Why are long-acting beta-adrenoceptor agonists long-acting?

The extended duration of bronchodilation due to formoterol and salmeterol greatly exceeds that of short acting beta 2-adrenoceptor agonists, such as salbutamol or terbutaline. This extended duration and their capacity to "reassert" airway smooth muscle relaxation in vitro despite repeated washing has prompted considerable debate on the underlying mechanism(s). The comparative pharmacology, and molecular modelling of these drugs and of the beta 2-adrenoceptor and its ligand binding core have cast doubt on the exosite/exoceptor model previously proposed to explain the behaviour of salmeterol. We present evidence supporting a unifying hypothesis that the duration of action both of formoterol and salmeterol is determined principally by their physicochemical interactions with membrane lipid bilayers (plasmalemma diffusion microkinetic model), rather than putative distinct exosite/exoceptor binding sites in or near the beta 2-adrenoceptor. This model provides a clearer understanding of the pharmacological profile of these drugs (rate of onset, duration, "reassertion", interaction with hydrophilic and hydrophobic beta 2-adrenoceptor antagonists), and explains why in human airway smooth muscle in vitro a true relaxation-concentration response may not exist for salmeterol.