Typical staining and destaining in response to Ca influx in a BHK cell expressing NCX1. Background fluorescence without dye is negligible. After incubating the cell with 10 μM FM 4-64 for 50 s, Ca influx was activated for 10 s. Thereafter, fluorescence at the cell surface is greatly increased. Upon washing out the FM dye after 2.2 min, fluorescence declines by 60%. When Ca is introduced briefly a second time, fluroescence decreases rapidly by ∼55%. This decrease corresponds to 28% of the total cell fluorescence before dye washout. Fluorescence micrographs and line scans at the time points indicated are provided above and below the fluorescence graph, respectively.