L-arginine abrogates salt-sensitive hypertension in Dahl/Rapp rats

Blunted acetylcholine relaxation and nitric oxide release in arteries from renal hypertensive rats

OBJECTIVE

Investigation of the effect of hypertension on endothelium-dependent relaxation and release of nitric oxide (NO) in normotensive and renal hypertensive rats.

DESIGN AND METHODS

Sprague-Dawley rats were randomly allocated into two groups: uninephrectomized controls and one-kidney one-clip (Goldblatt hypertension) hypertensive rats, a non-renin dependent model of hypertension. After 10 weeks and in the presence of the cyclooxygenase inhibitor indomethacin, simultaneous measurements of the NO concentration, measured with a NO-specific microelectrode and endothelium-dependent relaxation were performed in isolated rat superior mesenteric arteries.

RESULTS

Addition of the NO scavenger, oxyhaemoglobin, showed that basal NO concentration was unaltered in arterial segments from hypertensive rats. In norepinephrine-contracted arteries, acetylcholine increased the NO concentration and caused relaxations, and both parameters were significantly reduced in renal hypertensive arteries. Relaxations induced by the NO donor, S-nitroso-N-acetylpenicillamine were reduced. The superoxide scavenger, superoxide dismutase, and the NO synthase substrate, l-arginine, did not change the increase in NO concentration or acetylcholine relaxation in arteries from normotensive or renal hypertensive animals. In contrast, the NO synthase inhibitor, asymmetric dimethyl l-arginine, reduced the NO concentration and acetylcholine relaxation, while these responses were abolished in the presence of oxyhaemoglobin.

CONCLUSIONS

This study provides direct evidence that reduced endothelium-dependent relaxations in the superior mesenteric artery from renal hypertensive rats is due, at least in part, to diminished NO release. The reduced NO release and relaxation persist in the presence of excess of substrate for NO synthase.

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.

Magnesium influx enhanced by nitric oxide in hypertensive rat proximal tubule cells

An abnormal handling of renal magnesium has been suggested to cause salt-sensitive hypertension. The filtered magnesium is first reabsorbed in the proximal tubule. Amiloride has been shown to enhance renal magnesium conservation, but the regulatory mechanisms are unknown yet. High-salt (8% NaCl) diet decreased serum magnesium concentration, while increased urinary magnesium in Dahl salt-sensitive (DS) rat. Furthermore, the expression of nitric oxide synthase type 3 and nitric oxide (NO) content were decreased in high-salt loaded DS rat. In isolated proximal tubule cells, amiloride (0.1 mM) increased intracellular free magnesium concentration ([Mg(2+)](i)). However, the net [Mg(2+)](i) increase in the high-salt loaded DS rat was smaller than other groups. NOR1 (0.1 mM), a NO donor, restored the increase of [Mg(2+)](i) to the same level of other groups. On the contrary, L-NMMA (0.1 mM), an inhibitor of NO production, inhibited the increase of [Mg(2+)](i) in all groups. These results suggest that intracellular NO has an important role to up-regulate amiloride-elicited magnesium influx.

The nitric oxide system in glucocorticoid-induced hypertension

The blood pressure-raising effects of adrenocortical steroids with predominantly glucocorticoid activity, both naturally occurring and synthetic, are well known. Recent evidence suggests that the nitric oxide system plays a key role in the hypertension produced by glucocorticoids. Glucocorticoid actions at various sites in the nitric oxide synthase (NOS) pathway may result in elevated blood pressure. These include: alterations in l-arginine availability or transport; NOS2 and NOS3 downregulation; reduced cofactor bioavailability; NOS uncoupling; a concomitant elevation in reactive oxygen species and removal of nitric oxide (NO) from the vascular environment; alterations in whole body antioxidant status; and erythropoietin induced resistance to NO.

Decreased expression of arginase II in the kidneys of Dahl salt-sensitive rats

Arginase catalyzes the hydrolysis of arginine to urea and ornithine. Urea is not only an important solute for concentrating urine but also inhibits Na-K-2Cl cotransport. To elucidate the roles of arginase in the development of salt-sensitive hypertension, we examined arginase activity and expression in the kidney and other organs of Dahl/Rapp salt-sensitive (SS) and salt-resistant (SR) rats before and after 4 weeks' administration of a 4% NaCl or control diet. At 4 weeks of age, arginase activity in the kidney was lower in SS rats than in SR rats. Kidney arginase activity was lower in SS rats than in SR rats at 8 weeks of age, and salt loading did not alter arginase activity. Arginase II (the dominant isoform in the kidney) mRNA and protein in the kidney of salt-loaded SS rats were also lower than those of salt-loaded SR rats. Arginase activities in the liver and cerebellum did not differ between SS and SR rats. To examine the effect of urea, the product of arginase reaction, on the development of hypertension, SS rats were given a 4% NaCl diet containing 5% kaolin or 5% urea. Six-week urea supplementation attenuated the development of hypertension in SS rats. These findings suggest that decreased arginase expression in the kidney may be at least partially responsible for the salt-sensitive hypertension in SS rats.

The presence of African American race predicts improvement in coronary endothelial function after supplementary L-arginine

OBJECTIVES

The purpose of our study was to determine if the presence of African American ethnicity modulates improvement in coronary vascular endothelial function after supplementary L-arginine.

BACKGROUND

Endothelial dysfunction is an early stage in the development of coronary atherosclerosis and has been implicated in the pathogenesis of hypertension and cardiomyopathy. Amelioration of endothelial dysfunction has been demonstrated in patients with established coronary atherosclerosis or with risk factors in response to infusion of L-arginine, the precursor of nitric oxide. Racial and gender patterns in L-arginine responsiveness have not, heretofore, been studied.

METHODS

Invasive testing of coronary artery and microvascular reactivity in response to graded intracoronary infusions of acetylcholine (ACh) +/- L-arginine was carried out in 33 matched pairs of African American and white subjects with no angiographic coronary artery disease. Pairs were matched for age, gender, indexed left ventricular mass, body mass index and low-density lipoprotein cholesterol.

RESULTS

In addition to the matching parameters, there were no significant differences in peak coronary blood flow (CBF) response to intracoronary adenosine or in the peak CBF response to ACh before L-arginine infusion. However, absolute percentile improvement in CBF response to ACh infusion after L-arginine, as compared with before, was significantly greater among African Americans as a group (45 +/- 10% vs. 4 +/- 6%, p = 0.0016) and after partitioning by gender. The mechanism of this increase was mediated through further reduction in coronary microvascular resistance. L-arginine infusion also resulted in greater epicardial dilator response after ACh among African Americans.

CONCLUSIONS

We conclude that intracoronary infusion of L-arginine provides significantly greater augmentation of endothelium-dependent vascular relaxation in those of African American ethnicity when compared with matched white subjects drawn from a cohort electively referred for coronary angiography. Our findings suggest that there are target populations in which supplementary L-arginine may be of therapeutic benefit in the amelioration of microvascular endothelial dysfunction. In view of the excess prevalence of cardiomyopathy among African Americans, pharmacologic correction of microcirculatory endothelial dysfunction in this group is an important area of further investigation and may ultimately prove to be clinically indicated.

