R. Davis Manning

Oxidative Stress in Dahl Salt-Sensitive Hypertension

Abstract— The role of oxidative stress in the long‐term regulation of arterial pressure, renal hemodynamics, and renal damage was studied in Dahl salt‐sensitive rats. Twenty‐eight Dahl S/Rapp strain rats, equipped with indwelling arterial and venous catheters, were subjected to a 3‐week intravenous infusion of either low Na (0.9 mmol/d) or high Na (20.6 mmol/d) or the superoxide dismutase mimetic, 4‐hydroxyl‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl (Tempol), at 125 &mgr;mol · kg‐1 · h‐1 plus low Na or high Na. After 21 days, mean arterial pressure was 140±3 mm Hg in the high‐Na group, 118±1 mm Hg (P <0.05) in the high‐Na/Tempol group, and unchanged in the low‐Na/Tempol and low‐Na groups. Tempol did not change renal blood flow, glomerular filtration rate, or glomerular cross‐sectional area in rats subjected to the high‐Na intake but did decrease urinary protein excretion, the percentage of sclerotic glomeruli, and the kidney weight to body weight ratio. In 15 additional Dahl S rats subjected to high or low Na intake for 3 weeks, renal cortical and medullary O2·‐ release increased significantly in the high‐Na group when compared with the low‐Na group. Tempol decreased both renal cortical and medullary O2·‐ release in the high‐ and low‐Na rats, but the decrease in O2·‐ release was greater in high‐Na rats. The data suggest that oxidative stress contributes to Dahl salt‐sensitive hypertension and the accompanying renal damage.

Oxidative Stress and Antioxidant Treatment in Hypertension and the Associated Renal Damage

Reactive oxygen species (ROS) are elevated in humans with hypertension many of which develop end-stage renal disease (ESRD), and antioxidant capacity is decreased. About one-half of essential hypertensives have a salt-sensitive type of hypertension, and the amount of renal damage that occurs in salt-sensitive hypertensives greatly exceeds that of non-salt-sensitive hypertensives. Antioxidant therapy can improve cardiovascular outcomes in humans but only if sufficient doses are used. Salt-sensitive hypertensive animal models, especially Dahl salt-sensitive rats, have been used to investigate the relationship between hypertension, ROS and end-stage renal damage. In experimental salt-sensitive hypertension, ROS increase and significant renal damage occur. In the Dahl salt-sensitive (S) rat on high Na for 3 weeks, renal damage is mild, renal levels of superoxide dismutase are decreased, and treatment with Tempol reduces arterial pressure. In the Dahl S rat on high Na for 5 weeks, renal damage is severe, GFR and renal plasma flow are decreased, and renal superoxide production is high. Treatment with vitamins C and E decreases renal superoxide production and renal damage and prevents the decrease in renal hemodynamics. Antioxidant treatment reduces arterial pressure, aortic superoxide production and renal inflammation in DOCA-salt rats, and decreases blood pressure and aortic superoxide release and increases bioactive nitric oxide in SHR stroke-prone rats. In conclusion, in both human and experimental salt-sensitive hypertension, superoxide production and renal damage are increased, antioxidant capacity is decreased, and antioxidant therapy can be helpful.

Antioxidant Treatment Prevents Renal Damage and Dysfunction and Reduces Arterial Pressure in Salt-Sensitive Hypertension

