L-arginine administration normalizes pressure natriuresis in hypertensive Dahl rats

Animal models of hypertension: The status of nitric oxide and oxidative stress and the role of the renal medulla

Hypertension significantly contributes to overall morbidity and mortality worldwide, and animal models of hypertension provide important tools to verify the physiological and molecular mechanisms underlying the development of the disease. A review of the most important models available would provide an insight into the appropriate targets to be addressed in the treatment of different forms of human hypertension. In the animal models discussed a special attention is given to the status and pathophysiological role of nitric oxide and its interaction with reactive oxygen species and oxidative stress. Another focus of the review are the processes running in the renal medulla which are still insufficiently explored. Deficient nitric oxide synthesis and its reduced bioavailability are important determinants of hypertension since NO is recognized as a major control factor of vascular tone homeostasis. For decades perfusion of the renal medulla has also been regarded as one of the blood pressure control factors and, noteworthily, the renal medulla belongs to the tissues with the highest NO content. The list of most often applied animal hypertension models reviewed here includes variants of salt-induced hypertension, the models with genetic background: such as spontaneously hypertensive rats (SHR) and Dahl salt sensitive (SS/SR) rats, Goldblatt 2K-1C hypertensive rats, and also the pharmacologically-plus-dietary salt-induced model known as DOCA-salt hypertension.

l -Arginine Can Enhance the Beneficial Effect of Losartan in Patients with Chronic Aortic Regurgitation and Isolated Systolic Hypertension

Information about the effects of angiotensin II receptor blocker (ARB) therapy on the hemodynamic and cardiac structure in patients with chronic aortic regurgitation (CAR) and isolated systolic hypertension (ISH) is limited. This study planned to test the hypothesis that l -arginine could further enhance the beneficial effect of an ARB, losartan, and provide a favorable effect on the natural history of CAR and ISH. Sixty patients with CAR and ISH were enrolled in a randomized, double-blind trial comparing hemodynamic and ultrasonic change in two treatment arms: losartan +  l -arginine and losartan-only treated groups. Serial echocardiographic and hemodynamic studies were evaluated before and after treatment. Both groups had a significant reduction in systolic blood pressure (SBP) and diastolic blood pressure (DBP), left ventricular end-diastolic volume index (LVEDVI), LV end-systolic volume index (LVESVI), LV mass index (LVMI), and LV mean wall stress after 6- and 12-month treatment ( p <0.01 in all comparisons). Both groups had a significant increase in LV ejection fraction and exercise duration after 6- and 12-month treatment ( p  < 0.01 in all comparisons). Using multivariate linear regression analysis, only losartan +  l -arginine therapy achieved a significantly lower LVESVI (38.89 ± 0.23 mL/m ² ), LVEDVI (102.3 ± 0.3 mL/m ² ), LVMI (107.6 ± 0.3 g/m ² ), SBP (123.5 ± 1.0 mm Hg), and greater exercise duration (7.38 ± 0.02 minutes) than those of the losartan-only treated groups ( p <0.01 in all comparisons). These findings suggest that early co-administrative strategy provides a beneficial approach to favorably influence the natural history of CAR.

Arginine and Endothelial Function

Arginine (L-arginine), is an amino acid involved in a number of biological processes, including the biosynthesis of proteins, host immune response, urea cycle, and nitric oxide production. In this systematic review, we focus on the functional role of arginine in the regulation of endothelial function and vascular tone. Both clinical and preclinical studies are examined, analyzing the effects of arginine supplementation in hypertension, ischemic heart disease, aging, peripheral artery disease, and diabetes mellitus.

Norvaline Reduces Blood Pressure and Induces Diuresis in Rats with Inherited Stress-Induced Arterial Hypertension

Growing evidence suggests that increased arginase activity affects vital bioprocesses in various systems and universally mediates the pathogenesis of numerous metabolic diseases. The adverse effects of arginase are associated with a severe decline in L-arginine bioavailability, which leads to nitric oxide synthase substrate insufficiency, uncoupling, and, eventually, superoxide anion generation and substantial reduction of nitric oxide (NO) synthesis. In cooperation, it contributes to chronic oxidative stress and endothelial dysfunction, which might lead to hypertension and atherosclerosis. Recent preclinical investigations point arginase as a promising therapeutic target in ameliorating metabolic and vascular dysfunctions. In the present study, adult rats with inherited stress-induced arterial hypertension (ISIAH) were used as a model of hypertension. Wistar rats served as normotensive controls. Experimental animals were intraperitoneally administered for seven days with nonproteinogenic amino acid L-norvaline (30 mg/kg/day), which is a potent arginase inhibitor, or with the vehicle. Blood pressure (BP), body weight, and diuresis were monitored. The changes in blood and urine levels of creatinine, urea, and NO metabolites were analyzed. We observed a significant decline in BP and induced diuresis in ISIAH rats following the treatment. The same procedure did not affect the BP of control animals. Remarkably, the treatment had no influence upon glomerular filtration rate in two experimental groups, just like the daily excretion of creatinine and urea. Conversely, NO metabolite levels were amplified in normotonic but not in hypertensive rats following the treatment. The data indicate that L-norvaline is a potential antihypertensive agent and deserves to be clinically investigated. Moreover, we suggest that changes in blood and urine are causally related to the effect of L-norvaline upon BP regulation.

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.

Arginase upregulation and eNOS uncoupling contribute to impaired endothelium-dependent vasodilation in a rat model of intrauterine growth restriction

Individuals born after intrauterine growth restriction (IUGR) are at increased risk of developing cardiovascular diseases in adulthood, notably hypertension (HTN). Alterations in the vascular system, particularly impaired endothelium-dependent vasodilation, may play an important role in long-term effects of IUGR. Whether such vascular dysfunction precedes HTN has not been fully established in individuals born after IUGR. Moreover, the intimate mechanisms of altered endothelium-dependent vasodilation remain incompletely elucidated. We therefore investigated, using a rat model of IUGR, whether impaired endothelium-dependent relaxation precedes the development of HTN and whether key components of the l-arginine-nitric oxide (NO) pathway are involved in its pathogenesis. Pregnant rats were fed with a control (CTRL, 23% casein) or low-protein diet (LPD, 9% casein) to induce IUGR. Systolic blood pressure (SBP) was measured by tail-cuff plethysmography in 5- and 8-wk-old male offspring. Aortic rings were isolated to investigate relaxation to acetylcholine, NO production, endothelial NO synthase (eNOS) protein content, arginase activity, and superoxide anion production. SBP was not different at 5 wk but significantly increased in 8-wk-old offspring of maternal LPD (LP) versus CTRL offspring. In 5-wk-old LP versus CTRL males, endothelium-dependent vasorelaxation was significantly impaired but restored by preincubation with l-arginine or the arginase inhibitor S-(2-boronoethyl)-l-cysteine; NO production was significantly reduced but restored by l-arginine pretreatment; total eNOS protein, dimer-to-monomer ratio, and arginase activity were significantly increased; superoxide anion production was significantly enhanced but normalized by pretreatment with the NO synthase inhibitor N^ω-nitro-l-arginine. In this model, IUGR leads to early-impaired endothelium-dependent vasorelaxation, resulting from arginase upregulation and eNOS uncoupling, which precedes the development of HTN.

