MECHANISM OF IMPAIRED BETA-ADRENOCEPTOR RESPONSIVENESS IN ATOPIC SENSITIZED AIRWAY SMOOTH-MUSCLE

Decreased airway relaxation to Beta-adrenoceptor stimulation has been hypothesized as a potential mechanism leading to enhanced bronchoconst rictor responsiveness in asthma. In addressing potential mechanisms un derlying this phenomenon, the relative contributions of beta-adrenocep tor-coupled transmembrane signaling mechanisms were examined in isolat ed rabbit tracheal smooth muscle (TSM) passively sensitized with serum from atopic asthmatic patients and in TSM comparably exposed to non-a topic (control) human serum. During half-maximal isometric contraction of the tissues with acetylcholine, relative to control TSM, the sensi tized tissues exhibited significant attenuation of both their maximal relaxation (R(max)) and sensitivity (i.e., -log 50% R(max)) to cumulat ive administration of isoproterenol (P < 0.001) or prostaglandin (PG)E (2) (P < 0.001). In contrast, the relaxation responses to forskolin, a diterpene that directly activates adenylate cyclase, were similar in both tissue groups. Extended studies demonstrated that the attenuated relaxation to isoproterenol and PGE(2) in sensitized TSM was 1) ablate d by pretreatment with the muscarinic M(2)-receptor antagonists methoc tramine (10(-6) M) or gallamine (10(-4) M); 2) also inhibited by pretr eatment with pertussis toxin (100 ng/ml), which ADP ribosylates the in hibitory G protein (G(i)) negatively coupled to adenylate cyclase acti vation; and 3) associated with diminished adenosine 3',5'-cyclic monop hosphate accumulation in response to isoproterenol administration. Mor eover, based on Western immunoblot analysis, we found that G(i) protei n expression was increased in membrane fractions from sensitized TSM, related to enhanced expression of the G(i) alpha(3) subunit. Collectiv ely, these observations provide new evidence that the impaired beta-ad renoceptor-mediated relaxation in atopic sensitized airways is associa ted with increased muscarinic M(2) receptor/G(i) protein-coupled expre ssion and function.