CHRONIC CARBACHOL PRETREATMENT DECREASES ADENYLYL-CYCLASE ACTIVITY INAIRWAY SMOOTH-MUSCLE

In airway smooth muscle, the regulation of adenylyl cyclase, the enzym e that synthesizes adenosine 3',5'-cyclic monophosphate, is under dual regulation by G protein-coupled receptors. It is unknown if chronic a ctivation of muscarinic receptors in airway smooth muscle alters the s timulatory adenylyl cyclase cascade to decrease airway relaxation. Bov ine airway smooth muscle pretreated with carbachol for 18 h, but not f or 30 min or 2 h, showed decreased adenylyl cyclase activity under bas al conditions and in response to isoproterenol, prostaglandin E-1, GTP , and forskolin. The quantity of beta-adrenergic receptors or of G(i) alpha proteins was unaffected by carbachol pretreatment. The effect of carbachol pretreatment was blocked by the inclusion of atropine or th e protein kinase C (PKC) inhibitor staurosporine. These results sugges t that chronic but not acute agonist pretreatment of muscarinic recept ors decreases in adenylyl cyclase stimulation at a site distal to rece ptors and that this effect is mediated by the chronic activation of PK C via the M-3 muscarinic receptor.