Voltage-gated chloride currents in cultured canine tracheal epithelial cells

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Abstract
Chloride ions (Cl−) are concentrated in airway epithelial cells and subsequently secreted into the tracheal lumen by downhill flux through apical Cl− channels. We have studied Cl− currents in cultured canine tracheal cells using the whole-cell voltage-clamp technique. Ultrastructural techniques demonstrated that the cells used in the electrophysiological experiments possessed apical membrane specializations known to be present in the intact, transporting cell type. Cultured cells 2–6 days old were characterized by an input resistance of 3.4±0.8 GΩ (n=11) and a capacitance of 63.8±10.8 pF (n=26). A comparison of 3 and 4 day-old cells with 5 and 6 day-old cells showed that the input resistance decreased almost 50%, and the cell capacitance and the inward and outward currents increased concomitantly approximately 200%. Cultured cells 3–4 days old held at −40 mV produced currents of 196±22 pA at 50 mV and −246±27 pA at −90 mV (n=212) with pipette and bath solutions containing primarily 140 KCl and 140 NaCl, respectively. The chloride channel blocker diphenylamine-2-carboxylate (DPC, 100 μm) suppressed whole-cell currents by 76.8% at 60 mV; however, currents were unaffected by the stilbenes SITS (1mm) and DNDS (1–30 μm). Replacement of K+ with Cs+ in the pipette solution did not affect the outward current, the current reversal potential, or the input resistance of the cells, indicating that the current was not significantly K+ dependent when the intrapipette solution was buffered to a Ca2+ concentration of 20nm. The Cl−/Na+ permeability ratio was estimated to be greater than 11 as calculated from reversal potential measurements in the presence of an internal to external NaCl concentration ratio of 1∶2. Current equilibrium permeabilities, relative to Cl− were: I− (2.9)≫NO 3 − (1.1)≥Br− (1.1)≥Cl− (1.0)≥F− (0.93)≫MeSO 4 − (0.19)≥gluconate (0.18)≥aspartate (0.14). Depolarizations to potentials greater than 20 mV elicited a time-dependent component in the outward current in 71% of the cells studied. Currents inactivated with a double exponential time course at the most depolarized voltages. Recovery from inactivation was fast, holding potential-dependent, and followed a double exponential time course. Current amplitude was increased via a cAMP-dependent pathway as has been demonstrated for single Cl-selective channels in cell-attached patches from cultured canine and human tracheal epithelial cells. Forskolin, an activator of adenylate cyclase, produced a 260% increase in the outward current at +50 mV. In summary, cultured canine tracheal cells have a single resting conductance that is Cl− selective, voltage-dependent, and modulated by a cAMP-dependent mechanism. This preparation appears to be appropriate for analysis of cellular modulation of airway Cl− channels and Cl− secretion.