Comparison Of Endothelial-dependent Vasodilatory And Vasoconstrictor Responses Between Upper- And Lower-Limb Arteries In Older Adults

Abstract

Low-flow mediated constriction (L-FMC) provides important information about the acute effects of reduced shear stress on endothelial function. L-FMC has been mainly tested in upper-limb arteries, where an inverse association with VO2peak has been observed in younger adults. It is unclear if an L-FMC response exists in lower limb arteries, which are exposed to larger fluctuations in shear stress during traditional modes of aerobic exercise than the upper-limb. Furthermore, it is uncertain whether the same inverse relationship between L-FMC and VO2peak exists in older adults (OA). PURPOSE: Based on known differences in flow-mediated dilation (FMD) between upper- and lower limbs, we tested the hypothesis that heterogeneous L-FMC responses exist between the brachial- (BA) vs. popliteal (POP) arteries. We also expected that OA with greater aerobic fitness would have larger L-FMC responses in both arteries. METHODS: FMD and L-FMC were assessed in 47 OA (67±5 yr; 30F) using high-resolution duplex ultrasonography with commercial edge-detection and wall-tracking software. L-FMC was defined as the % decrease in lumen diameter in response to 5-min of distal ischemia. FMD was calculated as the % increase in lumen diameter following a reactive hyperemia. RESULTS: Larger FMD responses were observed in the BA vs. POP (4.7±1.6% vs. 3.3±2.0%; P<0.001), which were moderately correlated to each other (r=0.54; P<0.001). When allometrically scaled, the BA exhibited a greater L-FMC response than the POP (-1.3±1.6% vs. -0.4±1.6%; P=0.03). L-FMC responses in the BA vs. POP were not correlated with each other (r=0.22; P=0.14). As expected, VO2peak was associated with both BA-FMD (r=0.59; P<0.001) and POP-FMD (r=0.48; P=0.001). VO2peak was moderately correlated to BA L-FMC (r=-0.52; P<0.001) but strongly correlated to POP L-FMC (r=-0.73; P<0.001). CONCLUSION: The heterogeneous L-FMC responses between the BA and POP indicates that upper limb L-FMC responses do not represent a systemic measure of vasoconstrictor capacity. The stronger association between VO2peak and POP L-FMC suggests that larger local shear stress responses, induced by traditional lower-limb modes of aerobic exercise, may result in greater adaptations to low flow-mediated endothelial vasoconstrictor responsiveness.