Bronchomotor responses of isolated sheep airways to electrical field stimulation

To determine the functional innervation of sheep airway smooth muscle, we measured the isometric tension developed by sheep tracheal segments, bronchial rings, and lung parenchymal strips in response to electrical field stimulation (ES) in tissue chambers. The contractions caused by ES were abolished in trachea and bronchi by atropine 10(-6) M or tetrodotoxin 1.6 X 10(-6) M. The small contractions evoked in lung parenchymal strips were not affected by tetrodotoxin. Electrical stimulation in the presence of atropine 10(-6) M and phentolamine 10(-6) M caused a frequency-dependent reduction in serotonin-induced tension in tracheal segments and bronchial rings. Electrical stimulation had little or no effect on the tension evoked in lung parenchymal strips by acetylcholine 10(-5) M or to histamine 10(-6) M. Propranolol 10(-6) M or guanethidine 10(-5) M caused a shift to the right in the frequency-response curve of trachea and bronchi. These findings suggest the presence of cholinergic excitatory and adrenergic inhibitory innervation in larger sheep airways. A concomitant nonadrenergic inhibitory system may be present. The terminal bronchioles present in lung parenchymal strips do not appear to have an effective bronchomotor innervation.