The use of two different pharmacological principles to inhibit a cholinergic bronchospasm in guinea pig

The ability of inhaled atropine and clonidine to inhibit a bronchospasm produced by injected acetylcholine or by vagal stimulation, was studied in anaesthetized guinea pigs. Vagally mediated airway constriction was restrained in a concentration-related fashion by inhaled clonidine and reached nearly 100% inhibition (EC50 = 2 X 10(-5) mol/l in the nebulizer). Atropine expressed initially about the same level of spasmolytic effect but only a final prevention of 60% was possible to reach with this drug. On the contrary, inhaled atropine was able to produce a complete inhibition (EC50 = 2 X 10(-6) mol/l) when the bronchoconstriction was elicited by exogenous acetylcholine. This type of spasm was also to some degree inhibited by clonidine.