Nitric oxide in the mediation of pressure natriuresis

Malate reduced blood pressure and exerted differential effects on renal hemodynamics; role of the nitric oxide system and renal epithelial sodium channels (ENaC)

Malate regulates blood pressure via nitric oxide production in salt-sensitive rats, a genetic model of hypertension. This study investigated the possible contributions of malate to blood pressure regulation and renal haemodynamics in normotensive rats. Malate (0.1, 0.3 and 1 μg/kg, iv) was injected into rats or L-nitro-arginine methyl ester (L-NAME)-treated rats and mean arterial blood pressure (MABP), cortical blood flow (CBF), and medullary blood flow (MBF), was measured. The clearance study involved infusion of malate at 0.1 μg/kg/h into rats, and MABP, CBF, MBF, glomerular filtration rate (GFR), urine volume (UV) and sodium output (UNaV) were determined. Mechanistic studies to evaluate the role of renal sodium channels involved the treatment with malate (600 mg/kg, po), amiloride (2.5 mg/kg, po) or hydrochlorothiazide (HCTZ) (10 mg/kg, po), and UV and UNaV were determined. Malate elicited significant peak reductions in MABP (124 ± 6.5 vs 105 ± 3.1 mmHg) at 0.1 μg/kg), CBF (231 ± 18.5 vs 205 ± 10.9 PU). L-NAME did not reverse the effect of malate on MABP but tended to blunt the effect on CBF (40%) and MBF (87%) at 0.3 μg/kg. Infusion of malate reduced MABP, CBF, and MBF in a time-dependent manner (p<0.05). Malate exerted a three-fold decrease in GFR in a time-related fashion (p<0.05) as well as increased UV. UNaV increased by 86% in malate-treated-amiloride rats (p<0.05). These data indicate that malate modulates blood pressure and exerts vascular and tubular effects on renal function that may involve epithelial sodium channels (ENaC).

Endothelial and Vascular Smooth Muscle Dysfunction in Hypertension

The development of essential hypertension involves several factors. Vascular dysfunction, characterized by endothelial dysfunction, low-grade inflammation and structural remodeling, plays an important role in the initiation and maintenance of essential hypertension. Although the mechanistic pathways by which essential hypertension develops are poorly understood, several pharmacological classes available on the clinical settings improve blood pressure by interfering in the cardiac output and/or vascular function. This review is divided in two major sections. The first section depicts the major molecular pathways as renin angiotensin aldosterone system (RAAS), endothelin, nitric oxide signalling pathway and oxidative stress in the development of vascular dysfunction. The second section describes the role of some pharmacological classes such as i) RAAS inhibitors, ii) dual angiotensin receptor-neprilysin inhibitors, iii) endothelin-1 receptor antagonists, iv) soluble guanylate cyclase modulators, v) phosphodiesterase type 5 inhibitors and vi) sodium-glucose cotransporter 2 inhibitors in the context of hypertension. Some classes are already approved in the treatment of hypertension, but others are not yet approved. However, due to their potential benefits these classes were included.

Angiotensin II augments renal vascular smooth muscle soluble GC expression via an AT1 receptor-forkhead box subclass O transcription factor signalling axis

BACKGROUND AND PURPOSE

Reduced renal blood flow triggers activation of the renin-angiotensin-aldosterone system (RAAS) leading to renovascular hypertension. Renal vascular smooth muscle expression of the NO receptor, soluble GC (sGC), modulates the vasodilator response needed to control renal vascular tone and blood flow. Here, we tested if angiotensin II (Ang II) affects sGC expression via an AT1 receptor-forkhead box subclass O (FoxO) transcription factor dependent mechanism.

EXPERIMENTAL APPROACH

Using a murine two-kidney-one-clip (2K1C) renovascular hypertension model, we measured renal artery vasodilatory function and sGC expression. Additionally, we conducted cell culture studies using rat renal pre-glomerular smooth muscle cells (RPGSMCs) to test the in vitro mechanistic effects of Ang II treatment on sGC expression and downstream function.

KEY RESULTS

Contralateral, unclipped renal arteries in 2K1C mice showed increased NO-dependent vasorelaxation compared to sham control mice. Immunofluorescence studies revealed increased sGC protein expression in 2K1C contralateral renal arteries over sham controls. RPGSMCs treated with Ang II caused a significant up-regulation of sGC mRNA and protein expression as well as downstream sGC-dependent signalling. Ang II signalling effects on sGC expression occurred through an AT1 receptor and FoxO transcription factor-dependent mechanism at both the mRNA and protein expression levels.

CONCLUSION AND IMPLICATIONS

Renal artery smooth muscle, in vivo and in vitro, up-regulates expression of sGC following RAAS activity. In both cases, up-regulation of sGC leads to increased downstream cGMP signalling, suggesting a previously unrecognized protective mechanism to improve renal blood flow in the uninjured contralateral renal artery.

LINKED ARTICLES

This article is part of a themed issue on cGMP Signalling in Cell Growth and Survival. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.11/issuetoc.

Assessment the effects of insulin on adiponectin, nitric oxide, myeloperoxidase and lipid profile in type 1 diabetic patients

Type 1 diabetes (T1DM) is well recognized risk factor cardiovascular disease (CVD). Insulin therapy is recommended for all patients with type 1 diabetes. Previous findings showed that diabetes impairs endothelial function and increased glucose level reduces nitric oxide (NO) output and increases myeloperoxidase (MPO) activity. However, adiponectin (APN) decreases serum glucose levels. The current study evaluated effects of insulin therapy on circulating levels of oxidative stress and CVD biomarkers like NO, APN, MPO, AIP and lipid profile in type 1 diabetic patients. Fifty patients with T1DM and 18 healthy people were enrolled in this study. The recruited people with T1DM were classified into two groups: 22 newly diagnosed (untreated) type 1 diabetic patients and 28 insulin treated patients. In all groups, circulating NO, APN, MPO, AIP and lipids levels were measured. Compared to control, untreated diabetes revealed a significant increase in the serum levels of APN, MPO, TG, VLDL, TC, LDL and AIP, with a marked reduction in NO and HDL levels. However, insulin therapy significantly lowered MPO, TC and LDL, with no significant changes in the other biochemical parameters. As expected, oxidative stress and CVD-associated markers were significantly increased in untreated diabetes. Insulin therapy exhibited a relatively positive effect on oxidative stress and CVD biomarkers. Accordingly, insulin plus antioxidant supplementation required to normalize these parameters.

