Airway narrowing measured in the most distal and proximal airways within the bronchial airway preparation. Airway narrowing was induced by carbachol administered to either (A) adventitial (N = 4) or (B) luminal (N = 4) airway surface. Airway narrowing was quantified from the percentage decrease in luminal cross sectional area (% CSA). Airway narrowing recordings were somewhat later in the proximal airway since aOCT scans were initiated from the distal airway (i.e., proximal recordings occurred ~1 min and 3 min later for luminal and adventitial protocols, respectively). Airway narrowing was greater in distal airways irrespective of whether the drug was applied to the adventitial (P < 0.05) or luminal (P < 0.001) surface. (Two-way ANOVA).