High dietary sodium augments vascular tone and attenuates low-flow mediated constriction in salt-resistant adults

Mechanistic, participant, and movement-related factors that contribute to low-flow-mediated constriction

Endothelial function is commonly determined via the ultrasound-based flow-mediated dilation (FMD) technique which assesses arterial dilation in response to a hyperemia response following distal cuff occlusion. However, the low-flow-mediated constriction (L-FMC) response during cuff-induced ischemia is often overlooked. L-FMC provides unique information regarding endothelial function, but vascular researchers may be unclear on what this metric adds. Therefore, the objective of this review was to examine the mechanistic determinants and participant-level factors of L-FMC. Existing mechanistic studies have demonstrated that vasoreactivity to low flow may be mediated via non-nitric oxide vasodilators (i.e., endothelial hyperpolarizing factors and/or prostaglandins), inflammatory markers, and enhancement of vasoconstriction via endothelin-1. In general, participant-level factors such as aging and presence of cardiovascular conditions generally are associated with attenuated L-FMC responses. However, the influence of sex on L-FMC is unclear with divergent results between L-FMC in upper versus lower limb vessels. The ability of aerobic exercise to augment L-FMC (i.e., make more negative) is well supported, but there is a major gap in the literature concerning the mechanistic underpinnings of this observation. This review summarizes that while larger L-FMC responses are generally healthy, the impact of interventions to augment/attenuate L-FMC has not included mechanistic measures that would provide insight into non-nitric oxide-based endothelial function. Clarifications to terminology and areas of further inquiry as it relates to the specific pharmacological, individual-level factors, and lifestyle behaviors that impact L-FMC are highlighted. A greater integration of mechanistic work alongside applied lifestyle interventions is required to better understand endothelial cell function to reductions in local blood flow.

Inverse salt sensitivity in normotensive adults: role of demographic factors

BACKGROUND

Salt sensitivity and inverse salt sensitivity [ISS; a reduction in blood pressure (BP) on a high sodium diet] are each associated with increased incidence of hypertension. The purpose of this analysis was to determine the prevalence of ISS in normotensive adults and whether ISS is associated with any demographic characteristic(s).

METHODS

Healthy normotensive, nonobese adults [ n  = 84; 43 women; age = 37 ± 13 years; baseline mean arterial pressure (MAP) = 89 ± 8 mmHg] participated in a controlled feeding study, consuming 7-day low-sodium (20 mmol sodium/day) and high-sodium (300 mmol sodium/day) diets. Twenty-four-hour ambulatory BP was assessed on the last day of each diet. ISS was defined as a reduction in 24-h MAP more than 5 mmHg, salt sensitivity as an increase in MAP more than 5 mmHg and salt resistance as a change in MAP between -5 and 5 mmHg from low sodium to high sodium.

RESULTS

Using this cutoff, 10.7% were ISS, 76.2% salt resistant, and 13.1% salt sensitive. Prevalence of ISS was similar between sexes and age groups ( P  > 0.05). However, ISS was more prevalent in those with normal BMI (15.8% ISS) compared with those with overweight BMI (0% ISS; P  < 0.01). Interestingly, classification of participants using a salt sensitivity index (ΔMAP/Δ urinary sodium excretion) categorized 21.4% as ISS, 48.8% salt resistant, and 29.8% salt sensitive.

CONCLUSION

Overall, we found that the prevalence of ISS was 10.7% (5 mmHg cutoff) or 21.4% (salt sensitivity index), and that ISS was associated with lower BMI. These results highlight the importance of future work to understand the mechanisms of ISS and to standardize salt sensitivity assessment.

High sodium intake differentially impacts brachial artery dilation when evaluated with reactive versus active hyperemia in salt resistant individuals

