Chloride-activated water permeability in the frog corneal epithelium

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Abstract
We have previously reported that the isolated frog corneal epithelium (a Cl−-secreting epithelium) has a large diffusional water permeability (Pdw ≈ 1.8×10−4 cm/s). We now report that the presence of Cl− in the apical-side bathing solution increases the diffusional water flux, Jdw (in both directions) by 63% from 11.3 to 18.4 μl min−1 · cm−2 with 60 mm [Cl] exerting the maximum effect. The presence of Cl− in the basolateral-side bathing solution had no effect on the water flux. In Cl−-free solutions amphotericin B increased Jdw by 29% but only by 3% in Cl−-rich apical-side bathing solution, suggesting that in Cl−-rich apical side bathing solution, the apical barrier is no longer rate limiting. Apical Br− (75 mm) also increased Jdw by 68%. The effect of Cl− on Jdw was observed within 1 min after its addition to the apicalside bathing solution. HgCl2 (0.5 mm) reduced the Cl−-increased Pdw by 31%. The osmotic permeability (Pf) was also measured under an osmotic gradient yielding values of 0.34 and 2.88 (x 10−3 cm/s) in Cl−-free and Cl−-rich apical-side bathing solutions respectively. It seems that apical Cl−, or Cl− secretion into the apical bath could activate normally present but inactive water channels. In the absence of Cl−, water permeability of the apical membrane seems to be limited to the permeability of the lipid bilayer.