Endothelial dysfunction following prolonged sitting is mediated by a reduction in shear stress

Abstract

We and others have recently reported that prolonged sitting impairs endothelial function in the leg vasculature; however, the mechanism(s) remain unknown. Herein, we tested the hypothesis that a sustained reduction in flow-induced shear stress is the underlying mechanism by which sitting induces leg endothelial dysfunction. Specifically, we examined whether preventing the reduction in shear stress during sitting would abolish the detrimental effects of sitting on popliteal artery endothelial function. In 10 young healthy men, bilateral measurements of popliteal artery flow-mediated dilation were performed before and after a 3-h sitting period during which one foot was submerged in 42°C water (i.e., heated) to increase blood flow and thus shear stress, whereas the contralateral leg remained dry and served as internal control (i.e., nonheated). During sitting, popliteal artery mean shear rate was reduced in the nonheated leg (pre-sit, 42.9 ± 4.5 s(-1); and 3-h sit, 23.6 ± 3.3 s(-1); P < 0.05) but not in the heated leg (pre-sit, 38.9 ± 3.4 s(-1); and 3-h sit, 63.9 ± 16.9 s(-1); P > 0.05). Popliteal artery flow-mediated dilation was impaired after 3 h of sitting in the nonheated leg (pre-sit, 7.1 ± 1.4% vs. post-sit, 2.8 ± 0.9%; P < 0.05) but not in the heated leg (pre-sit: 7.3 ± 1.5% vs. post-sit, 10.9 ± 1.8%; P > 0.05). Collectively, these data suggest that preventing the reduction of flow-induced shear stress during prolonged sitting with local heating abolishes the impairment in popliteal artery endothelial function. Thus these findings are consistent with the hypothesis that sitting-induced leg endothelial dysfunction is mediated by a reduction in shear stress.

Keywords: blood flow; endothelial function; physical inactivity.

Fig. 1.

Schematic diagram of experimental protocol and positional changes over the course of study. Measurements taken at the time points of 0, 1, 2, and 3 h were made while the subject was in the seated position, whereas pre-sit and post-sit measurements were taken while the subject was in the supine position. FMD, flow-mediated dilation.

Fig. 2.

A: popliteal artery mean shear rate in nonheated and heated legs before, during, and after sitting for 3 h. B: mean shear rate individual subject responses from pre-sit to 3 h of sitting for both nonheated and heated legs. Data are expressed as means ± SE; *P < 0.05 vs. pre-sit; †P < 0.05, between legs.

Fig. 3.

Popliteal artery %FMD in nonheated and heated legs before and after sitting for 3 h. %FMD data are unadjusted for hyperemic shear rate area under the curve. Data are expressed as means ± SE; *P < 0.05 vs. pre-sit; †P < 0.05, between legs.