High salt induced augmentation of angiotensin II mediated hypertension is associated with differential expression of tumor necrosis factor-alpha receptors in the kidney

Aim: Chronic high salt (HS) intake causes minimal changes in blood pressure (BP) but it induces augmented hypertensive response to angiotensin II (AngII) administration in rodents. The mechanism of this augmentation is not clearly understood. As tumor necrosis factor-alpha (TNF-α) induces natriuresis by activating TNF-α receptor type 1 (TNFR1) but not type 2 (TNFR2), we hypothesize that TNFR1 activity is reduced when HS is given in combination of AngII that leads to enhanced sodium retention and thus, causing augmented hypertension. The aim of this study is to examine the responses to chronic HS intake and AngII administration on the renal tissue protein expressions of TNFR1 and TNFR2 in mice. Methods: Different groups of mice (n = 6–7 in each group) chronically treated with or without AngII (25 ng/min; implanted minipump) for 4 weeks which were fed either normal salt (NS; 0.4% NaCl) or high salt (HS; 4% NaCl) diets. Systemic BP was measured by tail-cuff plethysmography. At the end of treatment period, kidneys were harvested after sacrificing the mice with euthanasia. Immuno-histochemical analysis of TNFR1 and TNFR2 proteins in renal tissues was performed by measuring the staining area and the intensity of receptors’ immunoreactivities using NIS-Elements software. The results were expressed in percent area of positive staining and the relative intensity. Results: HS intake alone did not alter mean BP (HS; 77 ± 1 vs. NS; 76 ± 3 vs. mmHg; tail-cuff plethysmography) but AngII induced increases in BP were augmented in HS group (104 ± 2 vs. 95 ± 2 mmHg; P < 0.05). The area of TNFR1 staining was higher in HS than NS group (6.0 ± 0.9% vs. 3.2 ± 0.7%; P < 0.05) but it was lower in AngII + HS than in AngII + NS group (5.0 ± 0.7% vs. 6.3 ± 0.7%; P = 0.068). TNFR2 immunoreactivity was minimal in NS and HS groups but it was high in AngII + NS and even higher in AngII + HS group. Conclusions: These data suggest that the HS induced increased TNFR1 activity that facilitates enhanced sodium excretion is compromised in elevated AngII condition leading to salt retention and augmented hypertension.

Angiotensin II-induced natriuresis is attenuated in knockout mice lacking the receptors for tumor necrosis factor-α

Intravenous infusion of relatively higher doses of angiotensin II (AngII) elicits natriuresis as opposed to its usual anti-natruretic response. As AngII can induce tumor necrosis factor-α (TNFα) production which elicits natriuresis via its action on TNFα receptor type 1 (TNFR1), we hypothesize that the concomitant release of TNFα contributes to the natriuretic response to AngII. Responses to AngII infusion (1 ng min-1  g-1 for 75 min, iv) were evaluated in anesthetized knockout (KO) mice lacking TNFR1 (n = 6) and TNFR2 (TNFα receptor type 2; n = 6) and compared these responses with those in wild type (WT; n = 6) mice. Arterial pressure (AP) was recorded from a cannula placed in the carotid artery. Renal blood flow (RBF) and glomerular filtration rate (GFR) were measured by PAH and inulin clearances, respectively. Urine was collected from a catheter placed in the bladder. AngII caused similar increases (p < 0.05 vs basal values) in AP (WT, 37 ± 5%; TNFR1KO, 35 ± 4%; TNFR2KO, 30 ± 4%) and decreases (p < 0.05) in RBF (WT, -39 ± 5%; TNFR1KO, -28 ± 6%; TNFR2KO, -31 ± 4%) without significant changes in GFR (WT, -17 ± 7%; TNFR1KO, -18 ± 7%; TNFR2KO, -12 ± 7%). However, despite similar changes in AP and renal hemodynamics, AngII induced increases (p < 0.05) in urinary sodium excretion in WT (3916 ± 942%) were less in the KO strains, more or less in TNFR1KO (473 ± 170%) than in TNFR2KO (1176 ± 168%). These data indicate that TNF-α receptors, particularly TNFR1 are involved in the natriuretic response that occur during acute infusion of AngII and thus, plays a protective role in preventing excessive salt retention at clinical conditions associated with elevated AngII level.

Angiotensin II-induced renal angiotensinogen formation is enhanced in mice lacking tumor necrosis factor-alpha type 1 receptor

In hypertension induced by angiotensin II (AngII) administration with high salt (HS) intake, intrarenal angiotensinogen (AGT) and tumor necrosis factor-alpha (TNF-α) levels increase. However, TNF-α has been shown to suppress AGT formation in cultured renal proximal tubular cells. We examined the hypothesis that elevated AngII levels during HS intake reduces TNF-α receptor type 1 (TNFR1) activity in the kidneys, thus facilitating increased intrarenal AGT formation. The responses to HS diet (4% NaCl) with chronic infusion of AngII (25 ng/min) via implanted minipump for 4 weeks were assessed in wild-type (WT) and knockout (KO) mice lacking TNFR1 or TNFR2 receptors. Blood pressure was measured by tail-cuff plethysmography, and 24-h urine samples were collected using metabolic cages prior to start (0 day) and at the end of 2nd and 4th week periods. The urinary excretion rate of AGT (uAGT; marker for intrarenal AGT) was measured using ELISA. HS +AngII treatment for 4 weeks increased mean arterial pressure (MAP) in all strains of mice. However, the increase in MAP in TNFR1KO (77 ± 2 to 115 ± 3 mmHg; n = 7) was significantly greater (p < 0.01) than in WT (76 ± 1 to 102 ± 2 mmHg; n = 7) or in TNFR2KO (78 ± 2 to 99 ± 5 mmHg; n = 6). The increase in uAGT at 4th week was also greater (p < 0.05) in TNFR1KO mice (6 ± 2 to 167 ± 75 ng/24 h) than that in WT (6 ± 3 to 46 ± 16 ng/24 h) or in TNFR2KO mice (8 ± 7 to 65 ± 44 ng/24 h). The results indicate that TNFR1 exerts a protective role by mitigating intrarenal AGT formation induced by elevated AngII and HS intake.

