Dynamic regulation of CFTR bicarbonate permeability by [Cl ^-]_i and its role in pancreatic bicarbonate secretion

Background & Aims: Pancreatic bicarbonate (HCO₃ ^-) secretion is important for a healthy pancreas as well as digestive physiology. However, how human pancreatic duct cells secrete copious amounts of HCO ₃ ^- has long been a puzzle. Here, we report that a dynamic increase in the cystic fibrosis transmembrane conductance regulator (CFTR) HCO₃ ^- permeability by intracellular Cl^- concentration ([Cl^-]_i)-sensitive mechanisms plays a pivotal role in pancreatic HCO₃ ^- secretion. Methods: The role of [Cl^-]_i-sensitive kinases in CFTR-mediated HCO ₃ ^- transport was examined in heterologous expression systems, PANC1 human pancreatic duct cells, and human and guinea pig pancreatic tissues using an integrated molecular and physiologic approach. Results: In human pancreatic tissues, CFTR-positive duct cells abundantly expressed with-no-lysine (WNK1) kinase, oxidative stress-responsive kinase 1 (OSR1), and sterile 20/SPS1-related proline/alanine-rich kinase (SPAK), which are known to be activated by low [Cl^-]_i. Interestingly, CFTR activation rapidly decreased [Cl^-]_i in response to luminal Cl ^- depletion in polarized PANC1 human pancreatic duct cells. Notably, the WNK1-mediated OSR1 and SPAK activation by low [Cl^-]_i strongly increased CFTR HCO₃ ^- permeability in CFTR-transfected HEK 293T, PANC1, and guinea pig pancreatic duct cells, making CFTR primarily an HCO₃ ^- channel, which is essential for the secretion of pancreatic juice containing HCO₃ ^- at a concentration greater than 140 mmol/L. In contrast, OSR1 and SPAK activation inhibited CFTR-dependent Cl^-/HCO₃ ^- exchange activity that may reabsorb HCO₃ ^- from the high HCO ₃ ^--containing pancreatic juice. Conclusions: These results indicate that the [Cl^-]_i-sensitive activation of the WNK1-OSR1/SPAK pathway is the molecular switch to generate HCO₃ ^--rich fluid in the human pancreatic duct.