High affinity [ 3H]formoterol binding sites in lung: characterization and autoradiographic mapping

Agonist binding to the β 2-adrenoceptors and its mapping were studied using the newly developed radioligand [ 3H]formoterol. The results of [ 3H]formoterol saturation binding and formoterol inhibition of [ 3H]formoterol binding were consistent with binding to a single class of receptors ( K d = 1.34 ± 0.15 nM, B max = 154.9 ± 8.0 fmol/mg protein in guinea pig lung membranes, n = 8; K d = 1.05 ± 0.17 nM, B max = 67.8 ± 8.1 fmol/mg protein in human lung membranes, n = 5) and competition assays with other agonists and antagonists disclosed only a single class of site. The nonhydrolyzable GTP analogue GTPγS caused a reduction in both K d and B max, indicating that the receptors labelled by [ 3H]formoterol are coupled to a guanine nucleotide binding regulatory protein. Receptor mapping of [ 3H]formoterol binding sites shows that β 2-adrenoceptors were widely distributed in both guinea pig and human lung, with dense labelling over airway epithelium and uniformly over alveolar walls, and sparse labelling of airway and vascular smooth muscle. In addition, submucosal glands were also sparsely labelled in human bronchus. The distribution of β 2-adrenoceptors was similar to the pattern previously described with non-selective radiolabelled antagonists in the presence of selective β 1-adrenoceptor antagonists.