Bidirectional transepithelial water transport: chloride-dependent mechanisms.

We hypothesized that inhibition and activation of basolateral to luminal chloride transport mechanisms were associated with respective decreases and increases in basolateral to luminal water fluxes. The luminal to basolateral (J(W)(L-->B)) and basolateral to luminal (J(W)(B-->L)) water fluxes across ovine tracheal epithelia were measured simultaneously. The mean J(W)(L-->B) (6.5 microl/min/cm(2)) was larger than J(W)(B-->L) (6.1 microl/min/cm(2)). Furosemide reduced J(W)(B-->L) from 6.0 to 5.6 microl/min/cm(2). Diphenylamine-2-carboxylate (DPC) reduced J(W)(B-->L) from 7.9 to 7. 3 microl/min/cm(2) and reduced the membrane potential difference by 38%. Furosemide together with DPC decreased J(W)(L-->B) by 30% and J(W)(B-->L) by 15%. Norepinephrine increased J(W)(B-->L) from 4.9 to 6.0 microl/min/cm(2). Neuropeptide Y in the presence of norepinephrine decreased J(W)(L-->B) (6.4 to 5.2 microl/min/cm(2)) and returned J(W)(B-->L) to its baseline value. Vasopressin increased J(W)(B-->L) from 4.1 to 5.1 microl/min/cm(2). Endothelin-1 induced a simultaneous increase in J(W)(B-->L) (7.0 to 7.7 microl/min/cm(2)) and decrease in J(W)(L-->B) (7.4 to 6.4 microl/min/cm(2)); and decreased the membrane resistance. These data indicate that in tracheal epithelia under homeostatic conditions J(W)(B-->L) has a approximately 15% actively coupled component. Consistent with our hypothesis, inhibition and receptor-induced stimulation of chloride effluxes were associated with decreases and increases in J(W)(B-->L), respectively. However, as inhibition of transcellular chloride transport always decreased J(W)(L-->B) more than J(W)(B-->L), reducing transepithelial chloride transport did not result in less water being transported into the airway lumen.