High dietary salt intake increases urinary NGAL excretion and creatinine clearance in healthy young adults.

In rodents and older patients with elevated blood pressure (BP), high dietary sodium increases excretion of biomarkers of kidney injury, but it is unclear whether this effect occurs in healthy young adults. The purpose of this study was to determine whether short-term high dietary salt increases urinary excretion of the kidney injury biomarkers neutrophil gelatinase-associated lipocalin (NGAL) and kidney injury molecule-1 (KIM-1) in healthy young adults. Twenty participants participated in a double-blind, placebo-controlled, randomized crossover study. For 10 days each, participants were asked to consume salt (3,900 mg sodium) or placebo capsules. We measured BP during each visit, obtained 24-h urine samples for measurements of electrolytes, NGAL, and KIM-1, and assessed creatinine clearance. Compared with placebo, salt loading increased daily urinary sodium excretion (placebo: 130.3 ± 62.4 mmol/24 h vs. salt: 287.2 ± 72.0 mmol/24 h, P < 0.01). There was no difference in mean arterial BP (placebo: 77 ± 7 mmHg vs. salt: 77 ± 6 mmHg, P = 0.83) between conditions. However, salt loading increased the urinary NGAL excretion rate (placebo: 59.8 ± 44.4 ng/min vs. salt: 80.8 ± 49.5 ng/min, P < 0.01) and increased creatinine clearance (placebo: 110.5 ± 32.9 mL/min vs. salt: 145.0 ± 24.9 mL/min, P < 0.01). Urinary KIM-1 excretion was not different between conditions. In conclusion, in healthy young adults 10 days of dietary salt loading increased creatinine clearance and increased urinary excretion of the kidney injury biomarker marker NGAL but not KIM-1.NEW & NOTEWORTHY In healthy young adults, 10 days of dietary salt loading increased creatinine clearance and increased urinary excretion of the kidney injury biomarker marker neutrophil gelatinase-associated lipocalin despite no change in resting blood pressure.

Ten days of high dietary sodium does not impair cerebral blood flow regulation in healthy adults.

High dietary sodium impairs cerebral blood flow regulation in rodents and is associated with increased stroke risk in humans. However, the effects of multiple days of high dietary sodium on cerebral blood flow regulation in humans is unknown. Therefore, the purpose of this study was to determine whether ten days of high dietary sodium impairs cerebral blood flow regulation. Ten participants (3F/7M; age: 30 ± 10 years; blood pressure (BP): 113 ± 8/62 ± 9 mmHg) participated in this randomized, cross-over design study. Participants were placed on 10-day diets that included either low- (1000 mg/d), medium- (2300 mg/d) or high- (7000 mg/d) sodium separated by ≥four weeks. Urinary sodium excretion, beat-to-beat BP (finger photoplethysmography), middle cerebral artery velocity (transcranial Doppler), and end-tidal carbon dioxide (capnography) was measured. Dynamic cerebral autoregulation during a ten-minute baseline was calculated and cerebrovascular reactivity assessed by determining the percent change in middle cerebral artery blood flow velocity to hypercapnia (8% CO2, 21% oxygen, balance nitrogen) and hypocapnia (via mild hyperventilation). Urinary sodium excretion increased in a stepwise manner (ANOVA P = 0.001) from the low, to medium, to high condition. There were no differences in dynamic cerebral autoregulation between conditions. While there was a trend for a difference during cerebrovascular reactivity to hypercapnia (ANOVA P = 0.06), this trend was abolished when calculating cerebrovascular conductance (ANOVA: P = 0.28). There were no differences in cerebrovascular reactivity (ANOVA P = 0.57) or conductance (ANOVA: P = 0.73) during hypocapnia. These data suggest that ten days of a high sodium diet does not impair cerebral blood flow regulation in healthy adults.

A high salt meal does not impair cerebrovascular reactivity in healthy young adults.

