The responsiveness of airway smooth muscle in vitro from dogs with airway hyper-responsiveness in vivo

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Abstract
1. To determine whether smooth muscle from airways made hyper-responsive by ozone exposure is hyper-response in vitro, tracheal smooth muscle strips taken from five dogs with airway hyperresponsiveness induced by ozone exposure were compared with strips from five control animals. 2. On 1 day, airway responsiveness was assessed with dose-response curves of acetylcholine aerosol versus pulmonary resistance. On a second day, five dogs were exposed to ozone (3.0 p.p.m. for 2 h) and five were exposed to filtered air. Then airway responsiveness to acetylcholine was reassessed. 3. All dogs were then killed, the trachea was rapidly removed and strips of smooth muscle were prepared from the central one-third of the trachea. The responsiveness of each strip to electrical field stimulation (contractions inhibitable by atropine and tetrodotoxin) and exogenous acetylcholine was assessed after 2 and 6 h of incubation and washing. 4. Ozone caused a marked increase in responsiveness in vivo to acetylcholine with a fall in mean provocation concentration from 0.15 g% to 0.026 g% (P < 0.001) while sham exposure had no effect. The responsiveness of muscle strips to electrical field stimulation in ozone-exposed dogs after 2 h of incubation and washing was increased when compared with 6 h of incubation and washing and with the control dogs (P < 0.05 for EF50, the frequency of stimulation giving 50% maximum contraction). 5. However, responsiveness to exogenous acetylcholine was similar in all strips from both ozone-exposed and control dogs. 6. There was a significant correlation between the responsiveness in vivo of the airways just before killing and the mean log EF50 for field stimulation in vitro for each dog obtained 2 h later (r = 0.70, P < 0.05). 7. These data suggest that increased amounts of acetylcholine may be released from stimulated parasympathetic nerve endings in hyper-responsive, ozone-exposed animals, but the change is short lived once the tissue is removed from the in vivo milieu. There was no demonstrable change in the function of the tracheal smooth muscle itself.