Mechanism of sodium hyperabsorption in cultured cystic fibrosis nasal epithelium: A patch-clamp study

Transepithelial Na+ absorption is increased two to three times in cystic fibrosis (CF) compared with normal (NL) airway epithelia. This increase has been associated with a higher Na+ permeability of the apical membrane of airway epithelial cells. Because Na+ absorption is electrogenic and abolished by amiloride, Na+ channels are thought to dominate the apical membrane Na+ permeability. Three Na+ channel-related mechanisms may explain the increase in apical Na+ permeability in CF cells: increased number of channels, increased single-channel conductance, and increased single-channel open probability. We compared the properties of Na+-permeable channels in the apical membrane of confluent preparations of human NL and CF nasal epithelial cells cultured on permeable supports. Na+-permeable channels were studied using the patch-clamp technique in the excised inside-out and cell- attached configurations. The same types of Na+-permeable channels were recorded in CF and NL cells. In excised patches, nonselective (Na+/K+) cation channels were recorded, and no differences between CF and NL were found in the properties, incidence, single-channel conductance, and single- channel open probability. In cell-attached patches, channels with a higher Na+ vs. K+ selectivity dominated. There was no difference between CF and NL cells in the incidence (18.8 vs. 21.4%, respectively) and conductance (17.2 ± 2.8 vs. 21.4 ± 1.5 pS, respectively) of Na+-permeable channels. However, the open probability was higher in CF cells compared with NL cells (30.0 ± 3.4%, n = 6, vs. 15.0 ± 3.9%, n = 13; P < 0.05). We conclude that, in CF nasal epithelial cells, the increase in Na+ permeability of the apical membrane results from an increase in the open probability of Na+-permeable channels in the apical membrane.