Superoxide enhances tubuloglomerular feedback by constricting the afferent arteriole

Angiotensin II stimulates thick ascending limb superoxide production via protein kinase C(α)-dependent NADPH oxidase activation

Angiotensin II (Ang II) stimulates thick ascending limb (TAL) O₂ production, but the receptor(s) and signaling mechanism(s)involved are unknown. The effect of Ang II on O₂. is generally attributed to the AT₁receptor. In some cells, Ang II stimulates protein kinase C (PKC), whose α isoform (PKCα) can activate NADPH oxidase. We hypothesized that in TALs, Ang II stimulates O₂. via AT₁and PKC α-dependent NADPH oxidase activation.In rat TALs, 1 nM Ang II stimulated O₂. from 0.760.17 to 1.97 0.21 nmol/min/mg (p < 0.001). An AT₁antagonist blocked the stimulatory effect of Ang II on O₂. (0.87 0.25 nmol/min/mg; p < 0.006), whereas an AT₂ antagonist had no effect (2.16 0.133 nmol/min/mg; p < 0.05 versus vehicle). Apocynin, an NADPH oxidase inhibitor, blocked Ang II-stimulated O₂by 90% (p <0.01). Ang II failed to stimulate O₂. in TALs from p47(phox) -/- mice (p < 0.02). Monitored by fluorescence resonance energy transfer, Ang II increased PKC activity from 0.02 0.03 to 0.13 0.02 arbitrary units (p < 0.03). A general PKC inhibitor, GF109203X, blocked the effect of Ang II on O₂(1.47 +/- .21 versus 2.72 +/- .47 nmol/min/mg with Ang II alone; p < 0.03). A PKCα- and ß-selective inhibitor, Gö6976, also blocked the stimulatory effect of Ang II on O₂. (0.59 +/- 0.15 versus 2.05 +/- 0.28 nmol/min/mg with Ang II alone; p < 0.001). To distinguish between PKC α and PKC ß, we used tubules expressing dominant-negative PKC α or -ß. In control TALs, Ang II stimulated O2. by 2.17 0.44 nmol/min/mg (p < 0.011). In tubules expressing dominant-negative PKC α, Ang II failed to stimulate O2. (change: -0.30 +/- 0.27 nmol/min/mg). In tubules expressing dominant-negative PKC ß1, Ang II stimulated O2. by 2.080.69 nmol/min/mg (p < 0.002). We conclude that Ang II stimulates TAL O₂production via activation of AT₁receptors and PKC α-dependent NADPH oxidase.

Salt-sensitive splice variant of nNOS expressed in the macula densa cells

Neuronal nitric oxide synthase (nNOS), which is abundantly expressed in the macula densa cells, attenuates tubuloglomerular feedback (TGF). We hypothesize that splice variants of nNOS are expressed in the macula densa, and nNOS-beta is a salt-sensitive isoform that modulates TGF. Sprague-Dawley rats received a low-, normal-, or high-salt diet for 10 days and levels of the nNOS-alpha, nNOS-beta, and nNOS-gamma were measured in the macula densa cells isolated with laser capture microdissection. Three splice variants of nNOS, alpha-, beta-, and gamma-mRNAs, were detected in the macula densa cells. After 10 days of high-salt intake, nNOS-alpha decreased markedly, whereas nNOS-beta increased two- to threefold in the macula densa measured with real-time PCR and in the renal cortex measured with Western blot. NO production in the macula densa was measured in the perfused thick ascending limb with an intact macula densa plaque with a fluorescent dye DAF-FM. When the tubular perfusate was switched from 10 to 80 mM NaCl, a maneuver to induce TGF, NO production by the macula densa was increased by 38 +/- 3% in normal-salt rats and 52 +/- 6% (P < 0.05) in the high-salt group. We found 1) macula densa cells express nNOS-alpha, nNOS-beta, and nNOS-gamma, 2) a high-salt diet enhances nNOS-beta, and 3) TGF-induced NO generation from macula densa is enhanced in high-salt diet possibly from nNOS-beta. In conclusion, we found that the splice variants of nNOS expressed in macula densa cells were alpha-, beta-, and gamma-isoforms and propose that enhanced level of nNOS-beta during high-salt intake may contribute to macula densa NO production and help attenuate TGF.

