Effect of salt intake and inhibitor dose on arterial hypertension and renal injury induced by chronic nitric oxide blockade

Activation of TRPV1 improves natriuresis and salt sensitivity in high-fat diet fed mice

Western diet (WD) intake increases morbidity of obesity and salt-sensitive hypertension albeit mechanisms are largely unknown. We investigated the role of transient receptor potential vanilloid 1 (TRPV1) in WD intake-induced hypertension. TRPV1^-/- and wild-type (WT) mice were fed a normal (CON) or Western diet (WD) for 16-18 weeks. Mean arterial pressure (MAP) after normal sodium glucose (NSG) loading with or without L-NAME (a NO synthase inhibitor) or N-oleoyldopamine (OLDA, a TRPV1agonist) was not different between the two strains on CON.WT or TRPV1^-/- mice fed WD had increased MAP after NSG, with a greater magnitude in TRPV1^-/- mice. OLDA decreased while L-NAME increased MAP in WT-WD but not in TRPV1^-/--WD mice. The urinary nitrates plus nitrites excretion (UNOx), an indicator of renal NO production, was increased in both strains on CON after NSG. TRPV1 ablation with WD intake abolished NSG-induced increment in UNOx. OLDA further increased while L-NAME prevented NSG-induced increment in UNOx in WT-WD mice. Urinary sodium excretion was increased in both strains on CON and in WT-WD mice but not in TRPV1^-/--WD mice after NSG. OLDA further increased while L-NAME prevented NSG-induced increases in sodium excretion in WT-WD but not in TRPV1^-/--WD mice. Thus, TRPV1 ablation increases salt sensitivity during WD intake possibly via impaired renal NO production and sodium excretion. Activation of TRPV1 enhances renal NO production and sodium excretion, resulting in prevention of increased salt sensitivity during WD intake.

Hypoxia and Endothelial Dysfunction in Autosomal-Dominant Polycystic Kidney Disease

Autosomal-dominant polycystic kidney disease (ADPKD) is the most prevalent inherited kidney disease, characterized by growth of bilateral renal cysts, hypertension, and multiple extrarenal complications that eventually can lead to renal failure. It is caused by mutations in PKD1 or PKD2 genes encoding the proteins polycystin-1 and polycystin-2, respectively. Over the past few years, studies investigating the role of primary cilia and polycystins, present not only on the surface of renal tubular cells but also on vascular endothelial cells, have advanced our understanding of the pathogenesis of ADPKD and have shown that mechanisms other than cyst formation also contribute to renal functional decline in this disease. Among them, increased oxidative stress, endothelial dysfunction, and hypoxia may play central roles because they occur early in the disease process and precede the onset of hypertension and renal functional decline. Endothelial dysfunction is linked to higher asymmetric dimethylarginine levels and reduced nitric oxide bioavailability, which would cause regional vasoconstriction and impaired renal blood flow. The resulting hypoxia would increase the levels of hypoxia-inducible-transcription factor 1α and other angiogenetic factors, which, in turn, may drive cyst growth. In this review, we summarize the existing evidence for roles of endothelial dysfunction, oxidative stress, and hypoxia in the pathogenesis of ADPKD.

Low-dose L-NAME induces salt sensitivity associated with sustained increased blood volume and sodium-chloride cotransporter activity in rodents

To investigate the cause of salt sensitivity in a normotensive animal model, we treated rats with a low-dose of the nitric oxide synthase inhibitor, L-NAME, that does not elevate blood pressure per se or induce kidney fibrosis. A high salt diet increased the circulating blood volume both in L-NAME-treated and nontreated animals for the first 24 hours. Thereafter, the blood volume increase persisted only in the L-NAME-treated rats. Blood pressure was higher in the L-NAME-treated group from the start of high salt diet exposure. Within the first 24 hours of salt loading, the L-NAME treated animals failed to show vasodilation and maintained high systemic vascular resistance in response to blood volume expansion. After four weeks on the high salt diet, the slope of the pressure-natriuresis curve was blunted in the L-NAME-treated group. An increase in natriuresis was observed after treatment with hydrochlorothiazide, but not amiloride, a change observed in parallel with increased phosphorylated sodium-chloride cotransporter (NCC). In contrast, a change in blood pressure was not observed in L-NAME-treated NCC-deficient mice fed a high salt diet. Moreover, direct L-NAME-induced NCC activation was demonstrated in cells of the mouse distal convoluted tubule. The vasodilatator, sodium nitroprusside, downregulated phosphorylated NCC expression. The effect of L-NAME on phosphorylated NCC was blocked by both the SPAK inhibitor STOCK2S-26016 and the superoxide dismutase mimetic TEMPO which also attenuated salt-induced hypertension. These results suggest that the initiation of salt sensitivity in normotensive rodents could be due to hyporeactivity of the vasculature and that maintaining blood pressure could result in a high circulating volume due to inappropriate NCC activity in the low-dose L-NAME model. Thus, even slightly impaired nitric oxide production may be important in salt sensitivity regulation in healthy rodents.

Fumarase Overexpression Abolishes Hypertension Attributable to endothelial NO synthase Haploinsufficiency in Dahl Salt-Sensitive Rats

Human blood pressure salt sensitivity is associated with changes in urinary metabolites related to fumarase (Fh) and nitric oxide (NO) metabolism, and fumarase promotes NO production through an arginine regeneration pathway. We examined the role of the fumarase-NO pathway in the development of hypertension using genetically engineered rat models. Dahl salt-sensitive (SS) rats with heterozygous mutation of eNOS (endothelial NO synthase or Nos3; SS-Nos3^+/-) were bred with SS rats with a hemizygous Fh transgene. SS-Nos3^+/- rats without the Fh transgene (SS-Nos3^+/-/Fh^0/0) developed substantial hypertension with a mean arterial pressure of 134.2±3.7 mm Hg on a 0.4% NaCl diet and 178.0±3.5 mm Hg after 14 days on a 4% NaCl diet. Mean arterial pressure decreased remarkably to 123.1±1.4 mm Hg on 0.4% NaCl, and 143.3±1.5 mm Hg on 4% NaCl in SS-Nos3^+/- rats with a Fh transgene (SS-Nos3^+/-/Fh^0/1), and proteinuria, renal fibrosis, and tubular casts were attenuated in SS-Nos3^+/-/Fh^0/1 rats compared with SS-Nos3^+/-/Fh^0/0 rats. eNOS protein abundance decreased in rats with the Nos3 heterozygous mutation, which was not influenced by Fh overexpression in rats on the 0.4% NaCl diet. However, the decrease in NO metabolite in the renal outer medulla of SS-Nos3^+/-/Fh^0/0 rats on the 0.4% NaCl diet was reversed in SS-Nos3^+/-/Fh^0/1 rats, and levels of L-arginine, but not the other 12 amino acids analyzed, were significantly higher in SS-Nos3^+/-/Fh^0/1 rats than in SS-Nos3^+/+/Fh^0/0 rats. In conclusion, fumarase has potent effects in restoring NO production and blunting the development of hypertension attributable to eNOS haploinsufficiency.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

Role of asymmetrical dimethylarginine in the progression of renal disease

Asymmetric dimethylarginine (ADMA) is a naturally occurring amino acid found in tissues and cells that circulates in plasma and is excreted in urine. It inhibits nitric oxide synthases (NOs) and produces considerable cardiovascular biological effects. Several studies have suggested that plasma concentrations of ADMA provide a marker of risk for endothelial dysfunction and cardiovascular disease. In animal and in population studies ADMA has been associated with progression of CKD. Several mechanisms may be involved in this association, such as compromise of the integrity of the glomerular filtration barrier and development of renal fibrosis. This review summarizes the existing literature on the biology and physiology of ADMA focusing on its role in the progression of renal disease.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced hypertension

Objective:

The purpose of the present study was to evaluate the effects of melatonin on biochemical and cardiovascular changes resulting from exposure to 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD), a polychlorinated dibenzo- para-dioxin.

