Expression of BK channels and Na+-K+ pumps in the apical membrane of lacrimal acinar cells suggests a new molecular mechanism for primary tear-secretion

Abstract Purpose Primary fluid secretion in secretory epithelia relies on the unidirectional transport of ions and water across a single cell layer. This mechanism requires the asymmetric apico-basal distribution of ion transporters and intracellular Ca 2+ signaling. The primary aim of the present study was to verify the localization and the identity of Ca 2+ -dependent ion channels in acinar cells of the mouse lacrimal gland. Methods Whole-cell patch-clamp-electrophysiology, spatially localized flash-photolysis of Ca 2+ and temporally resolved digital Ca 2+ -imaging was combined. Immunostaining of enzymatically isolated mouse lacrimal acinar cells was performed. Results We show that the Ca 2+ -dependent K + -conductance is paxilline-sensitive, abundant in the luminal, but negligible in the basal membrane; and co-localizes with Cl − -conductance. These data suggest that both Cl − and K + are secreted into the lumen and thus they account for the high luminal [Cl − ] (∼141 mM), but not for the relatively low [K + ] ( + must be reabsorbed from the primary tear fluid by the acinar cells. We hypothesized that apically-localized Na + -K + pumps are responsible for K + -reabsorption. To test this possibility, immunostaining of lacrimal acinar cells was performed using anti-Na + -K + ATP-ase antibody. We found positive fluorescence signal not only in the basal, but in the apical membrane of acinar cells too. Conclusions Based on these results we propose a new primary fluid-secretion model in the lacrimal gland, in which the paracellular pathway of Na + secretion is supplemented by a transcellular pathway driven by apical Na + -K + pumps.