Na+ and Cl- transport across rabbit nonciliated bronchiolar epithelial (Clara) cells

Radioisotopic flux measurements were performed on rabbit Clara cell epithelium cultured in serum-free hormone-supplemented medium to identify the major ion transport pathways in the cell type. Clara cells cultured in serum-free hormone-supplemented medium exhibit a large short-circuit current compared with cells maintained in serum-containing medium (45 μA/cm2 vs. 15 μA/cm2). The responses to amiloride and isoproterenol, however, are similar for cells grown in the two media. A net amiloride-sensitive movement of Na+ in the mucosal (M)-to-serosal (S) direction under short- and open-circuit conditions is detected (1.48 and 0.67 μeq · h-1 · cm-2, respectively). No statistically significant difference in the unidirectional fluxes of Cl- is apparent in the basal state, but a net flux of Cl- in the S-to-M direction is observed after exposure of the apical membrane to amiloride (0.93 μeq · h-1 · cm-2). The partial ionic conductances for Na+ and Cl- estimated from the fluxes measured in the passive directions (J(S → M)(Na), J(M → S)(Cl)) exceed the total tissue conductance by 20%. Using flux ratio analyses of Cl- movements at clamped potentials between -60 and +20 mV show that Cl- movements are not strictly through passive conductive pathways at negative potentials. The movement of Cl- can be modeled by passive diffusion combined with Cl- - Cl- exchange equal to 20% of total passive fluxes of Na+ and Cl-. These observations indicate that 1) Na+ absorption is the major active ion transport pathway across cultured Clara cells, 2) active Cl- secretion is minimal in the basal state, and 3) ~ 20% of the unidirectional Cl- fluxes occur via nonconductive pathways.