Methods:

A total of 24 Sprague-Dawley rats were divided equally into the following four groups: (1) control group was administered with 0.5 mL corn oil by gavage and 0.5 cc vehicle of melatonin (proportionally nine parts physiological serum + one part ethyl alcohol) intraperitoneally for 4 weeks, (2) the melatonin group was given 5 mg/kg/day melatonin intraperitoneally for 4 weeks, (3) the TCDD group was given 500 ng/kg/day TCDD by gavage for 4 weeks and (4) the TCDD + melatonin group was given TCDD (500 ng/kg/day) by gavage and melatonin (5 mg/kg/day) intraperitoneally simultaneously for 4 weeks. Systolic blood pressure was evaluated by the tail-cuff method. Vascular responses to phenylephrine and acetylcholine were evaluated in the isolated thoracic aortas.

Results:

TCDD not only augmented the systolic blood pressure but also increased the contractile responses to phenylephrine in aorta. Melatonin reversed the blood pressure augmented by TCDD and decreased the contractile responses to phenylephrine in aorta. TCDD induced an increase in the malondialdehyde levels in kidney tissue and melatonin did not change it. Therefore, TCDD caused a decrease in glutathione levels in kidney tissues and melatonin reversed it.

Conclusion:

Present data demonstrated that TCDD may lead to an increase in blood pressure via increased renal oxidative stress and vascular reactivity. However, melatonin might ameliorate the blood pressure disturbed by TCDD in part by decreasing the oxidant activity induced by TCDD.

L-NAME in the cardiovascular system - nitric oxide synthase activator?

L-arginine analogues are widely used inhibitors of nitric oxide synthase (NOS) activity both in vitro and in vivo, with N(ω)-nitro-L-arginine methyl ester (L-NAME) being at the head. On the one hand, acute and chronic L-NAME treatment leads to changes in blood pressure and vascular reactivity due to decreased nitric oxide (NO) bioavailability. However, lower doses of L-NAME may also activate NO production via feedback regulatory mechanisms if administered for longer time. Such L-NAME-induced activation has been observed in both NOS expression and activity and revealed considerable differences in regulatory mechanisms of NO production between particular tissues depending on the amount of L-NAME. Moreover, feedback activation of NO production by L-NAME seems to be regulated diversely under conditions of hypertension. This review summarizes the mechanisms of NOS regulation in order to better understand the apparent discrepancies found in the current literature.

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.

Transient nitric oxide reduction induces permanent cardiac systolic dysfunction and worsens kidney damage in rats with chronic kidney disease

Left ventricular systolic dysfunction (LVSD) in patients with chronic kidney disease (CKD) is associated with poorer prognosis. Because patients with CKD often exhibit progressively decreased nitric oxide (NO) availability and inhibition of NO production can reduce cardiac output, we hypothesized that loss of NO availability in CKD contributes to pathogenesis of LVSD. Subtotally nephrectomized (SNX) rats were treated with a low dose of the NO synthase inhibitor Nω-nitro-l-arginine (l-NNA; 20 mg/l water; SNX+l-NNA) and compared with relevant control groups. To study permanent changes separate from hemodynamic effects, l-NNA was stopped after week 8 and rats were followed up to week 15, until blood pressure was similar in SNX+l-NNA and SNX groups. To study effects of NO depletion alone, a control group with high-dose l-NNA (l-NNA-High: 100 mg/l) was included. Mild systolic dysfunction developed at week 13 after SNX. In SNX+l-NNA, systolic function decreased by almost 50% already from week 4 onward, together with markedly reduced whole body NO production and high mortality. In l-NNA-High, LVSD was not as severe as in SNX+l-NNA, and renal function was not affected. Both LVSD and NO depletion were reversible in l-NNA-High after l-NNA was stopped, but both were persistently low in SNX+l-NNA. Proteinuria increased compared with rats with SNX, and glomerulosclerosis and cardiac fibrosis were worsened. We conclude that SNX+l-NNA induced accelerated and permanent LVSD that was functionally and structurally different from CKD or NO depletion alone. Availability of NO appears to play a pivotal role in maintaining cardiac function in CKD.

Does the Analysis Based on a Histological and Immunohistochemical Grading System in the Model of BDL Kidney Allow the Quantification of the Degree of Injury?

The aim of this study is to evaluate histopathological findings induced by Nomega-nitro-L-arginine methyl ester (L-NAME) and molsidomine (MOL) on the kidney of bile duct ligated rats. Forty Sprague-Dawley rats, each weighing 125 to 140 g, were included in the study. Extent of histological glomerular injury scores (GIS), arterial injury scores (AIS), and tubulointerstitial injury scores (TIS) in each animal were graded. Alpha-smooth muscle actin (alpha-SMA), tenascin, lectin (Ulex europaeus agglutinin-1), and vimentin were used to determine extent of the injury. The cholestasis was evidenced by a significant increase in the levels of serum total bilirubin in BDL rats (p < 0.01). Malondialdeyde MDA levels increased by the bile duct ligation (BDL) to 12.10 +/- 0.45. This value was significantly higher than the other groups (p < 0.01). Changes in the BDL kidney were marked at 7 days after surgery. GIS were observed to have the highest score, especially at juxtamedullary region in BDL/L-NAME rats, and AIS were also the highest score in this region. These observations were lower in BDL/MOL rats. There is a correlation between GIS and AIS scores (r = .2, p < .01). TIS revealed that BDL/L-NAME rats were significantly more damage than rats in the other groups (p<.001). MOL-treated rats showed considerably fewer lesions in the tubules and interstitium (p < .001). The tubular injuries observed in BDL and BDL/L-NAME rats were significantly attenuated by MOL treatment. Lectin was more and extensively stained in tubular epithelia of the BDL/L-NAME group than in the other (p <.05). Expression of tenascin in tubular epithelia was significantly higher in BDL and BDL/L-NAME as compared with controls (p < .01). Fibrous tissue was only observed in the BDL and BDL/L-NAME group. These areas were weakly stained with vimentin. alpha-SMA staining was more reduced in the L-NAME-treated arterioles than in BDL/MOL (p < .05). In conclusion, the analysis of cell injury based on a histological grading system in the model of BDL kidney allows the quantification of the degree of injury.

