Identification of Nonselective Cation Channels in Cultured Adult Rat Alveolar Type II Cells

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Abstract
There is evidence supporting the role of active transport of Na+ in the resolution of pulmonary edema, but the exact cellular mechanism(s) underlying this process remain unknown. This study demonstrated the presence of ion channels on adult rat alveolar type II cells that might be associated with this active transport of Na+. Patch-clamp techniques were used to characterize a nonselective cation channel in adult rat alveolar type II epithelial cells held in culture for 24 to 72 h. Single-channel currents were recorded from inside-out, cell-free membrane patches. The most common type of single channel had a linear slope conductance of 20.4 +/- 0.6 pS (n = 22) in symmetrical NaCl (150 mM) solutions. The channel was approximately equally permeable to Na+ and K+ ions (PK/PNa = 1.15) and was highly selective for cations (PCl/PNa < 0.05). Channel activity was Ca(2+)-dependent, and it required at least 10 microM Ca2+ on the cytosolic side of an inside-out patch to activate the channel. Amiloride (1 to 10 microM), a Na+ channel blocker in epithelial tissue, reduced the steady-state open probability of the channel 10-fold but had no significant effect on the magnitude of the single-channel conductance. Single channels with similar properties were not found in cultured rat alveolar macrophages. The possible role of this amiloride-sensitive, nonselective cation channel in Na+ transport and lung liquid clearance is discussed.