Superoxide scavenging attenuates renal responses to ANG II during nitric oxide synthase inhibition in anesthetized dogs

The renal antioxidative effect of losartan involves heat shock protein 70 in proximal tubule cells

Angiotensin II exerts a cardinal role in the pathogenesis of hypertension and renal injury via action of angiotensin II type 1 (AT1) receptors. Local renin-angiotensin system (RAS) activity is essential for the mechanisms mediating pathophysiological functions. Proximal tubular angiotensinogen and tubular AT1 receptors are augmented by intrarenal angiotensin II. Caveolin 1 plays an important role as a regulatory molecule for the compartmentalization of redox signaling events through angiotensin II-induced NADPH oxidase activation in the kidney. A role for the renin-angiotensin system in the development and/or maintenance of hypertension has been demonstrated in spontaneously hypertensive rats (SHRs). Many effects of angiotensin II are dependent on the AT1 stimulation of reactive oxygen species (ROS) production by NADPH oxidase. Angiotensin II upregulation stimulates oxidative stress in proximal tubules from SHR. The NADPH oxidase 4 (Nox4) is abundantly expressed in kidney proximal tubule cells. Induction of the stress response includes synthesis of heat shock protein 70, a molecular chaperone that has a critical role in the recovery of cells from stress and in cytoprotection, guarding cells from subsequent insults. HSP70 chaperones function in part by driving the molecular triage decision, which determines whether proteins enter the productive folding pathway or result in client substrate ubiquitination and proteasomal degradation. This review examines regulation of losartan-mediated antioxidative stress responses by the chaperone HSP70 in proximal tubule cells of spontaneously hypertensive rats.

Superoxide Formation and Interaction with Nitric Oxide Modulate Systemic Arterial Pressure and Renal Function in Salt-Depleted Dogs

To determine the role of superoxide (O2–) formation in the kidney during alterations in the renin-angiotensin system, we evaluated responses to the intra-arterial infusion of an O2–-scavenging agent, tempol, in the denervated kidney of anesthetized salt-depleted (SD, n = 6) dogs and salt-replete (SR, n = 6) dogs. As expected, basal plasma renin activity was higher in SD than in SR dogs (8.4 ± 1.0 vs. 2.3 ± 0.6 ng angiotensin 1/ml/hr). Interestingly, the basal level of urinary F2-isoprostanes excretion (marker for endogenous O2– activity) relative to creatinine (Cr) excretion was also significantly higher in SD compared to SR dogs (9.1 ± 2.8 vs. 1.6 ± 0.4 ng F2-isoprostanes/mg of Cr). There was a significant increase in renal blood flow (4.3 ± 0.5 to 4.9 ± 0.6 ml/min/g) and decreases in renal vascular resistance (38.2 ± 5.8 to 33.2 ± 4.7 mm Hg/ml/min/g) and mean systemic arterial pressure (148 ± 6 to 112 ± 10 mm Hg) in SD dogs but not in SR dogs during infusion of tempol at 1 mg/kg/min for 30 mins. Glomerular filtration rate and urinary sodium excretion (UNaV) did not change significantly during tempol infusion in both groups of dogs. Administration of the nitric oxide synthase inhibitor nitro-L-arginine (50 μg/kg/min) during tempol infusion caused a reduction in UNaV in SR dogs (47% ± 12%) but did not cause a decrease in SD dogs. These data show that low salt intake enhances O2– activity that influences renal and systemic hemodynamics and thus may contribute to the regulation of arterial pressure in the salt-restricted state.

Enhancement in cellular Na+K+ATPase activity by low doses of peroxynitrite in mouse renal tissue and in cultured HK2 cells

In the normal condition, endogenous formation of peroxynitrite (ONOOˉ) from the interaction of nitric oxide and superoxide has been suggested to play a renoprotective role. However, the exact mechanism associated with renoprotection by this radical compound is not yet clearly defined. Although ONOOˉ usually inhibits renal tubular Na⁺K⁺ATPase (NKA) activity at high concentrations (micromolar to millimolar range [μM–mM], achieved in pathophysiological conditions), the effects at lower concentrations (nanomolar range [nM], relevant in normal condition) remain unknown. To examine the direct effect of ONOOˉ on NKA activity, preparations of cellular membrane fraction from mouse renal tissue and from cultured HK2 cells (human proximal tubular epithelial cell lines) were incubated for 10 and 30 min each with different concentrations of ONOOˉ (10 nmol/L–200 μmol/L). NKA activity in these samples (n = 5 in each case) was measured via a colorimetric assay capable of detecting inorganic phosphate. At high concentrations (1–200 μmol/L), ONOOˉ caused dose‐dependent inhibition of NKA activity (−3.0 ± 0.6% and −36.4 ± 1.4%). However, NKA activity remained unchanged at 100 and 500 nmol/L ONOOˉ concentration, but interestingly, at lower concentrations (10 and 50 nmol/L), ONOOˉ caused small but significant increases in the NKA activity (3.3 ± 1.1% and 3.1 ± 0.6%). Pretreatment with a ONOOˉ scavenger, mercaptoethylguanidine (MEG; 200 μmol/L), prevented these biphasic responses to ONOOˉ. This dose‐dependent biphasic action of ONOO⁻ on NKA activity may implicate that this radical compound helps to maintain sodium homeostasis either by enhancing tubular sodium reabsorption under normal conditions or by inhibiting it during oxidative stress conditions.

