Effect of sodium delivery on superoxide and nitric oxide in the medullary thick ascending limb

Angiotensin II increases intrarenal transforming growth factor-beta1 in rats submitted to sodium overload independently of blood pressure

Angiotensin II (Ang II) promotes sodium-retention, cell growth and fibrosis in addition to its classical effects on blood pressure and fluid homeostasis. In this study we examined whether low and non-hypertensive doses of exogenous Ang II could enhance the intrarenal expression of transforming growth factor-beta1 (TGF-beta1) observed in rats submitted to sodium overload. Sprague-Dawley-rats were infused for 2 h with 0.1 and 5 microg kg(-1) h(-1) Ang II (Ang 0.1 and Ang 5, respectively) together with saline solution at four different concentrations (isotonic and Na 0.5 mol L(-1), Na 1.0 mol L(-1) and Na 1.5 mol L(-1)). Renal function and mean arterial blood pressure (BP) were measured. The renal distributions of TGF-beta1, alpha-smooth-muscle-actin (alpha-SMA) and nuclear factor-kappaB (NF-kappaB) were evaluated by immunohistochemistry. While the Ang 0.1 groups were normotensive, the Ang 5 groups developed arterial hypertension progressively, and the highest blood pressure values were observed when rats were simultaneously infused with Na 1.5 mol L(-1). Glomerular function was not altered in any group. In cortical tubules, all groups infused with Ang II (0.1 and 5) and hypertonic saline solution (HSS) showed an increase in TGF-beta1 immunostaining compared to those infused with HSS alone. In medullary tubules, only the Ang 5-Na 0.5 group showed a significant increase in TGF-beta 1 immunostaining compared to the Na 0.5 group. Peritubular positive staining for alpha-SMA was present in groups receiving Ang alone or Ang-Na, in a sodium concentration-dependent manner. In cortical-tubules, NF-kappaB immunostaining was significantly increased in the Ang groups in comparison with the control and in Ang-Na 0.5 and Ang-Na 1.0 groups in comparison with the Na 0.5 mol L(-1) and Na 1.5 mol L(-1) groups, respectively, except in the case of the Ang 0.1-Na 1.5 mol L(-1) and Ang 5-Na 1.5 mol L(-1) groups. Moreover, Ang II and sodium overload induced additional changes in TGF-beta1, alpha-SMA and NF-kappaB immunostanding in glomeruli, medullary tubules and renal vessels. In conclusion, the interaction of Ang II with acute-sodium overload exacerbated intrarenal TGF-beta1, alpha-SMA and NF-kappaB expression, independently from changes in blood pressure levels, in normal rats.

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.

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.

Cellular stretch increases superoxide production in the thick ascending limb

Superoxide (O(2)(-)) is an important regulator of kidney function. We have recently shown that luminal flow stimulates O(2)(-) production in the thick ascending limb (TAL), attributable in part to mechanical factors. Stretch, pressure and shear stress all change when flow increases in the TAL. We hypothesized that stretch rather than shear stress or pressure per se stimulates O(2)(-) production by TALs. We measured O(2)(-) production in isolated perfused rat TALs using fluorescence microscopy and dihydroethidium. Tubules were perfused with a Na-free solution to eliminate the confounding effect of Na transport. Flow induced an increase in O(2)(-) production from 29+/-4 to 90+/-8 AU/s (P<0.002; n=5). The response to flow is rapidly reversible. O(2)(-) production by TALs perfused at 10 nL/min decreased from 113+/-6 to 25+/-10 AU/s (P<0.003; n=4) 15 minutes after flow was stopped. Increasing pressure and stretch in the absence of shear stress caused a significant increase in O(2)(-) production (40+/-6 to 118+/-17 AU/s; P<0.02; n=5). In contrast, eliminating shear stress had no effect (107+/-9 versus 108+/-10 AU/s; n=5). Increasing stretch by 27+/-2% in the presence of flow while reducing pressure stimulated O(2)(-) production from 66+/-7 to 84+/-9 AU/s (29+/-8%; P<0.02; n=5). Tempol inhibited this increase (n=5). We conclude that increasing stretch rather than pressure or shear stress accounts for the mechanical aspect of flow-induced O(2)(-) production in the TAL. Stretch of the TAL during hypertension, diabetes, and salt loading may contribute to renal damage.

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

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

Oxidative stress as a common pathway to chronic tubulointerstitial injury in kidney allografts

A major challenge for kidney transplantation is to dissect out the identifiable causes of chronic allograft tubulointerstitial fibrosis and to develop cause-specific treatment strategies. There has been a recent interest in the role of oxidative stress (OS) as a mediator of injury in chronic allograft tubular atrophy (TA) and interstitial fibrosis (IF). A review of the literature and data from my laboratory studying chronic allograft TA/IF in rat, rhesus monkey, and human kidneys suggests that OS is increased in graft-infiltrating macrophages, activated myofibroblasts, interstitium, and areas of tubular injury. Chronic allograft OS may be induced by inflammation, abnormal tissue oxygenation, immunosuppressant drugs, and comorbid clinical conditions including diabetes, hypertension, proteinuria, anemia, and dyslipidemia. Moreover, OS-induced chronic TA/IF is associated with signaling pathways including inflammation, apoptosis, hypoxia, and epithelial-to-mesenchymal transition. Most of these injury pathways participate in a self-perpetuating cycle with OS. In conclusion, evidence suggests that OS is a common mechanism of injury in chronic allograft TA/IF. However, most available data demonstrate a correlation and no causal relationship. Furthermore, the extent to which TA/IF is dependent on OS is unknown. These questions may be answered by prospective randomized placebo-control trials examining the role of select antioxidants in the prevention of chronic allograft TA/IF.

