Membrane-bound carbonic anhydrase (CA IV) in human corneal epi- and endothelium

Purpose Active HCO3- transport through the corneal endothelial cell layer c auses a dehydration of the corneal stroma and is thought to be driven by Na /K- and HCO3--dependent ATPase as well as an electro-genic Na/HCO3- cotrans port. Transmembrane bicarbonate transport has also been associated with the recently characterised membrane-anchored isoform of carbonic anhydrase (CA IV) in various tissues. We investigated the localisation of CA IV in human fresh and cultured epi- and endothelium at the light- (LM) and electronmic roscopic (EM) level. Methods Postmortem corneas were obtained within 12 hours of death, stored i n formaldehyde and sectioned in paraffin. LM immunohisto-chemistry was perf ormed using the purified gamma-globulin fraction of a polyclonal chicken an tibody against CA IV isolated from human kidneys. Epi- and endothelial cell cultures were grown in uncoated flasks under standard conditions and proce ssed both for LM and EM immunohistochemistry using the same antibody. Results Light microscopy of fresh tissue showed membrane staining for CA IV in the whole circumference of the endothelium. Little staining was also ob served in some cells of the basal cell layer of the epithelium. Immunohisto chemical staining at the EM level was confined to the cell surface of confl uent cultures of both epi- and endothelial cells. Conclusion The localisation of CA IV to the cell surface of fresh and cultu red corneal endothelium suggests the presence of a membrane-bound ion excha nge mechanism which may be important for HCO3- transport and corneal hydrat ion. Compromising this mechanism by treatment with local carbonicanhydrase inhibitors may be of clinical importance in selected endothelial disease.