Chloride-activated water permeability in the frog corneal epithelium

Abstract

We have previously reported that the isolated frog corneal epithelium (a Cl⁻-secreting epithelium) has a large diffusional water permeability (P_dw ≈ 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, J_dw (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 J_dw 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 J_dw by 68%. The effect of Cl⁻ on J_dw was observed within 1 min after its addition to the apicalside bathing solution. HgCl₂ (0.5 mm) reduced the Cl⁻-increased P_dw by 31%. The osmotic permeability (P_f) 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.