Acid and base secretion in the Calu-3 model of human serous cells

Submucosal glands are the primary source of airway mucus, a critical component of lung innate defenses. Airway glands are defective in cystic fibrosis (CF), showing a complete absence of secretion to vasoactive intestinal peptide or forskolin, which increase intracellular cAMP concentration. This defect is attributed to gland serous cells, which express the cystic fibrosis transmembrane conductance regulator. Calu-3 cells, which mimic many features of serous cells, secrete Cl- and HCO3-, with HCO3- secretion predominating for forskolin stimulation and Cl- secretion predominating for stimuli that open basolateral K+ channels to hyperpolarize the cells. We used pH stat and ion substitution experiments to clarify the mechanisms and consequences of these two modes of secretion. We confirm that Calu-3 cells secrete primarily HCO3- in response to forskolin. Unexpectedly, HCO3- secretion continued in response to K+ channel openers, with Cl- secretion being added to it. Secretion of HCO3- from hyperpolarized cells occurs via the conversion of CO2 to HCO3- and is reduced by similar to50% with acetazolamide. A gap between the base equivalent current and short-circuit current was observed in all experiments and was traced to secretion of H+ via a ouabain-sensitive, K+-dependent process (possibly H+-K+-ATPase), which partially neutralized the secreted HCO3-. The conjoint secretion of HCO3- and H+ may help explain the puzzling finding that mucus secreted from normal and CF glands has the same acidic pH as does mucus from glands stimulated with forskolin or ACh. It may also help explain how human airway glands produce mucus that is hypotonic.