Cloning and immunolocalization of a rat pancreatic Na⁺ bicarbonate cotransporter

In the rat, pancreatic HCO₃^- secretion is believed to be mediated by duct cells with an apical Cl^-/HCO₃^- exchanger acting in parallel with a cAMP-activated Cl^- channel and protons being extruded through a basolateral Na⁺/H⁺ exchanger. However, this may not be the only mechanism for HCO₃^- secretion by the rat pancreas. Recently, several members of electrogenic Na⁺/HCO₃^- cotransporters (NBC) have been cloned. Here we report the cloning of a NBC from rat pancreas (rpNBC). This rpNBC is 99% identical to the longer, more common form of NBC [pNBC; 1079 amino acids (aa); 122 kDa in human heart, pancreas, prostate, and a minor clone in kidney]. The longer NBC isofoms are identical to the rat and human kidney-specific forms (kNBC; 1035 aa; 116 kDa) at the ~980 C-terminal aa's and are unique (with different lengths) at the initial N-terminus. Using polyclonal antibodies to the common N- and C-termini of rat kidney NBC, a ~130-kDa protein band was labeled by immunoblotting of rat pancreas homogenate and was enriched in the plasma membrane fraction. Immunofluorescence and immunoperoxidase light microscopy of rat pancreatic tissue with both antibodies revealed basolateral labeling of acinar cells. Labeling of both apical and basolateral membranes was found in centroacinar cells, intra- and extralobular duct, and main duct cells. The specificity of the antibody labeling was confirmed by antibody preabsorption experiments with the fusion protein used for immunization. The data suggest that rpNBC likely plays a more important role in the transport of HCO₃^- by rat pancreatic acinar and duct cells than previously believed.