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 permeabilit y (P-dw approximate to 1.8 x 10(-4) cm/s). We now report that the pres ence of Cl- in the apical-side bathing solution increases the diffusio nal water flux, J(dw) (in both directions) by 63% from 11.3 to 18.4 mu l min(-1). cm(-2) with 60 mM [Cl] exerting the maximum effect. The pr esence of Cl- in the basolateral-side bathing solution had no effect o n 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, sugge sting that in Cl--rich apical side bathing solution, the apical barrie r 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 t he 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 r espectively. It seems that apical Cl-, or Cl- secretion into the apica l bath could activate normally present but inactive water channels. In the absence of CT, water permeability of the apical membrane seems to be limited to the permeability of the lipid bilayer.