GTPase-Rac enhances depolarization-induced superoxide production by the macula densa during tubuloglomerular feedback

Superoxide (O(2)(-) ) enhances tubuloglomerular feedback (TGF) by scavenging nitric oxide at the macula densa (MD). The primary source of O(2)(-) in the MD during TGF is NADPH oxidase, which is activated by membrane depolarization. While Rac, a small GTP-binding protein, has been shown to enhance NADPH oxidase activity, its role in O(2)(-) generation by the MD is unknown. We hypothesized that depolarization of the MD leads to translocation of Rac to the apical membrane, and its activation, in turn, augments O(2)(-) generation during TGF. We tested this by measuring membrane potential and increased O(2)(-) levels during TGF responses in isolated, perfused tubules containing the intact MD plaque. Switching tubular NaCl from 10 to 80 mM, which induces TGF, depolarized membrane potential by 28.4 + or - 4.5% from control (P < 0.05) and O(2)(-) levels from 124 + or - 19 to 361 + or - 27 U/min. This NaCl-induced depolarization and O(2)(-) generation were blocked by a Cl(-) channel blocker, 5-nitro-2(3-phenylpropylamino) benzoic acid (NPPB; 10(-6) M). Inhibition of Rac blunted NaCl-induced O(2)(-) generation by 47%. When the NaCl content of the MD perfusate was increased from 10 to 80 mM, immunointensity of Rac on the apical side increased from 32 + or - 3.1 to 46 + or - 2.5% of the total immunofluorescence in the MD, indicating that high NaCl induces the translocation of Rac to the apical membrane. This NaCl-induced Rac translocation was blocked by a Cl(-) channel blocker, NPPB, indicating that depolarization of the MD induced Rac translocation. In conclusion, we found that depolarization of the MD during TGF leads to translocation of Rac to the apical membrane, which enhances O(2)(-) generation by the MD.

NOX2 is the primary source of angiotensin II-induced superoxide in the macula densa

Macula densa (MD)-mediated regulation of renal hemodynamics via tubuloglomerular feedback is regulated by interactions between factors such as superoxide (O(2)(-)) and angiotensin II (ANG II). We have reported that NaCl-induced O(2)(-) in the MD is produced by the NOX2 isoform of NADPH oxidase (NOX); however, the source of ANG II-induced O(2)(-) in MD is unknown. Thus we determined the pathways by which ANG II increased O(2)(-) in the MD by measuring O(2)(-) in ANG II-treated MMDD1 cells, a MD-like cell line. ANG II caused MMDD1 O(2)(-) levels to increase by more than twofold (P < 0.01). This increase was blocked by losartan (AT(1) receptor blocker) but not PD-123319 (AT(2) receptor antagonist). Apocynin (a NOX inhibitor) decreased O(2)(-) by 86% (P < 0.01), whereas oxypurinol (a xanthine oxidase inhibitor) and NS-398 (a cyclooxygenase-2 inhibitor) had no significant effect. The NOX-dependent increase in O(2)(-) was due to the NOX2 isoform; a short interfering (si)RNA against NOX2 blunted ANG II-induced increases in O(2)(-), whereas the NOX4/siRNA did not. Finally, we found that inhibiting the Rac1 subunit of NOX blunted ANG II-induced O(2)(-) production in NOX4/siRNA-treated cells but did not further decrease it in NOX2/siRNA-treated cells. Our results indicate that ANG II stimulates O(2)(-) production in the MD primarily via AT(1)-dependent activation of NOX2. Rac1 is required for the full activation of NOX2. This pathway may be an important component of ANG II enhancement of tubuloglomerular feedback.

Oxidative stress and hypertension

This review has summarized some of the data supporting a role of ROS and oxidant stress in the genesis of hypertension. There is evidence that hypertensive stimuli, such as high salt and angiotensin II, promote the production of ROS in the brain, the kidney, and the vasculature and that each of these sites contributes either to hypertension or to the untoward sequelae of this disease. Although the NADPH oxidase in these various organs is a predominant source, other enzymes likely contribute to ROS production and signaling in these tissues. A major clinical challenge is that the routinely used antioxidants are ineffective in preventing or treating cardiovascular disease and hypertension. This is likely because these drugs are either ineffective or act in a non-targeted fashion, such that they remove not only injurious ROS Fig. 5. Proposed role of T cells in the genesis of hypertension and the role of the NADPH oxidase in multiple cells/organs in modulating this effect. In this scenario, angiotensin II stimulates an NADPH oxidase in the CVOs of the brain, increasing sympathetic outflow. Sympathetic nerve terminals in lymph nodes activate T cells, and angiotensin II also directly activates T cells. These stimuli also activate expression of homing signals in the vessel and likely the kidney, which attract T cells to these organs. T cells release cytokines that stimulate the vessel and kidney NADPH oxidases, promoting vasoconstriction and sodium retention. SFO, subfornical organ. 630 Harrison & Gongora but also those involved in normal cell signaling. A potentially important and relatively new direction is the concept that inflammatory cells such as T cells contribute to hypertension. Future studies are needed to understand the interaction of T cells with the CNS, the kidney, and the vasculature and how this might be interrupted to provide therapeutic benefit.

