The impact of chronic isolated increases in superficial femoral artery shear stress on flow-mediated dilation
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A single layer of cells, called the endothelium, lines the arteries of the human body. In response to an increase in blood flow-associated shear stress, these cells stimulate an endothelial-dependent, flow-mediated dilation (FMD). FMD resulting from a transient vs. a sustained stimulus may provide distinct information regarding endothelial function. Previous findings have shown that repeated, isolated increases in shear stress via forearm heat training improve brachial artery FMD in response to a transient shear stress stimulus (TS-FMD). However, the effects of heat training on FMD assessed in response to a sustained shear stress stimulus (SS-FMD) in the lower limb vasculature has not been examined. Therefore, the purpose of this thesis was to determine the impact of heat training on superficial femoral artery (SFA) shear stress on SFA FMD, assessed via TS-FMD (created via reactive hyperemia following release of thigh occlusion) and SS-FMD (created via plantar flexion exercise). Thirteen healthy female participants (23 ± 2 years) underwent 8 weeks of unilateral lower limb heat training in 42.5C water for five 35-min sessions/wk. Biweekly assessments of TS- and SS-FMD were conducted in both the heated and unheated limbs. SFA diameter and blood velocity (for determination of FMD and the shear stress stimulus) were assessed using echo and Doppler ultrasound, respectively. The stimulus for TS-FMD and SS-FMD was stable over the 8 weeks of training (p = 0.113 and p = 0.540, respectively) and not different between limbs (p = 0.705 and p = 0.073, respectively). In the control limb, TS-FMD was lower at weeks 4 (p = 0.012) and 8 (p = 0.011) vs. week 0. SS-FMD did not change over time (p = 0.220) in either limb (p = 0.246). The main findings of this study suggest that 8 weeks of heat training and the associated shear stress stimulus in the SFA: 1) may have prevented a decline in %TS-FMD, and 2) did not affect the FMD response to a sustained increase in shear stress. This suggests that 8 weeks of repeated lower limb heating does not increase shear stress to levels that improve SFA endothelial function in healthy, young females.