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. 2019 Mar 7;126(5):1335–1342. doi: 10.1152/japplphysiol.01065.2018

Examining the acute effects of retrograde versus low mean shear rate on flow-mediated dilation

Joshua C Tremblay 1, Arman S Grewal 1, Kyra E Pyke 1,✉
PMCID: PMC6589808  PMID: 30844335

Abstract

Arterial endothelial function is acutely and chronically regulated by blood flow-associated shear stress. An acute intervention employing modest forearm cuff occlusion to simultaneously increase retrograde and decrease mean brachial artery shear rate for 30 min evokes transient impairments in flow-mediated dilation (FMD). However, the independent influence of the low mean versus the retrograde shear stress components is unclear. Healthy young adults [n = 24 (12 women, 12 men); 22 ± 2 yr, body mass index = 25 ± 2 kg/m2 (mean ± SD)] completed three laboratory visits within 1 wk. Visits consisted of 45 min of supine rest followed by a brachial artery FMD test (duplex ultrasound) before and after a 30-min intervention: control (shear rate unchanged), cuff (mean shear rate decreased, retrograde shear rate increased), or arterial compression (mean shear rate decreased, no increase in retrograde shear rate). The mean shear rate on the compression visit was targeted to match that achieved on the cuff visit. Cuff and compression trials decreased mean shear rate to a similar extent (cuff: 43 ± 22 s−1, compression: 43 ± 21 s−1; P = 0.850) compared with control (65 ± 21 s−1; both P < 0.001), with the retrograde component elevated only in the former (cuff: −83 ± 30 s−1, compression: −7 ± 5 s−1; P < 0.001). FMD decreased by 29 ± 30% (P < 0.001) after the cuff intervention and 32 ± 24% (P < 0.001) after the compression trial but was unchanged on the control visit (−0.3 ± 18%; P = 0.754). This was not altered by accounting for the shear rate stimulus. An increased retrograde shear stress does not appear to be obligatory for the transient reduction in FMD achieved after a 30-min exposure to low mean shear stress. These findings provide novel mechanistic insight on the regulation of endothelial function in vivo.

NEW & NOTEWORTHY Low mean and retrograde shear stress are considered atherogenic; however, their relative contribution to the acute regulation of endothelial function in humans is unclear. Matched reductions in mean shear stress (30 min), with and without increases in retrograde shear stress, elicited equivalent reductions in flow-mediated dilation in men and women. These findings afford novel insight regarding the shear stress components governing the acute (dys)regulation of conduit artery endothelial function in vivo.

Keywords: brachial artery, disturbed blood flow, endothelial function, oscillatory shear stress, shear stress

INTRODUCTION

There is considerable interest in examining the impact of shear stress pattern on arterial function and structure because arterial segments exposed to chronically low and oscillatory shear stress preferentially display atherosclerotic lesions (2, 8, 55). An intervention has been developed to examine the impact of acute shear stress disturbances in humans. By inflation of a pneumatic cuff on the forearm to a moderate pressure (50–75 mmHg), retrograde shear stress is increased and mean shear stress is typically reduced in the upstream brachial artery. Application of the cuff intervention for 20–30 min has repeatedly been shown to result in transient endothelial dysfunction in a range of populations and conditions (6, 20, 21, 28, 36, 37, 41, 42, 48, 49, 51).

Although the cuff intervention typically simultaneously decreases mean shear stress while it increases retrograde shear stress, the reduction in endothelial function is often specifically attributed to the increased retrograde component (37, 38, 41, 42). However, in vitro investigations demonstrate that within several hours of exposure both increased retrograde shear stress (i.e., the presence of flow reversal) (12–14, 18, 24, 56) and isolated low time-averaged mean shear stress (i.e., no flow reversal or oscillatory shear stress) evoke deleterious responses in endothelial cells (9–11). Given the potential for independent negative effects of low mean versus increased retrograde shear stress, it is unclear which shear stress component is the primary endothelial insult in the 30-min forearm cuff model.

