Calcium responses of endothelial cell monolayers subjected to pulsatile and steady laminar flow differ

The vascular endothelium is the primary transducer of hemodynamically imposed mechanochemical events. In this study, we measured the intracellular free calcium concentration ([Ca2+]i) using the fluorescent probe fura 2 and ratiometric digital imaging in cultured bovine aortic endothelial cells (BAEC) subjected to various laminar flow patterns. These were steady shear stress (0.2-70 dyn/cm2) and three types of sinusoidal pulsatile shear stress (nonreversing: 40 +/- 20 dyn/cm2; reversing: 20 +/- 40 dyn/cm2; and purely oscillatory: 0 +/- 20 dyn/cm2; flow frequencies: 0.4, 1.0, and 2.0 Hz) in a serum-containing medium. The most dramatic finding was failure of a purely oscillatory flow to increase [Ca2+]i in BAEC monolayers. In contrast, steady flow, as well as nonreversing and reversing pulsatile flows, increased [Ca2+]i. The dynamics of the response were dependent on the flow pattern. Both internal Ca2+ release and extracellular Ca2+ entry were involved in these [Ca2+]i increases. Also, switching from either a steady nonreversing pulsatile or reversing pulsatile flow back to a static condition resulted in a [Ca2+]i increase. However, switching from an oscillatory flow to a static condition did not induce any changes in average [Ca2+]i. This study shows that endothelial cells are able to sense different flow environments in terms of [Ca2+]i signaling and is relevant to further studies of the influence of hemodynamic forces on vascular pathophysiology.