High-salt intake reduces renal tissue levels of inflammatory cytokines in mice

High salt (HS) intake is usually considered as an aggravating factor to induce inflammatory renal injury. However, the changes in the renal levels of inflammatory cytokines during HS intake is not yet clearly defined. We hypothesize that HS increases renal levels of tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) but decreases interleukin-10 (IL-10; anti-inflammatory cytokine) and these responses exacerbate in NO deficient conditions. Both wild-type (WT) and endothelial NO synthase knockout (eNOSKO) mice (~8 weeks old, n = 6 in each group) were given normal-salt (NS; 0.3% NaCl) and HS (4% NaCl) containing diets for 2 weeks. Systolic blood pressure (SBP) was determined by tail-cuff plethysmography and urine collections were made using metabolic cages. Basal SBP was higher in eNOSKO than WT mice (131 ± 7 vs 117 ± 3 mmHg; p < .05). HS intake for 2 weeks increased SBP in eNOSKO (161 ± 5 mmHg) but not in WT mice. In NS groups, the cytokine levels in renal tissues (measured using ELISA kits and expressed in pg/mg protein) were significantly higher in eNOSKO than WT mice (TNF-α, 624 ± 67 vs. 325 ± 73; IL-6, 619 ± 106 vs. 166 ± 61; IL-10, 6,087 ± 567 vs. 3,929 ± 378). Interestingly, these cytokine levels in HS groups were significantly less both in WT (TNF-α, 114 ± 17; IL-6, 81 ± 14; IL-10, 865 ± 130) and eNOSKO (TNF-α, 115 ± 18; IL-6, 56 ± 7; IL-10, 882 ± 141) mice. These findings indicate that HS induces downregulation of cytokines in the kidney. Such HS-induced reduction in cytokines, particularly TNF-α (a natriuretic agent), would facilitate more salt-retention, and thus, leading to salt-sensitive hypertension in NO deficient conditions.

Low Nitric Oxide Bioavailability Increases Renin Production in the Collecting Duct

In the kidney, the stimulation of renin production by the collecting duct (CD-renin) contributes to the development of hypertension. The CD is a major nephron segment for the synthesis of nitric oxide (NO), and low NO bioavailability in the renal medulla is associated with hypertension. However, it is unknown whether NO regulates renin production in the CD. To test the hypothesis that low intrarenal NO levels stimulate the production of CD-renin, we first examined renin expression in the distal nephron segments of CD-eNOS deficient mice. In these mice, specific CD-renin immunoreactivity was increased compared to wild-type littermates; however, juxtaglomerular (JG) renin was not altered. To further assess the intracellular mechanisms involved, we then treated M-1 cells with either 1 mM L-NAME (L-arginine analog), an inhibitor of NO synthase activity, or 1 mM NONOate, a NO donor. Both treatments increased intracellular renin protein levels in M-1 cells. However, only the inhibition of NOS with L-NAME stimulated renin synthesis and secretion as reflected by the increase in Ren1C transcript and renin protein levels in the extracellular media, respectively. In addition, NONOate induced a fast mobilization of cGMP and intracellular renin accumulation. These response was partially prevented by guanylyl cyclase inhibition with ODQ (1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1]. Accumulation of intracellular renin was blocked by protein kinase G (PKG) and protein kinase C (PKC) inhibitors. Our data indicate that low NO bioavailability increases CD-renin synthesis and secretion, which may contribute to the activation of intrarenal renin angiotensin system.

Evidence for Prohypertensive, Proinflammatory Effect of Interleukin-10 During Chronic High Salt Intake in the Condition of Elevated Angiotensin II Level

IL-10 (interleukin-10) has been suggested to play a protective role in angiotensin II (AngII)-induced cardiovascular disorders. This study examined the role of endogenous IL-10 in salt-sensitive hypertension and renal injury induced by AngII. Responses to chronic AngII (400 ng/min per kilogram body weight; osmotic minipump) infusion were evaluated in IL-10 gene knockout mice fed with either normal salt diet (0.3% NaCl) or high salt (HS; 4% NaCl) diet, and these responses were compared with those in wild-type mice. Normal salt diets or HS diets were given alone for the first 2 weeks and then with AngII treatment for an additional 2 weeks (n=6 in each group). Arterial pressure was continuously monitored by implanted radio-telemetry, and a 24-hour urine collection was performed by metabolic cages on the last day of the experimental period. Basal mean arterial pressure was lower in IL-10 gene knockout mice than in wild-type (98±3 versus 113±3 mm Hg) mice. Mean arterial pressure responses to normal salt/HS alone or to the AngII+normal salt treatment were similar in both strains. However, the increase in mean arterial pressure induced by the AngII+HS treatment was significantly lower in IL-10 gene knockout mice (15±5% versus 37±3%) compared with wild-type mice. Renal tissue endothelial nitric oxide synthase expression (≈3-folds) and urinary excretion of nitric oxide metabolites, nitrate/nitrite (1.2±0.1 versus 0.2±0.02 µmol/L/24 hours) were higher in IL-10 gene knockout mice compared with wild-type mice. These results indicate that an increase in nitric oxide production helps to mitigate salt-sensitive hypertension induced by AngII and suggest that a compensatory interaction between IL-10 and nitric oxide exists in modulating AngII-induced responses during HS intake.

Tumor necrosis factor-α, kidney function, and hypertension

Hypertension is considered to be a low-grade inflammatory condition characterized by the presence of various proinflammatory cytokines. Tumor necrosis factor-α (TNF-α) is a constituent of the proinflammatory cytokines that is associated with salt-sensitive hypertension (SSH) and related renal injury. Elevated angiotensin II (ANG II) and other factors such as oxidative stress conditions promote TNF-α formation. Many recent studies have provided evidence that TNF-α exerts a direct renal action by regulating hemodynamic and excretory function in the kidney. The cytokine incites a strong natriuretic response and plays a part in regulation of the intrarenal renin-angiotensin system. The exact mechanistic role of TNF-α in the development of SSH is as yet poorly understood. While TNF-α antagonism has been shown to attenuate hypertensive responses in many hypertensive animal models, contrasting findings demonstrate that the direct systemic administration of TNF-α usually induces hypotensive as well as natriuretic responses, indicating a counterregulatory role of TNF-α in SSH. Differential activities of two cell surface receptors of TNF-α (receptor type 1 and type 2) may explain the contradictory functions of TNF-α in the setting of hypertension. This short review will evaluate ongoing research studies that investigate the action of TNF-α within the kidney and its role as an influential pathophysiological variable in the development of SSH and renal injury. This information may help to develop specific TNF-α receptor targeting as an effective treatment strategy in this clinical condition.