In agreement with in vitro work, acute endothelial responses in humans can be influenced by changes in mean shear stress independent of retrograde shear stress (22, 27, 32). For example, interventions with a longer duration than the forearm cuff model [prolonged sitting still (1–6 h)] that decrease mean shear stress in the lower limb conduit arteries without producing an increase in retrograde shear stress also result in decreased flow-mediated dilation (FMD) (23, 26–28, 34, 35, 43–45, 52). It is thus reasonable to speculate that even though it is a shorter, 30-min intervention the lowering of mean shear stress per se may contribute to the brachial artery FMD impairment observed with the cuff model.

The purpose of this study was to test whether an acute (30 min) reduction in mean shear stress, achieved with and without an increase in the retrograde component, decreases brachial artery FMD in young, healthy humans. We hypothesized that an acute reduction in mean shear stress would elicit a reduction in FMD when not accompanied by an increase in retrograde shear stress. The forearm cuff intervention is a highly utilized model to investigate the acute effects of shear stress pattern (6, 20, 21, 36, 37, 39, 41, 42, 48–51), and this study challenges the common assumption that the effects of this intervention can be fully attributed to an increase in retrograde shear stress (37, 38, 41, 42). Beyond specifically informing the appropriate interpretation of the effects of the cuff intervention, the findings of this study provide novel insight regarding the isolated impact of shear stress pattern components on the acute regulation of human endothelial function in vivo.

METHODS

Ethical Approval

All experimental procedures and measurements were reviewed and approved by the Queen’s University Health Sciences Research Ethics Board, which conforms to the standards set by the Declaration of Helsinki (with the exception that this study was not registered in a database). Written informed consent was obtained on forms approved by this board before study participation.

Participants

Healthy young men [n = 12; 23 ± 2 yr, body mass index = 24.9 ± 2.0 kg/m2 (mean ± SD)] and women (n = 12; 20 ± 2 yr, body mass index = 24.2 ± 3.0 kg/m2) participated in three counterbalanced experimental testing visits. Participants were normotensive nonsmokers without cardiovascular disease (self-reported) and were not taking any medications besides oral hormonal contraceptives (n = 8). Female participants had regular menstrual cycles (10+ in last 12 mo) and were tested in days 1–7 of their menstrual cycle.

Experimental Protocol

The study involved three laboratory visits completed within 1 wk (temperature controlled: 21 ± 1°C). Each visit was identical apart from the intervention. Baseline arterial blood pressure was acquired in the right arm after participants lay supine for 20 min. Heart rate was continuously monitored from a standard lead II ECG configuration. All FMD tests were performed on the left arm. The first FMD (preintervention FMD) was performed after participants lay supine for 45 min. After the preintervention FMD, the intervention was applied for 30 min. After the intervention, blood pressure measurements were repeated, and subsequently the postintervention FMD measurement was performed.

The three interventions (1 performed each visit) were control, cuff, and compression. On the control visit, the intervention consisted of 30 min of rest. The cuff visit consisted of inflation of a cuff immediately distal to the epicondyles of the left arm to 70 mmHg for 30 min to elicit a decrease in mean shear rate and concomitant increase in retrograde shear rate (5, 41). On the compression visit, the brachial artery pulse was palpated in the antecubital fossa and compressed to decrease mean shear rate to the same level achieved on the cuff visit without increasing retrograde shear rate (7, 30–33). The order of visits was counterbalanced, although the cuff visit always preceded the compression visit [i.e., 3 potential trial sequences; 1) control, cuff, compression; 2) cuff, control, compression; 3) cuff, compression, control]. This was done separately for men and women (i.e., 4 men and 4 women completed the study in each of the 3 orders). One-minute recordings of brachial artery diameter and velocity were performed at 10, 20, and 25 min into the intervention and were used to report the intervention shear rates.

Experimental Measurements

Brachial artery diameter.

