Sodium- and chloride-conductive pathways in cultured mouse tracheal epithelium

The utility of a transgenic murine model of cystic fibrosis (CF) lung disease will likely depend on whether the mouse's proximal airway epithelium is characterized by Na+- and Cl--conductive pathways comparable to those found in human airways. Therefore, the electrophysiological properties of primary cultures of mouse tracheal epithelium (MTE) were investigated using double-barreled, Cl--selective microelectrodes. Epithelial cells isolated from freshly excised mouse tracheae formed confluent polarized monolayers on permeable collagen supports and developed significant transepithelial potential differences (~-10 mV) within 5-6 days postseeding. Under basal conditions, the MTE monolayers had an equivalent short-circuit current (I(eq)) of -21.1 ± 2.1 μA/cm2 and a transepithelial resistance of 424 ± 49 Ω · cm2. Intracellular measurements indicated that the apical (V(a)) and basolateral (V(b)) membrane potential differences were -16.9 ± 1.5 and - 25.4 ± 1.5 mV, respectively; apical membrane fractional resistance was 0.36 ± 0.03; and intracellular Cl- activity was 56.1 ± 2.3 mM. The presence of an apical Na+ conductance was demonstrated by luminal amiloride application (10-4 M), which decreased I(eq), hyperpolarized V(a), and increased the fractional resistance of the apical membrane. The presence of an apical Cl- conductance was demonstrated by substitution of Cl- with gluconate in the luminal bath, which decreased intracellular Cl- activity and increased the fractional resistance of the apical membrane. Regulation of the Cl- conductance was tested by exposing MTE to isoproterenol (10-4 M, luminal), which increased I(eq) by activating a depolarizing conductance in the apical membrane. Luminal application of ATP (10-4 M) was also found to increase the rate of Cl- secretion. We conclude that ion transport in MTE, like normal human airway epithelia, is characterized by 1) a significant amiloride-sensitive Na+ conductance in the apical membrane and 2) an apical membrane Cl--conductive pathway that can be regulated by β-adrenergic agonists.