Conservation of bronchiolar wall area during constriction and dilation of human airways

We assessed the effect of smooth muscle contraction and relaxation on airway lumen subtended by the internal perimeter (Ai) and total cross-sectional area (Ao) of human bronchial explants in the absence of the potential lung tethering forces of alveolar tissue to test the hypothesis that bronchoconstriction results in a comparable change of Ai and Ao. Luminal area (i.e., Ai) and Ao were measured by using computerized videomicrometry, and bronchial wall area was calculated accordingly. Images on videotape were captured; areas were outlined, and data were expressed as internal pixel number by using imaging software. Bronchial rings were dissected in 1.0- to 1.5-mm sections from macroscopically unaffected areas of lungs from patients undergoing resection for carcinoma, placed in microplate wells containing buffered saline, and allowed to equilibrate for 1 h. Baseline, Ao [5.21 +/- 0.354 (SE) mm2], and Ai (0.604 +/- 0.057 mm2) were measured before contraction of the airway smooth muscle (ASM) with carbachol. Mean Ai narrowed by 0.257 +/- 0.052 mm2 in response to 10 microM carbachol (P = 0.001 vs. baseline). Similarly, Ao narrowed by 0.272 +/- 0.110 mm2 in response to carbachol (P = 0.038 vs. baseline; P = 0.849 vs. change in Ai). Similar parallel changes in cross-sectional area for Ai and Ao were observed for relaxation of ASM from inherent tone of other bronchial rings in response to 10 microM isoproterenol. We demonstrate a unique characteristic of human ASM; i.e., both luminal and total cross-sectional area of human airways change similarly on contraction and relaxation in vitro, resulting in a conservation of bronchiolar wall area with bronchoconstriction and dilation.