generate antenatal Bartter syndrome type 1 (type 1 BS), a life-threatening salt-losing nephropathy characterized by arterial hypotension, as well as electrolyte abnormalities. In contrast to the genetic inactivation of NKCC2, inappropriate increased NKCC2 activity has been associated with salt-sensitive hypertension. Given the importance of NKCC2 in salt-sensitive hypertension and the pathophysiology of prenatal BS, studying the molecular regulation of this Na-K-2Cl cotransporter has attracted great interest. Therefore, several studies have addressed various aspects of NKCC2 regulation, such as phosphorylation and post-Golgi trafficking. However, the regulation of this cotransporter at the pre-Golgi level remained unknown for years. Similar to several transmembrane proteins, export from the ER appears to be the rate-limiting step in the cotransporter's maturation and trafficking to the plasma membrane. The most compelling evidence comes from patients with type 5 BS, the most severe form of prenatal BS, in whom NKCC2 is not detectable in the apical membrane of thick ascending limb (TAL) cells due to ER retention and ER-associated degradation (ERAD) mechanisms. In addition, type 1 BS is one of the diseases linked to ERAD pathways. In recent years, several molecular determinants of NKCC2 export from the ER and protein quality control have been identified. The aim of this review is therefore to summarize recent data regarding the protein quality control of NKCC2 and to discuss their potential implications in BS and blood pressure regulation.

AUP1 Regulates the Endoplasmic Reticulum-Associated Degradation and Polyubiquitination of NKCC2

Inactivating mutations of kidney Na-K-2Cl cotransporter NKCC2 lead to antenatal Bartter syndrome (BS) type 1, a life-threatening salt-losing tubulopathy. We previously reported that this serious inherited renal disease is linked to the endoplasmic reticulum-associated degradation (ERAD) pathway. The purpose of this work is to characterize further the ERAD machinery of NKCC2. Here, we report the identification of ancient ubiquitous protein 1 (AUP1) as a novel interactor of NKCC2 ER-resident form in renal cells. AUP1 is also an interactor of the ER lectin OS9, a key player in the ERAD of NKCC2. Similar to OS9, AUP1 co-expression decreased the amount of total NKCC2 protein by enhancing the ER retention and associated protein degradation of the cotransporter. Blocking the ERAD pathway with the proteasome inhibitor MG132 or the α-mannosidase inhibitor kifunensine fully abolished the AUP1 effect on NKCC2. Importantly, AUP1 knock-down or inhibition by overexpressing its dominant negative form strikingly decreased NKCC2 polyubiquitination and increased the protein level of the cotransporter. Interestingly, AUP1 co-expression produced a more profound impact on NKCC2 folding mutants. Moreover, AUP1 also interacted with the related kidney cotransporter NCC and downregulated its expression, strongly indicating that AUP1 is a common regulator of sodium-dependent chloride cotransporters. In conclusion, our data reveal the presence of an AUP1-mediated pathway enhancing the polyubiquitination and ERAD of NKCC2. The characterization and selective regulation of specific ERAD constituents of NKCC2 and its pathogenic mutants could open new avenues in the therapeutic strategies for type 1 BS treatment.

The Role of Macula Densa Nitric Oxide Synthase 1 Beta Splice Variant in Modulating Tubuloglomerular Feedback

Abnormalities in renal electrolyte and water excretion may result in inappropriate salt and water retention, which facilitates the development and maintenance of hypertension, as well as acid-base and electrolyte disorders. A key mechanism by which the kidney regulates renal hemodynamics and electrolyte excretion is via tubuloglomerular feedback (TGF), an intrarenal negative feedback between tubules and arterioles. TGF is initiated by an increase of NaCl delivery at the macula densa cells. The increased NaCl activates luminal Na-K-2Cl cotransporter (NKCC2) of the macula densa cells, which leads to activation of several intracellular processes followed by the production of paracrine signals that ultimately result in a constriction of the afferent arteriole and a tonic inhibition of single nephron glomerular filtration rate. Neuronal nitric oxide (NOS1) is highly expressed in the macula densa. NOS1β is the major splice variant and accounts for most of NO generation by the macula densa, which inhibits TGF response. Macula densa NOS1β-mediated modulation of TGF responses plays an essential role in control of sodium excretion, volume and electrolyte hemostasis, and blood pressure. In this article, we describe the mechanisms that regulate macula densa-derived NO and their effect on TGF response in physiologic and pathologic conditions. © 2023 American Physiological Society. Compr Physiol 13:4215-4229, 2023.

Oxidative Stress Induced by High Salt Diet—Possible Implications for Development and Clinical Manifestation of Cutaneous Inflammation and Endothelial Dysfunction in Psoriasis vulgaris

Although oxidative stress is recognized as an important effector mechanism of the immune system, uncontrolled formation of reactive oxygen and nitrogen species promotes excessive tissue damage and leads to disease development. In view of this, increased dietary salt intake has been found to damage redox systems in the vessel wall, resulting in endothelial dysfunction associated with NO uncoupling, inflammation, vascular wall remodeling and, eventually, atherosclerosis. Several studies have reported increased systemic oxidative stress accompanied by reduced antioxidant capacity following a high salt diet. In addition, vigorous ionic effects on the immune mechanisms, such as (trans)differentiation of T lymphocytes are emerging, which together with the evidence of NaCl accumulation in certain tissues warrants a re-examination of the data derived from in vitro research, in which the ionic influence was excluded. Psoriasis vulgaris (PV), as a primarily Th17-driven inflammatory skin disease with proven inflammation-induced accumulation of sodium chloride in the skin, merits our interest in the role of oxidative stress in the pathogenesis of PV, as well as in the possible beneficial effects that could be achieved through modulation of dietary salt intake and antioxidant supplementation.

Hydrogen peroxide (H2O2) mediated activation of mTORC2 increases intracellular Na+ concentration in the renal medullary thick ascending limb of Henle

Hydrogen peroxide (H2O2) production in the renal outer medulla is an important determinant of renal medullary blood flow and blood pressure (BP) salt-sensitivity in Dahl salt-sensitive (SS) rats. The mechanisms and pathways responsible for these actions are poorly understood. Recently, we have discovered that the mTOR complex 2 (mTORC2) plays a critical role in BP salt-sensitivity of SS rats by regulating Na+ homeostasis. PP242, an inhibitor of mTORC1/2 pathways exhibits potent natriuretic actions and completely prevented salt-induced hypertension in SS rats. In the present study, we have found that chronic infusion of H2O2 into the single remaining kidney of Sprague Dawley (SD) rats (3 days) stimulated the functional marker (pAKTSer473/AKT) of mTORC2 activity measured by Western Blot analysis. No changes in mTORC1 activity in OM were observed as determined by pS6Ser235/236/S6. Using fluorescent microscopy and the Na+ sensitive dye Sodium Green, we have shown that H2O2 (100 µM added in the bath) increased intracellular sodium concentration ([Na+]i) in renal medullary thick ascending limbs (mTALs) isolated from SD rats. These responses were almost completely abolished by pretreatment of mTAL with 10 µM PP242, indicating that mTORC1/2 pathways were involved in the H2O2 induced increase of [Na+]i. mTAL cell volume remained unchanged (± 1%) by H2O2 as determined by 3D reconstruction confocal laser scanning microscopy techniques. Consistent with the microscopy data, Western Blot analysis of proteins obtained from freshly isolated mTAL treated with 100 µM H2O2 exhibited increased activity/phosphorylation of AKT (pAKTSer473/AKT) that was inhibited by PP242. This was associated with increased protein activity of the apical membrane cotransporter Na+-K+-2Cl- (NKCC2) and the Na/H exchanger (NHE-3). Na+-K+-ATPase activity was increased as reflected an increase in the ratio of pNa+-K+-ATPaseSer16 to total Na+-K+-ATPase. Overall, the results indicate that H2O2 mediated activation of mTORC2 plays a key role in transducing the observed increases of cytosolic [Na+]i despite associated increases of basolateral pump activity.