Nitric oxide production by glomerular podocytes

Nitric Oxide (NO), a potent vasodilator and vital signaling molecule, has been shown to contribute to the regulation of glomerular ultrafiltration. However, whether changes in NO occur in podocytes during the pathogenesis of salt-sensitive hypertension has not yet been thoroughly examined. We showed here that podocytes produce NO, and further hypothesized that hypertensive animals would exhibit reduced NO production in these cells in response to various paracrine factors, which might contribute to the damage of glomeruli filtration barrier and development of proteinuria. To test this, we isolated glomeruli from the kidneys of Dahl salt-sensitive (SS) rats fed a low salt (LS; 0.4% NaCl) or high salt (HS; 4% NaCl, 3 weeks) diets and loaded podocytes with either a combination of NO and Ca²⁺ fluorophores (DAF-FM and Fura Red, respectively) or DAF-FM alone. Changes in fluorescence were observed with confocal microscopy in response to adenosine triphosphate (ATP), angiotensin II (Ang II), and hydrogen peroxide (H₂O₂). Application of Ang II resulted in activation of both NO and intracellular calcium ([Ca²⁺]_i) transients. In contrast, ATP promoted [Ca²⁺]_i transients, but did not have any effects on NO production. SS rats fed a HS diet for 3 weeks demonstrated impaired NO production: the response to Ang II or H₂O₂ in podocytes of glomeruli isolated from SS rats fed a HS diet was significantly reduced compared to rats fed a LS diet. Therefore, glomerular podocytes from hypertensive rats showed a diminished NO release in response to Ang II or oxidative stress, suggesting that podocytic NO signaling is dysfunctional in this condition and likely contributes to the development of kidney injury.

Novel Paradigms of Salt and Hypertension

Salt resistance/sensitivity refers specifically to the effect of dietary sodium chloride (salt) intake on BP. Increased dietary salt intake promotes an early and uniform expansion of extracellular fluid volume and increased cardiac output. To compensate for these hemodynamic changes and maintain constant BP in salt resistance, renal and peripheral vascular resistance falls and is associated with an increase in production of nitric oxide. In contrast, the decline in peripheral vascular resistance and the increase in nitric oxide are impaired or absent in salt sensitivity, promoting an increase in BP in these individuals. Endothelial dysfunction may pose a particularly significant risk factor in the development of salt sensitivity and subsequent hypertension. Vulnerable salt-sensitive populations may have in common underlying endothelial dysfunction due to genetic or environmental influences. These individuals may be very sensitive to the hemodynamic stress of increased effective blood volume, setting in motion untoward molecular and biochemical events that lead to overproduction of TGF-β, oxidative stress, and limited bioavailable nitric oxide. Finally, chronic high-salt ingestion produces endothelial dysfunction, even in salt-resistant subjects. Thus, the complex syndrome of salt sensitivity may be a function of the endothelium, which is integrally involved in the vascular responses to high salt intake.

Exogenous L-arginine attenuates the effects of angiotensin II on renal hemodynamics and the pressure natriuresis-diuresis relationship

Administration of exogenous L-arginine (L-Arg) attenuates angiotensin-II (AngII)-mediated hypertension and kidney disease in rats. The present study assessed renal hemodynamics and pressure diuresis-natriuresis in anaesthetized rats infused with vehicle, AngII (20 ng/kg per min i.v.) or AngII + L-Arg (300 μg/kg per min i.v.). Experiments in isolated aortic rings were carried out to assess L-Arg effects on the vasculature. Increasing renal perfusion pressure (RPP) from ~100 to 140 mmHg resulted in a nine- to tenfold increase in urine flow and sodium excretion rate in control animals. In comparison, AngII infusion significantly reduced renal blood flow (RBF) and glomerular filtration rate (GFR) by 40-42%, and blunted the pressure-dependent increase in urine flow and sodium excretion rate by 54-58% at elevated RPP. Supplementation of L-Arg reversed the vasoconstrictor effects of AngII and restored pressure-dependent diuresis to levels not significantly different from control rats. Dose-dependent contraction to AngII (10(-10) mol/L to 10(-7) mol/L) was observed with a maximal force equal to 27 ± 3% of the response to 10(-5) mol/L phenylephrine. Contraction to 10(-7) mol/L AngII was blunted by 75 ± 3% with 10(-4) mol/L L-Arg. The influence of L-Arg to blunt AngII-mediated contraction was eliminated by endothelial denudation or incubation with nitric oxide synthase inhibitors. Furthermore, the addition of 10(-3) mol/L cationic or neutral amino acids, which compete with L-Arg for cellular uptake, blocked the effect of L-Arg. Anionic amino acids did not influence the effects of L-Arg on AngII-mediated contraction. These studies show that L-Arg blunts AngII-mediated vascular contraction by an endothelial- and nitric oxide synthase-dependent mechanism involving cellular uptake of L-Arg.

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Silencing of HIF prolyl-hydroxylase 2 gene in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats

BACKGROUND

In response to high salt intake, transcription factor hypoxia-inducible factor (HIF) 1α activates many antihypertensive genes, such as heme oxygenase 1 (HO-1) 1 and cyclooxygenase 2 (COX-2) in the renal medulla, which is an important molecular adaptation to promote extra sodium excretion. We recently showed that high salt inhibited the expression of HIF prolyl-hydroxylase 2 (PHD2), an enzyme that promotes the degradation of HIF-1α, thereby upregulating HIF-1α, and that high salt-induced inhibition in PHD2 and subsequent activation of HIF-1α in the renal medulla was blunted in Dahl salt-sensitive hypertensive rats. This study tested the hypothesis that silencing the PHD2 gene to increase HIF-1α levels in the renal medulla attenuates salt-sensitive hypertension in Dahl S rats.

METHODS

PHD2 short hairpin RNA (shRNA) plasmids were transfected into the renal medulla in uninephrectomized Dahl S rats. Renal function and blood pressure were then measured.

RESULTS

PHD2 shRNA reduced PHD2 levels by >60% and significantly increased HIF-1α protein levels and the expression of HIF-1α target genes HO-1 and COX-2 by >3-fold in the renal medulla. Functionally, pressure natriuresis was remarkably enhanced, urinary sodium excretion was doubled after acute intravenous sodium loading, and chronic high salt-induced sodium retention was remarkably decreased, and as a result, salt-sensitive hypertension was significantly attenuated in PHD2 shRNA rats compared with control rats.

CONCLUSIONS

Impaired PHD2 response to high salt intake in the renal medulla may represent a novel mechanism for hypertension in Dahl S rats, and inhibition of PHD2 in the renal medulla could be a therapeutic approach for salt-sensitive hypertension.

Hypertension: physiology and pathophysiology

Despite major advances in understanding the pathophysiology of hypertension and availability of effective and safe antihypertensive drugs, suboptimal blood pressure (BP) control is still the most important risk factor for cardiovascular mortality and is globally responsible for more than 7 million deaths annually. Short-term and long-term BP regulation involve the integrated actions of multiple cardiovascular, renal, neural, endocrine, and local tissue control systems. Clinical and experimental observations strongly support a central role for the kidneys in the long-term regulation of BP, and abnormal renal-pressure natriuresis is present in all forms of chronic hypertension. Impaired renal-pressure natriuresis and chronic hypertension can be caused by intrarenal or extrarenal factors that reduce glomerular filtration rate or increase renal tubular reabsorption of salt and water; these factors include excessive activation of the renin-angiotensin-aldosterone and sympathetic nervous systems, increased formation of reactive oxygen species, endothelin, and inflammatory cytokines, or decreased synthesis of nitric oxide and various natriuretic factors. In human primary (essential) hypertension, the precise causes of impaired renal function are not completely understood, although excessive weight gain and dietary factors appear to play a major role since hypertension is rare in nonobese hunter-gathers living in nonindustrialized societies. Recent advances in genetics offer opportunities to discover gene-environment interactions that may also contribute to hypertension, although success thus far has been limited mainly to identification of rare monogenic forms of hypertension.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

Hyperphosphorylation of Na-K-2Cl Cotransporter in Thick Ascending Limbs of Dahl Salt-Sensitive Rats