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.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Overexpression of HIF prolyl-hydoxylase-2 transgene in the renal medulla induced a salt sensitive hypertension

Renal medullary hypoxia-inducible factor (HIF)-1α and its target genes, such as haem oxygenase and nitric oxide synthase, have been indicated to play an important role in the regulation of sodium excretion and blood pressure. HIF prolyl hydroxylase domain-containing proteins (PHDs) are major enzymes to promote the degradation of HIF-1α. We recently reported that high salt intake suppressed the renal medullary PHD2 expression and thereby activated HIF-1α-mediated gene regulation in the renal medulla in response to high salt. To further define the functional role of renal medullary PHD2 in the regulation of renal adaptation to high salt intake and the longer term control of blood pressure, we transfected PHD2 expression plasmids into the renal medulla in uninephrectomized rats and determined its effects on pressure natriuresis, sodium excretion after salt overloading and the long-term control of arterial pressure after high salt challenge. It was shown that overexpression of PHD2 transgene increased PHD2 levels and decreased HIF-1α levels in the renal medulla, which blunted pressure natriuresis, attenuated sodium excretion, promoted sodium retention and produced salt sensitive hypertension after high salt challenge compared with rats treated with control plasmids. There was no blood pressure change in PHD2-treated rats that were maintained in low salt diet. These results suggested that renal medullary PHD2 is an important regulator in renal adaptation to high salt intake and a deficiency in PHD2-mediated molecular adaptation in response to high salt intake in the renal medulla may represent a pathogenic mechanism producing salt sensitive hypertension.

Renal collecting duct NOS1 maintains fluid-electrolyte homeostasis and blood pressure

Nitric oxide is a pronatriuretic and prodiuretic factor. The highest renal NO synthase (NOS) activity is found in the inner medullary collecting duct. The collecting duct (CD) is the site of daily fine-tune regulation of sodium balance, and led us to hypothesize that a CD-specific deletion of NOS1 would result in an impaired ability to excrete a sodium load leading to a salt-sensitive blood pressure phenotype. We bred AQP2-CRE mice with NOS1 floxed mice to produce flox control and CD-specific NOS1 knockout (CDNOS1KO) littermates. CDs from CDNOS1KO mice produced 75% less nitrite, and urinary nitrite+nitrate (NOx) excretion was significantly blunted in the knockout genotype. When challenged with high dietary sodium, CDNOS1KO mice showed significantly reduced urine output, sodium, chloride, and NOx excretion, and increased mean arterial pressure relative to flox control mice. In humans, urinary NOx is a newly identified biomarker for the progression of hypertension. These findings reveal that NOS1 in the CD is critical in the regulation of fluid-electrolyte balance, and this new genetic model of CD NOS1 gene deletion will be a valuable tool to study salt-dependent blood pressure mechanisms.

Overexpression of HIF-1α transgene in the renal medulla attenuated salt sensitive hypertension in Dahl S rats

Hypoxia inducible factor (HIF)-1α-mediated gene activation in the renal medulla in response to high salt intake plays an important role in the control of salt sensitivity of blood pressure. High salt-induced activation of HIF-1α in the renal medulla is blunted in Dahl S rats. The present study determined whether the impairment of the renal medullary HIF-1α pathway was responsible for salt sensitive hypertension in Dahl S rats. Renal medullary HIF-1α levels were induced by either transfection of HIF-1α expression plasmid or chronic infusion of CoCl₂ into the renal medulla, which was accompanied by increased expressions of anti-hypertensive genes, cyclooxygenase-2 and heme oxygenase-1. Overexpression of HIF-1α transgenes in the renal medulla enhanced the pressure natriuresis, promoted the sodium excretion and reduced sodium retention after salt overload. As a result, hypertension induced by 2-week high salt was significantly attenuated in rats treated with HIF-1α plasmid or CoCl₂. These results suggest that an abnormal HIF-1α in the renal medulla may represent a novel mechanism mediating salt-sensitive hypertension in Dahl S rats and that induction of HIF-1α levels in the renal medulla could be a therapeutic approach for the treatment of salt-sensitive hypertension.

Modulation of pressure-natriuresis by renal medullary reactive oxygen species and nitric oxide

The renal pressure-natriuresis mechanism is the dominant controller of body fluid balance and long-term arterial pressure. In recent years, it has become clear that the balance of reactive oxygen and nitrogen species within the renal medullary region is a key determinant of the set point of the renal pressure-natriuresis curve. The development of renal medullary oxidative stress causes dysfunction of the pressure-natriuresis mechanism and contributes to the development of hypertension in numerous disease models. The purpose of this review is to point out the known mechanisms within the renal medulla through which reactive oxygen and nitrogen species modulate the pressure-natriuresis response and to update the reader on recent advances in this field.

