High density of Ca(2+)-dependent K+ and Cl- channels on the luminal membrane of lacrimal acinar cells.

Tight-seal whole-cell recording and Ca2+ imaging were simultaneously performed on cell clusters or individual acinar cells of rat lacrimal glands during application of the secretagogue acetylcholine. Activation of Ca(2+)-dependent K+ and Cl- currents was selectively followed as a function of time by placing the cell potential near the equilibrium potential for Cl- or for K+ ion, respectively. Upon acetylcholine application to cell clusters, K(+)- and Cl(-)-selective currents displayed a distinctive initial rise ("hump"). At this time, there was only a small elevation of Ca2+ concentration, [Ca2+]i, that was restricted to the luminal end of acinar cells. A quantitative analysis of Ca2+ and current signals during the hump suggested that the luminal membrane contained high densities of K(+)- and Cl(-)-selective channels, roughly 10 times higher than those found in the basolateral domain. Distinct luminal and basolateral membrane domains were preserved in isolated cells, but with less contrasted densities than in cell clusters. The results suggest that Ca(2+)-dependent K+ channels are implicated not only in the transfer of salt from the blood compartment to the interior of acinar cells, as commonly accepted, but also in the electrolyte secretion from the cell interior to the acinar lumen.