The goal of this study was to test the hypothesis that oxidative stress in Dahl salt-sensitive (SS) rats on a high-sodium intake contributes to the progression of renal damage, the decreases in renal hemodynamics, and the development of hypertension. We specifically studied whether antioxidant therapy, using vitamins C and E, could help prevent renal damage and glomerular filtration rate (GFR) and renal plasma flow reductions and attenuate the increases in arterial pressure. Thirty-three 7- to 8-week old Dahl SS/Rapp strain rats were placed on either a high-sodium (8%) or a low-sodium (0.3%) diet with or without vitamin E (111 IU/d) in the food and 98 mg/d vitamin C in the drinking water for 5 weeks. Rats were equipped with indwelling arterial and venous catheters at day 21. By day 35 in the rats with high-sodium diet, vitamin C and E treatment significantly decreased renal cortical and medullary &OV0151; release, mean arterial pressure, urinary protein excretion, glomerular necrosis, and renal tubulointerstitial damage. At this time, GFR significantly decreased in the high-sodium diet group (1.6±0.2 mL/min) when compared with either the high-sodium plus vitamins C and E (2.9±0.2 mL/min) or the low-sodium diet group (2.9±0.3 mL/min). In SS rats on high-sodium diet, renal plasma flow decreased 40%, and this reduced flow was restored by vitamin treatment. In Dahl salt-sensitive hypertension, increased oxidative stress plays an important role in the renal damage, decreases in renal hemodynamics, and increases in arterial pressure that occur. Antioxidant treatment with vitamins C and E improves renal dysfunction, lessens renal injury, and decreases arterial pressure in Dahl salt-sensitive hypertension.

Associations of Glomerular Number and Birth Weight With Clinicopathological Features of African Americans and Whites

BACKGROUND Hypertension and its cardiovascular complications affect African Americans more severely than whites, a disparity variously ascribed to low birth weight, low glomerular number, an exaggerated arteriolonephrosclerotic blood pressure response, and inflammation-induced oxidative stress. STUDY DESIGN Case series. SETTING AND PARTICIPANTS Autopsy kidneys of 107 African Americans and 87 whites aged 18 to 65 years at a single medical center between 1998 and 2005. Excluded were persons with known premorbid kidney disease; pathological findings of severe arterioarteriolonephrosclerosis, nodular and diffuse diabetic glomerulosclerosis, or nonischemic cardiomyopathy. PREDICTORS & OUTCOMES Associations of: (1) race, age, sex, birth weight, obesity, and glomerular number (predictors) with hypertension and death from coronary artery (CAD) and cerebrovascular disease (CVD; outcomes); and (2) age, blood pressure, and race (predictors) with arteriolonephrosclerotic changes, including chronic tubulointerstitial inflammation (outcomes). MEASUREMENTS Hypertension ascertained from chart review and heart weight. Cause of death determined from chart review and autopsy findings. Birth weight obtained from birth records (115 persons). Total glomerular number (N(glom)) estimated by using the dissector/fractionator technique. Arteriolosclerosis, glomerulosclerosis, cortical fibrosis, and chronic inflammation by using CD68 density were measured morphometrically. RESULTS 59 African Americans (55%) and 32 whites (37%) were classified as hypertensive. CAD and CVD were the cause of death in 64 (33%) and 18 persons (9%), respectively. By using multiple linear regression, birth weight (P < 0.001) and sex (P < 0.01), but not race (P = 0.3) or age (P = 0.2), predicted N(glom) (P < 0.001; adjusted r(2) = 0.176). Hypertension was associated with African American race (P = 0.04), older age (P < 0.001), and male sex (P = 0.01), but not with N(glom) (P = 0.9), body mass index (P = 0.9), or birth weight (P = 0.4). Hypertension was the only significant factor associated with CAD and CVD (P < 0.001 for both). Interactions of age and blood pressure with race showed that although African Americans had more severe hypertension (P < 0.001) and arteriolosclerosis (P = 0.01) at a younger age than whites, there were no significant racial differences in degrees of arteriolosclerosis, glomerulosclerosis, cortical fibrosis, or CD68 density for any level of increased blood pressure. LIMITATIONS The study is observational and descriptive. CONCLUSIONS The more severe hypertension found in African Americans could not be attributed to racial differences in N(glom) or birth weight. CAD and CVD death and increased arteriolonephrosclerosis, including CD68 density, were determined by using blood pressure without a significant interacting contribution from race.