SOD1 deficiency causes salt sensitivity and aggravates hypertension in hydronephrosis

Hydronephrosis causes renal dysfunction and salt-sensitive hypertension, which is associated with nitric oxide deficiency and abnormal tubuloglomerular feedback (TGF) response. We investigated the role of oxidative stress for salt sensitivity and for hypertension in hydronephrosis. Hydronephrosis was induced in superoxide dismutase 1-transgenic (SOD1-tg), SOD1-deficient (SOD1-ko), and wild-type mice and in rats. In mice, telemetric measurements were performed during normal (0.7% NaCl) and high-sodium (4% NaCl) diets and with chronic tempol supplementation. The 8-iso-prostaglandin-F(2alpha) (F2-IsoPs) and protein excretion profiles and renal histology were investigated. The acute effects of tempol on blood pressure and TGF were studied in rats. In hydronephrosis, wild-type mice developed salt-sensitive hypertension (114 +/- 1 to 120 +/- 2 mmHg), which was augmented in SOD1-ko (125 +/- 3 to 135 +/- 4 mmHg) but abolished in SOD1-tg (109 +/- 3 to 108 +/- 3 mmHg). SOD1-ko controls displayed salt-sensitive blood pressure (108 +/- 1 to 115 +/- 2 mmHg), which was not found in wild types or SOD1-tg. Chronic tempol treatment reduced blood pressure in SOD1-ko controls (-7 mmHg) and in hydronephrotic wild-type (-8 mmHg) and SOD1-ko mice (-16 mmHg), but had no effect on blood pressure in wild-type or SOD1-tg controls. SOD1-ko controls and hydronephrotic wild-type and SOD1-ko mice exhibited increased fluid excretion associated with increased F2-IsoPs and protein excretion. The renal histopathological changes found in hydronephrotic wild-type were augmented in SOD1-ko and diminished in SOD-tg mice. Tempol attenuated blood pressure and normalized TGF response in hydronephrosis [DeltaP(SF): 15.2 +/- 1.2 to 9.1 +/- 0.6 mmHg, turning point: 14.3 +/- 0.8 to 19.7 +/- 1.4 nl/min]. Oxidative stress due to SOD1 deficiency causes salt sensitivity and plays a pivotal role for the development of hypertension in hydronephrosis. Increased superoxide formation may enhance TGF response and thereby contribute to hypertension.

Cellular mediators of renal vascular dysfunction in hypertension

The renal vasculature plays a major role in the regulation of renal blood flow and the ability of the kidney to control the plasma volume and blood pressure. Renal vascular dysfunction is associated with renal vasoconstriction, decreased renal blood flow, and consequent increase in plasma volume and has been demonstrated in several forms of hypertension (HTN), including genetic and salt-sensitive HTN. Several predisposing factors and cellular mediators have been implicated, but the relationship between their actions on the renal vasculature and the consequent effects on renal tubular function in the setting of HTN is not clearly defined. Gene mutations/defects in an ion channel, a membrane ion transporter, and/or a regulatory enzyme in the nephron and renal vasculature may be a primary cause of renal vascular dysfunction. Environmental risk factors, such as high dietary salt intake, vascular inflammation, and oxidative stress further promote renal vascular dysfunction. Renal endothelial cell dysfunction is manifested as a decrease in the release of vasodilatory mediators, such as nitric oxide, prostacyclin, and hyperpolarizing factors, and/or an increase in vasoconstrictive mediators, such as endothelin, angiotensin II, and thromboxane A(2). Also, an increase in the amount/activity of intracellular Ca(2+) concentration, protein kinase C, Rho kinase, and mitogen-activated protein kinase in vascular smooth muscle promotes renal vasoconstriction. Matrix metalloproteinases and their inhibitors could also modify the composition of the extracellular matrix and lead to renal vascular remodeling. Synergistic interactions between the genetic and environmental risk factors on the cellular mediators of renal vascular dysfunction cause persistent renal vasoconstriction, increased renal vascular resistance, and decreased renal blood flow, and, consequently, lead to a disturbance in the renal control mechanisms of water and electrolyte balance, increased plasma volume, and HTN. Targeting the underlying genetic defects, environmental risk factors, and the aberrant renal vascular mediators involved should provide complementary strategies in the management of HTN.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Isoforms and functions of NAD(P)H oxidase at the macula densa

Macula densa cells produce superoxide (O2-) during tubuloglomerular feedback primarily via NAD(P)H oxidase (NOX). The purpose of the present study was to determine NOXs expressed by the macula densa and the role of each one in NaCl-induced O2- production. To identify which isoforms are expressed, we applied single-cell RT-PCR to macula densa cells isolated by laser capture microdissection and to MMDD1 cells (a macula densa-like cell line). The captured cells expressed neuronal NOS (marker of macula densa), NOX2, and NOX4 but not NOX1. Expression of the NOXs and neuronal NOS was essentially identical in the MMDD1 cells. Thus, we used MMDD1 cells to investigate which isoform is responsible for NaCl-induced O2- production. We used small-interfering RNA to knock down NOX2 or NOX4 in MMDD1 cells and measured O2- exposed to low-salt solution (LS; 70 mmol/L of NaCl) or high-salt solution (HS; 140 mmol/L of NaCl). Exposing control cells (scrambled small-interfering RNA) to HS increased O2- concentrations from 0.75+/-0.28 to 1.48+/-0.46 U/min per 10(5) cells in LS and HS, respectively (P<0.001). Inhibiting NOX2 blocked the HS-induced increase in O2- (0.62+/-0.39 versus 0.76+/-0.31 U/min per 10(5) cells in LS and HS groups, respectively). Blocking NOX4 did not affect HS-induced O2- levels. O2- levels in the control cells during LS and HS were 0.80+/-0.30 and 1.56+/-0.49 U/min per 10(5) cells, respectively (P<0.001); whereas O2- levels in NOX4-small-interfering RNA-treated cells during LS and HS were 0.40+/-0.25 and 1.26+/-0.51 U/min per 10(5) cells, respectively (P<0.001). We conclude that, whereas macula densa cells express the NOX2 and NOX4 isoforms, NOX2 is primarily responsible for NaCl-induced O2- generation.

