Bicarbonate transport of airway surface epithelia in luminally perfused mice bronchioles

HCO₃^- secretion in distal airways is critical for airway mucosal defense. HCO₃^-/H⁺ transport across the apical membrane of airway surface epithelial cells was studied by measuring intracellular pH in luminally microperfused freshly dissected mice bronchioles. Functional studies demonstrated that CFTR, ENaC, Cl^--HCO₃^- exchange, Na⁺-H⁺ exchange, and Na⁺-HCO₃^- cotransport are involved in apical HCO₃^-/H⁺ transport. RT-PCR of isolated bronchioles detected fragments from Cftr, α, β, γ subunits of ENaC, Ae2, Ae3, NBCe1, NBCe2, NBCn1, NDCBE, NBCn2, Nhe1, Nhe2, Nhe4, Nhe5, Slc26a4, Slc26a6, and Slc26a9. We assume that continuous decline of intracellular pH following alkaline load demonstrates time course of HCO₃^- secretion into the lumen which is perfused with a HCO₃^--free solution. Forskolin-stimulated HCO₃^- secretion was substantially inhibited by luminal application of CFTR_inh-172 (5 μM), H₂DIDS (200 μM), and amiloride (1 μM). In bronchioles from a cystic fibrosis mouse model, basal and acetylcholine-stimulated HCO₃^- secretion was substantially impaired, but forskolin transiently accelerated HCO₃^- secretion of which the magnitude was comparable to wild-type bronchioles. In conclusion, we have characterized apical HCO₃^-/H⁺ transport in native bronchioles. We have demonstrated that cAMP-mediated and Ca²⁺-mediated pathways are involved in HCO₃^- secretion and that apical HCO₃^- secretion is largely mediated by CFTR and H₂DIDS-sensitive Cl^--HCO₃^- exchanger, most likely Slc26a9. The impairment of HCO₃^- secretion in bronchioles from a cystic fibrosis mouse model may be related to the pathogenesis of early lung disease in cystic fibrosis.