Hypoxia-inducible factor prolyl-hydroxylase 2 senses high-salt intake to increase hypoxia inducible factor 1alpha levels in the renal medulla

High salt induces the expression of transcription factor hypoxia-inducible factor (HIF) 1alpha and its target genes in the renal medulla, which is an important renal adaptive mechanism to high-salt intake. HIF prolyl-hydroxylase domain-containing proteins (PHDs) have been identified as major enzymes to promote the degradation of HIF-1alpha. PHD2 is the predominant isoform of PHDs in the kidney and is primarily expressed in the renal medulla. The present study tested the hypothesis that PHD2 responds to high salt and mediates high-salt-induced increase in HIF-1alpha levels in the renal medulla. In normotensive rats, high-salt intake (4% NaCl, 10 days) significantly inhibited PHD2 expressions and enzyme activities in the renal medulla. Renal medullary overexpression of the PHD2 transgene significantly decreased HIF-1alpha levels. PHD2 transgene also blocked high-salt-induced activation of HIF-1alpha target genes heme oxygenase 1 and NO synthase 2 in the renal medulla. In Dahl salt-sensitive hypertensive rats, however, high-salt intake did not inhibit the expression and activities of PHD2 in the renal medulla. Correspondingly, renal medullary HIF-1alpha levels were not upregulated by high-salt intake in these rats. After transfection of PHD2 small hairpin RNA, HIF-1alpha and its target genes were significantly upregulated by high-salt intake in Dahl salt-sensitive rats. Overexpression of PHD2 transgene in the renal medulla impaired renal sodium excretion after salt loading. These data suggest that high-salt intake inhibits PHD2 in the renal medulla, thereby upregulating the HIF-1alpha expression. The lack of PHD-mediated response to high salt may represent a pathogenic mechanism producing salt-sensitive hypertension.

Nitric oxide produced by endothelial nitric oxide synthase promotes diuresis

Extracellular fluid volume is highly regulated, at least in part, by peripheral resistance and renal function. Nitric oxide (NO) produced by NO synthase type 3 (NOS 3) in the nonrenal vasculature may promote fluid retention by reducing systemic vascular resistance and arterial pressure. In contrast, NO produced by renal NOS 3 promotes water excretion by reducing renal vascular resistance, increasing glomerular filtration, and inhibiting reabsorption along the nephron. Thus, the net effect of NO from NOS 3 on urinary volume (UV) is unclear. We hypothesized that NO produced by NOS 3 promotes water excretion primarily due to renal tubular effects. We gave conscious wild-type and NOS 3 -/- mice an acute volume load and measured UV, blood pressure, plasma renin concentration (PRC), Na(+), vasopressin, and urinary Na(+) and creatinine concentrations. To give the acute volume load, we trained mice to drink a large volume of water while in metabolic cages. On the day of the experiment, water was replaced with 1% sucrose, and mice had access to it for 1 h. Volume intake was similar in both groups. Over 3 h, wild-type mice excreted 62 +/- 10% of the volume load, but NOS 3 -/- excreted only 42 +/- 5% (P < 0.05). Blood pressure in NOS 3 -/- was 118 +/- 3 compared with 110 +/- 2 mmHg in wild-type mice (P < 0.05), but it did not change following volume load in either strain. PRC, vasopressin, and glomerular filtration rate were similar between groups. Urinary Na(+) excretion was 49.3 +/- 7.0 in wild-type vs. 37.8 +/- 6.4 mumol/3 h in NOS 3 -/- mice (P < 0.05). Bumetanide administration eliminated the difference in volume excretion between wild-type and NOS 3 -/- mice. We conclude that 1) NO produced by NOS 3 promotes water and Na(+) excretion and 2) the renal epithelial actions of NO produced by NOS 3 supersede the systemic and renal vascular actions.

Lack of hemodynamic effects after extended heme synthesis inhibition by succinylacetone in rats

Hypertyrosinemia (HT) is a life-threatening condition caused in large part by the buildup of tyrosine metabolites and their derivatives. One such metabolite is succinylacetone (SA), a potent irreversible inhibitor of heme biosynthesis. Heme is a key component of numerous enzymes involved in arterial blood pressure (BP) regulation, including nitric-oxide synthase (NOS) and its downstream mediator soluble guanylyl cyclase (sGC). Because NOS and sGC are important regulators of cardiovascular function, we hypothesized that inhibition of heme supply to these enzymes by SA would result in the induction of a measurable hypertensive response. Male Sprague-Dawley rats were treated with SA (80 mg x kg(-1) x day(-1) i.p.) for 14 days, resulting in a marked increase in urinary SA and delta-aminolevulinic acid (P < 0.001 for both parameters) and decreased heme concentrations in kidney, liver, spleen, and vascular tissues (P < 0.05 for all parameters). After SA treatment, systemic nitrite/nitrate excretion was reduced by 72% (P < 0.001), and renal NOS and sGC activities were decreased by 32 (P < 0.05) and 38% (P < 0.01), respectively. SA administration also compromised the ex vivo sensitivity of aorta to endothelium-dependent and -independent vasodilation. Despite these effects, SA treatment failed to induce any changes in BP, as assessed by radiotelemetry. Moreover, BP profiles in the SA-treated animals were less responsive to altered sodium intake. The present results demonstrate that extended inhibition of heme synthesis with SA affects hemoenzyme function, albeit without consequent effects on BP regulation and sodium excretion.

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.

Salt-sensitive hypertension induced by decoy of transcription factor hypoxia-inducible factor-1alpha in the renal medulla