Experimental animal models of hypertension

Hypertension (HTN) and cardiovascular disease are the most common causes of death in developed countries. The use of experimental animal models of HTN has provided valuable information regarding many aspects of HTN, including etiology, pathophysiology, complications, and treatment. Because the etiology of HTN is heterogeneous, many experimental animal models have been developed to mimic the many facets of human HTN. The choice of animal model will be determined by the research question, monetary limitations, and technical expertise. The categories of models of HTN are: renovascular, renal parenchymal, pharmacologically induced, environmentally induced, and genetic. There are considerable differences between HTN in animals and humans, including differences in homeostatic mechanisms and pathophysiology; therefore, a thorough understanding of the animal models and rigorous analysis is required before extrapolating the finding in animals to humans.

Salt inactivates endothelial nitric oxide synthase in endothelial cells

There is a 1-4 mmol/L rise in plasma sodium concentrations in individuals with high salt intake and in patients with essential hypertension. In this study, we used 3 independent assays to determine whether such a small increase in sodium concentrations per se alters endothelial nitric oxide synthase (eNOS) function and contributes to hypertension. By directly measuring NOS activity in living bovine aortic endothelial cells, we demonstrated that a 5-mmol/L increase in salt concentration (from 137 to 142 mmol/L) caused a 25% decrease in NOS activity. Importantly, the decrease in NOS activity was in a salt concentration-dependent manner. The NOS activity was decreased by 25, 45, and 70%, with the increase of 5, 10, and 20 mmol/L of NaCl, respectively. Using Chinese hamster ovary cells stably expressing eNOS, we confirmed the inhibitory effects of salt on eNOS activity. The eNOS activity was unaffected in the presence of equal milliosmol of mannitol, which excludes an osmotic effect. Using an ex vivo aortic angiogenesis assay, we demonstrated that salt attenuated the nitric oxide (NO)-dependent proliferation of endothelial cells. By directly monitoring blood pressure changes in response to salt infusion, we found that in vivo infusion of salt induced an acute increase in blood pressure in a salt concentration-dependent manner. In conclusion, our findings demonstrated that eNOS is sensitive to changes in salt concentration. A 5-mmol/L rise in salt concentration, within the range observed in essential hypertension patients or in individuals with high salt intake, could significantly suppress eNOS activity. This salt-induced reduction in NO generation in endothelial cells may contribute to the development of hypertension.

A role for the thromboxane receptor in L-NAME hypertension

Actions of the lipid mediator thromboxane (Tx) A2 acting through the TP receptor contribute to the pathogenesis of cardiovascular disease. To further explore the role of TxA2 in hypertension, we examined the consequences of deficiency of the TP receptor on the course of hypertension associated with endothelial dysfunction and salt sensitivity. To this end, the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) was administered to TP-deficient (Tp-/-) and wild-type (Tp+/+) control mice in drinking water for 21 wk along with a high-salt (HS; 6% NaCl) diet. Administration of L-NAME increased urinary excretion of TxB2 to a similar extent in both Tp+/+ and Tp-/- animals. L-NAME also caused significant and sustained elevations in blood pressure that reached a maximum between weeks 3 and 6. However, the severity of hypertension was attenuated in the Tp-/- mice throughout the study period (P<0.001). At the end of the study, the wild-type mice developed significant cardiac hypertrophy (23.6+/-2% increase in heart-to-body weight ratio). The severity of cardiac hypertrophy was attenuated in the TP-deficient group (11.1+/-2.6%; P<0.05). In contrast, kidney hypertrophy was exaggerated in the Tp-/- mice compared with controls (37.1+/-5.4 vs. 12.3+/-2.3%; P<0.01). Moreover, the severity of glomerulosclerosis, tubule vacuolization, and interstitial chronic inflammation was also enhanced in the Tp-/- group (P<0.01). Thus, in L-NAME hypertension, TP receptors contribute to elevated blood pressure and cardiac hypertrophy. In this model, TP receptors also provided unexpected protection against kidney injury.

Asymmetric dimethylarginine and lipid peroxidation products in early autosomal dominant polycystic kidney disease

BACKGROUND

Patients with autosomal dominant polycystic kidney disease (ADPKD) with normal renal function have endothelial dysfunction and decreased nitric oxide synthase activity in subcutaneous resistance vessels. We investigated asymmetric dimethylarginine (ADMA) as a marker of an inhibitor of nitric oxide synthase and the lipid peroxidation product 13-hydroxyoctadecadienoic acid (HODE) as a marker of oxidative stress in patients with early ADPKD.

STUDY DESIGN

Cross-sectional study.

SETTING & PARTICIPANTS

Patients with early ADPKD (n = 27) and age-matched volunteers (n = 30) from a single academic medical center.

FACTOR

Patients with ADPKD versus controls.

OUTCOMES & MEASUREMENT

Plasma (P) levels, urinary (U) excretion, and urinary clearance (C) of ADMA and HODE. Because of multiple comparisons, P for significance is considered less than 0.0167.

RESULTS

Patients with ADPKD had significantly increased P(ADMA) levels (604 +/- 131 versus 391 +/- 67 nmol/L; P < 0.01) and U(ADMA) excretion (22 +/- 4 versus 15.2 +/- 3 nmol/micromol creatinine; P = 0.01), decreased C(ADMA) (25 +/- 3 versus 33 +/- 4 mL/min; P = 0.01), increased P(HODE) levels (316 +/- 64 versus 230 +/- 38 nmol/L; P < 0.01) and U(HODE) excretion (467 +/- 67 versus 316 +/- 40 nmol/micromol creatinine; P < 0.01), and decreased plasma nitrite plus nitrate (P(NOx)) levels (21 +/- 5 versus 32 +/- 6 micromol/L; P < 0.01) and U(NOx) excretion (59 +/- 7 versus 138 +/- 27 micromol/micromol creatinine; P < 0.01).

LIMITATIONS

Small sample size, cross-sectional nature of study, and limited number of markers of oxidative stress.

CONCLUSIONS

P(ADMA) and P(HODE) levels are increased in patients with early ADPKD. Increased P(ADMA) level is related to decreased C(ADMA) and is accompanied by oxidative stress.

Role of nitric oxide deficiency in the development of hypertension in hydronephrotic animals

Hydronephrotic animals develop renal injury and hypertension, which is associated with an abnormal tubuloglomerular feedback (TGF). The TGF sensitivity is coupled to nitric oxide (NO) in the macula densa. The involvement of reduced NO availability in the development of hypertension in hydronephrosis was investigated. Hydronephrosis was induced by ureteral obstruction in young rats. Blood pressure and renal excretion were measured in adulthood, under different sodium conditions, and before and after chronic administration of either NG-nitro-l-arginine methyl ester (l-NAME) or l-arginine. Blood samples for ADMA, SDMA, and l-arginine analysis were taken and the renal tissue was used for histology and determination of NO synthase (NOS) proteins. TGF characteristics were determined by stop-flow pressure technique before and after administration of 7-nitroindazole (7-NI) or l-arginine. Hydronephrotic animals developed salt-sensitive hypertension, which was associated with pressure natriuresis and diuresis. The blood pressure response to l-NAME was attenuated and l-arginine supplementation decreased blood pressure in hydronephrotic animals, but not in the controls. Under control conditions, reactivity and sensitivity of the TGF response were greater in the hydronephrotic group. 7-NI administration increased TGF reactivity and sensitivity in control animals, whereas, in hydronephrotic animals, neuronal NOS (nNOS) inhibition had no effect. l-Arginine attenuated TGF response more in hydronephrotic kidneys than in controls. The hydronephrotic animals displayed various degrees of histopathological changes. ADMA and SDMA levels were higher and the renal expressions of nNOS and endothelial NOS proteins were lower in animals with hydronephrosis. Reduced NO availability in the diseased kidney in hydronephrosis, and subsequent resetting of the TGF mechanism, plays an important role in the development of hypertension.