2-Methoxyestradiol Reduces Angiotensin II-Induced Hypertension and Renal Dysfunction in Ovariectomized Female and Intact Male Mice

Cytochrome P450 1B1 protects against angiotensin II (Ang II)-induced hypertension and associated cardiovascular changes in female mice, most likely via production of 2-methoxyestradiol. This study was conducted to determine whether 2-methoxyestradiol ameliorates Ang II-induced hypertension, renal dysfunction, and end-organ damage in intact Cyp1b1-/-, ovariectomized female, and Cyp1b1+/+ male mice. Ang II or vehicle was infused for 2 weeks and administered concurrently with 2-methoxyestradiol. Mice were placed in metabolic cages on day 12 of Ang II infusion for urine collection for 24 hours. 2-Methoxyestradiol reduced Ang II-induced increases in systolic blood pressure, water consumption, urine output, and proteinuria in intact female Cyp1b1-/- and ovariectomized mice. 2-Methoxyestradiol also reduced Ang II-induced increase in blood pressure, water intake, urine output, and proteinuria in Cyp1b1+/+ male mice. Treatment with 2-methoxyestradiol attenuated Ang II-induced end-organ damage in intact Cyp1b1-/- and ovariectomized Cyp1b1+/+ and Cyp1b1-/- female mice and Cyp1b1+/+ male mice. 2-Methoxyestradiol mitigated Ang II-induced increase in urinary excretion of angiotensinogen in intact Cyp1b1-/- and ovariectomized Cyp1b1+/+ and Cyp1b1-/- female mice but not in Cyp1b1+/+ male mice. The G protein-coupled estrogen receptor 1 antagonist G-15 failed to alter Ang II-induced increases in blood pressure and renal function in Cyp1b1+/+ female mice. These data suggest that 2-methoxyestradiol reduces Ang II-induced hypertension and associated end-organ damage in intact Cyp1b1-/-, ovariectomized Cyp1b1+/+ and Cyp1b1-/- female mice, and Cyp1b1+/+ male mice independent of G protein-coupled estrogen receptor 1. Therefore, 2-methoxyestradiol could serve as a therapeutic agent for treating hypertension and associated pathogenesis in postmenopausal females, and in males.

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.

6β-Hydroxytestosterone, a Cytochrome P450 1B1-Testosterone-Metabolite, Mediates Angiotensin II-Induced Renal Dysfunction in Male Mice

6β-Hydroxytestosterone, a cytochrome P450 1B1-derived metabolite of testosterone, contributes to the development of angiotensin II-induced hypertension and associated cardiovascular pathophysiology. In view of the critical role of angiotensin II in the maintenance of renal homeostasis, development of hypertension, and end-organ damage, this study was conducted to determine the contribution of 6β-hydroxytestosterone to angiotensin II actions on water consumption and renal function in male Cyp1b1(+/+) and Cyp1b1(-/-) mice. Castration of Cyp1b1(+/+) mice or Cyp1b1(-/-) gene disruption minimized the angiotensin II-induced increase in water consumption, urine output, proteinuria, and sodium excretion and decreases in urine osmolality. 6β-Hydroxytestosterone did not alter angiotensin II-induced increases in water intake, urine output, proteinuria, and sodium excretion or decreases in osmolality in Cyp1b1(+/+) mice, but restored these effects of angiotensin II in Cyp1b1(-/-) or castrated Cyp1b1(+/+) mice. Cyp1b1 gene disruption or castration prevented angiotensin II-induced renal fibrosis, oxidative stress, inflammation, urinary excretion of angiotensinogen, expression of angiotensin II type 1 receptor, and angiotensin-converting enzyme. 6β-Hydroxytestosterone did not alter angiotensin II-induced renal fibrosis, inflammation, oxidative stress, urinary excretion of angiotensinogen, expression of angiotensin II type 1 receptor, or angiotensin-converting enzyme in Cyp1b1(+/+)mice. However, in Cyp1b1(-/-) or castrated Cyp1b1(+/+) mice, it restored these effects of angiotensin II. These data indicate that 6β-hydroxytestosterone contributes to increased thirst, impairment of renal function, and end-organ injury associated with angiotensin II-induced hypertension in male mice and that cytochrome P450 1B1 could serve as a novel target for treating renal disease and hypertension in male mice.

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.

Decrease in IL-10 and increase in TNF-α levels in renal tissues during systemic inhibition of nitric oxide in anesthetized mice

Earlier, we demonstrated that the inhibition of nitric oxide synthase (NOS) by nitro‐l‐arginine methyl ester (l‐NAME) infusion increases the endogenous production of proinflammatory cytokine, tumor necrosis factor (TNF‐α). In the present study, we examined the hypothesis that inhibition of nitric oxide (NO) production leads to the suppression of interleukin (IL)‐10 (anti‐inflammatory cytokine) generation which facilitates the enhancement of TNF‐α production endogenously. Using appropriate enzyme‐linked immunosorbent assay kits and immunohistochemical staining, the levels of IL‐10 and TNF‐α in plasma (P) and in renal tissues (R) were measured in anesthetized mice (C57BL/6; ~10 weeks age; n = 6/group) infused with or without l‐NAME (200 μg/min/kg; i.v. for 2 h). Compared to vehicle‐treated control mice, l‐NAME‐treated mice had a lower level of IL‐10 (P, 0.3 ± 0.1 vs. 2.6 ± 0.6 ng/mL; R, 0.5 ± 0.1 vs. 3 ± 0.1 ng/mg protein) and a higher level of TNF‐α (P, 432 ± 82 vs. undetected pg/mL; R, 58 ± 7 vs. 6 ± 5 pg/mg protein). IL‐10 protein expression, present mostly in the distal nephron segments in control mice, was markedly downregulated in l‐NAME‐treated mice. Compared to control mice, TNF‐α expression increased 2.5‐fold in renal cortical sections (mostly in the distal nephron segments) in l‐NAME‐treated mice. Coinfusion of a NO donor, S‐nitroso‐N‐acetyl‐penicillamine (SNAP; 25 μg/min/kg) with l‐NAME in a separate group of mice prevented these changes in IL‐10 and TNF‐α induced by l‐NAME. IL‐10 infusion (0.075 ng/min/g) in l‐NAME‐treated mice markedly attenuated l‐NAME‐induced increments in TNF‐α. Thus, these results demonstrate that NOS inhibition decreases endogenous IL‐10 generation and thus, minimizes its immune downregulating action on the TNF‐α production in the kidney.

In this study in mice, we examined the hypothesis that NOS inhibition reduces the endogenous production of anti‐inflammatory cytokine, IL‐10 which facilitates enhanced production of TNF‐α in the kidney. Using appropriate enzyme‐linked immunosorbent assay kits and immunohistochemical staining, levels of IL‐10 and TNF‐α in renal tissue were measured in anesthetized mice infused with or without l‐NAME (200 μg/min/kg) in the presence or absence of a NO donor, S‐nitroso‐N‐acetyl‐penicillamine (SNAP; 25 μg/min/kg) or IL‐10 infusion (0.075 ng/min/g). These results demonstrate that NOS inhibition decreases endogenous IL‐10 generation and thus, minimizes its immune downregulating action on the TNF‐α production in the kidney.

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.