Hypoxia inducible factor (HIF)-1alpha, a transcription factor, is abundantly expressed in the renal medulla and regulates many oxygen-sensitive genes such as nitric oxide synthase, cyclooxygenase-2, and heme oxygenase-1. Given the important roles of these genes in the control of arterial pressure, the present study was to test the hypothesis that HIF-1alpha-mediated gene activation serves as an antihypertensive pathway by regulating renal medullary function and sodium excretion. HIF-1alpha decoy oligodeoxynucleotides (ODNs) or scrambled ODNs were transfected into the renal medulla in uninephrectomized Sprague-Dawley rats. Two weeks after ODN transfection, the HIF-1alpha binding activities were significantly inhibited by 45%, and high salt-induced increases of nitric oxide synthase-2 and heme oxygenase-1 transcriptions were also inhibited by 70% and 61% in the renal medulla from decoy rats. The natriuretic responses and increases of renal medullary blood flow responding to the elevations of renal perfusion pressure were significantly blunted by 50% and 37% in decoy rats. Intravenously acute sodium loading increased medullary blood flow and urinary sodium excretion, which was remarkably attenuated in decoy rats. In decoy rats, high salt intake caused a greater positive sodium balance. Consequently, arterial pressure was remarkably increased (from 118+/-1.9 to 154+/-6.3 mm Hg) in decoy rats but not in control rats when the rats were challenged with a high salt diet. There was no blood pressure change in decoy rats that were maintained in normal salt diet. In conclusion, HIF-1alpha-mediated gene activation importantly participates in the regulation of renal medullary function and long-term arterial blood pressure.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effect of maturation on renal Na+/K+-atpase and its susceptibility to nitric oxide-deficient hypertension in rats

1. The present study deals with the effect of maturation on the kinetic properties of renal Na(+)/K(+)-ATPase and its susceptibility to nitric oxide (NO)-deficient hypertension induced by the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME). 2. Immature (4-week-old) and adult (12-week-old) male Wistar rats were administered L-NAME (40 mg/kg per day) in their drinking water for 4 weeks. 3. The properties of the ATP- and Na(+)-binding sites of Na(+)/K(+)-ATPase were investigated by activation of the enzyme with increasing concentrations of the energy substrate ATP and/or cofactor Na(+). Unchanged values of K(m) suggest that energy utilization by the enzyme in the kidney of control rats remains unaffected during maturation. Conversely, the decrease in K(Na) values (the concentration of Na(+) necessary to achieve half-maximal reaction velocity) indicates improved affinity for Na(+) in the older group of control rats. 4. Application of L-NAME to all young animals had no significant effect on the functional properties of Na(+)/K(+)-ATPase. 5. In adult animals, the V(max) values remained unchanged after treatment with L-NAME, but the affinities of the ATP- and Na(+)-binding sites were decreased, as indicated by significant increase in K(m) and K(Na) values. 6. Maturation of control rats was accompanied by an increase in the Na(+) affinity of renal Na(+)/K(+)-ATPase without affecting ATP utilization. However, maturation increased the susceptibility of renal Na(+)/K(+)-ATPase to the harmful effects of L-NAME.

Novel role of AQP-1 in NO-dependent vasorelaxation

Nitric oxide (NO) produced by endothelial cells diffuses to vascular smooth muscle cells to cause dilatation of the renal vasculature and other vessels. Although it is generally assumed that NO moves from cell to cell by free diffusion, we recently showed that aquaporin-1 (AQP-1) transports NO across cell membranes. AQP-1 is expressed in endothelial and vascular smooth muscle cells. We hypothesized that diffusion of NO into vascular smooth muscle cells and out of endothelial cells is facilitated by AQP-1, and transport of NO by AQP-1 is involved in endothelium-dependent relaxation. In intact aortic rings from AQP-1 −/− mice, vasorelaxation induced by acetylcholine (which increases endogenous NO) was reduced ( P < 0.0001 vs. control). No differences were found in the relaxation caused by intracellular delivery of NO or intracellular cGMP between strains. In endothelium-denuded aortic rings from AQP-1 −/− mice, the vasorelaxant capability of NO released in the extracellular space was reduced ( P < 0.0001 vs. control). Influx of NO (5 μM) into vascular smooth muscle cells was 0.17 ± 0.02 f.u./s for control and 0.07 ± 0.01 f.u./s for AQP-1 −/− mice, 62% lower ( P < 0.002). NO released by endothelial cells in response to 1 μM acetylcholine was 96.2 ± 17.7 pmol NO/mg for control and 41.9 ± 13.4 pmol NO/mg for AQP-1 −/− mice, 56% reduction ( P < 0.04). NOS3 expression was 1.33 ± 0.29 O.D. units for control and 3.84 ± 0.76 O.D. units for AQP-1 −/− mice, 188% increase ( P < 0.01). We conclude that 1) AQP-1 facilitates NO influx into vascular smooth muscle cells, 2) AQP-1 facilitates NO diffusion out of endothelial cells, and 3) transport of NO by AQP-1 is required for full expression of endothelium-dependent relaxation.

Renal angiotensin II AT2 receptors promote natriuresis in streptozotocin-induced diabetic rats

Angiotensin II AT2 receptors have been implicated to play a role in the regulation of renal/cardiovascular functions under pathological conditions. The present study is designed to investigate the function of the AT2 receptors on renal sodium excretion and AT(2) receptor expression in the cortical membranes of streptozotocin (STZ)-induced diabetic rats. The STZ treatment led to a significant weight loss, hyperglycemia, and decrease in plasma insulin levels compared with control rats. STZ-induced diabetic rats had significantly elevated basal urine flow, urinary sodium excretion rate (U(Na)V), urinary fractional sodium excretion, and urinary cGMP compared with control rats. Infusion of PD-123319, an AT2 receptor antagonist, caused a significant decrease in U(Na)V (mumol/min) in STZ-induced diabetic rats (1 +/- 0.09 vs. 0.45 +/- 0.1) but not in control rats (0.35 +/- 0.05 vs. 0.4 +/- 0.07). The decrease in U(Na)V was associated with a significant decrease in urinary cGMP levels (pmol/min) in STZ-induced diabetic rats (21 +/- 2 vs. 10 +/- 0.8) but not in control rats (11.75 +/- 3 vs. 12.6 +/- 2). The infusion of PD-123319 did not alter glomerular filtration rate (STZ: 0.3 +/- 0.02 vs. 0.25 +/- 0.03; control: 1.4 +/- 0.05 vs. 1.5 +/- 0.09 ml/min) or mean arterial pressure (STZ: 82 +/- 3 vs. 79 +/- 3.5; control: 90 +/- 4 vs. 89 +/- 4 mmHg), suggesting a tubular effect of the drug. Western blot analysis using an AT2 receptor antibody revealed a significantly enhanced expression of the AT2 receptor protein ( approximately 45 kDa) in brush-border ( approximately 50-fold) and basolateral membranes ( approximately 80-fold) of STZ-induced diabetic compared with control rats. In conclusion, our data suggest that the tubular AT2 receptors in diabetic rats are profoundly enhanced and possibly via a cGMP pathway promote sodium excretion in this model of diabetes.

