title: Intermittent exercise abolishes the diurnal variation in endothelial-dependent flow-mediated dilation in humans.
abstract: It is currently unclear to what extent diurnal variation and exercise effect endothelium-dependent nitric oxide (NO)-mediated vasodilation. Therefore, we measured brachial artery flow-mediated dilation (FMD) in 10 males (mean age = 28 yr, SD = 7), before and after a bout of intermittent cycling at 70% peak oxygen uptake on separate days beginning either at 0800 or 1600. Edge-detection and wall-tracking software was used to measure changes in arterial diameter, while shear rate (SR) was assessed using simultaneously derived blood flow velocity and B-mode diameter data. The FMD data were analyzed before and after normalization for SR with repeated-measures models. Before exercise, mean + or - SD FMD was 7 + or - 3% in the morning compared with 11 + or - 6% in the afternoon (P = 0.01). This diurnal variation persisted after data were normalized for SR, which was found to be unaffected by time of day (P = 0.33). Postexercise SR was higher than at baseline (pre-exercise) (P = 0.01) to a similar extent at both times of day. FMD was unaffected by exercise in the morning (P = 0.96) but decreased by 4 + or - 3% following exercise in the afternoon (P = 0.01) so that postexercise measurements did not differ between times of day. These data indicate that endothelium-dependent FMD is lower in the morning, and this finding was not altered by normalization of FMD for diurnal variation in SR. This infers a reduced function of the intrinsic endothelial NO-vasodilator system in the morning. We also report, for the first time, that a bout of intermittent exercise abolishes this diurnal variation in endothelium-dependent FMD.
authors.affiliation: Research Institute for Sport and Exercise Sciences, Liverpool John Moores Univ., Henry Cotton Campus, Webster St., Liverpool L3 2ET, UK. h.jones1@ljmu.ac.uk
authors.forename: Helen
authors.initials: H
authors.lastname: Jones
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Green
authors.forename: Keith
authors.initials: K
authors.lastname: George
authors.forename: Greg
authors.initials: G
authors.lastname: Atkinson
mesh_terms: D000328:Adult; D001642:Bicycling / Q000502:physiology; D001783:Blood Flow Velocity / Q000502:physiology; D001916:Brachial Artery / Q000000981:diagnostic imaging / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology*; D004730:Endothelium, Vascular / Q000502:physiology*; D015444:Exercise / Q000502:physiology*; D006801:Humans; D008297:Male; D010101:Oxygen Consumption / Q000187:drug effects; D014463:Ultrasonography; D014664:Vasodilation / Q000502:physiology*