Impact of the cAMP efflux and extracellular cAMP-adenosine pathway on airway smooth muscle relaxation induced by formoterol and phosphodiesterase inhibitors

β_2-adrenoceptors agonists and phosphodiesterase (PDE) inhibitors are effective bronchodilators, due to their ability to increase intracellular cyclic AMP (cAMP) levels and induce airway smooth muscle (ASM) relaxation. We have shown that increment of intracellular cAMP induced by β₂-adrenoceptors agonist fenoterol is followed by efflux of cAMP, which is converted by ecto-PDE and ecto-5'-nucleotidases (ecto-5'NT) to adenosine, leading to ASM contraction. Here we evaluate whether other classical bronchodilators used to treat asthma and chronic obstructive pulmonary disease (COPD) could induce cAMP efflux and, as consequence, influence the ASM contractility. Our results showed that β₂-adrenoceptor agonists formoterol and PDE inhibitors IBMX, aminophylline and roflumilast induced cAMP efflux and a concentration-dependent relaxation of rat trachea precontracted with carbachol. Pretreatment of tracheas with MK-571 (MRP transporter inhibitor), AMP-CP (ecto-5'NT inhibitor) or CGS-15943 (nonselective adenosine receptor antagonist) potentiated the relaxation induced by β₂-adrenoceptor agonists but did not change the relaxation induced by PDE inhibitors. These data showed that all bronchodilators tested were able to induce cAMP efflux. However, only β₂-adrenoceptor-induced relaxation of tracheal smooth muscle was affected by cAMP efflux and extracellular cAMP-adenosine pathway.