The early effects of uninephrectomy on rat kidney metabolic state using optical imaging

Uninephrectomy (UNX) is known to result in structural and metabolic changes to the remaining kidney, although it is uncertain if this alters the mitochondrial redox state and how soon such changes may occur. A custom-designed fluorescence cryo-imaging technique was used to quantitatively assess the effect of UNX by measuring the levels of nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) in the remaining kidney. Kidneys were snap-frozen 3 days following UNX, and the intrinsic fluorescence of NADH and FAD were optically acquired. The 3D images were created to characterize the NADH/FAD redox ratios (RR) of the right kidneys, which underwent UNX and the remaining kidneys 3 days following UNX. Both the NADPH-oxidases (Nox2 and Nox4) and the mitochondria are the main sources of reactive oxygen species (ROS) production in tubular epithelial cells. Responses to the UNX were obtained in kidneys of normal Sprague Dawley (SD) rats, Dahl salt-sensitive (SS) rats and SS rats in which NADPH-oxidase isoform 4 (Nox4) was knocked out (SS^Nox4-/- ). The results found that each of the strains exhibited similar increase in kidney weights averaging 17% after 3 days of UNX. SD and SS^Nox4-/- rats both exhibited global reductions of the RR (P < .05) with a similar tendency observed in SS rats (P < .08), indicating increased ROS production. The unexpected reduction of the RR in the remnant kidneys of SS^Nox4-/- rats indicates that mechanisms independent of H₂ O₂ produced from Nox4 may be responsible for this global increase of ROS. We propose that the reduced RR was largely a consequence of enhanced mitochondrial bioenergetics due to increased tubular workload of the remaining kidney. The data indicate that mitochondria become the dominant source of increased ROS following UNX and could represent an important hypertrophic signaling mechanism.

The roles of melatonin on kidney injury in obese and diabetic conditions

Obesity is a common and complex health problem worldwide and can induce the development of Type 2 diabetes. Chronic kidney disease (CKD) is a complication occurring as a result of obesity and diabetic conditions that lead to an increased mortality rate. There are several mechanisms and pathways contributing to kidney injury in obese and diabetic conditions. The expansion of adipocytes triggers proinflammatory cytokines release into blood circulation and bind with the receptors at the cell membranes of renal tissues leading to kidney injury. Obesity-mediated inflammation, oxidative stress, apoptosis, and mitochondrial dysfunction are the important causes and progression of CKD. Melatonin (N-acetyl-5-methoxytryptamine) is a neuronal hormone that is synthesized by the pineal gland and plays an essential role in regulating several physiological functions in the human body. Moreover, melatonin has pleiotropic effects such as antioxidant, anti-inflammation, antiapoptosis. In this review, the relationship between obesity, diabetic condition, and kidney injury and the renoprotective effect of melatonin in obese and diabetic conditions from in vitro and in vivo studies have been summarized and discussed.

Voltage gated proton channels modulate mitochondrial reactive oxygen species production by complex I in renal medullary thick ascending limb

Hv1 is a voltage-gated proton channel highly expressed in immune cells where, it acts to maintain NAD(P)H oxidase activity during the respiratory burst. We have recently reported that Hv1 is expressed in cells of the medullary thick ascending limb (mTAL) of the kidney and is critical to augment reactive oxygen species (ROS) production by this segment. While Hv1 is associated with NOX2 mediated ROS production in immune cells, the source of the Hv1 dependent ROS in mTAL remains unknown. In the current study, the rate of ROS formation was quantified in freshly isolated mTAL using dihydroethidium and ethidium fluorescence. Hv1 dependent ROS production was stimulated by increasing bath osmolality and ammonium chloride (NH₄Cl) loading. Loss of either p67phox or NOX4 did not abolish the formation of ROS in mTAL. Hv1 was localized to mitochondria within mTAL, and the mitochondrial superoxide scavenger mitoTEMPOL reduced ROS formation. Rotenone significantly increased ROS formation and decreased mitochondrial membrane potential in mTAL from wild-type rats, while treatment with this inhibitor decreased ROS formation and increased mitochondrial membrane potential in mTAL from Hv1^−/− mutant rats. These data indicate that NADPH oxidase is not the primary source of Hv1 dependent ROS production in mTAL. Rather Hv1 localizes to the mitochondria in mTAL and modulates the formation of ROS by complex I. These data provide a potential explanation for the effects of Hv1 on ROS production in cells independent of its contribution to maintenance of cell membrane potential and intracellular pH.

Role of Nox4 and p67phox subunit of Nox2 in ROS production in response to increased tubular flow in the mTAL of Dahl salt-sensitive rats

Nox4 and Nox2 are the most abundant NADPH oxidases (Nox) in the kidney and have been shown to contribute to hypertension, renal oxidative stress, and injury in Dahl salt-sensitive (SS) hypertensive rats. The present study focused on the role of Nox4 and p67phox/Nox2 in the generation of H2O2 and O2 (·-) in the renal medullary thick ascending limb of Henle (mTAL) of SS rats in response to increasing luminal flow (from 5 to 20 nl/min). Nox4 and p67phox/Nox2 genes were found to be expressed in the mTAL of SS rats. Responses of SS rats were compared with those of SS rats with knockout of Nox4 (SS(Nox4-/-)) or functional mutation of p67phox (SS(p67phox-/-)). Nox4 was the dominant source of increased intracellular H2O2 production in response to increased luminal flow as determined using the fluorescent dye peroxyfluor 6-AM (PF6-AM). The rate of mitochondrial H2O2 production [as determined by mitochondria peroxy yellow 1 (mitoPY1)] was also significantly reduced in SS(Nox4-/-) compared with SS rats, but not in SS(p67phox-/-) rats. In contrast, intracellular superoxide (O2 (·-)) production (the ratio of ethidium to dihydroethidium) in the mTAL of SS(Nox4-/-) rats was nearly identical to that of SS rats in response to luminal flow, indicating that Nox4 made no measurable contribution. mTAL O2 (·-) production was reduced in SS(p67phox-/-) compared with SS rats at the lower luminal flow of 5 nl/min and progressively increased when perfusion was changed to 20 nl/min. We conclude that increased mTAL luminal flow results in increases in intracellular and mitochondrial H2O2, which are dependent on the presence of Nox4, and that p67phox/Nox2 accounts solely for increases in O2 (·-) production.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

OS9 Protein Interacts with Na-K-2Cl Co-transporter (NKCC2) and Targets Its Immature Form for the Endoplasmic Reticulum-associated Degradation Pathway

Mutations in the renal specific Na-K-2Cl co-transporter (NKCC2) lead to type I Bartter syndrome, a life-threatening kidney disease featuring arterial hypotension along with electrolyte abnormalities. We have previously shown that NKCC2 and its disease-causing mutants are subject to regulation by endoplasmic reticulum-associated degradation (ERAD). The aim of the present study was to identify the protein partners specifically involved in ERAD of NKCC2. To this end, we screened a kidney cDNA library through a yeast two-hybrid assay using NKCC2 C terminus as bait. We identified OS9 (amplified in osteosarcomas) as a novel and specific binding partner of NKCC2. Co-immunoprecipitation assays in renal cells revealed that OS9 association involves mainly the immature form of NKCC2. Accordingly, immunocytochemistry analysis showed that NKCC2 and OS9 co-localize at the endoplasmic reticulum. In cells overexpressing OS9, total cellular NKCC2 protein levels were markedly decreased, an effect blocked by the proteasome inhibitor MG132. Pulse-chase and cycloheximide-chase assays demonstrated that the marked reduction in the co-transporter protein levels was essentially due to increased protein degradation of the immature form of NKCC2. Conversely, knockdown of OS9 by small interfering RNA increased NKCC2 expression by increasing the co-transporter stability. Inactivation of the mannose 6-phosphate receptor homology domain of OS9 had no effect on its action on NKCC2. In contrast, mutations of NKCC2 N-glycosylation sites abolished the effects of OS9, indicating that OS9-induced protein degradation is N-glycan-dependent. In summary, our results demonstrate the presence of an OS9-mediated ERAD pathway in renal cells that degrades immature NKCC2 proteins. The identification and selective modulation of ERAD components specific to NKCC2 and its disease-causing mutants might provide novel therapeutic strategies for the treatment of type I Bartter syndrome.

Proton channels and renal hypertensive injury: a key piece of the Dahl salt-sensitive rat puzzle?

