Intraluminal (A) and intracellular (B) mucus volume was determined in the proximal and distal airways. Differences were calculated with ANOVA on ranks; n = 12, 12, 12 and 9 animals for WT, Kcnn4+/+, Scnn1btg/+, and double mutants, respectively. Representative images of mucus attachment to the epithelial surface for Scnn1btg/+ (n = 15) and double mutants (n = 13) (C). Selected areas of main images (scale bar: 50 μm) are noted as 1–3 in red letters, and are shown amplified separately (scale bar: 20 μm). Summary of the percentage of epithelium surface covered my mucus in proximal and distal airways (D). Only paired samples from the same animal were included; n = 13, 16, 15, and 13 for WT, Kcnn4+/+, Scnn1btg/+, and double mutants, respectively. *P < 0.05 vs. WT and Kcnn4–/– ANOVA on ranks. The P values for each proximal vs. distal airways comparison were calculated by rank-sum test.