Salt-Sensitive Hypertension: Perspectives on Intrarenal Mechanisms

Salt sensitive hypertension is characterized by increases in blood pressure in response to increases in dietary salt intake and is associated with an enhanced risk of cardiovascular and renal morbidity. Although researchers have sought for decades to understand how salt sensitivity develops in humans, the mechanisms responsible for the increases in blood pressure in response to high salt intake are complex and only partially understood. Until now, scientists have been unable to explain why some individuals are salt sensitive and others are salt resistant. Although a central role for the kidneys in the development of salt sensitivity and hypertension has been generally accepted, it is also recognized that hypertension is of multifactorial origin and a variety of factors can induce, or prevent, blood pressure responsiveness to the manipulation of salt intake. Excess salt intake in susceptible persons may also induce inappropriate central and sympathetic nervous system responses and increase the production of intrarenal angiotensin II, catecholamines and other factors such as oxidative stress and inflammatory cytokines. One key factor is the concomitant inappropriate or paradoxical activation of the intrarenal renin-angiotensin system, by high salt intake. This is reflected by the increases in urinary angiotensinogen during high salt intake in salt sensitive models. A complex interaction between neuroendocrine factors and the kidney may underlie the propensity for some individuals to retain salt and develop salt-dependent hypertension. In this review, we focus mainly on the renal contributions that provide the mechanistic links between chronic salt intake and the development of hypertension.

Superoxide enhances Ca2+ entry through L-type channels in the renal afferent arteriole

Reactive oxygen species regulate cardiovascular and renal function in health and disease. Superoxide participates in acute calcium signaling in afferent arterioles and renal vasoconstriction produced by angiotensin II, endothelin, thromboxane, and pressure-induced myogenic tone. Known mechanisms by which superoxide acts include quenching of nitric oxide and increased ADP ribosyl cyclase/ryanodine-mediated calcium mobilization. The effect(s) of superoxide on other calcium signaling pathways in the renal microcirculation is poorly understood. The present experiments examined the acute effect of superoxide generated by paraquat on calcium entry pathways in isolated rat afferent arterioles. The peak increase in cytosolic calcium concentration caused by KCl (40 mmol/L) was 99±14 nmol/L. The response to this membrane depolarization was mediated exclusively by L-type channels because it was abolished by nifedipine but was unaffected by the T-type channel blocker mibefradil. Paraquat increased superoxide production (dihydroethidium fluorescence), tripled the peak response to KCl to 314±68 nmol/L (P<0.001) and doubled the plateau response. These effects were abolished by tempol and nitroblue tetrazolium, but not by catalase, confirming actions of superoxide and not of hydrogen peroxide. Unaffected by paraquat and superoxide was calcium entry through store-operated calcium channels activated by thapsigargin-induced calcium depletion of sarcoplasmic reticular stores. Also unresponsive to paraquat was ryanodine receptor-mediated calcium-induced calcium release from the sarcoplasmic reticulum. Our results provide new evidence that superoxide enhances calcium entry through L-type channels activated by membrane depolarization in rat cortical afferent arterioles, without affecting calcium entry through store-operated entry or ryanodine receptor-mediated calcium mobilization.

Protective role of the endothelial isoform of nitric oxide synthase in ANG II-induced inflammatory responses in the kidney

In the present study, we examine the hypothesis that the nitric oxide (NO) produced by endothelial NO synthase (eNOS) plays a protective role in the development of ANG II-induced hypertension and renal injury by minimizing oxidative stress and the inflammation induced by TNF-α. Systolic blood pressure (SBP) and renal injury responses to chronic infusions of ANG II (via implanted minipumps) were evaluated for 2 wk in wild-type (WT) and in eNOS knockout mice (KO) cotreated with or without a superoxide (O2(-)) scavenger, tempol (400 mg/l in the drinking water), or a TNF-α receptor blocker, etanercept (5 mg/kg/day ip). In study 1, when ANG II was given at a dose of 25 ng/min, it increased mean SBP in WT mice (Δ36 ± 3 mmHg; n = 7), and this effect was attenuated in mice pretreated with tempol (Δ24 ± 3 mmHg; n = 6). In KO mice (n = 9), this dose of ANG II resulted in severe renal injury associated with high mortality. To avoid this high mortality in KO, study 2 was conducted with a lower dose of ANG II (10 ng/min) that increased SBP slightly in WT (Δ17 ± 7 mmHg; n = 6) but exaggeratedly in KO (Δ48 ± 12 mmHg, n = 6) associated with severe renal injury. Cotreatment with either tempol (n = 6) or etanercept (n = 6) ameliorated the hypertensive, as well as the renal injury responses in KO compared with WT. These data demonstrate a protective role for eNOS activity in preventing renal inflammatory injury and hypertension induced by chronic increases in ANG II.

