Differential inhibitory effect of isoproterenol on contractions of canine airways.

We examined the inhibitory effect of isoproterenol on contractions of isolated canine airways induced by histamine, 5-hydroxytryptamine (5-HT), and acetylcholine. Airway contractions were matched by using agonist concentrations that produced approximately 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 X 10(-9), 3.6 X 10(-8), and 9.0 X 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 greater than or equal to 10(-4) M never totally relaxed airways contracted by acetylcholine. We conclude that isoproterenol inhibits airway contractions induced by histamine and 5-HT much more effectively than those induced by acetylcholine.