Role of NOX2 in the regulation of afferent arteriole responsiveness

NADPH oxidases (NOX) are the major source of reactive oxygen species (ROS) in the vasculature and contribute to the control of renal perfusion. The role of NOX2 in the regulation of blood pressure and afferent arteriole responsiveness was investigated in NOX2(-/-) and wild-type mice. Arteriole constrictions to ANG II (10(-14)-10(-6) mol/l) were weaker in NOX2(-/-) compared with wild types. N(omega)-nitro-l-arginine methyl ester (l-NAME; 10(-4) mol/l) treatment reduced basal diameters significantly more in NOX2(-/-) (-18%) than in wild types (-6%) and augmented ANG II responses. Adenosine (10(-11)-10(-4) mol/l) constricted arterioles of wild types but not of NOX2(-/-). However, simultaneous inhibition of adenosine type-2 receptors induced vasoconstriction, which was stronger in NOX2(-/-). Adenosine (10(-8) mol/l) enhanced the ANG II response in wild type, but not in NOX2(-/-). This sensitizing effect by adenosine was abolished by apocynin. Chronic ANG II pretreatment (14 days) did not change the ANG II responses in NOX2(-/-), but strengthened the response in wild types. ANG II pretreatment augmented the l-NAME response in NOX2(-/-) (-33%), but not in wild types. Simultaneous application of l-NAME and ANG II caused a stronger constriction in the NOX2(-/-) (-64%) than in wild types (-46%). Basal blood pressures were similar in both genotypes, however, chronic ANG II infusion elevated blood pressure to a greater extent in wild-type (15 +/- 1%) than in NOX2(-/-) (8 +/- 1%) mice. In conclusion, NOX2 plays an important role in the control of afferent arteriole tone and is involved in the contractile responses to ANG II and/or adenosine. NOX2 can be activated by elevated ANG II and may play an important role in ANG II-induced hypertension. NOX2-derived ROS scavenges nitric oxide, causing subsequent nitric oxide-deficiency.

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.

Heme oxygenase induction attenuates afferent arteriolar autoregulatory responses

Heme oxygenases (HO-1, HO-2) catalyze conversion of heme to iron, carbon monoxide (CO), and biliverdin/bilirubin. We studied the effects of renal HO-1 induction on afferent arteriole (Aff-Art) autoregulatory responses to increases in renal perfusion pressure (RPP). Rats were treated with hemin and SnCl2 to induce HO-1, and Aff-Art autoregulatory responses were evaluated using the rat blood-perfused juxtamedullary nephron preparation. Renal HO-1 expression was significantly increased in hemin- and SnCl2-treated rats, while HO-2 was not altered. Aff-Art autoregulatory constrictor responses to increases in RPP from 100 to 150 mmHg were attenuated in hemin- and SnCl2-treated rats compared with control rats (+1.1+/-3.3, n=9 and +4.4+/-5.3, n=9 vs. -14.2+/-1.5%, n=10, respectively) (P<0.05). Acute HO inhibition with chromium mesoporphyrin (CrMP; 15 micromol/l) restored Aff-Art autoregulatory responses in hemin- and SnCl2-treated rats. Superfusing Aff-Arts from control rats with 100 micromol/l biliverdin did not alter autoregulatory responses; however, superfusion with 1 mmol/l CO significantly attenuated autoregulatory responses to increases in RPP from 100 to 150 mmHg (+3.3+/-5.4 vs. -16.6+/-3.8%, n=6) (P<0.05). Acute soluble guanylate cyclase inhibition with 10 micromol/l ODQ restored Aff-Art autoregulatory responses in hemin-treated rats. Immunohistochemistry shows HO-2 to be expressed mainly in epithelial cells with weak staining in proximal tubules, interlobular arteries, and Aff-Arts. In hemin- and SnCl2-treated rats, HO-1 was induced in tubular epithelial cells but not interlobular arteries and Aff-Arts. We conclude that induction of renal HO-1 attenuates Aff-Art constrictor responses to increases in RPP via increasing CO production from tubular epithelial cells, suggesting that an augmented HO system in pathophysiological conditions modulates renal autoregulation.

