Sodium-permeable channels in the apical membrane of human nasal epithelial cells

We used patch-clamp techniques to study the channels that underlie the Na+ conductance of the apical membrane of human normal nasal epithelial cells. Cells were cultured on permeable supports and studied after confluence. In 172 of 334 (52%) excised membrane patches, we observed 20-pS Na(+)-permeable channels that do not discriminate between Na+ and K+ (pNa/pK = 1.33). These nonselective cation channels contained subpopulations that differed by dependence of open probability on voltage and bath Ca2+ activity, suggesting two or more channel types with similar electrical properties. In the presence of 10(-4) M amiloride in the pipette, the proportion of excised patches with nonselective cation channels decreased to 52 of 139 patches (37%), but the decrease was spread across all subpopulations of nonselective cation channels in excised patches. Thus no distinctive Na(+)-selective amiloride-sensitive channels were identified in excised patches. In cell-attached patches, Na(+)-permeable channels were recorded in 56 of 262 patches (21%). Their conductance was 21.4 +/- 1.5 pS (n = 25), and most were selective for Na+ over K+ (pNa/pK > 6). In the presence of amiloride (10(-4) M) in the pipette, the frequency of lambda Na(+)-permeable channels in cell-attached patches decreased to 8 of 134 patches (6%), revealing a population of Na(+)-selective channels recorded in cell-attached patches that was inhibited by amiloride. We conclude that, in excised patches, Na(+)-permeable channels are nonselective for Na+ over K+ and < 30% appear to be amiloride sensitive. In contrast, in cell-attached patches, most channels that conduct sodium are 1) selective for Na+ over K+ and 2) amiloride sensitive. Although we have not discovered the explanation for the discrepancy between cell-attached and excised patch data, we speculate that the channels recognized on cell account for the amiloride-sensitive Na+ conductance of the apical membrane, whereas the excision process alters the properties of the Na(+)-permeable channels and/or activate new channels.