Brachial artery diameter was obtained by two‐dimensional ultrasound in B mode (12 MHz; Vivid i2; GE Medical Systems). Ultrasound images were recorded with a VGA‐to‐USB frame grabber (Epiphan Systems) and saved as .avi files on a separate computer using commercially available software (Camtasia Studio; TechSmith) as previously described (19).

Brachial artery blood velocity.

Brachial artery blood velocity was obtained by Doppler ultrasound operating at 4 MHz (Vivid i2; GE Medical Systems). The sample volume encompassed the width of the artery, and the Doppler shift frequency spectrum derived from the Vivid i probe was analyzed with a Multigon 500P TCD spectral analyzer (Multigon Industries) to determine the mean blood velocity. The resulting voltage output from the Multigon was continuously recorded (LabChart; ADInstruments) for subsequent analysis (19). All scans were performed at an insonation angle of 68° as described in Pyke et al. (31). Briefly, our Doppler signal was subject to a calibration procedure at this angle. The ultrasound probe insonated tubing immersed in a water bath at an angle of 68°. A particulate solution was pumped through the tubing at several known flow rates, and the continuous voltage output from the Doppler signal (representing the mean blood velocity) was plotted against the known velocity to provide a linear calibration slope (31).

FMD.

FMD was performed in adherence with the recommended guidelines (16, 40). One minute of baseline brachial artery diameter and blood velocity were acquired via duplex ultrasound. Subsequently, a pneumatic cuff placed distal to the epicondyles was inflated to suprasystolic pressure (250 mmHg) for 5 min. The cuff was then rapidly deflated, and ultrasound imaging persisted for 3 min after deflation.

Data Analysis

Mean arterial pressure and heart rate.

Systolic and diastolic blood pressures were taken as the average of five measurements (first discarded; BpTRU BPM‐100; BpTRU Medical Devices). Mean arterial pressure (MAP) was calculated as (2 × diastolic blood pressure + systolic blood pressure)/3. Resting heart rate (HR) was taken as the average during the 1-min baseline period of each FMD test.

Brachial artery diameter.

Standardized software approaches were used to acquire and analyze the ultrasound recordings. A region of interest was placed around the highest-quality portion of the B-mode longitudinal image of the artery. The software automatically and continuously tracks the walls of the vessel and velocity trace within the regions of interest at a frequency of 30 Hz (FMD/BloodFlow Software version 5.1; Reed Electronics, Perth, WA, Australia) (54). The diameter data were averaged into 3-s time bins.

Brachial artery blood velocity.

Three-second average time bins of antegrade (positive), retrograde (negative), and mean (sum of antegrade and retrograde) blood velocity were analyzed off-line with data acquisition software (LabChart; ADInstruments) (31).

Shear rate.

Antegrade, retrograde, and mean shear rate were calculated as 4 × blood velocity/brachial artery diameter (15). The oscillatory shear index (OSI) was calculated as |retrograde shear rate|/(|antegrade shear rate| + |retrograde shear rate|) (25).

FMD.

Peak diameter was determined as the greatest 3-s average diameter after cuff deflation. FMD was calculated as the absolute (mm) and relative (%) change from baseline to peak diameter. The time to peak diameter was calculated as the time from cuff deflation to peak diameter. The stimulus for FMD was calculated as the shear rate area under the curve from cuff deflation to peak diameter (SRAUC) (33). In healthy participants assessed under normoxic conditions, the inclusion of whole blood viscosity to assess shear stress does not influence the interpretation of FMD (29).

