Airway compliance is a key factor in understanding lung mechanics and is used as a clinical diagnostic index. Understanding such mechanics in small airways both physiologically and clinically is critical. We developed a two-step method to visualize small airways (to as small as-150,μm internal diameter) in detail by staining the lung tissue with a radiopaque solution and then visualizing the tissue with a cone-beam micro-CT. And we determined the “morphometic change” and “localized compliance” of small airways. For the smaller airways (D < 300,μm), the diameter was 36% larger at end tidal inspiration (TV) and 89% larger at total lung capacity (TLC), (length was 18% larger at TV and 43% at TLC) compared with the values at functional residual capacity. With increasing lung pressure, the diameter changed dramatically at a particular pressure and the length changed approximately linearly during both inflation and deflation. The percentage of airway volume for the smaller airways did not behave linearly with that of lung volume. Smaller airways were generally more compliant than the larger airways and exhibited hysteresis in their diameter behavior. These results indicated that the smaller airways did not behave homogenously.

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Indocyanine green (ICG) is a fluorescent dye that has been used as a reagent to examine liver function and for imaging of the retina and choroidal vasculatures. The binding reaction of ICG with Dimyristoylphosphatidylcholine (DMPC) liposome as a model of the cell membrane was examined using a spectroscopic technique. The DMPC liposome was prepared in the unilamellar condition and the binding of ICG was evaluated utilizing a metachromasia phenomenon. It was found that the binding reaction of ICG to the DMPC liposome proceeds stoichiometrically and greatly differs at lower and higher temperatures compared to the phase transition temperature of the membrane. Corresponding to the change of the membrane state, binding of the ICG molecule to the liposome becomes unstable and the binding constant decreases with temperature. Near the phase transition temperature, the binding constant was at a minimum. It is suggested that the membrane fluidity greatly influences the binding properties of the ICG molecule.

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