Functional receptor coupling to G(i) is mechanism of agonist-promoted desensitization of the beta(2)-adrenergic receptor

The beta(2)-adrenergic receptor (beta(2)AR) couples to G(s), activating ade nylyl cyclase (AC) and increasing cAMP. Such signaling undergoes desensitiz ation with continued agonist exposure. beta(2)AR also couple to G(i) after receptor phosphorylation by the cAMP dependent protein kinase A, but the ef ficiency of such coupling is not known. Given the PKA dependence of beta(2) AR-G(i) coupling, we explored whether this may be a mechanism of agonist-pr omoted desensitization. HEK293 cells were transfected to express beta(2)AR or beta(2)AR and G(i alpha 2), and then treated with vehicle or the agonist isoproterenol to evoke agonist-promoted beta(2)AR desensitization. Membran e AC activities showed that G(i alpha 2) overexpression decreased basal lev els, but the fold-stimulation of the AC over basal by agonist was not alter ed. However, with treatment of the cells with isoproterenol prior to membra ne preparation, a marked decrease in agonist-stimulated AC was observed wit h the cells overexpressing G(i alpha 2). In the absence of such overexpress ion, beta(2)AR desensitization was 23+/-7%, while with 5-fold G(i alpha 2) overexpression desensitization was 58+/-5% (p<0.01, n=4). The effect of G(i ) on desensitization was receptor-specific, in that forskolin responses wer e not altered by G(i alpha 2) overexpression. Thus, acquired beta(2)AR coup ling to G(i) is an important mechanism of agonist-promoted desensitization, and pathologic conditions that increase G(i) levels contribute to beta(2)A R dysfunction.