Hv1 is a voltage-gated proton channel highly expressed in phagocytic cells, where it participates in the NADPH oxidase-dependent respiratory burst. We have recently identified Hv1 as a novel renal channel, expressed in the renal medullary thick ascending limb that appears to importantly contribute to the pathogenesis of renal hypertensive injury in the Dahl salt-sensitive rat model. The purpose of this review is to describe the experimental approaches that we have undertaken to identify the source of excess reactive oxygen species production in the renal outer medulla of Dahl salt-sensitive rats and the resulting evidence that the voltage-gated proton channel Hv1 mediates augmented superoxide production and contributes to renal medullary oxidative stress and renal injury. In addition, we will attempt to point out areas of current controversy, as well as propose areas in which further experimental studies are likely to move the field forward. The content of the following review was presented as part of the Water and Electrolyte Homeostasis Section New Investigator Award talk at Experimental Biology 2014.

Rubbing salt into wounded endothelium: sodium potentiates proatherogenic effects of TNF-α under non-uniform shear stress

Increased consumption of sodium is a risk factor for hypertension and cardiovascular diseases. In vivo studies indicated that high dietary sodium may have a direct negative influence on endothelium. We investigated the effects of high sodium on the endothelial activation during early steps of atherogenesis. Endothelial cells (HUVECs) grown in a model of arterial bifurcations were exposed to shear stress in the presence of normal or high (+ 30 mmol/l) sodium. Adherent THP-1 cells, and the adhesion molecule expression were quantified. Sodium channel blockers, pathways' inhibitors, and siRNA against tonicity-responsive enhancer binding protein (TonEBP) were used to identify the mechanisms of sodium effects on endothelium. ApoE-deficient mice on low-fat diet received water containing normal or high salt (8% w/v) for four weeks, and the influence of dietary salt on inflammatory cell adhesion in the common carotid artery and carotid bifurcation was measured by intravital microscopy. In vitro, high sodium dramatically increased the endothelial responsiveness to tumour necrosis factor-α under non-uniform shear stress. Sodium-induced increase in monocytic cell adhesion was mediated by reactive oxygen species and the endothelial nitric oxygen synthase, and was sensitive to the knockdown of TonEBP. The results were subsequently confirmed in the ApoE-deficient mice. As compared with normal-salt group, high-salt intake significantly enhanced the adhesion of circulating CD11b+ cells to carotid bifurcations, but not to the straight segment of common carotid artery. In conclusion, elevated sodium has a direct effect on endothelial activation under atherogenic shear stress in vitro and in vivo, and promotes the endothelial-leukocyte interactions even in the absence of increased lipid concentrations.

Mesenchymal stem cells improve medullary inflammation and fibrosis after revascularization of swine atherosclerotic renal artery stenosis

Atherosclerotic renal artery stenosis (ARAS) raises blood pressure and can reduce kidney function. Revascularization of the stenotic renal artery alone does not restore renal medullary structure and function. This study tested the hypothesis that addition of mesenchymal stem cells (MSC) to percutaneous transluminal renal angioplasty (PTRA) can restore stenotic-kidney medullary tubular transport function and attenuate its remodeling. Twenty-seven swine were divided into three ARAS (high-cholesterol diet and renal artery stenosis) and a normal control group. Six weeks after ARAS induction, two groups were treated with PTRA alone or PTRA supplemented with adipose-tissue-derived MSC (10×10⁶ cells intra-renal). Multi-detector computed tomography and blood-oxygenation-level-dependent (BOLD) MRI studies were performed 4 weeks later to assess kidney hemodynamics and function, and tissue collected a few days later for histology and micro-CT imaging. PTRA effectively decreased blood pressure, yet medullary vascular density remained low. Addition of MSC improved medullary vascularization in ARAS+PTRA+MSC and increased angiogenic signaling, including protein expression of vascular endothelial growth-factor, its receptor (FLK-1), and hypoxia-inducible factor-1α. ARAS+PTRA+MSC also showed attenuated inflammation, although oxidative-stress remained elevated. BOLD-MRI indicated that MSC normalized oxygen-dependent tubular response to furosemide (-4.3±0.9, −0.1±0.4, −1.6±0.9 and −3.6±1.0 s⁻¹ in Normal, ARAS, ARAS+PTRA and ARAS+PTRA+MSC, respectively, p<0.05), which correlated with a decrease in medullary tubular injury score (R² = 0.33, p = 0.02). Therefore, adjunctive MSC delivery in addition to PTRA reduces inflammation, fibrogenesis and vascular remodeling, and restores oxygen-dependent tubular function in the stenotic-kidney medulla, although additional interventions might be required to reduce oxidative-stress. This study supports development of cell-based strategies for renal protection in ARAS.

ABCC1 is related to the protection of the distal nephron against hyperosmolality and high sodium environment: possible implications for cancer chemotherapy

Aims

Glutathione (GSH) plays an important role in protecting cells against oxidative damage. ABCC1 protein transports GSH. Although this protein is largely studied in cancer, due to multidrug resistance phenotype, its role in the tubular cells of the kidney is unknown. The goal of this study was to find out whether ABCC1 has a role in protecting cells from the distal nephron against the stress caused by high medullar osmolality.

Main Methods

MA104 cells were treated with high concentrations of sodium chloride, urea, or both to raise the osmolality of the culture medium. Cell viability was accessed by MTT and trypan blue assays. ABCC1 expression and extrusion of carboxi-fluorescein (CF), a fluorescent ABCC1 substrate, were measured by flow cytometry.

Key Findings

Incubation of MA104 cells in a high sodium concentration medium resulted in changes in cell granularity and altered expression and activity of ABCC1. Urea did not alter ABCC1 expression or activity, but reversed the observed NaCl effects. High sodium concentrations also had a negative effect on cell viability and urea also protected cells against this effect.

Significance

Our findings demonstrate that ABCC1 plays a significant role in the protection of kidney epithelial cells against the stress caused by high sodium environment present in renal medulla.

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.

Salt sensitivity of nitric oxide generation and blood pressure in mice with targeted knockout of the insulin receptor from the renal tubule

To elucidate the role of the insulin receptor (IR) on kidney nitric oxide generation and blood pressure (BP) control, we generated mice with targeted deletion of renal tubule IR using loxP recombination driven by a Ksp-cadherin promoter. Male knockout (KO) and wild-type (WT) littermates (~4 mo old) were transitioned through three 1-wk treatments: 1) low-NaCl diet (0.085%); 2) high-NaCl diet (HS; 5%); and 3) HS diet plus 3 mM tempol, a superoxide dismutase mimetic, in the drinking water. Mice were then switched to medium-NaCl (0.5%) diet for 5 days and kidneys harvested under pentobarbital anesthesia. Twenty-four-hour urinary nitrates plus nitrites were significantly higher in the WT mice under HS (2,067 ± 280 vs. 1,550 ± 230 nmol/day in WT and KO, respectively, P < 0.05). Tempol attenuated genotype differences in urinary nitrates plus nitrites. A rise in BP with HS was observed only in KO mice and not affected by tempol (mean arterial pressure, dark period, HS, 106 ± 5 vs. 119 ± 4 mmHg, for WT and KO, respectively, P < 0.05). Renal outer medullary protein levels of nitric oxide synthase (NOS) isoforms by Western blot (NOS1-3 and phosphorylated-S1177-NOS3) revealed significantly lower band density for NOS1 (130-kDa isoform) in the KO mice. A second study, when mice were euthanized under HS conditions, confirmed significantly lower NOS1 (130 kDa) in the KO, with an even more substantial (>50%) reduction of the 160-kDa NOS1 isoform. These studies suggest that the loss of renal IR signaling impairs renal nitric oxide production. This may be important in BP control, especially in insulin-resistant states, such as the metabolic syndrome.

Role of renal medullary oxidative and/or carbonyl stress in salt-sensitive hypertension and diabetes

1. Salt-sensitive hypertension is commonly associated with diabetes, obesity and chronic kidney disease. The present review focuses on renal mechanisms involved in the development of this type of hypertension. 2. The renal medullary circulation plays an important role in the development of salt-sensitive hypertension. In vivo animal studies have demonstrated that the balance between nitric oxide (NO) and reactive oxygen species (ROS) in the renal medulla is an important element of salt-sensitive hypertension. The medullary thick ascending limb (mTAL) in the outer medulla is an important source of NO and ROS production and we have explored the mechanisms that stimulate their production, as well as the effects of NO superoxide and hydrogen peroxide on mTAL tubular sodium reabsorption and the regulation of medullary blood flow. 3. Angiotensin II-stimulated NO produced in the mTAL is able to diffuse from the renal mTAL to the surrounding vasa recta capillaries, providing a mechanism by which to increase medullary blood flow and counteract the direct vasoconstrictor effects of angiotensin II. Enhanced oxidative stress attenuates NO diffusion in this region. 4. Carbonyl stress, like oxidative stress, can also play an important role in the pathogenesis of chronic kidney disease, such as insulin resistance, salt-sensitive hypertension and renal vascular complications. 5. Despite the large number of studies undertaken in this area, there is as yet no drug available that directly targets renal ROS. Oxidative and/or carbonyl stress may be the next target of drug discovery to protect against salt-sensitive hypertension and associated end-organ damage.