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.

p22phox in the macula densa regulates single nephron GFR during angiotensin II infusion in rats

Angiotensin II (ANG II) infusion increases renal superoxide (O2−) and enhances renal vasoconstriction via macula densa (MD) regulation of tubuloglomerular feedback, but the mechanism is unclear. We targeted the p22 phox subunit of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) with small-interfering RNA (siRNA) to reduce NADPH oxidase activity and blood pressure response to ANG II in rats. We compared single nephron glomerular filtration rate (SNGFR) in samples collected from the proximal tubule (PT), which interrupts delivery to the MD, and from the distal tubule (DT), which maintains delivery to the MD, to assess MD regulation of GFR. SNGFR was measured in control and ANG II-infused rats (200 ng·kg−1·min−1 for 7 days) 2 days after intravenous injection of vehicle or siRNA directed to p22 phox to test the hypothesis that p22 phox mediates MD regulation of SNGFR during ANG II. The regulation of SNGFR by MD, determined by PT SNGFR-DT SNGFR, was not altered by siRNA in control rats (control + vehicle, 13 ± 1, n = 8; control + siRNA, 12 ± 2 nl/min, n = 8; not significant) but was reduced by siRNA in ANG II-treated rats (ANG II + vehicle, 13 ± 2, n = 7; ANG II + siRNA, 7 ± 1 nl/min, n = 8; P < 0.05). We conclude that p22 phox and NADPH oxidase regulate the SNGFR during ANG II infusion via MD-dependent mechanisms.

Angiotensin II–nitric oxide interaction in the kidney

PURPOSE OF REVIEW

The balance of angiotensin II and nitric oxide determines the sensitivity of the tubuloglomerular feedback mechanism, renal vascular resistance and filtration rate. Angiotensin II induces nitric oxide release, but the role of angiotensin II receptors here is not fully understood. Further, the angiotensin II-nitric oxide interaction can be modulated by reactive oxygen species. This review focuses on the angiotensin II-nitric oxide interaction and their modulation by reactive oxygen species in the control of renal blood flow.

RECENT FINDINGS

Ideas about the role of angiotensin II type 1 and angiotensin II type 2 receptors are extended by the observation of angiotensin II type 1-mediated nitric oxide release with direct effects on vascular tone, tubuloglomerular feedback and sympathetic neurotransmission. Angiotensin receptors elicit disparate effects on intrarenal circulation. Angiotensin II-nitric oxide interactions are modulated by reactive oxygen species, as shown by angiotensin II type 1-mediated activation of superoxide and depression of antioxidant enzymes leading to reduced nitric oxide concentration - mechanisms that may be also important in angiotensin II-dependent hypertension.

SUMMARY

Recent studies show that angiotensin II stimulates the nitric oxide system via angiotensin II type 1 and angiotensin II type 2 receptors, whereas receptors exert different effects on renal and medullary flow. The interaction via angiotensin II type 1 is modulated by reactive oxygen species.

Superoxide mediates acute renal vasoconstriction produced by angiotensin II and catecholamines by a mechanism independent of nitric oxide

NAD(P)H oxidases (NOX) and reactive oxygen species (ROS) are involved in vasoconstriction and vascular remodeling during hypertension produced by chronic angiotensin II (ANG II) infusion. These effects are thought to be mediated largely through superoxide anion (O2−) scavenging of nitric oxide (NO). Little is known about the role of ROS in acute vasoconstrictor responses to agonists. We investigated renal blood flow (RBF) reactivity to ANG II (4 ng), norepinephrine (NE, 20 ng), and α1-adrenergic agonist phenylephrine (PE, 200 ng) injected into the renal artery (ira) of anesthetized Sprague-Dawley rats. The NOX inhibitor apocynin (1–4 mg·kg−1·min−1 ira, 2 min) or the superoxide dismutase mimetic Tempol (1.5–5 mg·kg−1·min−1 ira, 2 min) rapidly increased resting RBF by 8 ± 1% ( P < 0.001) or 3 ± 1% ( P < 0.05), respectively. During NO synthase (NOS) inhibition ( Nω-nitro-l-arginine methyl ester, 25 mg/kg iv), the vasodilation tended to increase (apocynin 13 ± 4%, Tempol 10 ± 1%). During control conditions, both ANG II and NE reduced RBF by 24 ± 4%. Apocynin dose dependently reduced the constriction by up to 44% ( P < 0.05). Similarly, Tempol blocked the acute actions of ANG II and NE by up to 48–49% ( P < 0.05). In other animals, apocynin (4 mg·kg−1·min−1 ira) attenuated vasoconstriction to ANG II, NE, and PE by 46–62% ( P < 0.01). During NOS inhibition, apocynin reduced the reactivity to ANG II and NE by 60–72% ( P < 0.01), and Tempol reduced it by 58–66% ( P < 0.001). We conclude that NOX-derived ROS substantially contribute to basal RBF as well as to signaling of acute renal vasoconstrictor responses to ANG II, NE, and PE in normal rats. These effects are due to O2− rather than H2O2, occur rapidly, and are independent of scavenging of NO.