Thromboxane-induced renal vasoconstriction is mediated by the ADP-ribosyl cyclase CD38 and superoxide anion

The present renal hemodynamic study tested the hypothesis that CD38 and superoxide anion (O2(·-)) participate in the vasoconstriction produced by activation of thromboxane prostanoid (TP) receptors in the mouse kidney. CD38 is the major mammalian ADP-ribosyl cyclase contributing to vasomotor tone through the generation of cADP-ribose, a second messenger that activates ryanodine receptors to release Ca(2+) from the sarcoplasmic reticulum in vascular smooth muscle cells. We evaluated whether the stable thromboxane mimetic U-46619 causes less pronounced renal vasoconstriction in CD38-deficient mice and the involvement of O2(·-) in U-46619-induced renal vasoconstriction. Our results indicate that U-46619 activation of TP receptors causes renal vasoconstriction in part by activating cADP-ribose signaling in renal resistance arterioles. Based on maximal renal blood flow and renal vascular resistance responses to bolus injections of U-46619, CD38 contributes 30-40% of the TP receptor-induced vasoconstriction. We also found that the antioxidant SOD mimetic tempol attenuated the magnitude of vasoconstriction by U-46619 in both groups of mice, suggesting mediation by O2(·-). The degree of tempol blockage of U-46619-induced renal vasoconstriction was greater in wild-type mice, attenuating renal vasoconstriction by 40% compared with 30% in CD38-null mice. In other experiments, U-46619 rapidly stimulated O2(·-) production (dihydroethidium fluorescence) in isolated mouse afferent arterioles, an effect abolished by tempol. These observations provide the first in vivo demonstration of CD38 and O2(·-) involvement in the vasoconstrictor effects of TP receptor activation in the kidney and in vitro evidence for TP receptor stimulation of O2(·-) production by the afferent arteriole.

Renal protection of in vivo administration of tempol in streptozotocin-induced diabetic rats

The present study was carried out to investigate the protective effects of tempol on renal function and the underlying mechanism in streptozotocin-induced diabetic rats. The diabetic rats were randomly divided into the model group (without tempol) and tempol group (1 mM tempol in drinking water for 6 weeks). Nondiabetic rats were served as the Control group. The mRNA expression of canonical transient receptor potential 6 (TRPC6), transforming growth factor (TGF)-β1, and type IV collagen (Col IV) were examined. The malondialdehyde (MDA) level, activities of superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) in renal tissues were measured to assess redox status in kidneys. We found that tempol significantly reduced 24-h urine output and urine albuminuria excretion in the diabetic rats. Compared with the model group, the concentration of MDA was significantly lower in the tempol group. In addition, diabetes decreased activities of SOD and GSH-Px and these responses were prevented by tempol treatment. Moreover, in diabetic rats, the mRNA expression levels of TGF-β1 and Col IV were upregulated. TRPC6 mRNA expression level was down-regulated in diabetic kidneys. However, all of these diabetic effects were significantly suppressed by tempol treatment. These results suggest that chronic treatment of diabetic rats with tempol can protect kidneys, possibly by reducing expression of TGF-β1, Col IV, and upregulating TRPC6 expression level.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effects of tempol and candesartan on neural control of the kidney

We compared the effects of tempol (300 μmol kg(-1) plus 300 μmol kg(-1) h(-1), n=14) and candesartan (10 μg kg(-1) plus 10 μg kg(-1) h(-1), n=14) on renal haemodynamics, excretory function, and responses to electrical stimulation of the renal nerves (RNS) in lean and obese rabbits under pentobarbitone anaesthesia. Depressor responses to tempol (-16 ± 2 mmHg) and candesartan (-12 ± 1 mmHg) were similar. Candesartan, but not tempol, significantly increased basal renal blood flow (RBF; +36 ± 7%). Tempol, but not candesartan, significantly reduced glomerular filtration rate (GFR; -30 ± 10%) and sodium excretion (U(Na)V; -44 ± 14%). RNS induced frequency-dependent reductions in RBF (-20 ± 3% at 1 Hz), GFR (-28 ± 6% at 1 Hz) and U(Na)V (-55 ± 6% at 1 Hz). Candesartan blunted these responses. Tempol did not significantly alter RBF and GFR responses to RNS but blunted the U(Na)V response. Responses to RNS, and the effects of tempol and candesartan, were similar in lean compared with obese rabbits. Unlike candesartan, tempol did not induce renal vasodilatation, maintain GFR and U(Na)V during reductions in arterial pressure, or blunt neurally-mediated vasoconstriction. In conclusion, unlike the AT(1)-receptor antagonist candesartan, tempol does not blunt the effects of RNS on renal haemodynamic function. Furthermore, under the current experimental conditions superoxide appears to make little contribution to the actions of endogenous angiotensin II on baseline renal haemodynamics or excretory function, or their responses to RNS.

Development of oxidative stress in the peritubular capillary microenvironment mediates sepsis-induced renal microcirculatory failure and acute kidney injury

Acute kidney injury is a frequent and serious complication of sepsis. To better understand the development of sepsis-induced acute kidney injury, we performed the first time-dependent studies to document changes in renal hemodynamics and oxidant generation in the peritubular microenvironment using the murine cecal ligation and puncture (CLP) model of sepsis. CLP caused an increase in renal capillary permeability at 2 hours, followed by decreases in mean arterial pressure, renal blood flow (RBF), and renal capillary perfusion at 4 hours, which were sustained through 18 hours. The decline in hemodynamic parameters was associated with hypoxia and oxidant generation in the peritubular microenvironment and a decrease in glomerular filtration rate. The role of oxidants was assessed using the superoxide dismutase mimetic/peroxynitrite scavenger MnTMPyP [Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin]. At 10 mg/kg administered 6 hours after CLP, MnTMPyP did not alter blood pressure, but blocked superoxide and peroxynitrite generation, reversed the decline in RBF, capillary perfusion, and glomerular filtration rate, preserved tubular architecture, and increased 48-hour survival. However, MnTMPyP administered at CLP did not prevent capillary permeability or the decrease in RBF and capillary perfusion, which suggests that these early events are not mediated by oxidants. These data demonstrate that renal hemodynamic changes occur early after sepsis and that targeting the later oxidant generation can break the cycle of injury and enable the microcirculation and renal function to recover.

