Functional and molecular evidence for Na+-HCO3- cotransporter in human corneal endothelial cells

Although bicarbonate transport in corneal endothelium has been suggested to be coupled to Na+, the underlying molecular mechanism has not been clarified. In the present study we investigated whether a recently cloned Na+-NCO3- cotransporter (NBC-I) is responsible for this process, and, if so, whether the endothelium expresses a separate isoform or one of the other two isoforms that have recently been identified (kNBC-1 from kidney and pNBC-1 from pancreas). Using primers designed for specific and common regions we demonstrated by reverse transcriptase polymerase chain reaction (RT-PCR) that both kNBC-1 and pNBC-1 are expressed in cultured human corneal endothelial cells. In addition functional studies with a pi-I-sensitive fluorescence probe were performed. In the presence of HCO3-/CO2 a pH regulatory process was demonstrated which depends on the presence of Na+ and membrane potential, but is independent of Cl- and is inhibited by the disulfonic stilbene DIDS. These results support the presence of NBC-I as the major bicarbonate transport system in corneal endothelium.