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 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.

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.

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.

Keywords: Acinar cell; BK channel; Fluid secretion; Lacrimal gland; Na(+)-K(+) ATP-ase; Tear; maxiK.