Modulation of the myogenic response in renal blood flow autoregulation by NO depends on endothelial nitric oxide synthase (eNOS), but not neuronal or inducible NOS

Nitric oxide (NO) blunts the myogenic response (MR) in renal blood flow (RBF) autoregulation. We sought to clarify the roles of NO synthase (NOS) isoforms, i.e. neuronal NOS (nNOS) from macula densa, endothelial NOS (eNOS) from the endothelium, and inducible NOS (iNOS) from smooth muscle or mesangium. RBF autoregulation was studied in rats and knockout (ko) mice in response to a rapid rise in renal artery pressure (RAP). The autoregulatory rise in renal vascular resistance within the first 6 s was interpreted as MR, from ∼6 to ∼30 s as tubuloglomerular feedback (TGF), and ∼30 to ∼100 s as the third regulatory mechanism. In rats, the nNOS inhibitor SMTC did not significantly affect MR (67 ± 4 vs. 57 ± 4 units). Inhibition of all NOS isoforms by l-NAME in the same animals markedly augmented MR to 78 ± 4 units. The same was found when SMTC was combined with angiotensin II to reproduce the hypertension and vasoconstriction seen with l-NAME (58 ± 3 vs. 54 ± 7 units, l-NAME 81 ± 2 units), or when SMTC was replaced by the nNOS inhibitor NPA (57 ± 5 vs. 56 ± 7 units, l-NAME 79 ± 4 units) or by the iNOS inhibitor 1400W (50 ± 1 vs. 55 ± 4 units, l-NAME 81 ± 3 units). nNOS-ko mice showed the same autoregulation as wild-types (MR 36 ± 4 vs. 38 ± 3 units) and the same response to l-NAME (111 ± 9 vs. 114 ± 10 units). eNOS-ko had similar autoregulation as wild-types (44 ± 8 vs. 33 ± 4 units), but failed to respond to l-NAME (37 ± 7 vs. 78 ± 16 units). We conclude that the attenuating effect of NO on MR depends on eNOS, but not on nNOS or iNOS. In eNOS-ko mice MR is depressed by NO-independent means.

Abundance of TRPC6 protein in glomerular mesangial cells is decreased by ROS and PKC in diabetes

The present study was performed to investigate the underlying mechanism, particularly the roles of reactive oxygen species (ROS) and protein kinase C (PKC), in the diabetes-induced canonical transient receptor potential 6 (TRPC6) downregulation. We found that high glucose (HG) significantly reduced TRPC6 protein expression in cultured mesangial cells (MCs). TRPC6 protein was also significantly reduced in the glomeruli but not in the heart or aorta isolated from streptozotocin-induced diabetic rats. In the cultured MCs, H(2)O(2) suppressed TRPC6 protein expression in a dose- and time-dependent manner, which emulated the HG effect. Catalase as well as superoxide dismutase were able to prevent the inhibitory effect of HG on TRPC6. The antioxidant effect observed in cultured cells was also observed in diabetic rats treated with tempol for 2 wk, which exhibited a preservation of TRPC6 in the glomeruli. Specific knockdown of Nox4, a component of NADPH oxidase, increased TRPC6 protein expression. Furthermore, the PKC activator phorbol 12-myristate 13-acetate (PMA), but not its analog 4α-phorbol 12, 13-didecanoate (4α-PDD), suppressed TRPC6 expression, and this PMA effect was not affected by catalase. Moreover, Gö6976, but not LY333531, attenuated the negative effect of HG on TRPC6 expression. Gö6976 also inhibited H(2)O(2) effect on TRPC6. Furthermore, either knockdown of TRPC6 or HG treatment significantly decreased ANG II-stimulated MC contraction, and the HG-impaired MC contraction was rescued by overexpression of TRPC6. These results suggest that hyperglycemia in diabetes downregulated TRPC6 protein expression in MCs through a NADPH oxidase Nox4-ROS-PKC pathway, proving a mechanism for impaired MC contraction in diabetes.

