Transport of Na, Cl, and water by the rabbit corneal epithelium at resting potential

Theophylline (1 mM) produced a net transport of Na and Cl from aqueous humor to tears (.02-.04 μmol/cm² h) in the isolated rabbit cornea denuded of endothelium and in the presence of normal resting potential (25-35 mV). The active transport of Na (tears to aqueous) and of Cl (aqueous to tears), estimated with the Goldman constant field equations, was confirmed. A 10°C rise in temperature produced changes close to those predicted for passive processes in both undirectional fluxes of Na and in tears to aqueous flux of Cl, but not for the aqueous to tears flux of Cl. Theophylline treatment doubled Cl permeability but did not significantly affect Na or urea permeability, suggesting specificity of effect. In separate experiments it was shown that stromal thinning occurred in previously swollen corneas when the endothelium was blocked by silicone oil and the epithelium was treated with theophylline. These findings provide further support for the argument that the mammalian epithelium could have an active role in the regulation of corneal thickness in situ.