The Ile164 β 2-adrenoceptor polymorphism alters salmeterol exosite binding and conventional agonist coupling to G s

Abstract

β 2-adrenoceptors (β 2AR) are polymorphic at amino acid 164 (Thr or Ile) of the fourth transmembrane domain. In transfected fibroblasts, six agonists commonly used in the treatment of bronchospasm were studied. Isoproterenol, albuterol, metaproterenol, terbutaline, formoterol, and salmeterol displayed decreased binding affinities ( K is were 1.2–3.0-fold higher) and a significant degree of impaired maximal stimulation of adenylyl cyclase (∼40%), was observed with all agonists for the Ile164 receptor. The ratios of signal transduction efficiencies (Tau function, Ile164/Thr164) varied from a low of 0.17 for terbutaline to 0.49 for salmeterol. In addition, Ile164 bound salmeterol at the exosite, as delineated in perfusion washout studies, at a decreased level (31±4.8% vs. 49±4.4% retained salmeterol, respectively, P=0.02). In cAMP production studies under perfusion conditions, this decreased exosite binding caused a ∼50% decrease in the duration of action of salmeterol at Ile164 ( t 1/2=21.0±3.6 vs. 46.8±4.1 min for Thr164, P=0.001). The durations of action for isoproterenol and formoterol under similar perfusion conditions were not different between the two receptors. These in vitro results indicate the Ile164 polymorphic receptor represents a pharmacogenetic locus for the most commonly utilized agonists in the treatment of asthma with a unique phenotype for salmeterol.