Episodic bouts of hyperaemia and shear stress improve arterial blood flow and endothelial function.

INTRODUCTION: Exercise and heat stress lead to systemic improvements in arterial endothelial function, vascular stiffness, and cardiopulmonary capacity. The improvements in endothelial function may be primarily mediated via increases in shear stress. This study examined whether improvements in arterial function may be achieved in the absence of systemic vascular adaptations. Specifically, we hypothesized that repeated bouts of brief occlusion would improve arterial endothelial function via shear stress-dependent mechanisms. METHODS: Eleven healthy males underwent a shear stress intervention (5 s brachial occlusion, 10 s rest) for 30 min, five times weekly for 6 weeks on one arm while the other acted as an untreated control. Ultrasound was used to assess brachial arterial forearm blood flow (FBF) and vascular conductance (FVC), diameter, and shear rate (SR), while endothelial function was assessed by flow-mediated dilatation (FMD). Post-occlusive reactive hyperaemia and pulse wave velocity (PWV) were also measured. RESULTS: There were no changes in any of the measures in the control arm (all d < 0.2, p > 0.05). After 3 weeks of the intervention, FMD was increased from baseline (7.6 ± 0.6 vs. 5.9 ± 0.9%; d = 1.3, p = 0.038) and further increased after 6 weeks to 9.5 ± 2.6% (d = 1.7, p < 0.001). SR was also increased following the 6-week intervention (all d ≥ 0.6, p < 0.001). Resting and peak FBF and FVC were also increased in response to the intervention (all d ≥ 0.6, p < 0.001) and PWV was reduced. CONCLUSIONS: These data demonstrate that episodic increases in shear stress elicit marked increases in arterial endothelial function and vascular reactivity.

The role of shear stress on cutaneous microvascular endothelial function in humans.

PURPOSE: Previous studies suggest that exercise and heat stress improve cutaneous endothelial function, caused by increases in shear stress. However, as vasodilatation in the skin is primarily a thermogenic phenomenon, we investigated if shear stress alone without increases in skin temperature that occur with exercise and heat stress increases endothelial function. We examined the hypothesis that repeated bouts of brief occlusion would improve cutaneous endothelial function via shear stress-dependent mechanisms. METHODS: Eleven males underwent a shear stress intervention (forearm occlusion 5 s rest 10 s) for 30 min, five times·week-1 for 6 weeks on one arm, the other was an untreated control. Skin blood flow was measured using laser-Doppler flowmetry, and endothelial function was assessed with and without NOS-inhibition with L-NAME in response to three levels of local heating (39, 42, and 44 °C), ACh administration, and reactive hyperaemia. Data are cutaneous vascular conductance (CVC, laser-Doppler/blood pressure). RESULTS: There were no changes in the control arm (all d ≤ 0.2, p > 0.05). In the experimental arm, CVC to 39 °C was increased after 3 and 6 weeks (d = 0.6; p ≤ 0.01). Nitric oxide contribution was increased after 6 weeks compared to baseline (d = 0.85, p < 0.001). Following skin heating to 42 °C and 44 °C, CVC was not different at weeks 3 or 6 (d ≤ 0.8, p > 0.05). For both 42 and 44 °C, nitric oxide contribution was increased after weeks 3 and 6 (d ≥ 0.4, p < 0.03). Peak and area-under-the-curve responses to ACh increased following 6 weeks (p < 0.001). CONCLUSIONS: Episodic increases in shear stress, without changes in skin or core temperature, elicit an increase in cutaneous microvascular reactivity and endothelial function.