Bidirectional transepithelial water transport: Measurement and governing mechanisms

In the search for the mechanisms whereby water is transported across biolog ical membranes, we hypothesized that in the airways, the hydration of the p ericiliary fluid layer is regulated by luminal-to-basolateral water transpo rt coupled to active transepithelial sodium transport, The luminal-to-basol ateral (J(W)(L-->B)) and the basolateral-to-luminal (J(W)(B-->L)) transepit helial water fluxes across ovine tracheal epithelia were measured simultane ously. The J(W)(L-->B) (6.1 mu l/min/cm(2)) was larger than J(W)(B-->L) (4. 5 mu l/min/cm(2), p < 0.05, n = 30), The corresponding water diffusional pe rmeabilities were p(d)(L-->B) = 1.0 X 10(-4) cm/s and p(d)(B-->L) = 7.5 X 1 0(-5) cm/s. The activation energy (E-a) of J(W)(L-->B) (11.6 kcal/mol) was larger than the E-a of J(W)(B-->L) (6.5 kcal/mol, p < 0.05, n = 5), Acetyls trophanthidin (100 mu M basolateral) reduced J(W)(L-->B) from 6.1 to 4.4 mu l/min/cm(2) (p < 0.05, n = 5) and abolished the PD. Amiloride (10 mu M lum inal) reduced J(W)(L-->B) from 5.7 to 3.7 mu l/min/cm(2) (p < 0.05, n = 5) and reduced PD by 44%. Neither of these agents significantly changed J(W)(B -->L). These data indicate that in tracheal epithelia under homeostatic con ditions, J(W)(B-->L) was dominated by diffusion (E-a = 4.6 kcal/mol), where as similar to 30% of J(W)(L-->B) was coupled to the active Na+, K+-ATPase p ump (E-a = 27 kcal/mol).