Fig. 9. Shear stress changes the sensitivity of cells to ATP. Primary endothelial cells were either cultured on coverslips (unstressed) or exposed to a fluid shear stress of 20 dynes/cm² (stressed) for 24 hours in a parallel-plate flow chamber. The cells were loaded with Indo-1 and then exposed to the indicated concentrations of ATP while Ca²⁺-dependent Indo-1 fluorescence was continuously recorded. As little as 0.2 µM ATP was sufficient to stimulate a wave of Ca²⁺ release in unstressed cells, whereas 2 µM ATP was required to elicit a similar response in stressed cells.