Evidence for a Non-β2-Adrenoceptor Binding Site in Human Lung Tissue for a Subset of β2-Adrenoceptor Agonists

The aim of this study was to compare the binding profile of a range of β2-adrenoceptor (β2-AR) agonists and antagonists in human lung tissue. Radioligand saturation and competition binding experiments were performed by filtration with a β2-AR antagonist ([3H]propranolol) or agonist ([3H]vilanterol) radioligand and membrane fragments generated from lung parenchyma in the presence of 100 μM guanosine 5’-[β,γ-imido]triphosphate (Gpp(NH)p). In membranes prepared from human lung parenchyma, carmoterol, formoterol, ICI118551, propranolol and salbutamol resulted in inhibition of [3H]vilanterol binding to levels that were significantly different from indacaterol, salmeterol and vilanterol (ANOVA, Bonferroni post-test, P < 0.001 except formoterol vs indacaterol where P < 0.01). Indacaterol and salmeterol resulted in inhibition of [3H]vilanterol binding to levels that were not significantly different from vilanterol (ANOVA, Bonferroni post-test, P > 0.05). Indacaterol, salmeterol and vilanterol resulted in full inhibition of [3H]propranolol binding to levels not significantly different from ICI118551 (ANOVA, Bonferroni post-test, P > 0.05). Indacaterol, salmeterol and vilanterol bind to an additional site in human lung parenchyma membranes that is distinct from the β2-AR.