Interactions between thyroid disorders and kidney disease

There are several interactions between thyroid and kidney functions in each other organ's disease states. Thyroid hormones affect renal development and physiology. Thyroid hormones have pre-renal and intrinsic renal effects by which they increase the renal blood flow and the glomerular filtration rate (GFR). Hypothyroidism is associated with reduced GFR and hyperthyroidism results in increased GFR as well as increased renin - angiotensin - aldosterone activation. Chronic kidney disease (CKD) is characterized by a low T3 syndrome which is now considered a part of an atypical nonthyroidal illness. CKD patients also have increased incidence of primary hypothyroidism and subclinical hypothyroidism. The physiological benefits of a hypothyroid state in CKD, and the risk of CKD progression with hyperthyroidism emphasize on a conservative approach in the treatment of thyroid hormone abnormalities in CKD. Thyroid dysfunction is also associated with glomerulonephritis often by a common autoimmune etiology. Several drugs could affect both thyroid and kidney functions. There are few described interactions between thyroid and renal malignancies. A detailed knowledge of all these interactions is important for both the nephrologists and endocrinologists for optimal management of the patient.

Increase of sodium delivery stimulates the mitochondrial respiratory chain H2O2 production in rat renal medullary thick ascending limb

The mitochondria-rich epithelial cells of the renal medullary thick ascending limb (mTAL) reabsorb nearly 25% of filtered sodium (Na(+)) and are a major source of cellular reactive oxygen species. Although we have shown that delivery of Na(+) to the mTAL of rats increases superoxide (O(2)(·-)) production in mTAL, little is known about H(2)O(2) production, given the lack of robust and selective fluorescent indicators for determining changes within the whole cell, specifically in the mitochondria. The present study determined the effect of increased tubular flow and Na(+) delivery to mTAL on the production of mitochondrial H(2)O(2) in mTAL. H(2)O(2) responses were determined in isolated, perfused mTAL of Sprague-Dawley rats using a novel mitochondrial selective fluorescent H(2)O(2) indicator, mitochondria peroxy yellow 1, and a novel, highly sensitive and stable cytosolic-localized H(2)O(2) indicator, peroxyfluor-6 acetoxymethyl ester. The results showed that mitochondrial H(2)O(2) and cellular fluorescent signals increased progressively over a period of 30 min following increased tubular perfusion (5-20 nl/min), reaching levels of statistical significance at ∼10-12 min. Responses were inhibited with rotenone or antimycin A (inhibitors of the electron-transport chain), polyethylene glycol-catalase and by reducing Na(+) transport with furosemide or ouabain. Inhibition of membrane NADPH-oxidase with apocynin had no effect on mitochondrial H(2)O(2) production. Cytoplasmic H(2)O(2) (peroxyfluor-6 acetoxymethyl ester) increased in parallel with mitochondrial H(2)O(2) (mitochondria peroxy yellow 1) and was partially attenuated (∼65%) by rotenone and completely inhibited by apocynin. The present data provide clear evidence that H(2)O(2) is produced in the mitochondria in response to increased flow and delivery of Na(+) to the mTAL, and that whole cell H(2)O(2) levels are triggered by the mitochondrial reactive oxygen species production. The mitochondrial production of H(2)O(2) may represent an important target for development of more effective antioxidant therapies.

Osmoadaptation of Mammalian cells - an orchestrated network of protective genes

In mammals, the cells of the renal medulla are physiologically exposed to interstitial osmolalities several-fold higher that found in any other tissue. Nevertheless, these cells not only have the ability to survive in this harsh environment, but also to function normally, which is critical for maintenance of systemic electrolyte and fluid homeostasis. Over the last two decades, a substantial body of evidence has accumulated, indicating that sequential and well orchestrated genomic responses are required to provide tolerance to osmotic stress. This includes the enhanced expression and action of immediate-early genes, growth arrest and DNA damage inducible genes (GADDs), genes involved in cell cycle control and apoptosis, heat shock proteins, and ultimately that of genes involved in the intracellular accumulation of nonperturbing organic osmolytes. The present review summarizes the sequence of genomic responses conferring resistance against osmotic stress. In addition, the regulatory mechanisms mediating the coordinated genomic response to osmotic stress will be highlighted.

Production of reactive oxygen species in endothelial cells under different pulsatile shear stresses and glucose concentrations

A hemodynamic Lab-on-a-chip system was developed in this study. This system has two unique features: (1) it consists of a microfluidic network with an array of endothelial cell seeding sites for testing them under multiple conditions, and (2) the flow rate and the frequency of the culture medium in the microchannel are controlled by a pulsation free pump to mimic the flow profile of the blood in the blood vessel under different physiological conditions. The investigated physiological conditions were: (1) the resting condition in a normal shear stress of 15 dyne cm(-2) with a normal heart rate of 70 bpm, (2) an exhaustive exercise condition with a high shear stress of 30 dyne cm(-2) and a fast heart rate of 140 bpm, and (3) a constant high shear stress of 30 dyne cm(-2). Two chemical conditions were investigated (10 mM and 20 mM glucose) to mimic hyperglycemic conditions in diabetes patients. The effects of various shear stresses either alone or in combination with different glucose concentrations on endothelial cells were examined using the developed hemodynamic Lab-on-a-chip system by assessing two parameters. One is the intracellular level of reactive oxygen species (ROS) determined by a fluorescent probe, H(2)DCFDA. Another is the mitochondrial morphology revealed with a fluorescent dye, MitoTracker Green FM. The results showed that ROS level was elevated nearly 4-fold after 60 min of exhaustive exercise. We found that the pulsatile nature of the fluid was the determination factor for causing ROS generation in the cells as almost no increase of ROS was detected in the constant shear stress condition. Similarly, much higher level of ROS was detected when 10 mM glucose was applied to the cells under normal or high pulsatile shear stresses compared with under a static condition. These results suggest that it is necessary to use pulsatile shear stress to represent the physiological conditions of the blood flow, and demonstrate the advantage of utilizing this newly developed hemodynamic Lab-on-a-chip system over the conventional non-pulsatile system in the future shear stress related studies.

Effect of turmeric and its active principle curcumin on t(3)-induced oxidative stress and hyperplasia in rat kidney: a comparison

The present study was designed to compare the potential of turmeric and its active principle curcumin on T(3)-induced oxidative stress and hyperplasia. Adult male Wistar strain rats were rendered hyperthyroid by T(3) treatment (10 μg · 100 g(-1) · day(-1) intraperitoneal for 15 days in 0.1 mM NaOH) to induce renal hyperplasia. Another two groups were treated similarly with T(3) along with either turmeric or curcumin (30 mg kg(-1) body weight day(-1) orally for 15 days). The results indicate that T(3) induces both hypertrophy and hyperplasia in rat kidney as evidenced by increase in cell number per unit area, increased protein content, tubular dilation and interstitial edema. These changes were accompanied by increased mitochondrial lipid peroxidation and superoxide dismutase activity without any change in catalase activity and glutathione content suggesting an oxidative predominance. Both turmeric and curcumin were able to restore the level of mitochondrial lipid peroxidation and superoxide dismutase activity in the present dose schedule. T(3)-induced histo-pathological changes were restored with turmeric treatment whereas curcumin administration caused hypoplasia. This may be due to lower concentration of curcumin in the whole turmeric. Thus it is hypothesized that regulation of cell cycle in rat kidney by T(3) is via reactive oxygen species and curcumin reveres the changes by scavenging them. Although the response trends are comparable for both turmeric and curcumin, the magnitude of alteration is more in the later. Turmeric in the current dose schedule is a safer bet than curcumin in normalizing the T(3)-induced hyperplasia may be due to the lower concentration of the active principle in the whole spice.