NADPH oxidase contributes to renal damage and dysfunction in Dahl salt-sensitive hypertension

The goal of this study was to test the hypothesis that NADPH oxidase contributes importantly to renal cortical oxidative stress and inflammation, as well as renal damage and dysfunction, and increases in arterial pressure. Fifty-four 7- to 8-wk-old Dahl salt-sensitive (S) or R/Rapp strain rats were maintained for 5 wk on a high sodium (8%) or high sodium + apocynin (1.5 mmol/l in drinking water). Arterial and venous catheters were implanted on day 21. By day 35 in the high-Na S rats, mRNA expression of renal cortical gp91phox, p22phox, p47phox, and p67phox NADPH subunits in S rats increased markedly, and treatment of high-Na S rats with the NADPH oxidase inhibitor apocynin resulted in significant decreases in mRNA expression of these NADPH oxidase subunits. At the same time, in apocynin-treated S rats 1) renal cortical GSH/GSSG ratio increased, 2) renal cortical O2(.-) release and NADPH oxidase activity decreased, and 3) renal glomerular and interstitial damage markedly fell. Apocynin also decreased renal cortical monocyte/macrophage infiltration, and apocynin, but not the xanthine oxidase inhibitor allopurinol, attenuated decreases in renal hemodynamics and lowered arterial pressure. These data suggest that NADPH oxidase plays an important role in causing renal cortical oxidative stress and inflammation, which lead to decreases in renal hemodynamics, renal cortical damage, and increases in arterial pressure.

Role of Neuronal Nitric Oxide Synthase in Dahl Salt-Sensitive Hypertension

The goal of this study was to determine the role of neuronal nitric oxide synthase (nNOS) in the arterial pressure, renal hemodynamic, and renal excretory changes that occur in Dahl salt-resistant (DR) and salt-sensitive (DS) rats during changes in Na intake. Fifty-three DR and DS rats/Rapp strain of 7 to 8 weeks of age with indwelling arterial and venous catheters were subjected to low (0.87 mmol/d) or high (20.6 mmol/d) Na intake beginning 2 days before the start of the control period. Measurements were made during a 5-day control period followed by a 5-day period of nNOS inhibition with intravenous 7-nitroindazole (7NI, 1.67 mg. kg-1. h-1) or vehicle infusion. After 5 days of 7NI, mean arterial pressure increased to 120+/-6% control in the DR-high Na, 7NI rats compared with 98+/-1% control (P<0.05) in the DR-high Na alone rats. After 5 days of 7NI, DS-high Na rats, which had a control arterial pressure 31 mm Hg higher than the comparable DR rats, increased their arterial pressure to 114+/-3% control, which was not significantly different from the DS-high Na alone pressure of 110+/-2% control. No significant changes occurred in glomerular filtration rate, effective renal plasma flow, urinary Na excretion, or urine volume because of 7NI. However, plasma renin activity decreased significantly in DR and DS rats on low Na intake with 7NI infusion. The data demonstrate that the highly salt-resistant DR rat became salt-sensitive during nNOS inhibition with 7NI. However, the arterial pressure of the DS rat was not affected by 7NI. This suggests that nitric oxide produced by nNOS in the DR rat normally helps to prevent salt-sensitive hypertension and that low functional levels of nNOS in the DS rat may contribute to its salt-sensitivity.

Role of abnormal nitric oxide systems in salt-sensitive hypertension

A large percentage of human hypertensive patients are salt sensitive, referring to the dependence of hypertension on sodium intake, but the cause of the salt sensitivity is not known. Although several mechanisms may contribute to salt-sensitive hypertension, the nitric oxide (NO) system appears to play a major role. Studies in humans and Dahl salt-sensitive (S) rats indicate that NO production is decreased during hypertension. Intravenous L-arginine infusion in Dahl S rats increases NO production and prevents salt-sensitive hypertension. In the Dahl salt-resistant (R) rat, NO production by both inducible NO synthase (iNOS) and neuronal NOS (nNOS) help to prevent salt-sensitive hypertension. Experimental evidence is summarized, indicating that the Dahl S rat has a deficient production of NO by nNOS, although NO production by iNOS appears to moderately decrease salt sensitivity. Other evidence about the importance of NO in salt-sensitive hypertension is reviewed, including the role of the renal NO system.

