Mechanisms of bicarbonate secretion: lessons from the airways.

Early studies showed that airway cells secrete HCO(3)(-) in response to cAMP-mediated agonists and HCO(3)(-) secretion was impaired in cystic fibrosis (CF). Studies with Calu-3 cells, an airway serous model with high expression of CFTR, also show the secretion of HCO(3)(-) when cells are stimulated with cAMP-mediated agonists. Activation of basolateral membrane hIK-1 K(+) channels inhibits HCO(3)(-) secretion and stimulates Cl(-) secretion. CFTR mediates the exit of both HCO(3)(-) and Cl(-) across the apical membrane. Entry of HCO(3)(-) on a basolateral membrane NBC or Cl(-) on the NKCC determines which anion is secreted. Switching between these two secreted anions is determined by the activity of hIK-1 K(+) channels.