Statistical Analysis

Data were analyzed with a linear mixed model with a compound symmetry covariance structure. For analysis of the impact of the intervention on shear rate (mean, antegrade, retrograde, and OSI), the selected factors were time (preintervention versus intervention versus postintervention), trial (control versus cuff versus compression), and sex (male versus female). For analysis of baseline variables (HR, MAP) and FMD-associated parameters (baseline diameter, time to peak diameter, SRAUC, and FMD), the selected factors were time (preintervention versus postintervention), trial (control versus cuff versus compression), and sex (male versus female). Fisher’s least significant difference post hoc tests were performed when significant main effects or interactions were detected (P < 0.05). To account for potential effects of uncontrolled shear rate stimulus on FMD, analysis was also performed with the SRAUC as a covariate (17). Furthermore, to account for differences in baseline diameter, allometric scaling of FMD was performed with a linear mixed model with the difference between the natural log of peak and baseline diameters as the dependent variable with time, trial, and sex as factors and the natural log of baseline diameter as the covariate (3, 4). Linear mixed model-estimated means and standard errors were backtransformed to present allometrically scaled FMD and SD.

RESULTS

Baseline Parameters

Baseline MAP, HR, and diameter and associated P values are presented in Table 1. Brachial artery diameters were greater in men versus women and smaller after intervention versus before intervention.

Table 1.

Heart rate, mean arterial pressure, and parameters for flow-mediated dilation before and after control, cuff, and compression interventions

Men
 Women
Control Cuff Compression Control Cuff Compression
Baseline diameter, mm
    Pre 3.65 ± 0.58 3.60 ± 0.54 3.61 ± 0.59 2.96 ± 0.27 2.94 ± 0.26 2.98 ± 0.27
    Post 3.58 ± 0.59 3.59 ± 0.52 3.57 ± 0.57 2.92 ± 0.29 2.95 ± 0.27 2.95 ± 0.27
time, P = 0.012; trial, P = 0.838; sex, P = 0.002; time × trial, P = 0.149; time × sex, P = 0.539; trial × sex, P = 0.291; time × trial × sex, P = 0.901
FMD, mm
    Pre 0.19 ± 0.06 0.18 ± 0.07 0.17 ± 0.08 0.19 ± 0.05 0.20 ± 0.06 0.21 ± 0.07
    Post 0.19 ± 0.06 0.13 ± 0.05* 0.11 ± 0.06* 0.18 ± 0.06 0.13 ± 0.07* 0.15 ± 0.07*
time, P < 0.001; trial, P < 0.001; sex, P = 0.503; time × trial, P = 0.001; time × sex, P = 0.249; trial × sex, P = 0.062; time × trial × sex, P = 0.802
SRAUC, 103 au
    Pre 15.8 ± 5.1 15.6 ± 3.4 17.5 ± 5.1 21.2 ± 7.3 19.4 ± 3.7 20.3 ± 8.0
    Post 15.3 ± 5.1 13.8 ± 4.5 16.1 ± 4.6 19.7 ± 6.4 19.6 ± 6.5 20.7 ± 10.1
time, P = 0.186; trial, P = 0.079; sex, P = 0.051; time × trial, P = 0.924; time × sex, P = 0.412; trial × sex, P = 0.616; time × trial × sex, P = 0.438
SRAUC-corrected FMD, %
    Pre 5.6 ± 2.1 5.3 ± 2.1 4.9 ± 2.1 6.2 ± 2.1 6.7 ± 2.1 6.7 ± 2.1
    Post 5.7 ± 2.1 4.2 ± 2.1* 3.6 ± 2.1* 6.1 ± 2.1 4.2 ± 2.1* 4.8 ± 2.1*
time, P < 0.001; trial, P < 0.001; sex, P = 0.236; time × trial, P < 0.001; time × sex, P = 0.063; trial × sex, P = 0.090; time × trial × sex, P = 0.482
Allometrically scaled FMD, %
    Pre 6.5 ± 2.1 6.0 ± 2.1 5.9 ± 2.1 5.7 ± 2.1 5.9 ± 2.1 6.2 ± 2.1
    Post 6.4 ± 2.1 4.7 ± 2.1* 4.3 ± 2.1* 5.2 ± 2.1 3.5 ± 2.1* 4.1 ± 2.1*
time, P < 0.001; trial, P < 0.001; sex, P = 0.513; time × trial, P = 0.001; time × sex, P = 0.074; trial × sex, P = 0.079; time × trial × sex, P = 0.702
Time to peak, s
    Pre 48 ± 29 54 ± 34 54 ± 41 39 ± 9 40 ± 5 39 ± 8
    Post 45 ± 22* 37 ± 8* 46 ± 23* 40 ± 10 39 ± 12 48 ± 14
time, P = 0.174; trial, P = 0.183; sex, P = 0.369; time × trial, P = 0.155; time × sex, P = 0.005; trial × sex, P = 0.975; time × trial × sex, P = 0.460
MAP, mmHg
    Pre 76 ± 6 74 ± 6 75 ± 8 74 ± 5 75 ± 6 74 ± 7
    Post 78 ± 6 78 ± 6 81 ± 5 77 ± 6 75 ± 6 78 ± 7
time, P < 0.001; trial, P = 0.401; sex, P = 0.471; time × trial, P = 0.172; time × sex, P = 0.328; trial × sex, P = 0.682; time × trial × sex, P = 0.152
HR, beats/min
    Pre 60 ± 10 60 ± 10 59 ± 9 62 ± 9 62 ± 9 61 ± 7
    Post 60 ± 10 59 ± 9 59 ± 8 62 ± 9 61 ± 8 61 ± 7
time, P = 0.095; trial, P = 0.080; sex, P = 0.182; time × trial, P = 0.927; time × sex, P = 0.660; trial × sex, P = 0.793; time × trial × sex, P = 0.732
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Data are presented as means ± SD. au, Arbitrary units; FMD, flow-mediated dilation; HR, heart rate; MAP, mean arterial pressure; SRAUC, shear rate area under the curve from cuff deflation to peak diameter.