A high sodium (Na+ ) meal impairs peripheral vascular function. In rodents, chronic high dietary Na+ impairs cerebral vascular function, and in humans, habitual high dietary Na+ is associated with increased stroke risk. However, the effects of acute high dietary Na+ on the cerebral vasculature in humans are unknown. The purpose of this study was to determine if acute high dietary Na+ impairs cerebrovascular reactivity in healthy adults. Thirty-seven participants (20F/17M; 25 ± 5 years; blood pressure [BP]: 107 ± 9/61 ± 6 mm Hg) participated in this randomized, cross-over study. Participants were given a low Na+ meal (LSM; 138 mg Na+ ) and a high Na+ meal (HSM; 1,495 mg Na+ ) separated by ≥ one week. Serum Na+ , beat-to-beat BP, middle cerebral artery velocity (transcranial Doppler), and end-tidal carbon dioxide (PET CO2 ) were measured pre- (baseline) and 60 min post-prandial. Cerebrovascular reactivity was assessed by determining the percent change in middle cerebral artery velocity to hypercapnia (via 8% CO2 , 21% oxygen, balance nitrogen) and hypocapnia (via mild hyperventilation). Peripheral vascular function was measured using brachial artery flow-mediated dilation (FMD). Changes in serum Na+ were greater following the HSM (HSM: Δ1.6 ± 1.2 mmol/L vs. LSM: Δ0.7 ± 1.2 mmol/L, p < .01). Cerebrovascular reactivity to hypercapnia (meal effect: p = .41) and to hypocapnia (meal effect: p = .65) were not affected by the HSM. Contrary with previous findings, FMD was not reduced following the HSM (meal effect: p = .74). These data suggest that a single high Na+ meal does not acutely impair cerebrovascular reactivity, and suggests that despite prior findings, a single high Na+ meal does not impair peripheral vascular function in healthy adults.

Short-term water deprivation attenuates the exercise pressor reflex in older female adults.

Older adults have reduced fluid intake and impaired body fluid and electrolyte regulation. Older female adults exhibit exaggerated exercise blood pressure (BP) responses, which is associated with an increased risk of adverse cardiovascular events. However, it is unclear if dysregulated body fluid homeostasis contributes to altered exercise BP responses in older female adults. We tested the hypothesis that short-term water deprivation (WD) increases exercise BP responses in older female adults. Fifteen female adults (eight young [25 ± 6 years] and seven older [65 ± 6 years]) completed two experimental conditions in random crossover fashion; a euhydration control condition and a stepwise reduction in water intake over three days concluding with a 16-hr WD period. During both trials, beat-to-beat BP (photoplethysmography) and heart rate (electrocardiogram) were continuously assessed during rest, handgrip exercise (30% MVC), and post-exercise ischemia (metaboreflex isolation). At screening, older compared to young female adults had greater systolic and diastolic BP (p ≤ .02). Accelerometer-assessed habitual physical activity was not different between groups (p = .65). Following WD, 24-hr urine flow rate decreased, whereas thirst, urine specific gravity, and plasma osmolality increased (condition: p < .05 for all), but these WD-induced changes were not different between age groups (interaction: p ≥ .31 for all). Resting systolic and diastolic BP values were higher in older compared to young adults (p < .01 for both), but were not different between experimental conditions (p ≥ .20). In contrast to our hypothesis, WD was associated with attenuated systolic BP responses during handgrip exercise (post hoc: p < .01) and post-exercise ischemia (post hoc: p = .03) in older, but not young, female adults. These data suggest that reduced water intake-induced challenges to body fluid homeostasis do not contribute to exaggerated exercise BP responses in post-menopausal female adults.

High Salt Intake Augments Blood Pressure Responses During Submaximal Aerobic Exercise.

