AGONIST-DEPENDENT PHOSPHORYLATION AND DESENSITIZATION OF THE RAT A(3) ADENOSINE RECEPTOR - EVIDENCE FOR A G-PROTEIN-COUPLED RECEPTOR KINASE-MEDIATED MECHANISM

A(3) adenosine receptor (A(3)AR) activation contributes to both the cardioprotective and antihypertensive effects of adenosine. To date, no studies have examined the mechanisms by which this receptor undergoes rapid homologous desensitization. Therefore, a functional hemagglutinin epitope-tagged A(3)AR has been stably expressed in Chinese hamster ovary cells, and its regulation by the AR agonist 5'-N-ethylcarboxamidoadenosine (NECA) has been studied. Cellular exposure to NECA induces rapid (t1/2 = similar to 1 min) A(3)AR phosphorylation on serine and threonine residues. This is associated with a functional desensitization and a 30-40% reduction in the number of high affinity agonist binding sites as determined by radioligand binding assays. Activation of second messenger-regulated kinases could not mimic the effect of NECA, suggesting a role for G-protein-coupled receptor kinases (GRKs). In vitro phosphorylation assays demonstrate that phosphorylation of agonist-occupied A(3)ARs is enhanced by GRK2 and that cellular pretreatment with NECA dramatically inhibits subsequent GRB2-mediated phosphorylation in vitro. Therefore, the A(3)AR is phosphorylated in situ by a kinase similar or identical to GRK2, and this may be involved in rapid functional desensitization of the A(3)AR.