Renal segmental tubular response to salt during the normal menstrual cycle

BACKGROUND

It has been suggested that women gain weight and develop peripheral edema during the luteal phase of the menstrual cycle because they tend to retain sodium and water. However, there is actually no clear evidence for physiological, cyclic variations in renal sodium handling during the menstrual cycle. We prospectively assessed the changes in segmental renal sodium handling occurring during the menstrual cycle in response to changes in salt intake.

METHODS

Thirty-five normotensive women were enrolled. Seventeen women were randomized and studied in the follicular and 18 in the luteal phases of their menstrual cycle. All women were assigned at random to receive a low (40 mmol/day) or a high (250 mmol/day) sodium diet for seven days on two consecutive menstrual cycles. Renal sodium handling and hemodynamics were measured at the end of each diet period.

RESULTS

The changes in sodium intake induced comparable variations in sodium excretion in both phases of the menstrual cycle. In the follicular phase, the increase in salt intake was associated with no change in renal hemodynamics, an increased fractional excretion of lithium (FELi) and a decreased fractional distal reabsorption of sodium (FDRNa), suggesting that sodium reabsorption is reduced both in the proximal and the distal tubules. In contrast, in the luteal phase, the renal response to salt was characterized by a significant renal vasodilation and a marked salt escape from the distal nephron, compared to the women investigated in the follicular phase (P < 0.01). Sodium reabsorption by the proximal nephron was not reduced as indicated by the unchanged FELi.

CONCLUSIONS

These results show that the segmental renal handling of sodium differs markedly in the two phases of the menstrual cycle. They suggest that the female hormones modulate the renal handling of sodium at the proximal and distal segments of the nephron in young normotensive women.

Sex-specific acute effect of estrogen on endothelium-derived contracting factor in the renal artery of hypertensive Dahl rats

OBJECTIVE

To determine whether estrogen rapidly affects endothelium-derived contracting factor (EDCF) in the renal artery of hypertensive Dahl rats, and whether factors other than nitric oxide (NO) contribute to the effect of estrogen.

DESIGN

Acute effects of estrogen on the acetylcholine-induced vasomotor responses and on prostaglandin H2/thromboxane A2 mimetic, U46619,-induced contraction were examined in isolated arterial rings.

METHODS AND RESULTS

Dahl salt-sensitive male and female rats were fed an 8% NaCl diet for 4 weeks. The blood pressure increased more rapidly and to a greater extent in males than in females. Renal arterial rings were prepared for isometric tension recording. 17beta-Estradiol, but not the biologically less active stereoisomer, 17alpha-estradiol, improved the relaxation response to acetylcholine in renal arteries from females. Estrogen also rapidly decreased the contraction evoked by acetylcholine (10(-6) to approximately 10(-4) mol/l) in renal arteries from females and it was effective at a physiological concentration (10(-9) mol/l) in the presence of Nomega-nitro-l-arginine methyl ester (an NO synthase inhibitor). The estrogen receptor antagonist, ICI 182,780, abolished the effect of estrogen, whereas the cytochrome P450 inhibitor, miconazole, had no effect. The contraction induced by U46619 was also suppressed by estrogen, without any contribution from NO. Estrogen had no effect on either relaxation or contraction responses in renal arteries from males.

CONCLUSION

17beta-Estradiol antagonizes increases in vascular tone in hypertensive females by enhancing NO-dependent relaxation, and by suppressing EDCF-mediated mechanisms in an NO-independent manner.

Increased dietary salt activates rat aortic endothelium

The function of vascular endothelium as a biomechanical sensor permits alterations in gene expression in the vascular tree in response to wall stress. The present study explored the mechanism by which the arterial endothelium responds to changes in dietary salt. Normotensive rats were fed diets containing varying amounts of NaCl for 4 days. At that time, levels of phosphorylated p38 MAP kinase, p42/44 MAP kinase, and p46/54 JNK/SAP kinase increased when the diet contained > or = 3.0% NaCl. Kinase assays demonstrated dose-response relationships between dietary salt intake and the activities of p38 MAP kinase and p42/44 MAP kinase. Aortic segments from animals on the 8.0% NaCl diet produced greater amounts of total and active transforming growth factor-beta 1 (TGF-beta1) and nitric oxide. The MEK1 inhibitor, PD-098059, and the p38 MAP kinase inhibitor, SB-203580, decreased production of these bioactive compounds to background levels. Intravenous injection of tetraethylammonium chloride (TEA) into rats on the 8.0% NaCl diet decreased the activities of p38 MAP kinase and p42/44 MAP kinase, compared with rats on the same diet and given vehicle intravenously. These findings provided direct evidence that dietary salt modulated gene expression in the arterial wall through a tetraethylammonium-sensitive mechanism and activation of the p38 and p42/44 MAP kinase pathways.

Activation of the Fas/Fas ligand pathway in hypertensive renal disease in Dahl/Rapp rats

BACKGROUND

Hypertensive nephrosclerosis is the second most common cause of end-stage renal failure in the United States. The mechanism by which hypertension produces renal failure is incompletely understood. Recent evidence demonstrated that an unscheduled and inappropriate increase in apoptosis occurred in the Dahl/Rapp rat, an inbred strain of rat that uniformly develops hypertension and hypertensive nephrosclerosis; early correction of the hypertension prevents the renal injury. The present study examined the role of the Fas/FasL pathway in this process.

METHODS

Young male Dahl/Rapp salt-sensitive (S) and Sprague-Dawley rats were fed diets that contained 0.3% or 8.0% NaCl diets. Kidneys were examined at days 7 and 21 of the study.

RESULTS

An increase in Fas and FasL expression was observed in glomerular and tubular compartments of kidneys of hypertensive S rats, whereas dietary salt did not change expression of either of these molecules in normotensive Sprague-Dawley rats. Associated with this increase was cleavage of Bid and activation of caspase-8, the initiator caspase in this apoptotic pathway, by day 21 of the study.

CONCLUSIONS

Augmented expression of apoptotic signaling by the Fas/FasL pathway occurred during development of end-stage renal failure in this model of hypertensive nephrosclerosis.

1A-779 attenuates angiotensin-(1-7) depressor response in salt-induced hypertensive rats

Chronic infusion of angiotensin-(1-7) [Ang-(1-7)] lowers blood pressure in salt-induced and spontaneously hypertensive (SHR) rats. In the present study, we have examined the acute effect of Ang-(1-7) in salt-induced hypertension using Dahl salt-sensitive rats placed on low (0.3%) or high (8.0% NaCl) salt diets for 2 weeks. Rats fed a high salt diet showed a greater rise in BP than those fed a low salt diet. Ang-(1-7) (24 microg/kg) reduced mean arterial pressure (MAP), enhanced the release of prostacyclin and nitric oxide, and suppressed thromboxane A(2) levels. A-779 (48 microg/kg, i.v), a selective Ang-(1-7) antagonist, partially blocked these effects of Ang-(1-7). The Ang-(1-7)-induced depressor response observed in these animals was related to an increase in vasodilatory prostanoids, a decrease in the constrictor prostanoid thromboxane A(2), and an increase in nitric oxide levels in both plasma and isolated aortic rings.