Background High sodium (Na+) intake is a widespread cardiovascular disease risk factor. High Na+ intake impairs endothelial function and exaggerates sympathetic reflexes, which may augment exercising blood pressure (BP) responses. Therefore, this study examined the influence of high dietary Na+ on BP responses during submaximal aerobic exercise. Methods and Results Twenty adults (8F/12M, age=24±4 years; body mass index 23.0±0.6 kg·m-2; VO2peak=39.7±9.8 mL·min-1·kg-1; systolic BP=111±10 mm Hg; diastolic BP=64±8 mm Hg) participated in this randomized, double-blind, placebo-controlled crossover study. Total Na+ intake was manipulated via ingestion of capsules containing either a placebo (dextrose) or table salt (3900 mg Na+/day) for 10 days each, separated by ≥2 weeks. On day 10 of each intervention, endothelial function was assessed via flow-mediated dilation followed by BP measurement at rest and during 50 minutes of cycling at 60% VO2peak. Throughout exercise, BP was assessed continuously via finger photoplethysmography and every 5 minutes via auscultation. Venous blood samples were collected at rest and during the final 10 minutes of exercise for assessment of norepinephrine. High Na+ intake increased urinary Na+ excretion (placebo=140±68 versus Na+=282±70 mmol·24H-1; P<0.001) and reduced flow-mediated dilation (placebo=7.2±2.4 versus Na+=4.2±1.7%; P<0.001). Average exercising systolic BP was augmented following high Na+ (placebo=Δ30.0±16.3 versus Na+=Δ38.3±16.2 mm Hg; P=0.03) and correlated to the reduction in flow-mediated dilation (R=-0.71, P=0.002). Resting norepinephrine concentration was not different between conditions (P=0.82). Norepinephrine increased during exercise (P=0.002), but there was no Na+ effect (P=0.26). Conclusions High dietary Na+ augments BP responses during submaximal aerobic exercise, which may be mediated, in part, by impaired endothelial function.

The Impact of High Dietary Sodium Consumption on Blood Pressure Variability in Healthy, Young Adults.

BACKGROUND: High sodium (Na+) intake augments blood pressure variability (BPV) in normotensive rodents, without changes in resting blood pressure (BP). Augmented BPV is associated with end-organ damage and cardiovascular morbidity. It is unknown if changes in dietary Na+ influence BPV in humans. We tested the hypothesis that high Na+ feeding would augment BPV in healthy adults. METHODS: Twenty-one participants (10 F/11 M; 26 ± 5 years; BP: 113 ± 11/62 ± 7 mm Hg) underwent a randomized, controlled feeding study that consisted of 10 days of low (2.6 g/day), medium (6.0 g/day), and high (18.0 g/day) salt diets. On the ninth day of each diet, 24-h urine samples were collected and BPV was calculated from 24-h ambulatory BP monitoring. On the tenth day, in-laboratory beat-to-beat BPV was calculated during 10 min of rest. Serum electrolytes were assessed. We calculated average real variability (ARV) and standard deviation (SD) as metrics of BPV. As a secondary analysis, we calculated central BPV from the 24-h ambulatory BP monitoring. RESULTS: 24-h urinary Na+ excretion (low = 41 ± 24, medium = 97 ± 43, high = 265 ± 92 mmol/24 h, P < 0.01) and serum Na+ (low = 140.0 ± 2.1, medium = 140.7 ± 2.7, high = 141.7 ± 2.5 mmol/l, P = 0.009) increased with greater salt intake. 24-h ambulatory ARV (systolic BP ARV: low = 9.5 ± 1.7, medium = 9.5 ± 1.2, high = 10.0 ± 1.9 mm Hg, P = 0.37) and beat-to-beat ARV (systolic BP ARV: low = 2.1 ± 0.6, medium = 2.0 ± 0.4, high = 2.2 ± 0.8 mm Hg, P = 0.46) were not different. 24-h ambulatory SD (systolic BP: P = 0.29) and beat-to-beat SD (systolic BP: P = 0.47) were not different. There was a trend for a main effect of the diet (P = 0.08) for 24-h ambulatory central systolic BPV. CONCLUSIONS: Ten days of high sodium feeding does not augment peripheral BPV in healthy, adults. CLINICAL TRIALS REGISTRATION: NCT02881515.

A high-salt meal does not augment blood pressure responses during maximal exercise.

