The role of chloride in the lens of the eye

Until recently, the investigation of membrane ion transport mechanisms and their relationship with cataract formation has mainly focused on sodium, potassium and calcium. The specific role of chloride in solute transport within the lens has been given little attention. Rather, chloride was considered as simply the counterion to sodium. The purpose of this review is to emphasize the importance of chloride and its involvement in the membrane ion transport systems within the lens. We summarize the general physiological and chemical properties of the chloride ion in the lens, with reference to the regional ion fluxes generated by its special anatomical and electrical structure. We also present our current knowledge of the principal ion transport mechanisms associated with chloride, with particular emphasis on Cl- channels, and discuss their possible physiological significance. Maintenance of a constant cell volume is an evolutionarily ancient homeostatic process and we present some important findings associated with volume regulatory mechanisms in the whole lens and in single lens cells. Finally, we review and discuss the link between cataract formation and chloride channel dysfunction.