Active Na+ transport across Xenopus lung alveolar epithelium

Bioelectric properties and unidirectional ion fluxes of alveolar epithelium were studied by utilizing excised Xenopus lungs mounted in the flux chamber under short-circuited conditions. Results show that the alveolar epithelium generates a potential difference (PD) of 8 mV (lumen negative) with a tissue resistance (Rt) of 1000 ohm.cm2. The short-circuit current (Isc) is inhibitable by 80% with alveolar amiloride. Alveolar or pleural exposure of ouabain slowly decreases Isc to zero. Rt is slightly increased by either agent. Control tissues exhibit a greater unidirectional 22Na+ flux in the alveolar to pleural (A----P) direction than in the opposite (P----A) direction, indicating a net removal of Na+ from the alveolar fluid. Amiloride and ouabain both decrease the A----P Na+ flux to the level of the P----A flux, thereby abolishing net Na+ absorption. In contrast, unidirectional 36Cl fluxes are not different in either direction. Neither amiloride nor ouabain affected these 36Cl fluxes and tissue resistance appreciably, indicating that Cl- passively permeates the alveolar epithelium.