TNF-α type 2 receptor mediates renal inflammatory response to chronic angiotensin II administration with high salt intake in mice

Tumor necrosis factor-alpha (TNF-α) has been implicated in salt-sensitive hypertension and renal injury (RI) induced by angiotensin II (ANG II). To determine the receptor type of TNF-α involved in this mechanism, we evaluated the responses to chronic ANG II infusion (25 ng/min by implanted minipump) given with high-salt diet (HS; 4% NaCl) for 2 wk in gene knockout mice for TNF-α receptor type 1 (TNFR1KO; n = 6) and type 2 (TNFR2KO; n = 6) and compared the responses with those in wild-type (WT; C57BL/6; n = 6) mice. Blood pressure in these mice was measured by implanted radiotelemetry as well as by tail-cuff plethysmography. RI responses were assessed by measuring macrophage cell infiltration (CD68(+) immunohistochemistry), glomerulosclerosis (PAS staining), and interstitial fibrosis (Gomori's trichrome staining) in renal tissues at the end of the treatment period. The increase in mean arterial pressure induced by ANG II + HS treatment was not different in these three groups of mice (TNFR1KO, 114 ± 1 to 161 ± 7 mmHg; TNFR2KO, 113 ± 1 to 161 ± 3 mmHg; WT, 110 ± 3 to 154 ± 3 mmHg). ANG II + HS-induced RI changes were similar in TNFR1KO mice but significantly less in TNFR2KO mice (macrophage infiltration, 0.02 ± 0.01 vs. 1.65 ± 0.45 cells/mm(2); glomerulosclerosis, 26.3 ± 2.6 vs. 35.7 ± 2.2% area; and interstitial fibrosis, 5.2 ± 0.6 vs. 8.1 ± 1.1% area) compared with the RI changes in WT mice. The results suggest that a direct activation of TNF-α receptors may not be required in inducing hypertensive response to chronic ANG II administration with HS intake, but the induction of inflammatory responses leading to renal injury are mainly mediated by TNF-α receptor type 2.

Tumor necrosis factor-α receptor type 1, not type 2, mediates its acute responses in the kidney

Acute administration of tumor necrosis factor-α (TNF-α) resulted in decreases in renal blood flow (RBF) and glomerular filtration rate (GFR) but induced diuretic and natriuretic responses in mice. To define the receptor subtypes involved in these renal responses, experiments were conducted to assess the responses to human recombinant TNF-α (0.3 ng·min(-1)·g body wt(-1) iv infusion for 75 min) in gene knockout (KO) mice for TNF-α receptor type 1 (TNFαR1 KO, n = 5) or type 2 (TNFαR2 KO, n = 6), and the results were compared with those obtained in corresponding wild-type [WT (C57BL/6), n = 6] mice. Basal levels of RBF (PAH clearance) and GFR (inulin clearance) were similar in TNFαR1 KO, but were lower in TNFαR2 KO, than WT mice. TNF-α infusion in WT mice decreased RBF and GFR but caused a natriuretic response, as reported previously. In TNFαR1 KO mice, TNF-α infusion failed to cause such vasoconstrictor or natriuretic responses; rather, there was an increase in RBF and a decrease in renal vascular resistance. Similar responses were also observed with infusion of murine recombinant TNF-α in TNFαR1 KO mice (n = 5). However, TNF-α infusion in TNFαR2 KO mice caused changes in renal parameters qualitatively similar to those observed in WT mice. Immunohistochemical analysis in kidney slices from WT mice demonstrated that while both receptor types were generally located in the renal vascular and tubular cells, only TNFαR1 was located in vascular smooth muscle cells. There was an increase in TNFαR1 immunoreactivity in TNFαR2 KO mice, and vice versa, compared with WT mice. Collectively, these functional and immunohistological findings in the present study demonstrate that the activation of TNFαR1, not TNFαR2, is mainly involved in mediating the acute renal vasoconstrictor and natriuretic actions of TNF-α.

Renoprotection with pituitary adenylate cyclase-activating polypeptide in cyclosporine A-induced nephrotoxicity

BACKGROUND

Acute and long-term nephrotoxicity is the major dose-limiting factor for cyclosporine A (CsA). We evaluated the protective effects of pituitary adenylate cyclase-activating polypeptide (PACAP)38 on CsA-induced nephrotoxicity in human renal proximal tubule epithelial (human kidney-2) cells and in intact mice.

METHODS

Confluent (human kidney-2 cells were exposed to CsA (25-50 μmol/L) in the presence or absence of PACAP38 or vasoactive intestinal peptide (10(-10) to 10(-6) M). For studies in vivo, male BALB/c mice (n = 5 in each group) were given a single intraperitoneal injection of CsA (5 mg/kg body weight). Treatment group received 20 μg of PACAP38 2 hours before exposure to CsA and additional doses daily for 10 days.

RESULTS

Cyclosporine A caused oxidative injury, marked morphological alterations, apoptosis, and increased expression of transforming growth factor (TGF)-β1 in cell cultures. Pituitary adenylate cyclase-activating polypeptide 38 at 10(-8) mol/L restored cell confluency, reduced TGF-β1 secretion, and preserved cell integrity. In mice, CsA caused tubular injury characterized by loss of tubular epithelial cell brush border membranes, tubular collapse, cellular necrosis, interstitial fibrosis, increased production of TGF-β1, and elevated serum creatinine (3.39 ± 0.21 vs 0.13 ± 0.02 mg/dL in controls, P < 0.01). Treatment with PACAP38 reduced TGF-β1 and tumor necrosis factor-α production in kidney, prevented epithelial-mesenchymal transition of the renal cells, and reduced serum creatinine levels to 1.01 ± 0.18 mg/dL, P < 0.01 versus CsA group.

CONCLUSIONS

Pituitary adenylate cyclase-activating polypeptide 38 ameliorated renal tubular injury, reduced oxidative injury, and inhibited the expression of TGF-β1 in CsA-exposed murine kidneys. Pituitary adenylate cyclase-activating polypeptide could be a novel renoprotective and antifibrotic agent for CsA nephrotoxicity.