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

Background Salt-sensitive hypertension in humans and experimental animals causes progressive increases in renal damage and dysfunction. The Dahl salt-sensitive (S) rat closely mimics human salt-sensitive hypertension. Aim Our goal was to test the hypothesis that enhancing the glutathione system with dietary N-acetylcysteine administration in Dahl S rats on a high sodium intake for 5 weeks will attenuate the increases in arterial pressure, the decreases in renal hemodynamics and the increases in renal damage that normally occur in S rats on high sodium. Methods Forty-four 7- to 8-week-old Dahl S/Rapp strain rats were maintained on a high sodium (8%), high sodium + N-acetylcysteine (4 g/kg per day), or low sodium (0.3%) diet for 5 weeks. Rats had arterial and venous catheters implanted at day 21. Results By day 35 in the high-sodium rats, N-acetylcysteine treatment significantly increased the renal reduced-to-oxidized glutathione ratio, glomerular filtration rate, and renal plasma flow, and decreased renal cortical and medullary O2− release, urinary protein excretion, renal tubulointerstitial damage and glomerular necrosis. At this time, mean arterial pressure increased to 183 ± 1 mmHg, and N-acetylcysteine reduced this arterial pressure to 121 ± 4 mmHg. By day 35 in S high-sodium rats, N-acetylcysteine had caused a 91% decrease in glomerular necrosis and an 83% decrease in tubulointerstitial damage. Conclusions In Dahl S rats on high sodium intake, arterial pressure increases significantly and renal injury is pronounced. Treatment with N-acetylcysteine enhances the renal glutathione system, improves renal dysfunction and markedly decreases arterial pressure and renal injury in Dahl salt-sensitive hypertension.

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

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

Vascular endothelial growth factor receptor inhibitor enhances dietary salt-induced hypertension in Sprague-Dawley rats

Clinical evidence links the inhibition of VEGF to hypertension. However, the mechanisms by which VEGF affects the pathogenesis of hypertension remain in question. We determined 1) whether administration of VEGF receptor inhibitor SU5416 enhances dietary salt-induced hypertension in Sprague-Dawley (SD) rats, and 2) whether VEGF or SU5416 directly affects proliferation of cultured human renal proximal tubular epithelial cells (HRPTEC) and endothelial nitric oxide synthase (eNOS) expression in cultured human glomerular microvessel endothelial cells (HGMEC). Ten 10-wk-old male SD rats received a high sodium diet (HS; 8%) and the other 10 SD rats received a normal sodium diet (NS; 0.5%) for 4 wks. After 2 wks of the dietary program, five rats were administered with SU5416 at 10 mg x kg(-1) x day(-1) ip or DMSO (vehicle) for 14 days in HS and NS groups. Mean arterial pressure was significantly higher in rats treated with SU5416, as opposed to those treated with DMSO and fed with HS for 4 wk (157.6 +/- 3.9 vs. 125.9 +/- 4.3 mmHg, P < 0.01). Increased proteinuria and albuminuria were associated with marked renal histological abnormalities in HS group with SU5416 administration, compared with those in the vehicle HS group. 3H-thymidine incorporation assay showed that SU5416 blocked the actions of both exogenous and endogenous VEGF on the proliferation of HRPTEC. VEGF (10 ng/ml) significantly increased eNOS protein levels by 29% in cultured HGMEC, but its action was completely abolished by SU5416. These results suggest that VEGF receptor inhibition enhances dietary salt-induced hypertension and kidney injury, possibly by direct damage on renal cells and decreasing NO production by eNOS.