Diurnal variation of flow-mediated vasodilation in healthy premenopausal women.

The present study was designed to test the hypothesis of a diurnal variation of endothelial function. Sixteen healthy, nonsmoking women were studied, each on four occasions during one 24-h period (2:00 PM, 8:00 PM, 2:00 AM, and 8:00 AM). Endothelial function was assessed by ultrasound determinations of flow-mediated vasodilation (FMD%) in the brachial artery. FMD% was contrasted with endothelium-independent vasodilation, i.e., nitroglycerine-induced vasodilation (NTG%). Additionally, plasma concentrations and urinary excretion of nitrate and cGMP were analyzed. FMD% and NTG% displayed diurnal, albeit not parallel, patterns of variation. Whereas FMD% gradually increased from 2:00 PM and peaked at 2:00 AM (means +/- SE: 3.1 +/- 0.4, 4.4 +/- 0.4, 5.1 +/- 0.9, and 3.9 +/- 0.8%), the NTG% demonstrated a nadir at 2:00 AM. Plasma levels and urinary excretion of nitrate and cGMP did not display diurnal variation and no clear association with the variations seen in FMD% and NTG%. This study demonstrates a diurnal variation in both endothelium-dependent and -independent vasodilation in the brachial artery of healthy women. The background and possible implication of such a variation require further studies.