Differential desensitization and phosphorylation of three cloned and transfected α₂-adrenergic receptor subtypes

Genes encoding 3 distinct subtypes of human α₂-adrenergic receptor are known and are found, respectively, on chromosome 10, 4, and 2 (α₂-C10, α₂-C4, and α₂-C2 adrenergic receptors). All 3 receptors inhibit adenylyl cyclase via G(i) proteins. To study and compare their regulatory properties we assessed the ability of each to undergo agonist-promoted desensitization and phosphorylation. When Chinese hamster ovary cells stably expressing each of the three receptor genes were incubated with epinephrine for 20 min, a marked decrease in sensitivity to subsequent agonist-mediated inhibition of adenylyl cyclase was observed for the α₂-C10 and α₂-C2 receptors but not for the α₂-C4 receptors. When similar incubations were performed with ³²P(i)-labeled cells and the receptors were immunoprecipitated with specific antibodies, α₂-C10 and α₂-C2 receptors were found to undergo an ~3-fold increase in receptor phosphorylation after epinephrine exposure. When transfected into COS cells epinephrine also stimulated phosphorylation of α₂-C10 and α₂-C2 receptors while having only a slight effect on α₂- C4 receptors. Cotransfection of the cells with the cDNA encoding the β- adrenergic receptor kinase further increased receptor phosphorylation for α₂-C10 and α₂-C2 receptors while having little or no effect on α₂-C4 receptors. Moreover purified and reconstituted recombinant α₂-C10 receptors could be phosphorylated in an agonist-dependent fashion whereas α₂-C4 receptors could not. These observations suggest receptor subtype-specific differences in susceptibility to regulatory phosphorylation and desensitization.