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.

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 signaling. The primary aim of the present study was to verify the localization and the identity of Ca-dependent ion channels in acinar cells of the mouse lacrimal gland.

METHODS

Whole-cell patch-clamp-electrophysiology, spatially localized flash-photolysis of Ca and temporally resolved digital Ca-imaging was combined. Immunostaining of enzymatically isolated mouse lacrimal acinar cells was performed.

RESULTS

We show that the Ca-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.