Aim: During inspiration, expansion of alveolar sacs promotes clearance of alveolar fluid. We hypothesize that stretch-activated conductance channels on the alveolar (apical) side of type II pneumocytes(TIIPs) contribute to fluid clearance. The purpose of this initial study is to probe TIIPs for stretch-activated changes in membrane potential (DY).Methods: Vesicles prepared from the apical (AP) and basolateral (BL) membranes of juvenile porcine TIIPs were suspended in 250 mM buffered mannitol (pH 7.4). Vesicle DY was continuously monitored during vacuum-induced (-0.6 atm) membrane stretch in the presence of DY-sensitive, fluorescent indicator, DiOC5(3), and presence or absence of Na+, K+ or choline salts. DiOC5(3) was calibrated to DY by imposition of transmembrane H+ and K+ gradients. Results: Vacuum-induced stretch of AP membranes initiated depolarization of DY (32-48 mV) in the presence of 25 mM Na+ salts (NaCl, Na gluconate). Within 3-5 mins of depolarization and continued vacuum exposure, repolarization of membranes ensued. During repeated vacuum exposures, depolarization and repolarization were preserved. In the absence of salts or presence of non-Na+ salts (KCl, choline Cl), vacuum-induced depolarization of AP membrane DY was not observed. Gramicidin, a Na+-channel forming ionophore, and gadolinium, a blocker of stretch-dependent Na+ conductance, individually inhibited vacuum-induced, Na+-dependent depolarization. In contrast to AP vesicles, BL vesicles failed to exhibit vacuum-induced depolarizations under any of the tested conditions.Conclusions: Vacuum-induced stretch of AP - but not BL - membranes promotes Na+-dependent, gadolinium-sensitive membrane depolarization consistent with stretch-sensitive, Na+ conductance. We speculate that cyclic activation of this AP conductance contributes to alveolar fluid absorption during respiration.Funded by Children's Health Foundation