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 bronchoconstrictor responsiveness in asthma. In addressing potential mechanisms underlying this phenomenon, the relative contributions of beta-adrenoceptor-coupled transmembrane signaling mechanisms were examined in isolated rabbit tracheal smooth muscle (TSM) passively sensitized with serum from atopic asthmatic patients and in TSM comparably exposed to non-atopic (control) human serum. During half-maximal isometric contraction of the tissues with acetylcholine, relative to control TSM, the sensitized tissues exhibited significant attenuation of both their maximal relaxation (Rmax) and sensitivity (i.e., -log 50% Rmax) to cumulative administration of isoproterenol (P < 0.001) or prostaglandin (PG)E2 (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 PGE2 in sensitized TSM was 1) ablated by pretreatment with the muscarinic M2-receptor antagonists methoctramine (10(-6) M) or gallamine (10(-4) M); 2) also inhibited by pretreatment with pertussis toxin (100 ng/ml), which ADP ribosylates the inhibitory G protein (G(i)) negatively coupled to adenylate cyclase activation; and 3) associated with diminished adenosine 3',5'-cyclic monophosphate accumulation in response to isoproterenol administration. Moreover, based on Western immunoblot analysis, we found that G(i) protein expression was increased in membrane fractions from sensitized TSM, related to enhanced expression of the G(i) alpha 3 subunit. Collectively, these observations provide new evidence that the impaired beta-adrenoceptor-mediated relaxation in atopic sensitized airways is associated with increased muscarinic M2 receptor/G(i) protein-coupled expression and function.