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

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 mu A/cm(2), n = 77; CF: 16.7 +/- 1.5 mu A/cm(2), n = 42), nor R-T (non-CF: 288 +/- 15 Ohm . cm(2): CF: 325 +/- 20 Ohm . cm(2)) showed any significant differences, only VT 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.