ROLE OF G-PROTEINS AND K-CA CHANNELS IN THE MUSCARINIC AND BETA-ADRENERGIC REGULATION OF AIRWAY SMOOTH-MUSCLE

We have examined the functional consequences of G protein coupling to calcium-activated potassium (K-Ca) channels using isometric tension records from guinea pig tracheal smooth muscle. After incubation with 1 mu g/ml pertussis toxin (PTX) for 6 h, the contraction response to 1 mu M methacholine (MCh) was suppressed by 31.7 +/- 5.0% (n = 10). Similarly, the contraction was inhibited by 29.1 +/- 5.0% (n = 6) after application of 0.1 mu M AF-DX 116, an M(2)-selective muscarinic receptor antagonist. Cholera toxin (CTX, 2.0 mu g/ml for 6 h), which activates the stimulatory G protein of adenylyl cyclase (G(s)), also suppressed contraction by 43.9 +/- 3.3% (n = 11). The inhibitory effects of PTX, AF-DX 116, or CTX were reversed in the presence of 100 nM charybdotoxin (ChTX), a selective K-Ca channel inhibitor. These findings suggest that disruption of inhibitory coupling between muscarinic receptor and K-Ca channels mediated by PTX-sensitive G proteins, or K-Ca channel activation induced by G(s)/adenylyl cyclase-linked processes, antagonizes muscarinic contraction. The isoproterenol concentration-inhibition curves for precontracted trachea (1 mu M MCh) were shifted to the left after perfusion with PTX or AF-DX 116, and the leftward shift of the curve was blocked by ChTX. Thus direct or indirect regulation of K-Ca channels mediated by the inhibitory guanine nucleotide binding protein (G(i)) and G(s) may play a functionally important role in the mechanical antagonism by the two receptor agonists.