Salt-sensitive hypertension involves a renal defect preventing the kidney from eliminating excess NaCl. The thick ascending limb of Henle loop reabsorbs ≈30% of filtered NaCl via the apical Na-K-2Cl cotransporter (NKCC2). Higher NKCC2 activity and Cl reabsorption have been reported in the thick ascending limbs from Dahl salt-sensitive rats (DSS) fed normal salt. NKCC2 activity is primarily regulated by protein trafficking and phosphorylation at Thr 96 /Thr 101 via STE20- and SPS1-related proline and alanine-rich kinases and oxidative stress-responsive kinase 1. However, the mechanism for enhanced NKCC2 activity in DSS is unclear. We hypothesized that DSS exhibit enhanced NKCC2 trafficking and higher NKCC2 phosphorylation compared with Dahl salt-resistant rats on normal salt diet. We measured steady state surface NKCC2 expression and phosphorylation at Thr 96 and Thr 101 by surface biotinylation and Western blot. In DSS, the surface:total NKCC2 ratio was enhanced by 25% compared with Dahl salt-resistant rats ( P <0.05) despite lower NKCC2 expression. Total NKCC2 phosphorylation at Thr 96 and Thr 101 was enhanced ≈5-fold in DSS thick ascending limbs. Moreover, total STE20- and SPS1-related proline and alanine-rich kinases expression, kidney-specific STE20- and SPS1-related proline and alanine-rich kinases, and oxidative stress-responsive kinase 1 were not different between strains, although STE20- and SPS1-related proline and alanine-rich kinases/oxidative stress-responsive kinase 1 phosphorylation was enhanced by 60% ( P <0.05) in DSS rats, suggesting increased activity. We concluded that phosphorylation of NKCC2 Thr 96 and Thr 101 and surface:total NKCC2 ratio are enhanced in DSS rats. These differences in NKCC2 may be, in part, responsible for higher NKCC2 activity and abnormally enhanced thick ascending limb NaCl reabsorption in DSS rats.

The antihypertensive effect of arginine

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at increased risk of stroke, heart disease and kidney failure. Although the etiology of essential hypertension has a genetic component, lifestyle factors such as diet play an important role. Reducing dietary salt is effective in lowering blood pressure in salt-sensitive individuals. Insulin resistance and altered glucose metabolism are common features of hypertension in humans and animal models, with or without salt sensitivity. Altered glucose metabolism leads to increased formation of advanced glycation end products. Insulin resistance is also linked to oxidative stress, and alterations in the nitric oxide pathway and renin angiotensin system. A diet rich in protein containing the semiessential amino acid, arginine, and arginine treatment, lowers blood pressure in humans and in animal models. This may be due to the ability of arginine to improve insulin resistance, decrease advanced glycation end products formation, increase nitric oxide, and decrease levels of angiotensin II and oxidative stress, with improved endothelial cell function and decreased peripheral vascular resistance. The Dietary Approaches to Stop Hypertension (DASH) study demonstrated that the DASH diet, rich in vegetables, fruits and low-fat dairy products; low in fat; and including whole grains, poultry, fish and nuts, lowered blood pressures even more than a typical North American diet with similar reduced sodium content. The DASH diet is rich in protein; the blood pressure-lowering effect of the DASH diet may be due to its higher arginine-containing protein, higher antioxidants and low salt content.

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.

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Statins reverse renal inflammation and endothelial dysfunction induced by chronic high salt intake

High salt intake (HS) is a risk factor for cardiovascular and kidney disease. Indeed, HS may promote blood-pressure-independent tissue injury via inflammatory factors. The lipid-lowering 3-hydroxy 3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors exert beneficial lipid-independent effects, reducing the expression and synthesis of inflammatory factors. We hypothesized that HS impairs kidney structure and function in the absence of hypertension, and these changes are reversed by atorvastatin. Four groups of rats were treated for 6 wk in metabolic cages with their diets: normal salt (NS); HS, NS plus atorvastatin and HS plus atorvastatin. We measured basal and final body weight, urinary sodium and protein excretion (UProtV), and systolic blood pressure (SBP). At the end of the experimental period, cholesterolemia, creatinine clearance, renal vascular reactivity, glomerular volume, cortical and glomerular endothelial nitric oxide synthase (eNOS), and transforming growth factor (TGF)-β1 expression were measured. We found no differences in SBP, body weight, and cholesterolemia. HS rats had increased creatinine clearence, UProtV, and glomerular volume at the end of the study. Acetylcholine-induced vasodilatation decreased by 40.4% in HS rats ( P < 0.05). HS decreased cortical and glomerular eNOS and caused mild glomerular sclerosis, interstitial mononuclear cell infiltration, and increased cortical expression of TGF-β1. All of these salt-induced changes were reversed by atorvastatin. We conclude that long-term HS induces inflammatory and hemodynamic changes in the kidney that are independent of SBP. Atorvastatin corrected all, suggesting that the nitric oxide-oxidative stress balance plays a significant role in the earlier stages of salt induced kidney damage.

Vascular consequences of dietary salt intake

Animal and human studies support an untoward effect of excess dietary NaCl (salt) intake on cardiovascular and renal function and life span. Recent work has promoted the concept that the endothelium, in particular, reacts to changes in dietary salt intake through a complex series of events that are independent of blood pressure and the renin-angiotensin-aldosterone axis. The cellular signaling events culminate in the intravascular production of transforming growth factor-beta (TGF-beta) and nitric oxide in response to increased salt intake. Plasticity of the endothelium is integral in the vascular remodeling consequences associated with excess salt intake, because nitric oxide serves as a negative regulator of TGF-beta production. Impairment of nitric oxide production, such as occurs with endothelial dysfunction in a variety of disease states, results in unopposed excess vascular TGF-beta production, which promotes reduced vascular compliance and augmented peripheral arterial constriction and hypertension. Persistent alterations in vascular function promote the increase in cardiovascular events and reductions in renal function that reduce life span during increased salt intake.

Dietary salt activates an endothelial proline-rich tyrosine kinase 2/c-Src/phosphatidylinositol 3-kinase complex to promote endothelial nitric oxide synthase phosphorylation

Although many laboratories have shown that dietary NaCl (salt) intake increases NO production in rodents and humans, the mechanism has not been uncovered. In the present study, pharmacological and dominant-negative strategies were used to show that feeding a formulated diet containing increased amounts of salt to young male Sprague-Dawley rats induced the formation of an endothelial cell-signaling complex that contained proline-rich tyrosine kinase 2, c-Src (also known as pp60(c-src)), and phosphatidylinositol 3-kinase. In the setting of a high-salt diet, proline-rich tyrosine kinase 2 served as the scaffold for c-Src-mediated phosphatidylinositol 3-kinase activation. Phosphatidylinositol 3-kinase was the upstream activator of protein kinase B (Akt), which was responsible for phosphorylation of the rat endothelial isoform of NO synthase at S1176 and thereby promoted the increase in NO production. The combined findings illustrated the crucial role for a proline-rich tyrosine kinase 2-signaling complex in the endothelial response to salt intake.

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

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

Nitric oxide and superoxide interactions in the kidney and their implication in the development of salt-sensitive hypertension

1. Enhanced superoxide (O2(-)) activity as a result of the inhibition of the superoxide dismutase (SOD) enzyme results in vasoconstrictor and antinatriuretic responses in the canine kidney; these responses were shown to be greatly enhanced during inhibition of nitric oxide synthase (NOS). Glomerular filtration rate remained mostly unchanged during SOD inhibition in the intact nitric oxide (NO) condition, but was markedly reduced during NOS inhibition. These findings indicate that endogenous NO has a major renoprotective effect against O2(-) by acting as an anti-oxidant. Nitric oxide synthase inhibition was also shown to enhance endogenous O2(-) activity. 2. Experiments in our laboratory using dogs, rats and gene knockout mice have shown that renal vasoconstrictor and antinatriuretic responses to acute or chronic angiotensin (Ang) II administration are mediated, in part, by O2(-) generation. In the absence of NO, enhanced O2(-) activity largely contributes to AngII-induced renal tubular sodium reabsorption. Acute or chronic treatment with the O2(-) scavenger tempol in experimental models of hypertension (induced by chronic low-dose treatment with AngII and NO inhibitors) causes an improvement in renal haemodynamics and in excretory function, abolishes salt sensitivity and reduces blood pressure. 3. The present mini review also discusses related studies from many other laboratories implicating a role for O2(-) and its interaction with NO in the development of salt-sensitive hypertension. 4. Overall, the collective data support the hypothesis that an imbalance between the production of NO and O2(-) in the kidney primarily determines the condition of oxidative stress that alters renal haemodynamics and excretory function leading to sodium retention and, thus, contributes to the development of salt-sensitive hypertension.