Chronic nitric oxide blockade modulates renal Na–K–2Cl cotransporters

OBJECTIVE

The Na-K-2Cl cotransporter (NKCC2 isoform) of the thick ascending limb of Henle's loop (TAL) plays an important role in renal sodium handling, and the vascular isoform (NKCC1) participates in the response to vasoconstrictors. Both isoforms appear to be regulated by nitric oxide. This study aimed to analyze the effect of chronic nitric oxide deficiency on tubular and vascular Na-K-2Cl cotransporters in kidney and their potential role in the development of N-nitro-L-arginine-methyl ester (L-NAME) hypertension.

METHODS

Wistar rats were given L-NAME (vehicle, 10, 35 and 80 mg/100 ml drinking water) for 4 weeks. Blood pressure was measured by the tail-cuff method. NKCC2 activity was estimated as the bumetanide-sensitive Rb influx in fresh isolated TAL tubules. NKCC1-contractile function was estimated as the bumetanide-sensitive vasocontractile response to phenylephrine in isolated perfused kidneys. Acute effects of L-NAME and endothelium removal were also evaluated. NKCC2 and NKCC1 protein expression were assessed by western blot analysis.

RESULTS

Chronic L-NAME administration increased, in a dose-dependent manner, both blood pressure and NKCC2 activity, and these changes significantly correlated (r2 = 0.89, P < 0.01). NKCC1-contractile activity decreased with the highest dose of L-NAME (80 mg/100 ml drinking water group) but it was not affected by acute nitric oxide blockade or endothelium removal. This 80 mg group showed increased NKCC2 expression in the renal medulla and decreased NKCC1 expression in aorta.

CONCLUSIONS

Chronic nitric oxide deficiency stimulates tubular Na-K-2Cl cotransporter, suggesting that NKCC2 hyperactivity contributes to the inability to excrete sodium, and hence to the development of L-NAME hypertension. In contrast, L-NAME hypertension develops independently of vascular NKCC1-contractile activity.

Cross-sectional relations of serum aldosterone and urine sodium excretion to urinary albumin excretion in a community-based sample

Experimental models suggest that increased aldosterone and sodium intake are associated with renovascular damage and resultant proteinuria. We hypothesized that serum aldosterone and urinary sodium would be associated with urinary albumin excretion, an indicator of kidney damage. We evaluated 2700 participants (53% women, mean age 58 years) from the Framingham Offspring Study who attended a routine examination between 1995 and 1998, who were free of heart failure and renal failure, and underwent testing for serum aldosterone, spot urinary sodium, and urinary albumin excretion (urine albumin/creatinine ratio, UACR), the latter two indexed to urinary creatinine. Stepwise multivariable linear regression was used to evaluate the relations between UACR with urinary sodium index and serum aldosterone. In multivariable regression, log urinary sodium index was associated positively with log-UACR (P<0.0001). UACR levels in the fourth and fifth quintiles of urinary sodium index were 24% (95% confidence interval (CI) 3-49%), and twofold higher (95% CI 72-150%), respectively, relative to the lowest quintile (P-value for trend across quintiles <0.001). In multivariable models, log-transformed aldosterone was not related to log-UACR. The top quintile of serum aldosterone levels was associated with a 21% higher (95% 1-44%) UACR levels relative to the lowest quintile. Urinary albumin excretion was strongly and positively associated in a continuous fashion with urinary sodium excretion, whereas a weaker nonlinear positive relation with serum aldosterone was noted. Our cross-sectional observations raise the possibility that dietary salt intake may be associated with early renovascular damage.

Dose-dependent autonomic dysfunction in chronic L-NAME-hypertensive diabetic rats

This study investigated the effects of varying doses of L-NAME on arterial pressure (AP), baroreflex control, and heart rate (HR)/AP variability in the STZ-diabetic rat. Fifty-two male Wistar rats were injected with 50 mg/kg IV STZ (diabetes, D, n = 24) or citrate (controls, C, n = 28) 30 days before recordings. After 16 days, they received 14 days of oral L-NAME, 10 (H10) or 30 (H30) mg/kg, or water. Catheters were implanted into the femoral artery and vein (PE-10) for measurements in conscious rats; recorded data were analyzed on a beat-to-beat basis. Mean AP was higher in CH30 versus C and in DH10 and DH30 versus D rats. Reflex tachycardia was blunted in CH30 and DH30 rats (b = -1.81, -1.41, -0.48 in C, CH10, and CH30, respectively, P < 0.05 and b = -1.45, -1.19, -0.28 in D, DH10, and DH30, respectively, P < 0.05). Although HR and AP variability were reduced in CH30 and DH30 rats versus C and D rats, the DH30 rat had more accentuated dysfunction. All doses of L-NAME produced similar AP responses in experimental versus control groups, independent of the disease state (diabetes). Thus, autonomic dysfunction is more related to the L-NAME dose used and to the association of diabetes and hypertension than to AP values.

L-NAME- and ADMA-induced sympathetic neural activation in conscious rats

Although studies in anesthetized, sino-aortic denervated animals indicate that inhibition of central nitric oxide (NO) causes an excitatory influence on efferent sympathetic nerve activity (SNA) that is normally offset by baroreflex activation, studies in conscious animals have not provided clear-cut evidence for a sympathoexcitatory effect of N(omega)-nitro-l-arginine methyl ester (L-NAME) or the endogenous circulating NO synthase (NOS) inhibitor asymmetric dimethylarginine (ADMA). Thus our goals were to 1) use surgical sino-aortic denervation to test for a sympathoexcititatory effect of intravenous l-NAME in conscious rats, and 2) to determine whether SNA responses to intravenous L-NAME can be extrapolated directly to intravenous ADMA. We recorded mean arterial blood pressure and renal SNA in both intact and sino-aortic-denervated conscious rats during 3 h of continuous intravenous infusion with either L-NAME or ADMA. When we eliminated the confounding influence of the sino-aortic baroreceptors, L-NAME produced a progressive increase in SNA with the peak response exceeding the baseline level of nerve firing by 150%. The same type of frank sympathetic activation was observed with intravenous ADMA. Taken together, these data offer straightforward evidence for l-NAME, as well as ADMA-induced sympathetic activation with direct recordings of SNA in conscious animals. These data confirm and extend the concept that circulating endogenous NOS inhibitors can constitute an excitatory signal to SNA.