*

P < 0.05 versus preintervention.

Intervention Shear Rate

Representative Doppler traces and mean and retrograde shear rate during the control, cuff, and compression interventions are displayed in Fig. 1. Group means of shear rate parameters are displayed in Fig. 2. Measurements made at the 10, 20, and 25 min time points were not significantly different for each shear rate parameter and were averaged to provide one value for each intervention. A main effect of sex was present such that mean and antegrade shear rate were greater in women compared with men (P = 0.010 and 0.029). Preintervention shear rate parameters were similar between trials, with the exception that antegrade shear rate was greater before intervention during the compression trial compared with the cuff trial (P = 0.038). Shear rate did not change over time in the control intervention. As intended, mean shear rate was reduced from preintervention to approximately the same extent during the cuff and compression interventions (cuff versus compression P = 0.850). The mean shear rate was significantly lower during the intervention period of the cuff and compression trials compared with the control intervention period (both P < 0.001). Retrograde shear rate and OSI increased during the cuff intervention (both P < 0.001) and decreased during the compression intervention (both P < 0.001). Retrograde shear rate and OSI during the intervention were greater in the cuff condition and lower in the compression condition compared with during the control intervention period (all P < 0.001).

Fig. 1.

Fig. 1.

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A–C: representative blood velocity tracings for the control (A), cuff (B), and compression (C) interventions. D and E: mean (D) and retrograde (E) shear rates during the intervention are presented as means ± SD. *P < 0.05 versus control; †P < 0.05 versus control and cuff.

Fig. 2.

Fig. 2.

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Mean (A), antegrade (B), and retrograde (C) shear rate and oscillatory shear index [D; arbitrary units (au)] before (Pre), during (Intervention), and after (Post) the control, cuff, and compression interventions. Data from men and women were pooled for presentation. Data are presented as means ± SD. *P < 0.05 versus preintervention within condition and versus intervention in the control condition; †P < 0.05 versus preintervention within condition; ‡P < 0.05 versus preintervention during cuff; §P < 0.05 versus postintervention in control condition.

