(A–G) Illustration in A shows the design of confocal imaging studies of liquid transport in live alveoli of intact, perfused lungs of transgenic mice of the genotypes indicated in B–G. As indicated in A, we microinstilled alveolar airspaces with Alexa Fluor 647–conjugated dextran (AF-dextran, yellow, 10 kDa, 10 mg/mL) to carry out the determinations of liquid transport shown in B–F. At the conclusion of each experiment, we added fluorescein isothiocyanate-conjugated dextran (FITC-dextran, cyan, 20 kDa, 5 mg/mL) to the lung perfusate solution to define alveolar barrier properties. The confocal image in B shows AF-dextran fluorescence in airspaces of alveoli (magenta) of a TdTomato-expressing LMC-TdTomato mouse at 2 minutes after AF-dextran microinstillation. Air, example airspace. Arrowheads point out example AF-dextran pools at alveolar niches. Images and group data in C and D show change of AF-dextran fluorescence over time. Fluorescence of alveolar walls is not shown. In D, circles indicate n and each represent one mouse in which mean fluorescence change was quantified in imaging fields of at least 30 alveoli; data are shown as mean ± SEM; *P < 0.05 by ANOVA with post hoc Tukey testing. High power views in E show widths of the example AF-dextran pool (white dashed lines and text) and airspace (magenta dashed lines and text) indicated by the dashed square in B. Group data in F show pool and airspace widths over time. In F, circles and bars are as in D; P is as indicated by 2-tailed t test. The confocal image in G shows fluorescence of FITC-dextran added to the lung perfusate solution. Images in B and G represent images generated in 3 mice each of LMC-TdTomato, AT2-AT1-CFTR^null-GFP, AT2-CFTR^null-GFP, and AT1-CFTR^null-GFP mouse groups. Scale bars: 100 (B and G), 50 (C), 20 (E) μm.