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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/m 2  (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 &lt; 0.001), with the retrograde component elevated only in the former (cuff: −83 ± 30 s −1 , compression: −7 ± 5 s −1 ; P &lt; 0.001). FMD decreased by 29 ± 30% ( P &lt; 0.001) after the cuff intervention and 32 ± 24% ( P &lt; 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 &amp; 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.

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