Dose-dependent inhibition of cold air-induced bronchoconstriction by atropine

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Abstract
A study was undertaken to determine whether inhalation of atropine could inhibit cold air-induced bronchoconstriction in a dose-dependent fashion. In 7 asthmatic subjects the effects of placebo and of various doses of inhaled atropine (0.13-2.08 mg) on baseline specific airway resistance (sRaw) and on the increase in sRaw produced by 5 min of voluntary eucapnic hyperventilation with subfreezing air at -17.degree. C were assessed. The effect of the lowest doses of atropine on the increase in sRaw produced by 5 breaths of 1.0% methacholine were assessed. Atropine in doses of 0.13 or 0.26 mg caused a maximal reduction in baseline sRaw and completely inhibited the effect of 1.0% methacholine on sRaw, but it did not inhibit the bronchomotor response to cold air. Higher doses of atropine did inhibit the effect of cold air on sRaw in a dose-dependent fashion. The dose of atropine required to inhibit this effect of cold air varied with the increase in sRaw produced by cold air after placebo. Apparently, cold air causes bronchoconstriction through vagal pathways and higher doses of antimuscarinic agents are required to inhibit vagally mediated bronchoconstriction than those required to reduce baseline airway tone or to inhibit the effects of a large dose of an inhaled muscarinic agonist.