Dietary sodium intake modulates vasodilation mediated by nitroprusside but not by methacholine in the human forearm

The Role of Nitric Oxide in the Micro- and Macrovascular Response to a 7-Day High-Salt Diet in Healthy Individuals

This study aimed to investigate the specific role of nitric oxide (NO) in micro- and macrovascular response to a 7-day high-salt (HS) diet, specifically by measuring skin microvascular local thermal hyperemia and the flow-mediated dilation of the brachial artery, as well as serum NO and three NO synthase enzyme (NOS) isoform concentrations in healthy individuals. It also aimed to examine the concept of non-osmotic sodium storage in the skin following the HS diet by measuring body fluid status and systemic hemodynamic responses, as well as serum vascular endothelial growth factor C (VEGF-C) concentration. Forty-six young, healthy individuals completed a 7-day low-salt diet, followed by a 7-day HS diet protocol. The 7-day HS diet resulted in impaired NO-mediated endothelial vasodilation in peripheral microcirculation and conduit arteries, in increased eNOS, decreased nNOS, and unchanged iNOS concentration and NO serum level. The HS diet did not change the volume of interstitial fluid, the systemic vascular resistance or the VEGF-C serum level. These results indicate that the 7-day HS-diet induces systemic impairment of NO-mediated endothelial vasodilation, while dissociation in the eNOS and nNOS response indicates complex adaptation of main NO-generating enzyme isoforms to HS intake in healthy individuals. Our results failed to support the concept of non-osmotic sodium storage.

Mechanisms of Dietary Sodium-Induced Impairments in Endothelial Function and Potential Countermeasures

Despite decades of efforts to reduce sodium intake, excess dietary sodium remains commonplace, and contributes to increased cardiovascular morbidity and mortality independent of its effects on blood pressure. An increasing amount of research suggests that high-sodium diets lead to reduced nitric oxide-mediated endothelial function, even in the absence of a change in blood pressure. As endothelial dysfunction is an early step in the progression of cardiovascular diseases, the endothelium presents a target for interventions aimed at reducing the impact of excess dietary sodium. In this review, we briefly define endothelial function and present the literature demonstrating that excess dietary sodium results in impaired endothelial function. We then discuss the mechanisms through which sodium impairs the endothelium, including increased reactive oxygen species, decreased intrinsic antioxidant defenses, endothelial cell stiffening, and damage to the endothelial glycocalyx. Finally, we present selected research findings suggesting that aerobic exercise or increased intake of dietary potassium may counteract the deleterious vascular effects of a high-sodium diet.

Enhanced Antioxidative Defense by Vitamins C and E Consumption Prevents 7-Day High-Salt Diet-Induced Microvascular Endothelial Function Impairment in Young Healthy Individuals

This study aimed to examine whether the oral supplementation of vitamins C and E during a seven-day high salt diet (HS; ~14 g salt/day) prevents microvascular endothelial function impairment and changes oxidative status caused by HS diet in 51 (26 women and 25 men) young healthy individuals. Laser Doppler flowmetry measurements demonstrated that skin post-occlusive reactive hyperemia (PORH), and acetylcholine-induced dilation (AChID) were significantly impaired in the HS group, but not in HS+C+E group, while sodium nitroprusside-induced dilation remained unaffected by treatments. Serum oxidative stress markers: Thiobarbituric acid reactive substances (TBARS), 8-iso prostaglandin-F2α, and leukocytes’ intracellular hydrogen peroxide (H₂O₂) production were significantly increased, while ferric-reducing ability of plasma (FRAP) and catalase concentrations were decreased in the HS group. All these parameters remained unaffected by vitamins supplementation. Matrix metalloproteinase 9, antioxidant enzymes Cu/Zn SOD and glutathione peroxidase 1, and leukocytes’ intracellular superoxide production remained unchanged after the protocols in both HS and HS+C+E groups. Importantly, multiple regression analysis revealed that FRAP was the most powerful predictor of AChID, while PORH was strongly predicted by both FRAP and renin-angiotensin system activity. Hereby, we demonstrated that oxidative dis-balance has the pivotal role in HS diet-induced impairment of endothelial and microvascular function in healthy individuals which could be prevented by antioxidative vitamins consumption.

