Effective sites by sympathetic beta-adrenergic and vagal nonadrenergic inhibitory stimulation in constricted airways

In an attempt to identify the site and nature of nonadrenergic inhibitory nervous regulation in airways, we studied the relative effectiveness of the nervous system in reversing bronchoconstriction along various generations of airways by comparing it with that seen during sympathetic (beta-adrenergic) stimulation in 9 cats. After giving atropine (3 mg/kg), airway dimensions were obtained from roentgenograms after insufflation of powdered tantalum. Baseline bronchoconstriction was then induced by an intravenous infusion of serotonin (50 to 250 micrograms/kg/min). Electrical stimulation of the sympathetic nerves inhibited constriction throughout the airways from trachea down to bronchioli less than 0.5 mm in diameter, with the maximal effect in airways 1 to 4 mm in diameter. The vagi were then stimulated after blocking the adrenergic inhibitory nervous system with propranolol (2 mg/kg). The distribution and potency of the remaining nonadrenergic inhibitory nerve's effect on airway constriction induced by serotonin was comparable to that obtained with sympathetic stimulation. We conclude that the nonadrenergic inhibitory nerves preferentially dilate the midsized airways, which essentially correspond to the sites maximally constricted by vagal stimulation without atropine. The largest (tracheal) and the peripheral airways (less than 1 mm in diameter) are only slightly affected by either inhibitory system.