Autonomic response characteristics of porcine airway smooth muscle in vivo

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Abstract
The autonomic response characteristics of airways were studied in 65 swine in vivo. Tracheal smooth muscle response was measured isometrically in situ; bronchial response was measured simultaneously as change in airway resistance and dynamic compliance. To determine the optimal resting length at which maximal tracheal contraction was obtained, length-tension studies were generated in 4 animals using maximal electrical stimulation of the vagus nerves determined from stimulus-response characteristics in 8 other swine. Pharmacological studies were performed in 25 animals to determine the relative potency and intrinsic activity of agonists (acetylcholine > histamine .mchgt. norepinephrine) causing contraction of trachea and bronchial airways. In 13 swine, the effects of autonomic stimulation were studied by i.v. administration of dimethylphenylpiperazinium (DMPP) after muscarinic blockade with 1.5 mg/kg i.v. atropine. Tracheal contraction caused by topical application of 3.4 .times. 10-4 mol histamine (13.4 .+-. 1.54 g/cm) was 96 .+-. 7.2% blocked by 25 .mu.g/kg i.v. DMPP in adrenal-intact animals; minimal relaxation was demonstrated in adrenalectomized animals, indicating absence of substantial sympathetic innervation to porcine trachea. Nonadrenergic innervation was not demonstrated. After .beta.-adrenergic blockade, sympathetic stimulation caused .alpha.-adrenergic contraction in bronchial airways but not in trachea. The unique response characteristics of the airways of swine are defined and their utility for acute experimental study of airway responses in vivo is discussed.