Influence of time of day and intermittent aerobic exercise on vascular endothelial function and plasma endothelin-1 in healthy adults

Reduced flow-mediated dilation (FMD) and elevated plasma endothelin-1 (ET-1) levels may contribute to the higher incidence of adverse cardiovascular events observed in the morning hours. A single bout of intermittent exercise abolishes the diurnal variation in FMD. Studies examining the effects of exercise on vascular and plasma ET-1 responses at different times of day are lacking. We determined the effects of time of day and intermittent aerobic exercise on brachial artery FMD and plasma ET-1 levels in healthy adults. We hypothesized that lower brachial artery FMD in the morning (compared to the afternoon) will be accompanied by higher plasma ET-1 levels. Additionally, we hypothesized that the diurnal variation in brachial artery FMD and plasma ET-1 will be abolished by performing a single bout of intermittent aerobic exercise. Utilizing a randomized, cross-over design, healthy adults [n = 12; 22 ± 4 y; 25.2 ± 2.7 kg/m2] completed two separate trials: morning (08:00 h) and afternoon (16:00 h). Brachial artery FMD and plasma ET-1 were measured prior to and immediately following a bout of intermittent cycling performed at 70% peak Watts. Brachial artery FMD was lower (P < .05) at 08:00 h (4.4 ± 3.4%) compared to 16:00 h (6.3 ± 3.7%), but was unaffected by exercise (4.8 ± 3.9% and 5.7 ± 2.2% for 08:00 h and 16:00 h, respectively). Plasma ET-1 was unaffected by time of day. Compared to pre-exercise, plasma ET-1 decreased (P < .01) at both times of day. Our data indicate that circulating ET-1 levels do not explain the lower morning FMD in healthy adults. Further, a bout of intermittent exercise did not affect brachial artery FMD but decreased plasma ET-1 levels.