Precision-cut ectopic lung slices (275 μm) were prepared25 (link). After washes to remove residual agarose, slices were placed onto cell culture inserts (Millicell) within 35-mm dishes containing 1 ml recording medium, sealed with a coverslip and placed under a self-contained Olympus LV200 luminescence microscopy system fitted with a cooled Hamamatsu C9100-13 EM-CCD camera (Olympus, Japan) as previously described27 (link). Individual regions of interest were delineated using ImageJ software (v1.41o).