Distribution of alpha-adrenergic bronchoconstriction in feline airways

We studied the distribution of in vivo alpha-adrenergic bronchoconstriction roentgenographically and histologically as well as functionally, using 13 anesthetised, mechanically ventilated cats. We observed changes in airway diameter by employing tantalum bronchography and studied function by measuring pulmonary resistance (RL), static compliance (Cst), and dynamic compliance (Cdyn). alpha-adrenergic stimulation was produced by intravenous injection of norepinephrine (40 micrograms/kg) after administration of atropine (3 mg/kg, i.v.) and propranolol (2 mg/kg, i.v.). alpha-adrenergic stimulation produced a significant increase in RL (76.2 +/- 4.0 (mean +/- SE)%) and a significant decrease in Cst and Cdyn (23.7 +/- 2.2 and 27.9 +/- 4.2%, respectively). Tantalum bronchograms revealed that alpha-adrenergic stimulation produced a significant decrease in airways with bronchial diameter of 0.5-1.0 mm (9.5 +/- 3.7%). Histological study using a rapid-freezing technique with liquid nitrogen was added to ascertain the site of alpha-adrenergic bronchoconstriction, demonstrating bronchoconstriction in the airways from small bronchi to bronchioli, especially in bronchioli less than 0.5 mm in internal diameter. We conclude that alpha-adrenergic responsiveness can be observed roentgenographically and histologically as well as functionally in vivo, and that the main site of alpha-adrenergic responsiveness is bronchioli less than 1 mm in internal diameter.