Comparison of the potency of adenosine as an agonist at human adenosine receptors expressed in Chinese hamster ovary cells

The potency of adenosine and inosine as agonists at human adenosine receptors was examined in a functional assay using changes in cyclic AMP (cAMP) formation in intact Chinese hamster ovary (CHO) cells stably transfected with the human A(1), A(2A), A(2B), and A(3) receptors. Adenosine increased cAMP formation in cells expressing the A(2A) (Ec(50): 0.7 muM) and A(2B) (Ec(50): 24 muM) receptors and inhibited forskolin (0.3-3 muM)-stimulated cAMP formation in cells expressing the A(1) (Ec(50): 0.7 muM) and A(3) receptors (Ec(50): 0.29 muM). The potency of adenosine at the A(2A) and A(2B) receptors was not altered by the presence of the uptake inhibitor nitrobenzylthioinosine (NBMPR), whereas it was increased about h-fold by NBMPR at the A(1) and A(3) receptors. In the presence of NBMPR, inosine was a potent agonist (Ec(50): 7 and 0.08 muM at the A(1) and A(3) receptors, respectively), but with low efficacy especially at the A(3) receptors. No effect of inosine was seen at the A(2) receptors. Caffeine, theophylline, and paraxanthine shifted the dose-response curve for adenosine at the A(1), A(2A), and A(2B) receptors. These results indicate that adenosine is the endogenous agonist at all human adenosine receptors and that physiological levels of this nucleoside can activate A(1), A(2A), and A(3) receptors on cells where they are abundantly expressed, whereas pathophysiological conditions are required to stimulate A(2B) receptors to produce cyclic AMP. (C) 2001 Elsevier Science Inc. All rights reserved.