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.
Methods
Whole-cell patch-clamp-electrophysiology, spatially localized flash-photolysis of Ca2+ and temporally resolved digital Ca2+-imaging was combined. Immunostaining of enzymatically isolated mouse lacrimal acinar cells was performed.
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 Ca2+ signaling. The primary aim of the present study was to verify the localization and the identity of Ca2+-dependent ion channels in acinar cells of the mouse lacrimal gland.
Results
We show that the Ca2+-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+] (<17 mM) of the primary fluid. Accordingly, these results also imply that 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.