Salt intake, endothelial dysfunction, and salt-sensitive hypertension

Numerous epidemiologic and clinical studies have demonstrated a clear relationship between high salt intake and blood pressure. However, the mechanisms of a salt-induced increase in blood pressure--a phenomenon known as salt sensitivity--and the heterogeneity of this effect are far from being completely understood. Endothelial dysfunction, and especially the nitric oxide system, is implicated in both experimental and clinical hypertension. Animal studies indicate that endogenous nitric oxide plays an important role in renal hemodynamics and sodium homeostasis, inducing renal vasodilation and natriuresis. Studies of essential hypertensive patients have also suggested that both high salt intake and salt sensitivity are associated with impaired endothelial function. Although there are many hypotheses concerning the nature of salt sensitivity, clinical data indicate that salt-sensitive patients may be unable to up-regulate the production of nitric oxide in response to salt intake. This endothelial dysfunction, which is more frequent in salt-sensitive than in salt-resistant essential hypertensive patients, may partially explain the blood pressure increase in response to salt intake and may underlie the more pronounced target organ damage and cardiovascular risk in salt-sensitive patients.

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.

Intrarenal transport and vasoactive substances in hypertension

Blood pressure is influenced by several vasoactive factors that also regulate nephron transport. An imbalance in regulation of salt reabsorption by the nephron contributes to hypertension. In the spontaneously hypertensive rat (SHR), the responses to dopamine and angiotensin II in the proximal nephron are diminished and enhanced, respectively. This partially explains why the proximal tubule of SHR absorbs more salt and water than that of normotensive controls. In the Dahl salt-sensitive rat, defects in NO signaling and alterations in the arachidonic acid/cytochrome P450 pathways are associated with increased salt reabsorption by the thick ascending limb. In other animal models, such as the deoxycorticosterone acetate (DOCA)-salt rat, hypertension develops as the result of an induced hormonal imbalance. By mimicking the effects of aldosterone, DOCA stimulates sodium reabsorption in the collecting ducts, causing salt and fluid retention. Thus, this model is similar to inherited forms of human hypertension caused by abnormal regulation of transport by mineralocorticoids, such as apparent mineralocorticoid excess and glucocorticoid-remediable aldosteronism. Overall, these findings demonstrate the significance of vasoactive compounds in regulating nephron transport and controlling blood pressure. However, important questions regarding humoral control of nephron transport and its implications in hypertension remain unanswered, and intensive research in these areas is required.

Nitric oxide synthase (NOS2) mutation in Dahl/Rapp rats decreases enzyme stability

The pathogenesis of salt-sensitive hypertension remains poorly defined, but a role for nitric oxide (NO) has been suggested. The Dahl/Rapp salt-sensitive rat possesses a defect in NO synthesis that is overcome by supplementation with L-arginine, which increases NO and cGMP production and prevents salt-sensitive hypertension. An S714P mutation of inducible NO synthase (NOS2) was subsequently identified. The current report examined the functional significance of an S714P mutation in NOS2. COS-7 cells were transiently transfected with cDNA of wild-type NOS2 and S714P and S714A mutants of NOS2, and enzyme function was determined. Whereas steady-state mRNA levels did not differ, immunoblot analysis demonstrated decreased levels of NOS2 protein. Metabolic labeling experiments confirmed a reduced half-life of the S714P mutation. Nitrite production, which was dependent on the concentration of L-arginine in the medium, was diminished in cells transfected with the S714P mutant, compared with the wild type and the S714A mutant. These data provide a biochemical explanation of the physiological abnormalities of NOS2 in the Dahl/Rapp salt-sensitive rat and suggest that a posttranslational mechanism involving the proteasome may be responsible for the diminished NO production observed in response to increased dietary salt intake in these animals.

Differential nNOS gene expression in salt-sensitive and salt-resistant Dahl rats

BACKGROUND

Several indications exist to suggest that an impaired production of nitric oxide might have a role in the development of salt-sensitive hypertension.

OBJECTIVE

To examine whether the gene expression of the nitric oxide synthases (NOS) is altered in the salt-sensitive Dahl rat compared with that in the salt-resistant Dahl rat.

DESIGN AND METHODS

The abundance of NOS mRNA was measured by RNase protection assay in different organs of salt-resistant and salt-sensitive Dahl rats. In addition, the zonal expression of NOS genes in the kidney under salt load and salt restriction was determined.

RESULTS

The abundance of endothelial NOS mRNA was similar between the salt-resistant and salt-sensitive Dahl rat strains in all organs. Inducible NOS mRNA was not detectable by RNase protection assay in any organ. Neuronal NOS (nNOS) mRNA expression, however, was about 50% lower in brain and kidney of salt-sensitive Dahl rats than in salt-resistant Dahl rats. Within the kidney, nNOS mRNA levels were significantly decreased in salt-sensitive Dahl rats compared with those in salt-resistant Dahl rats, in cortex, outer and inner medulla (50, 40 and 30%, respectively) under all dietary conditions. A comparison of renal nNOS gene expression in Dahl rats with that in salt-insensitive Sprague- Dawley rats revealed that the abundance of renal nNOS was similar in salt-sensitive Dahl and Sprague-Dawley rats, but was increased in salt-resistant Dahl rats relative to that in Sprague-Dawley rats.

CONCLUSION

These data suggest that nNOS gene expression is increased in salt-resistant Dahl rats compared with that in salt-sensitive Dahl rats. This increased nNOS expression of the salt-resistant Dahl strain might play a part in compensating for a defect of renal salt excretion in the Dahl strains.

Dimethylarginines in chronic renal failure

BACKGROUND

Nitric oxide (NO) is a potent chemical mediator involved in many functions. In vivo production of NO is thought to be regulated by endogenous analogues of L-arginine: asymmetric dimethylarginine (ADMA).

AIM

To examine the effect of renal function and dialysis on the serum concentrations of ADMA and symmetric dimethylarginine (SDMA).

METHODS

Blood samples were obtained from nine healthy subjects, patients with renal failure before (n = 17) and after haemodialysis (n = 9), nine patients on chronic ambulatory peritoneal dialysis (CAPD), and 13 patients with chronic renal failure on conservative treatment. Serum samples were extracted using a solid phase cation exchange column and the extracts were analysed by high performance liquid chromatography (HPLC).

RESULTS

Serum concentrations of ADMA in patients with renal failure (mean, 1.04 micromol/litre; SD, 0.17) were significantly higher than those of controls (mean, 0.61 micromol/litre; SD, 0.13). Haemodialysis significantly decreased the serum concentration by 36% (before dialysis: mean 0.99 (SD, 0.25) micromol/litre; after dialysis: mean, 0.63 (SD, 0.15) micromol/litre). Serum SDMA concentrations were higher in patients with renal failure, and haemodialysis decreased the concentration by 60%. There was no difference in serum arginine concentrations between the groups.

CONCLUSION

Serum concentrations of ADMA are increased in renal failure and haemodialysis reduces the concentration.