Amino acids as modulators of endothelium-derived nitric oxide

To examine the mechanisms whereby amino acids modulate nitric oxide (NO) production and blood flow in the renal vasculature, chemiluminescence techniques were used to quantify NO in the renal venous effluent of the isolated, perfused rat kidney as different amino acids were added to the perfusate. The addition of 10−4or 10−3M cationic amino acids (l-ornithine, l-lysine, or l-homoarginine) or neutral amino acids (l-glutamine, l-leucine, or l-serine) to the perfusate decreased NO and increased renal vascular resistance. Perfusion with anionic amino acids (l-glutamate or l-aspartate) had no effect on either parameter. The effects of the cationic and neutral amino acids were reversed with 10−3M l-arginine and prevented by deendothelialization or NO synthase inhibition. The effects of the neutral amino acids but not the cationic amino acids were dependent on extracellular sodium. Cationic and neutral amino acids also decreased calcimycin-induced NO, as assessed by DAF-FM-T fluorescence, in cultured EA.hy926 endothelial cells. Inhibition of system y+or y+L by siRNA for the cationic amino acid transporter 1 or the CD98/4F2 heavy chain diminished the NO-depleting effects of these amino acids. Finally, transport studies in cultured cells demonstrated that cationic or neutral amino acids in the extracellular space stimulate efflux of l-arginine out of the cell. Thus the present experiments demonstrate that cationic and neutral amino acids can modulate NO production in endothelial cells by altering cellular l-arginine transport through y+and y+L transport mechanisms.

l-Arginine uptake affects nitric oxide production and blood flow in the renal medulla

Experiments were performed to determine whether l-arginine transport regulates nitric oxide (NO) production and hemodynamics in the renal medulla. The effects of renal medullary interstitial infusion of cationic amino acids, which compete with l-arginine for cellular uptake, on NO levels and blood flow in the medulla were examined in anesthetized rats. NO concentration in the renal inner medulla, measured with a microdialysis-oxyhemoglobin trapping technique, was significantly decreased by 26–44% and renal medullary blood flow, measured by laser Doppler flowmetry, was significantly reduced by 20–24% during the acute renal medullary interstitial infusion of l-ornithine, l-lysine, and l-homoarginine (1 μmol·kg−1·min−1each; n = 6–8/group). In contrast, intramedullary infusion of l-arginine increased NO concentration and medullary blood flow. Flow cytometry experiments with 4-amino-5-methylamino-2′,7′-difluorescein diacetate, a fluorophore reactive to intracellular NO, demonstrated that l-ornithine, l-lysine, and l-homoarginine decreased NO by 54–57% of control, whereas l-arginine increased NO by 21% in freshly isolated inner medullary cells (1 mmol/l each, n > 1,000 cells/experiment). The mRNA for the cationic amino acid transporter-1 was predominantly expressed in the inner medulla, and cationic amino acid transporter-1 protein was localized by immunohistochemistry to the collecting ducts and vasa recta in the inner medulla. These results suggest that l-arginine transport by cationic amino acid transport mechanisms is important in the production of NO and maintenance of blood flow in the renal medulla.

Identification of salt-sensitive genes in the kidneys of Dahl rats

OBJECTIVE

Inherited differences in renal function underlie the effect of high salt diets on blood pressure in Dahl rats. We probed the kidneys of inbred Dahl SS/Jr and SR/Jr for anonymous and candidate genes whose expression was regulated by dietary sodium.

METHODS

mRNA quantitation of both candidate genes implicated in sodium excretion and anonymous gene products found by differential hybridization in the kidneys of salt-resistant (SR) and salt sensitive (SS) inbred Dahl rats on high and low salt diets for 21 days.

RESULTS

Differential screening revealed a cDNA clone (H1) that showed increased dietary salt-dependent expression only in SS rats. Sequencing of the H1 cDNA showed it was the Dahl rat homologue to a perchloric acid soluble protein expressed in liver and kidney. Among candidate genes, transcript levels of arginosuccinate synthetase (AS) and arginosuccinate lyase (AL) were higher in SS on low salt diets, and AS mRNA increased in response to a high salt diet in SR. Renal mRNA for the ANP-A and the vasopressin type II receptors did not differ by strain or dietary conditions.

CONCLUSIONS

Three new salt-sensitive genes were detected in the kidneys of inbred Dahl rats. Two genes encode enzymes in the biosynthesis of L-arginine. The upregulation of these genes by dietary salt indicates increased demand and biosynthesis of L-arginine in Dahl SS rats. A third gene encodes a small acid-soluble protein thought to influence the transcription/translation of numerous genes. Further studies will be needed to determine the nature of the association of these genes with salt-sensitivity and blood pressure.

L-arginine attenuates hypertension in pregnant rats with reduced uterine perfusion pressure

A chronic reduction in uterine perfusion pressure in the pregnant rat is associated with significant elevations in mean arterial pressure, proteinuria, and reductions in kidney function as is chronic nitric oxide blockade, suggesting that nitric oxide deficiency may contribute to the clinical manifestations of preeclampsia. The purpose of this study was to determine whether supplementation with L-arginine, the precursor for nitric oxide, attenuates the hypertension produced in response to a chronic reduction in uterine perfusion pressure in the pregnant rat. Reduced uterine perfusion was initiated at day 14 of gestation with arterial pressure determined at day 19 of gestation in conscious, chronically instrumented rats. Arterial pressure was significantly elevated in pregnant rats with chronic reductions in uterine perfusion as compared with pregnant control rats (132+/-2 versus 109+/-2 mm Hg, P<0.01, respectively). Treatment with L-arginine (2%) in the drinking water was initiated at day 10 of gestation. l-arginine supplementation resulted in a significant decrease in arterial pressure in both pregnant rats with reduced uterine perfusion pressure (113+/-2 mm Hg treated, P<0.01 versus untreated pregnant with reduced uterine perfusion pressure) and pregnant control (97+/-3 mm Hg treated, P<0.01 versus untreated pregnant) rats. However, supplementation with L-arginine decreased blood pressure by 19 mm Hg in pregnant with reduced uterine perfusion pressure (untreated versus treated) as compared with 12 mm Hg in pregnant (untreated versus treated) rats. Thus, these results suggest that l-arginine supplementation may be beneficial in attenuating the hypertension in preeclampsia.

Influence of dietary NaCl on L-arginine transport in the renal medulla

Previous work demonstrated that l-arginine, the substrate for nitric oxide (NO) synthase, is carried into inner medullary collecting duct (IMCD) cells via system y+, that the major system y+ gene product in IMCD is the cationic amino acid transporter 1 (CAT1), and that blockade of l-arginine uptake in the renal medulla decreases NO and leads to systemic hypertension. The present study determined the influence of dietary sodium intake on l-arginine uptake in IMCD, on CAT1 immunoreactive protein in the renal medulla, and on the hypertensive response to blockade of l-arginine uptake in the renal medulla. Transport studies in bulk-isolated IMCD demonstrated that l-arginine uptake by IMCD was significantly greater (663 ± 100 pmol·mg-1· min-1, n = 6) in rats exposed to a low-sodium diet (0.4% NaCl) compared with rats on a normal (1% NaCl, 519 ± 78 pmol·mg-1·min-1, n = 6) or high-sodium diet (4.0% NaCl, 302 ± 27 pmol·mg-1·min-1, n = 6). Immunoblotting experiments demonstrated that CAT1 immunoreactive protein was significantly decreased by ∼30% in rats maintained on a high-NaCl diet ( n = 5) compared with rats on a low-NaCl diet ( n = 5). In contrast to the l-arginine transport and immunoblotting data, in vivo blockade of l-arginine uptake led to hypertension of equal magnitude in rats maintained on a low- or high-NaCl diet. These results indicate that sodium loading leads to a decrease in immunoreactive CAT1 protein in the rat renal medulla, resulting in decreased l-arginine uptake capacity. The decrease in l-arginine uptake capacity, however, does not alter the blood pressure response to l-arginine uptake inhibition in the renal medulla.