Rac1 mediates NaCl-induced superoxide generation in the thick ascending limb

Superoxide (O(2)(-)) produced by NADPH oxidase regulates Na absorption and renal hemodynamics. Increased NaCl in the thick ascending limb (TAL) stimulates O(2)(-) generation. However, we do not know whether physiological changes in NaCl concentration augment O(2)(-) generation, nor do we know the mediator(s) involved. In other cells, Rac1, a regulatory subunit of NADPH oxidase, is activated by elevated NaCl. We hypothesized that increasing luminal NaCl within the physiological range activates Rac1 and NADPH oxidase and, thereby, increases O(2)(-) production. We increased NaCl from 10 to 57 mM in medullary TAL suspensions and used lucigenin to measure O(2)(-) generation and Western blot to measure Rac1 activity. Increasing NaCl stimulated O(2)(-) generation from 1.41 +/- 0.16 to 2.71 +/- 0.30 nmol O(2)(-) x min(-1) x mg protein(-1) (n = 6, P < 0.05). This increase was blocked by the Na-K-2Cl cotransporter inhibitor furosemide and the NADPH oxidase inhibitor apocynin. To examine the role of Rac1 in NaCl-induced O(2)(-) production, we measured Rac1 translocation by Western blot. When we added NaCl, Rac1 in the particulate fraction increased from 6.8 +/- 0.8 to 11.7 +/- 2.4% of total Rac1 (n = 7, P < 0.05). Then we measured O(2)(-) generation in the presence and absence of the Rac1 inhibitor. In the absence of the Rac1 inhibitor, NaCl increased O(2)(-) generation from 1.07 +/- 0.24 to 2.02 +/- 0.49 nmol O(2)(-) x min(-1) x mg protein(-1), and this increase was completely blocked by the inhibitor. Similarly, in vivo treatment of TALs with adenovirus expressing dominant-negative Rac1 decreased NaCl-induced O(2)(-) generation by 60% compared with control (0.33 +/- 0.04 vs. 0.81 +/- 0.17 nmol O(2)(-) x min(-1) x mg protein(-1), n = 6, P < 0.05). We concluded that physiological increases in NaCl stimulate TAL O(2)(-) generation by activating Rac1.

PKC-dependent superoxide production by the renal medullary thick ascending limb from diabetic rats

Type 1 diabetes (T1D) is a state of oxidative stress accompanied by PKC activation in many tissues. The primary site of O2*- production by the normal rat kidney is the medullary thick ascending limb (mTAL). We hypothesized that T1D increases O2*- production by the mTAL through a PKC-dependent mechanism involving increased expression and translocation of one or more PKC isoforms. mTAL suspensions were prepared from rats with streptozotocin-induced T1D (STZ mTALs) and from normal or sham rats (normal/sham mTALs). O2*- production by STZ mTALs was fivefold higher than normal/sham mTALs (P < 0.05). PMA (30 min) mimicked the effect of T1D on O2*- production. Exposure to calphostin C or chelerythrine (PKC inhibitors), Gö6976 (PKCalpha/beta inhibitor), or rottlerin (PKCdelta inhibitor) decreased O2*- production to <20% of untreated baseline in both normal/sham and STZ mTALs. PKCbeta inhibitors had no effect. PKC activity was increased in STZ mTALs (P < 0.05 vs. normal/sham mTALs) and was unaltered by antioxidant exposure (tempol). PKCalpha protein levels were increased by 70% in STZ mTALs, with a approximately 30% increase in the fraction associated with the membrane (both P < 0.05 vs. sham). PKCbeta protein levels were elevated by 29% in STZ mTALs (P < 0.05 vs. sham) with no change in the membrane-bound fraction. Neither PKCdelta protein levels nor its membrane-bound fraction differed between groups. Thus STZ mTALs display PKC activation, upregulation of PKCalpha and PKCbeta protein levels, increased PKCalpha translocation to the membrane, and accelerated O2*- production that is eradicated by inhibition of PKCalpha or PKCdelta (but not PKCbeta). We conclude that increased PKCalpha expression and activity are primarily responsible for PKC-dependent O2*- production by the mTAL during T1D.

PP2B-dependent NO production in the medullary thick ascending limb during diabetes

Calcineurin (PP2B) has recently been shown to be upregulated in the medullary thick ascending limb (mTAL) during diabetes. The mTAL expresses all three isoforms of nitric oxide synthase (NOS), which are subject to phosphoregulation and represent substrates for PP2B. Therefore, we hypothesized that diabetes induces PP2B-dependent upregulation of NOS activity and NO production in the mTAL. Three weeks after injection of streptozotocin (STZ rats) or vehicle (sham rats), mTAL suspensions were prepared for use in functional and biochemical assays. PP2B activity and expression were increased in mTALs from STZ rats compared with sham. Nitrite production was significantly reduced in mTALs from STZ rats compared with sham. However, incubation with the free radical scavenger, tempol, unmasked a significant increase in nitrite production by mTALs from STZ rats. Inhibition of PP2B attenuated the increase in nitrite production and NOS activity evident in mTALs from STZ rats. Analysis of specific NOS isoform activity revealed increased NOS1 and NOS2 activities in mTALs from STZ rats. All three NOS isoform activities were regulated in a PP2B-dependent manner. Western blot analysis detected no differences in NOS isoform expression, although phosphorylation of pThr(495)-NOS3 was significantly reduced in mTALs from STZ rats. Phosphorylation of pSer(852)-NOS1, pSer(633)-NOS3, and pSer(1177)-NOS3 was similar in mTALs from STZ and sham rats. Inhibition of PP2B did not alter the phosphorylation of NOS1 or NOS3 at known sites. In conclusion, while NO bioavailability in mTALs is reduced during diabetes, free radical scavenging with tempol unmasks increased NO production that involves PP2B-dependent activation of NOS1 and NOS2.

Effects of renal perfusion pressure on renal medullary hydrogen peroxide and nitric oxide production

Studies were designed to determine the effects of increases of renal perfusion pressure on the production of hydrogen peroxide (H(2)O(2)) and NO(2)(-)+NO(3)(-) within the renal outer medulla. Sprague-Dawley rats were studied with either the renal capsule intact or removed to ascertain the contribution of changes of medullary blood flow and renal interstitial hydrostatic pressure on H(2)O(2) and NO(2)(-)+NO(3)(-) production. Responses to three 30-minute step changes of renal perfusion pressure (from approximately 85 to approximately 115 to approximately 145 mm Hg) were studied using adjustable aortic occluders proximal and distal to the left renal artery. Medullary interstitial H(2)O(2) determined by microdialysis increased at each level of renal perfusion pressure from 640 to 874 to 1593 nmol/L, as did H(2)O(2) urinary excretion rates, and these responses were significantly attenuated by decapsulation. Medullary interstitial NO(2)(-)+NO(3)(-) increased from 9.2 to 13.8 to 16.1 mumol/L, with parallel changes in urine NO(2)(-)+NO(3)(-), but decapsulation did not significantly blunt these responses. Over the range of renal perfusion pressure, medullary blood flow (laser-Doppler flowmetry) rose approximately 30% and renal interstitial hydrostatic pressure rose from 7.8 to 19.7 cm H(2)O. Renal interstitial hydrostatic pressure and the natriuretic and diuretic responses were significantly attenuated with decapsulation, but medullary blood flow was not affected. The data indicate that pressure-induced increases of H(2)O(2) emanated largely from increased tubular flow rates to the medullary thick-ascending limbs of Henle and NO largely from increased medullary blood flow to the vasa recta. The parallel pressure-induced increases of H(2)O(2) and NO indicate a participation in shaping the "normal" pressure-natriuresis relationship and explain why an imbalance in either would affect the blood pressure salt sensitivity.

