Prolonged sitting is not associated with altered shear‐mediated dilation of the internal carotid artery, despite impairing lower limb endothelial function

The present study aims to examine the effect of 4 h of continuous sitting on cerebral endothelial function, which is a crucial component of cerebral blood flow regulation. We hypothesized that 4 h of sitting may impair cerebral endothelial function similarly to how it affects lower limb vasculature. Thirteen young, healthy participants were instructed to remain seated for 4 h without moving their lower limbs. The blood flow and shear rate (SR) in the popliteal and internal carotid artery (ICA) were measured using duplex Doppler ultrasound. During the 4-h sitting, peripheral (popliteal artery) and cerebral (ICA) endothelial function were assessed every hour. We induced peripheral and cerebral flow-mediated dilation (pFMD and ICA FMD) using hyperemia (5 min of cuff inflation on lower limb, then deflation) or hypercapnia (30s of hypercapnia, end-tidal partial pressure of CO₂ + 9 mmHg), respectively. We then calculated each relative peak dilation from the baseline diameter to identify both pFMD and ICA FMD. We observed a significant decrease in pFMD starting at 2 h from the onset of sitting, and this reduction persisted throughout the 4-h sitting [Base (6.8 ± 4.2%) vs. 2-h (3.9 ± 2.0%), p = 0.044; vs. 3-h (3.2 ± 1.8%), p = 0.016; vs. 4-h (3.2 ± 1.9%), p = 0.005]. In contrast, during the 4-h sitting, ICA blood flow, SR, and ICA FMD remained unchanged (p = 0.062, p = 0.068, and p = 0.203, respectively). Unlike peripheral endothelial function, cerebral endothelial function remained stable during 4-h sitting. This suggests that the acute effect of prolonged sitting on cerebral vasculature differs from that of lower limb vasculature.