Relationship between intrapulmonary airway diameter and smooth muscle tone in excised lungs.

1. Intrapulmonary bronchi in excised dog lungs were outlined with tantalum dust and stereoscopic radiographs taken during deflation and inflation of the lung after rinsing with solutions of saline, histamine, isoprenaline or EDTA. Dimensions of airways were calculated from measurements of the stereoscopic X-ray images. 2. After treatment with EDTA to minimize bronchial smooth muscle activity, airway diameters increased at all transpulmonary pressures (Ptp) and lung volumes relative to their diameter after treatment with histamine; airway hysteresis in relation to Ptp decreased. 3. At low lung volumes, the per cent increase from histamine to EDTA for airways of different sizes was the same (24-30%) but at high volumes (30 cm H2O distending pressure) the dilatation induced by EDTA was 30% for airways less than 3.0 mm diameter and 13% for those greater than 5.0 mm diameter. 4. Even at high lung volumes, intrapulmonary airways are free to constrict or dilate in spite of the stiffness of the supporting parenchyma.