SALMETEROL-INDUCED DESENSITIZATION, INTERNALIZATION AND PHOSPHORYLATION OF THE HUMAN BETA(2)-ADRENOCEPTOR

1 Partial agonists of the beta(2)-adrenoceptor which activate adenylyl cyclase are widely used as bronchodilators for the relief of bronchoc onstriction accompanying many disease conditions, including bronchial asthma. The bronchodilator salmeterol has both a prolonged duration of action in bronchial tissue and the ability to reassert this activity following the temporary blockade of human beta(2)-adrenoceptors with a ntagonist. 2 We have compared the activation and desensitization of hu man beta(2)-adrenoceptor stimulation of adenylyl cyclase induced by sa lmeterol, adrenaline and salbutamol in a human lung epithelial line, B EAS-2B, expressing beta(2)-adrenoceptor levels of 40-70 fmol mg(-1), a nd in human embryonic kidney (HEK) 293 cell lines expressing 2-10 pmol mg(-1). The efficacy observed for the stimulation of adenylyl cyclase by salmeterol was only congruent to 10% of that observed for adrenali ne in BEAS-2B cells expressing low levels of beta(2)-adrenoceptor, but similar to adrenaline in HEK 293 cells expressing very high levels of receptors. Salmeterol pretreatment of these cells induced a rapid and stable activation of adenylyl cyclase activity which resisted extensi ve washing and beta(2)-adrenoceptor antagonist blockade, consistent wi th binding to a receptor exosite and/or to partitioning into membrane lipid. 3 The desensitization and internalization of beta(2)-adrenocept ors induced by the partial agonists salmeterol and salbutamol were con siderably reduced relative to the action of adrenaline. Consistent wit h these observations, the initial rate of phosphorylation of the recep tor induced by salmeterol and salbutamol was much reduced in compariso n to adrenaline. 4 Our data suggest that the reduction in the rapid ph ase of desensitization of beta(2)-adrenoceptors after treatment with s almeterol or salbutamol is caused by a decrease in the rate of beta(2) -adrenoceptor kinase (beta ARK) phosphorylation and internalization. I n contrast, the fate of cyclic AMP-dependent protein kinase (PKA)-medi ated phosphorylation by these partial agonists appears to be similar t o adrenaline.