Abundance of the Na-K-2Cl cotransporter NKCC2 is increased by high-fat feeding in Fischer 344 X Brown Norway (F1) rats

Insulin resistance is associated with hypertension by mechanisms likely involving the kidney. To determine how the major apical sodium transporter of the thick ascending limb, the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2) is regulated by high-fat feeding, we treated young male, Fischer 344 X Brown Norway (F344BN) rats for 8 wk with diets containing either normal (NF, 4%) or high (HF, 36%) fat, by weight, primarily as lard. HF-fed rats had impaired glucose tolerance, increased urine excretion of 8-isoprostane (a marker of oxidative stress), increased protein levels for NKCC2 (50-125%) and the renal outer medullary potassium channel (106%), as well as increased natriuretic response to furosemide (20-40%). To test the role of oxidative stress in this response, in study 2, rats were fed the NF or HF diet plus plain drinking water, or water containing N(G)-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor (100 mg/l), or tempol, a superoxide dismutase mimetic (1 mmol/l). The combination of tempol with HF nullified the increase in medullary NKCC2, while l-NAME with HF led to the highest expression of medullary NKCC2 (to 498% of NF mean). However, neither of these drugs dramatically affected the elevated natriuretic response to furosemide with HF. Finally, l-NAME led to a marked increase in blood pressure (measured by radiotelemetry), which was significantly enhanced with HF. Mean arterial blood pressure at 7 wk was as follows (mmHg): NF, 100 +/- 2; NF plus l-NAME, 122 +/- 3; and HF plus l-NAME, 131 +/- 2. Overall, HF feeding increased the abundance of NKCC2. Inappropriately high sodium reabsorption in the thick ascending limb via NKCC2 may contribute to hypertension with insulin resistance.

Interactions between thyroid and kidney function in pathological conditions of these organ systems: a review

Thyroidal status affects kidney function already in the embryonic stage. Thyroid hormones influence general tissue growth as well as tubular functions, electrolyte handling and neural input. Hyper- and hypo-functioning of the thyroid influences mature kidney function indirectly by affecting the cardiovascular system and the renal blood flow, and directly by affecting glomerular filtration, electrolyte pumps, the secretory and absorptive capacity of the tubuli, and the structure of the kidney. Hyperthyroidism accelerates several physiologic processes, a fact which is reflected in the decreased systemic vascular resistance, increased cardiac output (CO), increased renal blood flow (RBF), hypertrophic and hyperplastic tubuli, and increased glomerular filtration rate (GFR). Renal failure can progress due to glomerulosclerosis, proteinuria and oxidative stress. Hypothyroidism has a more negative influence on kidney function. Peripheral vascular resistance is increased with intrarenal vasoconstriction, and CO is decreased, causing decreased RBF. The influence on the different tubular functions is modest, although the transport capacity is below normal. The GFR is decreased up to 40% in hypothyroid humans. Despite the negative influences on glomerular and tubular kidney function, a hypothyroid state has been described as beneficial in kidney disease. Kidney disease is associated with decreased thyroid hormone concentrations caused by central effects and by changes in peripheral hormone metabolism and thyroid hormone binding proteins. Geriatric cats form an animal model of disease because both hyperthyroidism and chronic kidney disease (CKD) have high prevalence among them, and the link between thyroid and kidney affects the evaluation of clinical wellbeing and the possible treatment options.

Strain vessel hypothesis: a viewpoint for linkage of albuminuria and cerebro-cardiovascular risk

Albuminuria is closely associated with stroke and cardiovascular diseases (CVDs) as well as the salt sensitivity of blood pressure (BP). Although albuminuria may reflect generalized endothelial dysfunction, there may be more specific hemodynamic mechanisms underlying these associations. Cerebral hemorrhage and infarction occur most frequently in the area of small perforating arteries that are exposed to high pressure and that have to maintain strong vascular tone in order to provide large pressure gradients from the parent vessels to the capillaries. Analogous to the perforating arteries are the glomerular afferent arterioles of the juxtamedullary nephrons. Hypertensive vascular damage occurs first and more severely in the juxtamedullary glomeruli. Therefore, albuminuria may be an early sign of vascular damages imposed on 'strain vessels' such as perforating arteries and juxtamedullary afferent arterioles. Coronary circulation also occurs under unique hemodynamic conditions, in which the entire epicardial segments are exposed to very high pressure with little flow during systolic phases. From the evolutionary point of view, we speculate that such circulatory systems in the vital organs are mandatory for survival under the danger of hypoperfusion due to difficult access to salt and water as well as high risks of wound injuries. In addition, albuminuria would indicate an impairment of renal medullary circulation, downstream from the juxtamedullary glomeruli, and therefore an impaired pressure natriuresis, which would lead to salt sensitivity of BP. Our 'strain vessel hypothesis' may explain why hypertension and diabetes, unforeseen in the concept of evolution, preferentially affect vital organs such as the brain, heart and kidney.

Redox control of renal function and hypertension

Loss of redox homeostasis and formation of excessive free radicals play an important role in the pathogenesis of kidney disease and hypertension. Free radicals such as reactive oxygen species (ROS) are necessary in physiologic processes. However, loss of redox homeostasis contributes to proinflammatory and profibrotic pathways in the kidney, which in turn lead to reduced vascular compliance and proteinuria. The kidney is susceptible to the influence of various extracellular and intracellular cues, including the renin-angiotensin-aldosterone system (RAAS), hyperglycemia, lipid peroxidation, inflammatory cytokines, and growth factors. Redox control of kidney function is a dynamic process with reversible pro- and anti-free radical processes. The imbalance of redox homeostasis within the kidney is integral in hypertension and the progression of kidney disease. An emerging paradigm exists for renal redox contribution to hypertension.

Normal saline induces oxidative stress in peritoneal mesothelial cells

BACKGROUND

Peritoneal adhesions are the most common complication of the abdominal surgery. Normal saline is frequently used to rinse the peritoneal cavity during abdominal surgery, although there is no well-established data describing effect of such procedure on the process of formation of peritoneal adhesions.

METHODS

Effect of 0.9% NaCl solution on viability, oxidative stress, and fibrinolytic activity of human peritoneal mesothelial cells maintained in in vitro culture was evaluated.

RESULTS

Exposure of mesothelial cells to 0.9% NaCl induces oxidative stress, derangement of their structure with subsequent increased release of tissue factor (+75%) and plasminogen activator inhibitor-1 (+19%), and simultaneous suppression of tissue plasminogen activator release (-39%). In effect, ration tissue plasminogen activator/plasminogen activator inhibitor-1 was reduced in 0.9% NaCl-treated cells by 50%. Pretreatment of cells with precursor of glutathione synthesis: L-2-oxothiazolidine-4-carboxylic acid prevented these changes.

CONCLUSIONS

Oxidative stress in the peritoneal mesothelium caused by 0.9% NaCl activates their procoagulant activity and impairs fibrinolytic properties of these cells. These effects disqualify 0.9% NaCl as rinsing solution during abdominal surgery.

Intracellular pH regulates superoxide production by the macula densa

We hypothesized that elevated macula densa intracellular pH (pH(i)) during tubuloglomerular feedback enhances O(2)(-) production from NAD(P)H oxidase. Microdissected thick ascending limbs from rabbits with intact macula densa were cannulated and perfused with physiological saline. When luminal NaCl was switched from 10 to 80 mM, O(2)(-) production increased from 0.53 +/- 0.09 to 2.62 +/- 0.54 U/min (P < 0.01). To determine whether inhibiting the Na/H exchanger blocks O(2)(-) production, we used dimethyl amiloride (DMA) to block Na/H exchange. In the presence of DMA, O(2)(-) production induced by NaCl was blunted by 40%. To study the effect of pH(i) on O(2)(-) in intact macula densa cells, we measured O(2)(-) while pH(i) was changed by adjusting luminal pH. When the macula densa was perfused with 80 mM NaCl and the pH of the perfusate was switched to 6.8, 7.4, and 8.0, O(2)(-) production was significantly enhanced, but not at 10 mM NaCl. To ascertain the source of O(2)(-), we used the NAD(P)H oxidase inhibitor apocynin. In the presence of apocynin (10(-5) M), O(2)(-) production induced by elevating pH(i) was blocked. Finally, we measured the optimum pH for O(2)(-) production by the macula densa and found optimum extracellular pH is at 7.7 and optimum pH(i) is approximately 8 for O(2)(-) production. We found that elevated pH(i) enhances O(2)(-) production from NAD(P)H oxidase induced by increasing luminal NaCl when the lumen is perfused with 80 mM NaCl, not 10 mM, and O(2)(-) production is pH sensitive, with an optimum pH(i) of 8.

