Parasympathetic and adrenergic contractile responses in canine trachea and bronchus

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Abstract
Sympathetic and parasympathetic contractile responses of tracheal and 3rd-order bronchial smooth muscle were compared simultaneously in 26 dogs in situ. Stimulus-response curves were generated by bilateral stimulation of the cervical vagus nerves in 5 dogs to determine the parameters (20 V, 15 Hz, 2-ms duration) causing maximal parasympathetic contraction in trachea and bronchus. In 6 adrenal-intact (ADi) and 5 adrenalectomized (ADx) dogs, sympathetically mediated .alpha.-adrenergic contraction was studied after muscarinic and .beta.-adrenergic blockade by administering i.v. 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP). In ADi dogs the maximal .alpha.-adrenergic contractile response to i.v. DMPP was 67.3 .+-. 14.8% of the maximal parasympathetic response in trachea and 112 .+-. 21% of the maximal parasympathetic response in bronchus (P < 0.03). In ADx dogs the maximal .alpha.-adrenergic-to-parasympathetic stimulation ratios were 17.6 .+-. 1.3% in trachea and 41.4 .+-. 2.5% in bronchus (P < 0.001). Comparable relationships were also obtained in pharmacological studies of .alpha.-adrenergic and cholinergic responses in trachea and bronchus. There is substantial heterogeneity in the physiological and pharmacological cholinergic and .alpha.-adrenergic contractile properties in trachea and bronchus. Relative to cholinergic contraction, circulating catecholamines and sympathetic innervation cause substantially greater .alpha.-adrenergic contraction in bronchus than for tracheal smooth muscle.