Etiology and management of hypertension in chronic kidney disease

The kidneys are vital in the pathogenesis of hypertension and are also pathologically affected by the presence of hypertension. The prevalence of hypertension in chronic kidney disease (CKD) depends on age, the severity of renal failure, and proteinuria. The intricate and inextricable relationship between CKD and hypertension seems to cause cardiovascular disease that has assumed epidemic proportions. This article discusses the etiology and treatment of hypertension in CKD so that it can be better controlled.

Flow-mediated vasodilation and the risk of developing hypertension in healthy postmenopausal women

OBJECTIVES

This study provided the opportunity to assess the relationship between endothelial vasomotor function and incidence of hypertension in a cohort of postmenopausal women.

BACKGROUND

Both menopause and hypertension are associated with endothelial dysfunction and are well-known risk factors for atherosclerotic-related disease.

METHODS

We conducted a prospective cohort study that began in 1996 on 952 apparently healthy postmenopausal women, age 53 +/- 5 years (range 44 to 60 years), with initially normal levels of blood pressure and no history of hypertension. All participants were followed up for a mean period of 3.6 +/- 0.7 years (range 0.5 to 6.9 years). Endothelial function was measured as flow-mediated dilation of the brachial artery using high-resolution ultrasound.

RESULTS

During follow-up 112 women developed hypertension. The adjusted relative risk for women with flow-mediated dilation of 3.5 or less (lowest quartile) was 5.77 (95% confidence interval 4.34 to 8.10) versus women with flow-mediated dilation of 5.5 or greater (highest quartile, referent). Each one-unit decrease of flow-mediated dilation was associated with a significant 16% (95% confidence interval 12% to 33%) increase in the multiple-adjusted relative risk of incident hypertension.

CONCLUSIONS

These prospective data indicate a significant increase in the relative risk of hypertension with each unit decrease of flow-mediated dilation that is independent of age and baseline systolic and diastolic pressure values. This could suggest that an impaired endothelial vasomotor function precedes and predicts the future development of hypertension in postmenopausal women.

Renal kallikrein-kinin system damage and salt sensitivity: insights from experimental models

The importance of tubulointerstitial injury in the pathophysiology of human essential hypertension, and particularly salt sensitivity, is increasingly recognized. Since the renal kallikrein-kinin system (KKS) is located in the tubulointerstitial region of the kidney it is reasonable to expect that injury to this area, whatever the cause, may impair KKS production and compromise its role in blood pressure regulation. In this review we discuss evidence of injury in the renal kallikrein-producing structures in three different experimental models characterized by prominent tubulointerstitial lesions: subtotal nephrectomy; inhibition of nitric oxide synthase; and overload proteinuria. These three experimental models have in common the development of important tubulointerstitial damage and salt-sensitive hypertension expressed after the initial injury has ceased. In these three models, reduced KKS activity may contribute to the establishment of a pathophysiologic state characterized by unopposed hyperactivity of the renin-angiotensin system, resulting in salt retention.

Reversible suppression of nitric oxide system in essential hypertension

Despite enormous research in the field of hypertension, its pathophysiology still remains largely unresolved and appears to be multifactorial. In the present communication, we have analyzed the status of nitric oxide (NO) in the patients with essential hypertension and age matched controls. We have found that the levels of NO are lowered in essential hypertension. The normalization of blood pressure by administration of antihypertensive therapy causes rise in the NO level indicating that perturbed NO status in essential hypertension is reversible. Addition of antioxidant to the antihypertensive drugs causes a further, though non significant, rise in the levels of NO, suggesting that antioxidants may be combined with antihypertensive drugs as adjunct in the management of essential hypertension.

Differential renal response to Nomega-nitro-L-arginine methyl ester and L-arginine in rats with hypertensive or diabetic nephropathy

The present experiments were designed to assess the renal functional response to alterations in nitric oxide formation in animals with different forms of nephropathy. To address this issue, the effects of Nomega-nitro-L-arginine methyl ester (L-NAME) or L-arginine were assessed in animal models exhibiting arterial hypertension due to chronic nitric oxide inhibition (L-NAME, 50 mg/l in drinking water for 12 weeks) or diabetes mellitus (streptozotocin, 60 mg/kg IP). Vehicle-treated, age-matched animals served as controls. Following 12 weeks of pretreatment, mean arterial pressure (MAP), renal hemodynamics, urinary albumin, and electrolyte excretion were determined in standard clearance experiments prior to and following infusion of L-NAME (50 microg/kg/min), l-arginine (5 mg/kg/min), or saline vehicle. In control animals, L-NAME resulted in an increase in MAP and renal vascular resistance and a decline in glomerular filtration rate and renal plasma flow, as expected. L-arginine had no effect on renal hemodynamics. In nitric oxide-depleted hypertensive animals, L-NAME had no additional effect on MAP or renal hemodynamics. Infusion of L-arginine reduced elevated MAP but did not reverse changes in renal hemodynamics. Diabetic rats demonstrated glomerular hyperfiltration and proteinuria. No significant changes in MAP or renal hemodynamics were observed following infusion of L-NAME or L-arginine, respectively. However, L-NAME increased urinary albumin excretion in the absence of hemodynamic changes. The effects of nitric oxide on vascular tone were shown to be dependent on the vascular bed and the underlying disease. Variations in local nitric oxide formation and susceptibility may account for the differential response of the systemic and renal vasculature and contribute to the degree of renal functional impairment observed in different systemic diseases.

Inhibition of inducible nitric oxide synthase during high dietary salt intake

BACKGROUND

Previous studies indicate that nitric oxide (NO) is involved in the regulation of blood pressure (BP) and natriuresis in response to high sodium intake. We investigated the role of inducible NO synthase (iNOS) in response to an increased salt intake.

METHODS

Conscious, chronically catheterized rats were exposed to a high-salt (6%) diet for 14 days while receiving vehicle or aminoguanidine ([AG]; 250 mg/kg/24 h), which selectively inhibits iNOS. A group of rats on normal salt intake + AG were also studied.

RESULTS

Aminoguanidine had no impact on BP (120 +/- 2 v 116 +/- 1 mm Hg, control v day 14) or 24-h urinary nitrite and nitrate excretion (UNOxV), in rats on normal salt but prevented lipopolysaccharide-induced hypotension. High salt alone had no impact on BP (120 +/- 1 v 121 +/- 1 mm Hg), whereas UNaV (1.3 +/- 0.2 v 3.5 +/- 0.6 microeq/min, P < .001) and UNOxV increased with high salt intake. The natriuretic response persisted (1.5 +/- 0.2 v 4.3 +/- 0.8 microeq/min, P < .005), but the increase in UNOXV was prevented with chronic AG although BP fell slightly (121 +/- 1 v 115 +/- 1 mm Hg, P < .05). There was no change in plasma volume with high salt, and 24-h UNaV increased appropriately in the presence of AG. The in vitro NOS activity was not increased in kidney homogenates by high salt diet, nor was it affected by chronic AG treatment.

CONCLUSION

We conclude that NO from an iNOS source is not essential for the regulation of sodium excretion and BP in the presence of a high-salt diet in a normal rat.