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

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

Decreased renal expression of nitric oxide synthase isoforms in adrenocorticotropin-induced and corticosterone-induced hypertension

Administration of adrenocorticotropic hormone (ACTH) leads to the development of hypertension. Because glucocorticoids can affect the nitric oxide system at several sites, the present study tested the hypothesis that nitric oxide synthase (NOS) expression may be altered in ACTH-induced and corticosterone-induced hypertension in the rat. This was addressed by measuring Nos1, Nos2, and Nos3 mRNA in the kidney, adrenal gland, heart, and hypothalamus of 16 ACTH-treated and 16 vehicle-treated rats as well as in 10 corticosterone-treated and 10 control rats. In addition, in situ hybridization and immunohistochemistry were used to confirm changes by detection of Nos in RNA and NOS protein in tissues. Systolic blood pressure of ACTH and corticosterone rats was elevated (165+/-6 and 162+/-11 mm Hg; P<0.001 versus control). Each Nos isoform mRNA was measured by reverse transcriptase-polymerase chain reaction technique. In ACTH rats, mRNA for Nos2 was reduced in renal cortex by 58+/-5% and in medulla by 68+/-7%; for Nos3, mRNA reductions of 59+/-6% and 51+/-11% were seen (P<0.001 after Hochberg correction for multiple comparisons). In corticosterone rats, Nos2 mRNA decreased in cortex by 68+/-5% and in medulla by 62+/-6%; Nos3 mRNA by 50+/-8% in cortex, and Nos1 by 29+/-7% in medulla (all P<0.001 after Hochberg correction). Reductions seen in kidney were supported by in situ hybridization and immunohistochemistry. Apart from a 62+/-2% decrease in Nos2 mRNA in adrenal of ACTH rats (corrected P<0.05), no significant changes were seen in the other nonrenal tissues for any isoform. In conclusion, we have shown for the first time that the physiological components of glucocorticoid action (ACTH and corticosterone) when given chronically in vivo reduce Nos2 and Nos3 expression in the kidney. Such changes are consistent with a role in hypertension for ACTH and corticosterone.

Increased dietary salt accelerates chronic allograft nephropathy in rats

BACKGROUND

Chronic allograft nephropathy (CAN), a major problem in renal transplantation, is related to both alloantigen-dependent and -independent processes. Because dietary salt intake modulated glomerular production of transforming growth factor-beta, which has been shown to play an important role in CAN, we hypothesized that dietary salt would directly enhance renal injury in a rodent model of CAN.

METHODS

Dietary NaCl was increased from 1.0% (normal) to 8.0% in a group of Fisher/Lewis rats 25 days following orthotopic renal transplantation and was continued until 16 weeks after transplantation.

RESULTS

Blood pressure, which was recorded using radiotelemetry in the first eight-weeks post-transplantation, did not differ between the groups, but allograft recipients on the 8.0% NaCl diet rapidly demonstrated increased urinary albumin excretion. Renal function determined by dynamic functional imaging was worse in allograft recipients on the 8.0% NaCl diet by six weeks following transplantation. Histologic examination at 16 weeks confirmed a significant increase in allograft damage in the 8.0% NaCl group compared with allografts from rats on 1.0% NaCl diet. These findings included glomerulosclerosis and tubulointerstitial injury that consisted of fibrosis, tubular atrophy and dilation, intratubular casts, and tubular epithelial cell damage. Small arteries and arterioles did not show evidence of damage from hypertension or other abnormality.

CONCLUSIONS

In this model of CAN, renal allograft dysfunction preceded hypertension and was accelerated significantly by an increase in dietary salt.

Relative deficiency of nitric oxide-dependent vasodilation in salt-hypertensive Dahl rats: the possible role of superoxide anions

OBJECTIVE

The contribution of major vasoactive systems (renin-angiotensin system, sympathetic nervous system and nitric oxide) to blood pressure maintenance and the possible involvement of superoxide anions in the reduced efficiency of nitric oxide (NO)-dependent vasodilation to counterbalance sympathetic vasoconstriction were studied in salt-hypertensive Dahl rats.

DESIGN AND METHODS

We used Dahl salt-sensitive (SS/Jr) and salt-resistant (SR/Jr) female rats kept on a low-salt (0.3% NaCl) or high-salt diet (8% NaCl) for 6 weeks since weaning. Mean arterial pressure (MAP) was measured in conscious animals subjected to acute consecutive blockade of the renin-angiotensin system (RAS) [captopril, 10 mg/kg intravenously (i.v.)], the sympathetic nervous system (SNS) (pentolinium, 5 mg/kg i.v.) and NO synthase (Nomega-nitro-L-arginine methyl ester (L-NAME), 30 mg/kg i.v.). Before the consecutive blockade of vasoactive systems one-half of the animals in each experimental group was pre-treated with a stable membrane-permeable mimetic of superoxide dismutase (tempol, 25 mg/kg i.v.) which functions as a superoxide scavenger.

RESULTS

Compared to normotensive SR/Jr animals, salt-hypertensive SS/Jr rats were characterized by an enhanced blood pressure (BP) fall after ganglionic blockade (-104 +/- 8 versus -62 +/- 5 mm Hg, P < 0.001) and by higher residual blood pressure recorded after the blockade of both RAS and SNS (70 +/- 3 versus 43 +/- 3 mmHg, P < 0.01), but there was only a borderline elevation of their BP response to acute NO synthase inhibition (67 +/- 6 versus 49 +/- 4 mmHg, P < 0.05). The acute tempol pre-treatment elicited the most pronounced reduction of basal BP (-13 +/- 1 mmHg, P < 0.001) in the salt-hypertensive SS/Jr group in which the BP rise after L-NAME administration was augmented by about 50%. On the contrary, tempol pre-treatment did not affect norepinephrine- or angiotensin II-dependent vasoconstriction.

CONCLUSIONS

The NO system is not able to counterbalance effectively the hyperactivity of the sympathetic nervous system in salt-hypertensive Dahl rats. The predominance of sympathetic vasoconstriction over NO-dependent vasodilation could be explained partially by enhanced NO inactivation due to augmented superoxide anion formation in hypertensive animals.

Endothelial dysfunction in salt-sensitive essential hypertension

The aim of this study was to evaluate endothelium-dependent and -independent vasodilation, as well as endothelium biochemical markers, in a group of essential hypertensive patients classified on the basis of salt sensitivity. Changes in forearm blood flow in response to acetylcholine, sodium nitroprusside, and N(G)-monomethyl-L-arginine (L-NMMA) infusion were determined by means of strain-gauge plethysmography. Moreover, plasma and urinary concentrations of nitrates, cGMP, and endothelin were measured during low (50 mmol/d) and high (250 mmol/d) salt intake. Salt-sensitive hypertension was diagnosed in 26 patients who exhibited a significant increase in 24-hour mean blood pressure assessed by ambulatory blood pressure monitoring after 1 week of high salt intake. Nineteen patients were considered salt resistant. Compared with salt-resistant hypertensives, salt-sensitive patients presented a significant lower (P=0.005) maximal acetylcholine-induced vasodilation (21+/-6.3 versus 28+/-7.5 mL. 100 mL(-1). tissue. min(-1)). On the contrary, maximal sodium nitroprusside-induced vasodilation did not significantly differ between groups (22.4+/-4.5 versus 23.9+/-5.3 mL. 100 mL(-1). tissue. min(-1)). The decrease in maximal acetylcholine-induced vasodilation promoted by the coadministration of L-NMMA was significantly more pronounced in salt-resistant compared with salt-sensitive patients (P=0.003). Finally, high salt intake promoted a significant decrease in 24-hour urinary nitrate excretion in salt-sensitive patients (from 443+/-54 to 312+/-54 micromol/d; P=0.033) compared with salt-resistant hypertensives (from 341+/-50 to 378+/-54 micromol/d). We conclude that salt-sensitive hypertension is associated with endothelial dysfunction characterized by a defective endothelium-dependent vasodilation. Impairment of the L-arginine-nitric oxide pathway may be responsible for this abnormal endothelial response.