Inhibition of the formation of EETs and 20-HETE with 1-aminobenzotriazole attenuates pressure natriuresis

This study examined the effects of chronic blockade of the renal formation of epoxyeicosatrienoic acids and 20-hydroxyeicosatetraenoic acid with 1-aminobenzotriazole (ABT; 50 mg·kg−1· day−1ip for 5 days) on pressure natriuresis and the inhibitory effects of elevations in renal perfusion pressure (RPP) on Na+-K+-ATPase activity and the distribution of the sodium/hydrogen exchanger (NHE)-3 in the proximal tubule of rats. In control rats ( n = 15), sodium excretion rose from 2.3 ± 0.4 to 19.4 ± 1.8 μeq·min−1·g kidney weight−1when RPP was increased from 114 ± 1 to 156 ± 2 mmHg. Fractional excretion of lithium rose from 28 ± 3 to 43 ± 3% of the filtered load. Chronic treatment of the rats with ABT for 5 days ( n = 8) blunted the natriuretic response to elevations in RPP by 75% and attenuated the increase in fractional excretion of lithium by 45%. In vehicle-treated rats, renal Na+-K+-ATPase activity fell from 31 ± 5 to 19 ± 2 μmol Pi·mg protein−1·h−1and NHE-3 protein was internalized from the brush border of the proximal tubule after an elevation in RPP. In contrast, Na+-K+-ATPase activity and the distribution of NHE-3 protein remained unaltered in rats treated with ABT. These results suggest that cytochrome P-450 metabolites of arachidonic acid contribute to pressure natriuresis by inhibiting Na+-K+-ATPase activity and promoting internalization of NHE-3 protein from the brush border of the proximal tubule.

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-.

Role of nitric oxide and prostaglandin E2 in acute renal hypoperfusion

Although acute renal ischaemia alters the production of various paracrines, there has been little investigation examining the role of intrarenal vasoactive substances. In the present study, we investigated the role of intrarenal nitric oxide and prostaglandins in modulating the acute renal hypoperfusion-induced alterations in renal function. After a 90% clipping of the left renal artery for 60 min, the clip was released, and the renal haemodynamics and sodium excretion were evaluated in both clipped and non-clipped kidneys of anaesthetized dogs. Furthermore, the changes in renal contents of nitrate/nitrite (NOx) and prostaglandin E2 (PGE2) were assessed by using the renal microdialysis technique. The release of the clipping elicited a gradual recovery of renal plasma flow and glomerular filtration rate, and a sustained increase in fractional sodium excretion (FENa) in the clipped kidney. Renal interstitial NOx was reduced in both the cortex (from 8.2 +/- 1.1 to 2.5 +/- 0.3 micromol/L, P < 0.01) and medulla (from 10.1 +/- 0.9 to 3.1 +/- 0.2 micromol/L, P < 0.01), but the levels gradually elevated after declamping. The treatment with nitro-l-arginine methylester only modestly impaired the recovery of renal plasma flow (RPF; at hour 4) and glomerular filtration rate (GFR; at hours 3 and 4 after declamping), without affecting FENa. Conversely, the renal PGE2 levels increased prominently upon the onset of ischaemia (medulla, from 149 +/- 19 to 378 +/- 39 pg/mL, P < 0.01; cortex, from 107 +/- 13 to 302 +/- 34 pg/mL, P < 0.01). Furthermore, the pretreatment with a non-specific cyclo-oxygenase (COX) inhibitor, sulpyrine, and a COX-2-specific inhibitor, NS398, prominently inhibited the increases in FENa induced by the acute renal arterial clipping in a similar manner. In conclusion, in acute renal hypoperfusion, nitric oxide (NO) plays a permissive role in the recovery of the renal haemodynamics. In contrast, sustained increases in renal PGE2 in both clipped and non-clipped kidneys indicate that the COX-2-mediated PGE2 contributes importantly to the failure of the sodium reabsorption in response to acute renal hypoperfusion.

Volume expansion during acute angiotensin II receptor (AT(1)) blockade and NOS inhibition in conscious dogs

The responses to AT(1)-receptor blockade (candesartan 1 mg/kg) and to concomitant volume expansion (saline 35 ml/kg for 90 min) with and without nitric oxide synthase (NOS) inhibition (N(G)-nitro-L-arginine methyl ester 30 microg small middle dot kg(-1) small middle dot min(-1)) were investigated in separate experiments in normal dogs. AT(1) blockade decreased arterial pressure (106 +/- 4 to 96 +/- 5 mmHg) and increased glomerular filtration rate (GFR) by 17% and sodium excretion threefold. NOS inhibition increased arterial pressure (103 +/- 3 to 116 +/- 3 mmHg) and decreased GFR by 21% and reduced sodium excretion by some 80%. Volume expansion increased arterial pressure significantly in all series involving this procedure, most pronounced during combined AT(1) blockade and NOS inhibition (21 +/- 4 mmHg). Volume expansion during AT(1) blockade elicited marked natriuresis (26 +/- 11 to 274 +/- 55 micromol/min) that was severely reduced by concomitant NOS inhibition (10 +/- 3 to 45 +/- 11 micromol/min), but still much larger than that seen with volume expansion during NOS inhibition alone (2 +/- 1 to 23 +/- 7 micromol/min). Volume expansion during AT(1) blockade increased GFR (+30%), less so during combined AT(1) blockade and NOS inhibition (+13%), but it did not increase GFR significantly (P = 0.07) during NOS inhibition alone. Plasma ANG II increased greater than sevenfold with AT(1) blockade and doubled with NOS inhibition (paired t-test, P < 0.05), whereas it decreased by 50-80% during volume expansion irrespective of pretreatment, i.e., during NOS inhibition, volume expansion did not generate subnormal plasma ANG II concentrations. In conclusion, 1) acute AT(1) blockade leads to hyperfiltration, natriuresis, and hyperresponsiveness to volume expansion, 2) these responses are >85% inhibitable by unspecific NOS inhibition, and 3) NOS inhibition alone is followed by increases in plasma ANG II, hypofiltration, and severe antinatriuresis that may be counterbalanced but not overwhelmed by volume expansion. Thus NOS inhibition virtually abolishes the volume expansion natriuresis, at least in part, due to the lack of appropriate inhibition of the renin-angiotensin-aldosterone system.

