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¹ Department of Physiology, ² Department of Cell and Developmental Biology, and ³ Department of Pathology and Laboratory Medicine, Division of Medical Genetics, University of Pennsylvania, Philadelphia, PA 19104

Correspondence to J. Kevin Foskett: foskett{at}mail.med.upenn.edu

Airway submucosal glands contribute to airway surface liquid (ASL) composition and volume, both important for lung mucociliary clearance. Serous acini generate most of the fluid secreted by glands, but the molecular mechanisms remain poorly characterized. We previously described cholinergic-regulated fluid secretion driven by Ca²⁺-activated Cl^– secretion in primary murine serous acinar cells revealed by simultaneous differential interference contrast (DIC) and fluorescence microscopy. Here, we evaluated whether Ca²⁺-activated Cl^– secretion was accompanied by secretion of HCO₃^–, possibly a critical ASL component, by simultaneous measurements of intracellular pH (pH_i) and cell volume. Resting pH_i was 7.17 ± 0.01 in physiological medium (5% CO₂–25 mM HCO₃^–). During carbachol (CCh) stimulation, pH_i fell transiently by 0.08 ± 0.01 U concomitantly with a fall in Cl^– content revealed by cell shrinkage, reflecting Cl^– secretion. A subsequent alkalinization elevated pH_i to above resting levels until agonist removal, whereupon it returned to prestimulation values. In nominally CO₂–HCO₃^–-free media, the CCh-induced acidification was reduced, whereas the alkalinization remained intact. Elimination of driving forces for conductive HCO₃^– efflux by ion substitution or exposure to the Cl^– channel inhibitor niflumic acid (100 µM) strongly inhibited agonist-induced acidification by >80% and >70%, respectively. The Na⁺/H⁺ exchanger (NHE) inhibitor dimethylamiloride (DMA) increased the magnitude (greater than twofold) and duration of the CCh-induced acidification. Gene expression profiling suggested that serous cells express NHE isoforms 1–4 and 6–9, but pharmacological sensitivities demonstrated that alkalinization observed during both CCh stimulation and pH_i recovery from agonist-induced acidification was primarily due to NHE1, localized to the basolateral membrane. These results suggest that serous acinar cells secrete HCO₃^– during Ca²⁺-evoked fluid secretion by a mechanism that involves the apical membrane secretory Cl^– channel, with HCO₃^– secretion sustained by activation of NHE1 in the basolateral membrane. In addition, other Na⁺-dependent pH_i regulatory mechanisms exist, as evidenced by stronger inhibition of alkalinization in Na⁺-free media.

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