Chloride-activated water permeability in the frog corneal epithelium

We have previously reported that the isolated frog corneal epithelium (a Cl(-)-secreting epithelium) has a large diffusional water permeability (Pdw approximately 1.8 x 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 microliters 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 apical-side 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.