Short-term high salt intake reduces brachial artery and microvascular function in the absence of changes in blood pressure

OBJECTIVES

The aims of this study were to test the hypothesis that short-term high salt intake reduces macrovascular and microvascular endothelial function in the absence of changes in blood pressure and to determine whether acute exercise restores endothelial function after high salt in women.

MATERIALS AND METHODS

Twelve women were administered high salt (11 g of sodium chloride for 7 days) and then underwent a weightlifting session. Brachial artery flow-mediated dilation and nitroglycerin dilation were measured with ultrasound at baseline, after high salt, and after weightlifting. Subcutaneous fat tissue biopsies were obtained at baseline, after high salt, and after weightlifting. Resistance arteries from biopsies were cannulated for vascular reactivity measurements in response to flow [flow-induced dilation (FID)] and acetylcholine.

RESULTS

Blood pressure was similar before and after high salt diet. Brachial flow-mediated dilation was reduced after high salt diet but was not affected by acute weightlifting. Brachial nitroglycerin dilations were similar before and after high salt. FID and acetylcholine-induced dilation of resistance arteries were similar to that of before and after high salt diet. FID and acetylcholine-induced dilation was not altered by weightlifting after high salt diet. However, N-nitro-L-arginine methyl ester significantly reduced FID at baseline and after exercise but had no effect dilator reactivity after high salt diet alone.

CONCLUSION

These data suggest that high salt intake reduces brachial artery endothelial function and switches the mediator of vasodilation in the microcirculation to a non-nitric oxide-dependent mechanism in healthy adults and acute exercise may switch the dilator mechanism back to nitric oxide during high salt diet.

Salt, Angiotensin II, Superoxide, and Endothelial Function

Proper function of the vascular endothelium is essential for cardiovascular health, in large part due to its antiproliferative, antihypertrophic, and anti-inflammatory properties. Crucial to the protective role of the endothelium is the production and liberation of nitric oxide (NO), which not only acts as a potent vasodilator, but also reduces levels of reactive oxygen species, including superoxide anion (O2•-). Superoxide anion is highly injurious to the vasculature because it not only scavenges NO molecules, but has other damaging effects, including direct oxidative disruption of normal signaling mechanisms in the endothelium and vascular smooth muscle cells. The renin-angiotensin system plays a crucial role in the maintenance of normal blood pressure. This function is mediated via the peptide hormone angiotensin II (ANG II), which maintains normal blood volume by regulating Na+ excretion. However, elevation of ANG II above normal levels increases O2•- production, promotes oxidative stress and endothelial dysfunction, and plays a major role in multiple disease conditions. Elevated dietary salt intake also leads to oxidant stress and endothelial dysfunction, but these occur in the face of salt-induced ANG II suppression and reduced levels of circulating ANG II. While the effects of abnormally high levels of ANG II have been extensively studied, far less is known regarding the mechanisms of oxidant stress and endothelial dysfunction occurring in response to chronic exposure to abnormally low levels of ANG II. The current article focuses on the mechanisms and consequences of this less well understood relationship among salt, superoxide, and endothelial function.

The role of cyclo-oxygenase-1 in high-salt diet-induced microvascular dysfunction in humans

KEY POINTS

Recent studies have shown that some of the deleterious effects of a high-salt (HS) diet are independent of elevated blood pressure and are associated with impaired endothelial function. Increased generation of cyclo-oxygenase (COX-1 and COX-2)-derived vasoconstrictor factors and endothelial activation may contribute to impaired vascular relaxation during HS loading. The present study aimed to assess the regulation of microvascular reactivity and to clarify the role of COX-1 and COX-2 in normotensive subjects on a short-term HS diet. The present study demonstrates the important role of COX-1 derived vasoconstrictor metabolites in regulation of microvascular blood flow during a HS diet. These results help to explain how even short-term HS diets may impact upon microvascular reactivity without changes in blood pressure and suggest that a vasoconstrictor metabolite of COX-1 could play a role in this impaired tissue blood flow.