This study sought to determine if high sodium (HS) intake in salt resistant (SR) individuals attenuates upper limb arterial dilation in response to reactive (occlusion) and active (exercise) hyperemia, two stimuli with varying vasodilatory mechanisms, and the role of oxidative stress in this response. Ten young, SR participants (9 males, 1 female) consumed a 7-day HS (6,900 mg/day) and a 7-day recommended sodium intake (RI: 2,300 mg/day) diet in a randomized order. On the last day of each diet, brachial artery (BA) function was evaluated via reactive (RH-FMD: 5 min of cuff occlusion) and active [handgrip (HG) exercise] hyperemia after consumption of both placebo (PL) and antioxidants (AO). The HS diet significantly elevated sodium excretion (P < 0.05), but mean arterial blood pressure was unchanged. During the PL condition, the HS diet significantly reduced RH-FMD when compared with RI diet (P = 0.01), but this reduction was significantly restored (P = 0.01) when supplemented with AO (HS + PL: 5.9 ± 3.4; HS + AO: 8.2 ± 2.7; RI + PL: 8.9 ± 4.7; RI + AO: 7.0 ± 2.1%). BA shear-to-dilation slopes, evaluated across all HG exercise workloads, were not significantly different across sodium intervention or AO supplementation. In SR individuals, HS intake impaired BA function when assessed via RH-FMD, but was restored with acute AO consumption suggesting oxidative stress as a contributor to this dysfunction. However, exercise-induced BA dilation was unaltered, potentially implicating an inability of HS intake to influence the mechanisms responsible for effectively maintaining skeletal muscle perfusion during exercise.NEW & NOTEWORTHY This study examined if high sodium (HS) intake in salt resistant (SR) individuals attenuates brachial artery (BA) flow-mediated dilation in response to reactive (occlusion) and active (exercise) hyperemia. In SR individuals, HS intake impaired reactive hyperemia-induced BA dilation, but not exercise-induced BA dilation. This finding suggests that although brachial artery nitric oxide bioavailability may be reduced following HS intake, the redundant mechanisms associated with adequate upper limb blood flow regulation during exercise are maintained.

The World Hypertension League Science of Salt: a regularly updated systematic review of salt and health outcomes studies (Sept 2019 to Dec 2020)

The World Hypertension League Science of Salt health outcomes review series highlights high-quality publications relating to salt intake and health outcomes. This review uses a standardised method, outlined in previous reviews and based on methods developed by WHO, to identify and critically appraise published articles on dietary salt intake and health outcomes. We identified 41 articles published between September 2019 to December 2020. Amongst these, two studies met the pre-specified methodological quality criteria for critical appraisal. They were prospective cohort studies and examined physical performance and composite renal outcomes as health outcomes. Both found an association between increased/higher sodium intake and poorer health outcomes. Few studies meet criteria for high-quality methods. This review adds further evidence that dietary salt reduction has health benefits and strengthens evidence relating to health outcomes other than blood pressure and cardiovascular disease. We observe that most studies on dietary sodium do not have adequate methodology to reliably assess sodium intake and its association with health outcomes.

Dietary sodium and health: How much is too much for those with orthostatic disorders?

High dietary salt (NaCl) increases blood pressure (BP) and can adversely impact multiple target organs including the vasculature, heart, kidneys, brain, autonomic nervous system, skin, eyes, and bone. However, patients with orthostatic disorders are told to increase their NaCl intake to help alleviate symptoms. While there is evidence to support the short-term benefits of increasing NaCl intake in these patients, there are few studies assessing the benefits and side effects of long-term high dietary NaCl. The evidence reviewed suggests that high NaCl can adversely impact multiple target organs, often independent of BP. However, few of these studies have been performed in patients with orthostatic disorders. We conclude that the recommendation to increase dietary NaCl in patients with orthostatic disorders should be done with care, keeping in mind the adverse impact on dietary NaCl in people without orthostatic disorders. Modest, rather than robust, increases in NaCl intake may be sufficient to alleviate symptoms but also minimize any long-term negative effects.

What is the impact of aerobic fitness and movement interventions on low-flow-mediated vasoconstriction? A systematic review of observational and intervention studies