Therapeutic modification of the L-arginine-eNOS pathway in cardiovascular diseases

L-Arginine is the substrate for nitric oxide production. Endothelium dysfunction could be attributed to L-arginine deficiency or the presence of L-arginine endogenous inhibitors. This hypothesis leads to the assumption that provision of L-arginine could be the key for endothelial function improvement. Many studies have proven that L-arginine has a beneficial effect on endothelium dependent vasoreactivity, as well as on the interaction between vascular wall, platelets and leucocytes. Therefore, individuals with risk factors for atherosclerosis and patients with coronary artery disease or heart failure, could benefit from therapy with L-arginine.

Effect of oral L-arginine administration for three weeks in two kidney-two clip hypertensive rats

Nitric oxide (NO) has been identified as an effective vascular relaxant. This study analyses the contribution of the precursor L-arginine (L-arg) by oral administration in two kidney-two clip hypertension in the rat (2K-2C). Two groups were studied: sham (SH, n=21) and hypertensive (HT, n=15). After 4 weeks of surgery, a group of rats remained as controls (SHc and HTc, respectively), while others were supplemented with L-arg (1.25 g/L) in drinking water (SHa and HTa) for 3 weeks. Blood pressure was significantly increased in 2K-2C rats but remained unchanged after L-arg treatment. Plasma nitrite/nitrate concentrations were not different among groups. The contractile response of aorta to KCl, serotonin and the protein kinase C (PKC) stimulant, phorbol 12,13-dibutyrate (PDBu) was also evaluated. Higher contractile responses to PDBu (p<0.001) and lower relaxation to acetylcholine (Ach 10(-6) M, p<0.05 and 10(-5)M, p<0.02) were observed in aortic rings of HTc vs SHc; L-arg supplementation significantly diminished tension development to all agonists (p<0.05) but failed to modify the lower relaxation to Ach in HTa. Thromboxane (TxA(2)) - synthesis in rings of HTc was higher than in SHc under basal conditions (p<0.05). In the groups with supplement of L-arg, PDBu significantly stimulated prostacyclin (PGI(2)) synthesis more in HTa rats than in SHa ones (p<0.05). To conclude: 1) L-arg fails to modify hypertension development in 2K-2C rats; and 2) L-arg exerts a beneficial effect on the vascular wall, by reducing contractility in rings from HTa rats; it also improved PGI(2) synthesis under PDBu stimulation. 3) greater PKC activation and TxA(2) production rather than lower NO availability might result in systemic hypertension in 2K-2C rats.

Mechanisms of salt-sensitive hypertension: role of inducible nitric oxide synthase

The goal of this study was to determine the role of inducible nitric oxide synthase (iNOS) in the arterial pressure, renal hemodynamic, renal excretory, and hormonal changes that occur in Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats during changes in Na intake. Thirty-two R and S rats, equipped with indwelling arterial and venous catheters, were subjected to low (0.87 mmol/day) or high (20.6 mmol/day) Na intake, and selective iNOS inhibition was achieved with intravenous aminoguanidine (AG, 12.3 mg. kg(-1). h(-1)). After 5 days of AG, mean arterial pressure increased to 121 +/- 3% control in the R-high Na AG rats compared with 98 +/- 1% control (P < 0.05) in the R-high Na alone rats, and S-high Na rats increased their arterial pressure to 123 +/- 3% control compared with 110 +/- 2% control (P < 0.05) in S-high Na alone rats. AG caused no significant changes in renal hemodynamics, urinary Na or H(2)O excretion, plasma renin activity, or cerebellar Ca-dependent NOS activity. The data suggest that nitric oxide produced by iNOS normally helps to prevent salt-sensitive hypertension in the Dahl R rat and decreases salt sensitivity in the Dahl S rat.

Induction of apoptosis during development of hypertensive nephrosclerosis

BACKGROUND

As the biology of programmed cell death, or apoptosis, is clarified, a role for this process in the pathophysiology of organ dysfunction and fibrosis has been hypothesized. Hypertensive nephrosclerosis represents an important cause of end-stage renal disease. One model of the progressive, noninflammatory, sclerotic renal lesion of hypertension is the Dahl/Rapp salt-sensitive rat, which was examined in this study.

METHODS

Male, Dahl/Rapp salt-sensitive (SS) and Sprague-Dawley rats were placed on either 0.3 or 8.0% NaCl diets for three weeks. Blood pressure was determined, and the kidneys were harvested for histochemical analysis and to obtain total RNA for RNase protection assays and total protein for Western blotting.

RESULTS

An increase in apoptosis in the glomerular and tubular compartments was observed only in kidneys of SS rats on the high-salt diet. These findings occurred at a time when renal function was markedly impaired and irreversible changes in renal morphology developed. Temporally associated with this increase in apoptosis was augmented expression of pro-apoptotic molecules that included Fas, Bax, and Bcl-XS.

CONCLUSIONS

The inappropriate shift in expression of proteins that facilitate apoptosis in the nephron, along with ongoing cell death that manifested at a time when renal function was deteriorating, supported an important role for this process in development of hypertensive nephrosclerosis.

Role of nitric oxide in adrenocorticotrophin-induced hypertension: L-arginine effects reversed by N-nitro-L-arginine

1. L-arginine prevents adrenocorticotrophin (ACTH)-induced hypertension in the rat. To confirm that this effect is mediated through the nitric oxide (NO) system, we examined whether N-nitro-L-arginine (NOLA) could reverse the L-arginine-induced blockade of ACTH-induced hypertension. 2. Blood pressure and metabolic parameters were examined in sham-, ACTH-, L-arginine + sham-, NOLA + sham-, ACTH + L-arginine- and ACTH + L-arginine + NOLA-treated Sprague-Dawley rats (n = 40). 3. Adrenocorticotrophin treatment increased systolic blood pressure (SBP), water intake and urine output and decreased bodyweight. N-Nitro-L-arginine alone increased SBP without affecting metabolic variables. L-Arginine alone did not affect blood pressure. The SBP was lower in L-arginine + ACTH- than ACTH-treated rats (P < 0.001), but was higher following ACTH + L-arginine + NOLA than ACTH + L-arginine (P < 0.05). 4. N-Nitro-L-arginine reversed the blood pressure-lowering effect of L-arginine in ACTH-induced hypertension in the rat, supporting the notion that NO plays a role in the hypertension.

