Evidence for a greater elevation in vascular shear stress after morning exercise

The vascular endothelium plays an important role in the maintenance of vascular health and the modulation of vascular tone and blood pressure. Recently, it has been demonstrated that blood pressure reactivity to physical activity is greater in the morning, and possibly, diurnal variation in vascular function may also be evident. The aim of this study was to assess vascular responses after exercise at different times of day.

After 45 min of supine rest, 12 male normotensives completed a 30-min bout of cycling at 70% peak oxygen uptake beginning on separate days at 0800 and 1600 h. Edge detection and wall tracking of high-resolution arterial B-mode ultrasound images combined with synchronized Doppler waveform analysis were used to measure brachial and femoral conduit artery diameter and to calculate blood flow and shear rate. Measurements were recorded before and 20 min after exercise.

At 5 min after exercise, the mean +/- SE brachial shear rate was 72 +/- 21 arbitrary unit (AU) higher in the morning compared with the afternoon (P = 0.05), but this was not compensated for by enlargement of arterial diameter (P = 0.59). No diurnal variation was observed in the femoral artery measurements.

Diurnal difference in conduit artery regulatory control manifests as an elevated intravascular shear stress after morning exercise. Potentially, higher postexercise shear rate in the morning in at-risk individuals could contribute to the elevated cardiovascular risk evident in the postwaking hours.