Role of renal NO production in the regulation of medullary blood flow

The unique role of nitric oxide (NO) in the regulation of renal medullary function is supported by the evidence summarized in this review. The impact of reduced production of NO within the renal medulla on the delivery of blood to the medulla and on the long-term regulation of sodium excretion and blood pressure is described. It is evident that medullary NO production serves as an important counterregulatory factor to buffer vasoconstrictor hormone-induced reduction of medullary blood flow and tissue oxygen levels. When NO synthase (NOS) activity is reduced within the renal medulla, either pharmacologically or genetically [Dahl salt-sensitive (S) rats], a super sensitivity to vasoconstrictors develops with ensuing hypertension. Reduced NO production may also result from reduced cellular uptake of l-arginine in the medullary tissue, resulting in hypertension. It is concluded that NO production in the renal medulla plays a very important role in sodium and water homeostasis and the long-term control of arterial pressure.

Contributions of 20-HETE to the antihypertensive effects of Tempol in Dahl salt-sensitive rats

The present study evaluated whether reactive oxygen species-induced alterations in bioavailability of 20-HETE in the kidney contribute to the antihypertensive and renoprotective actions of antioxidant therapy with Tempol in the Dahl salt-sensitive (DS) rat. Superoxide inhibited the synthesis of 20-HETE by renal cortical microsomes and enhanced breakdown of 20-HETE to a more polar product. Addition of Tempol (1 mmol/L) to the drinking water reduced mean arterial pressure from 187+/-9 to 160+/-3 mm Hg in DS rats fed an 8%-NaCl diet for 2 weeks. 20-HETE excretion rose from 117+/-11 to 430+/-45 ng/day, and 8-isoprostane excretion fell from 14+/-1 to 8+/-1 ng/day. Tempol also increased creatinine clearance and reduced the severity of renal damage in DS rats fed a high-salt diet. Blockade of NO synthase with NG-nitro-L-arginine methyl ester (25 mg/kg per day) did not attenuate the antihypertensive or renoprotective actions of Tempol in DS rats. However, chronic blockade of the formation of 20-HETE with N-hydroxy-N'-(4-butyl-2 methylphenyl) formamidine (HET0016, 10 mg/kg per day) blunted the antihypertensive and renoprotective effects of Tempol. These findings indicate that the antihypertensive and renoprotective effects of reducing oxidative stress with Tempol depends in part on increasing the bioavailability of 20-HETE in the kidney.

Renal medullary nitric oxide deficit of Dahl S rats enhances hypertensive actions of angiotensin II

Studies were designed to examine the hypothesis that the renal medulla of Dahl salt-sensitive (Dahl S) rats has a reduced capacity to generate nitric oxide (NO), which diminishes the ability to buffer against the chronic hypertensive effects of small elevations of circulating ANG II. NO synthase (NOS) activity in the outer medulla of Dahl S rats (arginine-citrulline conversion assay) was significantly reduced. This decrease in NOS activity was associated with the downregulation of protein expression of NOS I, NOS II, and NOS III isoforms in this region as determined by Western blot analysis. In anesthetized Dahl S rats, we observed that a low subpressor intravenous infusion of ANG II (5 ng. kg(-1). min(-1)) did not increase the concentration of NO in the renal medulla as measured by a microdialysis with oxyhemoglobin trapping technique. In contrast, ANG II produced a 38% increase in the concentration of NO (87 +/- 8 to 117 +/- 8 nmol/l) in the outer medulla of Brown-Norway (BN) rats. The same intravenous dose of ANG II reduced renal medullary blood flow as determined by laser-Doppler flowmetry in Dahl S, but not in BN rats. A 7-day intravenous ANG II infusion at a dose of 3 ng. kg(-1). min(-1) did not change mean arterial pressure (MAP) in the BN rats but increased MAP in Dahl S rats from 120 +/- 2 to 138 +/- 2 mmHg (P < 0.05). ANG II failed to increase MAP after NO substrate was provided by infusion of L-arginine (300 microg. kg(-1). min(-1)) into the renal medulla of Dahl S rats. Intravenous infusion of L-arginine at the same dose had no effect on the ANG II-induced hypertension. These results indicate that an impaired NO counterregulatory system in the outer medulla of Dahl S rats makes them more susceptible to the hypertensive actions of small elevations of ANG II.

Sodium chloride loading does not alter endothelium-dependent vasodilation of forearm vasculature in either salt-sensitive or salt-resistant patients with essential hypertension

The purpose of this study was to determine whether a high NaCl intake impairs endothelium-dependent and -independent vasodilation of forearm circulation in salt sensitive (SS) patients with essential hypertension. We evaluated the effects of intra-arterial acetylcholine (ACh) and isosorbide dinitrate (ISDN) on forearm hemodynamics in 29 patients with essential hypertension, while consuming a low NaCl (50 mmol/d) or high Na Cl (340 mmol/d) diet for 1 week. The forearm blood flow (FBF) was measured by strain-gauge plethysmography. Patients were classified as SS (n=12) or salt resistant (SR; n=17) based on salt-induced changes in blood pressures. The FBF responses of ACh and ISDN were similar in the SS and SR patients while on either NaCl diet, and was not altered by salt loading (ACh, SS: low NaCl 22.8+/-4.3 vs. high NaCl 21.1+/-3.6 ml/min per 100 ml, SR: low NaCl 22.5+/-4.0 vs. high NaCl 23.3+/-4.1 ml/min per 100 ml; ISDN, SS: low NaCl 13.9+/-2.1 vs. high NaCl 14.1+/-2.2 ml/min per 100 ml, SR: low NaCl 13.8+/-2.3 vs. high NaCl 14.0+/-2.2 ml/min per 100 ml). There were no significant differences in the vascular responses to ACh and ISDN in the presence of N(G)-monomethyl-L-arginine, a nitric oxide synthase inhibitor, in either group for either NaCl diet. These findings suggest that forearm resistance artery endothelial function may not be influenced by salt loading in either SS patients which finding may play a role in determining salt sensitivity in patients with essential hypertension or SR patients.

Gender differences in the attenuation of salt-induced hypertension by angiotensin (1-7)

Chronic infusion of angiotensin (1-7) [Ang-(1-7)] lowers blood pressure in spontaneously hypertensive rats (SHR). To assess the role of Ang-(1-7) in salt-induced hypertension, Ang-(1-7) (24 microg/kg/hr) or saline was administered chronically via osmotic minipump into the jugular vein of 5-6 wk-old male (M) and female (F) Dahl salt-sensitive rats placed on a high-salt (8% NaCl) diet for 2 weeks. Blood pressure (BP) and heart rate were measured prior to the start of the diet and weekly thereafter. Ang-(1-7) significantly attenuated the BP increase after 1 wk on the diet in both M and F rats, but after 2 weeks only in F rats. Enhanced release of prostacyclin, (6-keto PGF1 alpha), following Ang-(1-7) treatment was observed in both M and F rats. In addition, significant increases in aortic blood flow and plasma levels of nitric oxide were observed in the F rats following Ang-(1-7) treatment. These findings demonstrate that the reduction in BP is due to both prostacyclin and NO and that there is a gender difference in the attenuation of salt-induced hypertension by Ang-(1-7).