Renal cortical remodelling by NO-synthesis blockers in rats is prevented by angiotensin-converting enzyme inhibitor and calcium channel blocker

The cortical remodelling was studied when chronically nitric oxide synthesis (NOs) blockade (L-NAME-induced) hypertensive rats are simultaneously treated, or not, with angiotensin-converting enzyme inhibitor or calcium channel blocker. Four groups of eight rats each were studied as follows: Control (C), L-NAME (L), L-NAME+Enalapril (L+E) and L-NAME+Verapamil (L+V). The systolic blood pressure (SBP) was weekly recorded. The cortex of the left kidneys was analysed according to the vertical section design. The volume-weighted mean glomerular volume (VWGV) was made through the "point-sampled intercepts" method. Enalapril and verapamil were efficient in reducing the SBP in rats submitted to NOs blockade. Glomeruli had considerable alterations in L group rats (glomerular hypertrophy or sclerosis) and tubular atrophy. The VWGV was 100% greater in L group rats than in the C group rats, while it was 30% smaller in L+E and L+V groups than in L group. The tubular volume was 30-50% greater, while the tubular length was 20-30% smaller in the L group than in the other groups. The renal cortical region showed glomerular sclerosis/hypertrophy and tubular remodelling in rats with NOs blockade that was efficiently prevented with the simultaneous treatment with enalapril or verapamil.

Nitric oxide dependency of arterial pressure-induced changes in renal interstitial hydrostatic pressure in dogs

A direct relationship between renal arterial pressure (RAP) and renal interstitial hydrostatic pressure (RIHP) has been shown under conditions of efficient renal blood flow autoregulation. Experiments were performed in six anesthetized dogs to evaluate whether these RIHP responses to changes in RAP were modified during nitric oxide (NO) inhibition with nitro-L-arginine (NLA) or after administration of NO donor agents. A microtip catheter transducer was placed underneath the renal capsule to measure RIHP. Stepwise reductions in RAP (140 to 80 mm Hg) during control conditions resulted in decreases in RIHP from its basal value of 4.7+/-1.1 mm Hg with a slope of 0.04+/-0.026 mm Hg. mm Hg(-)(1) along with decreases in urinary nitrate/nitrite excretion rate (U(NOx)V). Renal cortical and medullary blood flows, measured by laser-Doppler flowmetry, exhibited high autoregulatory efficiency over this RAP range. The changes in RIHP during alterations in RAP were positively correlated (r=0.743; P:<0.001) with the changes in U(NOx)V but not with cortical or medullary blood flow. NLA infusion decreased RIHP to 1.9+/-0.5 mm Hg and also reduced U(NOx)V from 1.8+/-0.2 to 0.9+/-0.01 nmol. min(-)(1). g(-)(1). Infusion of NO donors restored RIHP (4.3+/-0.9 mm Hg) and U(NOx)V (1.5+/-0.2 nmol. min(-)(1). g(-)(1)). During NLA infusion, the RIHP responses to reductions in RAP were markedly attenuated and were not restored even during constant-rate infusion of NO donors. The results suggest that changes in RIHP in response to alterations in RAP are associated with changes in intrarenal NO, suggesting a direct effect of NO to regulate RIHP.

Renal expression of constitutive NOS and DDAH: separate effects of salt intake and angiotensin

BACKGROUND

Nitric oxide (NO) is generated from NO synthase (NOS) isoforms. These enzymes can be inhibited by asymmetric dimethylarginine, which is inactivated by N(G)-N(G)-dimethylarginine dimethylaminohydrolase (DDAH). The neuroneal (nNOS) type I and endothelial (eNOS) type III constitutive NOS isoforms are expressed predominantly in the macula densa and microvascular endothelium of the renal cortex, respectively. DDAH is expressed at sites of NOS expression. Since NO may coordinate the renal responses to angiotensin II (Ang II) and changes in salt intake, we tested the hypothesis that salt intake regulates the expression of nNOS, eNOS and DDAH by Ang II acting on type 1 (AT(1)) receptors.

METHODS

Groups (N = 6) of rats were adapted to low-salt (LS) or high-salt (HS) intakes for 10 days. Other groups of LS and HS rats received the AT(1) receptor antagonist losartan for six days (to test the effects of salt independent of AT(1) receptors). A further group of HS rats received an infusion of Ang II for six days (to test the effect of Ang II independent of salt intake).

RESULTS

Compared with HS rats, there was a significant (P < 0.05) increase in LS rats of nNOS protein in kidney and immunohistochemical expression in the macula densa, and of eNOS protein expression and immunohistochemical expression in the microvascular endothelium, and of DDAH protein expression. Losartan prevented these effects of salt on the expression of eNOS or DDAH, both of which were also increased by Ang II infusions in HS rats. In contrast, losartan did not prevent the effects of salt on nNOS expression, which was unresponsive to Ang II infusion. The generation of NO(2)(-) released by slices of renal cortex, in the presence of saturating concentrations of L-arginine, was increased by LS, compared to HS, independent of losartan and by Ang II during HS.

