Local uncaging of caged Ca(2+) reveals distribution of Ca(2+)-activated Cl(-) channels in pancreatic acinar cells.

In exocrine acinar cells, Ca(2+)-activated Cl(-) channels in the apical membrane are essential for fluid secretion, but it is unclear whether such channels are important for Cl(-) uptake at the base. Whole-cell current recording, combined with local uncaging of caged Ca(2+), was used to reveal the Cl(-) channel distribution in mouse pancreatic acinar cells, where approximately 90% of the current activated by Ca(2+) in response to acetylcholine was carried by Cl(-). When caged Ca(2+) in the cytosol was uncaged locally in the apical pole, the Cl(-) current was activated, whereas local Ca(2+) uncaging in the basal or lateral areas of the cell had no effect. Even when Ca(2+) was uncaged along the whole inner surface of the basolateral membrane, no Cl(-) current was elicited. There was little current deactivation at a high cytosolic Ca(2+) concentration (Ca(2+)), but at a low Ca(2+) there was clear voltage-dependent deactivation, which increased with hyperpolarization. Functional Ca(2+)-activated Cl(-) channels are expressed exclusively in the apical membrane and channel opening is strictly regulated by Ca(2+) and membrane potential. Ca(2+)-activated Cl(-) channels do not mediate Cl(-) uptake at the base, but acetylcholine-elicited local Ca(2+) spiking in the apical pole can regulate fluid secretion by controlling the opening of these channels in the apical membrane.