The role of gender in salt-induced hypertension

To evaluate gender differences in salt-induced hypertension, female and male Dahl salt-sensitive rats were fed high (8.0% NaCl, HS) and low (0.3% NaCl, LS) salt diets. During a 3-week treatment period, blood pressure was significantly elevated in both female and male HS groups compared to their respective LS groups. The blood pressure and 4 week mortality rate of the female HS group, however, were significantly lower than those of the male HS group. Renal and aortic blood flows were reduced in male rats on HS diet compared to the LS group, while, in females, renal blood flow was elevated and aortic flow was maintained while on HS diet. Plasma prostaglandin E2 and prostacyclin levels were higher in females than males and unaffected by diet. In contrast, plasma nitric oxide levels were reduced by HS, regardless of gender. In isolated aortic rings, HS diet caused a smaller elevation in the stimulated norepinephrine release ratio in female rats than in males. Thus, salt-induced hypertension is associated with a reduction in levels of nitric oxide regardless of gender. Plasma prostaglandin E2 and prostacyclin levels were higher in females. Taken together, the higher plasma prostaglandin levels and reduced sympathetic activity in females may be contributing factors in their lower blood pressure and reduced mortality.

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.

Characterization of L-arginine transporters in rat renal inner medullary collecting duct

Previous work from our laboratory has demonstrated that the inner medullary collecting duct (IMCD) expresses a large amount of nitric oxide synthase (NOS) activity. The present study was designed to characterize the transport of NOS substrate, L-arginine, in a suspension of bulk-isolated IMCD cells from the Sprague-Dawley rat kidney. Biochemical transport studies demonstrated an L-arginine transport system in IMCD cells that was saturable and Na(+) independent (n = 6). L-Arginine uptake by IMCD cells was inhibited by the cationic amino acids L-lysine, L-homoarginine, and L-ornithine (10 mmol/l each) and unaffected by the neutral amino acids L-leucine, L-serine, and L-glutamine. Both L-ornithine (n = 6) and L-lysine (n = 6) inhibited NOS enzymatic activity in a dose-dependent manner in IMCD cells, supporting the important role of L-arginine transport for NO production by this tubular segment. Furthermore, RT-PCR of microdissected IMCD confirmed the presence of cationic amino acid transporter CAT1 mRNA, whereas CAT2A, CAT2B, and CAT3 were not detected. These results indicate that L-arginine uptake by IMCD cells occurs via system y(+), is encoded by CAT1, and may participate in the regulation of NO production in this renal segment.

Blood pressure and metabolic changes during dietary L-arginine supplementation in humans

Dietary L-arginine supplementation has been proposed to reverse endothelial dysfunction in such diverse pathophysiologic conditions as hypercholesterolemia, coronary heart disease, and some forms of animal hypertension. In particular, chronic oral administration of L-arginine prevented the blood pressure rise induced by sodium chloride loading in salt-sensitive rats. To investigate the effects of L-arginine-rich diets on blood pressure and metabolic and coagulation parameters we performed a single-blind, controlled, crossover dietary intervention in six healthy volunteers. The subjects (aged 39+/-4 years, body mass index [BMI] 26+/-1 kg/m2, mean +/- SEM) received, in random sequence, three different isocaloric diets, each for a period of 1 week (Diet 1: control; Diet 2: L-arginine enriched by natural foods; Diet 3: identical to Diet 1 plus oral L-arginine supplement). Sodium intake was set at a constant level (about 180 mmol/day) throughout the three study periods. A blood pressure decrease was observed with both L-arginine-rich diets (Diet 2 v 1, SBP: -6.2 mm Hg [95% CI: -0.5 to -11.8], DBP: -5.0 mm Hg [-2.8 to -7.2]; Diet 3 v 1, SBP: -6.2 mm Hg [-1.8 to -10.5], DBP: -6.8 mm Hg [-3.0 to -10.6]). A slight increase in creatinine clearance (P = .07) and a fall in fasting blood glucose (P = .008) occurred after Diet 3 and, to a lesser extent, after Diet 2. Serum total cholesterol (P = .06) and triglyceride (P = .009) decreased and HDL cholesterol increased (P = .04) after Diet 2, but not after Diet 3. These results indicate that a moderate increase in L-arginine significantly lowered blood pressure and affected renal function and carbohydrate metabolism in healthy volunteers.

Effects of sodium intake on the pressor and renal responses to nitric oxide synthesis inhibition in normotensive individuals with different sodium sensitivity

OBJECTIVE

The present study evaluated the role of nitric oxide (NO) in the systemic vascular and renal adaptation to changes in dietary sodium intake.

DESIGN AND METHODS

Seven healthy normotensive male subjects were randomized to high or low sodium diets in a double blind crossover design (7 days on each diet). The NO synthesis inhibitor, NGmonomethyl-L-arginine (L-NMMA) was infused systemically (1.8 mg/kg over 30 min) at the end of each dietary period and its effects on blood pressure, renal plasma flow, glomerular filtration rate, urinary flow rate and sodium excretion were measured.

RESULTS

Blood pressure increased in response to L-NMMA on a high sodium diet only (area under time curve percentage change in mean blood pressure, low sodium = -94.5 +/- 164.3; high sodium = 391.1 +/- 228.6; P < 0.05 low versus high). The increase in blood pressure was directly and significantly associated with the individual salt sensitivity, defined by the difference in systemic mean blood pressure between high and low sodium diets (r = 0.756; P < 0.05). L-NMMA also reduced renal plasma flow and urinary flow rate in subjects on high sodium diet.

CONCLUSIONS

The data support a significant influence of endogenous NO in the systemic and renal vascular adaptation to a high sodium diet in normotensive men. In addition, the direct association between the individual sodium-sensitivity and the pressor response to L-NMMA suggests that there is increased dependence of vascular tone on NO in normotensive subjects whose blood pressure is more sodium sensitive.

Abnormal pressure-natriuresis in hypertension: role of nitric oxide

The kidneys have a critical role in long-term control of arterial pressure by regulating extracellular fluid and plasma volume. According to the renal body fluid feedback mechanism for long-term control, persistent hypertension can only occur as a result of a reduction in renal sodium excretory function or a hypertensive shift in the pressure-natriuresis relationship. Although an abnormal relationship between renal perfusion pressure and renal sodium excretion has been identified in every type of hypertension where it has been sought, factors responsible for this effect are still unclear. Nitric oxide (NO) is produced within the kidney and plays an important role in the control of many intrarenal processes which regulate the renal response to changes in perfusion pressure and thus, help determine plasma volume and blood pressure. Numerous studies have shown that long-term inhibition of NO synthesis results in a chronic rightward shift and marked attenuation in renal pressure-natriuresis. Recent studies have shown that certain animal models of genetic hypertension and forms of human hypertension areas are associated with a decrease in NO synthesis. Reductions in NO synthesis reduces renal sodium excretory function not only through direct actions on the renal vasculature, but through modulation of other vasoconstrictor processes and through direct and indirect alterations in tubular sodium transport. The causes and consequences of the dysregulation of NO in hypertension and other renal disease processes remain an important area of investigation.