N-Acetylcysteine improves renal dysfunction, ameliorates kidney damage and decreases blood pressure in salt-sensitive hypertension

BACKGROUND

Salt-sensitive hypertension in humans and experimental animals causes progressive increases in renal damage and dysfunction. The Dahl salt-sensitive (S) rat closely mimics human salt-sensitive hypertension.

AIM

Our goal was to test the hypothesis that enhancing the glutathione system with dietary N-acetylcysteine administration in Dahl S rats on a high sodium intake for 5 weeks will attenuate the increases in arterial pressure, the decreases in renal hemodynamics and the increases in renal damage that normally occur in S rats on high sodium.

METHODS

Forty-four 7- to 8-week-old Dahl S/Rapp strain rats were maintained on a high sodium (8%), high sodium + N-acetylcysteine (4 g/kg per day), or low sodium (0.3%) diet for 5 weeks. Rats had arterial and venous catheters implanted at day 21.

RESULTS

By day 35 in the high-sodium rats, N-acetylcysteine treatment significantly increased the renal reduced-to-oxidized glutathione ratio, glomerular filtration rate, and renal plasma flow, and decreased renal cortical and medullary O2 release, urinary protein excretion, renal tubulointerstitial damage and glomerular necrosis. At this time, mean arterial pressure increased to 183 +/- 1 mmHg, and N-acetylcysteine reduced this arterial pressure to 121 +/- 4 mmHg. By day 35 in S high-sodium rats, N-acetylcysteine had caused a 91% decrease in glomerular necrosis and an 83% decrease in tubulointerstitial damage.

CONCLUSIONS

In Dahl S rats on high sodium intake, arterial pressure increases significantly and renal injury is pronounced. Treatment with N-acetylcysteine enhances the renal glutathione system, improves renal dysfunction and markedly decreases arterial pressure and renal injury in Dahl salt-sensitive hypertension.

NADPH Oxidase in the Renal Medulla Causes Oxidative Stress and Contributes to Salt-Sensitive Hypertension in Dahl S Rats

Dahl salt-sensitive (SS) rats exhibit increased renal medullary oxidative stress and blood pressure salt-sensitivity compared with consomic, salt-resistant SS-13BN rats, despite highly similar genetic backgrounds. The present study examined potential sources of renal medullary superoxide in prehypertensive SS rats fed a 0.4% NaCl diet by assessing activity and protein levels of superoxide producing and scavenging enzymes. Superoxide production was nearly doubled in SS rats compared with SS-13BN rats as determined by urinary 8-isoprostane excretion and renal medullary oxy-ethidium microdialysate levels. Medullary superoxide production in tissue homogenates was greater in SS rats, and the NADPH oxidase inhibitor diphenylene iodonium preferentially reduced SS levels to those found in SS-13BN rats. Dinitrophenol, a mitochondrial uncoupler, eliminated the remaining superoxide production in both strains, whereas inhibition of xanthine oxidase, NO synthase, and cycloxygenase had no effect. L-arginine, NO synthase, superoxide dismutase, catalase, and glutathione peroxidase activities between SS and SS-13BN rats did not differ. Chronic blood pressure responses to a 4% NaCl diet were then determined in the presence or absence of the NADPH oxidase inhibitor apocynin (3.5 microg/kg per minute), chronically delivered directly into the renal medulla. Apocynin infusion reduced renal medullary interstitial superoxide from 1059+/-130 to 422+/-80 (oxyethidium fluorescence units) and mean arterial pressure from 175+/-4 to 157+/-6 mm Hg in SS rats, whereas no effects on either were observed in the SS-13(BN). We conclude that excess renal medullary superoxide production in SS rats contributes to salt-induced hypertension, and NADPH oxidase is the major source of the excess superoxide.

Acute effects of the superoxide dismutase mimetic tempol on split kidney function in two-kidney one-clip hypertensive rats

OBJECTIVE

To investigate the acute effects of the superoxide dismutase mimetic 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol) on split kidney function, and renal haemodynamics, in two-kidney, one-clip (2K1C) hypertensive rats.

METHODS

Three weeks after clipping, or the sham procedure, the effects of intravenous tempol (200 micromol/kg per h) were evaluated on thiobutabarbital anaesthetized Sprague-Dawley rats.