Augmented blood pressure (BP) responses during exercise are predictive of future cardiovascular disease. High dietary sodium (Na+) increases BP responses during static exercise. It remains unclear if high dietary Na+ augments BP responses during dynamic exercise. The purpose of this study was to test the hypothesis that an acute high-Na+ meal would augment BP responses during dynamic exercise. Twenty adults (10 male/10 female; age, 26 ± 5 years; BP, 105 ± 10/57 ± 6 mm Hg) were given a high-Na+ meal (HSM; 1495 mg Na+) and a low-Na+ meal (LSM; 138 mg Na+) separated by at least 1 week, in random order. Serum Na+ and plasma osmolality were measured. Eighty minutes following the meal, participants completed a graded-maximal exercise protocol on a cycle ergometer. Heart rate, beat-by-beat BP, cardiac output, total peripheral resistance, and manual BP were measured at rest and during exercise. Both serum Na+ (HSM: Δ1.6 ± 2.0 vs LSM: Δ1.1 ± 1.8 mmol/L, P = 0.0002) and plasma osmolality (HSM: Δ3.0 ± 4.5 vs LSM: Δ2.0 ± 4.2 mOsm/(kg·H2O), P = 0.01) were higher following the HSM. However, the HSM did not augment BP during peak exercise (systolic BP: HSM: 170 ± 23 vs LSM: 171 ± 21 mm Hg, P = 0.81). These findings suggest that an acute high-salt meal does not augment BP responses during dynamic exercise in adults. Novelty The high-salt meal increased serum sodium and plasma osmolality compared with the low-salt meal. The high-salt meal did not augment blood pressure responses during maximal dynamic exercise. This is important as augmented blood pressure responses during exercise put individuals at greater risk for development of cardiovascular disease.

Short-term water deprivation does not increase blood pressure variability or impair neurovascular function in healthy young adults.

High dietary salt increases arterial blood pressure variability (BPV) in salt-resistant, normotensive rodents and is thought to result from elevated plasma [Na+] sensitizing central sympathetic networks. Our purpose was to test the hypothesis that water deprivation (WD)-induced elevations in serum [Na+] augment BPV via changes in baroreflex function and sympathetic vascular transduction in humans. In a randomized crossover fashion, 35 adults [17 female/18 male, age: 25 ± 4 yr, systolic/diastolic blood pressure (BP): 107 ± 11/60 ± 7 mmHg, body mass index: 23 ± 3 kg/m2] completed two hydration protocols: a euhydration control condition (CON) and a stepwise reduction in water intake over 3 days, concluding with 16 h of WD. We assessed blood and urine electrolyte concentrations and osmolality, resting muscle sympathetic nerve activity (MSNA; peroneal microneurography; 18 paired recordings), beat-to-beat BP (photoplethysmography), common femoral artery blood flow (Doppler ultrasound), and heart rate (single-lead ECG). A subset of participants (n = 25) underwent ambulatory BP monitoring during day 3 of each protocol. We calculated average real variability as an index of BPV. WD increased serum [Na+] (141.0 ± 2.3 vs. 142.1 ± 1.7 mmol/L, P < 0.01) and plasma osmolality (288 ± 4 vs. 292 ± 5 mosmol/kg H2O, P < 0.01). However, WD did not increase beat-to-beat (1.9 ± 0.4 vs. 1.8 ± 0.4 mmHg, P = 0.24) or ambulatory daytime (9.6 ± 2.1 vs. 9.4 ± 3.3 mmHg, P = 0.76) systolic BPV. Additionally, sympathetic baroreflex sensitivity (P = 0.20) and sympathetic vascular transduction were not different after WD (P = 0.17 for peak Δmean BP following spontaneous MSNA bursts). These findings suggest that, despite modestly increasing serum [Na+], WD does not affect BPV, arterial baroreflex function, or sympathetic vascular transduction in healthy young adults.

Salt Loading Blunts Central and Peripheral Postexercise Hypotension.

