INDUCTION OF MULTIPLE EFFECTS ON ADENYLYL-CYCLASE REGULATION BY CHRONIC ACTIVATION OF THE HUMAN A(3) ADENOSINE RECEPTOR

The A(3) adenosine receptor (A(3)AR) contributes to several cardiovasc ular effects of adenosine, including antihypertensive and cardioprotec tive effects. Although several studies have detailed the mechanisms un derlying agonist-mediated desensitization of the rat A(3)AR, the regul ation of the human A(3)AR, which displays only a 70% amino acid identi ty with the rat homologue, has not been addressed. Using a Chinese ham ster ovary cell line stably expressing a recombinant human A(3)AR, we demonstrated that prolonged treatment with the AR agonist 5'-N-ethylca rboxamidoadenosine induces uncoupling of the A(3)AR from G proteins an d functional desensitization. In addition to A(3)AR desensitization, a 1.5-2.5-fold increase was noted in the adenylyl cyclase (AC) activity achieved in the presence of GTP with or without forskolin. This sensi tization of AC activity was not a consequence of the down-regulation o f G(i) proteins induced by NECA treatment and was not associated with sustained or transient increases in the expression of G(s). Time cours e experiments revealed that the onset of sensitization was half-maxima l between 2 and 3 hr but was not due to the synthesis of new proteins because cycloheximide treatment failed to inhibit sensitization. The i nability of the sensitization process to alter the AC activity obtaine d in the presence of manganese chloride suggests that prolonged A(3)AR activation increases the coupling efficiency between G(s) and AC cata lytic units. This phenomenon has implications for long term cellular a daptation to agonist because in agonist-treated cells, the extent to w hich a suboptimal concentration of forskolin could increase phosphoryl ation of the cAMP-responsive element binding protein was elevated comp ared with vehicle-treated controls.