Role of female sex hormones in the development and reversal of dahl hypertension

Female sex hormones protect against the development of Dahl hypertension mediated by increases in dietary sodium. The role of female sex hormones in the reversal of Dahl hypertension mediated by decreases in dietary sodium is unknown. The goal of this study was to identify sex differences in the reversal of Dahl hypertension and the associated changes in water and electrolyte homeostasis. Male (M, n=8), female (F, n=8), and ovariectomized female (OVX, n=9) Dahl salt-sensitive rats were instrumented with an abdominal radiotelemetry device for 24-hour monitoring of blood pressure (BP) and heart rate. Daily measurements of food intake, water intake, and urine output were recorded as diet was changed from a low-sodium diet (0.15% NaCl) to a diet containing 8% NaCl. The diet was then changed back to 0.15% NaCl. The responses to changes in the salt diet were compared with responses observed in rats (M, n=4; F, n=4; OVX, n=4) that were maintained on 0.15% NaCl during the experiment. Sex differences in BP were observed when M, F, and OVX rats were fed 8% NaCl diet for 2 weeks (152+/-4, 141+/-3, and 154+/-5 mm Hg, respectively). BP was significantly greater (P<0.05) in M and OVX rats than in F rats. Fluid balance (water intake minus urine volume) and sodium balance (sodium intake minus sodium excretion) were similar in all groups on the 8% NaCl diet. BP in time-control M, F, and OVX rats was 121+/-3, 130+/-4, and 162+/-11 mm Hg, respectively. Compared with time-control groups, differences in BP while rats were eating the 8% NaCl diet were observed in M and F rats but not OVX rats. Reinstatement of an NaCl-deficient diet reversed the hypertension in M and F but not OVX rats (124+/-4, 124+/-2, and 145+/-5 mm Hg, respectively). The changes in dietary sodium caused similar changes in renal handling of sodium and water in all groups of rats; therefore, the effect on blood pressure was independent of renal excretory function. The inability to reverse the hypertension by decreasing sodium intake in OVX rats and the development of spontaneous hypertension in OVX females maintained on a low-sodium diet indicates that removal of the female sex hormones predisposes the animal to the development of hypertension that is sodium independent. We conclude that female sex hormones protect Dahl S rats against the development of sodium-dependent and -independent hypertension.

Inhibitory effect of nitric oxide on the renin-angiotensin system in Dahl salt-sensitive rats

1. To explore the role of nitric oxide (NO) in the regulation of the renin-angiotensin system (RAS) in Dahl salt-sensitive (DS) rats, the effects of NG-nitro-L-arginine methyl ester (L-NAME) on plasma renin activity (PRA), and concentrations of angiotensin (Ang)I and AngII in the plasma, aorta and kidney were investigated in DS and Dahl salt-resistant (DR) rats. 2. NG-Nitro-L-arginine methyl ester (12-18 mg/kg per day) administration for 1 week increased mean arterial pressure (MAP) in DS and DR rats fed a 0.3% NaCl diet and in DR rats fed an 8% NaCl diet compared with corresponding vehicle (water)-treated groups. However, L-NAME administration did not change MAP in DS rats fed an 8% NaCl diet. 3. NG-Nitro-L-arginine methyl ester administration increased PRA in DS rats fed an 8% NaCl diet, but not in DR rats fed an 8% NaCl diet. NG-Nitro-L-arginine methyl ester administration increased AngI and AngII concentrations in plasma, aorta and kidney only in DS rats fed an 8% NaCl diet. The ratio of AngI to AngII did not change following L-NAME administration in any rats. 4. These results suggest that NO has an inhibitory role on renin release in DS rats fed a high-salt diet.

Endothelial nitric oxide gene knockout mice: cardiac phenotypes and the effect of angiotensin-converting enzyme inhibitor on myocardial ischemia/reperfusion injury

We tested the hypothesis that nitric oxide (NO) released by endothelial NO synthase (eNOS) is not only important in blood pressure regulation but also involved in cardiac function and remodeling and in the cardioprotective effect of angiotensin-converting enzyme inhibitors (ACEi). With the use of a 2D Doppler echocardiography system equipped with a 15-MHz linear transducer, we evaluated left ventricular (LV) morphology and function in conscious eNOS knockout mice (eNOS(-/-); n=15) and their wild-type littermates (eNOS(+/+); n=16). We also studied whether in eNOS(-/-) mice (1) myocardial ischemia/reperfusion injury is more severe and (2) the cardioprotective effect of ACEi is diminished or absent. In comparison with the wild type, eNOS(-/-) mice had significantly increased systolic blood pressure (128+/-3 versus 108+/-5 mm Hg; P<0.001) and decreased heart rate (531+/-22 versus 629+/-18 bpm; P<0.001) associated with increased LV posterior wall thickness (0.80+/-0.04 versus 0.64+/-0.02 mm; P<0.001) and LV mass (18.3+/-0.9 versus 13.1+/-0.5 mg/10 g body weight; P<0.01). Despite hypertension and LV hypertrophy, LV chamber dimension, shortening fraction and ejection fraction (indicators of LV contractility), and cardiac output did not differ between the 2 strains, which indicates that LV function in eNOS(-/-) mice is well compensated. We also found that in eNOS(+/+) mice, ACEi decreased the ratio of myocardial infarct size to area at risk from 62.7+/-3.9% to 36.3+/-1.6% (P<0. 001), whereas in eNOS(-/-) mice this effect of ACEi was almost abolished: the ratio of myocardial infarct size to area at risk was 67.2+/-2.9% in the vehicle-treated group and 62.7+/-3.9% in mice treated with ACEi. Moreover, infarct size in vehicle-treated eNOS(-/-) mice was not significantly different from eNOS(+/+) mice given the same treatment. We concluded that (1) endothelium-derived NO plays an important role in the regulation of blood pressure homeostasis; (2) NO released under basal conditions has no significant impact on cardiac function; and (3) ACEi protect the heart against ischemia/reperfusion injury in mice and that this effect is mediated in part by endothelium-derived NO.

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.

Prolonged L-arginine on cardiovascular mass and myocardial hemodynamics and collagen in aged spontaneously hypertensive rats and normal rats

This study was designed to examine whether L-arginine could prevent hypertension- and age-related impairment of coronary hemodynamics and cardiac fibrosis in aged (80-week-old) rats. To differentiate between hypertension- and age-related changes, the study was performed in both normotensive Wistar-Kyoto rats (WKYs) and spontaneously hypertensive rats (SHR). Male 1-year-old rats of both strains were divided into 2 groups and given either placebo or L-arginine (1.2 g/L) in drinking water. After 6 months, systemic and coronary hemodynamics (radionuclide-labeled microspheres), right and left ventricular and aortic mass indexes, and ventricular hydroxyproline (an estimate of collagen) concentrations were determined. In the aged WKYs, L-arginine did not affect any of the examined variables except slightly reducing total peripheral resistance. In contrast, L-arginine diminished arterial pressure, total peripheral resistance, and left ventricular and aortic mass indexes in the SHRs; it also increased coronary flow reserve and reduced minimal coronary flow resistance and myocardial hydroxyproline concentration. These findings demonstrated that L-arginine ameliorated adverse cardiovascular effects of hypertension in aged SHRs, as demonstrated by reduced arterial pressure and total peripheral resistance, diminished left ventricular mass and collagen content, and improved coronary hemodynamics. There were no important effects in the old WKYs.

Role of nitric oxide in vascular hyper-responsiveness to norepinephrine in hypertensive Dahl rats

OBJECTIVE

To determine whether the abnormal vascular responses observed in salt-sensitive hypertension are caused by an impairment in vascular nitric oxide function.

DESIGN

Isometric tension was measured in aortic rings isolated from Dahl salt-sensitive and salt-resistant rats fed a regular-salt (0.4% NaCl) or a high-salt (8% NaCl) diet, with and without inhibition of endogenous nitric oxide synthesis.

METHODS AND RESULTS

Systolic arterial pressure, measured weekly by the tail-cuff method, increased markedly in DS rats with a high-salt diet but did not increase in the other groups. In aortic rings, norepinephrine evoked dose-dependent contractions which were significantly increased in rings from DS rats with a high-salt diet Pretreatment with Nomega-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, increased the norepinephrine-induced contraction in all groups and abolished differences in contractile responses between high-salt DS rats and the other groups. Acetylcholine induced endothelium-dependent relaxation, which was significantly depressed in high-salt DS rats. L-NAME attenuated the acetylcholine-induced relaxation in all groups and abolished the difference in relaxation response between high-salt DS rats and the other groups. Sodium nitroprusside-induced relaxation was significantly depressed in high-salt DS rats.

