Coordination of pancreatic HCO₃^- secretion by protein-protein interaction between membrane transporters

Increasing evidence suggests that protein-protein interaction is essential in many biological processes including epithelial transport. In this report, we discuss the significance of protein interactions to HCO₃^- secretion in pancreatic duct cells. In pancreatic ducts HCO₃^- secretion is mediated by cystic fibrosis transmembrane conductance regulator (CFTR) activated luminal Cl^-/HCO₃^-exchange activity and HCO₃^- absorption is achieved by Na⁺-dependent mechanisms including Na⁺/H⁺ exchanger 3 (NHE3). We found biochemical and functional association between CFTR and NHE3. In addition, protein binding through PDZ modules is needed for this regulatory interaction. CFTR affected NHE3 activities in two ways. Acutely, CFTR augmented the cAMP-dependent inhibition of NHE3. In a chronic mechanism, CFTR increases the luminal expression of Na⁺/H⁺ exchange in pancreatic duct cells. These findings reveal that protein complexes in the plasma membrane of pancreatic duct cells are highly organized for efficient HCO₃^- secretion.