RESULTS

Mean arterial pressure (MAP; 152 +/- 3 versus 122 +/- 3 mmHg, P < 0.001), plasma renin activity (28.7 +/- 3.0 versus 9.5 +/- 0.6 ng/ml per h, P < 0.001) and urinary 8-iso-prostaglandin F2alpha excretion (124 +/- 4 versus 92 +/- 10 pmol/24 h, P = 0.003) were significantly elevated in 2K1C rats compared with sham. Tempol reduced MAP by 15 +/- 1% compared with baseline (P < 0.001) in 2K1C rats. In clipped kidneys, tempol increased the glomerular filtration rate (GFR; +50 +/- 15% from baseline) and the effective renal plasma flow (ERPF; +37 +/- 13%, from baseline), and reduced renal vascular resistance (RVR; -32 +/- 6% from baseline) compared with saline-treated controls (P < 0.05). In non-clipped kidneys, tempol reduced RVR (-24 +/- 5% from baseline) compared with saline-treated controls (P = 0.001). In sham-operated rats, tempol produced a modest reduction in MAP (-8 +/- 2% from baseline, P = 0.003), but did not significantly affect renal haemodynamics or function.

CONCLUSION

Tempol reduced MAP and RVR in both clipped and non-clipped kidneys of 2K1C hypertensive rats. In addition, tempol increased ERPF and GFR in the clipped kidney. These findings suggest important roles for superoxide in the regulation of renal haemodynamics during the early maintenance phase of renovascular hypertension.

Angiotensin II, reactive oxygen species, and Ca2+signaling in afferent arterioles

In afferent arteriolar vascular smooth muscle cells, ANG II induces a rise in cytosolic Ca2+([Ca2+]i) via inositol trisphosphate receptor (IP3R) stimulation and by activation of the adenine diphosphate ribose (ADPR) cyclase to form cyclic ADPR, which sensitizes the ryanodine receptor (RyR) to Ca2+. We hypothesize that ANG II stimulation of NAD(P)H oxidases leads to the formation of superoxide anion (O2−·), which, in turn, activates ADPR cyclase. Afferent arterioles were isolated from rat kidney with the magnetized microsphere and sieving technique and loaded with fura-2 to measure [Ca2+]i. ANG II rapidly increased [Ca2+]iby 124 ± 12 nM. In the presence of apocynin, a specific inhibitor of NAD(P)H oxidase assembly, the [Ca2+]iresponse was reduced to 35 ± 5 nM ( P < 0.01). Tempol, a superoxide dismutase mimetic, did not alter the [Ca2+]iresponse to ANG II at a concentration of 10−4M (99 ± 12 nM), but 10−3M tempol reduced the response to 32 ± 3 nM ( P < 0.01). The addition of nicotinamide, an inhibitor of ADPR cyclase, to apocynin or tempol (10−3M) resulted in no further inhibition. Measurement of superoxide production with the fluorescent probe tempo 9-AC showed that ANG II caused an increase of 48 ± 20 arbitrary units; apocynin or diphenyl iodonium (an inhibitor of flavoprotein oxidases) inhibited the response by 94%. Hydrogen peroxide (H2O2) was studied at physiological (10−7M) and higher concentrations. In the presence of H2O2(10−7M), neither baseline [Ca2+]inor the response to ANG II was altered (125 ± 15 nM), whereas H2O2(10−6and 10−5M) inhibited the [Ca2+]iresponse to ANG II by 35 and 46%, respectively. We conclude that ANG II rapidly activates NAD(P)H oxidases of afferent arterioles, leading to the formation of O2−·, which then stimulates ADPR cyclase to form cADPR. cADPR, by sensitizing the RyR to Ca2+, augments the Ca2+response (calcium-induced calcium release) initiated by activation of the IP3R.

Kidney immune cell infiltration and oxidative stress contribute to prenatally programmed hypertension

BACKGROUND

Prenatal environment has been shown to modify adult blood pressure profile, but the underlying mechanisms are not well understood. The role of renal immune cell infiltration, oxidative stress, and nitric oxide bioavailability in the pathogenesis was investigated.

METHODS

Adult hypertension in rat offspring was induced by maternal low protein diet. Oxidative stress was determined by quantitative immunoblotting for nitrotyrosine, and T-cell and macrophage content by immunostaining, in offspring kidneys before and after the onset of hypertension. Nitric oxide metabolites (NOx) were measured in 24-hour urines. A group of offspring was treated with the immunosuppressive drug mycophenolate mofetil (MMF) to reduce inflammation, or with the superoxide dismutase mimetic Tempol to reduce oxidative stress, for a 3-week period before the onset of hypertension.

RESULTS

During the prehypertensive stage, at 4 weeks of age, the low protein diet pups exhibited an increase in kidney nitrotyrosine content and in number of immune cells, both of which persisted in untreated animals after hypertension was established, at 8 weeks of age. Urine NOx was increased at 4 weeks and unchanged at 8 weeks of age. Both MMF and Tempol treatment prevented the immune cell infiltration, the increase in kidney nitrotyrosine abundance, and the development of hypertension. The effect on blood pressure persisted throughout the 4- to 10-week observation period after discontinuation of the treatments.

