Effect of vagal tone on airway smooth muscle force and peribronchial pressure in dogs.

The main lobar bronchi were isolated airtight from the rest of the lung in excised dog lung. In the relaxed isolated lobar bronchi in excised dog lung, we simulated the percent narrowing of bronchial diameter which was observed with vagus nerves intact and stimulated relative to that with vagus nerves cut (relaxed state). The percent narrowing of bronchi with vagus nerve intact and stimulated were adapted from the data by Hahn et al. (J. appl. Physiol., 41, 581-589, 1976). The forces of airway smooth muscle (Psm) were obtained as the negative intrabronchial pressure (Pbr) when the isolated bronchi were narrowed to the same diameter as those with vagus intact and stimulated, and they were 5 to 10 cmH20 and 10 to 30 cmH20, respectively. Peribronchial pressure (Px) was obtained as the difference of Pbr in the conditions of intact and dissected parenchyma when the diameter of the isolated bronchi were simulated to those with vagus intact and stimulated, and they were positive relative to pleural pressure (Ppl) with vagus cut, negative to Ppl with vagus stimulated and near zero relative to Ppl with vagus intact. It was suggested that smooth muscle tone which narrowed the bronchi to the same extent as the diameter with vagus intact made homogeneous expansion between bronchi and lung parenchyma.