CONCLUSION

The expressions of eNOS in cortical microvascular endothelium and DDAH in kidney are enhanced by Ang II acting on AT(1) receptors. The expression of nNOS in the macula densa is enhanced by salt restriction independent of Ang II or AT(1) receptors.

Systemic hemodynamics and renal function in hemorrhaged dogs resuscitated with cross-linked hemoglobin

Cross-linked hemoglobin (XL-Hb) infused into dogs increases mean arterial pressure (MAP) but decreases blood flow to the renal (RBF), mesenteric (MBF), and iliac (IBF) circulations. These actions differ markedly from dextran infusion (which increases RBF, MBF, and IBF without altering MAP) and may be due to scavenging of nitric oxide by XL-Hb. However, because the hormonal milieu regulating regional circulation is altered during hemorrhage (when XL-Hb may be used), we studied whether systemic hemodynamics, RBF, MBF, IBF, and renal excretory function in hemorrhaged dogs was altered when resuscitated with XL-Hb compared with dextran (n = 6 each). Hemorrhage decreased MAP by 25% due to a 75% decline in cardiac output. RBF, MBF, and IBF all fell by 33, 64, and 72%, respectively (P<0.05 each). There was also a fall in glomerular filtration rate (GFR), urinary flow, and sodium excretion (P<0.05 each). After resuscitation, MAP, cardiac output, RBF, MBF, IBF, and GFR all recovered to basal values with either XL-Hb or dextran. Urinary flow and sodium excretion increased to above basal levels with dextran (both by 3.5-fold; P<0.05) or XL-Hb (by 7.5- and 10-fold, respectively; P<0.05). We conclude that resuscitation with XL-Hb after hemorrhage not only increases MAP, but also restores RBF, MBF, IBF, GFR, and urinary sodium and volume excretion analogously to dextran. The results contrast with those in normal dogs and suggest that nitric oxide inhibition does not impair hemodynamic and renal function recovery during hemorrhage.

Responses to acute changes in arterial pressure on renal medullary nitric oxide activity in dogs

A direct relationship between renal arterial pressure (RAP) and cortical tissue nitric oxide (NO) activity in the canine kidney was reported earlier. We have conducted further experiments to evaluate medullary NO responses to alterations in RAP with the use of a NO-selective microelectrode inserted into the renal medulla of 6 anesthetized, sodium-replete dogs. Graded reductions in RAP (from 140 to 80 mm Hg) elicited decreases in medullary tissue NO concentration, with a slope of 10.2+/-4.5 nmol x L(-1) x mm Hg(-1). These changes in NO levels were associated with decreases in urinary excretion rate of nitrate and nitrite (U(NOx)V; control value, 1.7+/-0.03 nmol x min(-1) x g(-1); slope, 0.02+/-0.004 nmol x min(-1) x g(-1) x mm Hg(-1)) and sodium excretion (U(Na)V; control, 3.2+/-0.7 micromol x min(-1) x g(-1); slope, 0.06+/-0.02 micromol x min(-1) x g(-1) x mm Hg(-1)) without changes in glomerular filtration rate control (0.84+/-0.06 mL x min(-1) x g(-1)). Intra-arterial administration of the NO synthase inhibitor N(omega)-nitro-L-arginine (NLA; 50 microg x kg(-1) x min(-1)) decreased medullary NO concentration by 218+/-55 nmol x L(-1) (n=5) and attenuated the relationship between RAP and NO concentration (slope, 2.7+/-2.2 nmol x L(-1) x mm Hg(-1)). NLA infusion decreased U(NOx)V (0.8+/-0.06 nmol x min(-1) x g(-1)) and U(Na)V (1.1+/-0.2 micromol x min(-1) x g(-1)) without changes in glomerular filtration rate and attenuated RAP versus U(Nox)V and U(Na)V relationships. Total and regional blood flows, as measured by electromagnetic and laser Doppler needle flow probes, respectively, remained autoregulated both before and during NLA infusion. These data support the hypothesis that acute changes in RAP elicit changes in intrarenal NO production, which may participate in the mediation of pressure natriuresis.

Angiotensin II and nitric oxide: a question of balance

The vasoconstrictor peptide angiotensin II (Ang II) and the endogenous vasodilator nitric oxide (NO) have many antagonistic effects, as well as influencing each other's production and functioning. In the short-term, Ang II stimulates NO release, thus modulating the vasoconstrictor actions of the peptide. In the long-term, Ang II influences the expression of all three NO synthase (NOS) isoforms, while NO downregulates the Ang II Type I (AT1) receptor, contributing to the protective role of NO in the vasculature. Within the cardiovascular system, Ang II and NO also have antagonistic effects on vascular remodeling and apoptosis. In the kidney, the distribution of the NOS isoforms coincides with the sites of the components of the renin-angiotensin system. NO influences renin secretion from the kidney, and NO-Ang II interactions are important in the control of glomerular and tubular function. In the adrenal gland, NO has been shown to affect Ang II-induced aldosterone synthesis, while in the brain NO appears to influence Ang II-induced drinking behavior, although conflicting data have been reported. In this review, we focus on the diverse ways in which Ang II and NO interact, and on the importance of maintaining a balance between these two important mediators.

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.