Angiotensin II stimulates thick ascending limb NO production via AT(2) receptors and Akt1-dependent nitric-oxide synthase 3 (NOS3) activation

Angiotensin II (Ang II) acutely stimulates thick ascending limb (TAL) NO via an unknown mechanism. In endothelial cells, activation of Ang II type 2 receptor (AT(2)) stimulates NO. Akt1 activates NOS3 by direct phosphorylation. We hypothesized that Ang II stimulates TAL NO production via AT(2)-mediated Akt1 activation, which phosphorylates NOS3 at serine 1177. We measured NO production by fluorescence microscopy. In isolated TALs, Ang II (100 nm) increased NO production by 1.1 +/- 0.2 fluorescence units/min (p < 0.01). Ang II increased cGMP accumulation by 4.9 +/- 1.3 fmol/microg (p < 0.01). Upon adding the AT(2) antagonist PD123319 (1 microm), Ang II failed to stimulate NO (0.1 +/- 0.1 fluorescence units/min; p < 0.001 versus Ang II); adding the AT(1) antagonist losartan (1 microm) resulted in Ang II stimulating NO by 0.9 +/- 0.1 fluorescence units/min. Akt inhibitor (5 microm) blocked Ang II-stimulated NO (-0.1 +/- 0.2 fluorescence units/min versus inhibitor alone). Phospho-Akt1 increased by 72% after 5 min (p < 0.006), returning to basal after 10 min. Phospho-Akt2 did not change after 5 min but increased by 115 and 163% after 10 and 15 min (p < 0.02). Phospho-Akt3 did not change. An AT(2) agonist increased pAkt1 by 78% (p < 0.02), PI3K inhibition blocked this effect. In TALs transduced with dominant negative Akt1, Ang II failed to stimulate NO (0.1 +/- 0.2 fluorescence units/min versus 1.2 +/- 0.2 for controls; p < 0.001). Ang II increased phospho-NOS3 at serine 1177 by 130% (p < 0.01) and 150% after 5 and 10 min (p < 0.02). Ang II increased phosphoNOS3 at serine 633 by 50% after 5 min (p < 0.01). Akt inhibition prevented NOS3 phosphorylation. We concluded that Ang II enhances TAL NO production via activation of AT(2) and Akt1-dependent phosphorylation of NOS3 at serines 1177 and 633.

Attenuation of renal excretory responses to ANG II during inhibition of superoxide dismutase in anesthetized rats

To examine the functional interaction between superoxide dismutase (SOD) and NADPH oxidase activity, we assessed renal responses to acute intra-arterial infusion of ANG II (0.5 ng x kg(-1) x min(-1)) before and during administration of a SOD inhibitor, diethyldithiocarbamate (DETC, 0.5 mg x kg(-1) x min(-1)), in enalaprilat-pretreated (33 microg x kg(-1) x min(-1)) rats (n = 11). Total (RBF) and regional (cortical, CBF; medullary; MBF) renal blood flows were determined by Transonic and laser-Doppler flowmetry, respectively. Renal cortical and medullary tissue NADPH oxidase activity in vitro was determined using the lucigenin-chemiluminescence method. DETC treatment alone resulted in decreases in RBF, CBF, MBF, glomerular filtration rate (GFR), urine flow (V), and sodium excretion (U(Na)V) as reported previously. Before DETC, ANG II infusion decreased RBF (-18 +/- 3%), CBF (-16 +/- 3%), MBF [-5 +/- 6%; P = not significant (NS)], GFR (-31 +/- 4%), V (-34 +/- 2%), and U(Na)V (-53 +/- 3%). During DETC infusion, ANG II also caused similar reductions in RBF (-20 +/- 4%), CBF (-19 +/- 3%), MBF (-2 +/- 2; P = NS), and in GFR (-22 +/- 7%), whereas renal excretory responses (V; -12 +/- 2%; U(Na)V; -24 +/- 4%) were significantly attenuated compared with those before DETC. In in vitro experiments, ANG II (100 muM) enhanced NADPH oxidase activity both in cortical [13,194 +/- 1,651 vs. 20,914 +/- 2,769 relative light units (RLU)/mg protein] and in medullary (21,296 +/- 2,244 vs. 30,597 +/- 4,250 RLU/mg protein) tissue. Application of DETC (1 mM) reduced the basal levels and prevented ANG II-induced increases in NADPH oxidase activity in both tissues. These results demonstrate that renal excretory responses to acute ANG II administration are attenuated during SOD inhibition, which seems related to a downregulation of NADPH oxidase in the deficient condition of SOD activity.

Cholesterol induces renal vasoconstriction and anti-natriuresis by inhibiting nitric oxide production in anesthetized rats

Although hypercholesterolemia is implicated in the pathophysiology of many renal disorders as well as hypertension, its direct actions in the kidney are not yet clearly understood. In the present study, we evaluated renal responses to administration of cholesterol (8 microg x min(-1).100 g body wt(-1); bound by polyethylene glycol) into the renal artery of anesthetized male Sprague-Dawley rats. Total renal blood flow (RBF) was measured by a Transonic flow probe, and glomerular filtration rate (GFR) was determined by Inulin clearance. In control rats (n = 8), cholesterol induced reductions of 10 +/- 2% in RBF [baseline (b) 7.6 +/- 0.3 microg x min(-1).100 g(-1)], 17 +/- 3% in urine flow (b, 10.6 +/- 0.9 microg x min(-1).100 g(-1)), 29 +/- 3% in sodium excretion (b, 0.96 +/- 0.05 mumol.min(-1).100 g(-1)) and 24 +/- 2% in nitrite/nitrate excretion (b, 0.22 +/- 0.01 nmol.min(-1).100 g(-1)) without an appreciable change in GFR (b, 0.87 +/- 0.03 ml.min(-1).100 g(-1)). These renal vasoconstrictor and anti-natriuretic responses to cholesterol were absent in rats pretreated with nitric oxide (NO) synthase inhibitor, nitro-l-arginine methylester (0.5 microg x min(-1).100 g(-1); n = 6). In rats pretreated with superoxide (O(2)(-)) scavenger tempol (50 microg x min(-1).100 g(-1); n = 6), the cholesterol-induced renal responses remained mostly unchanged, although there was a slight attenuation in anti-natriuretic response. This anti-natriuretic response to cholesterol was abolished in furosemide-pretreated rats (0.3 microg x min(-1).100 g(-1); n = 6) but remained unchanged in amiloride-pretreated rats (0.2 microg x min(-1).100 g(-1); n = 5), indicating that Na(+)/K(+)/2Cl(-) cotransport is the dominant mediator of this effect. These data demonstrate that cholesterol-induced acute renal vasoconstrictor and antinatriuretic responses are mediated by a decrease in NO production. These data also indicate that tubular effect of cholesterol on sodium reabsorption is mediated by the furosemide sensitive Na(+)/K(+)/2Cl(-) cotransporter.