INTRODUCTION: High salt intake is a widespread cardiovascular risk factor with systemic effects. These effects include an expansion of plasma volume, which may interfere with postexercise hypotension (PEH). However, the effects of high salt intake on central and peripheral indices of PEH remain unknown. We tested the hypothesis that high salt intake would attenuate central and peripheral PEH. METHODS: Nineteen healthy adults (7 female/12 male; age, 25 ± 4 yr; body mass index, 23.3 ± 2.2 kg·m; V[Combining Dot Above]O2peak, 41.6 ± 8.7 mL·min·kg; systolic blood pressure (BP), 112 ± 9 mm Hg; diastolic BP, 65 ± 9 mm Hg) participated in this double-blind, randomized, placebo-controlled crossover study. Participants were asked to maintain a 2300 mg·d sodium diet for 10 d on two occasions separated by ≥2 wk. Total salt intake was manipulated via ingestion of capsules containing either table salt (3900 mg·d) or placebo (dextrose) during each diet. On the 10th day, participants completed 50 min of cycling at 60% V[Combining Dot Above]O2peak. A subset of participants (n = 8) completed 60 min of seated rest (sham trial). Beat-to-beat BP was measured in-laboratory for 60 min after exercise via finger photoplethysmography. Brachial and central BPs were measured for 24 h after exercise via ambulatory BP monitor. RESULTS: Ten days of high salt intake increased urinary sodium excretion (134 ± 70 (dextrose) vs 284 ± 74 mmol per 24 h (salt), P < 0.001), expanded plasma volume (7.2% ± 10.8%), and abolished PEH during in-laboratory BP monitoring (main effect of diet, P < 0.001). Ambulatory systolic BPs were higher for 12 h after exercise during the salt and sham trials compared with the dextrose trial (average change, 3.6 ± 2.1 mm Hg (dextrose), 9.9 ± 1.4 mm Hg (salt), 9.8 ± 2.5 mm Hg (sham); P = 0.01). Ambulatory central systolic BP was also higher during the salt trial compared with dextrose trial. CONCLUSION: High salt intake attenuates peripheral and central PEH, potentially reducing the beneficial cardiovascular effects of acute aerobic exercise.

Water deprivation does not augment sympathetic or pressor responses to sciatic afferent nerve stimulation in rats or to static exercise in humans.

Excess dietary salt intake excites central sympathetic networks, which may be related to plasma hypernatremia. Plasma hypernatremia also occurs following water deprivation (WD). The purpose of this study was to test the hypothesis that WD induces hypernatremia and consequently augments sympathetic and pressor responses to sympathoexcitatory stimuli in rats and humans. Sympathetic nerve activity (SNA) and arterial blood pressure (ABP) responses to sciatic afferent nerve stimulation (2-20 Hz) and chemical stimulation of the rostral ventrolateral medulla (RVLM) were assessed in rats after 48 h of WD and compared with normally hydrated control rats (CON). In a parallel randomized-crossover human experiment (n = 13 healthy young adults), sympathetic (microneurography) and pressor (photoplethysmography) responses to static exercise were compared between 16-h WD and CON conditions. In rats, plasma [Na+] was significantly higher in WD versus CON [136 ± 2 vs. 144 ± 2 (SD) mM, P < 0.01], but sciatic afferent nerve stimulation produced similar increases in renal SNA [5 Hz, 174 ± 34 vs. 169 ± 49% (SD), n = 6-8] and mean ABP [5 Hz, 21 ± 6 vs. 18 ± 7 (SD mmHg, n = 6-8]. RVLM injection of l-glutamate also produced similar increases in SNA and ABP in WD versus CON rats. In humans, WD increased serum [Na+] [140.6 ± 2.1 vs. 142.1 ± 1.9 mM (SD), P = 0.02] but did not augment sympathetic [muscle SNA: change from baseline (Δ) 6 ± 7 vs. 5 ± 7 (SD) bursts/min, P = 0.83] or mean ABP [Δ 12 ± 5 vs. 11 ± 8 (SD) mmHg, P = 0.73; WD vs. CON for all results] responses during the final minute of exercise. These findings suggest that despite eliciting relative hypernatremia, WD does not augment sympathetic or pressor responses to sciatic afferent stimulation in rats or to static exercise in humans. NEW & NOTEWORTHY Excess dietary salt intake excites central sympathetic networks, which may be related to plasma hypernatremia. Plasma hypernatremia also occurs following water deprivation (WD). We sought to determine whether plasma hypernatremia/hyperosmolality induced by WD augments sympathetic and pressor responses to sympathoexcitatory stimuli. Our findings suggest that WD does not augment sympathetic or pressor responses to sciatic afferent nerve stimulation in rats or to static exercise in humans.

