Volume and ion transport by fetal rat alveolar and tracheal epithelia in submersion culture

The complex architecture of the mammalian lung has hindered measurements of permeability of and transport by the intact alveolar epithelium. We compared properties of fetal rat alveolar buds and tracheas in submersion culture by microelectrode and micropuncture techniques. Both alveolar buds and tracheas form cysts that accumulate liquid for up to 3 wk in culture. The transepithelial electric potential difference (PD) of alveolar buds and tracheas (lumen negative, 1.2-3.5 and 8-21 mV, respectively) was abolished by metabolic inhibitors or ouabain in the bath. Despite the difference in PDs, both epithelial regions, after 1 or 2 wk in culture, exhibited the following. 1) Cl- activities and concentrations in luminal liquid was 13 and 34% higher than the bathing medium; Na+ and K+ concentrations were not different. 2) Bumetanide inhibited PD by 70%, whereas terbutaline increased PD by 45%. 3) Amiloride injected into the cyst lumen induced a 20% decrease in the PD of 1-wk explants but did not affect 2-wk preparations. Replacement of bath Na+ by choline decreased the PD of tracheas by 85% but did not change alveolar PD in the presence or absence of bumetanide. The pattern of cyst liquid composition and PD responses to drugs suggests that fetal rat alveolar and tracheal epithelia secrete Cl-, which drives volume flow.