Effects of posture on shear rates in human brachial and superficial femoral arteries

Self-reported total sitting time on a non-working day is associated with blunted flow-mediated vasodilation and blunted nitroglycerine-induced vasodilation

We divided the 466 subjects into two groups based on information on sitting time on a non-working day and evaluated flow-mediated vasodilation (FMD) and nitroglycerine-induced vasodilation (NID). FMD was smaller in subjects with sitting time on a non-working day of ≥6 h/day than in subjects with sitting time on a non-working day of <6 h/day (2.5 ± 2.6% vs. 3.7 ± 2.9%; p < 0.001). NID was smaller in subjects with sitting time at non-working day of ≥ 8 h/day than in subjects with sitting time on a non-working day of < 8 h/day (10.1 ± 5.6% vs. 11.5 ± 5.0%; p = 0.01). After adjustment for confounding factors for vascular function, the odds of having the lowest tertile of FMD was significantly higher in subjects with sitting time on a non-working day of ≥6 h/day than in subjects with sitting time on a non-working day of <6 h/day. The odds of having the lowest tertile of NID was significant higher in subjects with sitting time on a non-working day of ≥ 8 h/day than in subjects with sitting time on a non-working day of < 8 h/day. These findings suggest that prolonged sitting time on a non-working day is associated with blunted FMD and blunted NID.

A single bout of exercise improves vascular insulin sensitivity in adults with obesity

OBJECTIVE

This crossover study explored the impact of a single bout of exercise on insulin-stimulated responses in conduit arteries and capillaries.

METHODS

Twelve sedentary adults (49.5 [7.8] years; maximal oxygen consumption [VO₂ max]: 23.7 [5.4] mL/kg/min) with obesity (BMI 34.5 [4.3] kg/m² ) completed a control and exercise bout (70% VO₂ max to expend 400 kcal). Sixteen hours later, participants underwent a 2-hour euglycemic-hyperinsulinemic clamp (90 mg/dL; 40 mU/m² /min) to determine vascular and metabolic insulin sensitivity. Endothelial and capillary functions were assessed by brachial artery flow-mediated dilation and contrast-enhanced ultrasound, respectively. Metabolized glucose infusion rate, substrate oxidation (indirect calorimetry), nonoxidative glucose disposal (NOGD), and inflammation were also determined.

RESULTS

Exercise increased insulin-stimulated preocclusion diameter (p = 0.01) and microvascular blood flow (condition effect: p = 0.04) compared with control. Furthermore, exercise improved metabolic insulin sensitivity by 21%, which paralleled rises in NOGD (p = 0.05) and decreases in soluble receptors for advanced glycation end products (condition effect: p = 0.01). Interestingly, changes in NOGD were related to increased insulin-stimulated microvascular blood flow (r = 0.57, p = 0.05).

CONCLUSIONS

A single bout of exercise increases vascular insulin sensitivity in adults with obesity. Additional work is needed to determine vascular responses following different doses of exercise in order to design lifestyle prescriptions for reducing chronic disease risk.

Improvements in vascular function in response to acute lower limb heating in young healthy males and females

Regular exposure to passive heat stress improves vascular function, but the optimal heating prescription remains undefined. Local limb heating is more feasible than whole body heating, but the evidence demonstrating its efficacy is lacking. The purpose of this study was to determine whether acute improvements in vascular function can be achieved with lower limb heating in 16 young healthy individuals (8 female, 8 male). In separate visits, participants underwent 45 min of ankle- and knee-level hot water immersion (45°C). A subset of seven participants also participated in a time-control visit. Endothelial function was assessed through simultaneous brachial and superficial femoral artery flow-mediated dilation (FMD) tests. Macrovascular function was quantified by %FMD, whereas microvascular function was quantified by vascular conductance during reactive hyperemia. Arterial stiffness was assessed through carotid-femoral and femoral-foot pulse wave velocity (PWV). Plasma concentrations of interleukin-6 and extracellular heat shock protein-72 (eHSP72) were used as indicators of inflammation. Our findings showed that 45 min of lower limb heating-regardless of condition-acutely improved upper limb macrovascular endothelial function (i.e., brachial %FMD; Pre: 4.6 ± 1.7 vs. Post: 5.4 ± 2.0%; P = 0.004) and lower limb arterial stiffness (i.e., femoral-foot PWV; Pre: 8.4 ± 1.2 vs. Post: 7.7 ± 1.1 m/s; P = 0.011). However, only knee-level heating increased upper limb microvascular function (i.e., brachial peak vascular conductance; Pre: 6.3 ± 2.7 vs. Post: 7.8 ± 3.5 mL/min ⋅ mmHg; P ≤ 0.050) and plasma eHSP72 concentration (Pre: 12.4 ± 9.4 vs. Post: 14.8 ± 9.8 ng/mL; P ≤ 0.050). These findings show that local lower limb heating acutely improves vascular function in younger individuals, with knee-level heating improving more outcome measures.NEW & NOTEWORTHY This study demonstrates that lower limb hot water immersion is an effective strategy for acutely improving vascular function in young, healthy males and females, thereby encouraging the development of accessible modes of heat therapy for vascular health.

Comparison of morphometric, structural, mechanical, and physiologic characteristics of human superficial femoral and popliteal arteries

Peripheral arterial disease differentially affects the superficial femoral (SFA) and the popliteal (PA) arteries, but their morphometric, structural, mechanical, and physiologic differences are poorly understood. SFAs and PAs from 125 human subjects (age 13-92, average 52±17 years) were compared in terms of radii, wall thickness, and opening angles. Structure and vascular disease were quantified using histology, mechanical properties were determined with planar biaxial extension, and constitutive modeling was used to calculate the physiologic stress-stretch state, elastic energy, and the circumferential physiologic stiffness. SFAs had larger radii than PAs, and both segments widened with age. Young SFAs were 5% thicker, but in old subjects the PAs were thicker. Circumferential (SFA: 96→193°, PA: 105→139°) and longitudinal (SFA: 139→306°, PA: 133→320°) opening angles increased with age in both segments. PAs were more diseased than SFAs and had 11% thicker intima. With age, intimal thickness increased 8.5-fold, but medial thickness remained unchanged (620μm) in both arteries. SFAs had 30% more elastin than the PAs, and its density decreased ~50% with age. SFAs were more compliant than PAs circumferentially, but there was no difference longitudinally. Physiologic circumferential stress and stiffness were 21% and 11% higher in the SFA than in the PA across all ages. The stored elastic energy decreased with age (SFA: 1.4→0.4kPa, PA: 2.5→0.3kPa). While the SFA and PA demonstrate appreciable differences, most of them are due to vascular disease. When pathology is the same, so are the mechanical properties, but not the physiologic characteristics that remain distinct due to geometrical differences.

Impact of proximal and distal cuff inflation on brachial artery endothelial function in healthy individuals

PURPOSE

In this study, we examined whether the decrease in endothelial function associated with short-term exposure to elevated retrograde shear rate (SR), could be prevented when combined with a concurrent drop in transmural pressure in humans.

METHODS

Twenty-five healthy individuals reported to our laboratory on three occasions to complete 30-min experimental conditions, preceded and followed by assessment of endothelial function using flow-mediated dilation (FMD). We used cuff inflation for 30-min to manipulate retrograde SR and transmural pressure in the brachial artery. Subjects underwent, in randomised order: (1) forearm cuff inflation to 60 mmHg (distal cuff; causing increase in retrograde SR), (2) upper arm cuff inflation to 60 mmHg (proximal cuff; causing increase in retrograde SR + decrease in transmural pressure), and (3) no cuff inflation (Control).

RESULTS

The distal and proximal cuff conditions both increased brachial artery retrograde SR (p < 0.001) and oscillatory shear index (p < 0.001). The Control intervention did not alter SR patterns or FMD (p > 0.05). A significant interaction-effect was found for FMD (p < 0.05), with the decrease during distal cuff (from 6.9 ± 2.3% to 6.1 ± 2.5%), being reversed to an increase with proximal cuff (from 6.3 ± 2.0 to 6.9 ± 2.0%). The proximal cuff-related increase in FMD could not be explained by the decrease in antegrade or increase in retrograde shear.

CONCLUSION

This study suggests that a decrease in transmural pressure may ameliorate the decline in endothelial function that occurs following exposure to elevated retrograde shear in healthy individuals.

Popliteal flow-mediated dilatory responses to an acute bout of prolonged sitting between earlier and later phases of natural menstrual and oral contraceptive pill cycles

We compared changes in popliteal artery endothelial function to a 3-h bout of sitting in females across their natural menstrual or oral contraceptive pill cycles. Pre-sitting endothelial-dependent vasodilation was greater in females who naturally menstruate during the later versus earlier phase but unchanged among contraceptive pill phases. Neither menstrual nor oral contraceptive pill phases attenuated the robust decline in conduit artery health following an acute period of uninterrupted sitting in young females.