AT1 receptor-mediated augmentation of angiotensinogen, oxidative stress, and inflammation in ANG II-salt hypertension

Augmentation of intrarenal angiotensinogen (AGT) synthesis, secretion, and excretion is associated with the development of hypertension, renal oxidative stress, and tissue injury during ANG II-dependent hypertension. High salt (HS) exacerbates hypertension and kidney injury, but the mechanisms remain unclear. In this study, we determined the consequences of HS intake alone compared with chronic ANG II infusion and combined HS plus ANG II on the stimulation of urinary AGT (uAGT), renal oxidative stress, and renal injury markers. Sprague-Dawley rats were subjected to 1) a normal-salt diet [NS, n = 5]; 2) HS diet [8% NaCl, n = 5]; 3) ANG II infusion in NS rats [ANG II 80 ng/min, n = 5]; 4) ANG II infusion in HS rats [ANG II+HS, n = 5]; and 5) ANG II infusion in HS rats treated with ANG II type 1 receptor blocker (ARB) [ANG II+HS+ARB, n = 5] for 14 days. Rats fed a HS diet alone did not show changes in systolic blood pressure (SBP), proteinuria, cell proliferation, or uAGT excretion although they did exhibit mesangial expansion, collagen deposition, and had increased NADPH oxidase activity accompanied by increased peroxynitrite formation in the kidneys. Compared with ANG II rats, the combination of ANG II infusion and a HS diet led to exacerbation in SBP (175 ± 10 vs. 221 ± 8 mmHg; P < 0.05), proteinuria (46 ± 7 vs. 127 ± 7 mg/day; P < 0.05), and uAGT (1,109 ± 70 vs.. 7,200 ± 614 ng/day; P < 0.05) associated with greater collagen deposition, mesangial expansion, interstitial cell proliferation, and macrophage infiltration. In both ANG II groups, the O(2)(-) levels were increased due to increased NADPH oxidase activity without concomitant increases in peroxynitrite formation. The responses in ANG II rats were prevented or ameliorated by ARB treatment. The results indicate that HS independently stimulates ROS formation, which may synergize with the effect of ANG II to limit peroxynitrite formation, leading to exacerbation of uAGT and greater injury during ANG II salt hypertension.

High-salt intake enhances superoxide activity in eNOS knockout mice leading to the development of salt sensitivity

A deficiency in nitric oxide (NO) generation leads to salt-sensitive hypertension, but the role of increased superoxide (O(2)(-)) in such salt sensitivity has not been delineated. We examined the hypothesis that an enhancement in O(2)(-) activity induced by high-salt (HS) intake under deficient NO production contributes to the development of salt-sensitive hypertension. Endothelial NO synthase knockout (eNOS KO; total n = 64) and wild-type (WT; total n = 58) mice were given diets containing either normal (NS; 0.4%) or high-salt (HS; 4%) for 2 wk. During this period, mice were chronically treated with a O(2)(-) scavenger, tempol (400 mg/l), or an inhibitor of NADPH oxidase, apocynin (1 g/l), in drinking water or left untreated (n = 6-8 per group). Blood pressure was measured by radiotelemetry and 24-h urine samples were collected in metabolic cages. Basal mean arterial pressure (MAP) in eNOS KO was higher (125 +/- 4 vs. 106 +/- 3 mmHg) compared with WT. Feeding HS diet did not alter MAP in WT but increased it in eNOS KO to 166 +/- 9 mmHg. Both tempol and apocynin treatment significantly attenuated the MAP response to HS in eNOS KO (134 +/- 3 and 139 +/- 4 mmHg, respectively). Basal urinary 8-isoprostane excretion rates (U(Iso)V), a marker for endogenous O(2)(-) activity, were similar (2.8 +/- 0.2 and 2.4 +/- 0.3 ng/day) in both eNOS KO and WT mice. However, HS increased U(Iso)V more in eNOS KO than in WT (4.6 +/- 0.3 vs. 3.8 +/- 0.2 ng/day); these were significantly attenuated by both tempol and apocynin treatment. These data indicate that an enhancement in O(2)(-) activity contributes substantially to the development of salt-sensitive hypertension under NO-deficient conditions.

Involvement of tumor necrosis factor-alpha in natriuretic response to systemic infusion of nitric oxide synthase inhibitor in anesthetized mice

Systemic infusion of TNF-alpha exerts renal vasoconstriction but caused marked natriuresis in mice. Similar renal responses were also observed during systemic infusion of nitric oxide (NO) synthase inhibitors as opposed to their usual antinatriuretic responses when administered intrarenally. In the present study, we examined the hypothesis that acute NO blockade systemically induces TNF-alpha generation. which induces this natriuretic response. Renal responses to intravenous infusion of the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME; 0.2 microg x min(-1) x g body wt(-1) for 85 min) and its impact on the plasma level of TNF-alpha were evaluated in anesthetized mice. Plasma TNF-alpha was undetected in untreated mice (n = 7) but was elevated in L-NAME-treated mice (109 +/- 22 pg/ml; P < 0.01 vs. untreated group; n = 7) along with an increase in TNF-alpha protein expression in kidney tissue. L-NAME infusion caused a usual increase in mean arterial pressure (MAP; 98 +/- 3 to 122 +/- 3 mmHg; P < 0.01) and decreases in renal blood flow (RBF; 8.6 +/- 0.3 to 4.4 +/- 0.2 ml x min(-1) x g(-1); P < 0.01) and glomerular filtration rate (GFR; 1.14 +/- 0.07 to 0.77 +/- 0.04 ml x min(-1) x g(-1); P < 0.01) with a marked increase in sodium excretion (U(Na)V; 0.48 +/- 0.10 to 3.52 +/- 0.85 micromol x min(-1) x g(-1); P < 0.01). Interestingly, in mice (n = 7) pretreated with the TNF-alpha blocker etanercept (5 mg/kg sc), the U(Na)V response to l-NAME infusion was markedly blunted (0.58 +/- 0.08 to 1.22 +/- 0.28 micromol x min(-1) x g(-1); P = NS) although responses for MAP, RBF, and GFR were mostly unchanged. However, pretreatment with the superoxide scavenger tempol in mice (n = 7) did not alter the U(Na)V response to L-NAME. These data demonstrate that L-NAME-induced natriuresis is mediated, at least in part, by concomitant generation of TNF-alpha during NO blockade.

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.

Chronic NF-{kappa}B blockade reduces cytosolic and mitochondrial oxidative stress and attenuates renal injury and hypertension in SHR

Nuclear factor-kappaB (NF-kappaB) plays an important role in hypertensive renal injury; however, its roles in perpetuating mitochondrial oxidative stress and renal dysfunction remain unclear. In this study, we assessed the effects of chronic NF-kappaB blockade with pyrrolidine dithiocarbamate (PDTC) on renal dysfunction and mitochondrial redox status in spontaneously hypertensive rats (SHR). PDTC (150 mg.kg body wt(-1).day(-1)) or vehicle was administered orally to 8-wk-old SHR and their respective controls for 15 wk. Systolic blood pressure (SBP) was measured by tail-cuff plethysmography at the start of and at every third week throughout the study. After 15 wk of treatment, anesthetized rats underwent acute renal experiments to determine renal blood flow and glomerular filtration rate using PAH and inulin clearance techniques, respectively. Following renal experiments, kidneys were excised from killed rats, and cortical mitochondria were isolated for reactive oxygen species (ROS) measurements using electron paramagnetic resonance. Tissue mRNA and protein levels of NF-kappaB and oxidative stress genes were determined using real-time PCR and immunofluorescence or Western blotting, respectively. PDTC treatment partially attenuated the increase in SBP (196.4 +/- 9.76 vs. 151.4 +/- 2.12; P < 0.05) and normalized renal hemodynamic and excretory parameters and ATP production rates in SHR. PDTC treatment also attenuated the higher levels of cytosolic and mitochondrial ROS generation and tissue mRNA and protein expression levels of NF-kappaB and oxidative stress genes in SHR without any comparable responses in control rats. These findings suggest that NF-kappaB activation by ROS induces the cytosolic and mitochondrial oxidative stress and tissue injury that contribute to renal dysfunction observed in SHR.

