Current-induced volume flow across bovine tracheal epithelium: evidence for sodium-water coupling.

The passage of a constant current from lumen to serosa (Il-s), in the range 0.5-2.0 mA, across ouabain-treated bovine tracheal epithelium, induced a stable volume flow (Jv) toward the serosa, proportional to the current. No consistent Jv occurred when current was applied from serosa to lumen. When the standard K+ (6 mM) in the bathing solution was omitted or replaced by choline, Jv was in the same direction as, and proportional to, the current, both with Is-l and with Il-s. The electro-osmotic permeability beta was in the range of 10-15 microl h-1 cm-2 mA-1, i.e. 3-4 X 10(-6) cm s-1 mA-1. The fluxes of Na+, Cl- and mannitol were measured in current-clamp (1 mA, passed from serosa to lumen or lumen to serosa) or voltage-clamp (-20, 0 and +20 mV) conditions, with and without K+. Net transepithelial Na+ fluxes toward the cathode were either smaller than (with Is-l) or equal to (with Il-s) the net fluxes of Cl- toward the anode. The total transepithelial conductance (Gt) increased with the applied electrical gradient, both with Is-l and with Il-s, the change in Gt being larger with Il-s than with Is-l. This increase of Gt was less pronounced when K+ was omitted. The analyses of partial ionic conductances (GNa and GCl) and of the flux ratios indicate the existence of non-conductive diffusion for Cl- and also for Na+. The direction of the electrical gradient influenced the permeability ratio PNa/PCl. With Is-l, PNa/PCl was consistently lower than 0.7, i.e. the mobility ratio of Na+ and Cl- in solution. With Il-s, PNa/PCl was closer to 0.7. The highest Cl- selectivity of the epithelium was observed with Is-l in the presence of K+, i.e. under conditions which failed to induce any conspicuous Jv. The passage of current at 1 mA induced a net flux of mannitol toward the cathode, i.e. in the same direction as Na+ net flux and Jv. However, this mannitol flux was significant only in the absence of K+. These results indicate that Jv was predominantly coupled to the migration of Na+ along the electrical gradient, through a paracellular pathway.