Terbutaline causes immobilization of single ß2-adrenergic receptor-ligand complexes in the plasma membrane of living A549 cells as revealed by single-molecule microscopy.

ABSTRACT

G-protein-coupled receptors are important targets for various drugs. After signal transduction, regulatory processes, such as receptor desensitization and internalization, change the lateral receptor mobility. In order to study the lateral diffusion of ß(2)-adrenergic receptors (ß(2)AR) complexed with fluorescently labeled noradrenaline (Alexa-NA) in plasma membranes of A549 cells, trajectories of single receptor-ligand complexes were monitored using single-particle tracking. We found that a fraction of 18% of all ß(2)ARs are constitutively immobile. About 2/3 of the ß(2)ARs moved with a diffusion constant of D(2) = 0.03 ± 0.001 µm(2)/s and about 17% were diffusing five-fold faster (D(3) = 0.15 ± 0.02 µm(2)/s). The mobile receptors moved within restricted domains and also showed a discontinuous diffusion behavior. Analysis of the trajectory lengths revealed two different binding durations with τ(1) = 77 ± 1 ms and τ(2) = 388 ± 11 ms. Agonistic stimulation of the ß(2)AR-Alexa-NA complexes with 1 µM terbutaline caused immobilization of almost 50% of the receptors within 35 min. Simultaneously, the mean area covered by the mobile receptors decreased significantly. Thus, we demonstrated that agonistic stimulation followed by cell regulatory processes results in a change in ß(2)AR mobility suggesting that different receptor dynamics characterize different receptor states.