Mechanism of substance P-induced bronchoconstriction in maturing rabbit

The maturational effects of substance P (SP) on airway function were quantitatively assessed in 30 anesthetized tracheotomized rabbits ranging in age from 2 to 29 days. Following paralysis, during mechanical ventilation with 100% O2 in a body plethysmograph, respiratory resistance (Rrs) and dynamic compliance (Cdyn) were continuously monitored. Noncumulative systemic infusions of SP (0.001-5.0 micrograms/g) produced dose-dependent decreases in pulmonary conductance (Grs), i.e., 1/Rrs, and Cdyn. The dose of SP producing a 50% decrease in Cdyn (PD50-Cdyn) significantly increased as a function of age indicating a diminution in airway sensitivity to SP. Bilateral cervical vagotomy and ganglionic blockade with hexamethonium had no effect on the airway response to SP. On the other hand, the response was significantly reduced following atropine sulfate infusion (2 mg/kg), suggesting a peripheral cholinergic contribution located distal to the airway parasympathetic ganglia. The magnitude of this cholinergic contribution increased as a function of age. Unlike atropine, antagonists to histamine and 5-hydroxytryptamine had no effect on the airway response to SP, however, the response was inhibited following infusion of the SP antagonist, D-Pro2,D-Trp7,9-SP. These findings indicate that the airway response to SP is age-related and mediated by binding of the agonist to airway smooth muscle coupled with an accelerated release of acetylcholine at the airway neuromuscular junction.