Abstract
Epithelia of the eye are located in the cornea, the lens, ciliary body, and pigment epithelium. In this review the relevant aspects of electrolyte transport properties in the isolated human cornea and lens are described. The main physiological function of both the cornea and the lens is the conservation of the transparency of these structures. (1) In the isolated human cornea sodium and chloride ions are pumped towards the tear side. Both net transports are sensitive to pH-changes and can be stimulated by cyclic AMP, epinephrine, and norepinephrine. The response to adrenergic stimulation is mainly mediated by β2 receptors. Propranolol, a β-blocking agent widely used for treatment of glaucoma, hypertension, and angina pectoris, inhibits net sodium transport. By measuring intracellular membrane potentials and ion activities active transport mechanisms can be localized within the epithelium. In addition, an active transport across the endothelial cell layer has to be postulated. The discrepancy between the measured net ion flux and the short circuit current indicates that other ions but Na+ and Cl− ions are transported across the human cornea. (2) In the human lens the epithelial layer is localized on the anterior side of the tissue. In the isolated human lens short circuit current can only be inhibited by ouabain when the glycoside is added to the anterior side of the lens. From the effect of ouabain and ion substitution experiments it can be concluded that about 30% of the translenticular ion transport is maintained by Na+ transport. In addition, short-circuit current is also dependent on K+, Cl−, and HCO3 −. Measurements of intracellular membrane potential and potassium activity reveal that the interior of the lens is an electrically well connected tissue. Translenticular electrolyte transport, ionic content, and bioelectric properties of the whole lens are controlled by the epithelial monolayer of the anterior lens side. (3) Some disorders of cornea and lens in patients with chronic renal failure are discussed.
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