Subpressor dose of L-NAME unmasks hypertensive effect of chronic hyperinsulinemia

We previously found that chronic exogenous hyperinsulinemia without sugar supplementation does not elevate blood pressure. This may be partially explained by the ability of insulin to release nitric oxide and cause vasodilatation. To test this hypothesis, we studied 4 groups of rats: 9 rats (body weight, 213+/-14 g) treated with a gradual increase of a sustained-release subcutaneous insulin pellet; 9 rats (body weight, 213+/-9 g) treated with N:(G)-nitro-L-arginine methyl ester (L-NAME) in drinking water 50 mg/L; 19 rats (body weight, 217+/-11 g) treated with the combination of L-NAME and insulin; and 9 control rats (body weight, 218+/-11 g). Blood pressure was followed weekly for 6 weeks, and then rats were studied in metabolic cages. Weight gain was not different during the 6 weeks. Renal function did not differ between the 4 groups, but 24-hour urinary nitrite/nitrate excretion was lower (P<0.02) in L-NAME-treated and higher in insulin-treated rats. Plasma insulin doubled (P<0.002) in the insulin-treated rats, but there was no hypoglycemia and, by week 6, fructosamine levels were 2.1+/-0.2, 2.1+/-0.2, 2.3+/-0.1, and 2.3+/-0.2 mmol/L in control rats and rats treated with L-NAME, insulin, and L-NAME plus insulin, respectively. Systolic blood pressure, which did not differ at baseline, at week 3 was 122+/-17, 118+/-17, and 118+/-24 mm Hg in the control, L-NAME, and insulin groups and 136+/-14 mm Hg (P<0.03) in the combination group. At week 6, systolic blood pressure was 128+/-14, 127+/-15, and 118+/-13 mm Hg in the control, L-NAME, and insulin groups, respectively, and 150+/-14 mm Hg (P<0.0005) in the combination group. In a subsequent experiment, L-arginine 2 g/L abrogated the effects of L-NAME and insulin combination. In conclusion, chronic exogenous hyperinsulinemia does not affect blood pressure but may cause hypertension when endothelial function is compromised.

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.

Renal cyclic 3',5'-guanosine monophosphate and sodium excretion in Dahl salt-resistant and Dahl salt-sensitive rats: comparison of the roles of bradykinin and nitric oxide

OBJECTIVE

The purpose of this study was to determine the relative importance of bradykinin and nitric oxide (NO) in mediating renal responses to altered sodium intake in Dahl salt-resistant (Dahl-SR) and salt-sensitive (Dahl-SS) rats.

DESIGN AND METHODS

Dahl-SR and Dahl-SS rats consumed a diet containing 0.15% (low) or 4.0% (high) sodium chloride for 10 days. A microdialysis technique was then used to measure renal cortical interstitial fluid (RIF) cyclic 3',5'-guanosine monophosphate (cGMP) production in anesthetized rats, under baseline conditions and during acute cortical infusion of either the bradykinin B2 receptor antagonist icatibant or the NO synthase inhibitor nitro-L-arginine methyl ester (L-NAME). Urine sodium excretion was monitored simultaneously by ureter cannulation. Results Baseline sodium excretion was similar in the two types of rats, but RIF cGMP was significantly elevated in Dahl-SR compared to Dahl-SS rats on both low and high sodium diets. Icatibant infusion significantly reduced both RIF cGMP and sodium excretion in Dahl-SR rats during low sodium intake, but had no effect in Dahl-SS rats on either diet L-NAME infusion significantly reduced sodium excretion in Dahl-SR and Dahl-SS rats, during both low and high sodium intake. L-NAME infusion caused a significant reduction in RIF cGMP in Dahl-SR and Dahl-SS rats on low sodium diet, but reduced RIF cGMP only in Dahl-SR rats on high sodium diet. Conclusion These data suggest a potential role for cortical bradykinin, but not NO, in mediating the differences in the renal response to low sodium intake between Dahl-SR and Dahl-SS rats.

Overexpression of chloride channel CLC-K2 mRNA in the renal medulla of Dahl salt-sensitive rats

OBJECTIVE

The present study aimed to characterize the influence of salt intake on the gene expression of the kidney specific chloride channels CLC-K1 and CLC-K2 in the kidneys of salt-resistant and salt-sensitive Dahl rats.

DESIGN

For this purpose Dahl salt-resistant (Dahl-R) and Dahl salt-sensitive rats (Dahl-S) were fed a low (0.02%), normal (0.6%) or high (4%) salt diet for 19 days and CLC-K1 and -K2 mRNA expression was semiquantitated in cortex, outer and inner medulla.

METHODS

Kidneys were macroscopically dissected, total RNA was isolated according to the guanidinium-thiocyanate-phenol-chloroform method and messenger RNAs for the kidney specific chloride channels CLC-K1 and CLC-K2 were measured by ribonuclease protection assay.

RESULTS

Systolic blood pressure in high salt-treated Dahl-S rats increased to 204 +/- 5 mmHg versus 150 +/- 7 mmHg in Dahl-S controls. Dahl R and low salt Dahl-S rats showed no increase in blood pressure. For CLC-K1 mRNA we found an order of abundance inner medulla >> outer medulla >> cortex. There was no difference in mRNA abundance between Dahl-R and -S, nor any effect of the rate of salt intake on CLC-K1 mRNA abundance in the different kidney zones. CLC-K2 mRNA expression in cortex and outer medulla was similar between Dahl-R and -S rats. In the inner medulla, however, CLC-K2 mRNA was 1.7-fold higher in Dahl-S than in Dahl-R rats. In the cortex we found no influence of salt intake on CLC-K2 mRNA. In outer and inner medulla of Dahl-R rats and Dahl-S rats high salt diet led to a marked downregulation of CLC-K2 mRNA expression. Consequently, CLC-K2 gene expression in the inner medulla was 2.2-fold higher in Dahl-S than in Dahl-R rats in states of high salt diet.

CONCLUSION

Given that the CLC-K2 chloride channel in the outer and inner medulla contributes to salt reabsorption, our findings would suggest that Dahl-S rats have an increased medullary salt reabsorption. This may contribute to the inability of these animals to excrete an increased salt load at a normal renal perfusion pressure leading to the development of hypertension.

Nitric oxide and salt sensitivity

Studies in laboratory animals suggest that altered nitric oxide (NO) production may be associated with salt sensitivity. In this investigation we determined whether the endogenous NO production was altered in salt-sensitive human subjects when salt intake was changed. Salt sensitivity was assessed from the magnitude of the blood pressure (BP) lowering obtained when the salt intake was reduced from high to a low intake. The combined urinary excretion of nitrites and nitrates, the major metabolites of NO, was employed as an index of endogenous NO production. Salt-sensitive subjects (n = 23) were older, heavier, and had greater waist-to-hip ratios and higher baseline BP than salt-resistant individuals (n = 25). In salt-sensitive subjects, mean blood pressure (MBP) decreased 11.8+/-0.7 mm Hg, and NO metabolite excretion increased from 823+/-102 to 1530+/-148 mmol/24 h, when salt intake was reduced from 316 to 28 micromol/day. NO metabolite excretion was 45% lower during high salt (0.66+/-0.1 micromol/mg creatinine) than during low salt intake (1.12+/-0.1 micromol/mg creatinine) (P < .001). In contrast, when salt intake was reduced, salt-resistant subjects exhibited no significant mean changes in BP or NO metabolite excretion. During low salt intake, NO metabolite excretion (micromol/ day) was significantly higher in salt-sensitive individuals. The magnitude of decrease of systolic blood pressure, diastolic blood pressure, or MBP induced by reducing salt intake was not related to the increase in urinary excretion of NO metabolite levels (r2 = 0.009; P = .66). In summary, to the extent that urinary NO metabolite levels reflect the activity of the endogenous NO system, our results support the view that salt sensitivity may in part be determined by an inability to increase or to sustain NO production in response to high salt. Insufficient NO production during high salt may in turn lead to altered pressure-natriuresis relationships and to an increase in BP. The possibility that the increase in BP induced by high salt intake in salt-sensitive individuals could be the key factor in reducing NO metabolite levels can not be ruled out.

