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

Abstract

Tight-seal whole-cell recording and Ca2+ uMaging were simultaneously performed on cell dusters or individual acinar cells of rat lacrimal glands during appliction of the secretagogue acetylcholine. Activation of 2+-dependent K+ and Cl-currents was selectively followed as a f i of time by placing the cell potential near the equilibrium potential for Cl-or for K+ ion, respectively. Upon acetylcholine application to cel clusters, K+-and a--selective currents displayed a distinctive initial rise ("hump"). At this time, there was only a small elevation of Ca+ concentration, [CaW+, that was restricted to the luminal end of acinar cells. A qantitative analysis of Ca2e and current signals during the hump sugsted that the luminal membrane contie high densities of K+-and Cl1-selective channels, roughly 10 times higher than those found in the basolateral domain. Distinct luminal and baso lateral membrane doma were preserved in isolated cells, but with less contrasted densities than in cell clusters. The results suggest that Ca2+-dependent K+ annels are implkcted 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.Acinar cells of exocrine glands transfer ions from the blood to the acinar lumen by a Ca2+-dependent process. Three types of Ca2+-dependent channels, the Na,K pump, and a Na/K/Cl cotransport system have been proposed to contribute to this transfer (1-3). Cell-attached patch-clamp recordings from partially dissociated cell aggregates indicate that basolateral membranes of salivary and lacrimal glands contain all three kinds of Ca2+-dependent channels, respectively selective for K+, for Cl-, and for monovalent cations (1-7). Very little information has been obtained on the nature ofion channels in the luminal membrane. Because ofthis lack of information, and of the remaining uncertainty concerning the exact composition of the fluid secreted in the acinar lumen, no agreement has been reached concerning the mechanism of secretion (1-3, 7, 8).Kasai and Augustine (9) recently reported Ca2+ imaging results indicating that the Ca2+ rise induced by an application of the secretagogue acetylcholine (AcCho) is not spatially homogeneous. In these experiments, partially dissociated clusters ofacinar cells were used. Cells in such clusters retain a marked polarity: secretory granules are exclusively found near the "luminal" region ofcell-to-cell contact, whereas the nuclei are located far from the contact area, in the "basolateral" part of the cells (9). Upon AcCho application, the Ca2+ signal starts in the luminal part of the cells and then spreads to the basolateral side. Kasai and Augustine also compared imaging results to whole-cell recordings and have inferred that basolateral and luminal domains contain different sets of ion channels: Cl--and cation-selective channels on the basolateral membrane and Cl--selective channels on the luminal membrane in their particular prepa...

Agonist-induced localized Ca2+ spikes directly triggering exocytotic secretion in exocrine pancreas.

“…It is interesting to note that rises in Ca2+ which lead to vesicle fusion may be mediated by Ca2+ release from organelles in the granular area (Marty, 1991). In fact, increases in [Ca2+]i always started at the granular area where secretory granules lie (Kasai and Augustine, 1990;Nathanson et al, 1992;Tan et al 1992;Toescu et al, 1992 Our study demonstrated that exocytotic secretion could be triggered directly by individual Ca2+ spikes, because exocytotic fusion was less sensitive to Ca2+, and because Ca2+ spikes were sufficient for triggering exocytotic fusion.…”

Apical maxi-K (KCa1.1) channels mediate K+ secretion by the mouse submandibular exocrine gland

“…Cook and colleagues (5, 6) hypothesized that fluid secretion can be driven by K channels in the apical membrane of acinar cells. In fact, there is direct evidence for an apical maxi-K-like current in the acinar cells of frog skin (41) and for apical Ca 2ϩ -activated K ϩ currents in rat lacrimal acinar cells (44 (30,50). Because the K ϩ and Na ϩ concentrations of both the acinar secretions and the venous plasma outflow from salivary gland (which reflects the interstitial ion concentration) are plasma-like, this suggests that K ϩ and Na ϩ likely enter the primary saliva across the "leaky" tight junctions of the acinus.…”

Flash photolysis studies of the localization of calcium release sites in rat parotid isolated acinar cells.

“…In contrast, Kasai & Augustine (1990) reported that in rat pancreatic acinar cells they were localized at the apical end of the cell. More recently, Ca2+ release has been shown to occur at the apical pole of mouse pancreatic and lacrimal acinar cells (Toescu, Lawrie, Petersen & Gallacher, 1992) and of rat lacrimal acinar cells (Tan, Marty & Trautmann, 1992). In rat parotid acinar cells, the elevation of [Ca2+]i resulting from stimulation of muscarinic receptors induces both an outward current, carried mainly by K+ ions through Ca2+-activated K+ channels, and an inward current, carried mainly through Ca2'-activated Cl-channels (Iwatsuki, Maruyama, Matsumuto & Nishiyama, 1985;Gray, 1988a).…”