Role of nitric oxide in modulating renal function and arterial pressure during chronic aldosterone excess

Chronic aldosterone (Aldo) excess is associated with transient sodium retention, extracellular fluid volume expansion, renal vasodilation, and hypertension. The purpose of this study was to determine the role of nitric oxide (NO) in mediating the renal vasodilation and the escape from the sodium-retaining actions of Aldo. To achieve this goal, we examined the long-term effects of Aldo (15 microgram. kg-1. min-1 for 7 days) in conscious, chronically instrumented control dogs (n = 9) and in dogs (n = 12) pretreated with the NO synthesis inhibitor NG-nitro-L-arginine methyl ester (L-NAME; 10 microgram. kg-1. min-1). In control dogs, Aldo caused a transient sodium retention (126 +/- 6 to 56 +/- 2 meq/day) followed by a return of sodium excretion to normal levels. Aldo also increased renal plasma flow by 15% (205 +/- 13 to 233 +/- 16 ml/min), glomerular filtration rate by 20% (72 +/- 3 to 87 +/- 5 ml/min), and arterial pressure from 90 +/- 3 to 102 +/- 3 mmHg. Aldo increased urinary nitrate/nitrite excretion by 60% in the control dogs. Although the sodium-retaining (144 +/- 7 to 56 +/- 7 meq/day) and arterial pressure (122 +/- 6 to 136 +/- 5 mmHg) responses to Aldo were the same in dogs pretreated with L-NAME compared with control, the renal hemodynamic response was markedly attenuated. The results of this study suggest that NO plays an important role in mediating the renal vasodilation during chronic Aldo excess.

Contrasting effects of vasodilators on blood pressure and sodium balance in the hypertension of autonomic failure

Supine hypertension, which is very common in patients with autonomic failure, limits the use of pressor agents and induces nighttime natriuresis. In 13 patients with severe orthostatic hypotension due to autonomic failure (7 women, 6 men, 72 +/- 3 yr) and supine hypertension, the effect of 30 mg nifedipine (n = 10) and 0.025 to 0.2 mg/h nitroglycerin patch (n = 11) on supine BP, renal sodium handling, and orthostatic tolerance was determined. Medications were given at 8 p.m.; patients stood up at 8 a.m. Nitroglycerin was removed at 6 a.m. Compared with placebo, nifedipine and nitroglycerin decreased systolic BP during the night by a maximum of 37 +/- 9 and 36 +/- 10 mmHg, respectively (P < 0.01). At 8 a.m., supine systolic BP was 23 +/- 7 mmHg lower with nifedipine than with placebo (P < 0.05), but was similar with nitroglycerin and placebo. Sodium excretion during the night was not reduced with nitroglycerin (0.13 +/- 0.02 mmol/mg creatinine [Cr] versus 0.15 +/- 0.03 mmol/mg Cr with placebo), but it was increased with nifedipine (0.35 +/- 0.06 mmol/mg Cr versus 0.13 +/- 0.02 mmol/mg Cr with placebo, P < 0.05). Nifedipine but not nitroglycerin worsened orthostatic hypotension in the morning. It is concluded that nifedipine and transdermal nitroglycerin are effective in controlling supine hypertension in patients with autonomic failure. However, nifedipine has a prolonged depressor effect and worsens orthostatic hypotension in the morning. The decrease in pressure natriuresis that would be expected with the substantial decrease in BP obtained with nitroglycerin and nifedipine may be offset by a direct effect of both drugs on renal sodium handling.

Intrarenal nitric oxide activity and pressure natriuresis in anesthetized dogs

Recent studies have indicated that changes in intrarenal nitric oxide (NO) production participate in mediating arterial pressure-induced changes in urinary sodium excretion. Until recently, however, the means to measure changes in intrarenal NO activity in vivo have not been available. For the present study, changes in renal tissue NO activities were assessed directly using an NO-selective microelectrode inserted into the cortical tissue of anesthetized dogs. Control studies demonstrated that the electrode was responsive to intrarenal bolus injections of acetylcholine and to the NO donor S-nitroso-acetylpenicillamine (SNAP). Intrarenal nitro-L-arginine (50 microg x kg(-1) x min(-1)) decreased renal tissue NO concentration by 593+/-127 nmol/L (P<0.05; n=7). Infusions of SNAP (1, 2, and 3 microg x kg(-1) x min(-1) for 25 minutes) in nitro-L-arginine-treated dogs (n=5) resulted in dose-dependent increases in renal tissue NO activity, which showed a positive correlation with changes in urinary excretion rates of NO metabolites, nitrates and nitrites, (r=0.62, P<0.05) and sodium (r=0.78, P<0.01). During graded reductions of renal arterial pressure within the autoregulatory range (144+/-3 to 73+/-2 mm Hg; n=10), there were decreases in tissue NO activity that were positively correlated with changes in renal arterial pressure (r=0.45; P<0.05), urinary nitrate/nitrite excretion (r=0.64, P<0.005), and urinary sodium excretion (r=0.46; P<0.05). These data support the hypothesis that acute changes in renal arterial pressure result in alterations in intrarenal NO activity, which may be responsible for the associated changes in sodium excretion.

Nitrite and nitrate analyses: a clinical biochemistry perspective

OBJECTIVE

To review the assays available for measurement of nitrite and nitrate ions in body fluids and their clinical applications.

DESIGN AND METHODS

Literature searches were done of Medline and Current Contents to November 1997.

RESULTS

The influence of dietary nitrite and nitrate on the concentrations of these ions in various body fluids is reviewed. An overview is presented of the metabolism of nitric oxide (which is converted to nitrite and nitrate). Methods for measurement of the ions are reviewed. Reference values are summarized and the changes reported in various clinical conditions. These include: infection, gastroenterological conditions, hypertension, renal and cardiac disease, inflammatory diseases, transplant rejection, diseases of the central nervous system, and others. Possible effects of environmental nitrite and nitrate on disease incidence are reviewed.

CONCLUSIONS

Most studies of changes in human disease have been descriptive. Diagnostic utility is limited because the concentrations in a significant proportion of affected individuals overlap with those in controls. Changes in concentration may also be caused by diet, outside the clinical investigational setting. The role of nitrite and nitrate assays (alongside direct measurements of nitric oxide in breath) may be restricted to the monitoring of disease progression, or response to therapy in individual patients or subgroups. Associations between disease incidence and drinking water nitrate content are controversial (except for methemoglobinemia in infants).