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 ≈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+)](c)), but at a low [Ca(2+)](c) 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+)](c) and membrane potential. Ca(2+)-activated Cl(−) channels do not mediate Cl(−) uptake at the base, but acetylcholine-elicited local [Ca(2+)](c) spiking in the apical pole can regulate fluid secretion by controlling the opening of these channels in the apical membrane.