Simultaneous alpha2B- and beta2-adrenoceptor activation sensitizes the alpha2B-adrenoceptor for agonist-induced down-regulation.

We recently reported that alpha(2A)-adrenoceptor (AR) desensitization and down-regulation occurs after 24-h treatment with epinephrine (EPI) (0.3 microM) in BE(2)-C cells that express both alpha(2)- and beta(2)-ARs. The same concentration of norepinephrine (NE) has no effect. The effect of EPI is prevented by beta(2)-AR blockade and is associated with an increase in G protein-coupled receptor kinase 3 (GRK3) expression. Because differences in agonist-induced down-regulation of the alpha(2A)-versus alpha(2B)-ARs have been reported, the present study examines the effects of simultaneous activation of alpha(2B)- and beta(2)-ARs on alpha(2B)-AR number and signaling. We studied NG108 cells that naturally express alpha(2B)-ARs, and BN17 cells, NG108 cells transfected to express the human beta(2)-AR. In NG108 cells, alpha(2B)-AR desensitization and down-regulation require treatment with 20 microM EPI or NE; GRK expression was not changed. In BN17 cells expressing beta(2)-ARs, the threshold EPI concentration for alpha(2B)-AR desensitization and down-regulation was reduced to 0.3 microM; 10 microM NE was required for the same effect. Furthermore, 24-h EPI or NE treatments that produced desensitization also resulted in a selective 2-fold up-regulation of GRK3; GRK2 was unchanged. The beta-AR antagonist alprenolol (1 microM) and GRK3 antisense (but not sense) DNA blocked 0.3 microM EPI- and 10 microM NE-induced desensitization and down-regulation of the alpha(2B)-AR as well as GRK3 up-regulation. In conclusion, simultaneous activation of alpha(2B)- and beta(2)-ARs results in a 67-fold decrease in the threshold concentration of EPI required for alpha(2B)-AR down-regulation. This lower threshold for down-regulation is associated with alpha(2B)- and beta(2)-AR dependent up-regulation of GRK3 expression.