Dysregulation of angiotensin II synthesis is associated with salt sensitivity in the spontaneous hypertensive rat

(1) Salt sensitive hypertension, which occurs as a result of treatment with nitric oxide synthase inhibitors, is associated with a loss of the usual down-regulatory effect of dietary sodium on angiotensin II (Ang II) synthesis. In the spontaneous hypertensive rat (SHR), which suffers a relative NO deficiency, the hypertension is in part salt sensitive. We sought to determine therefore whether the salt sensitive component to the hypertension was associated with a loss of the regulatory effect of dietary sodium on Ang II synthesis. (2) Male SHR were placed on low, intermediate or high salt diets for 4 weeks and their blood pressure recorded. After 4 weeks, blood was collected for determination of renin, angiotensinogen, Ang I, Ang II and aldosterone concentrations, as well as ACE activity. (3) The increase in systolic blood pressure in rats on the high salt diet was significantly greater than in those on the low (P < 0.005) and intermediate salt diets (P < 0.0005). Plasma renin and aldosterone concentrations and ACE activity decreased with increasing dietary sodium. However, the concentrations of Ang II and angiotensinogen both increased in the rats on the high salt diet (Ang II: P < 0.05; angiotensinogen: P < 0.05). (4) We conclude that the hypertension in the SHR is in part salt sensitive and that this salt sensitive component is associated with a loss of the normal down-regulatory effect of dietary sodium on Ang II and angiotensinogen synthesis.

Nitric oxide synthase blockade and body fluid volumes

The influence of chronic nitric oxide synthase inhibition with N G-nitro-L-arginine methyl ester (L-NAME) on body fluid distribution was studied in male Wistar rats weighing 260-340 g. Extracellular, interstitial and intracellular spaces, as well as plasma volume were measured after a three-week treatment with L-NAME (approximately 70 mg/kg per 24 h in drinking water). An increase in extracellular space (16.1 +/- 1.1 vs 13.7 +/- 0.6 ml/100 g in control group, N = 12, P<0.01), interstitial space (14.0 +/- 0.9 vs 9.7 +/- 0.6 ml/100 g in control group, P<0.001) and total water (68.7 +/- 3.9 vs 59.0 +/- 2.9 ml/100 g, P<0.001) was observed in the L-NAME group (N = 8). Plasma volume was lower in L-NAME-treated rats (2.8 +/- 0.2 ml/100 g) than in the control group (3.6 +/- 0.1 ml/100 g, P<0.001). Blood volume was also lower in L-NAME-treated rats (5.2 +/- 0.3 ml/100 g) than in the control group (7.2 +/- 0.3 ml/100 g, P<0.001). The increase in total ratio of kidney wet weight to body weight in the L-NAME group (903 +/- 31 vs 773 +/- 45 mg/100 g in control group, P<0.01) but not in total kidney water suggests that this experimental hypertension occurs with an increase in renal mass. The fact that the heart weight to body weight ratio and the total heart water remained constant indicates that, despite the presence of high blood pressure, no modification in cardiac mass occurred. These data show that L-NAME-induced hypertension causes alterations in body fluid distribution and in renal mass.

Nitric oxide inhibition enhances platelet aggregation in experimental anti-Thy-1 nephritis

In the present paper we studied the role of nitric oxide radicals (NO) on platelet aggregation, fibrinogen deposition, superoxide formation, peroxynitrite formation, hemodynamics, and leukocyte migration in the Thy-1 model of glomerulonephritis. To first study the baseline kinetics of these parameters, groups of anti-Thy-1-treated rats were sacrificed at 1 h, 4 h, 24 h, 3 days, 7 days, and 14 days and compared to controls. Urinary protein excretion was significantly elevated in Thy-1 nephritis at 3 and 7 days. Glomerular macrophages, PMNs, and superoxide anion-positive cells were significantly increased in Thy-1 nephritis. Nitrotyrosine immunoreactivity was absent during the entire study period. Glomerular platelet aggregation was significantly increased in anti-Thy-1 injected rats at 1 h, 4 h, 24 h, and 3 days. Glomerular fibrinogen deposition was significantly elevated at all time points. To elucidate the role of NO in this process, additional groups of anti-Thy-1-injected rats were treated with the NOS inhibitor l-NAME and studied at 24 h. Urinary protein excretion was significantly higher in l-NAME treated Thy-1 rats compared to nontreated Thy-1 rats. Plasma and urine nitrite/nitrate levels were significantly lower in l-NAME-treated Thy-1 rats compared to nontreated Thy-1 rats. Compared to nontreated Thy-1 rats, there were no differences in intraglomerular leukocyte accumulation after treatment with l-NAME. In contrast, we observed a marked increase in platelet aggregation following l-NAME treatment. From these data we conclude that the inflammatory infiltrate in Thy-1 nephritis develops independent of NO radical production, whereas NO radicals prevent the accumulation of platelet aggregates.

Low-Dose Nitric Oxide Inhibition Produces a Negative Sodium Balance in Conscious Dogs

Abstract. Nitric oxide modulates renal hemodynamics and salt and water handling. Studies on the latter have provided conflicting results, however. Electrolyte and water balances were therefore studied in 28 beagles for 4 d, to determine the various effects of nitric oxide synthase (NOS) inhibition on renal function. The dogs were chronically equipped with aortic occluders to reduce renal perfusion pressure (RPP), bladder catheters, and catheters for measurements of RPP and mean arterial BP. A swivel system allowed free movement within the kennels. In a first set of experiments, a nonpressor dose of L-Nω-nitroarginine (LN) (3 μg/min per kg body wt) was administered, to prevent increases in mean arterial BP and thus pressure effects on renin release and natriuresis. Remarkably, the nonpressor dose of LN caused a negative sodium balance. The natriuretic effect may involve reduced plasma renin activity, reduced aldosterone concentrations, and increased atrial natriuretic peptide concentrations. Changes in aldosterone levels, however, were the only parameters to parallel the time course of sodium excretion. In a second set of experiments, a sodium-retaining challenge was elicited by reduction of RPP. Dogs without NOS inhibition escaped sodium retention during RPP reduction after 2 d (“pressure escape”). LN neither ameliorated nor aggravated the sodium-retaining effect of reduced RPP, nor did it compromise the accomplishment of pressure escape. In conclusion, inhibition of NOS with a low dose of LN results in a reduction of total-body sodium. This effect mainly relies on reduced aldosterone concentrations. Furthermore, LN does not change the regulatory response to long-term RPP reduction.

Effects of aging and alterations in dietary sodium intake on total nitric oxide production

Animal studies suggest that nitric oxide (NO) deficiency is linked to salt-sensitive hypertension and that NO activity decreases during normal aging. This study investigates the impact of increasing age and manipulations in dietary salt intake on biochemical indices of the NO system in healthy humans. We measured NO(2) + NO(3) (NO(X); stable oxidation products of NO) and cyclic guanosine monophosphate (cGMP; major second messenger) in plasma and urine of 30 healthy subjects aged 22 to 77 years. Subjects were maintained on controlled low NO(X) and low-, normal-, or high-salt diets for 3 days. Salt sensitivity of blood pressure was seen only in the oldest subjects. Plasma renin activity was suppressed by a high salt intake in all age groups, and baseline values declined with advancing age. Neither age nor salt intake correlated with indices of NO activity over the third 24-hour period of controlled salt intake. In a subgroup of subjects aged 33 +/- 4 years challenged with ultrahigh sodium intake (400 mEq/24 h), again there was no increase in NO(2) + NO(3) or cGMP measures. In contrast to animal studies, there is no correlation in humans between either salt intake or age and total NO production and activity, indicated by NO(2) + NO(3) and cGMP measures. This does not preclude undetected alterations occurring in NO production and/or activity in strategic locations in the kidney and cardiovascular system. Limitations of blood and urine measurements of NO(2) + NO(3) and cGMP as indices of NO activity are discussed.