Renal structural properties in prehypertensive Dahl salt-sensitive rats

In 10- to 12-week-old Dahl salt-sensitive (DS) and salt-resistant (DR) rats fed a 0.3% salt diet (n=10 in each group), flow-pressure and pressure-glomerular filtration rate (F-P and P-GFR, respectively) relationships were established for maximally vasodilated perfused kidneys. From these relationships, 3 indices of vascular structural properties were estimated: slope of F-P (minimal renal vascular resistance reflecting overall luminal dimensions of preglomerular and postglomerular vasculature), slope of P-GFR (glomerular filtration capability against pressure), and threshold pressure for beginning filtration at P-GFR (preglomerular-to-postglomerular vascular resistance ratio). Thereafter, maximal renal vascular resistance was determined to assess wall-to-lumen ratios of the resistance vessels in half of each group. In the remainder, the kidneys were perfusion-fixed for histological analysis. Mean arterial pressure did not differ between the DS and DR rats. There were no significant differences in the slopes of F-P between the 2 groups. In contrast, the slope of P-GFR was significantly lower (33%) in DS rats than in DR rats, although the DS kidneys began filtering at a threshold pressure similar to that of the DR kidneys. Thus, in DS rats, there were no abnormalities in luminal dimensions at preglomerular and postglomerular vascular segments, but the kidney filtration capacity decreased at any given increase in pressure. Maximal vascular resistance was greater in DS than in DR rats, a finding compatible with the histological appearance, which showed vascular hypertrophy with little, if any, vascular narrowing in the interlobular arteries of DS rats. In conclusion, hypertrophic remodeling without vascular narrowing at preglomerular resistance vessels and structural defects in filtering at the glomeruli could occur in prehypertensive DS rats.

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.

Potassium augments vascular relaxation mediated by nitric oxide in the carotid arteries of hypertensive Dahl rats

The present study was designed to determine whether and how potassium supplementation improves the endothelial function of carotid arteries of hypertensive Dahl rats. Dahl salt-sensitive rats were fed a high sodium diet, a high sodium plus potassium-supplemented diet, a normal rat chow, or a potassium-supplemented diet for 4 weeks. High sodium intake significantly increased the blood pressure, which was attenuated by potassium supplementation. The isometric tension of rat-isolated carotid rings was measured. In norepinephrine-precontracted rings, the relaxation in response to acetylcholine, adenosine 5'-diphosphate (ADP), and isoproterenol were significantly attenuated in hypertensive Dahl rats, which was improved by potassium supplementation. Pretreatment with N(G)-nitro-L-arginine methyl ester blocked the responses to acetylcholine and ADP, and eliminated the difference in relaxation in response to isoproterenol. The endothelium-independent relaxation in response to forskolin, S-nitroso-N-acetyl-DL-penicillamine, and sodium nitroprusside was significantly attenuated in hypertensive Dahl rats, which was not affected by potassium supplementation. The results indicated that salt-induced hypertension was associated with marked alterations in the endothelial and vascular smooth muscle functions of the carotid arteries of Dahl rats. Potassium supplementation ameliorated the endothelial but not the smooth muscle function. The protective effect of potassium appeared to be achieved through increased endothelial nitric oxide production. The current studies, in conjunction with our recent studies on nitric oxide synthase activity in the kidney, strongly suggest that potassium attenuates development of hypertension by increasing nitric oxide production in Dahl rats.

L-Arginine increases nitric oxide production in isolated lungs of chronically hypoxic newborn pigs

Previously, our laboratory found that pulmonary hypertension developed and lung nitric oxide (NO) production was reduced when piglets were exposed to chronic hypoxia (Fike CD, Kaplowitz MR, Thomas CJ, and Nelin LD. Am J Physiol Lung Cell Mol Physiol 274: L517-L526, 1998). The purposes of this study were to determine whether L-arginine addition augments NO production and to evaluate whether L-arginine uptake is impaired in isolated lungs of chronically hypoxic newborn piglets. Studies were performed by using 1- to 3-day-old piglets raised in room air (control) or 10% O(2) (chronic hypoxia) for 10-12 days. Lung NO production was assessed in isolated lungs from both groups by measuring the perfusate accumulation of nitrites and nitrates (collectively termed NO(-)(x)) before and after addition of L-arginine (10(-2) M) to the perfusate. The rate of perfusate NO(-)(x) accumulation increased by 220% (from 0.8 +/- 0.4 to 2.5 +/- 0.5 nmol/min, P < 0.05) after L-arginine addition to chronic hypoxic lungs but remained unchanged (3.2 +/- 0. 8 before vs. 3.3 +/- 0.4 nmol/min after L-arginine) in control lungs. In the second series of studies, L-arginine uptake was evaluated by measuring the perfusate concentration of L-[(3)H]arginine at fixed time intervals. The perfusate concentration of L-[(3)H]arginine at each time point was less (P < 0.05) in control than in chronic hypoxic lungs. Thus L-arginine uptake was impaired and may underlie in part the reduction in lung NO production that occurs when piglets are exposed to 10-12 days of chronic hypoxia. Moreover, these findings in isolated lungs lead to the possibility that L-arginine supplementation might increase in vivo lung NO production in piglets with chronic hypoxia-induced pulmonary hypertension.

Dysfunctional renal nitric oxide synthase as a determinant of salt-sensitive hypertension: mechanisms of renal artery endothelial dysfunction and role of endothelin for vascular hypertrophy and Glomerulosclerosis

This study investigated the role of renal nitric oxide synthase (NOS), endothelin, and possible mechanisms of renovascular dysfunction in salt-sensitive hypertension. Salt-sensitive (DS) and salt-resistant (DR) Dahl rats were treated for 8 wk with high salt diet (4% NaCl) alone or in combination with the ET(A) receptor antagonist LU135252 (60 mg/kg per d). Salt loading markedly increased NOS activity (pmol citrulline/mg protein per min) in renal cortex and medulla in DR but not in DS rats by 270 and 246%, respectively. Hypertension in DS rats was associated with renal artery hypertrophy, increased vascular and renal endothelin-1 (ET-1) protein content, and glomerulosclerosis. In the renal artery but not in the aorta of hypertensive DS rats, endothelium-dependent relaxation to acetylcholine was unchanged; however, endothelial dysfunction due to enhanced prostanoid-mediated, endothelium-dependent contractions and attenuation of basal nitric oxide release was present. Treatment with LU135252 reduced hypertension in part, but completely prevented activation of tissue ET-1 without affecting ET-3 levels. This was associated with a slight increase of renal NOS activity, normalization of endothelial dysfunction and renal artery hypertrophy, and marked attenuation of glomerulosclerosis. Thus, DS rats fail to increase NOS activity in response to salt loading. This abnormality may predispose to activation of the tissue ET-1 system, abnormal renal vasoconstriction, and renal injury. Chronic ET(A) receptor blockade normalized salt-induced changes in the renal artery and reduced glomerular injury, suggesting therapeutic potential for ET antagonists in salt-sensitive forms of hypertension.

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.

