Characterization of GTP-binding proteins coupled to inhibition of adenylyl cyclase in guinea pig tracheal epithelial cells.

Many important airway epithelial cell functions are regulated by intracellular cAMP. Adenylyl cyclase, the enzyme that synthesizes cAMP, is under dual regulation in many cells, but muscarinic agonists have not been shown to inhibit adenylyl cyclase in human and dog epithelial cells, despite the presence of muscarinic receptors. We question whether the lack of inhibition was related to the absence of a component of the inhibitory pathway or a lack of coupling between the components. The GTP-binding regulatory proteins (G proteins) that regulate adenylyl cyclase activity in airway epithelium have not been well characterized. We used primary cultures of guinea pig tracheal epithelial cells as a model system and identified the G proteins that modulate adenylyl cyclase activity. Immunoblot analysis demonstrated the presence of alpha subunits corresponding to stimulatory (Gs alpha) and inhibitory [Gi alpha (2) and Gi alpha (3)] G proteins as well as beta chains. These G proteins were functionally coupled to stimulation and inhibition of adenylyl cyclase in epithelial membrane preparations. Pertussis toxin-catalyzed [32P]ADP-ribosylation of Gi alpha was significantly reduced by 100 microM GTP gamma S (78.4 +/- 3.6% of control), by 100 mM NaF (41.9 +/- 9.1% of control), and by carbachol (100 microM) (29.2 +/- 9.0% of control). Atropine (10 microM) inhibited the carbachol effect by greater than 90%, suggesting that the muscarinic receptors were functionally coupled to Gi proteins. beta-Adrenergic agonists increased adenylyl cyclase activity, but muscarinic agonists failed to inhibit this enzyme. In summary, guinea pig tracheal epithelial membranes contain muscarinic receptors, Gi alpha (2) and adenylyl cyclase, which are appropriately coupled.(ABSTRACT TRUNCATED AT 250 WORDS)