Effects of Unilateral Arm Warming or Cooling on the Modulation of Brachial Artery Shear Stress and Endothelial Function during Leg Exercise in Humans

Aim: We examined the effect of modulating the shear stress (SS) profile using forearm warming and cooling on subsequent endothelial function in the brachial artery (BA) during exercise.

Methods: Twelve healthy young subjects immersed their right forearm in water (15°C or 42°C) during a leg cycling exercise at 120–130 bpm for 60 min. The same exercise without water immersion served as a control. The BA diameter and blood velocity were simultaneously recorded using Doppler ultrasonography to evaluate the antegrade, retrograde, and mean shear rates (SRs, an estimate of SS) before, during, and after exercise. The endothelial function in the right BA was evaluated using flow-mediated dilation (FMD) (%) using two-dimensional high-resolution ultrasonography before (baseline) and 15 and 60 min after exercise.

Results: During exercise, compared with the control trial, higher antegrade and mean SRs and lower retrograde SRs were observed in the warm trial; conversely, lower antegrade and mean SRs and higher retrograde SRs were observed in the cool trial. At 15 min postexercise, no significant change was observed in the FMD from baseline in the warm (Δ%FMD: +1.6%, tendency to increase; p = 0.08) and control trials (Δ %FMD: +1.1%). However, in the cool trial, the postexercise FMD at 60 min decreased from baseline (Δ%FMD: −2.7%) and was lower than that of the warm (Δ%FMD: +1.5%) and control (Δ%FMD: +1.2%) trials. Accumulated changes in each SR during and after exercise were significantly correlated with postexercise FMD changes.

Conclusion: Modulation of shear profiles in the BA during exercise appears to be associated with subsequent endothelial function.

Evaluation of forearm vascular resistance during orthostatic stress: Velocity is proportional to flow and size doesn't matter

Background

The upright posture imposes a significant challenge to blood pressure regulation that is compensated through baroreflex-mediated increases in heart rate and vascular resistance. Orthostatic cardiac responses are easily inferred from heart rate, but vascular resistance responses are harder to elucidate. One approach is to determine vascular resistance as arterial pressure/blood flow, where blood flow is inferred from ultrasound-based measurements of brachial blood velocity. This relies on the as yet unvalidated assumption that brachial artery diameter does not change during orthostatic stress, and so velocity is proportional to flow. It is also unknown whether the orthostatic vascular resistance response is related to initial blood vessel diameter.

Methods

We determined beat-to-beat heart rate (ECG), blood pressure (Portapres) and vascular resistance (Doppler ultrasound) during a combined orthostatic stress test (head-upright tilting and lower body negative pressure) continued until presyncope. Participants were 16 men (aged 38.4±2.3 years) who lived permanently at high altitude (4450m).

Results

The supine brachial diameter ranged from 2.9–5.6mm. Brachial diameter did not change during orthostatic stress (supine: 4.19±0.2mm; tilt: 4.20±0.2mm; -20mmHg lower body negative pressure: 4.19±0.2mm, p = 0.811). There was no significant correlation between supine brachial artery diameter and the maximum vascular resistance response (r = 0.323; p = 0.29). Forearm vascular resistance responses evaluated using brachial arterial flow and velocity were strongly correlated (r = 0.989, p<0.00001) and demonstrated high equivalency with minimal bias (-6.34±24.4%).

Discussion

During severe orthostatic stress the diameter of the brachial artery remains constant, supporting use of brachial velocity for accurate continuous non-invasive orthostatic vascular resistance responses. The magnitude of the orthostatic forearm vascular resistance response was unrelated to the baseline brachial arterial diameter, suggesting that upstream vessel size does not matter in the ability to mount a vasoconstrictor response to orthostasis.

Heterogeneity in the vasodilatory function of individual extremities

Objectives

Heterogeneity and homogeneity in the flow-mediated dilation of the human body's individual extremities are not fully understood, and the relationship between flow-mediated dilation and local muscle activity is unclear. We assessed the flow-mediated dilation of four individual extremities and sought to determine the contribution of local muscle activity (evaluated as muscle strength) to the flow-mediated dilation in each extremity.

Methods

Thirteen healthy young right-handed nonactive males participated. The flow-mediated dilation in the brachial and popliteal arteries at both arms and legs was assessed by ultrasound Doppler. Muscle strength was evaluated as the grip strength and knee extension.

Results

There was a significant difference in the brachial artery (BA)-FMD values between the subjects' dominant and non-dominant sides (8.0 ± 2.8 vs. 5.5 ± 2.2%, p < 0.05), whereas the two sides showed similar popliteal artery (PA)-FMD values. There was no significant correlation in flow-mediated dilation between the dominant brachial artery and popliteal artery. The BA-FMD was significantly correlated with the grip strength in both upper extremities (dominant: r = 0.562, non-dominant: r = 0.548; p < 0.05, respectively).

Conclusion

These results demonstrated heterogeneity in the flow-mediated dilation of individual extremities. We observed that local muscle activity can affect the local vascular function. Measurements of vasodilatory function in individual extremities should thus be carefully considered.

Relationship between brachial and popliteal artery low-flow-mediated constriction in older adults: impact of aerobic fitness on vascular endothelial function

We previously observed that brachial artery (BA) low-flow-mediated constriction (L-FMC) is inversely related to aerobic fitness (i.e., V̇o2peak) in older adults (OA). However, it is unclear if an L-FMC response is elicited in the popliteal artery (POP) or if a similar inverse relationship with aerobic fitness exists. Considering that the POP experiences larger shear stress fluctuations during sedentary behaviors and traditional lower limb modes of aerobic exercise, we tested the hypotheses that 1) heterogeneous L-FMC responses exist between the BA versus POP of OA, and 2) that aerobic fitness will be inversely related to POP L-FMC. L-FMC was assessed in 47 healthy OA (30 women, 67 ± 5 yr) using duplex ultrasonography and quantified as the percent decrease in diameter (from baseline) during the last 30 s of a 5-min distal cuff occlusion period. When allometrically scaled to baseline diameter, the BA exhibited a greater L-FMC response than the POP (–1.3 ± 1.6 vs. –0.4 ± 1.6%; P = 0.03). Furthermore, L-FMC responses in the BA and POP were not correlated ( r = 0.22; P = 0.14). V̇o2peak was strongly correlated to POP L-FMC ( r = –0.73; P < 0.001). The heterogeneous BA versus POP L-FMC data indicate that upper limb L-FMC responses do not represent a systemic measure of endothelial-dependent vasoconstrictor capacity in OA. The strong association between V̇o2peak and POP L-FMC suggests that localized shear stress patterns, perhaps induced by lower limb dominant modes of aerobic exercise, may result in greater vasoconstrictor responsiveness in healthy OA.

NEW & NOTEWORTHY We compared low-flow-mediated constriction responses between the brachial and popliteal arteries of healthy older adults. Vasoconstrictor responses were not correlated between arteries. A strong relationship between aerobic fitness and low-flow-mediated vasoconstriction was observed in the popliteal artery. These findings suggest that brachial vasoconstrictor responsiveness is not reflective of the popliteal artery, which is exposed to larger shear stress fluctuations during bouts of sedentary behavior and traditional lower limb modes of exercise.

UBC-Nepal expedition: upper and lower limb conduit artery shear stress and flow-mediated dilation on ascent to 5,050 m in lowlanders and Sherpa

The study of conduit artery endothelial adaptation to hypoxia has been restricted to the brachial artery, and comparisons with highlanders have been confounded by differences in altitude exposure, exercise, and unknown levels of blood viscosity. To address these gaps, we tested the hypothesis that lowlanders, but not Sherpa, would demonstrate decreased mean shear stress and increased retrograde shear stress and subsequently reduced flow-mediated dilation (FMD) in the upper and lower limb conduit arteries on ascent to 5,050 m. Healthy lowlanders (means ± SD, n = 22, 28 ± 6 yr) and Sherpa ( n = 12, 34 ± 11 yr) ascended over 10 days, with measurements taken on nontrekking days at 1,400 m (baseline), 3,440 m ( day 4), 4,371 m ( day 7), and 5,050 m ( day 10). Arterial blood gases, blood viscosity, shear stress, and FMD [duplex ultrasound of the brachial and superficial femoral arteries (BA and SFA, respectively)] were acquired at each time point. Ascent decreased mean and increased retrograde shear stress in the upper and lower limb of lowlanders and Sherpa. Although BA FMD decreased in lowlanders from 7.1 ± 3.9% to 3.8 ± 2.8% at 5,050 versus 1,400 m ( P < 0.001), SFA FMD was preserved. In Sherpa, neither BA nor SFA FMD were changed upon ascent to 5,050 m. In lowlanders, the ascent-related exercise may favorably influence endothelial function in the active limb (SFA); selective impairment in FMD in the BA in lowlanders is likely mediated via the low mean or high oscillatory baseline shear stress. In contrast, Sherpa presented protected endothelial function, suggesting a potential vascular aspect of high-altitude acclimatization/adaptation. NEW & NOTEWORTHY Upper and lower limb arterial shear stress and flow-mediated dilation (FMD) were assessed on matched ascent from 1,400 to 5,050 m in lowlanders and Sherpa. A shear stress pattern associated with vascular dysfunction/risk manifested in both limbs of lowlanders and Sherpa. FMD was impaired only in the upper limb of lowlanders. The findings indicate a limb-specific impact of high-altitude trekking on FMD and a vascular basis to acclimatization wherein endothelial function is protected in Sherpa on ascent.