Renal hemodynamic and excretory responses to intra-arterial infusion of peroxynitrite in anesthetized rats

Peroxynitrite (ONOO(-)) is formed endogenously by the reaction of nitric oxide (NO) and superoxide (O(2)(-)). To examine the hypothesis that OONO(-) cause renal vasodilation at low concentrations but cause vasoconstriction at higher concentrations, we examined renal responses to intra-arterial infusion of incremental doses of OONO(-) (10, 20, and 40 microg.kg(-1).min(-1); 45 min each) in anesthetized rats. Renal blood flow (RBF) and glomerular filtration rate (GFR) were determined by PAH and inulin clearance. In control rats (n = 6), low dose (10 microg.kg(-1).min(-1)) of OONO(-) increased RBF by 10 +/- 3% and GFR by 15 +/- 5%. The higher doses (20 and 40 microg.kg(-1).min(-1)) mostly reversed these responses which were -7 +/- 4 and -27 +/- 7% (P < 0.05) in RBF and -0.1 +/- 4.8 and -14 +/- 12% in GFR, respectively. There were no appreciable changes in urine flow (V) and sodium excretion (U(Na)V) during OONO(-) infusion. However, in rats pretreated with NO synthase (NOS) inhibitor, l-NAME (50 microg.kg(-1).min(-1); n = 5), these doses of ONOO(-) significantly reduced RBF (-26 +/- 7, -27 +/- 6, and -44 +/- 3%) and GFR (-21 +/- 6, -25 +/- 8, and -32 +/- 12%) with variable increases in V or U(Na)V. Long-term infusion of OONO(-) (10 microg.kg(-1).min(-1) for 75 min) in another set of control rats (n = 5) also showed similar vasodilator and hyperfiltration responses. These data indicate that ONOO(-) acts as an oxidant at high concentration but provides renoprotective function at low concentration that depends on intact NOS activity.

Tumor necrosis factor-alpha induces renal vasoconstriction as well as natriuresis in mice

Tumor necrosis factor-α (TNF-α) has been implicated in the pathogenesis of hypertension and renal injury. However, the direct effects of TNF-α on renal hemodynamic and excretory function are not yet clearly defined. We examined the renal responses to infusion of TNF-α (0.33 ng·g⁻¹·min⁻¹) in anesthetized mice. Renal blood flow (RBF) and glomerular filtration rate (GFR) were determined by PAH and inulin clearance. The urine was collected from a cannula inserted into the bladder. Following the 60-min control clearance period, TNF-α infusion was initiated and 15 min were given for stabilization followed by another 60-min clearance period. TNF-α alone (n = 7) caused decreases in RBF (7.9 ± 0.3 to 6.4 ± 0.3 ml·min⁻¹·g⁻¹) and GFR (1.04 ± 0.06 to 0.62 ± 0.08 ml·min⁻¹·g⁻¹) as well as increases in absolute (0.8 ± 0.3 to 1.4 ± 0.3 μmol·min⁻¹·g⁻¹) and fractional excretion of sodium (0.5 ± 0.2 to 1.5 ± 0.4%) without affecting arterial pressure. TNF-α also increased 8-isoprostane excretion (8.10 ± 1.09 to 11.13 ± 1.34 pg·min⁻¹·g⁻¹). Pretreatment with TNF-α blocker etanercept (5 mg/kg sc; 24 and 3 h before TNF-α infusion; n = 6) abolished these responses. However, TNF-α induced an increase in RBF and caused attenuation of the GFR reduction in mice pretreated with superoxide (O₂⁻) scavenger tempol (2 μg·g⁻¹·min⁻¹; n = 6). Pretreatment with nitric oxide (NO) synthase inhibitor nitro-l-arginine methyl ester (0.1 μg·g⁻¹·min⁻¹; n = 6) resulted in further enhancement in vasoconstriction while natriuresis remained unaffected in response to TNF-α. These data suggest that TNF-α induces renal vasoconstriction and hypofiltration via enhancing the activity of O₂⁻ and thus reducing the activity of NO. The natriuretic response to TNF-α is related to its direct effects on tubular sodium reabsorption.

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.

Involvement of tumor necrosis factor-alpha in angiotensin II-mediated effects on salt appetite, hypertension, and cardiac hypertrophy

Hypertension is considered a low-grade inflammatory condition induced by various proinflammatory cytokines, including tumor necrosis factor (TNF)-alpha. Recent studies have implicated an involvement of TNF-alpha in the development of salt-sensitive hypertension induced by angiotensin II (Ang II). To understand further the relationship between TNF-alpha and Ang II, we examined the responses to Ang II in TNF-alpha knockout (TNF-alpha(-/-)) mice in the present study. A continuous infusion of Ang II (1 microg/kg per minute) for 2 weeks was given to both TNF-alpha(-/-) and wild-type (WT) mice with implanted osmotic minipumps. Daily measurement of water intake, salt intake, and urine output were performed using metabolic cages. Blood pressure was monitored continuously with implanted radiotelemetry. Ang II administration for 2 weeks caused increases in salt (0.2+/-0.07 to 5.6+/-0.95 mL/d) and water (5.4+/-0.34 to 11.5+/-1.2 mL/d) intake and in mean arterial pressure (115+/-1 to 151+/-3 mm Hg) in wild-type mice, but these responses were absent in TNF-alpha(-/-) mice (0.2+/-0.04 to 0.3+/-0.09 mL/d, 5.5+/-0.2 to 6.1+/-0.07 mL/d, and 113+/-2 to 123+/-3 mm Hg, respectively). Cardiac hypertrophy induced by Ang II was significantly attenuated in TNF-alpha(-/-) mice compared with wild-type mice. In a group of TNF-alpha(-/-) mice, when replacement therapy was made with recombinant TNF-alpha, Ang II induced similar responses in salt appetite, mean arterial pressure, and cardiac hypertrophy, as observed in wild-type mice. These results suggest that TNF-alpha plays a mechanistic role in mediating chronic Ang II-induced effects on salt appetite and blood pressure, as well as on cardiac hypertrophy.

