SODIUM-PROTON TRANSPORT BY APICAL MEMBRANES OF TYPE-II PNEUMOCYTES

Recent studies fail to confirm the coexistence of Na+ channels and Na/H+ exchange at the apical membranes of lower airway epithelia. Availa bility of plasma membrane vesicles simplifies the investigation of mem brane transport processes. Apical and basolateral plasma membrane vesi cles of disrupted type-II pneumocytes were fractionated upon nonlinear , continuous sucrose gradients. To investigate sodium transport, Na-22 + uptake by apical membrane vesicles was assayed in the presence and a bsence of transmembrane sodium diffusion potentials. Interior-negative sodium diffusion potentials promoted Na-22+ uptake 1.5-fold. Internal ly-directed H+ gradients or NH4+ gradients inhibited Na-22+ uptake 40- 50%. Amiloride (1-1000 muM) inhibited uptake 10-79%. To investigate H transport, decay of transmembrane pH gradients was monitored with pH probe acridine orange. In the presence or absence of externally-direct ed H+ gradients, external sodium promoted internal alkalinization, exc ept in the presence of external amiloride. These observations of amilo ride-sensitive, electrogenic Na+ uptake and amiloride-sensitive, elect roneutral, Na+/H+ coupling indicate coexistence of Na+ channels and Na +/H+ exchange at the apical membrane of type-II pneumocytes.