Effects of prior aerobic exercise on sitting-induced vascular dysfunction in healthy men

INTRODUCTION

Acute aerobic exercise prevents sitting-induced impairment of flow-mediated dilation (FMD). Further, evidence suggests that sitting-induced impairment of FMD occurs via an oxidative stress-dependent mechanism that disrupts endothelial function.

PURPOSE

We hypothesized that acute aerobic exercise would prevent impairment of femoral artery FMD by limiting oxidative stress responses that increase endothelin-1 (ET-1) levels and disrupt nitric oxide (NO) status.

METHODS

In a randomized, cross-over study, healthy men (n = 11; 21.2 ± 1.9 years) completed two 3 h sitting trials that were preceded by 45 min of either quiet rest (REST) or a single bout of continuous treadmill exercise (65% maximal oxygen consumption) (EX). Superficial femoral artery FMD, plasma glucose, malondialdehyde (MDA), ET-1, arginine (ARG) and its related metabolites [homoarginine (HA), asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA)] were assessed at baseline, 1 h following EX (or REST) (0 h), and at 1 h intervals during 3 h of uninterrupted sitting. Data were analyzed using repeated measures ANOVA.

RESULTS

During REST, femoral artery FMD declined from baseline (2.6 ± 1.8%) at 1, 2, and 3 h of sitting and resting shear rate decreased at 3 h. In contrast, when sitting was preceded by EX, femoral artery FMD (2.7 ± 2.0%) and resting shear rate responses were unaffected. No between trial differences were detected for plasma glucose, MDA, ET-1, ARG, HA, ADMA, or SDMA.

CONCLUSION

Prior aerobic exercise prevented the decline in femoral artery FMD that is otherwise induced by prolonged sitting independent of changes in oxidative stress, ET-1, and NO status.

Effect of aging on mechanical stresses, deformations, and hemodynamics in human femoropopliteal artery due to limb flexion

Femoropopliteal artery (FPA) reconstructions are notorious for poor clinical outcomes. Mechanical and flow conditions that occur in the FPA with limb flexion are thought to play a significant role, but are poorly characterized. FPA deformations due to acute limb flexion were quantified using a human cadaver model and used to build a finite element model that simulated surrounding tissue forces associated with limb flexion-induced deformations. Strains and intramural principal mechanical stresses were determined for seven age groups. Computational fluid dynamics analysis was performed to assess hemodynamic variables. FPA shape, stresses, and hemodynamics significantly changed with age. Younger arteries assumed straighter positions in the flexed limb with less pronounced bends and more uniform stress distribution along the length of the artery. Even in the flexed limb posture, FPAs younger than 50 years of age experienced tension, while older FPAs experienced compression. Aging resulted in localization of principal mechanical stresses to the adductor hiatus and popliteal artery below the knee that are typically prone to developing vascular pathology. Maximum principal stresses in these areas increased threefold to fivefold with age with largest increase observed at the adductor hiatus. Atheroprotective wall shear stress reduced after 35 years of age, and atheroprone and oscillatory shear stresses increased after the age of 50. These data can help better understand FPA pathophysiology and can inform the design of targeted materials and devices for peripheral arterial disease treatments.

A tale of three cuffs: the hemodynamics of blood flow restriction

INTRODUCTION

The blood flow response to relative levels of blood flow restriction (BFR) across varying cuff widths is not well documented. With the variety of cuff widths and pressures reported in the literature, the effects of different cuffs and pressures on blood flow require investigation.

PURPOSE

To measure blood pressure using three commonly used BFR cuffs, examine possible venous/arterial restriction pressures, and measure hemodynamic responses to relative levels of BFR using these same cuffs.

METHODS

43 participants (Experiment 1, brachial artery blood pressure assessed) and 38 participants (Experiment 2, brachial artery blood flow assessed using ultrasound, cuff placed at proximal portion of arm) volunteered for this study.

RESULTS

Blood pressure measurement was higher in the 5 cm cuff than in the 10 and 12 cm cuffs. Sub-diastolic relative pressures appear to occur predominantly at <60% of arterial occlusion pressure (AOP). Blood flow under relative levels of restriction decreases in a non-linear fashion, with minimal differences between cuffs [resting: 50.3 (44.2) ml min^-1; 10% AOP: 42.0 (36.8); 20%: 33.6 (28.6); 30%: 23.6 (20.4); 40%: 17.1 (15.9); 50%: 12.5 (9.4); 60%: 11.5 (8.1); 70%: 11.4 (7.0); 80%: 10.3 (6.3); 90%: 7.9 (4.8); 100%: 1.5 (2.9)]. Peak blood velocity remains relatively constant until higher levels (>70% of AOP) are surpassed. Calculated mean shear rate decreases in a similar fashion as blood flow.

CONCLUSIONS

Under relative levels of restriction, pressures from 40 to 90% of AOP appear to decrease blood flow to a similar degree in these three cuffs. Relative pressures appear to elicit a similar blood flow stimulus when accounting for cuff width and participant characteristics.

Endogenous endothelin-1 and femoral artery shear rate: impact of age and implications for atherosclerosis

BACKGROUND

Both altered shear rate and endothelin-1 (ET-1) are associated with the age-related development of atherosclerosis. However, the role of ET-1, a potent endogenous vasoconstrictor, in altering shear rate in humans, especially in the atherosclerotic-prone vasculature of the leg, is unknown. Therefore, this study examined the contribution of ET-1 to the age-related alterations in common femoral artery (CFA) shear rate.

METHOD

BQ-123, a specific endothelin type A (ET(A)) receptor antagonist, was infused into the CFA, and diameter and blood velocity were measured by Doppler ultrasound in young (n = 8, 24 ± 2 years) and old (n = 9, 70 ± 2 years) study participants.

RESULTS AND CONCLUSION

The old had greater intima-media thickening in the CFA, indicative of a preatherogenic phenotype. Prior to infusion, the old study participants exhibited reduced mean shear rate (27 ± 3/s) compared with the young study participants (62 ± 9/s). This difference was likely driven by attenuated antegrade shear rate in the old as retrograde shear rate was similar in the young and old. Inhibition of ETA receptors, by BQ-123, increased leg blood flow in the old, but not in the young, abolishing age-related differences. Older study participants had a larger CFA (young: 0.82 ± 0.03 cm, old: 0.99 ± 0.03 cm) in which BQ-123 induced significant vasodilation (5.1 ± 1.0%), but had no such effect in the young (-0.8 ± 0.8%). Interestingly, despite the age-specific, BQ-123-induced increase in leg blood flow and CFA diameter, shear rate patterns remained largely unchanged. Therefore, ET-1, acting through the ETA receptors, exerts a powerful age-specific vasoconstriction. However, removal of this vasoconstrictor stimulus does not augment mean shear rate in the old.

Influence of chronic endurance exercise training on conduit artery retrograde and oscillatory shear in older adults

PURPOSE

With aging, there tends to be an increase in retrograde and oscillatory shear in peripheral conduit arteries of humans. Whether the increase in shear rate is due to the aging process or an effect of a less active lifestyle that often accompanies aging is unknown. Therefore, we examined whether chronic endurance exercise training attenuates conduit artery retrograde and oscillatory shear in older adults.

METHODS

Brachial and common femoral artery mean blood velocities and diameter were determined via Doppler ultrasound under resting conditions, and shear rate was calculated in 13 young (24 ± 2 years), 17 older untrained (66 ± 3 years), and 16 older endurance exercise-trained adults (66 ± 7 years).