Interaction between nitric oxide and mineralocorticoids in the long-term control of blood pressure

We analyzed the effects of a possible interaction between nitric oxide deficiency and mineralocorticoids on the long-term control of blood pressure and renal and endocrine variables. Six groups of uninephrectomized male Wistar rats were used: control animals and rats that received (1) N(G)-nitro-L-arginine methyl ester (L-NAME) subpressor (0.5 mg/100 mL drinking fluid), (2) L-NAME pressor (35 mg/100 mL drinking fluid), (3) deoxycorticosterone acetate (DOCA; 12. 5 mg/wk per rat), (4) DOCA plus L-NAME subpressor, or (5) L-NAME pressor plus DOCA. For all groups, the drinking fluid was tap water or 1% NaCl solution. We measured the time course of tail systolic blood pressure (SBP) and body weight for 3 weeks in all rats. At the end of the experimental period, we measured mean arterial pressure (direct recording) and endocrine and renal variables. Tail SBP rose significantly in the DOCA plus L-NAME subpressor-treated group but remained at normotensive levels in the DOCA-treated group. The addition of L-NAME to the subpressor dose accelerated the blood pressure increase in DOCA-salt hypertensive rats. The simultaneous administration of DOCA and L-NAME increased blood pressure and mortality rates in rats that drank water or saline compared with the rats treated with L-NAME alone. The subpressor dose of L-NAME did not increase blood pressure in saline-drinking rats. We conclude that impaired NO synthesis results in increased sensitivity to the pressor effect of mineralocorticoids in the presence or absence of an increased saline intake. Hence, nitric oxide contributes to the adaptative response to mineralocorticoid excess, perhaps through the facilitation of natriuresis and, thus, control of blood pressure.

Increased endothelium--monocyte interactions in salt-sensitive hypertension: effect of L-arginine

Although adhesion of monocytes to endothelial cells is considered as one of the initial factors leading in the long term to the development of atherosclerosis, the effects of hypertension on monocyte-endothelial cell interactions are still largely unknown. Thus we evaluated whether hypertension affects adhesion of monocytes on rat carotid endothelium, and whether this adhesion may be modified by long-term treatment with L-arginine, the physiologic precursor of nitric oxide (NO). Hypertension was induced in Dahl rats using a sodium-rich diet (8%), in the absence or the presence of L-arginine (1.25 g/L in drinking water). After 1 month, the carotid arteries were isolated, opened longitudinally, and incubated in the presence of 2 x 10(6) monocytes previously rendered fluorescent by incubation with tetramethyl rhodamine isothiocyanate (TRITC), and adherent cells were counted under fluorescence microscopy. In parallel, the production of NO was evaluated in vitro in isolated aorta and isolated hearts. Hypertension markedly increased adhesion of monocytes on carotid endothelium, and this was reduced by L-arginine. Hypertension also reduced an index of NO release at the level of the aorta and the coronary circulation. This impaired release of NO was partially prevented by L-arginine. Thus hypertension was associated with an increased adhesion of monocytes, which is probably due at least in part to a decreased production of NO. The increased adhesion was partly reduced by L-arginine, possibly secondary to an increased production of NO. Such an increased adhesion of monocytes may contribute the increased cardiovascular risk in hypertension.

Study on the relationship between plasma nitrite and nitrate level and salt sensitivity in human hypertension : modulation of nitric oxide synthesis by salt intake

BACKGROUND

High salt intake suppresses the effect of nitric oxide (NO) in the peripheral resistance vessels in animal models. We tested the hypothesis that the modulation of endogenous NO is related to salt sensitivity in human hypertension.

METHODS AND RESULTS

Inpatients with essential hypertension (n=24) were maintained on a normal-salt diet (12 g/d NaCl) for 3 days, a low-salt diet (2 g), a high-salt diet (20 to 23 g), and a low-salt diet for 7 days. Normotensive subjects (n=16) were maintained on the first 2 salt diets. The hypertensive patients whose average 24-hour blood pressure was increased by >5% by salt loading were assigned to group 1 (n=8) and the others to group 2 (n=16). Nitrate plus nitrite (NO(x)) was measured by the Griess method, and asymmetrical dimethylarginine (ADMA) by high-performance liquid chromatography. The plasma NO(x) level during the normal-salt diet was lower in group 1 than in group 2 and the normotensive group. After salt loading, the plasma NO(x) level was decreased and reversed after the second salt restriction. Plasma ADMA level was increased after salt loading and decreased after salt restriction. The change in plasma NO(x) level was correlated inversely with those in blood pressure (r=-0.59, P=0.0007) and plasma ADMA level (r=-0.64, P=0.003) after salt loading and restriction.

CONCLUSIONS

Modulation of NO synthesis by salt intake may be involved in a mechanism for salt sensitivity in human hypertension, presumably via the change in ADMA.

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.

Effects of NO donors and synthase agonists on endothelial cell uptake of L-Arg and superoxide production

It is commonly believed that the activity of NO synthase (NOS) solely controls NO production from its substrates, L-Arg and O(2). The Michaelis-Menten constant (K(m)) of NOS for L-Arg is in the micromolar range; cellular levels of L-Arg are much higher. However, evidence strongly suggests that cellular supply of L-Arg may become limiting and lead to reduced NO and increased superoxide anion (O(-)(2)*) formation, promoting cardiovascular dysfunction. Uptake of L-Arg into cells occurs primarily (approximately 85%) through the actions of a Na(+)-independent, carrier-mediated transporter (system y(+)). We have examined the effects of NOS agonists (substance P, bradykinin, and ACh) and NO donors (S-nitroso-N-acetyl-penicillamine and dipropylenetriamine NONOate) on transport of L-Arg into bovine aortic endothelial cells (BAEC). Our results demonstrate that NOS agonists increase y(+) transporter activity. A rapidly acting NO donor initially increases L-Arg uptake; however, after longer exposure, L-Arg uptake is suppressed. Exposure of BAEC without L-Arg to substance P and a Ca(2+) ionophore (A-23187) increased O(-)(2)* formation, which was blocked with concurrent presence of L-Arg or the NOS antagonist N(omega)-nitro-L-arginine methyl ester. We conclude that factors including NO itself control y(+) transport function and the production of NO and O(-)(2)*.

Control of arterial blood pressure and renal sodium excretion by nitric oxide synthase in the renal medulla

Work in our laboratory has focused on the role of nitric oxide synthase (NOS) in the regulation of renal medullary function. Biochemical studies demonstrated that the renal medulla is enriched in immunoreactive NOS protein and NOS enzymatic activity when compared with the renal cortex. Further experiments showed large amounts of NOS activity in the inner medullary collecting ducts, while moderate NOS activity was found in glomeruli and vasa recta and minimal NOS activity was detected in other nephron segments examined. In subsequent functional studies, selective renal medullary infusion of NOS stimulators (bradykinin or acetylcholine) or inhibitors (L-NAME) preferentially altered medullary blood flow. The alterations in medullary flow were associated with parallel changes in sodium and water excretion. Similar to the effects observed in anaesthetized rats, chronic infusion of L-NAME directly into the renal medullary interstitial space of conscious, uninephrectomized SD rats selectively decreased renal medullary blood flow throughout a 5-day L-NAME infusion period. The decrease in medullary blood flow was associated with retention of sodium and the development of hypertension, and the effects were reversible. In contrast to the effects of chronic NOS inhibition, renal medullary infusion of NOS substrate L-arginine prevented the development of sodium-sensitive hypertension in the Dahl salt-sensitive rat placed on a high sodium diet. The data reviewed in this paper indicate that NOS isoforms expressed in the renal medulla have a potent influence on renal medullary tubular and vascular function with consequential effects on fluid and electrolyte homeostasis and arterial blood pressure.