Flow increases superoxide production by NADPH oxidase via activation of Na-K-2Cl cotransport and mechanical stress in thick ascending limbs

Superoxide (O2−) regulates renal function and is implicated in hypertension. O2−production increases in response to increased ion delivery in thick ascending limbs (TALs) and macula densa and mechanical strain in other cell types. Tubular flow in the kidney acutely varies causing changes in ion delivery and mechanical stress. We hypothesized that increasing luminal flow stimulates O2−production by NADPH oxidase in TALs via activation of Na-K-2Cl cotransport. We measured intracellular O2−in isolated rat TALs using dihydroethidium in the presence and absence of luminal flow and inhibitors of NADPH oxidase, Na-K-2Cl cotransport, and Na/H exchange. In the absence of flow, the rate of O2−production was 5.8 ± 1.4 AU/s. After flow was initiated, it increased to 29.7 ± 4.3 AU/s ( P < 0.001). O2−production was linearly related to flow. Tempol alone and apocynin alone blocked the flow-induced increase in O2−production (3.5 ± 1.7 vs. 4.5 ± 2.8 AU/s and 8.2 ± 2.1 vs. 10.6 ± 2.8 AU/s, respectively). Furosemide decreased flow-induced O2−production by 55% (37.3 ± 5.2 to 16.8 ± 2.8 AU/s; P < 0.002); however, dimethylamiloride had no effect. Finally, we examined whether changes in mechanical forces are involved in flow-induced O2−production by using a Na-free solution to perfuse TALs. In the absence of NaCl, luminal flow enhanced O2−production (1.5 ± 0.5 to 13.5 ± 1.1 AU/s; P < 0.001), ∼50% less stimulation than when flow was increased in the presence of luminal NaCl. We conclude that flow stimulates O2−production in TALs via activation of NADPH oxidase and that NaCl absorption due to Na-K-2Cl cotransport and flow-associated mechanical factors contribute equally to this process.

NO synthase uncoupling in the kidney of Dahl S rats: role of dihydrobiopterin

NO synthase (NOS) can paradoxically contribute to the production of reactive oxygen species when l-arginine or the cofactor R-tetrahydrobiopterin (BH(4)) becomes limited. The present study examined whether NOS contributes to superoxide production in kidneys of hypertensive Dahl salt-sensitive (SS) rats compared with an inbred consomic control strain (SS-13(BN)) and tested the hypothesis that elevated dihydrobiopterin (BH(2)) levels are importantly involved in this process. This was assessed by determining the effects of l-nitroarginine methyl ester (l-NAME) inhibition of NOS on superoxide production and by comparing tissue concentrations of BH(4) and BH(2). A reverse-phase high-performance liquid chromatography method was applied for direct measurements of BH(4) and BH(2) using (S)-tetrahydrobiopterin as an internal standard. Superoxide concentrations were measured in vivo from medullary microdialysis fluid using dihydroethidine and in vitro using lucigenin. The results indicate the following: (1) that superoxide levels were elevated in the outer medulla of SS rats fed a 4% salt diet and could be inhibited by l-NAME. In contrast, l-NAME resulted in elevated superoxide production in consomic SS-13(BN) rats because of higher NOS activity; (2) SS rats showed a reduced ratio of BH(4)/BH(2) in the outer medulla that was driven by increased concentrations of BH(2); and (3) lower superoxide dismutase and catalase activities contributed to elevated reactive oxygen species in SS samples. Based on the shift of BH(4) to BH(2) and the observation of l-NAME inhibitable superoxide production, we conclude that NOS uncoupling occurs in the renal medulla of hypertensive SS rats fed a high-salt diet.

BOLD-MRI assessment of intrarenal oxygenation and oxidative stress in patients with chronic kidney allograft dysfunction

Blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) uses deoxyhemoglobin as an endogenous contrast agent for the noninvasive assessment of tissue oxygen bioavailability. We hypothesized that intrarenal oxygenation was impaired in patients with chronic allograft nephropathy (CAN). Ten kidney-transplant recipients with CAN and nine healthy volunteers underwent BOLD-MRI. Medullary R2* (MR2*) and cortical R2* (CR2*) levels (measures directly proportional to tissue deoxyhemoglobin levels) were determined alongside urine and serum markers of oxidative stress (OS): hydrogen peroxide (H(2)O(2)), F(2)-isoprostanes, total nitric oxide (NO), heat shock protein 27 (HSP27), and total antioxidant property (TAOP). Mean MR2* and CR2* levels were significantly decreased in CAN (increased local oxyhemoglobin concentration) compared with healthy volunteers (20.7 +/- 1.6 vs. 23.1 +/- 1.8/s, P = 0.03 and 15.9 +/- 1.9 vs. 13.6 +/- 2.3/s, P = 0.05, respectively). There was a significant increase in serum and urine levels of H(2)O(2) and serum HSP27 levels in patients with CAN. Conversely, urine NO levels and TAOP were significantly increased in healthy volunteers. Multiple linear regression analyses showed a significant association between MR2* and CR2* levels and serum/urine biomarkers of OS. BOLD-MRI demonstrated significant changes in medullary and cortical oxygen bioavailability in allografts with CAN. These correlated with serum/urine biomarkers of OS, suggesting an association between intrarenal oxygenation and OS.