Enhanced amiloride-sensitive superoxide production in renal medullary thick ascending limb of Dahl salt-sensitive rats

The aims of the present study were to determine whether superoxide (O(2)(-)) production is enhanced in medullary thick ascending limb (mTAL) of Dahl salt-sensitive (SS) rats compared with a salt-resistant consomic control strain (SS.13(BN)) and to elucidate the cellular pathways responsible for augmented O(2)(-) production. Studies were carried out in 7- to 10-wk-old male SS and SS.13(BN) rats fed either a 0.4% NaCl diet or a 4.0% NaCl diet for 3 days before tissue harvest. Tissue strips containing mTAL were isolated from the left kidney, loaded with the O(2)(-)-sensitive fluorescent dye dihydroethidium, superfused with modified Hanks' solution, and imaged at x60 magnification on a heated microscope stage. O(2)(-) production was stimulated in mTAL by incrementing superfusate NaCl concentration from 154 to 254 to 500 mM. O(2)(-) production was enhanced in mTAL of SS rats compared with SS.13(BN) rats in response to incrementing bath NaCl. Addition of N-methyl-amiloride (100 muM) or inhibition of NAD(P)H oxidase reduced O(2)(-) production in SS mTAL to levels observed in SS.13(BN) rats. Both amiloride- and ouabain-sensitive pathways of O(2)(-) production were elevated following 3 days of high (4.0%) NaCl feeding in mTAL of SS and SS.13(BN) rats. We conclude that mTAL from SS rats exhibit enhanced amiloride-sensitive O(2)(-) production. The amiloride-sensitive O(2)(-) response in mTAL is independent of active Na(+) transport and appears to be mediated by NAD(P)H oxidase. Amiloride-sensitive O(2)(-) production is likely to contribute to augmented outer medullary O(2)(-) production observed in SS rats during both normal and high NaCl diets.

Nitric oxide decreases expression of osmoprotective genes via direct inhibition of TonEBP transcriptional activity

During antidiuresis, renal medullary cells adapt to the hyperosmotic interstitial environment by increased expression of osmoprotective genes, which is driven by a common transcriptional activator, tonicity-responsive enhancer binding protein (TonEBP). Because nitric oxide (NO) is abundantly produced in the renal medulla, the present studies addressed the effect of NO on expression of osmoprotective genes and TonEBP activation in MDCK cells. Several structurally unrelated NO donors blunted tonicity-induced up-regulation of TonEBP target genes involved in intracellular accumulation of organic osmolytes. These effects were mediated by reduced transcriptional activity of TonEBP, as assessed by tonicity-responsive elements- and aldose reductase promoter-driven reporter constructs. Neither total TonEBP abundance nor nuclear translocation of TonEBP was affected by NO. Furthermore, 8-bromo-cGMP and peroxynitrite failed to reproduce the inhibitory effect of NO, indicating that NO acts directly on TonEBP rather than through classical NO signaling pathways. In support of this notion, electrophoretic mobility shift assays showed reduced binding of TonEBP to its target sequence in nuclear extracts prepared from MDCK cells treated with NO in vivo and in nuclear extracts exposed to NO in vitro. Furthermore, immunoprecipitation of S-nitrosylated proteins and the biotin-switch method identified TonEBP as a target for S-nitrosylation, which correlates with reduced DNA binding and transcriptional activity. These observations disclose a novel direct inhibitory effect of NO on TonEBP, a phenomenon that may be relevant for regulation of osmoprotective genes in the renal medulla.

High perfusion pressure accelerates renal injury in salt-sensitive hypertension

Renal injury in the Dahl salt-sensitive rat mimics human salt-sensitive forms of hypertension that are particularly prevalent in black individuals, but the mechanisms that lead to the development of this injury are incompletely understood. We studied the impact of renal perfusion pressure (RPP) on the development of renal injury in this model. During the development of salt-induced hypertension over 2 wk, the RPP to the left kidney was maintained at control levels (125 +/- 2 mmHg) by continuous servocontrol inflation of an aortic balloon implanted between the renal arteries; during the same period, the RPP to the right kidney rose to 164 +/- 8 mmHg. After 2 wk of a 4% salt diet, DNA microarray and real-time PCR identified genes related to fibrosis and epithelial-to-mesenchymal transition in the kidneys exposed to hypertension. The increased RPP to the right kidney accounted for differences in renal injury between the two kidneys, measured by percentage of injured cortical and juxtamedullary glomeruli, quantified proteinaceous casts, number of ED-1-positive cells per glomerular tuft area, and interstitial fibrosis. Interlobular arteriolar injury was not increased in the kidney exposed to elevated pressure but was reduced in the control kidney. We conclude that elevations of RPP contribute significantly to the fibrosis and epithelial-to-mesenchymal transition found in the early phases of hypertension in the salt-sensitive rat.

Tempol improves renal hemodynamics and pressure natriuresis in hyperthyroid rats

Hyperthyroidism in rats is associated with increased oxidative stress. These animals also show abnormal renal hemodynamics and an attenuated pressure-diuresis-natriuresis (PDN) response. We analyzed the role of oxidative stress as a mediator of these alterations by examining acute effects of tempol, a superoxide dismutase mimetic. The effects of increasing bolus doses of tempol (25-150 micromol/kg) on mean arterial pressure (MAP), renal vascular resistance (RVR), and cortical (CBF) and medullary (MBF) blood flow were studied in control and thyroxine (T4)-treated rats. In another experiment, tempol was infused at 150 micromol.kg(-1).h(-1) to analyze its effects on the glomerular filtration rate (GFR) and on PDN response in these animals. Tempol dose dependently decreased MAP and RVR and increased CBF and MBF in control and T4-treated rats, but the T4 group showed a greater responsiveness to tempol in all of these variables. The highest dose of tempol decreased RVR by 13.5 +/- 2.1 and 5.5 +/- 1.2 mmHg.ml(-1).min(-1) in hyperthyroid (P < 0.01) and control rats, respectively. GFR was not changed by tempol in controls but was significantly increased in the hyperthyroid group. Tempol did not change the absolute or fractional PDN responses of controls but significantly improved those of hyperthyroid rats, although without attaining normal values. Tempol increased the slopes of the relationship between renal perfusion pressure and natriuresis (T4+tempol: 0.17 +/- 0.05; T4: 0.09 +/- 0.03 microeq.min(-1).g(-1).mmHg(-1); P < 0.05) and reduced 8-isoprostane excretion in hyperthyroid rats. These results show that antioxidant treatment with tempol improves renal hemodynamic variables and PDN response in hyperthyroid rats, indicating the participation of an increased oxidative stress in these mechanisms.

Ectodomain shedding of pro-TGF-alpha is required for COX-2 induction and cell survival in renal medullary cells exposed to osmotic stress

In the renal medulla, cyclooxygenase (COX)-2 is induced by osmotic stress as present in this kidney region during antidiuresis. Increasing evidence suggests that EGF receptor (EGFR) signaling is involved in this process. The aim of the present study was to examine the mechanisms responsible for COX-2 expression and PGE(2) production during hypertonic conditions and to identify potential autocrine/paracrine EGFR ligands. Immunohistochemisty and Western blot analysis revealed abundant expression of the pro-EGFR ligand pro-transforming growth factor (TGF)-alpha in renal medullary cells in vivo and in cultured Madin-Darby canine kidney cells. In Madin-Darby canine kidney cells, hypertonicity rapidly increased TNF-alpha converting enzyme (TACE)-dependent ectodomain shedding of pro-TGF-alpha; phosphorylation of EGFR, p38, and ERK1/2; expression of COX-2; and production of PGE(2). Conversely, TACE inhibition prevented TGF-alpha release; EGFR, p38, and ERK1/2 activation; and COX-2 expression. Furthermore, cell survival was reduced substantially, a response that could be reversed by the addition of PGE(2). Simultaneous addition of recombinant TGF-alpha during TACE inhibition restored EGFR and MAPK phosphorylation, COX-2 expression, PGE(2) production, and cell survival during osmotic stress. These results indicate that hypertonicity induces TACE-mediated ectodomain shedding of pro-TGF-alpha, which subsequently activates COX-2 expression in an autocrine/paracrine fashion, via EGFR and MAPKs. We conclude that tonicity-induced TGF-alpha release is required for COX-2 expression, PGE(2) synthesis, and survival of renal medullary cells during osmotic stress.