RESULTS

Brachial artery retrograde (-9.1 ± 6.4 vs. -12.6 ± 9.4 s(-1); P = 0.35) and oscillatory (0.14 ± 0.08 vs. 0.14 ± 0.08 arbitrary units; P = 0.99) shear were similar between the older trained and untrained groups, whereas brachial artery retrograde and oscillatory shear were greater in older untrained compared to young adults (-5.0 ± 3.4, 0.08 ± 0.05 s(-1) arbitrary units, P = 0.017 and 0.048, respectively). There was no difference between the young and older trained brachial retrograde (P = 0.29) and oscillatory (P = 0.07) shear. Common femoral artery retrograde (-6.3 ± 2.9 s(-1)) and oscillatory (0.21 ± 0.08 arbitrary units) shear were reduced in older trained compared to the older untrained group (-10.4 ± 4.1 and 0.30 ± 0.09 s(-1) arbitrary units, both P = 0.005 and 0.006, respectively), yet similar to young adults (-7.1 ± 3.5 and 0.19 ± 0.06 s(-1) arbitrary units, P = 0.81 and 0.87, respectively).

CONCLUSION

Our results suggest that chronic endurance exercise training in older adults ameliorates retrograde and oscillatory shear rate patterns, particularly in the common femoral artery.

Effect of prolonged sitting and breaks in sitting time on endothelial function

Sitting time (ST) is associated with cardiovascular disease risk factors, whereas breaking ST has been reported to be beneficial for reducing cardiovascular risk.

PURPOSE

The objective of this study is to examine the effects of breaking ST on superficial femoral artery (SFA) endothelial function.

HYPOTHESES

1) Prolonged sitting would induce endothelial dysfunction and changes in shear forces, and 2) breaking ST with brief periods of activity would prevent attenuation in endothelial function.

METHODS

Twelve nonobese men (24.2 ± 4.2 yr) participated in two randomized 3-h sitting trials. In the sitting (SIT) trial, subjects were seated on a firmly cushioned chair for 3 h without moving their lower extremities. In the breaking ST trial (ACT), subjects sat similar to the SIT trial but walked on a treadmill for 5 min at 2 mph at 30 min, 1 h 30 min, and 2 h 30 min during the sitting interval. SFA flow-mediated dilation (FMD) was assessed at baseline, 1 h, 2 h, and 3 h in each trial. Statistical analyses were performed using dependent variables SFA FMD and shear rates. Significance was set at P ≤ 0.05.

RESULTS

In the SIT trial, there was a significant decline in SFA FMD from baseline to 3 h (baseline, 4.72% ± 3.78%; 1 h, 0.52% ± 0.85%; 2 h, 1.66% ± 1.11%; 3 h, 2.2% ± 2.15; P < 0.05 by ANOVA) accompanied by a decline in mean shear rate and antegrade shear rate but no difference in shear rate (area under the curve). By two-way repeated-measures ANOVA, ACT prevented the sitting-induced decline in FMD (baseline, 4.5% ± 2.3%; 1 h, 5.04% ± 2.85%; 2 h, 5.28% ± 5.05%; 3 h, 6.9% ± 4.5%) along with no decline in shear rates.

CONCLUSION

Three hours of sitting resulted in a significant impairment in shear rate and SFA FMD. When light activity breaks were introduced hourly during sitting, the decline in FMD was prevented.

Impact of prolonged sitting on vascular function in young girls

NEW FINDINGS

What is the central question of this study? Children are spending more than 60% of their waking day sedentary. The consequences of excessive sedentary behaviour are not well understood in the child, but there is growing evidence that with increasing sedentary time, cardiovascular risk in childhood also increases. What is the main finding and its importance? Our findings show that a 3 h period of uninterrupted sitting causes a profound (33%) reduction in vascular function in young girls. Importantly, we also demonstrate that breaking up sitting with regular exercise breaks can prevent this. Excessive sedentary behaviour has serious clinical and public health implications; however, the physiological changes that accompany prolonged sitting in the child are not completely understood. Herein, we examined the acute effect a prolonged period of sitting has upon superficial femoral artery function in 7- to 10-year-old girls and the impact of interrupting prolonged sitting with exercise breaks. Superficial femoral artery endothelium-dependent flow-mediated dilatation, total shear rate, anterograde and retrograde shear rates and oscillatory shear index were assessed before and after two experimental conditions: a 3 h uninterrupted period of sitting (SIT) and a 3 h period of sitting interrupted each hour with 10 min of moderate-intensity exercise (EX). A mixed-model analysis of variance was used to compare between-condition and within-condition main effects, controlling for the within-subject nature of the experiment by including random effects for participant. Superficial femoral artery endothelium-dependent flow-mediated dilatation decreased significantly from pre- to post-SIT (mean difference 2.2% flow-mediated dilatation; 95% confidence interval = 0.60-2.94%, P < 0.001). This relative decline of 33% was abolished in the EX intervention. Shear rates were not significantly different within conditions. Our data demonstrate the effectiveness of short but regular exercise breaks in offsetting the detrimental effects of uninterrupted sitting in young girls.

Impact of retrograde shear rate on brachial and superficial femoral artery flow-mediated dilation in older subjects

An inverse, dose-dependent relationship between retrograde shear rate and brachial artery endothelial function exists in young subjects. This relationship has not been investigated in older adults, who have been related to lower endothelial function, higher resting retrograde shear rate and higher risk of cardiovascular disease.

AIM

To investigate the impact of a step-wise increase in retrograde shear stress on flow-mediated dilation in older males in the upper and lower limbs.

METHODS

Fifteen older (68 ± 9 years) men reported to the laboratory 3 times. We examined brachial artery flow-mediated dilation before and after 30-min exposure to cuff inflation around the forearm at 0, 30 and 60 mmHg, to manipulate retrograde shear rate. Subsequently, the 30-min intervention was repeated in the superficial femoral artery. Order of testing (vessel and intervention) was randomised.

RESULTS

Increases in cuff pressure resulted in dose-dependent increases in retrograde shear in both the brachial and superficial femoral artery in older subjects. In both the brachial and the superficial femoral artery, no change in endothelial function in response to increased retrograde shear was observed in older males ('time' P = 0.274, 'cuff*time P = 0.791', 'cuff*artery*time P = 0.774').

CONCLUSION

In contrast with young subjects, we found that acute elevation in retrograde shear rate does not impair endothelial function in older humans. This may suggest that subjects with a priori endothelial dysfunction are less responsive or requires a larger shear rate stimulus to alter endothelial function.

Lower limb conduit artery endothelial responses to acute upper limb exercise in spinal cord injured and able-bodied men

Vascular improvements in the nonactive regions during exercise are likely primarily mediated by increased shear rate (SR). Individuals with spinal cord injury (SCI) experience sublesional vascular deconditioning and could potentially benefit from upper body exercise-induced increases in lower body SR. The present study utilized a single bout of incremental arm-crank exercise to generate exercise-induced SR changes in the superficial femoral artery in an effort to evaluate the acute postexercise impact on superficial femoral artery endothelial function via flow-mediated dilation (FMD), and determine regulatory factors in the nonactive legs of individuals with and without SCI. Eight individuals with SCI and eight age, sex, and waist-circumference-matched able-bodied (AB) controls participated. Nine minutes of incremental arm-crank exercise increased superficial femoral artery anterograde SR (P = 0.02 and P < 0.01), retrograde SR (P < 0.01 and P < 0.01), and oscillatory shear index (OSI) (P < 0.001 and P < 0.001) in both SCI and AB, respectively. However, these SR alterations resulted in acute postexercise increases in FMD in the AB group only (SCI 6.0 ± 1.2% to 6.3 ± 2.7%, P = 0.74; AB 7.5 ± 1.4% to 11.2 ± 1.4%, P = 0.03). While arm exercise has many cardiovascular benefits and results in changes in SR patterns in the nonactive legs, these changes are not sufficient to induce acute changes in FMD among individuals with SCI, and therefore are less likely to stimulate exercise training-associated improvements in nonactive limb endothelial function. Understanding the role of SR patterns on FMD brings us closer to designing effective strategies to combat impaired vascular function in both healthy and clinical populations.

Differences in brachial and femoral artery responses to prolonged sitting

Introduction

It is unknown if there are limb differences in vascular function during prolonged sitting.

Purpose

This study was designed to test whether the effects of prolonged sitting on brachial artery (BA) and the superficial femoral artery (SFA) are similar.

Methods

Twelve men (24.2 ± 4 yrs.) participated in a 3 hr prolonged sitting trial (SIT). SFA and BA flow mediated dilation (FMD) and respective flow patterns were measured at baseline, 1 hr, 2 hr and 3 hr.