ABSTRACT

The present study aimed to assess the effect of a 1-week high-salt (HS) diet on the role of cyclo-oxygenases (COX-1 and COX-2) and the vasoconstrictor prostaglandins, thromboxane A2 (TXA2 ) and prostaglandin F2α (PGF2α ), on skin microcirculatory blood flow, as well as to detect its effect on markers of endothelial activation such as soluble cell adhesion molecules. Young women (n = 54) were assigned to either the HS diet group (N = 30) (∼14 g day(-1) NaCl ) or low-salt (LS) diet group (N = 24) (<2.3 g day(-1) NaCl ) for 7 days. Post-occlusive reactive hyperaemia (PORH) in the skin microcirculation was assessed by laser Doppler flowmetry. Plasma renin activity, plasma aldosterone, plasma and 24 h urine sodium and potassium, plasma concentrations of TXB2 (stable TXA2 metabolite) and PGF2α , soluble cell adhesion molecules and blood pressure were measured before and after the diet protocols. One HS diet group subset received 100 mg of indomethacin (non-selective COX-1 and COX-2 inhibitor), and another HS group subset received 200 mg of celecoxib (selective COX-2 inhibitor) before repeating laser Doppler flowmetry measurements. Blood pressure was unchanged after the HS diet, although it significantly reduced after the LS diet. Twenty-four hour urinary sodium was increased, and plasma renin activity and plasma aldosterone levels were decreased after the HS diet. The HS diet significantly impaired PORH and increased TXA2 but did not change PGF2α levels. Indomethacin restored microcirculatory blood flow and reduced TXA2 . By contrast, celecoxib decreased TXA2 levels but had no significant effects on blood flow. Restoration of of PORH by indomethacin during a HS diet suggests an important role of COX-1 derived vasoconstrictor metabolites in the regulation of microvascular blood flow during HS intake.

Role of the epithelial sodium channel in salt-sensitive hypertension

The epithelial sodium channel (ENaC) is a heteromeric channel composed of three similar but distinct subunits, α, β and γ. This channel is an end-effector in the rennin-angiotensin-aldosterone system and resides in the apical plasma membrane of the renal cortical collecting ducts, where reabsorption of Na(+) through ENaC is the final renal adjustment step for Na(+) balance. Because of its regulation and function, the ENaC plays a critical role in modulating the homeostasis of Na(+) and thus chronic blood pressure. The development of most forms of hypertension requires an increase in Na(+) and water retention. The role of ENaC in developing high blood pressure is exemplified in the gain-of-function mutations in ENaC that cause Liddle's syndrome, a severe but rare form of inheritable hypertension. The evidence obtained from studies using animal models and in human patients indicates that improper Na(+) retention by the kidney elevates blood pressure and induces salt-sensitive hypertension.

Low-renin (volume dependent) mild-hypertensive patients have impaired flow-mediated and glyceryl-trinitrate stimulated vascular reactivity

BACKGROUND

Low-renin (volume-dependent) hypertension represents 25-30% of all cases of primary hypertension. Endothelial dysfunction and vascular remodeling are associated with hypertension but their relevance to volume-dependent hypertension (VDH) is not yet known. To evaluate this, flow-mediated dilation (FMD) of the brachial artery and the carotid intima-media thickness in the distal common carotid artery were measured and compared between renin-dependent mild-hypertensive patients (RDH) and controls.