Circulating endothelial nitric oxide synthase inhibitory factor in some patients with chronic renal disease

BACKGROUND

Chronic renal disease (CRD) is associated with hypertension and reduced synthesis of nitric oxide (NO). Here, we investigated whether there is a circulating endothelial NO synthase (eNOS) inhibitory factor(s) in some patients with CRD that might directly influence endothelial NOS.

METHODS

Human dermal microvascular endothelial cells (HDMECs) were incubated for six hours with 20% plasma from subjects with normal renal function (PCr = 0.8 +/- 0.2 mg%), and patients with moderate renal insufficiency of various causes (PCr = 4.0 +/- 1.5 mg%) and impact on NOS activity, transport of L-arginine, and abundance of eNOS protein were measured. Plasma concentrations of asymmetric and symmetric dimethyl L-arginine (ADMA and SDMA) were also measured.

RESULTS

There was no effect of any human plasma on L-arginine transport. The NOS activity was variable in CRD patients and fell into two subgroups: CRD I, individual values similar to control, and CRD II, individual values lower than control mean. The effect of CRD plasma on NOS activity in cultured cells was not related to the primary disease, but was predicted by plasma ADMA levels since plasma ADMA was elevated in CRD II versus both control and CRD I. Blood urea nitrogen and creatinine levels were uniformly elevated in CRD plasma. The abundance of eNOS protein was unaffected by plasma.

CONCLUSION

High plasma levels of ADMA in CRD patients are independent of reduced renal clearance, suggesting an alteration in ADMA synthesis and/or degradation. High ADMA is a marker and is partly responsible for the inhibition of eNOS activity in cultured cells and may also result in reduced eNOS activity in vivo, with consequent hypertension.

Mechanisms of albuminuria in the chronic nitric oxide inhibition model

Chronic nitric oxide (NO) inhibition causes hypertension and renal injury. Concomitant salt overload promotes massive albuminuria. We investigated the mechanisms whereby these treatments impair glomerular permselectivity. Adult male Munich-Wistar rats received either a standard-salt (SS; 0.5% Na) or high-salt (HS; 3.1% Na) diet and either no treatment or the NO inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). At 30 days, albuminuria was moderate, the density of fixed anionic sites at the glomerular basement membrane (GBM), estimated by cationic ferritin binding, declined by approximately 35%, and the fractional clearance of 70-kDa neutral dextran (phi) rose moderately in rats receiving L-NAME and SS. Rats given L-NAME and HS exhibited massive albuminuria, whereas phi was nearly tripled. Depletion of GBM anionic sites was also seen in these rats. The GBM was thickened in both L-NAME-treated groups. These abnormalities were largely reversed after cessation of treatments. These results indicate that chronic L-NAME treatment promotes reversible albuminuria by impairing both glomerular size and charge selectivity. These effects likely reflect functional rather than structural disruption of the glomerular wall.

Association of renal injury with increased oxygen free radical activity and altered nitric oxide metabolism in chronic experimental hemosiderosis

Chronic iron (Fe) overload is associated with a marked increase in renal tissue iron content and injury. It is estimated that 10% of the American population carry the gene for hemochromatosis and 1% actually suffer from iron overload. The mechanism of iron overload-associated renal damage has not been fully elucidated. Iron can accelerate lipid peroxidation leading to organelle membrane dysfunction and subsequent cell injury/death. Iron-catalyzed generation of reactive oxygen species (ROS) is responsible for initiating the peroxidatic reaction. We investigated the possible association of oxidative stress and its impact on nitric oxide (NO) metabolism in iron-overload-associated renal injury. Rats were randomized into Fe-loaded (given 0.5 g elemental iron/kg body weight as iron dextran; i.v.), Fe-depleted (given an iron-free diet for 20 weeks), and control groups. Renal histology, tissue expression of endothelial and inducible nitric oxide synthases (eNOS and iNOS), renal tissue expression of nitrotyrosine, plasma, and renal tissue lipid peroxidation product, malondialdehyde (MDA), and plasma and urinary NO metabolites (NOx) were examined. Iron overload was associated with mild proteinuria, tissue iron deposition together with significant glomerulosclerosis, tubular atrophy, and interstitial fibrosis. Rare focal glomerulosclerosis and tubulointerstitial changes were noted in normal controls. No renal lesions were observed in Fe-depleted rats. Iron deposits were seen in glomeruli, proximal tubules, and interstitium. The iron staining in the distal tubules was negligible. Both plasma and renal tissue MDA and renal tissue nitrotyrosine were increased significantly in Fe-loaded rats compared with control rats. In contrast, Fe-depleted animals showed a marked reduction in plasma and renal tissue MDA and nitrotyrosine together with significant elevation of urinary NOx excretion. In addition, iron-overload was associated with up-regulation of renal eNOS and iNOS expressions when compared with the control and Fe-depleted rats that showed comparable values. In conclusion, chronic iron overload resulted in iron deposition in the glomeruli and proximal tubules with various renal lesions and evidence of increased ROS activity, enhanced ROS-mediated inactivation, and sequestration of NO and compensatory up-regulation of renal eNOS and iNOS expressions. However, iron depletion was associated with reduced MDA and tissue nitrotyrosine abundance, increased urinary NOx excretion, normal nitric oxide synthase (NOS) expression, and absence of renal injury. These findings point to the possible role of ROS in chronic iron overload-induced renal injury.

Glomerular expression of endothelial nitric oxide synthase in deoxycorticosterone acetate-salt-treated rats

1. In the present study, we investigated the relationship between chronic volume loading and glomerular endothelial nitric oxide synthase (eNOS) expression. Immunohistochemical expression of eNOS in glomeruli was semiquantified by graded scores of staining intensity. Each glomerulus was isolated by microdissection and mRNA expression was detected by reverse transcription-polymerase chain reaction (RT-PCR) in desoxycorticosterone acetate (DOCA)-salt-treated rats. 2. Glomerular expansion and dilatation of tubules were the main histological findings and glomerular and vascular injury scores were significantly higher in the DOCA-salt-treated group than in the control group. Endothelial NOS staining in glomeruli in DOCA-salt-treated animals was 81.4% lower than in control animals. Endothelial NOS mRNA was also detected at a very low rate in the kidney of treated rats compared with control rats (22/80 vs 74/80 glomeruli, respectively). 3. These results suggest that eNOS protein and mRNA expression in glomeruli was reduced by DOCA-salt loading. Chronic volume loading may damage the glomerulus and this may be mediated, at least in part, by disruption of eNOS.

