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The amount of smooth muscle shortening required to cause narrowing of the bronchial lumen and to significantly alter airflow in an airway has not not been measured. Accordingly, in this study, the lumens of isolated fluid-filled bronchial segments from pigs were directly visualized using a fibreoptic endoscope and video-recording. The extent of lumen narrowing was related to smooth muscle contraction, determined morphometrically, and to bronchial lumen flow. Narrowing was produced by electrical field stimulation (EFS), (70 V, 20 Hz, 2-3 ms) and acetylcholine (ACh), (10(-5)-10(-2) M). The diameter of the relaxed bronchial lumen and the reduction in diameter produced by maximum electrical field stimulation were both greater when the transmural pressure was increased (from -8 to 20 cmH2O). The percentage change in diameter produced by electrical field stimulation was not, however, significantly different over this pressure range. Electrical field stimulation caused the bronchial lumen to narrow by 28.5 +/- 3.3% in diameter, and by 45.7 +/- 3.8% in cross-sectional area, at zero transmural pressure. Maximum doses of acetylcholine (10(-2) M) reduced the lumen diameter by 48.3 +/- 3.3%, and the area by 70.3 +/- 4.8%. In separate experiments, maximum doses of ACh stopped the flow of Krebs solution perfused through the lumen of bronchial segments. Morphometric measurements indicated that smooth muscle shortened by a maximum of 32.5 +/- 2.8% to acetylcholine 10(2) M, relative to fully-relaxed dimensions. Simultaneous imaging of the lumen and adventitia of bronchi showed that the luminal margin narrowed more than the adventitia.(ABSTRACT TRUNCATED AT 250 WORDS)

Video-imaging of lumen narrowing; muscle shortening and flow responsiveness in isolated bronchial segments of the pig