Apically localized increase in [Ca²⁺] activates K and Cl channels. Experiments were performed with the buffering conditions established in Fig. 2 (C–F). (A) A bright field and pseudocolored image series in which the [Ca²⁺] was increased close to the apical PM (red dot). (B) In these images and kinetic plot, a marked increase in signal was evoked in the apical domain (red line) and remained highly localized. A minimal increase is observed in the basal region (blue line) after laser exposure (green arrow). (C; top) The increase in whole cell K⁺ current evoked by the Ca²⁺ transient in B is shown. (Bottom) The increase in Cl⁻ current after a subsequent apical laser exposure after the bath solution had been exchanged for 140 mM TEA-Cl. A robust increase in whole cell Cl⁻ current is evoked. (D)Paired experiments before and after flash.