Alveolar epithelial ion and fluid transport: β-adrenoceptor-mediated control of apical membrane conductive properties in fetal distal lung epithelia

Distal lung epithelial cells isolated from fetal rats were cultured (48 h) on permeable supports so that transepithelial ion transport could be quantified electrometrically. Unstimulated cells generated a short-circuit current (I _sc) that was inhibited (∼80%) by apical amiloride. The current is thus due, predominantly, to the absorption of Na ⁺ from the apical solution. Isoprenaline increased the amiloride-sensitive I _sc about twofold. Experiments in which apical membrane Na ⁺ currents were monitored in basolaterally permeabilized cells showed that this was accompanied by a rise in apical Na ⁺ conductance (G _Na ⁺). Isoprenaline also increased apical Cl ^- conductance (G _Cl ^-) by activating an anion channel species sensitive to glibenclamide but unaffected by 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS). The isoprenaline-evoked changes in G _Na ⁺ and G _Cl ^- could account for the changes in I _sc observed in intact cells. Glibenclamide had no effect upon the isoprenaline-evoked stimulation of I _sc or G _Na ⁺ demonstrating that the rise in G _Cl ^- is not essential to the stimulation of Na ⁺ transport.