The Effect of β-adrenargic Agonists on Ca^2+ Sensitivity in Tracheal Smooth Muscle

To determine whether a reduction in the sensitivity to Ca 2 + is involved in relaxation by β-adrenergic receptor agonists, we examined correlation between force generation and intracellular Ca 2 + concentration in the inhibitory effects of isoproterenol (ISO) on methacholine (MCh)-induced contraction. Fura-2 was loaded in tracheal smooth muscle of guinea pigs and tension and fluorescence ratio F 3 4 0 /F 3 8 0 as an index of intracellular Ca 2 + concentration were recorded simultaneously. Addition of 0.3 μM ISO caused an inhibition in 1 μM MCh-induced contraction and a reduction in intracellular Ca 2 + concentration. The reduction in intracellular Ca 2 + concentration was, however, much smaller than that of tension. In conclusion, Ca 2 + sensitivity in addition to Ca 2 + influx plays an important role in β-adrenergic action in airway smooth muscle.