Tubulointerstitial injury and loss of nitric oxide synthases parallel the development of hypertension in the Dahl-SS rat

OBJECTIVE

Alterations in renal nitric oxide (NO) are involved in the hypertension of the Dahl salt-sensitive (Dahl-SS) rat We sought to identify the kinetics and sites of expression of the major NO synthase (NOS) isoforms.

DESIGN

The renal expression of the major NOS were examined in Dahl-SS and salt-resistant rats (Dahl-SR) while on a low salt (0.1% NaCl) diet at 3 and 9 weeks of age.

METHODS

Renal biopsies from Dahl-SS and Dahl-SR rats were compared for evidence of renal injury and for alterations in expression of the NOS enzymes by quantitative immunohistochemistry.

RESULTS

At 3 weeks of age Dahl-SS and Dahl-SR rats have normal renal histology and similar immunohistochemical expression of NOS1, -2, and -3. At 9 weeks Dahl-SS rats had significantly higher blood pressure than Dahl-SR rats (P< 0.005 ), and lower macula densa NOS1 (P< 0.05) and cortical and medullary NOS3 (P< 0.05). NOS2 was reduced in cortical tubules in biopsies showing severe tubulointerstitial damage, but was not significantly different between Dahl-SS and Dahl-SR groups as a whole. Dahl-SS rats also manifested glomerular and tubulointerstitial injury. Tubular expression of osteopontin (OPN), which is an inhibitor of NOS2, correlated with the systolic BP in individual Dahl-SS rats (r2 = 0.80, P < 0.0001 ).

CONCLUSION

Tubulointerstitial injury and the loss of NOS occur after birth and parallel the development of hypertension. We suggest that the structural and functional changes that occur with renal injury in the Dahl-SS rat may contribute to the development of hypertension.

Dexamethasone worsens nitric oxide inhibition-induced hypertension and renal dysfunction

Chronic nitric oxide (NO) inhibition with Nomega-nitro-L-arginine methyl ester (L-NAME) has previously been reported to produce systemic hypertension, renal vasoconstriction, and renal damage. In this study we investigated whether a compensatory restoration of NO synthesis occurs in chronic L-NAME hypertension and whether chronic treatment with dexamethasone (Dex) (which inhibits inducible NO synthase [iNOS]) can influence the course of the hypertension. We found that in the conscious chronically L-NAME-treated (approximately =10 mg/kg/24 h) hypertensive rats, acute systemic NOS inhibition elicited a further increase in blood pressure (BP), indicating partial restoration of NO production. Chronic Dex in a dose previously reported to inhibit iNOS (5 microg/24 h), amplified the hypertension (within 2 days), renal vasoconstriction, and reduction in glomerular filtration rate because of L-NAME. In contrast, chronic Dex alone had no effects on renal hemodynamics or BP during the first week, although by the end of week 2 a small increase in BP (approximately =10 mm Hg) was evident. These results show that BP continues to increase with chronic L-NAME despite partial restoration of NO production. An iNOS, which might be stimulated and escaped inhibition by L-NAME, may be responsible for the compensatory restoration of NO synthesis, serving to attenuate the development of hypertension and renal dysfunction.

Salt-induced hypertension in Dahl salt-resistant and salt-sensitive rats with NOS II inhibition

Although recent evidence suggests that reduced nitric oxide (NO) production may be involved in salt-induced hypertension, the specific NO synthase (NOS) responsible for the conveyance of salt sensitivity remains unknown. To determine the role of inducible NOS (NOS II) in salt-induced hypertension, we treated Dahl salt-resistant (DR) rats with the selective NOS II inhibitor 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT) for 12 days. Tail-cuff systolic blood pressures rose 29 +/- 6 and 42 +/- 8 mmHg in DR rats given 150 and 300 nmol AMT/h, respectively (P < 0.01, 2-way ANOVA) after 7 days of 8% NaCl diet. We observed similar results with two other potent selective NOS II inhibitors, S-ethylisourea (EIT) and N-[3-(aminomethyl)benzyl]acetamidine hydrochloride (1400W). Additionally, AMT effects were independent of alterations in endothelial function as assessed by diameter change of mesenteric arterioles in response to methacholine using videomicroscopy. We, therefore, conclude from these data that NOS II is important in salt-induced hypertension.

Role of nitric oxide in the control of renal function and salt sensitivity

Nitric oxide (NO) plays critical roles in the control of renal and glomerular hemodynamics, tubuloglomerular feedback response, release of renin and sympathetic transmitters, tubular ion transport, and renal water and sodium excretion. This paper explores the importance of NO in the control of renal water and sodium excretion and in the long-term control of arterial blood pressure. Synthesis of NO, characteristics of NO tissue redox forms, NO synthase activity, and NO synthase isoforms in the kidney are reviewed. To define the role of NO as a natriuretic and antihypertensive factor, the most supportive evidence is summarized, and some contradictory results are also noted. Given the evidence that high salt intake results in high NO concentrations and great NO synthase expression and activity selectively in the renal medulla of the kidney, as well as evidence of a deficiency of the NO synthase activity in Dahl salt-sensitive rats confined in the renal medulla, this report emphasizes the mechanisms by which the renal medullary l-arginine/NO system controls sodium excretion and arterial blood pressure. Other mechanisms for the action of NO on sodium homeostasis such as the action on glomerular filtration rate and the direct effect on tubules are also discussed. We conclude that there is strong evidence that under physiologic conditions, NO plays an important role in the regulation of renal blood flow to the renal medulla and in the tubular regulation of sodium excretion. It is thereby involved in the long-term control of arterial blood pressure, and inhibition or deficiency of NO synthase may result in a sustained hypertension.

Nitric oxide inhibits transcription of the Na+-K+-ATPase alpha1-subunit gene in an MTAL cell line

Nitric oxide (NO) has been implicated as an autocrine modulator of active sodium transport. To determine whether tonic exposure to NO influences active sodium transport in epithelial cells, we established transfected medullary thick ascending limb of Henle (MTAL) cell lines that overexpressed NO synthase-2 (NOS2) and analyzed the effects of deficient or continuous NO production [with or without NG-nitro-L-arginine methyl ester (L-NAME) in the culture medium, respectively] on Na+-K+-ATPase function and expression. The NOS2-transfected cells exhibited high-level NOS2 expression and NO generation, which did not affect cell viability or cloning efficiency. NOS2-transfected cells were grown in the presence of vehicle, NG-nitro-D-arginine methyl ester (D-NAME), or L-NAME for 16 h, after which 86Rb+ uptake assays, Northern analysis, or nuclear run-on transcription assays were performed. The NOS2-transfected cells allowed to produce NO continuously (vehicle or D-NAME) exhibited lower rates of ouabain-sensitive 86Rb+ uptake ( approximately 65%), lower levels of Na+-K+-ATPase alpha1-subunit mRNA ( approximately 60%), and reduced rates of de novo Na+-K+-ATPase alpha1-subunit transcription compared with L-NAME-treated cells. These results have uncovered a novel effect of NO to inhibit transcription of the Na+-K+-ATPase alpha1-subunit gene.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.