Results

By a one-way ANOVA there was a significant decline in SFA FMD during 3 hrs of SIT (p < 0.001). Simultaneously, there was a significant decline in antegrade (p = 0.04) and mean (0.037) shear rates. By a one way ANOVA there were no significant differences in BA FMD during 3 hrs of sitting. There were no changes in the shear rates in the BA except for a significant decrease in antegrade shear rate (p = 0.029) and a significant increase in oscillatory shear index (p = 0.034) during 3 hrs of sitting. Furthermore, there was no correlation between BA and SFA FMD measurements.

Conclusion

Three hours of sitting resulted in impaired SFA FMD but not BA FMD. Although 3 hours of sitting did not impair BA FMD, it impaired shear patterns in the BA.

Superficial femoral artery endothelial responses to a short-term altered shear rate intervention in healthy men

In animal and in-vitro models, increased oscillatory shear stress characterized by increased retrograde shear-rate (SR) is associated with acutely decreased endothelial cell function. While previous research suggests a possible detrimental role of elevated retrograde SR on endothelial-function in the brachial artery in humans, little research has been conducted examining arteries in the leg. Examinations of altered shear pattern in the superficial femoral artery (SFA) are important, as this vessel is both prone to atherosclerosis and leg exercise is a common form of activity in humans. Seven healthy men participated; bilateral endothelial-function was assessed via flow-mediated-dilation (FMD) before and after 30-minute unilateral inflations of a thigh blood pressure cuff to either 75 mmHg or 100 mmHg on two separate visits. Inflation of the cuff induced increases in maximum anterograde (p<0.05), maximum retrograde (p<0.01), and oscillatory shear index (OSI) (p<0.001) in the cuffed leg at both inflation pressures. At 100 mmHg the increases in SR were larger in the retrograde than the anterograde direction evidenced by a decrease in mean SR (p<0.01). There was an acute decrease in relative FMD in the cuffed leg alone following inflation to both pressures. These results indicate that in the SFA, altered SR profiles incorporating increased retrograde and OSI influence the attenuation in FMD after a 30-minute unilateral thigh-cuff inflation intervention. Novel information highlighting the importance of OSI calculations and assessments of flow profiles add to current body of knowledge regarding the influence of changes in SR patterns on FMD. Findings from the current study may provide additional insight when designing strategies to combat impaired vascular function in the lower extremity where blood vessels are more prone to atherosclerosis in comparison to the upper extremity.

Impact of age and body position on the contribution of nitric oxide to femoral artery shear rate: implications for atherosclerosis

Reduced shear stress and augmented oscillatory shear rate are associated with the proatherogenic phenotype observed with aging. To date, mechanisms contributing to the age-related alterations in shear rate in humans have only been examined in the conduit vessels of the arm. Therefore, this study sought to examine the contribution of nitric oxide (NO) bioavailability to age-related alterations in shear rate and the impact of common body positions (supine and seated) in the atherosclerotic-prone conduit artery of the leg. Inhibition of NO synthase (NOS) was accomplished by intra-arterial infusion of N(G)-monomethyl-l-arginine (L-NMMA), and common femoral artery diameter and blood velocity were measured by Doppler ultrasound in healthy young (n=8, 24±1 years) and old (n=8, 75±3 years) men. Old subjects exhibited reduced mean shear rate in the supine (18±3 s(-1)) and seated positions (17±3 s(-1)) compared with young subjects (supine: 42±6 s(-1); seated: 32±4 s(-1)). This reduced mean shear in the old was driven by attenuated antegrade shear as there were no differences in retrograde shear. Inhibition of NOS reduced antegrade shear in the young such that age-related differences were abolished. In contrast, NOS-induced reductions in retrograde shear rate were similar between groups. The seated position reduced mean shear rate in the young to that normally observed in old. Overall, this study reveals that age-related reductions in mean shear rate, assessed in the atherosclerotic-prone vasculature of the leg, are largely explained by reductions in antegrade shear as a result of reduced NO bioavailability in the elderly.

Acute impact of retrograde shear rate on brachial and superficial femoral artery flow-mediated dilation in humans

Retrograde shear rate (SR) in the brachial artery (BA) is associated with endothelial dysfunction; a precursor to atherosclerosis. The BA does not typically manifest clinical atherosclerosis, whereas the superficial femoral artery (SFA) is more prone to developing plaque. Examine whether the impact of incremental levels of retrograde SR differs between atherosclerosis-prone (i.e., SFA) and -resistant vessels (i.e., BA) in healthy men. Thirteen healthy young men reported three times to the laboratory. We examined BA flow-mediated dilation (FMD) before and after 30-min exposure to cuff inflation around the forearm at 0, 30, and 60 mmHg, to manipulate retrograde SR. Subsequently, the 30-min intervention was repeated in the SFA, using the same cuff pressure as in the forearm. Order of testing (vessel and intervention) was randomized among subjects. We found a dose-dependent increase in retrograde SR with 30 and 60 mmHg cuff inflation, which was present in both the BA and SFA (all P < 0.05). BA and SFA FMD decreased after the 30-min intervention ("time": P = 0.012), and this was dependent on cuff pressure ("cuff × time": P = 0.024). A significant decrease in FMD was observed after 60 mmHg only and this change was similarly present in both arteries ("time × artery": P = 0.227). Moreover, the BA and SFA demonstrate a similar relationship between changes in retrograde SR and FMD (r = 0.498 and 0.475, respectively). Our study demonstrates that acute exposure to an increase in retrograde shear leads to comparable decreases in FMD in atherosclerotic-prone and -resistant conduit arteries in humans.

Noninvasive estimation of central blood pressure and the augmentation index in the seated position: a validation study of two commercially available methods

OBJECTIVES

The aim of this study was to examine the utility of the generalized transfer function (GTF) and the late systolic shoulder of the radial pressure waveform (SBP2) for estimating seated central systolic blood pressure (c-SBP) and seated c-augmentation index (c-AIx).

METHODS

In 199 participants (182 cardiovascular disease patients and 17 normal individuals; mean age: 65.7 ± 10.6 years), c-SBP and c-AIx were estimated using the SphygmoCor and Omron HEM-9000AI systems. Both estimates were compared with carotid SBP and augmentation index (AIx). All measurements were performed with the participants in the seated position.

RESULTS

Mean c-SBP was 121.7 ± 18.6 mmHg and 122.6 ± 19.8 mmHg according to the GTF and SBP2 methods, respectively, and carotid SBP was 124.4 ± 19.2 mmHg. There were close correlations between the three values (GTF vs. carotid SBP: r = 0.986, SBP2 vs. carotid SBP: r = 0.975, GTF vs. SBP2: r = 0.989, all P < 0.0001), all of which met the AAMI SP10 criteria and the grade A British Society of Hypertension criteria. The relationship between c-AIx estimated using the GTF method and carotid AIx was comparable to that reported previously for supine individuals (r = 0.703, P < 0.0001). The seated carotid-radial artery amplifications of the first and second harmonics were identical to those reported in previous supine studies. Both methods produced reproducible estimates of c-SBP and c-AIx (r = 0.997-0.910, all P < 0.0001).

CONCLUSION

The present study indicates that the GTF and SBP2 methods can be used to estimate c-SBP and c-AIx in seated individuals.

Self-reported sitting time is associated with higher pressure from wave reflections independent of physical activity levels in healthy young adults

BACKGROUND

Time spent in sedentary pursuits such as sitting (SIT) is associated with an increased risk of cardiovascular disease independent of physical activity (PA). The purpose of this study was to examine the associations of PA and SIT with central hemodynamic burden in young adults.

METHODS

Aortic pressure waveforms were obtained using radial applanation tonometry and a generalized transfer function in 70 young healthy men (n = 41) and women (n = 29) (mean age = 23±1 years; body mass index = 24±1kg/m(2)). A wave separation technique that uses a physiologic pseudoaortic flow waveform was used to derive incident/forward wave pressure (Pf) and reflected/backward wave pressure (Pb). Levels of PA (metabolic equivalent, minutes per week) and SIT (sitting time per day) were obtained by self-report using the International Physical Activity Questionnaire.

RESULTS

A negative correlation existed between PA and Pf (r = -0.30; P < 0.05) and Pb (r = -0.36; P < 0.05). A positive correlation existed between SIT and Pf (r = 0.39; P < 0.05) and Pb (r = 0.44; P < 0.05). According to results from multiple regression, after adjusting for potential confounders (age, sex, height, heart rate, mean pressure) and PA, associations between SIT and Pf (P < 0.05) and SIT and Pb (P < 0.05) remained.

CONCLUSIONS

SIT is associated with higher forward wave pressure and backward wave pressure, 2 novel hemodynamic correlates of cardiovascular disease risk and target organ damage, in young apparently healthy men and women. This association is independent of PA.