The cardiovascular benefits of physical exercise are well established. The vasoreactivity that occurs during reductions in local arterial blood flow, termed low-flow-mediated constriction (L-FMC), is a measure of endothelial-dependent vasoconstrictor function. It is unclear whether aerobic fitness and movement (or lack thereof) influences L-FMC. We systematically reviewed studies examining the impact of physical behaviours on L-FMC. To be included, cross-sectional and interventional studies had to examine the impact of a physical behaviour on L-FMC in adults. There were no language or date of publication restrictions. Sources were searched in May, 2021 and included Scopus, Embase, MEDLINE, CINAHL, and Academic Search Premier. National Institutes of Health quality assessment tools were used. Fourteen studies (15 arms; 313 participants; 398 total observations from four arteries) met the inclusion criteria. The study quality varied from four out of 14 (controlled intervention scoring) to nine out of 12 (longitudinal intervention with no control group scoring) with the total points dependent upon the study design. Conflicting results were reported for acute prolonged sitting studies (attenuated L-FMC: n = 1; no change: n = 1) and resistance exercise (increased L-FMC: n = 2; no change: n = 2). Most observational studies examining aerobic fitness (3/4 studies) and aerobic exercise interventions (4/5 studies) observed a favourable effect on L-FMC. Overall, the included studies support that higher aerobic fitness and engaging in aerobic exercise training may augment L-FMC responses. Our systematic review highlights the heterogeneity between studies and identifies current gaps and future directions to better our understanding of (in)activity, exercise, and posture on endothelial vasoconstrictor function. PROSPERO Registration No.: CRD42021248241.

High Na+ Salt Diet and Remodeling of Vascular Smooth Muscle and Endothelial Cells

Our knowledge on essential hypertension is vast, and its treatment is well known. Not all hypertensives are salt-sensitive. The available evidence suggests that even normotensive individuals are at high cardiovascular risk and lower survival rate, as blood pressure eventually rises later in life with a high salt diet. In addition, little is known about high sodium (Na⁺) salt diet-sensitive hypertension. There is no doubt that direct and indirect Na⁺ transporters, such as the Na/Ca exchanger and the Na/H exchanger, and the Na/K pump could be implicated in the development of high salt-induced hypertension in humans. These mechanisms could be involved following the destruction of the cell membrane glycocalyx and changes in vascular endothelial and smooth muscle cells membranes’ permeability and osmolarity. Thus, it is vital to determine the membrane and intracellular mechanisms implicated in this type of hypertension and its treatment.

Influence of prostaglandins and endothelial-derived hyperpolarizing factors on brachial and popliteal endothelial-dependent function in young adults

Heterogeneous flow-mediated dilation (FMD) and low-flow-mediated constriction (L-FMC) responses have been reported between upper- and lower-limb arteries. Radial artery L-FMC, but not FMD, responses are blunted when endothelial-derived hyperpolarizing factors (EDHFs) or prostaglandin production is inhibited in young adults. However, it is unknown if these mechanisms similarly impact endothelial-dependent responses in the brachial (BA) and popliteal (POP) arteries. We tested whether BA- and POP-L-FMC and FMD would be influenced by independent EDHF and prostaglandin inhibition. Eighteen participants (23 ± 3 yr; 6♀) completed three randomized and double-blinded ultrasound assessments following ingestion of an opaque capsule containing maltodextrin (control), 150 mg of fluconazole (EDHF inhibition), or 500 mg of aspirin (prostaglandin inhibition). POP resting diameter was reduced following fluconazole administration (6.13 ± 0.63 mm vs. 6.19 ± 0.65 mm in control, P = 0.03). Compared with control, fluconazole also blunted the relative L-FMC responses in both the BA (-2.1 ± 0.8% vs. -0.8 ± 1.0%, P = 0.001) and POP (-1.7 ± 1.1% vs. -0.8 ± 0.9%, P = 0.009). In contrast, aspirin did not impact either the BA (-1.9 ± 0.7%) or POP-L-FMC (-1.3 ± 0.6%) responses (both, P > 0.35). The FMD response was unchanged following fluconazole or aspirin administration in either artery (both, P > 0.36). Our findings demonstrate that EDHF mediates L-FMC responses in both the brachial and popliteal arteries. Complementary to the nitric oxide-mediated FMD response, L-FMC appears to provide information regarding the EDHF pathway. Future research should uncover if these mechanisms impact older adults and/or patient populations characterized by vascular endothelial dysfunction associated with low aerobic fitness and habitual physical activity levels.NEW & NOTEWORTHY We compared changes in upper- and lower-limb artery endothelial-dependent vasodilatory and vasoconstrictor responses between control, prostaglandin inhibition, and endothelial-derived hyperpolarizing factor inhibition conditions. Neither prostaglandins nor endothelial-derived hyperpolarizing factor influenced flow-mediated dilation responses in either the brachial or popliteal artery. In contrast, endothelial-derived hyperpolarizing factor, but not prostaglandins, reduced resting brachial artery blood flow and shear rate and resting popliteal artery diameter, as well as low-flow-mediated constriction responses in both the popliteal and brachial arteries.