Desensitization of α(2A)-adrenoceptor signalling by modest levels of adrenaline is facilitated by β(2)-adrenoceptor-dependent GRK3 up-regulation

1. Adrenaline (ADR) and noradrenaline (NA) can simultaneously activate inhibitory α(2)- and stimulatory β-adrenoceptors (AR). However, ADR and NA differ significantly in that ADR is a potent β(2)-AR agonist while NA is not. Only recently has the interaction resulting from the simultaneous activation of α(2)- and β(2)-AR been examined at the cellular level to determine the mechanisms of α(2)-AR regulation following concomitant activation of both α(2)- and β(2)-ARs by chronic ADR. 2. This study evaluates β(2)-AR regulation of α(2A)-AR signalling following chronic ADR (300 nM) and NA (1 and 30 μM) treatments of BE(2)-C human neuroblastoma cells that natively express both β(2)- and α(2A)-ARs. 3. Chronic (24 h) treatment with ADR (300 nM) desensitized the response to the α(2A)-AR agonist, brimonidine, in BE(2)-C cells. Addition of the β-AR antagonist, propranolol, blocked the ADR-induced α(2A)-AR desensitization. Unlike ADR, chronic NA (1 μM) treatment had no effect on the α(2A)-AR response. However if NA was increased to 30 μM for 24 h, α(2A)-AR desensitization was observed; this desensitization was partially reversed by propranolol. 4. Chronic ADR (300 nM) treatment reduced α(2A)-AR binding levels, contributing to the α(2A)-AR desensitization. This decrease was prevented by addition of propranolol during ADR treatment. Chronic NA (30 μM), like ADR, treatment lowered specific binding, whereas 1 μM NA treatment was without effect. 5. Chronic ADR treatment produced a significant increase in GRK3 levels and this was blocked by propranolol or GRK2/3 antisense DNA treatment. This antisense DNA, common to both GRK2 and GRK3, also blocked chronic ADR-induced α(2A)-AR desensitization and down-regulation. 6. Acute (1 h) ADR (300 nM) or NA treatment (1 μM) produced α(2A)-AR desensitization. The desensitization produced by acute treatment was β-AR independent, as it was not blocked by propranolol. 7. We conclude that chronic treatment with modest levels of ADR produces α(2A)-AR desensitization by mechanisms that involve up-regulation of GRK3 and down-regulation of α(2A)-AR levels through interactions with the β(2)-AR