Relation between resting sympathetic outflow and vasoconstrictor responses to sympathetic nerve bursts: sex differences in healthy young adults.

Previous studies have demonstrated an inverse relation between resting muscle sympathetic nerve activity (MSNA) and vasoconstrictor responsiveness (i.e., sympathetic transduction), such that those with high resting MSNA have low vascular responsiveness, and vice versa. The purpose of this investigation was to determine whether biological sex influences the balance between resting MSNA and beat-to-beat sympathetic transduction. We measured blood pressure (BP) and MSNA during supine rest in 54 healthy young adults (27 females: 23 ± 4 yr, 107 ± 8/63 ± 8 mmHg; 27 males: 25 ± 3 yr, 115 ± 11/64 ± 7 mmHg; means ± SD). We quantified beat-to-beat fluctuations in mean arterial pressure (MAP, mmHg) and limb vascular conductance (LVC, %) for 10 cardiac cycles after each MSNA burst using signal averaging, an index of sympathetic vascular transduction. In females, there was no correlation between resting MSNA (burst incidence; burst/100 heartbeats) and peak ΔMAP (r = -0.10, P = 0.62) or peak ΔLVC (r = -0.12, P = 0.63). In males, MSNA was related to peak ΔMAP (r = -0.50, P = 0.01) and peak ΔLVC (r = 0.49, P = 0.03); those with higher resting MSNA had blunted increases in MAP and reductions in LVC in response to a burst of MSNA. In a sub-analysis, we performed a median split between high- versus low-MSNA status on ΔMAP and ΔLVC within each sex and found that only males demonstrated a significant difference in ΔMAP and ΔLVC between high- versus low-MSNA groups. These findings support an inverse relation between resting MSNA and sympathetic vascular transduction in males only and advance our understanding on the influence of biological sex on sympathetic nervous system-mediated alterations in beat-to-beat BP regulation.

Reducing Dietary Sodium to 1000 mg per Day Reduces Neurovascular Transduction Without Stimulating Sympathetic Outflow.

The American Heart Association recommends no more than 1500 mg of sodium/day as ideal. Some cohort studies suggest low-sodium intake is associated with increased cardiovascular mortality. Extremely low-sodium diets (≤500 mg/d) elicit activation of the renin-angiotensin-aldosterone system and stimulate sympathetic outflow. The effects of an American Heart Association-recommended diet on sympathetic regulation of the vasculature are unclear. Therefore, we assessed whether a 1000 mg/d diet alters sympathetic outflow and sympathetic vascular transduction compared with the more commonly recommended 2300 mg/d. We hypothesized that sodium reduction from 2300 to 1000 mg/d would not affect resting sympathetic outflow but would reduce sympathetic transduction in healthy young adults. Seventeen participants (age: 26±2 years, 9F/8M) completed 10-day 2300 and 1000 mg/d sodium diets in this randomized controlled feeding study (crossover). We measured resting renin activity, angiotensin II, aldosterone, blood pressure, muscle sympathetic nerve activity, and norepinephrine. We quantified beat-by-beat changes in mean arterial pressure and leg vascular conductance (femoral artery ultrasound) following spontaneous sympathetic bursts to assess sympathetic vascular transduction. Reducing sodium to 1000 mg/d increased renin activity, angiotensin II, and aldosterone ( P<0.01 for all) but did not alter mean arterial pressure (78±2 versus 77±2 mm Hg, P=0.56), muscle sympathetic nerve activity (13.9±1.3 versus 13.9±0.8 bursts/min, P=0.98), or plasma/urine norepinephrine. Sympathetic vascular transduction decreased ( P<0.01). These data suggest that reducing sodium from 2300 to 1000 mg/d stimulates the renin-angiotensin-aldosterone system, does not increase resting basal sympathetic outflow, and reduces sympathetic vascular transduction in normotensive adults.