Chronic carbachol pretreatment decreases adenylyl cyclase activity in airway smooth muscle

In airway smooth muscle, the regulation of adenylyl cyclase, the enzyme that synthesizes adenosine 3',5'-cyclic monophosphate, is under dual regulation by G protein-coupled receptors. It is unknown if chronic activation of muscarinic receptors in airway smooth muscle alters the stimulatory adenylyl cyclase cascade to decrease airway relaxation. Bovine airway smooth muscle pretreated with carbachol for 18 h, but not for 30 min or 2 h, showed decreased adenylyl cyclase activity under basal conditions and in response to isoproterenol, prostaglandin E1, GTP, and forskolin. The quantity of beta-adrenergic receptors or of Gi alpha proteins was unaffected by carbachol pretreatment. The effect of carbachol pretreatment was blocked by the inclusion of atropine or the protein kinase C (PKC) inhibitor staurosporine. These results suggest that chronic but not acute agonist pretreatment of muscarinic receptors decreases in adenylyl cyclase stimulation at a site distal to receptors and that this effect is mediated by the chronic activation of PKC via the M3 muscarinic receptor.