Calcium release from NP-EGTA is dependent on exposure time and laser intensity. A and B, neurons were loaded with NP-EGTA and Fluo-4. Uncaging of NP-EGTA was achieved with a continuous low level UV exposure over the whole scan frame. The characteristics of the calcium rise were controlled (A) by the exposure time (8, 16 and 32 msec, I0 = 0.5 mW) or (B) by the laser intensity (0.5, 0.63 and 0.75 mW, t0 = 16 msec). C, control cells were loaded with Fluo-4 only (I0 = 0.75 mW). The UV light was switched on for the indicated period (dotted line) although UV exposure occurred for only a fraction of each acquired frame (see Methods). The average of all cells (n) in the scan frame is shown. All traces represent normalized fluorescence of Fluo-4.