Muscarinic cholinergic inhibition of cyclic AMP accumulation in airway smooth muscle. Role of a pertussis toxin-sensitive protein.

Muscarinic agonists are potent constrictors of airway smooth muscle. In many tissues, muscarinic agonists also reduce intracellular cyclic AMP by inhibiting its synthesis. In airway smooth muscle, the role muscarinic agonists have in the regulation of cyclic AMP content is not established. The hypothesis of our study was that muscarinic agonists reduce cyclic AMP accumulation in dog tracheal smooth muscle, and that this reduction involves a pertussis toxin-sensitive regulatory protein (Gi) that couples occupancy of the muscarinic receptor by the agonist to inhibition of adenylate cyclase. We measured cyclic AMP accumulation in tracheal smooth muscle from 4 dogs, and found that acetylcholine (10(-4) M) diminished basal and isoproterenol-stimulated cyclic AMP accumulation by 37.6 +/- 12.1% and 39.4 +/- 1.9%, respectively (mean +/- SEM, p less than 0.05). This reduction of cyclic AMP was dose-dependent and inhibited by atropine (10(-5) M). Incubation of dog tracheal smooth muscle with pertussis toxin (12.5 micrograms/ml) for 21 h catalyzed covalent modification of a membrane protein with an approximate Mr of 40,000. In control strips, acetylcholine decreased isoproterenol-stimulated cyclic AMP content by 33.7 +/- 5.6% (p less than 0.05). However, in strips treated with pertussis toxin (10 micrograms/ml), acetylcholine decreased cyclic AMP by only 7.9 +/- 4.8%; this change was not significant. Thus, pertussis toxin (10 micrograms/ml) attenuated muscarinic cholinergic regulation of cyclic AMP. These findings are consistent with muscarinic cholinergic regulation of adenylate cyclase via Gi in dog tracheal smooth muscle. In addition, the techniques we employed should permit the evaluation of other functions of pertussis toxin-sensitive G proteins in airway smooth muscle.