Functional antagonism in canine tracheal smooth muscle: inhibition by methacholine of the mechanical and biochemical responses to isoproterenol.

The biochemical basis for the functional interaction between bronchoconstricting and bronchodilating pathways was investigated. Contracting canine trachealis strips with increasing concentrations of methacholine resulted in a progressive shift to the right of isoproterenol concentration-response curves. Thus, the EC50 for the relaxant response to isoproterenol was nearly 500-fold higher in preparations exposed to 3.0 microM methacholine than in tissues exposed to 0.03 microM methacholine. The maximum relaxation produced by isoproterenol was also dependent upon the initial muscarinic cholinergic tone. For example, isoproterenol reversed completely the contraction induced by 0.03 microM methacholine but did not relax trachealis strips contracted with 30 microM methacholine. To identify the molecular mechanism responsible for this functional antagonism, experiments were conducted to determine the effect of methacholine on isoproterenol-stimulated cyclic AMP accumulation and cyclic AMP-dependent protein kinase activation. Methacholine did not alter basal cyclic AMP content but did reduce cyclic AMP accumulation in response to isoproterenol. Furthermore, the ability of isoproterenol to activate cyclic AMP-dependent protein kinase was inhibited by methacholine in a concentration-dependent manner. This inhibition paralleled the decrease in mechanical responsiveness to isoproterenol. These results suggest that muscarinic cholinergic stimulation of canine tracheal smooth muscle functionally antagonizes the relaxant responses to beta adrenergic agonists and that a portion of this antagonism may be due to a suppression of catecholamine-stimulated cyclic AMP accumulation and cyclic AMP-dependent protein kinase activation.