MECHANISM OF CELLULAR REGULATION OF BICARBONATE TRANSPORTER FUNCTION : AN APPROACH WITH RUMINANT PAROTID TOWARD ITS UNDERSTANDING

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Although the electrogenic Na^+- HCO_<3^-> cotransporter (NBCe) is believed to play a crucial role in vectorial HCO_<3^-> transport by mediating a basolateral HCO_<3^-> influx or efflux pathway in various epithelia, its transport properties and cellular regulation under native conditions have not been well understood. The main aim of the present project was to identify and characterize native NBCe currents in acinar cells (BPA cells) acutely isolated from bovine parotid, which is known to produce an enormous amount of HCO_<3^-> - rich fluid. Using the whole-cell patch-clamp technique, we could first demonstrate that BPA cells indeed express a membrane current, which is dependent on the presence of both extracellular Na^+ and HCO_<3^->, is blocked by extracellular DIDS, and has an apparent coupling ratio of 1 Na^+ to 2 HCO_<3^->. Using molecular biological techniques, we could also show that BPA cells express transcripts of NBCe1-B, and that electrophysiological and pharmacological properties of the native cotransporter current are similar to those of recombinant current of NBCe1-B expressed in HEK293 cells. Futhermore, we could provide evidence that the cotransporter activity likely contributes to the resting membrane potential of BPA cells. The results together not only provide the first electrophysiological characterization of native NBCe currents in a ruminant parotid, but also form a first step toward understanding cellular regulation mechanism of the NBCe in a native HCO_<3^-> transporting epithelium.