FMD

Individual and group FMD responses are presented in Fig. 3 and FMD parameters in Table 1. FMD decreased significantly (both P < 0.001) and similarly (P = 0.612) after the cuff and compression interventions but was unchanged on the control visit (P = 0.752 after versus before intervention). Postintervention FMD was lower in both the cuff and compression conditions compared with the same time point in the control condition (P < 0.05). There was no effect of sex, and when data from men and women were analyzed separately the pattern and direction of results were the same in both sexes (data not shown). SRAUC was not different before versus after intervention. Following the same pattern as the uncorrected results, SRAUC-corrected FMD and allometrically scaled FMD were reduced similarly after the cuff and compression interventions but were unchanged during the control trial.

Fig. 3.

Fig. 3.

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Mean (bars) and individual (lines) flow-mediated dilation (FMD) before (Pre) and after (Post) intervention for control (A), cuff (B), and compression (C) trials in men and women. *P < 0.05 versus preintervention within condition and versus the same time point in control condition.

DISCUSSION

The present investigation sought to determine whether matched reductions in mean shear stress, with and without an elevated retrograde component, elicit similar reductions in FMD. A 30-min exposure to low mean shear stress with and without a concomitant increase in retrograde shear stress provoked an ~30% reduction in brachial artery FMD. An increased retrograde and oscillatory shear stress does not appear to be obligatory for the transient reduction in FMD achieved after a 30-min exposure to low mean shear stress. These findings have implications for the interpretation of studies employing the cuff intervention and afford novel insight regarding the shear stress components governing the acute (dys)regulation of conduit artery endothelial function in vivo.

Impact of Cuff Intervention on FMD

A 30-min exposure to reduced mean shear stress with increased retrograde shear stress elicited a reduction in FMD. The observed reduction aligns with most previous investigations employing the forearm cuff intervention in young, healthy adults (21, 36, 37, 41, 42, 48, 49). Furthermore, this reduction was evident in both men and women. A previous study from our laboratory identified a decrease in reactive hyperemia-stimulated FMD in women following the forearm cuff intervention; however, there was a concomitant reduction in SRAUC such that covariate-correcting for the shear stress stimulus abolished the reduction in FMD (50). Through implementation of a longer duration of supine rest before preintervention measures (45 min versus 30 min), we were able to mitigate changes in the shear stress stimulus from before to after intervention. As such, the findings suggest, for the first time, that men and women are both susceptible to reductions in reactive hyperemia-stimulated FMD following 30 min of imposed oscillatory shear stress.

Impact of Increased Retrograde versus Isolated Reductions in Mean Shear Stress on FMD

Implementing brachial artery compression distal to the site of ultrasound imaging permitted isolated reductions in mean shear stress while essentially abolishing retrograde shear stress. A 30-min exposure to isolated low mean shear stress produced a reduction in FMD equivalent to that provoked by the forearm cuff intervention, a condition in which the same reduction in mean shear stress was produced in conjunction with increased retrograde shear stress. Therefore, these results indicate that the impairment in FMD following the often-used cuff intervention cannot clearly be attributed to increases in retrograde shear stress alone when mean shear stress is simultaneously reduced. Indeed, these findings suggest that low mean shear stress is an equally potent stimulus for reducing brachial artery endothelial function. Prolonged sitting and leg bending interventions (1–6 h) similarly elicit reductions in mean shear stress absent increased retrograde shear stress in the lower limb conduit arteries and also result in impaired FMD (43–45, 53). The present findings further define the impact of an isolated low mean shear stress, extending the negative impact to a shorter 30-min exposure and the upper limb. However, elevations in retrograde shear stress without significant decreases in mean shear stress have also been reported to reduce FMD with the forearm cuff intervention (21, 37, 48), indicating the potential for independent acute effects of reversing flow.

