Transgenic overexpression of beta(2)-adrenergic receptors in airway smoothmuscle alters myocyte function and ablates bronchial hyperreactivity

beta(2)-Adrenergic receptors (beta(2)AR) act to relax airway smooth muscle and can serve to counteract hyperresponsiveness, although the effect may no t be ablative even in the presence of exogenous agonist, Within this signal ing cascade that ultimately transduces smooth muscle relaxation, a signific ant "spare receptor" pool has been hypothesized to be present in the airway . In order to modify the relationship between beta(2)AR and downstream effe cters, transgenic mice (TG) were created overexpressing beta(2)AR similar t o 75-fold in airway smooth muscle using a mouse smooth muscle a-actin promo ter. While >90% of these receptors were expressed on the smooth muscle cell surface, the percentage of receptors able to form the agonist-promoted hig h affinity complex was less than that found with nontransgenic (NTG) cells (R-H = 18 versus 36%). Nevertheless, beta(2)AR signaling was found to be en hanced. Intact airway smooth muscle cells from TG had basal cAMP levels tha t were greater than NTG cells. A marked increase in agonist-stimulated cAMP levels was found in the TG (similar to 200% stimulation over basal) compar ed with NTG (similar to 50% over basal) cells. Adenylyl cyclase studies gav e similar results and also showed a 10-fold lower EC50 for TC: cells. Trach eal rings from TG mice that were precontracted with acetylcholine had an en hanced responsiveness (relaxation) to beta-agonist, with a 60-fold decrease in the ED50, indicating that the enhanced signaling imposed by overexpress ion results in an increase in the coordinated function of the intact airway cells. In vivo studies showed a significantly blunted airway resistance re sponse to the inhaled bronchoconstrictor methacholine in the TG mice. Indee d, with beta-agonist pretreatment, the TG mice displayed no response whatso ever to methacholine. These results are consistent with beta(2)AR being the limiting factor in the transduction system. Increases in the initial compo nent of this transduction system (the beta(2)AR) are sufficient to markedly alter signaling and airway smooth muscle function to the extent that bronc hial hyperresponsiveness is ablated, consistent with an antiasthma phenotyp e.