We compared the time courses of lung mechanical changes with intravenous (i
v) injection vs, aerosol administration of histamine, methacholine, and ACh
in dogs. We interpret these results in terms of a spring-and-dashpot model
of airway smooth muscle receiving activation via a tissue compartment when
agonist is delivered by the iv route and through an additional airway wall
compartment when it is delivered by the aerosol route. The model accuratel
y accounts for the principal features of the respiratory system elastance r
esponse curves. It also accounts for the differences between iv and aerosol
responses, supporting the notion that agonist delivered by aerosol has to
traverse a longer pathway to the airway smooth muscle than does agonist del
ivered iv. The time constants representing diffusive exchange of agonist be
tween compartments were not significantly different for the three agonists,
suggesting that the three agonists shared a common principal means of clea
rance, which was presumably blood flow.