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.

Oxidant stress and blood pressure responses to angiotensin II administration in rats fed varying salt diets

To examine the hypothesis that NAD(P)H oxidase (Nox)-derived superoxide generation is involved in the development of angiotensin II (ANG II)-induced hypertension, we evaluated the responses to ANG II infusion (65 ng/min; osmotic mini-pump) for 2 weeks in rats treated with or without apocynin (APO) (inhibitor of Nox subunits assembly) in drinking water (12 mmol/L). Rats were grouped according to their diets with varying salt content (normal salt [NS], 0.4%; high salt [HS], 8%; low salt [LS], 0.03%) given during the 2-week experimental period. The variation in salt intake did not alter mean arterial pressure (MAP, recorded via pre-implanted arterial catheter) but showed proportionate levels in urinary excretion rate of Isoprostaglandin(2alpha) (U(ISO)V; NS, 179 +/- 26; HS, 294 +/- 38; LS, 125 +/- 7 ng/kg/24 h). Treatment with ANG II increased MAP proportional to salt intake (NS, 126 +/- 3 to 160 +/- 5; HS, 116 +/- 4 to 184 +/- 5; LS, 125 +/- 1 to 154 +/- 5 mm Hg). However, ANG II increased U(ISO)V only in NS rats (250 +/- 19 ng/kg/24 h) but not in HS or LS rats. In response to ANG II, Nox subunits protein expression increased in HS but not in the NS or LS rats. Apocynin treatment partially ameliorated these changes in Nox proteins in HS rats but did not alter ANG II-induced increases in MAP or U(ISO)V. These data suggest that Nox activation may not be the sole factor or alternatively, that a constitutively active isoform of Nox is involved in oxidative stress mechanism that is associated with dietary salt or ANG II-induced hypertension.

Mechanisms mediating pressure natriuresis: what we know and what we need to find out

1. It is well established that pressure natriuresis plays a key role in long-term blood pressure regulation, but our understanding of the mechanisms underlying this process is incomplete. 2. Pressure natriuresis is chiefly mediated by inhibition of tubular sodium reabsorption, because both total renal blood flow and glomerular filtration rate are efficiently autoregulated. Inhibition of active sodium transport within both the proximal and distal tubules likely makes a contribution. Increased renal interstitial hydrostatic pressure (RIHP) likely inhibits sodium reabsorption by altering passive diffusion through paracellular pathways in 'leaky' tubular elements. 3. Nitric oxide and products of cytochrome P450-dependent arachidonic acid metabolism are key signalling mechanisms in pressure natriuresis, although their precise roles remain to be determined. 4. The key unresolved question is, how is increased renal artery pressure 'sensed' by the kidney? One proposal rests on the notion that blood flow in the renal medulla is poorly autoregulated, so that increased renal artery pressure leads to increased renal medullary blood flow (MBF), which, in turn, leads to increased RIHP. An alternative proposal is that the process of autoregulation of renal blood flow leads to increased shear stress in the preglomerular vasculature and, so, release of nitric oxide and perhaps products of cytochrome P450-dependent arachidonic acid metabolism, which, in turn, drive the cascade of events that inhibit sodium reabsorption. 5. Central to the arguments underlying these opposing hypotheses is the extent to which MBF is autoregulated. This remains highly controversial, largely because of the limitations of presently available methods for measurement of MBF.

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.

Renal interstitial fluid ATP responses to arterial pressure and tubuloglomerular feedback activation during calcium channel blockade

A close relationship between changes in renal interstitial fluid (RIF) ATP concentrations and renal autoregulatory or tubuloglomerular feedback (TGF)-dependent changes in renal vascular resistance (RVR) has been demonstrated, but it has not been determined whether the changes in RIF ATP are a consequence or the cause of the changes in RVR. The present study was performed in anesthetized dogs to assess the changes in RIF ATP following changes in renal arterial pressure (RAP) or stimulation of the TGF mechanism under conditions where changes in RVR were prevented by nifedipine, a calcium channel blocker. RIF ATP levels were measured by using microdialysis probes. Intra-arterial infusion of nifedipine (0.36 microg x kg(-1) x min(-1)) increased renal blood flow (RBF: from 4.49 +/- 0.27 to 5.34 +/- 0.39 ml x min(-1) x g(-1)) and glomerular filtration rate (GFR: from 0.84 +/- 0.07 to 1.09 +/- 0.11 ml x min(-1) x g(-1)). Under conditions of nifedipine infusion, autoregulatory adjustments in RBF, GFR, and RVR were not observed during stepwise reductions in RAP within the autoregulatory range (from 135 +/- 7 to 76 +/- 1 mmHg, n = 7). Furthermore, stimulation of the TGF mechanism with intra-arterial infusion of acetazolamide (100 microg x kg(-1) x min(-1)) did not alter RBF, GFR, and RVR (n = 7). During treatment with nifedipine, RIF ATP levels were significantly decreased in response to reductions in RAP (10.7 +/- 0.7, 5.8 +/- 0.7 and 2.8 +/- 0.3 nmol/l at 135 +/- 7, 101 +/- 4, and 76 +/- 1 mmHg, n = 7) and increased by acetazolamide infusion (from 8.8 +/- 0.8 to 17.0 +/- 1.8 nmol/l, n = 7). These results are similar to those that occurred in dogs not treated with nifedipine and thus demonstrate that the changes in RIF ATP can occur in the absence of autoregulatory or TGF-mediated changes in RVR. The data provide further support to the hypothesis that RIF ATP contributes to adjustments in RVR associated with renal autoregulation and changes in activity of the TGF mechanism.

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.

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

To assess the role of superoxide (O2−) and nitric oxide (NO) interaction in mediating the renal actions of ANG II, we examined the renal responses to intra-arterial infusion of ANG II (0.5 ng·kg−1·min−1) before and during administration of a superoxide dismutase mimetic, tempol (0.5 mg·kg−1·min−1), in the presence or absence of NO synthase inhibitor, nitro-l-arginine (NLA; 50 μg·kg−1·min−1), in anesthetized dogs pretreated with enalaprilat (33 μg·kg−1·min−1). In one group of dogs ( n = 7), ANG II infusion before tempol infusion caused decreases of 24 ± 4% in renal blood flow (RBF), 55 ± 7% in urine flow (V), and 53 ± 8% in urinary sodium excretion (UNaV) with a slight decrease in glomerular filtration rate (GFR; −7.8 ± 3.4%). Tempol infusion alone did not cause significant alterations in RBF, GFR, V, or UNaV; however, ANG II in the presence of tempol caused a smaller degree of decreases in RBF (−12 ± 2%), in V (−16 ± 5%), and in UNaV (−27 ± 10%) with a slight increase in GFR (6.6 ± 2.8%) than the responses observed before tempol. In another group of NLA-treated dogs ( n = 6), tempol infusion also caused significant attenuation in the ANG II-induced responses on RBF (−13 ± 3% vs. −22 ± 7%), GFR (−19 ± 5% vs. −33 ± 3), V (−15 ± 12% vs. −28 ± 4%), and UNaV (−11 ± 14% vs. −32 ± 7%). These data demonstrate that renal responses to ANG II are partly mediated by O2−generation and its interaction with NO. The sodium-retaining effect of ANG II is greatly influenced by O2−generation, particularly in the condition of NO deficiency.

