Amitriptyline inhibits bronchoconstriction independent of direct receptor binding and reduces number of caveolae

Bronchial asthma is a chronic inflammatory disease with rising prevalence worldwide. Apart from the immunological role of the tricyclic antidepressant amitriptyline in bronchial asthma, there is emerging evidence that inhaled amitriptyline directly reduces acute bronchoconstriction. However, the mechanism by which amitriptyline influences bronchial tone remains poorly understood. To influence bronchoconstriction, rat precision-cut lung slices treated with varying concentrations of amitriptyline (0-5 µM) and incubated with inhibitors targeting different signaling pathways. Amitriptyline reduces acetylcholine- and serotonin-induced bronchoconstriction. Neither the muscarinic antagonist ipratropium nor the phospholipase C inhibitor U73122, nor the protein kinase C inhibitor chelerythrine diminished the effect of amitriptyline. Inhibition of calcium sensitizing and induction failed to alter amitriptyline's effect on bronchoconstriction. Caveolae-as part of the plasma membrane-display a microenvironment, where regulation of signal transduction takes place. Similar to methyl ß cyclodextrin (MBCD), a common substance to destroy caveolae, amitriptyline dramatically reduced the number of caveolae in lung tissue. However, unlike MBCD, this effect could not be explained by cholesterol depletion alone, as cholesterol repletion did not reverse amitriptyline's effect. Furthermore, neither simvastatin (a lipid lowering agent) nor cytochalasin D (an inhibitor of actin polymerization), influenced the inhibitory effect of amitriptyline on bronchoconstriction. In conclusion, amitriptyline inhibits bronchoconstriction independently of direct receptor binding or interaction. It also reduces the total number of caveolae without effects on cholesterol lowering pathways or actin depolymerization. A more general mechanism seems likely, as inhibition of single signal transduction pathways failed. Further studies are required to elucidate the underlying mechanisms.