Dilution potential (DP) across rat alveolar epithelial cell monolayers

Abstract

In this study, we estimated permeability ratio (PNa/PCl) between Na and Cl for paracellular pathways of primary rat alveolar epithelial cell monolayers (RAECM). PNa/PCl was estimated using Goldman‐Hodgkin‐Katz equation and DP measured across RAECM at 5–7 days. To inhibit Na pump activities, RAECM were bathed on both sides with K‐free Ringer's solution (KFRS, in which 5 mM KCl is replaced by equimolar NaCl). DP was measured after lowering apical [NaCl] tenfold by replacing apical fluid with a 9:1 mixture of isotonic K‐free mannitol solution and KFRS. Osmolarity of all solutions was adjusted to 300 mOsm with mannitol. DP was further corrected for liquid junction potential of voltage‐sensing electrodes across blank filters. Results show that equivalent short‐circuit current across RAECM bathed on both sides with KFRS is not significantly different from zero with electrical resistance >1,000 Ωcm2. When apical [NaCl] was lowered tenfold, corrected DP ∼ −13 mV (apical as reference) was observed, yielding PNa/PCl ∼1.8. For comparison, the free solution diffusivity ratio of Na to Cl is ∼0.7, while paracellular PNa/PCl of various epithelial barriers ranges from ∼0.2 and ∼0.5 for frog skin and bullfrog lung to ∼1.3 and ∼10 for rabbit proximal tubule and rat jejunum, respectively. These data indicate that passive ion conductance through tight junctions of RAECM is greater for Na than Cl. (Support: NIH and Hastings Foundation.)