CONCLUSIONS

Vascular hypercontractile responses to norepinephrine in DS hypertensive rats can, in part, be explained by an impairment in endothelial nitric oxide production.

Renal medullary interstitial infusion of L-arginine prevents hypertension in Dahl salt-sensitive rats

Studies were designed to examine the effects of renal medullary interstitial infusion of L-arginine (L-Arg) on the development of high-salt-induced hypertension in Dahl salt-sensitive/Rapp (DS) rats. The threshold dose of L-Arg (300 micrograms . kg-1 . min-1) that increased the renal medullary blood flow without altering the cortical blood flow was first determined in anesthetized DS rats. Studies were then carried out to determine the effects of this dose of L-Arg on salt-induced hypertension in DS rats. In the absence of chronic medullary L-Arg infusion, mean arterial pressure (MAP) increased in DS rats from 125 +/- 2 to 167 +/- 5 mmHg by day 5 of a high-salt diet (4.0%), with no change observed in Wistar-Kyoto (WKY) or Dahl salt-resistant/Rapp (DR) rats. MAP did not change significantly with medullary infusion of L-Arg alone in DR rats (control = 104 +/- 1 mmHg) or in WKY rats (control = 120 +/- 3 mmHg) and was not significantly changed from these levels during the 7 days of L-Arg infusion combined with high-NaCl diet. The same amount of L-Arg that prevented salt-induced hypertension in DS rats when infused into the renal medulla (300 micrograms . kg-1 . min-1) failed to blunt salt-induced hypertension when administered intravenously to DS rats. DS rats receiving L-Arg (300 micrograms . kg-1 . min-1 iv) exhibited an increase in plasma L-Arg from control concentrations of 138 +/- 11 to 218 +/- 4 micromol/l, while MAP, which averaged 124 +/- 3 mmHg during the 3-day control period, rose to 165 +/- 5 mmHg by day 5 of high salt (4%) intake. These results indicate that the prevention of salt sensitivity in DS rats was due specifically to the action of L-Arg on renal medullary function and that DS rats may have a deficit of medullary substrate availability and NO production.

Dietary salt enhances glomerular endothelial nitric oxide synthase through TGF-beta1

Dietary salt controls production of nitric oxide (NO), a potent paracrine relaxation factor involved in glomerular filtration and salt excretion. We hypothesized that glomerular NO production was enhanced through endothelial nitric oxide synthase (NOS3). Rats in metabolic cages were studied after 4 days on 0.3% (Lo-salt) or 8.0% (Hi-salt) NaCl diet. Steady-state mRNA and protein levels of NOS3 and calcium-dependent NO production of isolated glomeruli from Hi-salt animals were greater than those values observed in glomeruli from Lo-salt rats. Because dietary salt enhanced glomerular production of transforming growth factor-beta1 (TGF-beta1) [W.-Z. Ying and P. W. Sanders. Am. J. Physiol. 274 (Renal Physiol. 43): F635-F641, 1998], studies were then conducted to examine the interaction between NOS3 and TGF-beta1. Glomerular steady-state levels of mRNA of NOS3 and TGF-beta1 directly correlated (r2 = 0. 946; P < 0.0001). A neutralizing antibody to TGF-beta reduced NOS3 protein and NO production in cultured glomeruli from Hi-salt animals to levels seen in the Lo-salt glomeruli. Thus dietary salt increased glomerular expression of TGF-beta1, which in turn augmented NO production through NOS3.

Genetic mapping of soluble guanylyl cyclase genes: implications for linkage to blood pressure in the Dahl rat

The nitric oxide (NO) signaling system, consisting of NO synthases, soluble guanylyl cyclase, and cGMP, plays a prominent role in salt handling and regulation of blood pressure. Soluble guanylyl cyclases are heme-containing heterodimers (alpha/beta). The alpha1/beta1 isoform has greater NO sensitivity than the alpha1/beta2. It has recently been shown that expression of the beta subunits is altered in the kidney of the Dahl salt-sensitive rat, ie, the beta1 subunit is decreased and the beta2 subunit increased. However, whether soluble guanylyl cyclase is linked to salt sensitivity is not known. In the present study, we investigated linkage of guanylyl cyclase genes to blood pressure. Alpha1 and beta1 gene loci for soluble guanylyl cyclase were mapped to rat chromosome 2, and the beta2 gene locus was mapped to rat chromosome 5 using fluorescent in situ metaphase hybridization. By use of a rat radiation hybrid panel, the gene loci were then further mapped with respect to known quantitative trait locus markers of salt-sensitive hypertension in the Dahl rat on chromosomes 2 and 5. Genes for alpha1 and beta1 were closely linked by two-point analysis to Na+,K+-ATPase alpha1 isoform (LOD of 15.1 and 14.0, respectively) and calmodulin-dependent protein kinase II-delta loci (LOD of 14.3 and 12.9, respectively), which have been previously shown to flank a quantitative trait locus for blood pressure in the Dahl rat. The alpha1 and beta1 genes were closely linked (LOD of 11.3; theta, 0.4). The beta2 gene locus was closely linked to the endothelin-2 (ET-2) locus (LOD of 13.0), which has been shown to cosegregate with blood pressure. We conclude that soluble guanylyl cyclase subunit loci, ie, alpha1, beta1, and beta2, are good candidates for genes controlling salt-sensitive hypertension in the Dahl rat.

Is the nitric oxide system involved in genetic hypertension in Dahl rats?

To address this issue, a series of genetic tests were carried out. Linkage studies showed that the inducible nitric oxide synthase (Nos2) locus cosegregated with blood pressure in three F2 populations originated from crosses of Dahl salt-sensitive (S) rats with rats of various normotensive strains. However, the brain nitric oxidase synthase (Nos1) and endothelial nitric oxide synthase (Nos3) loci did not cosegregate with blood pressure in five F2 populations. Thus, only Nos2, but not Nos1 and Nos3, was considered as a candidate gene for being a quantitative trait locus (QTL) for blood pressure in the S rat. To further test this hypothesis, congenic strains were constructed by substituting regions on Chromosome 10 of the S rat with the homologous regions of the Milan normotensive (MNS) rat. Results showed that the chromosome region including Nos2 did not contain a blood pressure QTL. In consequence, Nos2 per se is not supported as a candidate QTL capable of causing a blood pressure difference between the S and MNS rats. Nevertheless, the nitric oxide system appears to be involved secondarily in the regulation of blood pressure in the S rat, as evidenced by physiological data.

Vascular smooth muscle nitric oxide synthase anomalies in Dahl/Rapp salt-sensitive rats

Salt-sensitive hypertension in the Dahl/Rapp rat (S strain) is prevented by L-arginine. Based on the observations that dexamethasone prevented the antihypertensive effect of L-arginine in these animals and the suggestion that a locus in or near an inducible nitric oxide synthase (NOS) gene on chromosome 10 cosegregated with hypertension in some F2 crosses that utilized the S rat, the present study explored the hypothesis that the vascular smooth muscle isoform of inducible NOS (NOS2) was abnormal in S rats. Primary cultures of aortic smooth muscle cells from S rats demonstrated impaired inducible NO production, which improved with increased L-arginine in the medium. Sequence analysis identified a single T-->C transversion that produced an amino acid substitution (S714P) between the FAD and FMN binding sites and a restriction fragment length polymorphism. This restriction fragment length polymorphism was present only in S rats. The mutation of NOS2 and the role of this enzyme in the pathogenesis of salt-sensitive hypertension in the Dahl/Rapp rat require further investigation.