Fig. 5. Guinea pig tracheal ring muscle force generated in response to substance P expressed as percent change from peak contraction and response to intravenous anesthetics. (A ) Propofol, 0.5–100 μm (n = 4), and thiopental, 10–200 μm (n = 4), versus substance P time control after C-fiber neurotransmitter depletion by capsaicin analog N -vanillylnonanamide, and (B ) propofol, 10–100 μm (n = 4), and thiopental, 10–200 μm (n = 4), versus substance P time control after mast cell neurotransmitter depletion by compound 48/80. Only propofol causes significant relaxation of substance P-induced contraction after mast cell and nonadrenergic, noncholinergic nerve neurotransmitter depletion, suggesting a role for propofol-induced relaxation of substance P-induced contraction at the airway smooth muscle itself.