In vitro evidence suggests that both low mean and retrograde shear elicit proatherogenic effects on the endothelium and that these effects may be largely, although not entirely, overlapping. Both low unidirectional and oscillatory shear stress (i.e., with flow reversal) have been shown to reduce endothelial nitric oxide synthase (10, 14) and promote atherosclerotic plaque development (9), although the mechanisms responsible may differ. However, most gene expression changes in human umbilical vein endothelial cells exposed to matched low mean shear stress with and without flow reversal (i.e., retrograde shear stress) were regulated by the low mean shear stress and not the reversing flow pattern per se (11). In humans in vivo, the reduction in FMD following prolonged sitting (low mean shear stress) and the forearm cuff intervention (oscillatory shear stress) is abolished with prior vitamin C administration (21, 44), suggesting that the impairment in FMD may be mediated by similar mechanisms. In the present study, the magnitude of endothelial dysfunction elicited by exposure to low shear stress was not influenced by concomitant retrograde/oscillatory shear stress exposure; however, our experimental design does not allow us to determine whether there were distinctions in the mechanisms by which function was impaired.

Are Women Susceptible to Oscillatory and Low Mean Shear Stress-Induced Reductions in FMD?

Female participants exhibited 38 ± 30% and 34 ± 28% reduction in FMD after the cuff and compression interventions, respectively. This is in contrast with previous reports suggesting that female conduit arteries may be protected against proatherogenic perturbations in shear stress (52). For instance, Tremblay et al. (50) observed a reduction in FMD stimulated by a sustained increase in shear stress in men, but not women, after a similar 30-min forearm cuff intervention. Likewise, Vranish et al. (52) observed preserved popliteal artery reactive hyperemia-induced FMD in women after 3 h of uninterrupted sitting (isolated low mean shear stress) compared with a consistent, considerable reduction in men. Protection in women may be limb or shear stress stimulus specific, and whether other endothelial functions [i.e., circulating microparticles (20)] are affected differently in men versus women after exposure to oscillatory and/or low mean shear stress merits future investigation.

Methodological Considerations

We achieved matched reductions in mean shear rate measured with conventional Doppler ultrasound mean velocity; we did not assess wall shear rate. Recent advancement in multigate Doppler permits the description of more complex and dynamic shear rate events at the vessel wall (1, 47), and it is unknown whether other shear rate parameters differed between the cuff and compression interventions. Advancements in ultrasound technology should be harnessed for future determination of whether specific complex wall shear rate events (i.e., turbulence) promote the acute dysregulation of endothelial function in vivo. Although we did not assess endothelium-independent vasodilation (i.e., sublingual nitroglycerine), acute perturbations in shear do not influence endothelium-independent vasodilation (46). The sex comparison was not the primary objective of this study, and although the pattern and direction of responses were the same in both sexes, we did not power to detect small sex differences in the magnitude of the effect of the interventions. Additionally, whether clinical populations exhibit a similar endothelial susceptibility to acute periods of low mean shear stress, or a selective sensitivity to distinct shear stress patterns, cannot be determined from this study.

Conclusions

We sought to examine the effects of 30 min of reduced mean shear stress on FMD when this reduction is achieved with and without increases in retrograde shear stress. Matched reductions in mean shear stress evoked similar reductions in FMD with and without retrograde shear stress exposure. These findings provide in vivo evidence that 1) brief reductions in mean shear stress are a potent stimulus for the acute dysregulation of endothelial function and 2) impairments in endothelial function following interventions that simultaneously reduce mean shear stress and increase retrograde shear stress cannot be fully attributed to the retrograde component.

GRANTS

J. C. Tremblay was supported by an Alexander Graham Bell Doctoral Canada Graduate Scholarship [Natural Sciences and Engineering Research Council of Canada (NSERC)], and this study was funded by an NSERC Discovery Grant to K. E. Pyke (05270-2014).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.C.T. and K.E.P. conceived and designed research; J.C.T. and A.S.G. performed experiments; J.C.T. and A.S.G. analyzed data; J.C.T. and K.E.P. interpreted results of experiments; J.C.T. prepared figures; J.C.T. drafted manuscript; J.C.T., A.S.G., and K.E.P. edited and revised manuscript; J.C.T., A.S.G., and K.E.P. approved final version of manuscript.

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