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.

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.

Reduced renal responses to nitric oxide synthase inhibition in mice lacking the gene for gp91phox subunit of NAD(P)H oxidase

Both short-term and long-term nitric oxide (NO) blockade were shown to cause an increase in O(2)(-) activity. To assess the contribution of such enhanced O(2)(-) activity in the kidney, responses to administration of the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 200 microg.min(-1).kg body wt(-1)) were assessed in knockout mice the lacking NAD(P)H oxidase subunit gp91(phox) (KO; n = 10) and in wild-type (WT; n = 10) mice. Renal blood flow (RBF) and glomerular filtration rate (GFR) were determined by PAH and inulin clearances, respectively. Baseline RBF was higher in KO compared with WT mice (5.8 +/- 0.5 vs. 4.5 +/- 0.3 ml.min(-1).g(-1); P < 0.04) without significant differences in GFR (0.62 +/- 0.04 vs. 0.73 +/- 0.05 ml.min(-1).g(-1)) and in mean arterial pressure (MAP; 91 +/- 6 vs. 88 +/- 4 mmHg). L-NAME infusion for 60 min caused similar increases in MAP (114 +/- 6 vs. 113 +/- 3 mmHg) in both groups but resulted in a lesser degree of reduction in RBF in KO compared with WT mice (-7 +/- 3 vs. -17 +/- 3%; P < 0.02), although GFR remained unchanged in both groups. The natriuretic response to systemic L-NAME infusion was attenuated in KO compared with WT mice (Delta: 3.1 +/- 0.7 vs. 5.2 +/- 0.6 micromol.min(-1).g(-1)). L-NAME increased urinary 8-isoprostane excretion rate in WT (5.9 +/- 1 to 7.7 +/- 1 pg.min(-1).g(-1); P < 0.02) but not in KO mice (5.6 +/- 1 to 4.9 +/- 0.3 pg.min(-1).g(-1)). In contrast, responses to another vasoconstrictor, norepinephrine, were similar in both strains of mice. These data indicate that activation of NAD(P)H oxidase results in the enhancement of O(2)(-) activity that influences renal hemodynamics and excretory function in the condition of NO deficiency.

Nitric oxide in the kidney : its physiological role and pathophysiological implications

Nitric oxide has been implicated in many physiologic processes that influence both acute and long-term control of kidney function. Its net effect in the kidney is to promote natriuresis and diuresis, contributing to adaptation to variations of dietary salt intake and maintenance of normal blood pressure. A pretreatment with nitric oxide donors or L-arginine may prevent the ischemic acute renal injury. In chronic kidney diseases, the systolic blood pressure is correlated with the plasma level of asymmetric dimethylarginine, an endogenous inhibitor of nitric oxide synthase. A reduced production and biological action of nitric oxide is associated with an elevation of arterial pressure, and conversely, an exaggerated activity may represent a compensatory mechanism to mitigate the hypertension.

Reactive oxygen species participate in acute renal vasoconstrictor responses induced by ETAand ETBreceptors

Reactive oxygen species (ROS) play important roles in renal vasoconstrictor responses to acute and chronic stimulation by angiotensin II and norepinephrine, as well as in long-term effects of endothelin-1 (ET-1). Little is known about participation of ROS in acute vasoconstriction produced by ET-1. We tested the influence of NAD(P)H oxidase inhibition by apocynin [4 mg·kg−1·min−1, infused into the renal artery (ira)] on ETAand ETBreceptor signaling in the renal microcirculation. Both receptors were stimulated by ET-1, ETAreceptors by ET-1 during ETBantagonist BQ-788, and ETBby ETBagonist sarafotoxin 6C. ET-1 (1.5 pmol injected ira) reduced renal blood flow (RBF) 17 ± 4%. Apocynin raised baseline RBF (+10 ± 1%, P < 0.001) and attenuated the ET-1 response to 10 ± 2%, i.e., 35 ± 9% inhibition ( P < 0.05). Apocynin reduced ETA-induced vasoconstriction by 42 ± 12% ( P < 0.05) and that of ETBstimulation by 50 ± 8% ( P < 0.001). During nitric oxide (NO) synthase inhibition ( Nω-nitro-l-arginine methyl ester), apocynin blunted ETA-mediated vasoconstriction by 60 ± 8% ( P < 0.01), whereas its effect on the ETBresponse (by 87 ± 8%, P < 0.001) was even larger without than with NO present ( P < 0.05). The cell-permeable superoxide dismutase mimetic tempol (5 mg·kg−1·min−1ira), which reduces O2−and may elevate H2O2, attenuated ET-1 responses similar to apocynin (by 38 ± 6%, P < 0.01). We conclude that ROS, O2−rather than H2O2, contribute substantially to acute renal vasoconstriction elicited by both ETAand ETBreceptors and to basal renal vasomotor tone in vivo. This physiological constrictor action of ROS does not depend on scavenging of NO. In contrast, scavenging of O2−by NO seems to be more important during ETBstimulation.

