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

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Abstract
In exocrine acinar cells, Ca2+-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 Ca2+, was used to reveal the Cl− channel distribution in mouse pancreatic acinar cells, where ≈90% of the current activated by Ca2+ in response to acetylcholine was carried by Cl−. When caged Ca2+ in the cytosol was uncaged locally in the apical pole, the Cl− current was activated, whereas local Ca2+ uncaging in the basal or lateral areas of the cell had no effect. Even when Ca2+ 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 Ca2+ concentration ([Ca2+]c), but at a low [Ca2+]c there was clear voltage-dependent deactivation, which increased with hyperpolarization. Functional Ca2+-activated Cl− channels are expressed exclusively in the apical membrane and channel opening is strictly regulated by [Ca2+]c and membrane potential. Ca2+-activated Cl− channels do not mediate Cl− uptake at the base, but acetylcholine-elicited local [Ca2+]c spiking in the apical pole can regulate fluid secretion by controlling the opening of these channels in the apical membrane.