Bidirectional transepithelial water transport: measurement and governing mechanisms.

In the search for the mechanisms whereby water is transported across biological membranes, we hypothesized that in the airways, the hydration of the periciliary fluid layer is regulated by luminal-to-basolateral water transport coupled to active transepithelial sodium transport. The luminal-to-basolateral (JWL-->B) and the basolateral-to-luminal (JWB-->L) transepithelial water fluxes across ovine tracheal epithelia were measured simultaneously. The JWL-->B (6.1 microliter/min/cm2) was larger than JWB-->L (4.5 microliter/min/cm2, p < 0.05, n = 30). The corresponding water diffusional permeabilities were PdL-->B = 1.0 x 10(-4) cm/s and PdB-->L = 7.5 x 10(-5) cm/s. The activation energy (Ea) of JWL-->B (11.6 kcal/mol) was larger than the Ea of JWB-->L (6.5 kcal/mol, p < 0.05, n = 5). Acetylstrophanthidin (100 microM basolateral) reduced JWL-->B from 6.1 to 4.4 microliter/min/cm2 (p < 0. 05, n = 5) and abolished the PD. Amiloride (10 microM luminal) reduced JWL-->B from 5.7 to 3.7 microliter/min/cm2 (p < 0.05, n = 5) and reduced PD by 44%. Neither of these agents significantly changed JWB-->L. These data indicate that in tracheal epithelia under homeostatic conditions, JWB-->L was dominated by diffusion (Ea = 4.6 kcal/mol), whereas approximately 30% of JWL-->B was coupled to the active Na+,K+-ATPase pump (Ea = 27 kcal/mol).