IMPAIRED BETA-ADRENERGIC-RECEPTOR ACTIVATION OF ADENYLYL-CYCLASE IN AIRWAY SMOOTH-MUSCLE IN THE BASENJI-GREYHOUND DOG-MODEL OF AIRWAY HYPERRESPONSIVENESS

Previous studies in human asthmatics have suggested a defect in the beta-adrenergic pathway leading to cyclic adenosine monophosphate (cAMP) generation. Although these studies have suggested normal or increased numbers of beta-adrenergic receptors, limitations in the quantity of tissue available have not allowed further delineation of the biochemical or molecular mechanisms of human asthma. The basenji-greyhound (BG) dog model of nonspecific airway hyperreactivity displays similarities to human asthma, and altered functional response to beta-adrenergic agonists has been previously shown in airway tissue from this model. We have now correlated this functional impairment in beta-adrenergic response with a decreased generation of cAMP in response to isoproterenol. Organ bath studies and adenylyl cyclase assays of trachealis muscle revealed impaired responses to isoproterenol in the BG dog as compared with control dogs. Pretreatment of muscle strips from BG dogs with isoproterenol had no effect on subsequent dose-response curves to methacholine (pD2 = 7.17 +/- 0.13, 7.34 +/- 0.12, and 7.14 +/- 0.17 for control, 10(-6) M isoproterenol, and 10(-5) M isoproterenol, respectively), while muscle strips from mongrel dogs had a significant shift in methacholine responses after isoproterenol pretreatment (pD2 = 7.91 +/- 0.23, 7.48 +/- 0.29, and 6.98 +/-0.33 for control, 10(-6) M isoproterenol, and 10(-5) M isoproterenol, respectively). Adenylyl cyclase activity in response to isoproterenol was 54% in the BG trachealis membranes as compared with mongrels. Functional and biochemical responses to forskolin, NaF, prostaglandin, and dibutyryl cAMP, and the quantity of G(s)alpha were similar in the BG and control dogs. Beta-adrenergic receptors were increased in number (B(max) = 76.6 +/- 12.1 and 48.5 +/- 3.7 fmol/mg protein for BG and mongrel, respectively) but showed decreased affinity (63.8 +/- 12.6 and 34.7 +/- 6.1 pM for BG and mongrel, respectively) for I-125-cyano-pindolol in the BG. This suggests that a defect exists in ligand binding to the beta-adrenergic receptor or in receptor-G protein coupling.