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In the present study, we examined the hypothesis that dynamic characteristics of flow modulate the production of vasoactive mediators, namely nitric oxide (NO) and endothelin-1 (ET-1), by human umbilical vein endothelial cells (HUVECs). Cells were exposed for 6 hours in a cone-and-plate apparatus to different types of flow: steady laminar, with shear stresses of 2, 8, and 12 dyne/cm²; pulsatile laminar, with shear stress from 8.2 to 16.6 dyne/cm² and a frequency of 2 Hz; periodic laminar, with square wave cycles of 15 minutes and shear stress from 2 to 8 dyne/cm²; and turbulent, with shear stress of 8 dyne/cm² on average. A second culture dish was kept in a normal incubator as a static control for each experiment. Laminar flow induced synthesis of NO by HUVECs that was dependent on shear-stress magnitude. Laminar shear stress at 8 dyne/cm² also upregulated the level of NO synthase mRNA. As observed …