Manipulation of arterial stiffness, wave reflections, and retrograde shear rate in the femoral artery using lower limb external compression

Exposure of the arterial wall to retrograde shear acutely leads to endothelial dysfunction and chronically contributes to a proatherogenic vascular phenotype. Arterial stiffness and increased pressure from wave reflections are known arbiters of blood flow in the systemic circulation and each related to atherosclerosis. Using distal external compression of the calf to increase upstream retrograde shear in the superficial femoral artery (SFA), we examined the hypothesis that changes in retrograde shear are correlated with changes in SFA stiffness and pressure from wave reflections. For this purpose, a pneumatic cuff was applied to the calf and inflated to 0, 35, and 70 mmHg (5 min compression, randomized order, separated by 5 min) in 16 healthy young men (23 ± 1 years of age). Doppler ultrasound and wave intensity analysis was used to measure SFA retrograde shear rate, reflected pressure wave intensity (negative area [NA]), elastic modulus (Ep), and a single-point pulse wave velocity (PWV) during acute cuff inflation. Cuff inflation resulted in stepwise increases in retrograde shear rate (P < 0.05 for main effect). There were also significant cuff pressure-dependent increases in NA, Ep, and PWV across conditions (P < 0.05 for main effects). Change in NA, but not Ep or PWV, was associated with change in retrograde shear rate across conditions (P < 0.05). In conclusion, external compression of the calf increases retrograde shear, arterial stiffness, and pressure from wave reflection in the upstream SFA in a dose-dependent manner. Wave reflection intensity, but not arterial stiffness, is correlated with changes in peripheral retrograde shear with this hemodynamic manipulation.

Altered resting hemodynamics in lower-extremity arteries of individuals with spinal cord injury

OBJECTIVE

To investigate lower-extremity arterial hemodynamics in individuals with spinal cord injury (SCI). We hypothesized that oscillatory shear index would be altered and resting mean shear would be higher in the lower-extremity arteries of SCI.

RESEARCH

Cross-sectional study of men and women with SCIs compared to able-bodied controls.

SUBJECTS

Subjects included 105 ages 18-72 years with American Spinal Injury Association (ASIA) Impairment Scale grades A, B, or C and injury duration at least 5 years. Subjects were matched for age and cardiovascular disease risk factors with 156 able-bodied controls.

METHODS

Diameter and blood velocity were determined with subject at rest via ultrasound in superficial femoral, popliteal, brachial, and carotid arteries. Mean shear, antegrade shear, retrograde shear, and oscillatory shear index were calculated.

RESULTS

Oscillatory shear index was lower in SCI compared to controls for superficial femoral (0.16 ± 0.10 vs. 0.26 ± 0.06, P < 0.01) and popliteal arteries (0.20 ± 0.11 vs. 0.26 ± 0.05, P < 0.01). Mean shear rate was higher in SCI compared to controls for superficial femoral (43.54 ± 28.0 vs. 20.48 ± 13.1/second, P < 0.01) and popliteal arteries (30.43 ± 28.1 vs. 11.68 ± 9.5/second, P < 0.01).

CONCLUSIONS

The altered resting hemodynamics in SCI are consistent with an atheroprotective hemodynamic environment.

Disturbed blood flow acutely induces activation and apoptosis of the human vascular endothelium

There is strong and consistent evidence from in vitro studies that disturbed blood flow produces a proatherogenic vascular endothelial phenotype. However, data from human studies are lacking. To address this, a 220 mm Hg occlusion cuff was placed on the distal forearm of 10 young, healthy men to induce a localized region of disturbed blood flow in the proximal vasculature for 20 minutes. We hypothesized that disturbed blood flow would induce endothelial activation and apoptosis as indicated by increases in local concentrations of CD62E(+) and CD31(+)/CD42b(-) endothelial microparticles, respectively. Distal cuff occlusion induced reductions in mean blood flow, mean shear, and antegrade shear, and increases in retrograde flow, retrograde shear, and oscillatory shear stress, confirming that our protocol produced a disturbed blood flow stimulus in the experimental arm. Relative to baseline (0 minutes), CD62E(+) endothelial microparticles increased by ≈3-fold at 10 minutes and ≈4-fold at 20 minutes in the experimental arm (P<0.05). CD31(+)/CD42b(-) endothelial microparticles were elevated by ≈9-fold at the 20 minutes time point (P<0.05). There were no changes in the concentrations of either endothelial microparticle population throughout the experiment in the contralateral arm, exposed to normal resting blood flow (no cuffs). These findings indicate that disturbed blood flow acutely induces endothelial activation and apoptosis in humans, as reflected by release of microparticles from activated (CD62E(+)) and apoptotic (CD31(+)/CD42b(-)) endothelial cells. These data provide the first in vivo experimental evidence of disturbed blood flow-induced endothelial injury in humans.

Sitting and endothelial dysfunction: the role of shear stress

Sedentary activity is a modifiable life-style behavior and a key component in the etiology of atherosclerotic cardiovascular disease (ACVD). US adults and children spend more than half their waking time in sedentary pursuits. Sedentary activity has been shown to result in impaired insulin sensitivity, impaired metabolic function and attenuated endothelial function, which are classic markers of ACVD. Sedentary activity is defined as 'sitting without otherwise being active.' This behavior promotes reduced muscular activity of the lower extremities which decreases leg blood flow, increases blood pooling in the calf, augments mean arterial pressure, and deforms arterial segments resulting in low mean shear stress (SS). SS activates distinct physiological mechanisms which have been proposed to be protective against ACVD; specifically through a SS-induced endothelium-derived nitric oxide mechanism. Reduced bioavailability of nitric oxide creates a pro-oxidant milieu resulting in increased oxidative stress. There is sufficient evidence which demonstrates that endothelial function is attenuated in the presence of oxidative stress. Sedentary activity results in low SS in the lower extremities which may result in increased oxidative stress and impaired endothelial function. This review furthers the use of sitting as model to study the effects of inactivity, discusses possible physiological mechanisms and suggests future directions.

Vitamin C prevents the acute decline of flow-mediated dilation after altered shear rate patterns

Oscillatory and retrograde shear rate (SR) impairs endothelial function, potentially through shear-induced oxidative stress. We tested the hypothesis that acute vitamin C supplementation would prevent the attenuation of brachial artery flow-mediated dilation (FMD) after a period of augmented oscillatory and retrograde SR. Twelve healthy men (aged 26 ± 3 years) participated in two 30-min study visits in which one arm was subjected to increased oscillatory and retrograde SR, using 60 mm Hg of forearm cuff compression, and the contralateral arm served as the control. Subjects ingested capsules containing either placebo (sucrose) or vitamin C at 90 and 120 min (1000 mg total vitamin C) prior to cuff compression periods in a randomized placebo-controlled double-blind crossover study. Oscillatory and retrograde SR in the cuffed arms increased during the compression periods in the placebo and vitamin C study visits (p < 0.01 for both), with no difference between studies (p > 0.05). Antegrade SR remained unchanged throughout the compression periods (p > 0.05), and mean SR was lower in the cuffed arm than in the control arm for both study visits (p < 0.05). FMD decreased after cuff compression in the placebo cuffed arm (precompression vs. postcompression, 5.2% ± 1.4% vs. 3.5% ± 1.4%; p < 0.05), but remained unchanged after vitamin C therapy in the cuffed arm (precompression vs. postcompression, 5.3% ± 2.4% vs. 5.7% ± 2.6%; p > 0.05). No FMD changes were observed in the control arm for either study visit (p > 0.05). These data demonstrate that acute vitamin C supplementation prevents the attenuation of FMD due to altered SR patterns, suggesting that oxidative stress contributes to the oscillatory and retrograde SR-induced impairment of FMD.

α-adrenergic vasoconstriction contributes to the age-related increase in conduit artery retrograde and oscillatory shear

Aging is associated with increased retrograde and oscillatory shear in peripheral conduit arteries of humans. Although the mechanisms responsible for these age-related changes are not completely understood, augmented downstream α-adrenergic tone likely plays a significant role in this phenomenon. Therefore, in protocol 1, brachial artery diameter and blood velocity were measured via Doppler ultrasound during (1) rest (control), (2) endogenous norepinephrine release via intra-arterial infusions of tyramine, and (3) α-adrenergic blockade via infusions of phentolamine in young healthy humans (n=12). Tyramine increased brachial artery retrograde (-4.0±1.4 to -9.5±1.4 s(-1)) and oscillatory shear (0.05±0.02 to 0.18±0.05 arbitrary units), whereas phentolamine abolished retrograde and oscillatory shear (P<0.05). Additionally, in protocol 2, we examined brachial artery shear patterns in young (n=12; 29±2 years) and older (n=13; 69±2 years) healthy adults during (1) rest (control), (2) sympathetic activation via lower body negative pressure, and (3) infusion of phentolamine. At rest, older adults exhibited greater brachial artery retrograde and oscillatory shear (-9.9±2.7 s(-1) and 0.11±0.03 arbitrary units, respectively) compared with younger adults (-3.1±1.0 s(-1) and 0.05±0.02 arbitrary units, respectively; P<0.05 for both). Lower body negative pressure increased retrograde and oscillatory shear in young (P<0.05), but not older adults (P=0.85-0.97), such that differences between young and older were eliminated (P>0.05). During infusion of phentolamine, retrograde and oscillatory shear were abolished in young adults (P<0.05) and markedly reduced, yet still persistent, in older adults (P<0.01). Our data indicate that α-adrenergic vasoconstriction is a major contributor to age-related discrepancies in conduit artery shear-rate patterns at rest.

