Differential inhibitory effect of isoproterenol on contractions of canine airways

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Abstract
The inhibitory effect of isoproterenol on contractions of isolated canine airways induced by histamine, 5-hydroxytryptamine (5-HT) and acetylcholine were examined. Airway contractions were matched by using agonist concentrations that produced .apprx. 60% of the contractile force generated during a previous exposure to 10-4 M acetylcholine. Concentrations of isoproterenol causing 50% relaxations of histamine-, 5-HT-, and acetylcholine-induced contractions were 9.9 .times. 10-9, 3.6 .times. 10-8, and 9.0 .times. 10-8 M, respectively. Total relaxation of histamine- and 5-HT-induced contractions occurred at 10-7 and 10-6 M isoproterenol, respectively, whereas isoproterenol concentrations .gtoreq. 10-4 M never totally relaxed airways contracted by acetylcholine. Apparently, isoproterenol inhibits airway contractions induced by histamine and 5-HT much more effectively than those induced by acetylcholine.