Inhibition of nitric oxide synthase enhances superoxide activity in canine kidney

To evaluate the role of a potential interaction between superoxide anion (O2−) and nitric oxide (NO) in regulating kidney function, we examined the renal responses to intra-arterial infusion of a superoxide dismutase mimetic, tempol (0.5 mg·kg−1·min−1), in anesthetized dogs treated with or without NO synthase inhibitor, Nω-nitro-l-arginine (NLA; 50 μg·kg−1·min−1). In one group of dogs ( n = 10), tempol infusion alone for 30 min before NLA infusion did not cause any significant changes in renal blood flow (RBF; 5.2 ± 0.4 to 5.0 ± 0.4 ml·min−1·g−1), glomerular filtration rate (GFR; 0.79 ± 0.04 to 0.77 ± 0.04 ml·min−1·g−1), urine flow (V; 13.6 ± 2.1 to 13.9 ± 2.5 μl·min−1·g−1), or sodium excretion (UNaV; 2.4 ± 0.3 to 2.2 ± 0.3 μmol·min−1·g−1). Interestingly, when tempol was infused in another group of dogs ( n = 12) pretreated with NLA, it caused increases in V (4.4 ± 0.4 to 9.7 ± 1.4 μl·min−1·g−1) and in UNaV (0.7 ± 0.1 to 1.3 ± 0.2 μmol·min−1·g−1) without affecting RBF or GFR. Although NO inhibition caused usual qualitative responses in both groups of dogs, the antidiuretic (47 ± 5 vs. 26 ± 4%) and antinatriuretic (67 ± 4 vs. 45 ± 11%) responses to NLA were seen much less in dogs pretreated with tempol. NLA infusion alone increased urinary excretion of 8-isoprostane (13.9 ± 2.7 to 22.8 ± 3.6 pg·min−1·g−1; n = 7), which returned to the control levels (11.6 ± 3.4 pg·min−1·g−1) during coadministration of tempol. These data suggest that NO synthase inhibition causes enhancement of endogenous O2−levels and support the hypothesis that NO plays a protective role against the actions of O2−in the kidney.

Assessment of renal functional phenotype in mice lacking gp91PHOX subunit of NAD(P)H oxidase

To determine the role of endogenous superoxide (O2-) in the kidney, we assessed renal hemodynamics and excretory function in gp91(PHOX) (a NAD(P)H oxidase subunit) gene knockout (KO) mice and compared these findings with those of wild-type (WT) strain C57BL/6 mice. Renal blood flow (RBF) and glomerular filtration rate (GFR) were determined by PAH and inulin clearances respectively in anesthetized mice (n=8 in each group). There were higher baseline RBF (4.3+/-0.4 versus 2.5+/-0.2 mL/min per gram; P<0.002) and lower renal vascular resistance (RVR) (16+/-1.4 versus 29+/-2.3 mm Hg/mL/min per gram; P<0.0001) in KO compared with WT without a significant difference in mean arterial pressure (MAP) (67+/-2 versus 71+/-2 mm Hg) and GFR (0.66+/-0.09 versus 0.73+/-0.05 mL/min per gram) between the strains. Intravenous infusion of angiotensin II (Ang II) (2 ng/min per gram of body weight) for 30 minutes caused a lesser degree of decreases in RBF (-8% versus -33%) and of increases in RVR (+73% versus +173%) in KO compared with WT. GFR was increased (43%) in KO but not in WT during Ang II infusion. Urinary excretion of nitrate/nitrite was higher in conscious KO (n=5) than in WT (n=5), indicating an increase in nitric oxide bioavailability that could be the cause of high RBF and low RVR in KO. These data indicate that gp91(PHOX), a subunit of NAD(P)H oxidase, plays a regulatory role in the maintenance of renal vascular tone. These results also suggest that the mechanism of Ang II-mediated renal vascular action involves concomitant generation of O2-.

Nitric oxide blockade enhances renal responses to superoxide dismutase inhibition in dogs

To examine the potential role of superoxide anion (O(2)(-)) and its interaction with NO in the regulation of renal hemodynamics and excretory function, we have evaluated the renal responses to enhancement in O(2)(-) activity before and during NO synthase inhibition in anesthetized dogs (n=6). Intraarterial infusion of a superoxide dismutase (SOD) inhibitor, diethyldithiocarbamate (DETC; 0.1 and 0.5 mg/kg per min) was made to enhance O(2)(-) activity in the kidney. Cortical (CBF), medullary (MBF), and total renal blood flow (RBF) responses were assessed using laser-Doppler needle flow probes and an electromagnetic flow probe. DETC caused dose-dependent changes in renal parameters, which were recovered within 30 minutes after the termination of DETC infusion. The high-dose infusion of DETC for 25 minutes resulted in an increase of 29 +/- 10% in renal vascular resistance (control, 35.4 +/- 4.4 mm Hg/mL per min per g) and decreases of 21 +/- 5% in RBF (control, 3.5 +/- 0.5 mL/min per g), 20 +/- 5% in CBF, 21 +/- 7% in MBF, 62 +/- 11% in urine flow (control, 10.5 +/- 2.2 microL/min per g), and 47 +/- 11% in sodium excretion (control, 2.1 +/- 0.2 micromol/min per g), without a significant change (-10 +/- 6%) in glomerular filtration rate (control, 0.74 +/- 0.09 mL/min per g). During NO synthase inhibition with intraarterial administration of nitro-L-arginine (50 microg/kg per min), the same dose of DETC showed a greater increase in renal vascular resistance (73 +/- 15%) and reductions in RBF (39 +/- 4%), CBF (32 +/- 5%), MBF (34 +/- 6%), urine flow (78 +/- 5%), and sodium excretion (67 +/- 0%), with a marked reduction in glomerular filtration rate (59 +/- 7%). These data indicate that O(2)(-) exerts renal vasoconstriction as well as antidiuretic and antinatriuretic effects. These responses are enhanced during NO synthase blockade, suggesting that NO serves a renoprotective effect against these action of O(2)(-).