METHOD AND RESULTS

The study group comprised 40 mild-hypertensive patients and 25 controls. Plasma renin activity (PRA), plasma aldosterone concentration, angiotensin II and nitrite/nitrate plasma levels were measured. According to PRA, subjects were classified as VDH (<0.6 ng . ml (-1) . h(-1)), or RDH (>0.6 ng . ml(-1) . h (-1)). Vascular function was evaluated by FMD before and after reactive hyperemia (RH) and glyceryl-trinitrate (GTN) administration. FMD in response to RH and GTN in the VDH group when compared with RDH group was 10.2+/-2.8% vs 13.3+/-3.6% (p=0.01); and 16.0+/-3.5% vs 19.9+/-4.5% (p=0.01), respectively.

CONCLUSION

This study showed impaired FMD and reduced GTN response in mildly hypertensive patients with low-renin plasma levels.

Nitric oxide inhibition as a mechanism for blood pressure increase during salt loading in normotensive postmenopausal women

OBJECTIVES

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO), which plays an important role in natriuresis. We determined whether changes in endothelium-dependent vasodilation (EDD) and plasma ADMA predict changes in blood pressure (BP) after salt loading in normotensive postmenopausal women (PMW).

METHODS

In 15 normotensive PMW (age 50-60 years), not receiving estrogen, ambulatory 24-h BP, plasma lipids, and ADMA were measured after 4 days of a low-salt diet (70 mEq/day) and following 7 days of high-salt intake (260 mEq/day). Brachial artery diameter at rest, during reactive hyperemia, i.e. EDD, and after sublingual nitroglycerin, i.e. non-EDD, were measured by ultrasound. The 24-h urinary NO metabolite (NOx) was measured by Griess reaction. Plasma ADMA was measured by high-pressure liquid chromatography.

RESULTS

During low-salt, 24-h BP levels averaged 121 +/- 11 and 69 +/- 7 mmHg for systolic BP (SBP) and diastolic BP (DBP), respectively. After salt loading, average 24-h BP increases were: 7.6 mmHg for SBP, 2.2 mmHg for DBP, and 5.5 mmHg for pulse pressure (PP). Increases of 24-h SBP and 24-h PP after salt loading correlated directly with changes in ADMA (partial R2 = 0.16 for 24-h SBP and 0.17 for 24-h PP, P < 0.05 for both) and inversely with changes in EDD (partial R2 = 0.13, P = 0.09 for 24 h SBP and partial R2 = 0.15, P = 0.07 for 24-h PP), after adjustment for age and cholesterol.

CONCLUSIONS

Inhibition of NO bioavailability by ADMA and a subsequent reduction in EDD contribute to the increase in BP during high-salt intake in normotensive PMW not receiving estrogen.

Nitric oxide production decreases after salt loading but is not related to blood pressure changes or nitric oxide-mediated vascular responses

BACKGROUND

Nitric oxide production is a homeostatic mechanism that may regulate blood pressure during salt loading. Salt-sensitive hypertension in animal models and in humans is characterized by increased blood pressure and decreased nitric oxide production after salt loading. It is not known if this impaired nitric oxide production is the result of hypertension or is a mechanism contributing to the blood pressure response to salt.

METHODS AND RESULTS

The effects of salt loading on blood pressure, nitric oxide-mediated vasodilation and nitric oxide production were measured in 25 normotensive subjects after 6 days on either a high (400 mmol/day) or low (10 mmol/day) sodium, low nitrate diet. Mean arterial pressure increased during the high-salt diet [4 +/- 1 mmHg (mean +/- SEM)] in 12 subjects and remained unchanged or decreased (-4 +/- 1 mmHg) in 13 subjects. Plasma nitrite and nitrate, a measure of nitric oxide production, decreased significantly from 39 +/- 3.3 micromol/l during the low-salt diet to 22.4 +/- 2.4 micromol/l during the high-salt diet (P = 0.0001). However, changes in mean arterial pressure from low- to high-salt diet did not correlate with changes in plasma nitrite and nitrate (r = 0.14, P = 0.51). Forearm blood flow increased significantly (P <0.0001) in response to mental stress, a nitric oxide-mediated response, but was not affected by sodium intake (from 7.8 +/- 0.9 to 11.2 +/- 1.4 ml/min per 100 ml during low salt versus 8.5 +/- 1.2 to 10.4 +/- 1.3 ml/min per 100 ml during high salt,P = 0.3).

