Evidence for apical sodium channels in frog lung epithelial cells

To reveal the mechanism of Na+ transport across Xenopus lung epithelium, we recorded short-circuit current (Isc), transepithelial resistance (Rt), and current noise spectra while the isolated lung tissues were mounted in an Ussing-type chamber. Mean values of Isc and Rt obtained while the tissue was bilaterally incubated with NaCl-Ringer solution were Isc = 11.57 +/- 1.19 microA.cm-2 and Rt = 0.82 +/- 0.07 k omega.cm2. Amiloride added to the mucosal (apical) side depressed Isc by 61 to 99%. Ouabain abolished Isc totally when added to the basolateral compartment. Adenosine 3',5'-cyclic monophosphate (cAMP), epinephrine, and a variety of other compounds did not alter Isc significantly. Transepithelial depolarization with serosal KCl solution reduced Isc to 6.22 +/- 1.37 microA.cm-2. Amiloride-sensitive current and the kinetics of amiloride interaction were not significantly affected by depolarization. Fluctuation analysis of Isc in the presence of amiloride revealed a Lorentzian component in the power density spectrum indicating apical Na+ channels. Assuming pseudo-first order kinetics, we calculated single channel currents (iNa) and channel density (M): iNa = 0.29 +/- 0.04 pA and M = 0.24 +/- 0.04 micron 2. Our results show that the route for Na+ transport through lung epithelial cells follows the classical Koefoed-Johnson-Ussing model for tight epithelia.