Nitric oxide and superoxide interactions in the kidney and their implication in the development of salt-sensitive hypertension

1. Enhanced superoxide (O2(-)) activity as a result of the inhibition of the superoxide dismutase (SOD) enzyme results in vasoconstrictor and antinatriuretic responses in the canine kidney; these responses were shown to be greatly enhanced during inhibition of nitric oxide synthase (NOS). Glomerular filtration rate remained mostly unchanged during SOD inhibition in the intact nitric oxide (NO) condition, but was markedly reduced during NOS inhibition. These findings indicate that endogenous NO has a major renoprotective effect against O2(-) by acting as an anti-oxidant. Nitric oxide synthase inhibition was also shown to enhance endogenous O2(-) activity. 2. Experiments in our laboratory using dogs, rats and gene knockout mice have shown that renal vasoconstrictor and antinatriuretic responses to acute or chronic angiotensin (Ang) II administration are mediated, in part, by O2(-) generation. In the absence of NO, enhanced O2(-) activity largely contributes to AngII-induced renal tubular sodium reabsorption. Acute or chronic treatment with the O2(-) scavenger tempol in experimental models of hypertension (induced by chronic low-dose treatment with AngII and NO inhibitors) causes an improvement in renal haemodynamics and in excretory function, abolishes salt sensitivity and reduces blood pressure. 3. The present mini review also discusses related studies from many other laboratories implicating a role for O2(-) and its interaction with NO in the development of salt-sensitive hypertension. 4. Overall, the collective data support the hypothesis that an imbalance between the production of NO and O2(-) in the kidney primarily determines the condition of oxidative stress that alters renal haemodynamics and excretory function leading to sodium retention and, thus, contributes to the development of salt-sensitive hypertension.

Role of the T cell in the genesis of angiotensin II induced hypertension and vascular dysfunction

Hypertension promotes atherosclerosis and is a major source of morbidity and mortality. We show that mice lacking T and B cells (RAG-1^−/− mice) have blunted hypertension and do not develop abnormalities of vascular function during angiotensin II infusion or desoxycorticosterone acetate (DOCA)–salt. Adoptive transfer of T, but not B, cells restored these abnormalities. Angiotensin II is known to stimulate reactive oxygen species production via the nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase in several cells, including some immune cells. Accordingly, adoptive transfer of T cells lacking the angiotensin type I receptor or a functional NADPH oxidase resulted in blunted angiotensin II–dependent hypertension and decreased aortic superoxide production. Angiotensin II increased T cell markers of activation and tissue homing in wild-type, but not NADPH oxidase–deficient, mice. Angiotensin II markedly increased T cells in the perivascular adipose tissue (periadventitial fat) and, to a lesser extent the adventitia. These cells expressed high levels of CC chemokine receptor 5 and were commonly double negative (CD3⁺CD4⁻CD8⁻). This infiltration was associated with an increase in intercellular adhesion molecule-1 and RANTES in the aorta. Hypertension also increased T lymphocyte production of tumor necrosis factor (TNF) α, and treatment with the TNFα antagonist etanercept prevented the hypertension and increase in vascular superoxide caused by angiotensin II. These studies identify a previously undefined role for T cells in the genesis of hypertension and support a role of inflammation in the basis of this prevalent disease. T cells might represent a novel therapeutic target for the treatment of high blood pressure.

Cardioprotective effect of angiotensin II type 1 receptor antagonist associated with bradykinin-endothelial nitric oxide synthase and oxidative stress in Dahl salt-sensitive hypertensive rats

OBJECTIVES

The interactions between eNOS or oxidative stress and bradykinin under long-term treatment of angiotensin II type 1 receptor antagonists (ATRA) remain unknown. To elucidate the molecular mechanisms of the cardioprotective effect of ATRA, we evaluated whether valsartan affects the bradykinin-eNOS and nicotinamide adenine dinucleotide (NAD(P)H) oxidase pathway.

METHODS

After 5 weeks of feeding an 8% NaCl diet to 6-week-old Dahl salt-sensitive hypertensive (DS) rats, a distinct stage of concentric left ventricular hypertrophy (LVH) was noted. Six-week-old DS rats were treated with one of the following drug combinations for 5 weeks until the onset of LVH: vehicle; bradykinin B2 receptor antagonist FR172,357 alone; high-dose hydralazine; low-dose hydralazine; high-dose valsartan; low-dose valsartan; high and low-dose valsartan plus FR172,357. Age-matched Dahl salt-resistant rats fed the same diet served as controls.

RESULTS

eNOS expression and activity, which was decreased in hypertrophy, was increased by high or low-dose valsartan, but not by high and low-dose valsartan plus FR172,357 or FR172,357 alone or high and low-dose hydralazine. The increased expression of NAD(P)H oxidase p22phox, p47phox, p67phox, and gp91phox in DS rats was suppressed by high or low-dose valsartan, but not by high or low-dose valsartan plus FR172,357 or FR172,357 alone or high and low-dose hydralazine. Valsartan effectively inhibited vascular lesion formation and suppressed the expression of transforming growth factor-beta1, connective tissue growth factor, and type I collagen, but not valsartan plus FR172,357 or FR172,357 alone or high and low-dose hydralazine.

CONCLUSION

These findings suggest that valsartan may have cardioprotective effects in this model, partly associated with the bradykinin-eNOS and oxidative stress pathway.

Depolarization of the macula densa induces superoxide production via NAD(P)H oxidase

Superoxide (O2−) enhances tubuloglomerular feedback by scavenging nitric oxide at the macula densa. However, the singling pathway of O2−production in the macula densa is not known. We hypothesized that the increase in tubular NaCl concentration that initiates tubuloglomerular feedback induces O2−production by the macula densa via NAD(P)H oxidase, which is activated by macula densa depolarization. We isolated and microperfused the thick ascending limb of the loop of Henle and attached macula densa in rabbits. A fluorescent dye, dihydroethidium, was used to detect O2−production at the macula densa. When luminal NaCl was switched from 10 to 80 mM, a situation of initiating maximum tubuloglomerular feedback response, O2−production significantly increased. To make sure that the shifts in the oxyethidium/dihydroethidium ratio were due to changes in O2−, we used tempol (10−4M), a stable membrane-permeant superoxide dismutase mimetic. With tempol present, when we switched from 10 to 80 mM NaCl, the increase in oxyethidium/dihydroethidium ratio was blocked. To determine the source of O2−, we used the NAD(P)H oxidase inhibitor apocynin. When luminal NaCl was switched from 10 to 80 mM in the presence of apocynin, O2−production was inhibited by 80%. To see whether the effect of increasing luminal NaCl involves Na-K-2Cl cotransporters, we inhibited them with furosemide. When luminal NaCl was switched from 10 to 80 mM in the presence of furosemide, O2−production was blocked. To test whether depolarization of the macula densa induces O2−production, we artificially induced depolarization by adding valinomycin (10−6M) and 25 mM KCl to the luminal perfusate. Depolarization alone significantly increases O2−production. We conclude that increasing luminal NaCl induces O2−production during tubuloglomerular feedback. O2−generated by the macula densa is primarily derived from NAD(P)H oxidase and is induced by depolarization.

Melatonin is more effective than taurine and 5-hydroxytryptophan against hyperglycemia-induced kidney-cortex tubules injury

The antioxidative effects of melatonin (Mel), 5-hydroxytryptophan (5-HTP) and taurine (TAU) on hyperglycemia-induced oxidative stress was investigated in primary cultures of kidney-cortex tubule cells grown in metabolically and hormonally defined medium. In the presence of 30 mm glucose (hyperglycemic conditions), cell viability was decreased by about 35% in comparison with that estimated in the glucose-depleted medium probably as a result of induction of apoptosis, as concluded from: (i) chromatin condensation and DNA fragmentation assays, (ii) a significant enhancement of reactive oxygen species (ROS) production, (iii) 8-hydroxydeoxyguanosine (8-OHdG) generation, (iv) an increased protein peroxidation and (v) a decline of reduced glutathione (GSH) levels leading to a disturbed glutathione redox state. The addition of 100 microm Mel to the hyperglycemic medium resulted in a twofold decrease in both 8-OHdG accumulation and protein peroxidation as well as restoration of the control intracellular ROS levels accompanied by a substantial increase in GSH/oxidized glutathione (GSSG) ratio due to a decline in GSSG content. ROS elimination was also achieved in the presence of 1 mm TAU which diminished protein and DNA injuries by about 25% and 30%, respectively. On the contrary, the action of 100 microm 5-HTP on ROS level, 8-OHdG generation, protein peroxidation and GSH/GSSG ratio was negligible. Thus, in contrast to 5-HTP and TAU, Mel might be considered as beneficial for diabetes therapy, particularly in terms of reduction of hyperglycemia-induced kidney injury.