Cystic fibrosis and non-cystic-fibrosis human nasal epithelium show analogous Na⁺ absorption and reversible block by phenamil

Abstract

 Transepithelial short-circuit current (I _SC), potential (V _T) and resistance (R _T) of confluent monolayers of human nasal epithelium cultured from patients with and without cystic fibrosis (CF) were measured. In our Ussing chamber experiments with monolayers derived from non-CF and CF patients neither I _SC (non-CF: 14.1 ± 1.0 μA/cm², n = 77; CF: 16.7 ± 1.5 μA/cm², n = 42), nor R _T (non-CF: 288 ± 15 Ω· cm²; CF: 325 ± 20 Ω· cm²) showed any significant differences, only V _T showed moderate but significant different values (non-CF: –3.6 ± 0.4 mV; CF: –5.6 ± 0.7 mV, respectively). Total I _SC in CF cells was nearly completely inhibited by amiloride (92 ± 9.6%), while in non-CF tissue amiloride-insensitive conductances mediated a considerable amount of the I _SC (36.3 ± 6.1%), indicating a lower activity of amiloride-sensitive Na⁺ conductances in non-CF cells. In both tissues the amiloride-sensitive I _SC could also be blocked by the amiloride analogues benzamil, phenamil and 5-(N-ethyl-N-isopropyl)2’,4’-amiloride (EIPA) with different affinities. However, amiloride had a significant lower affinity in CF tissue (half-maximal blocker concentration, K _1/2 = 586 ± 59 nM) compared with non-CF tissue (K _1/2 = 294 ± 22 nM). Astonishingly, phenamil, a blocker which irreversibly blocks all epithelial Na⁺ channels hitherto described, inhibited the Na⁺ conductances of human nasal epithelium in a completely reversible way, but nevertheless with high affinity (non-CF: K _1/2 = 12.5 ± 1.2 nM; CF: K _1/2 = 17.1 ± 1.1 nM). Even in high doses none of these blockers had any effect on intracellular Ca²⁺ concentration as measured with Fura-2. From these findings, we conclude that the epithelial Na⁺ conductances of human CF nasal epithelium show modified regulation or are functionally different from those of other tissues.