Superoxide and its interaction with nitric oxide modulates renal function in prehypertensive Ren-2 transgenic rats

OBJECTIVE

The present study was performed to examine the role of superoxide (O2*) and its interaction with nitric oxide (NO) in the regulation of renal function in prehypertensive heterozygous Ren-2 transgenic rats (TGR).

METHODS

Renal responses to the O2* scavenger, tempol (150 microg/min per 100 g), and/or the NO synthase inhibitor, nitro-L-arginine methylester (L-NAME; 5 microg/min per 100 g), infused alone or in combination directly into the renal artery were evaluated in anesthetized heterozygous male TGR and aged-matched Hanover Sprague-Dawley rats (HanSD).

RESULTS

There were no differences in arterial pressure (122 +/- 3 versus 115 +/- 2 mmHg), renal plasma flow (RPF; 2.09 +/- 0.1 versus 2.07 +/- 0.1 ml/min per g), glomerular filtration rate (GFR; 0.73 +/- 0.1 versus 0.74 +/- 0.1 ml/min per g) or sodium excretion (0.63 +/- 0.13 versus 0.67 +/- 0.16 micromol/min per g) between TGR and HanSD. Tempol alone caused significant increases in RPF and GFR (10 +/- 4% and 12 +/- 2%, respectively) in TGR but not in HanSD. Tempol also caused greater sodium excretory responses in TGR compared to HanSD (112 +/- 16% versus 43 +/- 7%; P < 0.05). 8-Isoprostane excretion was significantly higher in TGR than in HanSD (10.2 +/- 0.8 versus 6.5 +/- 0.7 pg/min per g), which was attenuated by tempol. L-NAME caused greater decreases in RPF and GFR in TGR (-34 +/- 4% and -22 +/- 4%, respectively) than in HanSD (-19 +/- 3% and -10 +/- 4%, respectively). Co-infusion of tempol partially attenuated the renal hemodynamic and excretory responses to L-NAME in TGR.

CONCLUSIONS

These data suggest that the enhanced O2* activity and its interaction with NO during the prehypertensive phase in TGR modulates renal hemodynamic and excretory function, which may contribute to the development of hypertension in this transgenic rat model.

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.

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.

Mechanisms of Hypertension Induced by Nitric Oxide (NO) Deficiency: Focus on Venous Function

Loss of endothelial cell-derived nitric oxide (NO) in hypertension is a hallmark of arterial dysfunction. Experimental hypertension created by the removal of NO, however, involves mechanisms in addition to decreased arterial vasodilator activity. These include augmented endothelin-1 (ET-1) release, increased sympathetic nervous system activity, and elevated tissue oxidative stress. We hypothesized that increased venous smooth muscle (venomotor) tone plays a role in Nomega-nitro-L-arginine (LNNA) hypertension through these mechanisms. Rats were treated with the NO synthase inhibitor LNNA (0.5 g/L in drinking water) for 2 weeks. Mean arterial pressure of conscious rats was 119 +/- 2 mm Hg in control and 194 +/- 5 mm Hg in LNNA rats (P<0.05). Carotid arteries and vena cava were removed for measurement of isometric contraction. Maximal contraction to norepinephrine was modestly reduced in arteries from LNNA compared with control rats whereas the maximum contraction to ET-1 was significantly reduced (54% control). Maximum contraction of vena cava to norepinephrine (37% control) also was reduced but no change in response to ET-1 was observed. Mean circulatory filling pressure, an in vivo measure of venomotor tone, was not elevated in LNNA hypertension at 1 or 2 weeks after LNNA. The superoxide scavenger tempol (30, 100, and 300 micromol kg(-1), IV) did not change arterial pressure in control rats but caused a dose-dependent decrease in LNNA rats (-18 +/- 8, -26 +/- 15, and -54 +/- 11 mm Hg). Similarly, ganglionic blockade with hexamethonium caused a significantly greater fall in LNNA hypertensive rats (76 +/- 9 mm Hg) compared with control rats (35 +/- 10 mm Hg). Carotid arteries, vena cava, and sympathetic ganglia from LNNA rats had higher basal levels of superoxide compared with those from control rats. These data suggest that while NO deficiency increases oxidative stress and sympathetic activity in both arterial and venous vessels, the impact on veins does not make a major contribution to this form of hypertension.