CONCLUSION

Renal oxidative stress and infiltrating immune cells may play a pathogenetic role in prenatally programmed hypertension. Nitric oxide bioavailability does not appear impaired.

Increased intracellular pH at the macula densa activates nNOS during tubuloglomerular feedback

BACKGROUND

The macula densa senses increasing NaCl concentrations in tubular fluid and increases afferent arteriole tone by a process known as tubuloglomerular feedback (TGF). Nitric oxide (NO) production by macula densa neuronal nitric oxide synthase (nNOS) is enhanced by increasing NaCl in the macula densa lumen, and the NO thus formed inhibits TGF. Blocking apical Na(+)/H(+) exchange with amiloride augments TGF and mimics the effect of nNOS inhibition. We hypothesized that increasing NaCl in the macula densa lumen raises macula densa intracellular pH (pH(i)) and activates nNOS.

METHODS

The thick ascending limb and a portion of the distal tubule with intact macula densa plaque adherent to the glomerulus were microdissected and perfused. Macula densa perfusate was changed from a low (10 mmol/L) to high NaCl solution (80 mmol/L) to mimic the conditions that induce TGF. Osmolality of both solutions was 180 mOsm, so that changing the solutions did not alter cell volume.

RESULTS

Macula densa pH(i) increased significantly from 7.0 +/- 0.5 to 7.8 +/- 0.6 when the perfusate was changed from low to high (P < 0.05; N= 5). When amiloride was added to inhibit Na(+)/H(+) exchange, the increase in pH(i) during TGF was blocked (N= 5). Fluorescence intensity of DAF-2, an NO-sensitive dye, increased by 28.8 +/- 4.1% after increasing luminal NaCl (N= 5), indicating an increase in NO production. In the presence of the Na(+)/H(+) exchanger inhibitor amiloride or the nNOS inhibitor 7-NI, the increase in NO induced by switching the macula densa perfusate from low to high was blunted. To study whether changes in pH(i) can directly alter NO production, we used nigericin, a K(+)/H(+) ionophore, to equilibrate luminal and intracellular pH. When macula densa pH was raised from 7.3 to 7.8 in the presence of 10(-5) mol/L nigericin in the low NaCl solution, fluorescence of DAF-2 in the macula densa increased by 17.9 +/- 1.3% (P < 0.01; N= 5). In the presence of 7-NI, the increase in NO induced by raising pH(i) was blocked (N= 5).

CONCLUSION

We concluded that macula densa pH(i) increases during TGF, and this increase in pH(i) activates nNos.

Effects of isoprostane on tubuloglomerular feedback: roles of TP receptors, NOS, and salt intake

A thromboxane prostanoid receptor (TP-R) agonist U-46,619 enhances tubuloglomerular feedback (TGF). Glomerular expression of TP-R and enhancement of TGF by U-46,619 increase with salt intake. We investigated the hypothesis that 8-isoprostaglandin F(2alpha) (8-Iso) activates TGF via TP-R. The maximal TGF response in rats was assessed from the fall in proximal stop flow pressure (PSF; an index of glomerular capillary pressure) during loop of Henle (LH) microperfusion of artificial tubular fluid (ATF) at 40 nl/min. Microperfusion of 8-Iso (10(-4) M) into the efferent arteriole (EA) enhanced TGF responses by 20 +/- 3% (P < 0.01). TGF response to 8-Iso was independent of dietary salt [DeltaTGF%, low salt (LS): 21 +/- 5%; normal salt (NS): 17 +/- 4%; high salt (HS): 29 +/- 8%, not significant (ns)], unlike the salt-dependent effect of U-46,619 (DeltaTGF%, LS: 41 +/- 5%; NS: 52 +/- 4%; HS: 112 +/- 21%). Ifetroban, the TP-R antagonist, abolished TGF responses to 8-Iso and U-46,619 at all levels of salt intake. During luminal perfusion of N-monomethyl-l-arginine (l-NMA), the effect of 8-Iso on TGF was enhanced in NS and HS but not in LS (LS: 22 +/- 6 vs. LS + l-NMA: 28 +/- 6%, ns; NS: 18 +/- 4 vs. NS + l-NMA: 40 +/- 4, P < 0.01; HS: 27 +/- 3 vs. HS + l-NMA: 65 +/- 6, P < 0.01). However, U-46,619 did not further increase TGF after l-NMA in all salt groups (LS: 43 +/- 7 vs. LS + l-NMA: 51 +/- 6, ns; NS: 52 +/- 7 vs. NS + l-NMA: 48 +/- 8, ns; HS: 114 +/- 21 vs. HS + l-NMA: 74 +/- 22, ns). In conclusion, activation of TP receptors by U-46,619 and 8-Iso-PGF(2alpha) enhances TGF. In addition, the effect of U-46,619 was salt dependent, whereas the effect of 8-Iso-PGF(2alpha) was salt independent. However, stimulation of NO by 8-isoprostanes masks its salt-sensitive effect on TGF.