CONCLUSIONS

Salt loading results in a decrease in nitric oxide production in both salt-sensitive and salt-resistant normotensive subjects, which is independent of changes in blood pressure and does not affect the nitric oxide-mediated vascular response to mental stress. In contrast to salt-resistant animal models, salt loading in healthy subjects does not increase nitric oxide production. Therefore, the increased blood pressure response to salt loading may occur through mechanisms other than nitric oxide, or salt-sensitive individuals are more sensitive to the reduced nitric oxide production that occurs after salt loading in both salt-sensitive and salt-resistant subjects.

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.

Endothelial function during stimulation of renin-angiotensin system by low-sodium diet in humans

We examined whether physiological stimulation of the endogenous renin-angiotensin system results in impaired endothelium-dependent vasodilatation in forearm resistance vessels of healthy subjects and whether this impairment can be prevented by angiotensin II type 1 receptor blockade. A low-sodium diet was administered to 27 volunteers who were randomized to concomitant treatment with losartan (100 mg once daily) or matched placebo in a double-blind fashion. Forearm blood flow was assessed by venous occlusion plethysmography at baseline and after 5 days. Endothelium-dependent and -independent vasodilation was assessed by intra-arterial infusion of methacholine and verapamil, respectively. The low-sodium diet resulted in significantly decreased urine sodium excretion (placebo: 146 +/- 64 vs. 10 +/- 9 meq/24 h, P < 0.001; losartan: 141 +/- 56 vs. 14 +/- 14 meq/24 h, P < 0.001) and increased plasma renin activity (placebo: 1.0 +/- 0.5 vs. 5.0 +/- 2.5 ng x ml(-1) x h(-1), P < 0.001; losartan: 3.8 +/- 7.2 vs. 19.1 +/- 11.2 ng x ml(-1) x h(-1), P = 0.006) in both the losartan and placebo groups. With the baseline study as the reference, the diet intervention was not associated with any significant change in endothelium-dependent vasodilation to methacholine in either the placebo (P = 0.74) or losartan (P = 0.40) group. We conclude that short-term physiological stimulation of the renin-angiotensin system does not cause clinically significant endothelial dysfunction. Losartan did not influence endothelium-dependent vasodilation in humans with a stimulated renin-angiotensin system.

Role of angiotensin II in regulation of basal and sympathetically stimulated vascular tone in early and advanced cirrhosis

BACKGROUND & AIMS

The renin-angiotensin and sympathetic nervous systems are activated in cirrhosis. This study aimed to establish the role of angiotensin II (ANG II) in the regulation of basal and sympathetically stimulated vascular tone in preascitic cirrhotic patients and patients with diuretic-refractory ascites compared with age- and sex-matched healthy controls.

METHODS

Forearm blood flow (FBF) responses to lower body negative pressure (LBNP) and to subsystemic, intrabrachial infusions of losartan, an angiotensin II type 1 (AT(1)) receptor antagonist, norepinephrine, and ANG II were measured using venous occlusion plethysmography.

RESULTS

In all groups, ANG II and norepinephrine caused dose-dependent reductions in FBF (P < 0.001); responses to norepinephrine were similar across the 3 groups but those to ANG II were less in both cirrhotic groups than in controls (P < 0.01). Losartan caused a dose-dependent increase in FBF only in patients with refractory ascites (P < 0.01). LBNP caused less reduction in FBF in refractory ascites patients than in both preascitic patients and controls (P < 0.01).

CONCLUSIONS

Despite hyporesponsiveness to exogenous ANG II in both early and advanced cirrhosis, endogenous ANG II contributes to the maintenance of basal vascular tone only in advanced cirrhosis. These findings suggest a role of ANG II in the pathogenesis of ascites. Attenuated LBNP responses occurred only in advanced cirrhosis, without apparent interaction with endogenous ANG II.

