Loss of basolateral HCO₃⁻ transport gives WT a CF mucus phenotype. (A) The mucus from the mounted WT (n = 5) ileal specimens was removed. The apical surface was exposed to buffers with 23 mM NaHCO₃ and the serosal surface exposed to buffers with 23 mM (Control) or 0 mM HCO₃⁻. After 15 min, mucus release was stimulated by serosal exposure to CCh and PGE₂ for 40 min. The total mucus thickness was measured (Total), followed by aspiration of the mucus and measurement of the remaining thickness (Remaining; **, P = 0.008). (B) Bright field images of WT mouse ileal tissue in the absence of serosal HCO₃⁻ before and after mucus aspiration. See Video 3 to watch the difficulty in removing the charcoal-labeled mucus. (C) WT (n = 5) ileal tissue was treated with (5 mM) or without serosal Ba²⁺ before stimulation with CCh and PGE₂. The total mucus thickness was measured (Total), followed by aspiration of the mucus and measurement of the remaining thickness (Remaining; **, P = 0.008). (D) WT ileal tissue was treated with apical or serosal DIDS before stimulation with CCh and PGE₂ (n = 5 in all three groups; *, P < 0.05; **, P < 0.01). Data are presented as mean ± SEM.