Brachial artery flow-mediated dilation following exercise with augmented oscillatory and retrograde shear rate

Background

Acute doses of elevated retrograde shear rate (SR) appear to be detrimental to endothelial function in resting humans. However, retrograde shear increases during moderate intensity exercise which also enhances post-exercise endothelial function. Since SR patterns differ with the modality of exercise, it is important to determine if augmented retrograde SR during exercise influences post-exercise endothelial function. This study tested the hypothesis that (1) increased doses of retrograde SR in the brachial artery during lower body supine cycle ergometer exercise would attenuate post-exercise flow-mediated dilation (FMD) in a dose-dependent manner, and (2) antioxidant vitamin C supplementation would prevent the attenuated post-exercise FMD response.

Methods

Twelve men participated in four randomized exercise sessions (90 W for 20 minutes) on separate days. During three of the sessions, one arm was subjected to increased oscillatory and retrograde SR using three different forearm cuff pressures (20, 40, 60 mmHg) (contralateral arm served as the control) and subjects ingested placebo capsules prior to exercise. A fourth session with 60 mmHg cuff pressure was performed with 1 g of vitamin C ingested prior to the session.

Results

Post-exercise FMD following the placebo conditions were lower in the cuffed arm versus the control arm (arm main effect: P < 0.05) and without differences between cuff pressures (20 mmHg: 5.7 ± 2.2%; 40 mmHg: 4.7 ± 1.3%; 60 mmHg: 5.4 ± 2.4%) (P > 0.05). Following vitamin C treatment, post-exercise FMD in the cuffed and control arm increased from baseline (P < 0.05) but were not different (control: 7.1 ± 3.5% vs. cuffed: 6.6 ± 3.3%) (P > 0.05).

Conclusions

These results indicate that augmented oscillatory and retrograde SR in non-working limbs during lower body exercise attenuates post-exercise FMD without an evident dose–response in the range of cuff pressures evaluated. Vitamin C supplementation prevented the attenuation of FMD following exercise with augmented oscillatory and retrograde SR suggesting that oxidative stress contributes to the adverse effects of oscillatory and retrograde shear during exercise on FMD.

Exercise-induced Signals for Vascular Endothelial Adaptations: Implications for Cardiovascular Disease

This article reviews recent advances in our understanding of hemodynamic signals, external/compressive forces, and circulating factors that mediate exercise training-induced vascular adaptations, with particular attention to the roles of these signals in prevention and treatment of endothelial dysfunction and cardiovascular (CV) diseases.

Heterogeneity of endothelial cell phenotype within and amongst conduit vessels of the swine vasculature

The purpose of this study was to investigate the extent of endothelial cell phenotypic heterogeneity throughout the swine vasculature, with a focus on the conduit vessels of the arterial and venous circulations. We tested the hypothesis that atheroprone arteries exhibit higher expression of markers of inflammation and oxidative stress than do veins and atheroresistant arteries. The study sample included tissues from 79 castrated, male swine. Immediately after the animals were killed, endothelial cells were mechanically scraped from isolated segments of the thoracic and abdominal aorta, carotid, brachial, femoral and renal arteries, and the vein regionally associated with each of these vessels, as well as the internal mammary and right coronary arteries. Cells were also taken from two regions of the aortic arch contrasted by atheroprone versus atherosusceptible haemodynamics. Endothelial cell phenotype was assessed by either immunoblotting or quantitative real-time PCR for a host of both pro- and anti-atherogenic markers (e.g. endothelial nitric oxide synthase, p67phox, cyclo-oxygenase-1 and superoxide dismutase 1). Marked heterogeneity across the vasculature was observed in the expression of both pro- and anti-atherogenic markers, at both the protein and transcriptional levels. In particular, the coronary vascular endothelium expressed higher levels of the oxidative stress marker p67phox (P < 0.05 versus other arteries). In addition, differential expression of endothelial nitric oxide synthase and KLF4 was evident between atheroprone and atherosusceptible regions of the aorta, while expression of endothelial nitric oxide synthase, KLF2, KLF4 and cyclo-oxygenase-1 was lower in both areas of the aortic arch compared with the internal mammary artery. Conduit arteries typically expressed higher levels of both pro- and anti-atherogenic markers relative to their associated veins. We show, for the first time, that endothelial cell phenotype is variable within vessels, across six major vascular territories, and between the arterial and venous circulations. Importantly, even straight vessel segments from systemic conduit arteries (e.g. brachial and carotid arteries) exhibited regional phenotypic heterogeneity; a finding not expected on the basis of local haemodynamic forces alone.

Acute impact of intermittent pneumatic leg compression frequency on limb hemodynamics, vascular function, and skeletal muscle gene expression in humans

The mechanisms by which intermittent pneumatic leg compression (IPC) treatment effectively treats symptoms associated with peripheral artery disease remain speculative. With the aim of gaining mechanistic insight into IPC treatment, the purpose of this study was to investigate the effect of IPC frequency on limb hemodynamics, vascular function, and skeletal muscle gene expression. In this two study investigation, healthy male subjects underwent an hour of either high-frequency (HF; 2-s inflation/3-s deflation) or low-frequency (LF; 4-s inflation/16-s deflation) IPC treatment of the foot and calf. In study 1 (n = 11; 23.5 ± 4.7 yr), subjects underwent both HF and LF treatment on separate days. Doppler/ultrasonography was used to measure popliteal artery diameter and blood velocity at baseline and during IPC treatment. Flow-mediated dilation (FMD) and peak reactive hyperemia blood flow (RHBF) were determined before and after IPC treatment. In study 2 (n = 19; 22.0 ± 4.6 yr), skeletal muscle biopsies were taken from the lateral gastrocnemius of the treated and control limb at baseline and at 30- and 150-min posttreatment. Quantitative PCR was used to assess mRNA concentrations of genes associated with inflammation and vascular remodeling. No treatment effect on vascular function was observed. Cuff deflation resulted in increased blood flow (BF) and shear rate (SR) in both treatments at the onset of treatment compared with baseline (P < 0.01). BF and SR significantly diminished by 45 min of HF treatment only (P < 0.01). Both treatments reduced BF and SR and elevated oscillatory shear index compared with baseline (P < 0.01) during cuff inflation. IPC decreased the mRNA expression of cysteine-rich protein 61 from baseline and controls (P <0 .01) and connective tissue growth factor from baseline (P < 0.05) in a frequency-dependent manner. In conclusion, a single session of IPC acutely impacts limb hemodynamics and skeletal muscle gene expression in a frequency-dependent manner but does not impact vascular function.

Brachial artery vasodilatation during prolonged lower limb exercise: role of shear rate

We recently observed a marked increase in brachial artery (BA) diameter during prolonged leg cycling exercise. The purpose of the present study was to test the hypothesis that this increase in BA diameter during lower limb exercise is shear stress mediated. Accordingly, we determined whether recapitulation of cycling-induced BA shear rate with forearm heating, a known stimulus evoking shear-induced conduit artery dilatation, would elicit comparable profiles and magnitudes of BA vasodilatation to those observed during cycling. In 12 healthy men, BA diameter and blood velocity were measured simultaneously using Doppler ultrasonography at baseline and every 5 min during 60 min of either steady-state semi-recumbent leg cycling (120 W) or forearm heating. At the onset of cycling, the BA diameter was reduced (-3.9 ± 1.2% at 5 min; P < 0.05), but it subsequently increased throughout the remainder of the exercise bout (+15.1 ± 1.6% at 60 min; P < 0.05). The increase in BA diameter during exercise was accompanied by an approximately 2.5-fold rise in BA mean shear rate (P < 0.05). Similar increases in BA mean shear with forearm heating elicited an equivalent magnitude of BA vasodilatation to that observed during cycling (P > 0.05). Herein, we found that in the absence of exercise the extent of the BA vasodilator response was reproduced when the BA was exposed to comparable magnitudes of shear rate via forearm heating. These results are consistent with the hypothesis that shear stress plays a key role in signalling brachial artery vasodilatation during dynamic leg exercise.

