A single dose of an inhaled muscarinic antagonist will protect against methacholine induced bronchoconstriction and increase the dose of methacholine required to cause bronchoconstriction about 4 doubling doses. Beta agonist bronchodilators also shift the dose response curve to the right but the protection against methacholine induced bronchoconstriction is lost (i.e. tolerance develops) with regular use. This study will determine if bronchoprotection is lost following regular use of tiotropium.~This will be a randomized, double-blind placebo controlled study to assess the development of tolerance following regular use of tiotroium in a population of mild asthmatics.~Tiotropium will be administered at the clinically recommended dose (5mcg/day) for a total of 8 days. Methacholine challenges will be performed at baseline (pre-treatment), at one hour post first dose and on day 8, one hour after the final dose. The same will dosing schedule and assessments will occur with an identical placebo inhaler.
The study will assess if regular use of tiotropium (i.e. daily) results in loss of bronchoprotection against methacholine induced bronchoconstriction in those with mild asthma.