Dietary sodium affects systemic and renal hemodynamic response to NO inhibition in healthy humans

Animal studies have indicated that increased nitric oxide (NO) synthesis plays a significant role in the renal adaptation to increased sodium intake. To investigate the role of NO during increased sodium intake in humans, we studied the effect of acute, systemic injection of NG-monomethyl-L-arginine (L-NMMA) on renal hemodynamics [glomerular filtration rate and renal plasma flow (GFR and RPF, respectively)], urinary sodium excretion (FENa), systemic hemodynamics [mean arterial blood pressure and heart rate (MAP and HR)], and plasma levels of several vasoactive hormones in 12 healthy subjects during high (250 mmol/day) and low (77 mmol/day) sodium intake in a crossover design. The sodium diets were administered for 5 days before the L-NMMA treatments, in randomized order, with a washout period of 9 days between each diet and L-NMMA treatment. GFR and RPF were measured using the renal clearance of 51Cr-labeled EDTA and 125I-labeled hippuran by the constant infusion technique in clearance periods of 30-min duration. Two baseline periods were obtained, after which L-NMMA was given (3 mg/kg over 10 min), and the effect of treatment was followed over the next five clearance periods. During high sodium intake, L-NMMA induced a more pronounced relative decrease in RPF (P = 0.0417, ANOVA), a more pronounced relative decrease in FENa (P = 0.0032, ANOVA), and a more pronounced relative increase in MAP (P = 0.0231, ANOVA). During low sodium intake, the effect of L-NMMA on FENa was abolished. During low sodium intake, L-NMMA induced a sustained drop in plasma renin (31 +/- 5 vs. 25 +/- 5 microU/ml, P < 0.001), which was not seen during high sodium intake. The data indicate that increased production of NO is an important part of the adaptation to increased dietary sodium intake in healthy humans, with respect to renal hemodynamics, sodium excretion, and the secretion of renin.

Dietary sodium intake modulates systemic but not forearm norepinephrine release

INTRODUCTION

Sodium intake has profound effects on systemic and renal sympathetic activity, but its effects on sympathetic activity in skeletal muscle vascular beds, a site at which local regulatory mechanisms could alter vascular tone directly, are unclear.

METHODS

To determine the effect of dietary sodium intake on basal and isoproterenol-stimulated systemic and forearm norepinephrine kinetics, we studied seven healthy male volunteers twice, 4 weeks apart, while they were receiving a low-sodium (10 mmol sodium/24 hours) diet and a high-sodium diet (250 mmol sodium/24 hours). Forearm blood flow, measured by plethysmography, and systemic and forearm norepinephrine spillover, measured by radioisotope dilution, were determined before and after intra-arterial infusion of 60 and 400 ng/min isoproterenol.

RESULTS

Baseline (before isoproterenol) systemic norepinephrine spillover was higher when subjects received the low-sodium diet (448.1 +/- 55.7 ng/min) compared with the high-sodium diet (269.7 +/- 42.7 ng/min; p < 0.05). In contrast, sodium intake did not affect local forearm norepinephrine spillover, either at baseline (low-sodium diet, 2.05 +/- 0.48 ng/min versus high-sodium diet, 2.63 +/- 0.79 ng/min; p = 0.50) or after stimulation with isoproterenol in doses of 60 ng/min (low-sodium diet, 8.84 +/- 2.2 ng/min versus high-sodium diet, 6.1 +/- 1.9 ng/min; p = 0.38) or 400 ng/min (low-sodium diet, 16.4 +/- 4.5 ng/min versus high-sodium diet, 16.7 +/- 2.5 ng/min; p = 0.93).

CONCLUSIONS

Under conditions of low sodium intake, systemic norepinephrine spillover was increased but forearm norepinephrine spillover was not, suggesting that alteration in sodium intake may produce a differential effect on norepinephrine spillover in different tissues but that decreased local sympathetic activity in skeletal muscle is not the likely mechanism by which a low-sodium diet may lower blood pressure or attenuate stress-induced pressor responses.