Relationship between upper and lower limb conduit artery vasodilator function in humans

Brachial artery flow-mediated dilation (FMD) is a strong predictor of future cardiovascular disease and is believed to represent a "barometer" of systemic endothelial health. Although a recent study [Padilla et al. Exp Biol Med (Maywood) 235: 1287-1291, 2010] in pigs confirmed a strong correlation between brachial and femoral artery endothelial function, it is unclear to what extent brachial artery FMD represents a systemic index of endothelial function in humans. We conducted a retrospective analysis of data from our laboratory to evaluate relationships between the upper (i.e., brachial artery) vs. lower limb (superficial femoral n = 75; popliteal artery n = 32) endothelium-dependent FMD and endothelium-independent glyceryl trinitrate (GTN)-mediated dilation in young, healthy individuals. We also examined the relationship between FMD assessed in both brachial arteries (n = 42). There was no correlation between brachial and superficial femoral artery FMD (r(2) = 0.008; P = 0.46) or between brachial and popliteal artery FMD (r(2) = 0.003; P = 0.78). However, a correlation was observed in FMD between both brachial arteries (r(2) = 0.34; P < 0.001). Brachial and superficial femoral artery GTN were modestly correlated (r(2) = 0.13; P = 0.007), but brachial and popliteal artery GTN responses were not (r(2) = 0.08; P = 0.11). Collectively, these data indicate that conduit artery vasodilator function in the upper limbs (of healthy humans) is not predictive of that in the lower limbs, whereas measurement of FMD in one arm appears to be predictive of FMD in the other. These data do not support the hypothesis that brachial artery FMD in healthy humans represents a systemic index of endothelial function.

Impact of aging on conduit artery retrograde and oscillatory shear at rest and during exercise: role of nitric oxide

Aging has been recently associated with increased retrograde and oscillatory shear in peripheral conduit arteries, a hemodynamic environment that favors a proatherogenic endothelial cell phenotype. We evaluated whether nitric oxide (NO) bioavailability in resistance vessels contributes to age-related differences in shear rate patterns in upstream conduit arteries at rest and during rhythmic muscle contraction. Younger (n=11, age 26 ± 2 years) and older (n=11, age 61 ± 2 years) healthy subjects received intra-arterial saline (control) and the NO synthase inhibitor N(G)-Monomethyl-L-arginine. Brachial artery diameter and velocities were measured via Doppler ultrasound at rest and during a 5-minute bout of rhythmic forearm exercise. At rest, older subjects exhibited greater brachial artery retrograde and oscillatory shear (-13.2 ± 3.0 s(-1) and 0.11 ± .0.02 arbitrary units, respectively) compared with young subjects (-4.8 ± 2.3 s(-1) and 0.04 ± 0.02 arbitrary units, respectively; both P<0.05). NO synthase inhibition in the forearm circulation of young, but not of older, subjects increased retrograde and oscillatory shear (both P<0.05), such that differences between young and old at rest were abolished (both P>0.05). From rest to steady-state exercise, older subjects decreased retrograde and oscillatory shear (both P<0.05) to the extent that no exercise-related differences were found between groups (both P>0.05). Inhibition of NO synthase in the forearm circulation did not affect retrograde and oscillatory shear during exercise in either group (all P>0.05). These data demonstrate for the first time that reduced NO bioavailability in the resistance vessels contributes, in part, to age-related discrepancies in resting shear patterns, thus identifying a potential mechanism for increased risk of atherosclerotic disease in conduit arteries.

Impact of chronic intermittent external compressions on forearm blood flow capacity in humans

During dynamic exercise, the vasculature embedded within skeletal muscle intermittently collapses due to increased intramuscular pressure (IMP). The aim of this study was to ascertain whether oscillations in IMP during muscle contractions independently contribute to exercise training-induced increases in blood flow capacity (BFC). Based on IMP measurements during handgrip exercise, we attempted to mimic the action of repeated vascular compressions by using external inflatable cuffs. Thus, 24 healthy young male subjects underwent a 4-week program (5 days/week, 1 h/day) of application of external compressions of the non-dominant forearm, while the dominant limb served as an internal control. To evaluate the impact of compression pressures of different magnitudes, subjects were randomly assigned to one of three groups: 50, 100 and 150 mmHg of external compression. Prior to the intervention and after 2 and 4 weeks of treatment, we measured peak forearm blood flow (PBF) (Doppler ultrasound) and calculated peak vascular conductance (PVC) following 10 min of forearm ischemia. In the 50 and 100 mmHg groups, application of intermittent compressions did not alter PBF in either control or intervention forearms. In the 150 mmHg group, there was a trend (P = 0.04) for greater increases in PBF from baseline after 4 weeks in the intervention forearm compared to the control forearm (delta PBF: 4.2 ± 2.5 vs. -2.1 ± 2.0 (ml(100 ml)(-1) min(-1)), in the intervention and control forearms, respectively), but the changes in PVC were not significant (P = 0.1). These findings suggest that repeated oscillations in IMP contribute minimally to exercise-induced increase in forearm BFC in healthy young humans.

Increased muscle sympathetic nerve activity acutely alters conduit artery shear rate patterns

Escalating evidence indicates that disturbed flow patterns, characterized by the presence of retrograde and oscillatory shear stress, induce a proatherogenic endothelial cell phenotype; however, the mechanisms underlying oscillatory shear profiles in peripheral conduit arteries are not fully understood. We tested the hypothesis that acute elevations in muscle sympathetic nerve activity (MSNA) are accompanied by increases in conduit artery retrograde and oscillatory shear. Fourteen healthy men (25 +/- 1 yr) performed three sympathoexcitatory maneuvers: graded lower body negative pressure (LBNP) from 0 to -40 Torr, cold pressor test (CPT), and 35% maximal voluntary contraction handgrip followed by postexercise ischemia (PEI). MSNA (microneurography; peroneal nerve), arterial blood pressure (finger photoplethysmography), and brachial artery velocity and diameter (duplex Doppler ultrasound) in the contralateral arm were recorded continuously. All maneuvers elicited significant increases in MSNA total activity from baseline (P < 0.05). Retrograde shear (-3.96 +/- 1.2 baseline vs. -8.15 +/- 1.8 s(-1), -40 LBNP, P < 0.05) and oscillatory shear index (0.09 +/- 0.02 baseline vs. 0.20 +/- 0.02 arbitrary units, -40 LBNP, P < 0.05) were progressively augmented during graded LBNP. In contrast, during CPT and PEI, in which MSNA and blood pressure were concomitantly increased (P < 0.05), minimal or no changes in retrograde and oscillatory shear were noted. These data suggest that acute elevations in MSNA are associated with an increase in conduit artery retrograde and oscillatory shear, an effect that may be influenced by concurrent increases in arterial blood pressure. Future studies should examine the complex interaction between MSNA, arterial blood pressure, and other potential modulatory factors of shear rate patterns.

Impact of acute exposure to increased hydrostatic pressure and reduced shear rate on conduit artery endothelial function: a limb-specific response

Unlike quadrupeds, humans exhibit a larger hydrostatic pressure in the lower limbs compared with the upper limbs during a major part of the day. It is plausible that repeated episodes of elevated pressure in the legs may negatively impact the endothelium, hence contributing to the greater predisposition of atherosclerosis in the legs. We tested the hypothesis that an acute exposure to increased hydrostatic pressure would induce conduit artery endothelial dysfunction. In protocol 1, to mimic the hemodynamic environment of the leg, we subjected the brachial artery to a hydrostatic pressure gradient (∼15 mmHg) by vertically hanging the arm for 3 h. Brachial artery flow-mediated dilation (FMD) was assessed in both arms before and following the intervention. In protocol 2, we directly evaluated popliteal artery FMD before and after a 3-h upright sitting (pressure gradient ∼48 mmHg) and control (supine position) intervention. Our arm-hanging model effectively resembled the hemodynamic milieu (high pressure and low shear rate) present in the lower limbs during the seated position. Endothelium-dependent vasodilation at the brachial artery was attenuated following arm hanging ( P < 0.05); however, contrary to our hypothesis, upright sitting did not have an impact on popliteal artery endothelial function ( P > 0.05). These data suggest an intriguing vascular-specific response to increased hydrostatic pressure and reduced shear rate. Further efforts are needed to determine if this apparent protection of the leg vasculature against an acute hydrostatic challenge is attributable to posture-induced chronic adaptations.