Enhanced superoxide activity modulates renal function in NO-deficient hypertensive rats

An enhancement of superoxide (O2-) activity was shown to contribute to the development of hypertension induced by NO deficiency. To better understand the mechanistic role of O2- in this NO-deficient hypertension, we evaluated the renal responses to acute intraarterial administration of an O2- scavenger, tempol (50 microg/min per 100 g of body weight) in anesthetized male Sprague-Dawley rats treated with NO synthase inhibitor nitro-L-arginine methyl ester (15 mg/kg per day in drinking water, n=7) for 4 weeks, which caused increases in mean arterial pressure (146+/-3 versus 124+/-2 mm Hg) compared with normotensive control rats (n=6). Hypertensive rats had higher renal vascular resistance (29+/-2 versus 20+/-1 mm Hg/mL per minute per gram), as well as lower renal blood flow (5.2+/-0.3 versus 6.3+/-0.2 mL/min per gram; cortical blood flow, 153+/-13 versus 191+/-8 perfusion units; medullary blood flow, 43+/-2 versus 51+/-3 perfusion units) and glomerular filtration rate (0.69+/-0.04 versus 0.90+/-0.05 mL/min per gram) without a significant difference in urinary sodium excretion (0.81+/-0.07 versus 0.86+/-0.12 micromol/min per gram) compared with normotensive rats. Urinary 8-isoprostane excretion rate (6.8+/-0.7 versus 4.5+/-0.3 pg/min per gram) was higher in hypertensive than normotensive rats. Intraarterial infusion of tempol did not alter renal function in normotensive rats. However, tempol significantly decreased renal vascular resistance by 12+/-2% and urinary 8-isoprostane excretion rate by 24+/-4% and increased renal blood flow by 10+/-2%, cortical blood flow by 9+/-2%, medullary blood flow by 15+/-6%, glomerular filtration rate by 11+/-3%, and urinary sodium excretion by 19+/-5% in hypertensive rats. These data indicate that enhanced O2- activity modulates renal hemodynamics and excretory function during reduced NO production and, thus, contributes to the pathophysiology of the NO-deficient form of hypertension.

Enhanced superoxide generation modulates renal function in ANG II-induced hypertensive rats

This study was performed to examine the role of superoxide formation in the regulation of renal hemodynamic and excretory function and to assess its contribution in the pathogenesis of ANG II-dependent hypertension. Renal responses to acute intra-arterial infusion of the O2− scavenger tempol (50 μg·min−1·100 g body wt−1) with or without catalase (1,500 U·min−1·100 g−1; both native and polyethylene glycol-catalase), which reduces H2O2, were evaluated in anesthetized male Sprague-Dawley rats treated chronically with ANG II (65 ng/min) for 2 wk and compared with nontreated control rats. In ANG II-treated hypertensive rats, tempol caused increases in medullary (13 ± 2%), cortical (5 ± 2%), and total renal blood flow (9 ± 2%) without altering systemic arterial pressure. There were also increases in glomerular filtration rate (9 ± 2%), urine flow (17 ± 4%), and sodium excretion (26 ± 5%). However, tempol infusion in nontreated normotensive rats did not cause significant changes in any of these renal parameters. Coinfusion of catalase with tempol did not alter the responses observed with tempol alone, indicating that the observed renal responses to tempol in ANG II-treated rats were attributed to its O2− scavenging effects without the involvement of H2O2. Tempol infusion also significantly decreased 8-isoprostane excretion in ANG II-treated rats (39 ± 6%) without changes in H2O2 excretion. However, coinfusion of catalase reduced H2O2 excretion in both ANG II-treated (41 ± 6%) and nontreated rats (28 ± 5%). These data demonstrate that enhanced generation of O2− modulates renal hemodynamic and tubular reabsoptive function, possibly leading to sodium retention and thus contributing to the pathogenesis of ANG II-induced hypertension.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of 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.

Superoxide contributes to development of salt sensitivity and hypertension induced by nitric oxide deficiency

This study was performed to examine the role of superoxide (O2-) in the development of salt sensitivity and hypertension induced by inhibition of nitric oxide (NO) generation. Male Sprague-Dawley rats were fed with diet containing either normal salt (NS) (0.4% NaCl) or high salt (HS) (4% NaCl). These rats were treated with or without an NO synthase inhibitor, nitro-L-arginine methylester (L-NAME) (15 mg/kg/d) and O2- scavenger, tempol (30 mg/kg per day) in the drinking water for 4 weeks. Systolic blood pressure (SBP) was measured by tail-cuff plethysmography and urine collection was performed during the course of experimental periods. At the end of 4 weeks, L-NAME treatment resulted in greater increases in SBP in HS rats (127+/-2 to 172+/-3 mm Hg; n=8) than in NS rats (130+/-2 to 156+/-2 mm Hg; n=9). Co-administration of tempol with L-NAME markedly attenuated these SBP responses to a similar level in both HS (128+/-3 to 147+/-2 mm Hg; n=8) and NS rats (126+/-2 to 142+/-3 mm Hg; n=8). Urinary 8-isoprostane excretion (UIsoV) increased in response to L-NAME treatment that was higher in HS (10.6+/-0.5 to 21.5+/-0.8 ng/d) than in NS rats (10.8+/-0.7 to 16.9+/-0.6 ng/d). Co-treatment with tempol completely abolished these UIsoV responses to L-NAME in both HS and NS rats but did not alter urinary H2O2 excretion rate. The decreases in urinary nitrate/nitrite excretion in response to L-NAME treatment were not altered by co-administration of tempol in both HS and NS rats. These data suggest that enhancement of O2- activity during NO inhibition contributes to the development of salt sensitivity that is associated with NO-deficient hypertension.